U.S. patent application number 12/844427 was filed with the patent office on 2010-11-18 for method and apparatus for active control of golf club impact.
This patent application is currently assigned to HEAD USA, INC.. Invention is credited to NESBITT W. HAGOOD, JASON HORODEZKY.
Application Number | 20100292024 12/844427 |
Document ID | / |
Family ID | 34138923 |
Filed Date | 2010-11-18 |
United States Patent
Application |
20100292024 |
Kind Code |
A1 |
HAGOOD; NESBITT W. ; et
al. |
November 18, 2010 |
METHOD AND APPARATUS FOR ACTIVE CONTROL OF GOLF CLUB IMPACT
Abstract
A method and apparatus for actively controlling the impact
between a club head and a golf ball. A golf club head has a face
with an actuator material or device mechanically coupled to
influence face motion. The face actuation controls impact
parameters, impact properties, or resulting ball parameters such as
speed, direction and spin rates resulting from the impact event
between the face of the club and the golf ball. Further, the
apparatus has a control device for determining the actuation of the
face. Several embodiments are presented for controlling parameters
such as ball speed and direction. The invention can use energy
derived from the ball impact, converted into electrical energy, and
then reapplied in a controlled fashion to influence an aspect of
the face, such as position, velocity, deformation, stiffness,
vibration, motion, temperature, or other physical parameter.
Inventors: |
HAGOOD; NESBITT W.;
(Wellesley, MA) ; HORODEZKY; JASON; (Newbury Park,
CA) |
Correspondence
Address: |
LEONARD TACHNER, A PROFESSIONAL LAW;CORPORATION
17961 SKY PARK CIRCLE, SUITE 38-E
IRVINE
CA
92614
US
|
Assignee: |
HEAD USA, INC.
Norwalk
CT
|
Family ID: |
34138923 |
Appl. No.: |
12/844427 |
Filed: |
July 27, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10915804 |
Aug 9, 2004 |
7780535 |
|
|
12844427 |
|
|
|
|
60494739 |
Aug 14, 2003 |
|
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Current U.S.
Class: |
473/329 ;
473/332; 473/409 |
Current CPC
Class: |
A63B 53/047 20130101;
A63B 53/0487 20130101; A63B 53/0462 20200801; A63B 2220/10
20130101; A63B 2225/50 20130101; A63B 53/0458 20200801; A63B
53/0454 20200801; A63B 53/0466 20130101; A63B 60/00 20151001; A63B
69/362 20200801; A63B 2220/40 20130101; A63B 53/04 20130101; A63B
69/3685 20130101; A63B 2220/53 20130101; A63B 60/54 20151001; A63B
2220/30 20130101; A63B 60/42 20151001; A63B 53/0433 20200801; A63B
69/3632 20130101; A63B 2209/14 20130101; A63B 53/045 20200801; A63B
53/08 20130101 |
Class at
Publication: |
473/329 ;
473/332; 473/409 |
International
Class: |
A63B 53/08 20060101
A63B053/08; A63B 53/04 20060101 A63B053/04 |
Claims
1. A golf club head having a hitting surface for impacting a golf
ball; the head comprising: a transducer for converting mechanical
energy from said surface during a golf ball impact event to
electrical energy; a circuit coupled to said transducer for
selectively generating a triggering signal responsive to said
electrical energy; and an actuator mechanically coupled to said
hitting surface and actuated responsive to said triggering signal,
said actuator mechanically altering said hitting surface and
thereby altering golf ball impact in adaptively controlled manner
during said impact event in response to said electrical energy.
2. The golf club head recited in claim 1 wherein at least one of
said transducer and said actuator comprises a piezoelectric
element.
3. The golf club head recited in claim 2 wherein said piezoelectric
element is affixed to said hitting surface.
4. The golf club head recited in claim 3 wherein said piezoelectric
element is coupled to said hitting surface.
5. The golf club head recited in claim 3 wherein said piezoelectric
element is interconnected to said hitting surface by an
intermediate member.
6. The golf club head recited in claim 3 wherein said piezoelectric
element is braced against said hitting surface.
7. The golf club head recited in claim 1 further comprising a
support structure within said head, said support structure
maintaining firm contact between said hitting surface and said
actuator.
8. The golf club head recited in claim 3 further comprising a
support structure within said head, said support structure
maintaining firm contact between said hitting surface and said
piezoelectric element.
9. The golf club head recited in claim 7 wherein said support
structure comprises a conically shaped housing.
10. The golf club head recited in claim 8 wherein said support
structure comprises a conically shaped housing.
11. The golf club head recited in claim 1 wherein said actuator is
configured to cause said hitting surface to vibrate at a selected
frequency while said golf ball is being impacted by said hitting
surface.
12. The golf club head recited in claim 1 wherein said actuator is
configured to cause said hitting surface to vibrate at an
ultra-sonic frequency.
13. The golf club head recited in claim 1 wherein said actuator is
configured to cause said hitting surface to vibrate at a frequency
and with an amplitude sufficient to interrupt contact between said
hitting surface and said golf ball.
14. The golf club head recited in claim 1 wherein said circuit
comprises a reactive impedance for storing said electrical
energy
15. The golf club head recited in claim 1 wherein said circuit
comprises a reactance for storing said electrical energy.
16. The golf club head recited in claim 1 wherein said circuit
comprises an inductor for storing said electrical energy.
17. The golf club head recited in claim 1 wherein said circuit
comprises an inductor and a capacitor for storing said electrical
energy.
18. The golf club head recited in claim 1 wherein said circuit
comprises a switch for selectively applying said electrical energy
in response to a threshold parameter of said hitting surface
impacting said golf ball.
19. The golf club head recited in claim 18 wherein said parameter
is the magnitude of an electrical voltage produced by said
transducer in response to said impacting.
20. A method of reducing the effective coefficient of friction
between the face of a golf club head and a golf ball; the method
comprising the steps of: establishing a transducer in said golf
club head; and, automatically actuating said transducer to convert
the energy upon ball impact with said face during an impact event
into an electro-mechanically actuated ultra-sonic vibration of said
face to thereby mechanically alter said face for the interaction of
said face and said ball in adaptively controlled manner during said
impact event.
21. The method recited in claim 20 wherein said converting step
comprises the steps of: converting said ball impact energy into
electrical energy; and converting said electrical energy into said
ultra-sonic vibration.
22. The method recited in claim 21 wherein said converting steps
are each carried out using a piezoelectric element mechanically
coupled to said face.
23. A golf club head comprising: a hitting surface; and, a variable
stiffening element coupled to said hitting surface for automatic
electro-mechanical actuation, said variable stiffening element
selectively increasing and decreasing the stiffness of said hitting
surface of said head in adaptively controlled manner during an
impact event responsive to impact with a ball during said impact
event.
24. The golf club head recited in claim 23 wherein said stiffening
element comprises a piezoelectric element having a first level of
stiffness when a short circuit configuration is generated
thereacross and a second level of stiffness when a open circuit
configuration is generated thereacross.
25. The golf club head recited in claim 24 wherein the
configuration of said piezoelectric element is determined by a
switch controlled by a sensor responsive to the level of impact of
said golf club head with a golf ball.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Continuation of currently-pending U.S.
Non-provisional patent application Ser. No. 10/915,804, filed Aug.
9, 2004, which claims priority from U.S. Provisional Patent
Application Ser. No. 60/494,739 filed Aug. 14, 2003.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention pertains to the field of advanced
sporting equipment design and in particular to the design and
operation of a golf club head system for control of the impact
between a club head and a golf ball.
[0004] 2. Background Art
[0005] The present invention pertains to achieving an increase in
the accuracy and distance of a golf club (e.g., a driver) through
the application of controls techniques and actuation technology to
the design of the club. There have been many improvements over the
years which have had measurable impact on the accuracy and distance
which a golfer can achieve. These have typically focused on the
design of passive systems; those which do not have the ability to
change any of their physical parameters under active control during
the swing and in particular during the impact event with the golf
ball. Typical passive performance improvements such as head shape
and volume, weight distribution and resulting components of the
inertia tensor, face thickness and thickness profile, face
curvatures and CG locations, all pertain to the selection of
optimum constant physical and material parameters for the golf
club. The present invention pertains to the development of an
active system where critical parameters of the golf club and head
(for example surface position/shape/curvature or effective
coefficient of friction, or face stiffness) can be selectively
controlled in response to the actual state of the physical
head-ball system. Such states can be head velocity, impact force,
intensity, impact duration and timing, absolute location of the
head or relative location of the ball on the face, orientation of
the head relative to the ball and swing path or parameters,
physical deformation of the face, or any of numerous physically or
electrically measurable conditions.
[0006] The present invention relies on the field of controls
technologies and in particular structural or elastic system
actuation technologies and control algorithms for such systems. See
for example: Fuller, C. R. et al., Active Control of Vibration
Academic Press, San Diego, Calif. 1996. A particular embodiment of
one controlled system relies on friction control using ultrasonic
vibration (Katoh). An alternate embodiment of one controlled system
relies on changing the effective stiffness of the face to control
impact with the ball. The present invention also relies on the
concept of piezoelectric energy harvesting and/or simultaneous
energy harvesting from and actuation of mechanical systems.
Piezoelectric energy harvesting is described in the following U.S.
Pat. Nos. 4,504,761; 4,442,372; 5,512,795; 4,595,856, 4,387,318;
4,091,302; 3,819,963; 4,467,236; 5,552,657; and 5,703.474.
[0007] The impact between the ball and the head can be interpreted
in terms of the idealized impact between two elastic bodies each
having freedom to translate and rotate in space i.e. full 6 degrees
of freedom (DOF) bodies, each having the ability to deform at
impact, and each having fully populated mass and inertia tensors.
The typical initial condition for this event is a stationary ball
and high velocity head impacting the ball at a perhaps eccentric
point substantially on or substantially off the face of the club
head. The impact results in high forces both normal and tangential
to the contact surfaces between the head and the ball. These forces
integrate over time to determine the speed and direction, forming
velocity vector and spin vectors of the ball after it leaves the
face, hereafter called the impact resultants. These interface
forces are determined by many properties including elasticity of
the two bodies, material properties and dissipation, surface
friction coefficients, body masses and inertia tensors.
[0008] Some of these properties and conditions of the face can be
actively controlled during the impact resulting in some measure of
control over the impact resultants. For example, in a specific
embodiment, the surface can be ultrasonically vibrated under some
predetermined condition so as to create an effectively lower
friction coefficient between the ball and the face resulting in
decreased spin rates and longer flight of the ball when a trigger
condition is present. One such trigger condition might be high head
ball impact forces (and large face deformation), indicating a high
velocity impact where too much spin could create excess aerodynamic
lift producing a decreased flight distance.
[0009] In another embodiment, the position and/or orientation of
the face can be actively controlled relative to the ball and the
body of the club under some predetermined condition so as to create
a better presentation of the face to the ball for more accurate
ball flight or to reduce side spin by counteracting club head
rotation during eccentric impact events. One such triggering
condition might be highly eccentric impact events (off center hits)
that can be detected by deformation sensors on the face or angular
acceleration sensors in the body. Such sensor signals could be
processed to determine the necessary motion of the face to
compensate and correct the resulting ball flight.
[0010] In another embodiment, the effective stiffness of the face
during impact can be controlled so as to produce a more desirable
impact event. For example, the system can be designed to make the
face stiffer during a hard impact and make the face softer during a
less intense impact so as to tailor the face behavior under the
impact loads to the particular event. This can be accomplished by,
for example, shorting or opening the leads of a piezoelectric
transducer which has been surface bonded or otherwise mechanically
coupled to a face. The piezoelectric is softer (low modulus) when
it is electrically shorted and stiffer (high effective modulus)
when it is open circuited. A sensor attached to the face can
measure a quantity proportional to impact intensity (e.g., face
deflection, face strain, head deceleration etc). In the "hard" hit
case, the normally shorted piezoelectric can be open circuited to
make the face stiffer, while softer hits result in the circuit
leaving the piezoelectric in the short circuited condition and
therefore less stiff.
[0011] The trigger can be provided by an external sensor or by the
actual piezoelectric transducers bonded to the face itself by
triggering off of the current or voltage level achieved on the
transducer prior to the triggering event. As an example, circuitry
for using the piezoelectric element as a charge sensor can be
attached to the transducer leads. When the charge reaches a
critical level a circuit can be triggered which disconnects the
leads from the circuitry effectively enforcing the open circuit
condition.
[0012] A critical element of the ability to control the ball-head
impact is the ability to actuate the system in a beneficial manner.
Since the head and ball are a mechanical system, this entails the
application of some force or thermal energy to the system so as to
create a change in some mechanical physical attribute. The present
invention pertains principally to mechanical actuation
techniques.
[0013] U.S. Pat. No. 6,102,426 to Lazarus, et. al, discloses the
use of a piezoceramic sheet on a ski to affect its dynamic
performance such as limiting unwanted vibration at higher speeds or
on irregular surfaces: The disclosure mentions the application to
golf clubs to dampen vibrations or alter shaft stiffness or "to
affect its head".
[0014] U.S. Pat. Nos. 6,196,935, 6,086,490 and 6,485,380 to
Spangler et. al, disclose the use of piezoceramic sheets on golf
clubs to alter stiffness and to effect a dampening of vibration.
FIG. 9G illustrates the placement of piezo elements on a golf club
head to capture strain energy to be dissipated in a circuit for a
dampening effect.
[0015] U.S. Pat. No. 6,048,276 to Vandergrift relates to the use of
piezoelectric devices to stiffen the shaft of a golf club after
capturing energy from the swinging and flexing of the shaft.
[0016] The issue of reducing friction using ultrasonic vibration is
discussed by Katoh in an article entitled "Active Control of
Friction Using Ultrasonic Vibration" Japanese Journal of Tribology
Vol. 38 No. 8 (1993) pp 1019-1025. See also K. Adachi et al "The
Micromechanism of Friction Drive with Ultrasonic Wave", Wear 194
(1996) pp 137-142.
SUMMARY OF THE INVENTION
[0017] The present invention pertains to a system for the control
of the impact event between the ball and the club face using
actuation and control of the face position or properties to
influence the progression of the impact event between the ball and
the face. In particular, it pertains to the reuse of energy
generated and converted to electrical energy from the mechanical
energy of the impact event. Such reuse beneficially controls the
impact event. In a particular embodiment, the energy converted from
impact by a piezoelectric element is converted into ultrasonic face
deformations/oscillations which have the ability to effectively
lower the friction coefficients between the ball and the face. In
an alternate embodiment, the stiffness of the piezo-coupled face
under impact is controlled to a certain behavior upon the
occurrence of predetermined impact parameters. For example, the
face is made stiff under hard hits and soft under lower intensity
hits. All these cases pertain to putter, drivers and irons equally
and club-head will be taken to mean all of these without
prejudice.
[0018] The face actuator can be any of a number of actuators
capable of converting electrical energy to mechanical energy. These
include electromagnetic types such as a solenoid, as well as a
family of actuation technologies using electric and magnetic
induced fields to effect material size changes; electrostrictive,
piezoelectric, magnetostrictive, ferromagnetic shape memory alloys,
shape memory magnetic and shape memory ceramic materials, or
composites of any of the above. Included in the possible actuation
schemes are thermal actuators using resistive heating or shape
memory alloys which use applied thermal energy to induce a phase
change within the material to induce a resulting size change or
stress. All can be used to convert electrical energy into face
deformation or face positioning in a controlled fashion.
[0019] In such a system using a pure actuator there must be an
electric energy source, battery or other electrical generator
converting motion or impact energy into the electrical energy which
is used by the face actuator. The system can include a power
source, electronics, and an actuator mechanically coupled to the
head.
[0020] In a further definition there is alternately a class of
system in which a transducer is coupled to the face. A transducer
is capable of generation of electrical energy from mechanical
energy as well as vice versa. Examples of transducer materials
include electromagnetic coil system, piezoelectric and
electrostrictive materials operating under a biased electric field,
and magnetic field biased magnetostrictive materials and
ferromagnetic shape memory alloy materials, and or composites of
the above with themselves or other constituents. These will
hereafter be called piezoelectric materials generally and the use
of the word piezoelectric shall in no way be taken as limiting. In
systems employing such transducers, the transducer element can be
coupled to the face such that deformation or motion of the club
generates electrical energy which can be used via the converse
actuation function to control aspects of the head-ball impact.
[0021] Piezoelectric actuators are the most common of the class of
transducer materials. In general, they change size in response to
applied electric field and conversely they generate charge in
response to applied loads and stress. They can be used both as
electrically driven actuators and electrical generators.
[0022] Control of the impact involves putting forces on the head
and/or face so as to beneficially change a property of the system
which influences the impact event. For example, if the force
applied is proportional to the face-acceleration, then the control
acts to apparently increase the mass or inertia of the system. It
does this by putting the same force on the head that a mass at that
location would put under that particular face motion. The applied
force can be applied to effectively create forces which mimic
elastic and dissipative as well as inertial forces of the system.
For example, if the force put in the center of the face were to be
proportional to the velocity and opposing the velocity at the
center of the face, then it would effectively act as a dashpot at
the center of the face and create a viscous damper at the center of
the face. Similarly, if one could apply a force which was
essentially proportional and opposed to the deflection of the
center of the face, then it would look like a spring applied at the
center of the face-effectively stiffening it. Likewise if the force
was proportional and in the direction of the deflection then it
would look like a negative spring applied at the center of the
face--effectively softening the face. The actively controlled
system (if one can control the force), can mimic many different
dynamic effects in the system. The challenge is to develop a device
and system which can put those types of forces on the system even
if some other constraints prohibit that.
[0023] The idea of applying some forces that mimic other types of
forces that would result from inertias or masses, is one
manifestation of the forces that can be applied. In such control
systems there can be an arbitrary phase relationship between the
applied force and input and that relationship can be frequency
dependent. Essentially the control function can be a linear or
nonlinear dynamic system between some sensor and the output force
applied by the actuator. In a classic controlled system, there is a
control system which takes sensor outputs and puts forces on the
body to achieve some desired effect. That's the general area of
dynamic systems control and more specifically, structure control
for elastic systems and is well defined in the art.
[0024] Ultrasonic, or high frequency, oscillations of contacting
surfaces can result in lower effective coefficients of friction
between the two surfaces. The oscillations must be of sufficient
amplitude and frequency such that the surfaces lose contact briefly
during at least one portion of the oscillation. This breaking of
contact lowers the effective coefficient of friction.
[0025] An actuator coupled to the club face can be configured to
excite high frequency oscillation of the face when driven with high
frequency electrical input. If the excitation occurs at a frequency
at or near a resonant frequency of the club/face body, then the
amplitude can be maximized.
[0026] In scenarios such as a golf ball impact where the normal
forces are high during impact, the key requirement is that the
acceleration of the face away from the ball during the oscillatory
motion should be high enough that the ball cannot "catch up" and
surface contact is broken. The acceleration is proportional to the
amplitude of the oscillatory motion multiplied by the square of the
excitation frequency. This can be considered a figure of merit of
the design of the actuation system. Since the amplitude of
oscillation for an actuated system tends to roll off due to system
inertial effects, there is a tradeoff between driving at higher
frequency and achieving the highest possible oscillatory amplitude.
The figure of merit helps balance these to maximize the friction
control effect. For example, in the preferred embodiment of the
present invention, it was found advantageous to excite a face
surface mode at 120,000 Hz which is coupled to the actuation driver
described hereinafter.
[0027] In systems where an external source of power is not
available, a portion of the energy of impact (converted from
mechanical to electrical by a transducer coupled to the face) can
be stored and returned to the face in the form of ultrasonic
excitation of a high order face mode, high frequency oscillations
of the face which are well coupled to the transducer. The energy
can be stored in the transducer material itself, for example in the
charge stored in the capacitance of a piezoelectric material or it
can be stored primarily in auxiliary circuit elements such as
storage capacitors or inductors or tank circuits, etc, which are
electrically coupled to the transducer. After a triggering effect
releases the energy, an electrical drive circuit can be configured
so that when connected to the transducer, it induces a high
amplitude face oscillation which effectively reduces the impact
friction coefficient between the ball and the face at a critical
point in time during the impact event such critical point in time
being selected by a control algorithm. The face oscillation and
controlled friction result in a control of ball spin which can be
selectively triggered under certain impact conditions (such as high
impact force levels).
[0028] The exiting ball speed can also be controlled by applying
forces to the face proportional to face deflection. With
appropriate sign these forces can effectively soften the face by
increasing the duration of the impact thereby lessening the impact
loading and resulting ball deflection. The lower ball deflection
results in reduced dissipation by inelastic deformation of the ball
and increased recoverable energy from the impact event, thus
achieving higher coefficients of restitution (COR) and higher ball
velocities. Conversely, impact energy converted into electrical
energy can be dissipated to decrease the effective COR in selected
impact scenarios.
[0029] By selectively applying forces electrically to mimic the
effects of tailored compliance, portions of the face can be
selectively made to deform greater than others during the impact
event thus controlling the exit direction of the ball. The exit
direction is controlled because the final ball velocity (speed and
direction) is determined by the forces generated by the elastic
impact. Uneven deformation of the face (due to unbalanced
compliance) changes the direction of the normal reaction of the
ball and therefore the final direction the ball will travel. In
addition to this direct control of ball direction, indirect control
of ball direction can be achieved by reducing spin including
sidespin and thereby reducing cross range travel. Similar control
features can be achieved by actively positioning an actuated
clubface during impact in response to some measured impact variable
such as location of the impact or angular acceleration of the head
(caused by eccentric impact).
[0030] Forces can also be applied to the head to mimic the effects
of a higher moment of inertia. In other words, the forces would be
similar to those that an additional mass at a given location would
exert on the head during impact. Such forces can be triggered in
miss hit scenarios resulting in straighter shots. For instance, one
way of doing that would be to create a force on the head through
action with a reaction mass. The actuator reacts between the head
and the reaction mass. It reacts in such a way that it minimizes
head rotation under impact. It acts to effectively increase the
moment of inertia of the body and therefore keeps the face
straighter and therefore the ball flight straighter during the
impact event. Because the impact event is of a finite duration, one
can put that kind of force on the body within that finite duration.
A central post and an annular bimorph ring would be segmented so
that one can actually detect and sense which way the head is moving
relative to the reaction mass. Whether it is up, down, left or
right, basically which way the face is rotating could be used as a
sensor input to a compensator/controller to allow the applied force
to compensate for that resulting face motion. Multiple piezo
elements or configurations with multiple electrodes on a single
piezoelectric element would allow detection of a broader range of
impacts. One can actually determine where the ball is impacting on
the face and use the control circuitry to compensate accordingly,
for instance by slightly rotating the face to compensate for head
rotation during eccentric impacts. In the preferred embodiment
there is one voltage coming out of one piezo making it difficult to
determine the impact location from the variety of possible impact
locations. But that is not necessarily a limitation of the present
invention. It is possible to include a uniform piezo bonded to the
face where the electrodes are segmented to allow detection of
impact location. In that scenario, essentially there would be
multiple piezoelectric elements that are bonded to the face. There
would be multiple electrodes for example in a square array. For
example there might be actually nine electrode patterns in a
3.times.3 square array on the back of the face. Those voltages
would be applied to a control circuit that would determine where
the ball has impacted and the resulting appropriate response to
that impact. Switching on the voltage on some of the electrodes on
the transducers as opposed to others in response, could tailor the
response depending upon impact location.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] The various embodiments, features and advantages of the
present invention will be understood more completely hereinafter as
a result of a detailed description thereof in which reference will
be made to the following drawings:
[0032] FIGS. 1-5 illustrate various conceptual embodiments of the
invention wherein different forms of elastic coupling of a
piezoelectric actuator to a golf club head face are shown;
[0033] FIGS. 6-8 illustrate various conceptual embodiments of the
invention wherein different forms of inertial coupling of a
piezoelectric actuator to a golf club head face are shown;
[0034] FIG. 9 illustrates a conceptual embodiment of the invention
wherein piezoelectric transducers are disposed between the face and
body of the club positioning the face relative to the body;
[0035] FIGS. 10a and 10b are a block diagrams of a piezo actuator
with controlled switch, inductor, and control circuit;
[0036] FIG. 11 is a schematic diagram of the circuit of FIG. 10b
showing the control circuit in more detail;
[0037] FIG. 12 is a graphical presentation of an actuator output
voltage signal under ball impact showing un-triggered and triggered
voltage time histories;
[0038] FIG. 13 is a graphical presentation of the time histories of
key parameters in the ball to club impact showing A) impact normal
force, B) impact tangential (friction) force, C) transducer voltage
time histories, D) transducer current time histories, and E)
resulting ball spin time histories;
[0039] FIGS. 14-15 are section illustrations of a golf club head
employing the conceptual piezo coupling embodiment of FIG. 2 to
reduce the spin rate of a golf ball by converting ball impact
energy into a head face vibration to reduce friction between the
head and the golf ball;
[0040] FIGS. 16a and 16b together comprise an illustration of a
golf club head employing the conceptual piezo coupling embodiment
of FIG. 2 detailing the removable sole plate with system
electronics;
[0041] FIGS. 17-19 are detailed illustrations of the face assembly
showing piezoelectric transducer to face coupling hardware for
conceptual piezoelectric coupling embodiment of FIG. 2;
[0042] FIG. 20 is a graphical presentation of the friction model
for the interaction between the face and the ball;
[0043] FIG. 21 is a frequency response function showing the voltage
response of an open circuit piezoelectric transducer undergoing
periodic loading on the face of the club;
[0044] FIG. 22 is a frequency response function showing the face
surface acceleration as a function of the amplitude of time varying
voltage-excitation of the piezoelectric transducer; and
[0045] FIG. 23 is a circuit block diagram of a electrical system
for achieving variable stiffness which stiffens upon mechanical
excitation of the piezoelectric of sufficient intensity.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0046] The following description assumes that there is an
understanding of the fundamentals of piezoelectric materials,
operations and modes such as described in "Piezoelectric Ceramics"
by Jaffe, Cook and Jaffe, Academia Press, 1971 and the references
cited therein. The content of that publication is hereby
incorporated herein in its entirely by reference. Another useful
reference which describes the field of piezoelectric mechanics is
"Piezoelectric Shells" by H. S. Tzou, Kluwer, Academic Publishers,
MASS., 1993 and is also hereby incorporated herein by
reference.
Actuator Coupling to Face
[0047] There are several methods of coupling actuation elements and
transducers to the club face, the interaction surface between the
ball and the head. The transducer can be directly coupled to 1) the
face relative deformation (elastic), 2) absolute motion (inertial)
using a variety of techniques or 3) relative motion between the
face and the head body. Eight are described here which alternately
couple the actuator or transducer to elastic deformation of the
face or inertial motion of the head. For the actuation function the
goal is to enable maximal control over face deflection at the
desired frequency of actuation. For the transducer, the goal is to
maximally couple into either the absolute motion (deceleration) of
the head (or face) or into the deformation pattern induced in the
head and face by the ball impact. The two techniques tap into the
pool of kinetic or elastic energy available during impact. This
energy is then converted by the transducer into electrical energy
which is usable for face and interface actuation. A description of
eight alternative systems for coupling a transducer element to the
golf club face follows.
[0048] There are three classes of actuator face coupling. The first
class pertains to elastic piezo face actuation wherein transducer
size changes and deformations are directly mechanically coupled
into relative deformation along or between two structural points on
the face. This type of elastic actuation is generally known in the
art of structural control where piezoelectric elements
(predominately) are mounted on or embedded within structures to
effect beneficial structural deformation. The four embodiments of
elastically coupled actuators are as follows: [0049] Concept
1--Piezo wafer attached directly to the face to actuate bending as
shown in FIG. 1. [0050] Concept 2--Piezo stack and/or tube mounted
on the face with housing as shown in FIGS. 2a, 2b and 3. [0051]
Concept 3--Piezo disposed between the face and a stiff backing as
shown in FIG. 4. [0052] Concept 4--Piezo operated in shear mode and
disposed between the face and a stiff constraining layer as shown
in FIG. 5a 5b.
[0053] The second class of actuator face coupling is actuator
coupling to the face's absolute motion or those that rely on
inertial forces generated by face and head motion on impact with
the ball. These typically entail a reaction mass and an actuator or
transducer element acting between the reaction mass and the face.
These types of face couplings are generally related to proof mass
or reaction mass actuators. The concepts in this category are
described as follows: [0054] Concept 5--Direct piezo coupled
between the face and an inertial mass as shown in FIG. 6. [0055]
Concept 6--Motion amplified piezo between the face and an inertial
mass as shown in FIG. 7. [0056] Concept 7--Bimorph type piezo with
tip mass and mounted on the face as shown in FIG. 8.
[0057] The third class of actuator-face coupling is actuator
coupling between the face and the body of the club. The actuator
can be the sole or one of a number of parallel load paths between
the face and the body. This is similar to Concept 3 but the face is
treated more like a rigid body that can be positioned rather than
deformed as in Concept 3. The transducer positioned between the
face and the body supports the majority of the loads between the
face and the body and can therefore participate to a large extent
in the impact event. In addition, actuation induced positioning of
the face relative to the body in essence uses the body itself as a
large reaction mass to effect changes in the location or
orientation of the face during impact. [0058] Concept
8--piezoelectric transducer positioned between the face and body of
the club as shown in FIG. 9.
[0059] For transducer applications, to produce maximal available
actuation power and maximally available coupling (for instance
actuating high amplitude high frequency face oscillations for spin
control) it is desirable to achieve good coupling to both 1) impact
deformation pattern as well as 2) a high frequency mode. For face
positioning applications (rather than friction reduction
applications) it is desirable to achieve good coupling to both 1)
impact loading patterns as well as 2) impact-timescale motion
between the face and the body.
[0060] In general for the elastically coupled concepts (1-4), face
motion/loading generates loading on the transducer material and
corresponding electrical energy generation. Conversely, electrical
energy put on the transducer controls face motion. It is desirable
to have high electro-mechanical coupling between face
loading/motion and electrical voltages and currents. This coupling
can be measured in terms of the fraction of input mechanical energy
from the impact that is converted into stored electrical energy
(for instance on the piezoelectric element or a shunting circuit)
or conversely, by the fraction of input electrical energy that is
converted into strain energy in the actuation induced deformation
of the face.
Concept 1
[0061] In this face coupling embodiment an actuator, 21, capable of
planar size changes, (also called a 3-1 actuator, although a
variety of interdigitated piezoelectric wafer or composite
actuators are capable of planar size changes) is coupled to the
plane of the face, 10, onto or buried within the face itself. The
actuator can also be packaged using techniques known in the art.
Since the actuator is not exactly on the centerline, it couples
into bending deformation of the face and acts to impact a bending
moment on the face, 105, when electrically excited. Alternately for
in plane actuators near the centerline coupled preferably into in
plane deformation rather than bending, coupling into out-of-plane
motion can be achieved in large deformation scenarios using
parametric forcing. The actuation loading can be thought of as a
combination of in-plane forces and a curvature moment couple, 105,
acting on the face at the boundaries of the actuator as is shown in
FIG. 1. Some critical parameters are the spatial extent (length) of
the actuation element as well as its thickness. The spatial x-y
extent is determined by maximizing the coupling into a given
desired face deformation shape. Good coupling can be equated to the
integration of the transverse strain field times the electric field
times the piezoelectric constants over the domain of the actuator.
The coupling into some shapes and therefore some structural modes
is maximized at corresponding actuator shapes and extents.
[0062] For example, for an axially symmetric plate with a circular
actuation patch covering a given radius, coupling into the second
axisymmetric plate mode (one nodal circle) is maximized when the
extent of the actuation disk extends to that node radius but no
further. If the disk had a radius larger than the nodal circle's,
then material outside the circle would see strain of opposite sign
to the material inside the circle and there would be a partial
cancellation of the piezoelectric response when integrated over the
entire disk.
[0063] For the particular case in which a transducer is coupled and
it is desired to harvest energy from impact as well as potentially
excite a high frequency mode (to control friction), the actuator
must be designed in extent and thickness to achieve both: 1)
coupling into the shape produced by the impacting ball (roughly the
first mode deformation shape for center hits); and 2) coupling into
the deformation shape associated with a high frequency mode.
[0064] Because faces are relatively thick structural elements,
modeling suggests relatively thick piezoelectric elements on the
order of 1 mm are required to produce significant actuation of the
2-3 mm face. Typical face designs have shown that a piezo element a
few centimeters in diameter (1-5) can achieve the desired dual
objective of coupling to both the energy generating first impact
shape as well as a high frequency mode to be excited for friction
control. A typical implementation of this type of face coupling is
a 3-1 mode piezoelectric disk with electric field applied through
its thickness and disk directly bonded to the face 10 (usually
inside).
[0065] It is important to note that the piezoelectric element 21
can be prepackaged with polymer encapsulation and potential
electrode patterns on such polymer or flex circuit. The patterns
can define various active regions and produce segmented, uniform,
or interdigitated electrode patterns in potentially curvilinear
arrays. The key factor is to maximize electromechanical coupling
(as defined above) between the piezoelectric and the face
deformation.
Concept 2
[0066] The preferred method and system for coupling of an actuator
or transducer to a face will now be described. In this method the
actuation element 21 (preferably piezoelectric, but possibly
electrostrictive or magnetostrictive or any of a number of
actuation or transducer technologies described previously) is
attached to the face though the use of a housing 12 or support
structure attached to the face. A particular depiction is shown in
FIG. 2a and in sectioned assembly in FIG. 2b.
[0067] In this case the actuation element 21 is configured to
elongate or change size axially in response to input electrical
energy (voltage or current). For a piezoelectric system this can be
accomplished in a variety of ways. In particular, one can use a
piezoelectric stack to couple applied voltage to length changes.
This is known as 3-3 coupling and is a high mode of response of
piezoelectric materials. A 3-3 stack is an arrangement of multiple
piezo material layers with electrodes between the layers so that
the electric field is aligned with a central axis to produce a
longitudinal piezoelectric effect. This is shown in detail as
subassembly 15 in FIG. 18. The actuator can also be configured as
elongated transverse or 3-1 type actuator in which the field is
applied perpendicularly to the axial direction. This can be
achieved by a rod with electrodes along its length on opposite
sides, or a tubular actuator with the load being applied along its
length and the field being applied through the wall thickness by
electrodes on the inner and outer walls of the tube. There are
numerous other axially elongating actuator/transducer
configurations known in the art.
[0068] The second element is a housing 12 which serves to
mechanically connect the back end of the actuation element to the
face. It serves as a stiff load return path coupling elongation of
the actuation to deformation of the face. Face deformation causes
relative motion between the point (potentially at the face center)
where the actuator makes contact and the point where the housing is
attached to the face shown in FIG. 2a by the applied forces at
these points 106. The stiff housing then translates that relative
motion into relative motion between the two ends of the actuator.
The housing 12 thus acts as a mechanical attachment which couples
the actuator length changes to face differential motion
(deformation). It is therefore in the elastic class of face
couplings.
[0069] It is important that the housing be stiff (ideally rigid but
at least on the order of the stiffness of the piezoelectric
element), since any elongation of the housing under actuation loads
will reduce the load transferred to the face and the resulting face
deformation. To see this, one should consider the limiting case of
a very flexible housing. Then, as the actuation element starts to
elongate, the housing just stretches with it with little load and
therefore little deformation is induced into the face. In reality,
the condition generally is that the housing must be stiffer by at
least 1 to 20 times greater than the face under an equal but
opposite loading at the housing attachment and the actuator
attachment in order to insure that the load is effectively coupled
to face deformation rather than housing elongation. The housing
should also be as light as possible to avoid adding a large mass
and thereby significantly changing the center of gravity of the
head or its inertia tensor.
[0070] The housing 12 consists of a conical or cylindrical wall 52
with a back plate 13 that provides a contact with the actuator and
a circular end which establishes contact with the face at a ring
56. See FIGS. 17-19 for detailed drawings of a preferred embodiment
of Concept 2. The housing 12 can be screw attached 29, brazed or
welded to the face, or use any of a number of other techniques. The
end plate can be permanently attached, machined as one piece with
the wall or configured as a screw part 13 for ease of actuator
system assembly and removable for repair. It is important that all
the compliances of the housing, including back face bending and
other deformation of the housing, be taken into account when
considering its stiffness under actuation loads. That is why a
conical structure is very efficient, it reduces the bending of the
back plate and provides a more direct load path to the face.
Typical dimensions are .about.1 mm for the housing wall 52 and
.about.3 mm for the housing back 13. The transducer assembly 15,
consisting of piezoelectric layered actuator 21 and end pieces 23,
is .about.16 mm long (total) as shown in FIG. 18 (of which 10 mm is
active material 21). The cross-section is a 7 mm.times.7 mm square
stack or a preferred 9 mm diameter circular stack.
[0071] Of particular design importance is the selection of the
contact point locations between the housing and the actuator and
the face. If the actuator is arranged to make contact with the
center of the face, the housing can be configured to attach to the
face at a selected distance away from the center at either discrete
points or a continuous (circular) ring at a fixed radius. Selection
of this attachment radius is very important to maximize the
performance requirements for a given control application. The end
pieces 23 are preferably made of steel or alumina or other very
stiff material and have some curvature 26 to provide a centered
point contact with the face 33 and with the back of the housing 26
on nearly matching curvature (indentations).
[0072] In the particular case of friction control, an objective is
to excite high frequency oscillations as described above. The
diameter must be chosen to satisfy the need for: 1) good coupling
to the impact deformation shape to generate electrical energy; and
2) good coupling to a high frequency mode. This can be accomplished
by placing the attachment radius to correspond approximately to the
radius of an anti-node of the face mode of interest. The anti-node
should have preferentially opposite deformation direction at the
center to maximize relative motion.
[0073] The design considerations in optimization are as follows--if
the radius is too small, the piezo center force and the reaction
force are imposed on the face very close together. The face is very
stiff between these spaced points and little motion can be
introduced. Conversely, the differential deformation between those
attachment points under the impact deformation shape, is very
small, since it determined by the curvature under impact loading,
so little voltage is generated at impact. If the radius is made too
large, then there is good coupling to the impact, but it becomes
difficult to build a stiff housing structure and it becomes
difficult to generate high amplitudes in a high frequency mode
because of housing modes starting to participate, effectively
lowering the dynamic stiffness of the housing. In the preferred
embodiment, a diameter of attachment of approximately 35 mm was
chosen for the face ring 56 as optimum for maximizing the dual
objective of coupling to the ball impact face deformation and
coupling into a high frequency face mode at .about.120 kHz.
[0074] In evaluating particular designs it is necessary to take
into consideration stresses in the face and housing and actuator
during impact. Very high stress level can lead to low fatigue life
of the housing. In addition, the high compressive stresses imposed
on the actuator during ball impact can cause a permanent
"depolarization" of the material, a permanent reduction in actuator
properties. The mechanical system must be analyzed for its loads
during a variety of ball impact events to determine that these
critical load levels for life of the housing or stress induced
depolarization of the piezoelectric element have not been
exceeded.
[0075] One can either have the piezo at the center or one can use a
bolt welded at the center of the face and use a piezo cylinder or
multiple piezo-elements (for example stacks) radially spaced from
the bolt as shown in FIG. 3. One can couple to the lowest impact
deformation shape as well as high frequency mode shape in this
configuration. Because of the axial arrangements relative to the
face normal, it is easy to preload the transducer elements 21 for
robustness using a centrally located face anchor 205 threaded to
accept a preload bolt 206 and backing plate 212 and its easy to
design for desired surface excitation amplitude.
Concept 3
[0076] A third embodiment is shown in FIG. 4. In this embodiment
the piezo 21 acts between the face 10 center and a stiff
backing/support structure 207. The support structure must be stiff
for high reaction force--on order of 1-10.times.the stiffness of
the face so that actuation induces deformation of the face instead
of the backing structure. There is a potential to use an
intermittent contact between the piezo and the face. Because of the
requirements of high stiffness, the backing structure tends to be
heavy as well.
[0077] In Concept 3 shown in FIG. 4, there is a piezo element 21
configured between the face 10 and backing structure 207 which then
passes the face interface load to another piece of the club head,
i.e. the rear, the body 11, or the perimeter around the face. When
the face moves in about a millimeter during impact of the ball and
therefore compresses the piezo, it generates a charge and
electrical energy that can be used to power the system and for
example excite an ultrasonic device. Because it generates
electrical energy through relative motion and load between the face
and backing structure, the design must have a stiff backing
structure to resist the motion of the face and provide high piezo
loading. If the backing structure were soft, it would deform with
the face under low load and wouldn't actually squeeze or apply load
to the piezo. This would imply poor piezoelectric electromechanical
coupling to the impact.
[0078] This concept couples to axial motion (or normal motion) of
the deformation of the face. That can be done by a single stack
element or single piezoelectric monolithic element with a polling
direction and the loading is basically aligned with the surface
normal to the face. In this configuration the actuator would use
the 3-3 mode of actuation. It could be a 1-3 mode actuator or it
could be a tube with the electrodes on the inner or outer wall of
the tube as described for Concept 2. The stress is therefore in the
direction perpendicular to the polling direction. The basic
reaction force is trying to inhibit motion of the face. The backing
structure therefore needs to be stiff to accomplish this effect.
This stiffness requirement can lead to relatively heavy structural
elements which can by design be located relatively close to the CG.
The added mass, however, would decrease the moment of inertia of
the head for a fixed mass head since less mass would be available
at the periphery.
[0079] In another embodiment of Concept 3, the piezoelectric
element is initially not in contact with the backing structure.
Upon ball impact, the deforming face would bring the piezoelectric
into contact with the backing structure and load the piezoelectric
element. The piezoelectric element for instance attaches to the
face which is perhaps a half millimeter off from the backing
structure. No contact is made until the ball hits. In this way the
system can be designed so that only high amplitude impacts load the
piezoelectric element and trigger the control function. Such
impacting has been used to achieve damping in structural systems.
It can also be used to change effective stiffness and the effective
face reaction in different ball loading scenarios and therefore for
different head speeds. For instance, if there is a small gap
between the face and the backing structure, (even if there is no
transducer there) low intensity impacts might leave the face
unsupported, not forcing contact. For high intensity impacts,
contact between the face and the backing will be established during
impact; and the backing structure will support the face and reduce
face deflection
Concept 4--Shear Mode Piezo
[0080] In the previous concepts the loading on the piezoelectric
element has been primarily in the form of an applied normal stress.
In Concept 4, the piezoelectric is loaded in shear and coupled into
the electric field using the shear mode of piezoelectric operation.
More information on shear mode and the major modes of operation of
piezoelectric transducers can be found in the product literature
for Piezo Systems Inc. of Cambridge, Mass. The shear mode
piezoelectric element involves shear stresses about the axis of
polarization in the material as shown in FIG. 5a For example, if
the polarization is in the x direction in the material, the shear
stresses would be in the x-z plane about the y axis as shown in
FIG. 5a. In this mode of piezoelectric operation, the electric
field, E, is applied perpendicular to the poling axis, x. This mode
of piezoelectric response is sometimes called 1-5 mode of
operation.
[0081] In Concept 4, the mechanism using a shear mode piezo
actually works very much like a constrained layered damping
treatment used commonly for damping of vibratory response of
bending structures. The piezoelectric element 21 that is intended
to be loaded in shear is located between the face and a stiff
backing layer called the constraining layer 208. As the face bends
under the impact loading as shown in FIG. 5b, the constraining
layer resists that bending deformation putting the intermediate
piezoelectric elements in shear. In Concept 4, one or multiple
shear-mode piezo elements are located between the backing structure
208 and the face 10 as shown in FIG. 5b so that as the face bends,
it induces a shear stress on the piezo which then can be coupled
into the electrical field by the piezoelectric transducer. In the
typical configuration the electrical field is aligned with the
surface normal and the 1-5 mode piezoelectric elements are
polarized in the plane of the face. For instance one of the
elements can be placed on each side of the plate at points of high
curvature, then a bar or plate acting as the constraining layer is
bonded between these piezoelectric elements. When the face deforms,
the bar tries to keep it from deforming and that puts a large shear
load on the piezos using the 1-5 mode of actuation.
[0082] In another embodiment, the shear mode piezoelectric element
is a ring, polarized radially outward or inward. The ring can be
bonded about the center of the face. The electric field would act
through the thickness of the ring between the face and the
constraining layer. In this embodiment, the constraining layer
would be a disk with the same outer diameter as the ring bonded to
the ring about its circumference. This is an axisymmetric version
of the concepts presented above and acts to couple drumhead like
face motion into the piezoelectric element.
[0083] The shear mode of operation is a very effective, very high
coupling coefficient mode of operation for piezo transducers.
Coupling coefficients for 3-3 mode of actuation and 1-5 mode of
actuation are very similar. The coupling coefficient is defined
loosely as the fraction of the mechanical energy input that is
converted into electrical energy under a predefined loading
cycle.
[0084] Concepts 1, 2, 3, and 4 are elastically coupled systems. The
piezo is squeezed because of relative deformation between two parts
of an elastic body. Since the face-piezo system is part of that
elastic body, deformation of the face imparts deformation of the
piezoelectric. For Concept 1 as the face (an elastic body) deforms,
it deforms the piezo because it is bonded to the face. Concept 2
uses a support structure housing which connects to the face at a
different place than the piezoelectric element (e.g., the
piezoelectric element contacts the face at the center and the
housing contacts the face in a ring at a defined radius out from
the center). Because distinct contact points are established,
relative motion effectively squeezes the piezo. In this manner the
piezoelectric is coupled into the face motion. In Concept 3, motion
of the deformation of the face squeezes the piezo attached between
the face and the backing structure. In Concept 4, deformation of
the face induces a shear stress in the piezoelectric element. All
of these concepts rely on coupling into the elastic deformation of
the face-body structure that represents the head of the golf club.
For this reason these concepts are referred to collectively as
having elastically coupled transducers.
Concepts 5, 6 and 7--Inertial Coupling Concepts
[0085] The next class, consisting of Concepts 5, 6 and 7,
represents a different way of getting a load into the transducer
that utilizes inertial forces during impact. These concepts utilize
the load necessary to accelerate a mass to load a piezoelectric
element. The piezo loading is thus a function of acceleration
rather than relative deformation of the face. In the simplest
embodiment, there is a reaction mass 209 (sometimes called a proof
mass) and a piezo 21 is attached between that reaction mass and the
face 10 as shown in FIG. 6. The system is analogous to a
mass-spring system with the piezoelectric being the loaded spring.
The moving face is analogous to a moving base in the spring-mass
system. As the face moves under ball impact, inertial forces
inhibit the motion of the reaction mass and the piezoelectric
"spring" is loaded by the differential displacement between the
face and the mass. As it is loaded, it generates the charge and
voltage that can then be used to control the face as will be
described hereinafter.
[0086] In these concepts it is important to tune the mass and piezo
"spring" to couple well with the face motion during impact. In the
scenario that the face moves slowly in comparison to the period of
the first natural frequency of the spring-mass system, there is
little relative motion between the face and the mass and therefore
little piezo loading. In this scenario the mass follows the face
well since elastic forces of the spring are much larger than the
inertial resistance. In the alternate scenario, if the face moves
very quickly, the mass can't respond and the piezoelectric "spring"
is squeezed by the amount that the wall moves. Thus the load that
the piezo sees and therefore the amount of coupling to face motion
depends on the relative mass and spring constant of the system and
the timescale of the forcing.
[0087] To illustrate the system behavior, consider the case when
the face is moved with a 1/2 sine wave similar to an impact motion,
the center of the face moves a distance inward (about 1 mm) under
ball loading and comes back to normal position in a certain period
of time known as the impact duration. If the impact event takes a
1/2 millisecond, it would correspond to an input wave form
corresponding to one half the cycle of a one kHz input. If the
piezo 21, the mass 209 and the spring (face 10) have a natural
frequency which is significantly greater than that one kHz, that
system looks like a rigid body under that base (face) motion. In
this scenario, there is not a lot of relative deformation in the
piezo. The relative motion corresponds to the amount of strain the
piezo sees and thus the voltage the piezo sees in open circuit.
With this as the metric, the achievable open circuit voltage under
impact drops off to zero at very low frequency inputs (long
duration impacts and stiff piezo-mass systems). It rises up to a
resonant peak when the input is commensurate with the time constant
of the spring mass system with the face held rigid. If the first
fundamental mode of the spring mass system is below the forcing
frequency, then as the face moves the piezo gets squeezed by an
amount of the relative deformation between the moving face and the
inertial mass. This is because the mass in unable to move fast
enough to respond to the relatively high frequency face motion.
[0088] A typical 1 cm by 1 cm by 1 cm cube piezo with a typical 10
gram mass on the end, might have a frequency in the 20-40 kHz
range. That would be too stiff to couple well into that .about.1
kHz face motion unless a very large reaction mass is used. So what
that implies then is that the designer must try to create a system
where there is smaller mass and much smaller effective piezo
element stiffness, supporting that mass. If well designed, the
mass-piezo natural frequency is commensurate and thus well coupled
into that ball impact.
[0089] To achieve this frequency tuning, the designer must soften
the piezo element by either making it thinner or using some
mechanism to make it effectively have a lower spring constant.
Concepts 6 and 7 shown in FIGS. 7 and 8 respectively demonstrate
some manifestations of this using mechanically amplified
piezoelectric transducer configurations. These concepts act by
lowering the effective spring constant of the piezo element, lower
than for a stack element. Stack elements can be very stiff. The
mechanical amplification increases piezoelectric transducer stroke
while lowering its blocked force, essentially reducing the
effective stiffness of the transducer, lowering the spring
stiffness between proof mass or the reaction mass and the wall of
the face.
[0090] If the surface of the face moves slowly relative to the
natural vibration of the effective piezo spring and mass system,
then there is relatively little deformation of the piezo and little
charge buildup. If it moves fast relative to the time constant,
then the piezo element is squeezed by about the deflection of the
face. To get energy into the piezoelectric transducer, the question
is how you design the spring and how large the mass has to be? If
the spring and the mass have a natural frequency that's tuned to
the time constant of the face motion, for instance a time constant
of a 1/2 ms, then you want the natural frequency of that spring
mass system to be about 1 kHz, and then loading in the
piezoelectric element is maximized. At high frequency, the mass
looks like more of an inertial reaction mass. The piezoelectric
element pushes off from that reaction mass. This allows excitation
of direct surface motion in the face by force between the reaction
mass 209 and the face 10.
[0091] Concept 5 has the obvious problem of the piezo tied directly
to a mass which ends up being a very stiff system, requiring a
large mass to get the natural frequency down to the range best
suited for ball impact coupling. There are numerous techniques for
lowering the stiffness of the piezoelectric by mechanical design.
For example, piezo rods consisting of very thin small diameter
pillars can be embedded in an epoxy to lower the effective
stiffness but keep the piezo charge coefficients in place. That's
called a 1-3 piezo composite. A composite also works well with a
particulate composite using a piezoelectric particulate in epoxy.
By selecting the appropriate particulate volume fraction a
transducer can be designed to lower the effective material
stiffness. Other ways of lowering the effective piezo spring
constant without sacrificing coupling coefficient are other
configurations of the piezo system, such as having the piezo
element mechanically amplified. Concept 6 shown in FIG. 7
illustrates the general idea of a mechanical amplifier 210 to lower
the effective stiffness of the amplified piezoelectric. There are
thousands of different types of mechanical amplifiers that take
very large force and very small stroke piezo motion and turn it
into much larger stroke, but lower force output. Basically, the
effective coupling coefficient of the mechanically amplified piezo
is always lower than the effective coupling coefficient of the
piezo by itself. Concept 6 represents an approach which uses a
concept called aflex-tensional piezo. In that scenario, axial
deformation of the motion amplifier (in the directing perpendicular
to the face) creates horizontal motion and deformation of the
piezo. As the piezo changes size side to side, (i.e., as the piezo
gets longer, shorter), it pushes or pulls between the reaction mass
and the face. Amplification ratios may be anywhere from a factor of
2 to 100. Very small motion creates a very large motion of the
system. A mechanically amplified piezo actuator produces higher
stroke and lower force output. Therefore a softer spring can be
used between the face and the action mass to lower the needed
reaction mass, lower than required if you had a piezo without
mechanical amplification.
[0092] Concept 7 shown in FIG. 8 is a bender configuration. One
possible manifestation of the bimorph bender 211 is a rectangular
strip with one central layer of shim and 2 layers of piezo on
either side. Sometimes there is no shim, just 2 layers of piezo.
The piezos are actuated so that the top expands and the bottom
contracts. That causes a bending of the element very similar to
bending of a bimetallic strip due to different coefficients of
thermal expansion of the top and bottom layers. The output of this
device 211 is force and deflection of the tips. It's a bending mode
actuator that essentially turns a small piezo motion in the plane
of the bi-morph into large tip deflection out-of-plane. It works in
a manner similar to the mechanical amplifier. Typically, the
bi-morphs have much larger tip deflection than in the axial stroke
piezo. Basically the tip deflection of the beam that represents the
bi-morph bender turns into the axial compression or tension on the
piezoelectric element. Those are typically 1-3 mode elements where
there is a piezo wafer with electrodes and loading in the plane of
the bending element. Some have used piezo fiber composite (PFC)
actuators for the bimoph piezoelectric layers. These PFCs can be
configured to put the electric fields in the plane of the system
using inter-digitated electrodes and the fibers in the plane of the
system to couple to the planar fields. Two piezo fiber composites
can be attached (bonded or laminated) onto each other and can be
configured as a bi-morph bender. It's an element with high coupling
coefficient but has much better force deflection characteristics.
In this concept, the bi-morph is typically situated between the
proof mass 209 and the face structure 10.
[0093] FIG. 8 shows the single bi-morph in a proof mass off to the
side. You could have two on opposite sides. Bi-morph transducers
have properties making them efficient as electromechanical
transducers. Instead of having a beam pure rectangular plane form
so the beam is constant width, the width and/or thickness of the
bimorph can be changed as a function of the length along the beam.
It actually is advantageous to taper the bimorph so that it's wider
at the base and reduces down to a much narrower platform at the
point where the load is applied. This works as a more efficiently
coupled system to tip motion. Also it's advantageous to change the
thickness of the beam as a function of it's position along the
length of the bi-morph. It is best to have the thicker beam at the
root and thinner beam at the outside. That maximizes the stress in
the device and minimizes the mass of the device necessary to
achieve a stated level of energy coupling. You equalize the stress
level of the piezo so you don't have one highly loaded section of
the piezo and one very lightly loaded section. Relatively uniform
loading increases its effective coupling coefficient.
[0094] The bi-morphs don't have to be rectangular elements. They
could be tapered or round. They could have variable thickness. They
have also been fabricated as curved structures. There are many
different configurations for piezo bi-morphs. Of particular note is
the possibility of a disk shaped (round) bimoph configuration. The
piezoelectric bimorph disc is attached at the center of the disk to
the face with a standoff. The proof mass is a ring attached at the
outer radius of the piezoelectric bimorph. The electrodes on the
bimorph can be axis-symmetric and uniform or sectored
circumferentially (pie piece shaped sectors) so that differential
tilt can be actuated/responded to by the piezoelectric element.
[0095] The Concept 5 embodiment is shown in FIG. 6. The piezo 21
acts between the face center 10 and a reaction mass 209 sized such
that a first natural frequency of the mass on the piezo is
commensurate with twice the impact duration (tuned). This implies a
need for amplified or less stiff piezo if little reaction mass is
used. It is a challenge to make the piezo soft enough to accept
high impact energy but stiff enough to impact high force at high
frequency. A heavy reaction mass may be required.
[0096] The Concept 6 embodiment is shown in FIG. 7. This is like
Concept 5 except one substitutes a mechanically amplified 210
piezoelectric actuator. A motion amplifier 210 converts small piezo
motion to larger relative motion between the face center and the
reaction mass. One may solve an impedance miss-match problem but
there is a potentially heavier and more complex mechanism.
[0097] The Concept 7 embodiment is shown in FIG. 8. A bimorph
bender 211 acts between a mass 209 and the center of the face 10.
It's like Concepts 5 and 6 but uses a bimorph piezo between the
face and a mass. It can use an axisymmetric bimorph disk and ring
mass. It can use multiple rectangular or triangular shaped bimorphs
and masses. One must tune the first mass natural frequency to the
impact event and then segment electrodes to help locate the ball
impact on the face. There's an indeterminate high frequency force
output.
Concept 8--Actuator Coupled Between Face and Body
[0098] The Concept 8 embodiment is shown in FIG. 9. In this
embodiment the actuator or transducer 21 with electrical leads 22
is disposed between the body of the club 11 and the face 10. In
this manner, loads between the face and the body at impact can be
converted into electrical energy by the transducer during impact
and the face can be positioned relative to the body during impact
by selective controlled actuation of the transducer element(s).
These actuations can be used to change the position such as
rotation of the face relative to the body to counteract the
rotation induced in the system by eccentric impacts.
[0099] There are multiple modes of operations possible with this
configuration of the system. The first is quasi-static positioning.
In this mode of operation, the face is repositioned from its
initial orientation to an alternate position relative to the body
and ball. For instance, the face angle is adjusted slightly in
off-center impact events. The angle adjustment is pre-calibrated to
achieve a reduction in miss distance--for instance compensating for
a hook or slice by re-pointing the face. The benefit is accrued by
changing the static (with respect to the impact event) positioning
of the face.
[0100] In an alternate mode of operation, the face is repositioned
during the impact event so that the induced motion itself causes a
desirable effect on the impact outcome. For instance, the face can
be moved tangentially (perpendicular to the face normal) such that
the face tangential velocity during impact beneficially effects the
ball spin through the frictional interface between the ball and the
now tangentially moving surface. The face can be forced to have a
tangential velocity which has the effect of reducing or increasing
the ball spin resulting from the impact event. This spin control
can have desirable effects on the subsequent ball flight or ball
bounce and roll behavior after it hits the ground.
[0101] In a particular example, the face can be moved upward
tangentially to the face normal axis during the impact event. This
can be controlled to occur only in high impact events that would
otherwise produce too high a spin during impact. That too high spin
can result in excess lift and decreased flight distance as is known
in the art. The velocity of the upward motion can be a fraction of
the ball tangential velocity in this same coordinate frame. In this
case there will be less relative motion between the ball surface
and the face surface resulting in less spin up of the ball during
impact and therefore more distance during flight.
The Currently Preferred Embodiment (Concept 2)
Principle of Operation
[0102] As the ultimate design goal, the head is designed to convert
impact energy into high frequency, high amplitude vibrations of the
club face. High frequency excitation of the face reduces face/ball
effective friction coefficient using the techniques disclosed in
the Katoh and Adachi references and known in the art. The reduction
in the effective ball/face friction coefficient during the face
oscillation, acts to reduce ball spin induced by frictional contact
with the face at impact. Simulations of ball flight have shown that
reduced ball spin resulting from impact leads to increased ball
travel in a high effective ball velocity scenario. These scenarios
are those associated with high effective ball velocities i.e.--high
head speed and/or high headwind. In these conditions the excess
lift caused by high spin on the ball results in a ballooning
trajectory which results in a considerable reduction in down range
trajectory. Studies have shown that a 25% reduction in ball spin
can increase down range flight distance by 10-20 yards in some high
relative velocity scenarios.
[0103] Reduced friction between the ball and the face can also
result in reduced sidespin on the ball resulting from impact.
Reduced ball sidespin leads to reduced cross range scatter and
increased accuracy in the drive. It is therefore the intention of
the invention to provide a system that can impart the requisite
surface oscillations on the clubface so as to achieve the known
desirable benefits of controlled spin reduction. The system is
controlled in the sense that only the high velocity impacts (those
which exhibit the undesirable excess spin) will trigger the spin
reducing oscillations. It is furthermore the intention of the
invention to power this controlled friction reduction system
entirely from the energy available at impact between the golf club
head and the ball thereby requiring no external power supply such
as a battery.
[0104] Simulations indicate the ability of a high frequency driven
club face oscillating with a 5-10 micron amplitude near or above
120 kHz to dramatically lower ball spin rate. Simulations of a
ball--club impact are shown in FIGS. 12 and 13. FIG. 12 shows the
voltage time history of a piezoelectric transducer coupled to the
face during impact. The voltage rises until it reaches a critical
trigger level (set in the electronics) at which point an
oscillation is excited which is tuned to the face mode of interest
(120 Kz). These high frequency oscillations are shown in FIG. 13 to
reduce the friction coefficient and tangential force between the
ball and the face--thereby reducing the rate of spinup at impact
and the resulting ball spin. Curve C in FIG. 13 shows the voltage
time history analogous to that shown in FIG. 12. FIG. 13B shows the
tangential (friction) force between the ball and the face
indicating the reduction afforded by the high frequency oscillation
in C. The ball spin rate is shown in 13E wherein the ball spin does
not increase during the time that the tangential force is reduced
due to the oscillations of the face. The effect is predicated on
the hitting surface reaching a critical peak acceleration during
the oscillation cycle. The critical parameter for friction
reduction is that the hitting surface (clubface) has to
intermittently break contact with the impacting ball. For that to
happen in a ball-face impact scenario, the acceleration of the face
away from the ball has to be large enough to break that contact. In
effect, the face must move out from under the ball. This only needs
to happen for a short fraction of the impact event in order to
effect the ball-face friction as shown in FIG. 13. Since during the
ball-face impact there is a high preload, there is a high
compressive load between the ball and the head, shown in FIG. 13A.
This ball-face normal load causes the ball to accelerate in the
direction of the eventual ball flight. The ball is initially at
rest and then it has to undergo a high acceleration rate to reach
its peak velocity after the impact event. In order to break
contact, the face must accelerate at a level on the order of this
ball acceleration for at least a portion of the cycle.
[0105] The face has to reach a sufficient acceleration backwards
away from the ball in order to break contact. The amplitude of
oscillatory motion of the face times the frequency of that
oscillatory motion squared is proportional to the peak surface
acceleration. It has been found that surface oscillatory motions in
the range of 5-20 microns amplitude at frequencies in the
50-120+KHz range have sufficient surface acceleration to break the
contact between the face and the ball in a wide range of impact
conditions. Lower surface motion amplitudes are needed if the
oscillation occurs at higher frequency (all else being equal)
[0106] When this occurs, the face moves back away from the ball at
very high acceleration rates for very brief periods of time. The
principle of operation is that the induced surface motion has a
great enough amplitude and frequency and the surface acceleration
will be high enough to overcome the compressive loading due to ball
impact and actually break contact between the ball and face. The
face actually moves away from the ball surface faster than the ball
can respond to the lowering of interface force. It moves out from
underneath the ball.
[0107] The breaking of contact resets the micro-slip region used in
a common model of interfacial friction. In this friction model
(Katoh) shown in FIG. 20, there is a small amount of relative
tangential motion, u, allowed between the bodies (surfaces) before
the friction forces build up to the levels associated with Coulomb
(sliding) friction. FIG. 20 which is a plot of effective friction
coefficient (tangential coefficient), .phi..sub.t, as a function of
relative displacement between the bodies u. This region of lowering
frictional coefficient is due to tangential elasticity at the
interface. As the surfaces slide past each other, the friction
grows rapidly (in the course of a few microns travel, noted by
u.sub.1 in FIG. 20) up to the asymptotic level associated with the
Coulomb friction between two sliding surfaces. This friction model
represents micro-deformation that occurs to accommodate the
relative motion between the surfaces before the interfaces begin to
slip. This interface model is presented in the
Adachi-reference.
[0108] By breaking contact between the ball and face repetitively
before the objects have had enough relative motion to be in the
asymptotic region, the sliding between the surfaces occurs only in
the micro-slip region which has much lower effective coefficient of
friction. Over multiple cycles of breaking contact, the sliding
motion is therefore integrated to a lower average friction
coefficient between the ball and the face.
[0109] There are number of dynamic interactions which occur during
the ball--face impact. The forces can be thought of as active
normal to the face and tangential to the face. Normal forces act
through the center of mass of the ball and so to first order
accelerate the ball and do not directly induce spin. The tangential
forces that arise from the friction between the ball and the face
act both to affect the tangential component of velocity as well as
the ball spin.
[0110] In the tangential direction during the course of the impact
event, the ball is starting a slide up the face as it starts to
roll. By the time it leaves the face it's usually rolling up the
face with little sliding component, i.e. the ball is rolling
(spinning) at a rate such that the point of contact at the surface
of the ball and the face is not moving relative to the face contact
point. By controlling the effective coefficient of friction between
the ball and the face, the degree to which the ball spins up during
impact is controlled as shown in FIG. 13 trace E If the friction is
reduced enough, the tangential forces will not be sufficient to
spin up the ball to the point of pure rolling. Therefore since the
tangential (friction) forces directly effect the ball spin,
controlling these forces can lead to ball spin control.
System Implementation
[0111] The system is designed to capture the energy from the ball
club head collision and use it to excite high frequency
(ultrasonic) vibrations of the face, using these to control
friction between the face and ball as described above. It is
implemented using piezoelectric elements elastically coupled to
face deformations. In the preferred embodiment the same piezo
transducer (in the most general sense as defined for piezo above)
is used both to extract energy from the impact for powering the
system as well as using the extracted energy to excite ultrasonic
vibrations in the club face. In operation, the impact deforms the
club face onto which the piezoelectric transducer is elastically
coupled such that face deformations are converted to electrical
energy (charge and voltage on the piezoelectric element) for
example the elements P10 or P11 in FIG. 10. The electronics that
are coupled to the piezoelectric transducer are configured such
that the piezo is initially in the open circuit condition while it
is charging up during the impact. At some point the piezoelectric
voltage reaches a critical level (trigger level) pre-defined in the
system at which point a switch Q10 or Q11 in FIG. 10 is closed
thereby connecting an inductor L10 or L11 across the piezoelectric
electrodes. The inductor is configured such that the resulting LRC
circuit (the C being the capacitance of the piezoelectric element,
and the L being the shunt inductor) responds in an oscillation
(ring down) that initiates upon connection of the inductor circuit
across the piezo electrodes. The component values are selected such
that the frequency of the ring down is approximately tuned (as
described below) to a high frequency dynamic structural mode of the
face/piezo system such as the mode highlighted in the frequency
response function in FIG. 22--thereby causing high frequency face
motion/oscillation by virtue of the piezo electro-mechanical
coupling. The system is designed such that the high frequency face
motion is sufficient to control the friction between the ball and
the face as described above.
[0112] The system has a number of design issues that will now be
discussed. The system is designed to maximally charge up the piezo
to obtain maximum electrical energy stored in the piezo capacitance
prior to initiation of the ring down/oscillation. This maximizes
the oscillation amplitude. In addition the system is designed
structurally and electrically so as to maximize the coupling of the
piezoelectric to high frequency face motion as will be described
below.
[0113] Piezoelectric element (21) shown in FIGS. 2a and 2b is
elastically coupled to high frequency face mode so as to excite
high frequency vibrations. The electrical circuit is designed to
harvest the impact electrical energy and use it to drive an
oscillator approximately tuned to the selected the face modal
frequency. The electronics convert a small portion of the impact
energy into high frequency oscillations of the clubface. As the
piezo charges up, when it reaches a threshold (trigger level), the
control switch (Q10 and Q11 in FIG. 10 and Q3 in FIG. 11 is turned
on shunting an inductor across the previously open circuit
piezoelectric and initiating a high frequency oscillation at the
frequency determined by the inductor and piezoelectric capacitance
as illustrated in FIG. 12.
[0114] The frequency is determined by an LC time constant. The
inductor is sized for high frequency resonance and should have very
low resistance to reduce energy loss, and appropriate magnetic core
or air core to reduce magnetic hysteresis loss and magnetic field
saturation effects. The switch can most easily be implemented with
MOSFET transistors although other switches with the characteristics
of potentially rapid turn on time (sub 1 microsecond) and low
resistance when closed. There are many other desirable
characteristics of the switch which will be discussed
hereinafter.
Face and Piezoelectric Design
[0115] The piezoelectric transducer is coupled to the face motion
such that deformation of the face results in piezoelectric voltages
and charges. The objective of the design is to maximally couple the
piezoelectric transducer simultaneously to achieve two effects: 1)
maximum coupling (and resulting voltages) to face deformations
resulting from ball impact on the face--both impacts at the center
of the face as well as impacts off center, and 2) maximum coupling
to a high frequency mode of oscillation of the coupled piezo-face
structural system. The coupling from face loading to the
piezoelectric open circuit (OC) voltage is represented in FIG. 21
which shows the transfer function from a distributed loading
representing a ball impact to the piezoelectric open circuit
voltage. The curve represents the response to center hits and there
is a different curve for each hit location located 0.5 in from the
center location in each of the squared directions (above=north,
below=south, toe-ward=west, heel-ward=east). The quasi-static open
circuit voltage for a 10,000 N loading proportional to a 95 MPH
head swing is represented by the lower frequency asymptote of the
transfer function noted in FIG. 21. This figure of merit (FOM) can
be averaged over a series of hit locations to yield a design FOM
that attempts to maximize the piezoelectric voltage that is
generated by a range of center and off center hits.
[0116] The coupling to high frequency face mechanical oscillations
is represented by the transfer function in FIG. 22. This figure
represents the transfer function from applied sinusoidal
piezoelectric voltage to face surface acceleration at the center of
the face (and at points 0.5 inches away in each of the before noted
directions). In a like manner to the voltage response transfer
function mentioned above in FIG. 22, the motion/acceleration at a
range of locations can be used as the figure of merit for the
design--averaged or weighted. As is seen, the high frequency
acceleration response is maximized at a vibration mode of the face
and coupled piezoelectric system ("Excited mode" in FIG. 22). In
the preferred embodiment this mode occurs at 127 KHz. Driving the
face at this frequency will maximize surface acceleration. In a
like manner, a ring down of the piezoelectric oscillating in the
range of frequencies associated with the high acceleration response
will lead to maximal surface acceleration.
[0117] The goal in the design is to maximize both achieved open
circuit voltage due to center and off-center hits as well as to
maximize surface acceleration during the subsequent ring down
response from this voltage after the circuit has been triggered.
The geometry of the system is selected to maximize these two
figures of merit resulting in maximal high frequency response of
the surface due to the system activation.
[0118] The piezoelectric element, club face, and conical housing
elements described below are all configured such that the resulting
coupled system exhibits these qualities. It is a coupled system
design since the surface response to impacts and resulting voltages
are a function of the housing, piezoelectric transducer, as well as
the face geometry and material. In addition, the high frequency
mode shapes and frequencies are very much a function of all three
elements of the design. In the following sections, the
piezoelectric transducer will be described followed by the housing
and face structures.
Stack and Endcap Design
[0119] The piezoelectric element is shown in exploded view of the
face subassembly in FIG. 18 and in section view of the face
subassembly in FIG. 19. The piezoelectric stack itself is denoted
as element 21 while the actuator assembly consisting of the stack
21 leads 22, stack end caps 23 and strain relief 25 is together
taken as subassembly 15 in FIG. 18. The piezoelectric actuator 21
is preferably configured as a multi-layer stack, 3-3 type actuator.
It can alternately be a monolithic rod, tube, or bar, such that
electrical input generates axial actuation (motion and stress)
predominately and conversely axial loads generate voltage and
charge on the element. Note that 1-3 (transverse) coupled tube or
system also has this effect but using a 3-3 stack minimizes
voltages because the layers can be made thin and the 3-3 mode
multi-layer stack utilizes the high piezoelectric coupling
coefficients associated with the 3-3 mode of operation. A centrally
positioned piezoelectric stack is placed between the face 10 and a
backing plate (cap 13) that is structurally coupled to the face at
carefully determined locations. The piezo stack has convex endcaps
23 that provide a point contact with the face thereby minimizing
bending moments induced on the stack due to eccentric placement in
the system. This is important in this highly stressed system since
it is desirable to operate the piezoelectric near its maximum
allowable stress to minimize system weight while maximizing
electromechanical coupling. In addition, the convex endcaps 26 are
designed so as to distribute the stress more uniformly though the
stack resulting in more ideal stack operation and minimizing stress
inhomogeneity in the stack which can cause fracture or induce stack
failure under impact. The endcap thickness is determined to ensure
sufficient homogeneity. In the preferred embodiment, the endcaps
have a radius of curvature of 12.5 mm on the rounded end and
measure 3 mm from the top to the interface with the piezoelectric
stack, They are formed of a stiff material such as alumina or steel
to more efficiently distribute the stress to the stack in a minimal
thickness/mass part. Alternately they can be composed of
laminations of these materials for ease of fabrication.
[0120] The stacks 21 consist of co-fired multilayer piezoelectric
elements with layer thickness in the range from 15 to 150+ microns.
The systems with thinner layers have much higher capacitance and
thereby have a lower necessary inductance for tuning to a given
frequency than the system using thicker layers. For example, for a
9 mm diameter circular stack of 1 cm total length, if it is
assembled from 90 micron layers then the stack capacitance=550 nF,
while if it is assembled from 35 micron layers then the stack
capacitance=3442 nF.
[0121] The stacks with thinner layers conversely also have much
higher current during triggering. The higher current can lead to
excess loss. The thinner layers also lead to lower voltage systems
under comparable stresses that can simplify and lighten the
electronics design. The preferred embodiment uses 90-100 micron
thick layers. The piezoelectric material is a "hard" composition
similar to typical PZT-4. It is selected so as to minimize
piezoelectric hysteretic losses as well as maximize stack
robustness and tolerance to high axial stresses during impact. The
leads are attached such that all the piezoelectric layers act in
parallel. The leads are attached to the side of the stack as shown
in FIG. 18. The piezoelectric element is .about.1 cm long and 9 mm
in diameter. It is attached with a strong epoxy to the curved
endcaps with a very thin layer (so as to maximize coupling) such
that the overall piezo/endcap assembly 15 is .about.16 mm long.
Face and Cone Design
[0122] The objective is to couple to the face deformation during
impact to maximize generated voltage and charge during impact
(generated electrical energy) and also couple to a high frequency
mode of the face system which can be excited by high frequency
oscillations of the actuator. The system converts impact energy
into high frequency oscillation of the face. High frequency face
oscillation can be used to control the frictional interface between
the ball and the face using concepts of reduction in interface
friction by surface vibration.
[0123] The face structure is titanium of carefully controlled
thickness so as to create the desirable modal structure having a
high frequency mode easily excited by the piezoelectric element.
The general configuration of the face, housing and piezoelectric
(together the face assembly 14) is shown in assembled view in FIG.
17, exploded view in FIG. 18 and section view in FIG. 19. It
consists of a piezoelectric element 21 with endcaps 23 (described
above) attached to the face 10 and loaded against it, by a conical
housing structure 12. The piezoelectric element interfaces to the
face at the center point for impacts 33. The face is manufactured
with a small dimple 33 with a radius of curvature slightly larger
than that of the endcap, around 13 mm, so as to provide for
positive location of the stack on the face.
[0124] A conical housing 12 with an optional threaded independent
endpiece 13 is configured to interface with the distal end of the
piezo/endcap actuator assembly 15 (opposite the face end). It
likewise has a curved interface to provide for positive location of
the piezoelectric endcap. The conical endcap has a threaded base 29
that screws into the threaded ring 37 on the face of the club 10
(inside surface) as shown. By threading the cone onto the face, the
piezoelectric element is mechanically coupled to the face, and
piezoelectric axial size changes are coupled to the face bending.
The radius of the ring 56 as well as the thickness and geometry of
the conical housing are carefully determined so as to minimize
elastic losses and deformation between the face and the distal end
of the piezoelectric element. The axial stiffness of the housing
must be as high as possible to maximize piezoelectric coupling to
the face deformation.
[0125] The conical housing can be configured with access holes in
its sides as shown in FIG. 18 element 32. These allow stack
positioning and lead egress to the electronics located elsewhere
inside the club head. Care must be taken in the structural design
on the face, conical housing, and piezoelectric element so as to
avoid critical stress levels in these components under the repeated
high impact loads. The system is designed so that the housing can
be screwed onto the face to press the piezoelectric stack securely
onto the face and provide a sufficiently high compressive preload
on the piezoelectric element. The goal is to keep the actuation
element in compression during impact and operation since
piezoelectric elements do not have high tensile strengths.
[0126] The face thickness is 2.4 mm inside the cone ring 39 and 2.7
mm outside the ring in a step 35 with a gradual taper 36 down to
2.2 mm minimal thickness 34 moving radial outward from the ring.
Higher thickness outside the ring is due to the increased stress
due to the stiff conical housing, necessitating thicker walls in
these areas. The threaded ring can be welded onto or formed with
the face. It is approximately 2 mm thick and 3.5 mm high, at 38.
The conical housing 12 wall thickness is approximately 1 mm.
[0127] A critical dimension is the diameter of the housing at the
face attachment ring 38. This diameter is chosen as large as
possible while still allowing the system to have a clean
axisymmetric vibration mode at a high enough frequency so as to
allow excitation of high accelerations in the face structure. In
the preferred embodiment the ring 38 has approximately a 35 mm
diameter and a height of 4 mm. The face thickness inside the ring,
39, is 2.4 mm and is chosen to match one of its component modes (as
if it were a circular plate vibrating unattached to the
piezoelectric) to the first axial extension mode of the
piezoelectric element. This face--piezo mode matching creates a
coupled system (once the piezo is attached to the face) which has a
high modal amplitude at that design frequency.
[0128] The conical housing may have a threaded endcap 13 at its
distal end, the housing threaded surface 30 mating with the endcap
threaded surface 27. The opening in the housing allows for a
simplified assembly process. With the removable endcap design, the
conical housing is attached to the face first. Then the
piezoelectric element is inserted and the endcap screwed onto the
conical housing preloading the piezoelectric against the face. The
endcap can have a concave curved surface to mate with the
piezoelectric convex endcap. The endcap 13 can have a threaded
attachment 27 to the conical housing 12.
Electrical Circuitry
[0129] The general system is one which converts electrical
energy--which has been "quasi-statically" generated during impact
by an elastically coupled piezoelectric element which is loaded
during impact. As the stress/load is applied to a piezoelectric
element, the voltage and stored electrical energy builds up on that
piezoelectric element. The electronics shown in FIG. 10 and FIG. 11
convert that stored electrical energy on the piezoelectric element,
into a high frequency oscillatory motion of the piezoelectric
element. To accomplish this conversion, there is a
"switching-event" that switches an inductor L1 in FIG. 11 and L10
or L11 in FIG. 10 across the electrodes of the charged
piezoelectric element at a predetermined voltage threshold. The
voltage level can be predetermined to correspond to an impact of a
certain magnitude or intensity and thereby only trigger the system
in the event of a sufficiently intense impact so as to warrant
corrective action on the spin of the ball.
[0130] The switch can also be triggered by events other than a
critical voltage level. For instance the trigger can occur at the
peak of the loading during impact by using peak detection circuitry
which initiates when the piezoelectric voltage starts to retreat
from its previous value (peak detection circuitry).
[0131] The inductor is sized such that the capacitor and the
inductor oscillate at a predetermined frequency, (on the order of
120 KHz). Piezoelectric element capacitance is approximately 480
nF-600 nF, for 100 micron layer thickness at 9 mm diameter and 1 cm
total length of the stack. In this system the optimal inductor L10,
L11, L1 value is .about.1-10 microHenries.
[0132] In summary, the circuit design, from a high level
functionality, is such that it will sense voltage level on the
piezo when the piezo electrodes are open circuit, and then at a
predetermined voltage level will close a switch connecting an
inductor to that circuit thereby causing the piezo (which has
voltage on it prior to triggering) to oscillate at high frequencies
as the voltage and charge on the piezo discharge through the
inductor which causes a ringing as shown in FIG. 12.
[0133] The circuits depicted in FIGS. 10, and 11 have this simple
functionality of a triggered switch. As the transducer
(piezoelectric) is stressed during the impact, charge and voltage
build up on its electrodes, essentially storing the mechanical
energy of impact that has been converted by the transducer into
electrical energy. The particular circuit operates so that when the
voltage reaches a critical threshold, a switch is closed to connect
the capacitive piezoelectric element to an inductor. The inductor
is sized such that the LC time constant of the closed electrical
circuit (the electrical resonance frequency) is very near the
resonant frequency of a structural mode--in this case the selected
face flexural mode.
[0134] The high frequency ringing must be as efficient as possible
in converting "quasi-static" energy in the piezo capacitor into the
energy of the oscillation. This requires a very low loss
oscillation, so that the ring-down has very low damping ratio, very
high quality factor typically less then 10% of critical, preferably
less than 5% of critical. This, in turn, requires very low "on"
resistance switches and very low--no loss elements such as low loss
inductors and no resistors in the primary connection path.
[0135] High performance in the system also implies avoidance of any
parasitic loses. A typical parasitic loss is due to the charge
necessary to drive the switch control circuitry or any electrical
system elements such as capacitors that act to reduce the open
circuit voltage that the piezo would normally be generating at
impact.
[0136] Typical voltage expected to be seen on the piezo before
triggering is on the order of 400 v (system could see 100 v to 600
v). A lot of these components are going to be high voltage
components, and therefore must have high breakdown voltages but at
the same time very low on resistances for very little losses.
[0137] So in general the system consists of four components: 1) a
piezoelectric transducer 21 with some capacitance, 2) a switch Q3
in FIG. 11 that is controlled by 3) control circuitry, and which
connects an 3) inductor L1 in FIG. 11 across the piezoelectric
electrodes.
[0138] It is very important that this main switch turns on very
fast when the voltage on the piezoelectric element electrodes
reaches a critical level (pre-determined threshold level). Having
the switch turn on fast is important for reducing losses because at
120 kHz if it turns on relatively slowly, if it were to take a few
micro-seconds to turn on, the loss in piezo voltage before a true
ringdown could occur can be quite substantial. In essence the piezo
charge is bled off prior to fully connecting the inductor. This
severely limits the initial and subsequent voltages of the
oscillation. An ideal circuit connects the inductor onto the piezo
with little or no drop in piezo voltage from its original open
circuit state (prior to the initiation of the switching). In
summary, in operation the system reaches a trigger threshold level
and then rapidly closes a high voltage switch so that it has very
little loss and the ringdown initiates at the open circuit voltage
level determined by the trigger event.
[0139] The block diagram of the circuit is shown in FIG. 10 a and b
showing the control circuit driving the switch to connect the
inductor element to the terminals of the piezoelectric element.
FIG. 10a shows a configuration in which the switch is between the
piezoelectric and the inductor (high side) while 10b is a
configuration in which the switch drain is nominally at ground (low
side). The detailed circuit of the configuration in 10b is shown in
FIG. 11. In the following section, its operation will be described
making reference to the element numbers found in that figure. The
operation of the principal components of the circuit is as
follows:
Piezo (P1):
[0140] The circuit is connected to a piezoelectric device P1, with
the high electrode of the piezoelectric device (positive voltage
under stack compression) being connected to inductor L1 (FIG. 11).
In FIG. 11, the piezoelectric element can be represented by a
voltage source in series with a representative capacitance, C. In
actuality these elements are not part of the circuit and only serve
to represent the piezoelectric element for tuning purposes. This
representation neglects the coupling from the electrical energy to
mechanical energy and really only reflects the effects of
mechanical forcing on the piezoelectric element (mechanical to
electrical coupling). The capacitor C is sized to reflect the
piezoelectric's open circuit capacitance; while the voltage source
inputs sized to represent the open circuit voltage excursion that
the piezoelectric would see under mechanical forcing in the open
circuit condition (nothing attached). A more complete model for the
piezoelectric would include electrical analogues to the mechanical
properties such as stiffness and inertia of the piezoelectric
device, as well as a transformer or gyrator coupling the mechanical
and electrical domains.
Inductor (L1):
[0141] The inductor, L1, is connected to the piezoelectric element
P1. It is initially floating (not connected to ground) since the
switch, Q3, is open and so no current flows through it. Upon the
triggering event and the subsequent closing of the main switch
(Q3), the floating side of L1 is connected to ground and a closed
circuit is created between the piezoelectric element and the
inductor--now connected in parallel with the piezoelectric
capacitance. This creates a closed LRC circuit, with the piezo
acting as the capacitance, L1 acting as an inductance, and the
series resistance of L1 as well as any on resistance of the main
switch Q3 (and any lead resistance) acting as the R. The
fundamental goal of the design is to create a highly resonant
electrical circuit (low R and low damping) to allow coupling from
the electrical oscillations into the mechanical oscillations of the
piezo and face. For this reason, the inductor must have very low
series resistance at the frequency of oscillation of the LRC
circuit. This is typically in the range from 50-200 kHz. It is
essential to use high quality, low loss inductors rated for high
frequency operation such as in switching power supplies. For our
systems, the piezoelectric capacitance is on the order from 200-600
nF (with .about.400 nF most typical) and inductance values in the
range from 1-12 .mu.H are typically used to set the oscillation
frequency (with .about.6 pH most usual) as given by the
formula=1/sqrt(LC), where f is the desired electrical resonance
frequency (formula works for lightly damped systems). In our system
we have chosen 3.3 pH power choke coils from Vishay
IHLP5050FDRZ3R3M1 or alternately coils from Panasonic PCC-F126F
(N6), which for a 8.2 pH value has a DC resistance of .about.11
m.OMEGA. (and a very compact package). The tradeoff to be
considered is low resistance vs. package size. Both these weigh
about 3 grams each. Since the inductance value is typically a
function of frequency, it is important to select an inductor which
has the right value at the frequency of the resonant circuit.
[0142] Since saturation effects can be important upon switching
(since the currents can be large) care must be taken to choose an
inductor which will not saturate the core. The saturation changes
the effective tuning and inductance value and greatly complicates
the tuning process. At high current levels the magnetic fields in
the coil saturate, effectively lowering the coil inductance. This
can lead to difficulties in tuning the resonance, which is now
amplitude dependent, and lead to excess losses on switching since
the lower inductance of the saturated inductor does not act as an
effective choke to limit the high currents on switching. It is
desirable to choose an inductor which minimizes nonlinear effects
complication tuning, such as saturation and hysteretic losses in
the core
Main Switch (Q3):
[0143] The main switch is one of the most critical elements of the
circuit. When a predetermined threshold voltage is reached, control
circuitry turns on the mosfet, Q3, by raising the gate voltage of
this N-channel mosfet. Above a critical gate voltage, (.about.5-10
volts) the "on" resistance of the mosfet drops dramatically. The
mosfet changes from an open circuit to a low on-resistance
connection to ground for the inductor. Resistor, R4, is sized so
that the gate is nominally at ground even in the presence of a
leakage charging current from the mosfet, Q2. When the control
circuit fires, the gate of Q3 is rapidly charged up to the
threshold voltage and the "on" resistance of Q3 drops rapidly,
essentially closing the switch. Since the charge necessary to fire
the switch is derived from the piezo itself, this firing charge is
completely parasitic and should be minimized to maximize initial
piezo voltage levels. To this effect, a primary requirement of this
mosfet is a low gate drive charge and low total gate capacitance.
The mosfet also needs to operate at high source-to-drain
voltages--i.e., support the piezo voltage without breakdown prior
to reaching the trigger condition and firing. High breakdown
voltage is therefore important. Low on resistance, typically less
than 0.1 Ohms is also important since this contributes to damping
in the electrical oscillation and is perhaps the primary loss
mechanism for electrical energy in the system. It is also important
to note that mosfets have an intrinsic diode from source to drain.
This provides a reverse current path during upswings in the
electrical oscillations after switching. In the present circuit,
the switch, Q3, is held on during the electrical oscillations by
the diode D3 which allows charge to flow onto the gate when it
fires but not flow off the gate during subsequent voltage
excursions during oscillation. The time constant of how long Q3
stays on after firing is determined by the combination of the gate
capacitance and the resistor R4. After firing, the charge will
begin to slowly leak off of the gate until the voltage threshold is
passed, dramatically increasing the drain source resistance and in
effect opening the switch.
[0144] Several high voltage mosfets have been sourced and evaluated
there are currently two baselines, the APT30M75 from Advanced Power
Technologies, and the SI4490 from Vishay Siliconex. Their
comparative properties are shown below:
TABLE-US-00001 Diode Gate source Ron at Forward Device Vds Max
Charge Vg = 10 V voltage APT30M75 300 V 57nC 0.075 1.3 SI4490 200 V
34nC 0.070 Ohm 0.75
These were selected based on their low gate charge and low "on"
resistance while still having high voltage capability. For very
high voltage systems, the preferred switch is however the STY60NM50
from ST Microelectronics, rated for 500 volts and 60 amps.
Control Circuitry:
[0145] The control circuitry is designed to raise the voltage on
the gate of Q3 rapidly when a critical threshold voltage level is
reached on the piezoelectric. Rapid turn on (and high gain in the
control circuit) is needed to prevent high energy loss during the
transition to the on state--too slow a transition limits the peak
negative voltage excursion of the circuit and the subsequent
ringing.
[0146] Another feature of the control circuit is that it is
latching, meaning that once Q3 is turned on it stays on regardless
of the piezo voltage excursions. It stays on for a period
determined by the leakage of the Q3 gate drive charge through R4.
R4 is typically 3 megaOhms.
[0147] The control circuit operation is as follows: Q3 is initially
open so the voltage at the source terminal (top) of Q3 is
essentially the open circuit voltage of the piezo. At a critical
voltage, determined by the Zener diodes, D4 D5 and D6, which will
collectively start to conduct at the sum of the rated voltages
(plus the diode drop associated with D1) current will start to
conduct through D4-D6, charging up capacitor C3 and turning on
transistor Q1. It is important that D4-D6 be low leakage since
small leakage prematurely through D4-D6 can cause the capacitor C3,
to charge up and turn Q1 on partially or prematurely. R2 is sized
(typically 100 kOhm) to limit the voltage rise associated with the
leakage current of the Zeners, D4-D6, and allow a discharge path
for capacitor C3 (between hits). The transistor Q1, need only be
rated for low voltage since its source is connected to the control
supply capacitor C4 which is maintained at no higher than 28 volts
by Zener D2.
[0148] The control supply capacitor C4, is charged up during the
initial high voltage excursion of the piezo. It charges with a rate
determined by resistor R3 (typically 5 k.OMEGA.s). In the present
system, this is set at about 5k.OMEGA.s allowing a charge time of
approximately 100-200 .mu.sec for a C4 value in the range of about
47 nF. In design, the resistor R3, is sized for rapid charge up
after the capacitor C4 is sized. The capacitor C4 is sized such
that when it is connected to the main switch Q3 gate (when Q2
switches on) it dumps its charge into the as yet uncharged Q3 gate,
lowering the voltage on C4 and raising the gate voltage on Q3 to
the full on condition. Therefore C4 is sized to be large enough to
supply the gate charge of Q3 up to the needed ON level. Since the
charge on C4 is parasitic to the piezo charge and effectively
lowers the piezo voltage, it is desirable to have C4 as small as
possible yet still enable needed gate voltage rise on Q3. For the
selected M1s, this value can be as low as 3.3 nF, but for some of
the larger main mosfets, 47 nF was needed. In practice the
capacitor C4 peak voltage which is limited by the Zener, D2, is set
as high as practicable while keeping the control mosfets and
transistors low cost and low loss. In our circuit we chose 28 volts
for the supply capacitor C4. Testing has shown that at these
component values, the control circuits reduced the piezo voltage by
only a small fraction of the total open circuit piezo voltage.
[0149] When the critical voltage is reached and switch Q1 is turned
on, this in turn pulls down the gate of the P channel mosfet Q2,
rapidly turning it on and connecting the charged capacitor C4, to
the main mosfet Q3 gate. This, in turn, charges the Q3 gate up and
turns Q3 on rapidly. A Fairchild BSS110 was used for the p-channel
mosfet Q2. The mosfet version of the circuit has much lower leakage
through from C4 to the Q3 gate. This leakage occurs when C4 is
charged but switches Q2 and Q3, are nominally open. This leakage of
charge onto the gate of Q3 caused premature partial switching ON of
Q3. Using the mosfet in Q2 eliminates this leakage and leads to
clean switching. Once the gate of Q3 is charged up, it stays
charged since it charges through diode D3 and only switches back
open after the gate charge has drained through R4.
[0150] Electrical Concluding Overview: The piezoelectric element,
essentially and initially an open circuit, charges up. As low
parasitic losses dragging the piezo voltage down reaches some
threshold level that is user controllable, an electrical switch
connects an inductor across the piezo and starts it oscillating at
very high frequencies. That switch has to switch very rapidly to
avoid losses during the transition from open circuit to closed
circuit. It has to have very low on resistance and a circuit is
required that fires that and powers that switch and doesn't have a
lot of capacitive drain because that would lower the voltage on the
piezo. The energy used to turn the switch on, is energy not
available for the oscillation.
[0151] It is desirable to have the ability to be able to tune,
switch out or under electrical control, switch in and change the
inductors to provide variable tuning frequency.
[0152] Some circuits have a self-locking oscillation. They
automatically fall into an oscillation frequency determined by
feedback gain or delay gain in the circuit. It's possible this
would allow locking to the piezo vibration.
[0153] It has been found useful for the system to have some
external interfaces that allow probing of the voltages and signals
in the system during operation. Various leads/sensors/probe points
(external interfaces from the board) allow one to tune and examine
the system states and conditions throughout testing and operations.
The signals can be carried out by external wires, etc. without
disturbing the system, or can be brought out wirelessly. The
interfaces to external electronics (wired or wireless) can also be
used for monitoring/telemetry and also for reprogramming of the
system performance or diagnostics and data downloading.
[0154] These electrical circuit elements (external to the
piezoelectric element coupled to the face) are configured in a
single or multiple boards on a single or multiple sides. The board
is preferentially configured inside the head of the golf club or
external to the club, connected by transducer leads running out of
the head to the board as shown in FIGS. 13 and 14. Some or all of
the components can be located on the external board to allow for
easy access to the circuitry for changing trigger levels or other
tuning of the circuitry. Alternately, the board 18 can be
configured on a sole plate 54 (or other removable part) as part of
a sole plate assembly 16 shown in FIGS. 14 and 15 attached to the
head and in FIGS. 16a and 15b detached from the club head. The sole
plate assembly 16 can be configured with leads 22 or plug
connectors 20 so that electrical connection is made on assembly of
the removable piece to the main body of the club. Such an
arrangement is shown in FIGS. 14 and 15 in section view and in
FIGS. 16a and 16b with sole plate assembly detached. These figures
illustrate an electrical circuit board 18 mounted on a removable
sole plate 54 by standoffs 45 such that when the sole plate is
inserted and connected to the dub body 11 by fasteners 47, an
electrical connection is made between a connector on the primary
board 49 and a connector 20 on a secondary "connector" board 19
which is permanently mounted in the head 11 by standoffs 44 and
electrically connected to the transducer 21 and face assembly
14.
[0155] This arrangement allows for the simple removal and
tuning/maintenance/repair of the electrical circuit and board. The
connector and the connector board permanently mounted in the head
allow the simple removal of the primary board. Additional
connectors can be configured on the primary board to allow for
external monitoring/diagnostics during club swings and impact.
Alternately, such information can be wirelessly transmitted to a
receiver and stored for later examination. Alternately the data
taken during the impact event can be stored on the board in
on-board memory for later dumping/downloading upon a command
prompt. The telemetry transmission can occur over wireless or wired
channels. Such information that can be stored and monitored
includes swing speed, impact force, ball face impact location and
intensity, club head deceleration and resultant ball acceleration
or any of a number of system states that are associated with the
dynamics and conditions of the club swing and impact (or resulting
vibration of response of the ball-head system).
Assembly Procedure
[0156] In assembly the sequence of events can proceed in many
orders of which one is presented below. [0157] 1) Form the face 10
with appropriately configured ring. Perform post forging machining
operations to set inner diameter and thread 37 on the inner
diameter of the ring. Also form and polish the dimple 33 at the
location that the stack will interface when in contact with the
face [0158] 2) Put a dummy threaded piece into face ring thread to
hold its shape and then weld the face onto the body 11. Then remove
the supporting dummy threaded piece.
[0159] 3) Screw on cone 12 until tight
[0160] 4) Insert piezo stack/piezo endcap assembly 15 into cone to
make contact with the face. There may be a supporting element made
of plastic or other flexible material, designed to hold the piezo
in place/position until the endcap of the cone can be screwed on
and the piezo can thereby be preloaded and against the face and
locked into position. The leads of the piezoelectric 22 must be
routed through the holes in the housing walls 32. These should have
an appropriate grommet or strain relief to avoid abrasion during
impact induced motion. [0161] 5) The endcap 13 is then screwed onto
the cone (curved side interfacing with the piezoelectric stack
assembly) until the piezo is securely seated and preloaded against
the face sufficiently to avail breaking of contact between the face
and the piezo endcaps during impact (around 1000 N compressive
preload). A thin layer of machine oil can be used between the
endcaps of the piezo assembly and the face and the cone endcap to
aid in seating. [0162] 6) The screw on cone endcap 13 is than
locked in place with a set screw, epoxy or other method of
fixation. [0163] 7) The leads of the piezo are then soldered onto a
small connector board 19 that holds the connector 20 for
interfacing with the primary (removable) board 18. The connector
board is permanently attached into the head with epoxy or screws on
a standoff 44 The connector board is positioned so as to interface
to the primary board without interference. [0164] 8) The crown of
the club head 43 is then bonded to the head body 11 in a 160 degree
C. epoxy bonding operation. [0165] 9) The primary board 18 and
connector 49 are attached to the removable sole plate 54. And the
entire removable assembly 17 is then inserted into the club head
and screwed in. The system is now operational.
Alternate Embodiment
Face Stiffness Control
[0166] In the forgoing sections, a method and system for achieving
face-ball friction control using ultrasonic vibrations was
presented. In this section an alternate embodiment using a piezo
(or other) transducer coupled to a face of a golf club (putter,
driver, iron) to effect stiffness control will be presented. By
varying the effective face stiffness, the course and result of the
ball-face impact is effected/controlled and so this is generally
one example of an impact control system using solid-state
transducer materials. The concepts presented in this section are
described in terms of a piezoelectric transducer coupled to a face
but apply more generally to a system with any transducer coupled to
face motion--as long as the transducer is capable of converting
mechanical energy to electrical energy and vice versa i.e. exhibits
electro-mechanical coupling.
General Principle
[0167] The general concept is to utilize the aforementioned
electromechanical coupling of a face-coupled transducer to change
the effective stiffness of the face under prescribed conditions. In
essence, one controls the stiffness of the face to produce a
desirable effect from the resulting ball--face impact (with the
controlled stiffness). The stiffness can be controlled because in a
system with electro-mechanical coupling, changing the boundary
conditions on the electrical side (ports) of the system changes the
effective stiffness of the mechanical side of the system. For
example, it is well known in the art that the stiffness of a
shorted piezoelectric element is lower than the corresponding
stiffness of an equivalent piezoelectric element with the
electrodes open. This effect can be used to change the effective
stiffness (longitudinal i.e., in the poling direction or shear
mode, i.e., transverse to the poling direction) of the
piezoelectric material and piezoelectric element. Since the
piezoelectric element is mechanically coupled to the face, this
change in piezoelectric element stiffness results in a change in
the stiffness of the face.
[0168] In any of the transducer-face mechanical coupling
embodiments presented above (Concepts 1-8), the transducer is
mechanically coupled to the face in such a way that a change in the
stiffness of the transducer changes the behavior of the face. In
the case of the elastically coupled embodiments (Concepts 1-4), it
can be said that a change in stiffness of the transducer directly
changes the stiffness of the face to ball impact. This equivalently
changes the deflection of the face under impact. In the inertial
coupled cases (Concepts 5-8) changes in the transducer stiffness
result in changes to a coupling between the face motion and an
inertial mass (for Concept 8 this is the remainder of the club
head)--changing the dynamic stiffness of the face if not the
quasi-static stiffness (DC). This is because these inertial coupled
concepts are not DC coupled. They have no effect on the system at
very low frequencies since there is little inertial force from the
proof mass at low frequencies. They are designed to have effect on
the system at the impact timescales, however, and so a change in
the transducer stiffness in these concepts results in a change in
the stiffness of the system in the frequency range associated with
ball impact (around 0.5 milliseconds and 1 kHz). Thus any of the
Concepts 1-8 can be used to change the effective stiffness of the
face under impact by varying the stiffness of the transducer.
Transducer Configurations
[0169] As mentioned above any of the previously described
transducer configurations can be used as the basis for this impact
control concept. For example, one embodiment uses a piezoelectric
stack coupled to the face as in Concept 2. In the mechanical design
presented previously for Concept 2 and shown schematically in FIGS.
2a and 2b and in detail in FIGS. 13-19, the face DC stiffness (to
central ball forces normal to the face) increases approximately 25%
from the short circuit case to the and open circuit scenarios. An
alternate configuration to using a stack transducer is to use a
planar (potentially packaged) piezoelectric transducer (or other
solid state transducer material) bonded to the face and thereby
coupled to face motion through coupling to face extension and
bending. The face bending stiffness and thereby overall stiffness
to the ball forces can be changes by changing the electrical
circuit boundary conditions (open circuit or short circuit).
System Circuitry Operation
[0170] To enable the control, the transducer electrical boundary
conditions must be determined (controlled) based on some response
or behavior of the system.
[0171] This can be determined based on the transducer itself (i.e.,
voltage or charge under loading) or it can be determined by an
independent sensor for example face strain or face deflection
sensor. An accelerometer can also be used to determine club head
deceleration under impact and trigger the system accordingly.
[0172] In operation, the transducer is placed into an open circuit
or short circuit condition depending on the sensor. For example the
electrical connections can be controlled based on impact
intensity--making the system stiffer under more intense ball
impacts and less stiff under softer ball impact. This can be
especially important in conditions requiring enhanced feel, longer
ball dwell time and an increase in topspin or launch angle such as
in putting and putters, or wedges and short iron shots.
[0173] In putting it is known in the art that the key to reducing
skid is to give the ball as much topspin as possible before it
leaves the putter face and it is advantageous to minimize the
distance that the ball skids before it starts to roll.
[0174] The impact of a putter compresses the golf ball front to
back while widening the girth for an instant. The ball then
rebounds to its initial shape, causing it to propel forward from
the club face. A perfect scenario would have the golf ball
rebounding in a direction determined only by the direction the
putter is traveling and the angle of the putter face relative to
that direction. Since golf balls are not perfectly balanced,
imperfections in the ball can cause deviation in the rebound
direction called compression deflection. A reduction to the amount
that the ball is compressed at impact reduces compression
deflection. A softer face reduces interface loading and decreases
the ball compression. Therefore, when properly tuned the desired
effect of the system reduces ball compression deflection and
optimizes launch and roll conditions. For example in putters, the
combination of having a relatively soft clubface with a high
rebound resilience increases control both in distance and
direction.
[0175] The elastic deformation of both the ball and face materials
has a tremendous influence on the direction, velocity and manner a
golf ball will propel, launch or spring from a clubface after being
compressed during the impact event. The effective resilience of a
clubface striking a ball is a combination of the resilience of the
ball and clubface. To maximize control, in putters and wedges it is
better for a substantial portion of the effective resilience to
come from the clubface, not from compression of the ball, to reduce
compression deflection.
[0176] In contrast with this desire for more compliance in the face
to increase control, in putting and shorter golf shots as the
velocity of impact increases the amount of control could
potentially decrease with a more compliant face due to the
intensity of the impact and force of the stroke relative to
percussion point. Impact induced deformations can contribute to
ball trajectory errors and stroke inconsistency especially in non
ideal impacts at high intensity. Essentially the increased
compliance can lead to a loss of control in higher intensity impact
scenarios.
[0177] To increase the control of the shot and reduce scatter, it
is therefore desirable to have a clubface which has lower stiffness
in lower impact intensity events but higher stiffness in higher
impact intensity events.
[0178] In the preferred embodiment when the Piezo is in shorted
condition and an increase in the amount of time in which the ball
remains in contact with the clubface, "Dwell Time" is coupled to a
clubface with high coefficient of friction, an appreciable increase
in control and optimization of ball launch conditions result.
[0179] Increased dwell time enables the clubface an extended
opportunity to hold the ball for the purpose of imparting topspin.
It is also known that a longer dwell time improves feel.
[0180] For example in low velocity impacts with a putter the
shorted Piezo enables the clubface to cradle the ball during
contact, resulting in more dwell time and less skidding onto the
green. Additionally this performance characteristic translates to
an enhanced feel and control which is also known in the art to
improve accuracy, consistency and confidence.
[0181] In contrast stiffening the face in higher velocity impacts
can also increase accuracy and consistency by reducing elastic
deformation induced errors. Additionally the variable stiffening
effect presents a significant range of performance characteristics
out of one golf club using only simple electrical circuit
variations. Whereas the same range of performance characteristics
in a passive golf club design would require several identically
designed golf clubs with varying clubface material boundary
conditions to perform at this range. Thus the idea of a
electrically tunable or fittable club system is possible Wherein
changing a resistor or trigger level can be used to change the club
behavior to match a particular player, or playing condition.
[0182] By making the system stiffer under certain conditions during
the course of impact, the impact result is being controlled.
Alternately the stiffness change can be configured and fixed by the
user prior to the shot, thereby enabling a kind of fitting of the
club to the user. The user can select the most desirable stiffness
setting and have it set at the factory or in a user controllable
system, the stiffness can be set by the user prior to
play--depending on the user's desires or condition of the game
(weather, wind, etc). The switch or other electrical setting device
can be configured for easy user access, for instance at the end of
the grip.
[0183] A schematic of a preferred embodiment which uses the
piezoelectric itself as the impact sensor is shown in FIG. 23.
[0184] In operation, the circuit acts to open the piezoelectric
electrodes in harder impact scenarios and leave them shorted in
softer impact scenarios. The transducer (coupled to the face) is
electrically connected to charge or voltage sensing circuitry. In
essence it is configured as a sensor. The sensing circuitry keeps
the piezoelectric high lead at ground, essentially shorting the
piezoelectric. In this condition the piezoelectric transducer
exhibits short circuit mechanical properties. If the sensor output
voltage reaches a critical level, then the circuit is triggered and
the switch (normally closed) which connects the piezo to the
circuitry is opened, essentially opening the electrodes of the
piezoelectric transducer. Upon triggering the electronics, the
piezoelectric transducer then has open-circuit stiffness and the
face to which it is mechanically coupled will now have higher
stiffness for the remainder of the impact.
[0185] A circuit which implements this is very similar to the
circuitry described above for the friction control application. The
circuit is modified by replacing the inductor L1, with a resistor,
R12 in FIG. 23, and the switch M1, which is an n-channel
enhancement mode mosfet in the friction control circuit--is
replaced with a new mosfet which is an n-channel depletion mode
mosfet Q12. With a depletion mode n channel mosfet Q12, the circuit
is initially in the short circuit condition i.e. the switch Q12 is
closed. Upon lowering the voltage at the mosfet gate (when it
triggers) the depletion mode mosfet opens the circuit, thereby
disconnecting the resistor and thereby the piezoelectric
electrodes. The circuit is now open circuit. The control circuit
operates to lower rather than raise the gate voltage as in the
friction control circuit. Such voltage driven mosfet drive circuits
are common in the art.
[0186] The trigger event is set when the voltage on the
piezoelectric reaches a threshold voltage sent by the Zener diode.
The voltage rises because the piezo is forced to discharge through
the resistor, R12, and therefore not perfectly shorted. This
provides the opportunity to trigger off the voltage rise that
occurs when the piezo is forced. If the piezo were truly shorted,
the voltage would not rise and the trigger would not occur. Since
the piezo is initially shunted by the resistor, R12 (the switch Q12
being initially closed), the voltage will rise as long as the
forcing occurs at a rate on par with or greater than the RC time
constant of the system. Forcing at frequencies below that
associated with the RC time constant, the voltage will not rise
much since the resistor appears as a short. Above this time
constant (i.e., for relatively rapid forcing) the resistor appears
as an open circuit and the voltage rises. The piezo essentially
does not have the time to discharge through the resistor during the
course of the event.
[0187] The circuit thus has the effect that impacts of sufficient
rate or intensity that raise the voltage on the resistor-shunted
piezo, trigger the circuit and open the depletion mode mosfet
effectively opening the circuit and putting the piezo in a open
circuit electrical situation. The system thus stiffens the system
upon sufficiently intense or rapid impacts. The system can be tuned
by selection or an appropriate shunting resistor, or (primarily) by
selecting the appropriate triggering Zener breakdown voltage.
[0188] The above mentioned system is self sensing and self powering
in that it draws power from no external source but rather from the
charges of the face-coupled transducer itself. It should be noted
that the triggering signal could be derived from an alternate
sensor. In addition the feedback logic could be more complicated,
perhaps even determined by a programmable microprocessor. This
microprocessor could be powered from energy extracted by the
circuitry from the impact event. The microprocessor could be
externally programmed as a result of a fitting system to respond
under predetermined conditions particular to an individual golfers
characteristics and capabilities. This is the concept of a
programmable smart club designed to maximize the benefit from
impact derived from a given golfer's swing. The programming
essentially allows the club behavior to be tuned and customized to
the individual golfer and his characteristics and capabilities. For
example correcting for hooks or slices.
[0189] Having thus disclosed various embodiments of the invention,
it will now be apparent that many additional variations are
possible and that those described therein are only illustrative of
the inventive concepts. Accordingly, the scope hereof is not to be
limited by the above disclosure but only by the claims appended
hereto and their equivalents.
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