U.S. patent application number 11/418762 was filed with the patent office on 2007-11-08 for vibration control of free piston machines through frequency adjustment.
This patent application is currently assigned to Sunpower, Inc.. Invention is credited to Douglas E. Keiter, Reuven Z-M Unger.
Application Number | 20070256428 11/418762 |
Document ID | / |
Family ID | 38659977 |
Filed Date | 2007-11-08 |
United States Patent
Application |
20070256428 |
Kind Code |
A1 |
Unger; Reuven Z-M ; et
al. |
November 8, 2007 |
Vibration control of free piston machines through frequency
adjustment
Abstract
A method and apparatus for minimizing the amplitude of
mechanical vibrations of a mechanical apparatus including a linear,
freely reciprocating, prime mover coupled to and driving a
reciprocating mass of a driven machine in reciprocation at a
driving frequency. The coupled prime mover and driven machine have
a spring applying a force upon the reciprocating mass to form a
resonant main system having a main system resonant frequency of
reciprocation. A driving frequency range over which the driven
machine operates at an acceptable efficiency of operation is
determined and stored. A parameter of the operation of the
mechanical apparatus, such as the amplitude of vibrations or an
operating temperature, is sensed and the prime mover is driven in
response to the sensed parameter at a driving frequency that is
offset from the main system resonant frequency of reciprocation, is
within the driving frequency range of acceptable efficiency of
operation and reduces or minimizes the amplitude of mechanical
vibration of the mechanical apparatus under existing operating
conditions.
Inventors: |
Unger; Reuven Z-M; (Athens,
OH) ; Keiter; Douglas E.; (Athens, OH) |
Correspondence
Address: |
KREMBLAS, FOSTER, PHILLIPS & POLLICK
7632 SLATE RIDGE BOULEVARD
REYNOLDSBURG
OH
43068
US
|
Assignee: |
Sunpower, Inc.
|
Family ID: |
38659977 |
Appl. No.: |
11/418762 |
Filed: |
May 5, 2006 |
Current U.S.
Class: |
62/6 ;
62/295 |
Current CPC
Class: |
H02K 11/33 20160101;
F25B 2309/001 20130101; F04B 2201/0806 20130101; H02P 25/032
20160201; H02K 11/21 20160101; F16F 15/22 20130101; F16F 15/02
20130101; F04B 35/045 20130101; F04B 2203/0404 20130101 |
Class at
Publication: |
062/006 ;
062/295 |
International
Class: |
F25B 9/00 20060101
F25B009/00; F25D 19/00 20060101 F25D019/00 |
Claims
1. A method for minimizing the amplitude of mechanical vibrations
of a mechanical apparatus including a linear, freely reciprocating,
prime mover coupled to and driving a reciprocating mass of a driven
machine in reciprocation at a driving frequency, the coupled prime
mover and driven machine having a spring applying a force upon the
reciprocating mass to form a resonant main system having a main
system resonant frequency of reciprocation, the method comprising:
(a) determining and storing a driving frequency range over which
the driven machine operates at an acceptable efficiency of
operation; (b) sensing a parameter of the operation of the
mechanical apparatus; and (c) driving the prime mover in response
to the sensed parameter at a driving frequency that (i) is offset
from the main system resonant frequency of reciprocation; (ii) is
within the driving frequency range of acceptable efficiency of
operation; and (iii) reduces or minimizes the amplitude of
mechanical vibration of the mechanical apparatus under existing
operating conditions.
2. A method in accordance with claim 1 wherein the sensing step
comprises sensing the amplitude of vibration of the mechanical
apparatus and wherein the method further comprises: (a) sweeping
the driving frequency over a frequency range that includes the main
system resonant frequency of reciprocation; (b) storing the sensed
amplitude of vibration in association with a plurality of the
sweeping drive frequencies; and wherein the prime mover is driven
at a driving frequency that is a stored driving frequency
associated with the smallest, sensed, stored amplitude.
3. A method in accordance with claim 2 wherein the method is
periodically repeated.
4. A method in accordance with claim 1 for controlling a mechanical
apparatus in which the driven machine is a free piston, Stirling,
heat pumping apparatus, wherein (a) the driving frequency range is
determined and stored in response to testing of at least one
component of said mechanical apparatus; (b) at least one heat
pumping apparatus of the mechanical apparatus is operated during a
test at a plurality of operating temperatures, for each of the
operating temperatures the driving frequency is varied within the
acceptable driving frequency range and the driving frequency
resulting in the least amplitude of vibration of the mechanical
apparatus is stored in association with each operating temperature;
(c) the operating temperatures and associated driving frequencies
are stored as a lookup table in a memory device connected in a
frequency control system of replications of the tested mechanical
apparatus; (d) the sensing step comprises sensing the operating
temperature of the replications of the tested mechanical apparatus;
and (e) the prime mover is driven at the stored driving frequency
associated with the sensed temperature.
5. A computer or logic circuit control system for minimizing the
amplitude of mechanical vibrations of a mechanical apparatus
including a main machine having a linear, freely reciprocating
prime mover driving a driven machine in reciprocation, the main
machine including a mass and springs applying a force upon the mass
to provide a resonant mechanical oscillator having a resonant
frequency near which the main machine is designed to be operatively
driven, the control system comprising: (a) a sensor for sensing a
parameter of machine operation; (b) a data storage for storing a
driving frequency range over which the driven machine operates at
an acceptable efficiency of operation; and (c) a microcontroller
connected to receive inputs from the sensor and the data storage
for controlling the prime mover and programmed for driving the
prime mover in response to the sensed parameter at a driving
frequency that is offset from the main machine's resonant frequency
of reciprocation, is within the stored driving frequency range of
acceptable efficiency of operation and minimizes the amplitude of
mechanical vibration of the mechanical apparatus under existing
operating conditions.
6. A control system according to claim 5 wherein the sensor senses
the amplitude of vibrations of the mechanical apparatus.
7. A control system according to claim 6 wherein the sensor is an
accelerometer.
8. A control system according to claim 5 wherein the sensor is a
temperature sensor.
9. A control system in accordance with claim 8 wherein the sensor
is connected to sense the temperature of a spring of a vibration
balancer.
10. A control system in accordance with claim 5 wherein the prime
mover is a linear, electric motor.
11. A control system in accordance with claim 5 wherein the prime
mover is a free piston Stirling engine.
12. A control system in accordance with claim 5 wherein the driven
machine is a free piston Stirling cooler.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates generally to minimizing mechanical
vibrations of a mechanical apparatus that includes one or more
masses driven in reciprocation by a linear, freely reciprocating,
prime mover and using the electronic controller that controls that
prime mover.
[0003] 2. Description of The Related Art
[0004] Linear, freely reciprocating machines are often used because
they provide improved durability, reduced wear, controllability and
efficiency. Freely reciprocating machines include linear
compressors, free piston Stirling engines, Stirling cooler,
cryocoolers and heat pumps, linear motors and linear alternators.
Linear, freely reciprocating machines reciprocate with a
controllable stroke and are unconfined by conventional crankshafts
and connecting rods. However, linear, freely reciprocating machines
cause substantial vibration because they have one or more masses
that are linearly reciprocating within a common housing and/or are
attached to a common support frame.
[0005] Typically, a main machine or system consists of multiple,
freely reciprocating machines connected together. One reciprocating
machine is a linear, freely reciprocating, prime mover such as an
electric, linear motor or free piston Stirling engine which can
also be termed a Stirling linear motor. The second reciprocating
machine is a linear, freely reciprocating load driven through a
mechanical link by the prime mover and may be, for example, a free
piston compressor, a Stirling heat pump or cooler or an electric
alternator. The composite reciprocating masses of both the prime
mover and the load it drives contribute to the vibration. This
vibration is ordinarily undesirable and a variety of systems have
been developed to minimize the amplitude of such vibrations.
[0006] Typically, free piston and other linear, freely
reciprocating machines are constructed with one or more springs
applying a spring force to the reciprocating masses. Both the prime
mover and the machine it drives may include springs. The springs
may include one or a combination of mechanical springs as well as
gas springs and magnetic springs. The gas and magnetic springs may
be devices designed to provide a spring force or, more commonly,
they are the result of gas acting upon a component of the machine
and/or magnetic forces from electromagnetic devices or permanent
magnet system used in the machines, such as electric linear motors
and alternators. Together, the masses and springs of the linear,
freely reciprocating prime mover and the driven, linear, freely
reciprocating machine form a main machine that is a resonant
system
[0007] Commonly, the main machines are designed to be operated at
or near a resonant frequency because that maximizes their
efficiency. The natural frequency of such a system is described in
accordance with the equation: f = 1 2 .times. .pi. .times. K m Eq .
.times. 1 ##EQU1##
[0008] where f=the resonant frequency in cycles per second or
Hertz, K is the composite spring constant in Newton/meter and m is
the composite mass in Kg. The word "composite" is used to designate
the sum of the respective masses and springs of the main machine
and the terms "mass" and "spring" are used to include the composite
mass or spring when their effects sum together.
[0009] The vibration problem can be further complicated if a
mechanical apparatus consists of a main machine or system,
comprising a prime mover driving a driven machine, that is also
coupled to other equipment that includes one or more secondary
vibrating systems. Such secondary systems may be coupled to the
main machine by mounting the secondary vibrating system so it is
mechanically connected to the main system, for example because both
systems are mounted to the same support frame. Secondary vibrating
systems can be devices that are designed with masses and springs to
oscillate during their operation or they can be devices that are
not intended to oscillate as part of their normal function but
nonetheless have a mass connected to a structure acting as a
spring. A secondary vibrating system that is coupled to the main
system is a parasitic resonant system if it is not intended to
vibrate during its normal operation. If the resonant frequency of
the parasitic resonant system is sufficiently near the driving
frequency of the main machine, the parasitic resonant system may
vibrate at an excessive amplitude. If the parasitic resonant system
vibrates at the drive frequency and at less than 90.degree. out of
phase with the main system, it can increase the total vibration of
the mechanical apparatus.
[0010] The prior art has developed a variety of devices for
reducing the vibration of a main machine. These are known by a
variety of names including "vibration absorbers", although they are
more accurately called vibration balancers because they do not
"absorb" vibration. A vibration balancer is a secondary vibrating
system that is mechanically coupled to the main system usually by
direct connection to it. Although the purpose of vibration
balancers is to diminish the vibration resulting from the
reciprocations of the main machine, they are desirably viewed as a
form of a secondary vibrating system because they are not a part of
the main machine or system. One common vibration balancing system
seeks to drive a reciprocating, counterbalancing mass in a manner
that applies forces to the vibrating main machine that are equal
but opposite to the forces generated by the vibrating masses of the
main machine. The driven mass of the vibration balancer can be
driven by its own prime mover or, alternatively, it can be driven
by the vibrations of the vibrating main machine and tuned to be
resonant at the same driving frequency but designed to reciprocate
180.degree. out of phase with the vibrations of the vibrating main
machine. An example of a system of the former nature is shown in
U.S. Pat. No. 5,620,068.
[0011] Another system for reducing vibration is illustrated in U.S.
Pat. No. 6,040,672. A waveform induced in an electric motor drive
signal is sensed, translated into a control waveform and summed
with the motor drive current to reduce the vibration.
[0012] Although these systems perform satisfactorily under
relatively stable operating conditions, under extreme variations in
operating conditions they can encounter difficulties. For example,
a Stirling cycle cooler may undergo extreme ambient temperature
variations. It may operate anywhere in the range of -40.degree. C.
to +60.degree. C. If a Stirling cooler has a vibration balancer
attached to it, these variations in temperature change the
stiffness of its springs, thereby varying their spring constant,
and therefore cause the natural frequency of the vibration balancer
to change. The effective spring stiffness of the spring forces in
the cooler may also vary somewhat, although these variations
usually have less effect because Stirling coolers typically have a
relatively low Q while vibration balancers typically have a high Q,
(i.e. sharp resonant peak). Therefore, a relatively small variation
in the natural frequency of the vibration balancer results in a
large variation in its effective amplitude of oscillation if the
driving frequency remains the same. Consequently the ability of the
vibration balancer to cancel the vibrations of the vibrating main
machine is substantially diminished. Similarly, changes of
temperature can also result in changes in electrical parameters
which, in turn, can change the effective spring constant of
magnetic spring effects. Temperature can also change the dynamic
behavior of a Stirling engine resulting in a shift of its operating
frequency. Non-linear behavior of mechanical springs or structural
components in response to varying strokes may shift the natural
frequency as well.
[0013] As a result, a mechanical apparatus with a vibration
balancer can be well balanced and exhibit an acceptable amplitude
of vibration under some operating conditions, but if the operating
conditions depart sufficiently from the preset operating
conditions, the vibration balancer will become less effective
because the change in operating conditions changes the resonant or
natural frequency of the vibration balancer or changes it phase
relation to the main system or both. If the vibration balancer
becomes less effective, the amplitude of the vibrations
increases.
[0014] Similarly, the resonant frequency of secondary parasitic
vibrating systems coupled to the main system can also change as a
result of changes in operating conditions. As a result, a secondary
system that does not aggravate the vibration of the mechanical
apparatus under some operating conditions can become a problem when
the operating conditions change sufficiently. A component of a
mechanical apparatus that was not a vibration problem can become a
problem when operating conditions change sufficiently. A parasitic
vibration system can also be discovered after a machine is
constructed.
[0015] Although it is possible to construct a vibration balancer
that would be able to vary its spring constant or otherwise vary
its natural frequency of oscillation, such a vibration balancer
would be even more expensive than conventional vibration balancers.
Vibration balancers are not only a considerable cost, they also
take up space and add weight to a product.
[0016] It is a feature and object of the invention to supplement a
vibration balancer by electrically compensating for variations in
the ability of the vibration balancer to cancel vibrations as a
result of variations in operating conditions.
[0017] Another object and feature of the invention is to provide a
control system for a linear, freely reciprocating, main machine
that can compensate for secondary, parasitic vibration systems.
[0018] Another object and feature of the invention is to reduce
vibration electronically by altering the controlled operating
characteristics of a linear, freely reciprocating prime mover to
compensate for any of the variety of causes of changes in the
resonant frequency associated with a linear, freely reciprocating
main machine and any vibration balancer connected to it where the
uncompensated changes resulting from changes in operating
conditions would otherwise result in an increase in vibration.
[0019] Yet another object of the invention is to compensate for
changes in the resonant frequency of a linear, freely reciprocating
main machine independently of, and in response to, changes in the
operating conditions of the machine.
BRIEF SUMMARY OF THE INVENTION
[0020] The invention is a method and apparatus for minimizing the
amplitude of mechanical vibrations of a mechanical apparatus that
includes a linear motor coupled to and driving a reciprocating mass
of a driven machine in reciprocation at a driving frequency. The
coupled motor and driven machine have one or more springs applying
a force upon the composite reciprocating mass to form a resonant
main system having a main system resonant frequency of
reciprocation. A driving frequency range over which the driven
machine operates at an acceptable efficiency of operation is
determined and stored. A parameter of the operation of the
mechanical apparatus is sensed and the linear motor is driven in
response to the sensed parameter at a driving frequency that is
offset from the main system resonant frequency of reciprocation, is
within the driving frequency range of acceptable efficiency of
operation and reduces or minimizes the amplitude of mechanical
vibration of the mechanical apparatus under existing operating
conditions.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0021] FIG. 1 is a block diagram illustrating an example of an
embodiment of the invention.
[0022] FIG. 2 is a block diagram illustrating a second example of
an embodiment of the invention.
[0023] FIG. 3 is a graphical plot in the frequency domain of
resonant peaks and illustrating the operation of the invention.
[0024] FIG. 4 is a block diagram showing an example of a control
circuit embodying the invention and using a temperature sensor.
[0025] FIG. 5 is a block diagram showing an example of a control
circuit embodying the invention and using a vibration amplitude
sensor.
[0026] In describing the preferred embodiment of the invention
which is illustrated in the drawings, specific terminology will be
resorted to for the sake of clarity. However, it is not intended
that the invention be limited to the specific term so selected and
it is to be understood that each specific term includes all
technical equivalents which operate in a similar manner to
accomplish a similar purpose.
DETAILED DESCRIPTION OF THE INVENTION
[0027] The invention makes use of the observation that, for a
mechanical apparatus that includes a vibrating or reciprocating
main machine coupled to a secondary vibrating system, which can
include a vibration balancer, there are three frequencies that are
important. There are the resonant (or natural) frequency of the
vibrating main machine, the resonant (or natural) frequency of the
secondary vibrating system and the operating frequency of the main
machine. The operating frequency of the main machine is also the
operating frequency of the vibration balancer and any other
secondary vibrating system coupled to the main machine.
[0028] If a graph is made of frequency vs. amplitude of vibration
for any resonant system, the plotted amplitudes form a resonant
peak centered at a resonant frequency. These peaks can rise and
fall in a range extending from a broad, gradual manner, to a sharp,
steep manner. The sharper the peak, the higher the quality factor
"Q" of the resonant system, as known to those skilled in the
art.
[0029] The invention also makes use of the observation that those
main machines, that have a linear, freely reciprocating, prime
mover driving a linear, freely reciprocating driven machine and
that operate efficiently at or near their composite resonant
frequency, nonetheless ordinarily have a band of driving
frequencies over which they can operate at an acceptable
efficiency. They are not confined to operating exactly at their
resonant frequency. In part this is because a typical main machine,
such as a linear motor driving a Stirling cooler, ordinarily
exhibits a low Q resonance peak. Although this is helpful, the
driving frequency range over which the driven machine operates at
an acceptable efficiency of operation is determined not only by the
Q of the mechanically resonant, reciprocating components of the
main machine, but is also dependent upon other design and operating
characteristics of the main machine. However, a designer of any
particular machine is able to determine an acceptable range of
driving frequency by applying ordinary engineering principles to
the design of a particular main machine and its application.
[0030] FIG. 1 diagrammatically illustrates a mechanical apparatus
10 that has a main machine 12, consisting of an electromagnetic
linear motor 14 driving a Stirling cooler 16 in reciprocation, a
motor control circuit 18 that has a data storage 20 and controls
the operation of the linear motor 14. The main machine 10 may also
include secondary vibrating systems 24, such as a vibration
balancer 26 and parasitic resonant systems 28. All of the
illustrated components are mechanically coupled together to form
the mechanical apparatus 10. For example, they may be physically
connected together in the same housing or on the same frame or they
may be linked together by intermediate physical structures that can
transmit vibrations.
[0031] The control system also has a temperature sensor 22 that
senses the temperature of the vibrating systems 24, which may be
sensed at the casing, and inputs the temperature data to the motor
control 18. The temperature sensor may sense the temperature of the
spring of the vibration balancer because that is the principal
component for which temperature changes most directly affect the
resonant frequency of the vibration balancer. Alternatively, the
temperature of the ambient environment or a component in thermal
connection to the spring can be sensed to approximate the spring
temperature.
[0032] The motor control 18 can be of a conventional type which
typically is a microprocessor based computing system or
microcontroller or a digital signal processor and may also include
additional sensors. Although the preferred control circuit is a
microprocessor controller, many alternative means exist to provide
the control circuit functions. As known to those skilled in the
art, there are a variety of commercially available,
non-microprocessor based controllers that also can provide the
controller functions and therefore are equivalent and can be
substituted for the microprocessor controller. The sensing
functions can be performed by separate circuitry or can be provided
on-board a controller. Suitable controllers can include equivalent
digital and analog circuits available in the commercial
marketplace. Examples of controllers that can be used for the
control circuit of the invention include microprocessors,
microcontrollers, programmable gate arrays, digital signal
processors, field programmable analog arrays and logic gate arrays.
Such circuits can be elementary digital logic circuits and can be
constructed of discrete components such as diodes and transistors.
Therefore the term "controller" is used to generically refer to any
of the combinations of digital logic and analog signal processing
circuits that are available or known and can be constructed,
programmed or otherwise configured for performing the logic
functions of the control circuit as described above.
[0033] As shown widely in the prior art, the linear motor has a
reciprocating set of magnets that are located to reciprocate within
a stationary armature winding. The magnets are driven in
reciprocation by an alternating magnetic field generated by an
alternating current applied to the armature winding. A support for
the magnets is connected to the piston of the Stirling cooler 16
and drives it in reciprocation. This reciprocation causes the
Stirling cooler to pump heat energy from one area of the cooler to
another where it is rejected. As also known in the art, such
Stirling devices are also more generally known as heat pumps
because of their above described heat pumping capability. Stirling
heat pumps can be used either to heat objects from the rejected
heat or to cool objects by accepting heat at their cooled area and
rejecting it into the ambient environment. Therefore, the latter
are often referred to as coolers and this includes coolers that
cool to cryogenic temperatures. The details of the linear motor and
the driven machine, such as a Stirling heat pump, compressor or
fluid pump, are not illustrated because they are not the invention
and are illustrated in numerous examples in the prior art. The
principles of this circuit can also be applied for other linear,
freely reciprocating prime movers and driven loads such as linear
compressors and free-piston Stirling engines which use vibration
balancers.
[0034] FIG. 2 illustrates an example of an alternative embodiment
of the invention. The mechanical apparatus 30 has a main machine 32
that comprises a linear motor 34 driving a Stirling cooler 36 in
reciprocation, and a motor control 38 for controlling the linear
motor 34, including control of its driving frequency. The motor
control has a vibration amplitude sensor 40, such as an
accelerometer, to sense and input to the motor control 38 a signal
representing the amplitude of the vibrations of the mechanical
apparatus 30. All of these devices are physically coupled together
as described in connection with FIG. 1. The embodiment of FIG. 2
also may be coupled to one or more secondary vibrating systems,
such as a vibration balancer 46 and parasitic vibrating systems
48.
[0035] FIG. 3 illustrates the principles upon which the invention
operates. It refers to frequency values and curves that are
representative and typical but the invention is not limited to
those values and curves. For example, it is common for a
reciprocating main machine to be designed to be resonant at and to
operate at 60 Hz. However, many other frequencies of operation are
practical, such as 50 Hz, 120 Hz or 400 Hz. Main machines of the
type described are typically designed to be resonant at a natural
frequency of vibration or resonant frequency f.sub.0 which
corresponds to f in the above equation 1. Resonant peak M
illustrates a typical resonant peak for the mechanical vibrations
of a main machine that has a resonant frequency of 60 Hz. Its
resonant peak is relatively broad about its resonant frequency,
thus exhibiting a relatively low Q characteristic. Resonant peaks
S1 and S2 illustrate a typical resonant peak for a secondary
vibrating system. They are relatively sharp and steep, thus
exhibiting a relatively high Q characteristic.
[0036] Although half power points (70.7% of amplitude) are one well
known measure of the width of a resonant peak, this measure is
applicable only to the mechanically resonant aspects of the system.
Other operating characteristics of the main machine, such as its
cooling efficiency or coefficient of performance, determine the
operating efficiency of the driven machine. Therefore, the driving
frequency range in which the driven machine operates with
acceptable efficiency may, and usually is, different from the width
of the resonant peak of the mechanically oscillating system.
However, this acceptable driving frequency range can be, and
ordinarily is, determined by the designer. In FIG. 3, an example of
the acceptable driving frequency range R is illustrated as between
58 Hz and 62 Hz although it will be different for different main
machines.
[0037] The resonant peaks S1 and S2 can be used, in explaining the
operation of the invention, to represent either a secondary
vibrating system that is a vibration balancer or a secondary
vibrating system that is a parasitic vibration system. Each is
addressed in sequence.
[0038] If the secondary vibrating system is a vibration balancer
and the main machine is operating under its nominal or design
conditions, it will be operating with the peak M representing the
main machine and the peak S1 representing the vibration balancer.
Under that condition, the driven machine can be driven at the
nominal resonant frequency of the main machine, 60 Hz in the
example, because that frequency coincides with the resonant
frequency of the vibration balancer. However, if the operating
conditions, such as temperature, change sufficiently that the
physical parameters of the main machine or the secondary vibration
system cause a change in the resonant frequency of one or both,
that change will appear on the graph of FIG. 3 as one or both peaks
becoming displaced horizontally with respect to each other. For
example, the peak S1 may move to the position of peak S2, although
it could move in either direction and different distances.
[0039] The displacement of the peak S1 to the position of peak S2
causes the vibration balancer to become far less effective if the
main machine continues to be driven at the resonant frequency of
the main machine. However, if the driving frequency of the main
machine were changed to be closer to the center of peak S2, the
vibration balancer would become more effective at that frequency,
62 Hz in the example, so long as the changed operating conditions
remained. If the vibration balancer peak were to shift to a center
at 61 Hz or 62 Hz, the driving frequency would be moved to 61 Hz or
62 Hz respectively. Thus, in the invention the motor control system
18 or 38 drives the linear motor at a driving frequency that is
offset from the resonant frequency of the main machine or system,
that is nearer to or at the displaced resonant frequency of the
vibration balancer but is within the driving frequency range of
acceptable efficiency of operation of the main system.
[0040] Therefore, one aspect of the invention is that, in response
to changes in operating conditions that cause divergence of the
center frequency of the resonant peaks of the main machine and a
vibration balancer, the driving frequency of the linear motor is
moved closer to the altered center frequency of the vibration
balancer. Although the driving frequency can be changed to bring it
closer to the resonant frequency of the shifted peak S2, it can not
be moved beyond the limits of the acceptable driving frequency
range R because doing so would cause an unacceptable deterioration
of the operation of the main machine.
[0041] If the secondary vibration system is a parasitic vibration
system, hopefully its resonant peak is and remains sufficiently far
from the center frequency f.sub.0 of the main machine that it never
becomes a factor in the vibration. However, other equipment mounted
to the mechanical apparatus 10 or 30 can introduce one or more
parasitic vibration systems that either has a resonant peak
unexpectedly near the center frequency f.sub.0 or that moves near
it as a result of changes in operating conditions. The peaks S1 and
S2 can represent such resonant peaks of parasitic, secondary
vibration systems. If peak S1 is the peak of the parasitic
vibration system, FIG. 3 illustrates that the vibration of the
secondary, parasitic system can be substantially reduced by
changing the driving frequency to either side of the center
frequency of peak S1 but retaining it within the range R.
Consequently, the driving frequency would optimally be made 58 Hz
or 62 Hz. If the peak S2 is the peak of a parasitic vibration
system, the vibration of the parasitic system would be minimized by
moving the driving frequency to 58 Hz in the example, which is as
far as possible from the center frequency of peak S2, but not
beyond the range R.
[0042] There are multiple ways to design and construct a control
system to vary the driving frequency of the linear, freely
reciprocating, prime mover in accordance with the above principles
and six examples will be described. All include sensing a parameter
of the operation of the mechanical apparatus, such as the amplitude
of vibration of the mechanical apparatus or a temperature of a
component part of the mechanical apparatus. The sensed parameter
can be sensed by a sensor provided for practicing the invention or
it can be a sensor that is part of another control system.
[0043] FIG. 2 is the first of the six examples. The vibration
amplitude sensor 40 senses the amplitude of the vibration of the
mechanical apparatus. Generally, it can be placed on any of the
component parts of the mechanical apparatus that are mechanically
coupled together because the vibrations of any one part are
ordinarily transmitted to the other component parts. The motor
control 38 drives the linear motor 34 at each of several
representative frequencies distributed within the driving frequency
range of acceptable efficiency of operation. Each frequency is
stored in association with the amplitude of the sensed, resulting
vibrations. This is done on start up, periodically after start up,
in response to a sensed vibration amplitude above a selected level
and/or in response to other conditions or algorithms. The software
or logic circuitry of the motor control 38 then selects the least
vibration amplitude and drives the linear motor at the frequency
associated with the least vibration amplitude. The vibrations are
therefore reduced or minimized for the existing operating
conditions at the time this procedure is performed. Repetitions of
the procedure permits the control system to respond to changed
operating conditions.
[0044] More specifically, the motor control circuit 38 finds the
frequency of least vibration by driving the linear motor at a
plurality of frequencies within the acceptable range R by any of
several techniques which are referred to herein as dithering or
sweeping the frequency across the frequency range R. The most
common form of sweeping the frequency is to progressively vary the
frequency from one side of the range R to the other, either
continuously or in incremental steps. However, this sweep
alternatively can be performed in a non-sequential or in a random
manner and can be done at selected spaced intervals.
[0045] FIG. 1 illustrates a second example of the invention. The
temperature sensor 22 senses the temperature of the vibration
balancer 26, or its ambient environment, and inputs the temperature
data to the motor control 18. As with all embodiments of the
invention, the driving frequency range of acceptable efficiency of
operation is determined and stored in response to testing of at
least one mechanical apparatus. The testing is ordinarily performed
in a laboratory setup but can be based upon engineering design
specifications. This experimentally determined operating frequency
range of acceptable efficiency of operation is then stored in
production replications of the mechanical apparatus.
[0046] At least one heat pumping mechanical apparatus is also
tested, ordinarily in a laboratory environment, by being operated
at a plurality of different operating temperatures and, for each of
the operating temperatures, the driving frequency is swept within
the acceptable driving frequency range. The driving frequency
resulting in the least amplitude of vibration of the mechanical
apparatus is stored in association with each operating temperature.
As a result, for each operating temperature that is sensed, there
is a stored driving frequency that provides the minimum vibration.
These operating temperatures and their associated driving
frequencies are stored as a lookup table in a memory device 20
connected in the frequency control system of production
replications of the tested mechanical apparatus. Alternatively, the
lookup table can store a spring constant for the spring of the
vibration balancer in association with each measured temperature
and an algorithm used to convert the spring constant to an
operating frequency. As yet another alternative, instead of a
lookup table, an equation can be developed using well know
mathematical techniques, such as a polynomial series, for
approximating a plot of the lookup table and thereby relating the
vibration absorber spring constant or operating frequency to the
sensed temperature with the result computed by the motor control
microprocessor.
[0047] During operation of the production replications, the
corresponding operating temperature of the replications are sensed
and the associated driving frequency or spring constant is fetched
from the data storage 20 or, alternatively, computed by the
equation. The linear motor is then driven at the stored or computed
driving frequency associated with the sensed temperature. This
process is repeated during the operation of the production machines
so that the mechanical apparatus is always driven at the driving
frequency that provides the least vibration amplitude for the most
recently sensed temperature.
[0048] For mechanical apparatus that has both parasitic secondary
vibration systems and a vibration balancer, there will be more
resonant peaks to represent on a graph similar to FIG. 3. However,
the method and procedures of the invention remain the same.
[0049] FIG. 4 illustrates a third example of an embodiment of the
invention. It shows a prior art mechanical apparatus and control
system to which components have been added for implementing the
invention.
[0050] The illustrated prior art system has a main machine
consisting of an electric linear motor 50 mechanically linked to
drive internal moving masses 52 of itself and a driven load and
mechanically connected to secondary vibrating systems 54. A sensor
56 senses a parameter of main machine operation, such as the top
dead center (TDC) or piston position of one of the moving masses 52
and motor current and voltage are also detected. These signals are
applied to a signal conditioner 58 and applied as a feedback signal
to the summing junction 60 of a feedback control system, which are
implemented in the software of a microcontroller or digital signal
processor 62. A reference value is also applied to the summing
junction 60 to provide an error signal that is applied to a
transfer function and develop a control signal in accordance with
well known feedback control system principles. For controlling the
linear motor 50, the control signal is applied to a variable
frequency inverter duty cycle generator circuit 64 that generates a
square wave for which both the duty cycle and the frequency of the
square wave can be controllably varied. The duty cycle is
controlled in the prior art manner by the control signal. The
output of the generator circuit 64 is applied to an inverter output
stage 66 that converts the square wave to oppositely directed dc
pulses for driving the linear motor 50, the pulses having a pulse
width corresponding to the duty cycle of the square wave. A variety
of such circuits are known in the prior art and the invention is
not limited to any particular drive circuit having these general
characteristics.
[0051] In order to implement the present invention, a temperature
sensor 68 is mounted close to the vibration balancer 54A and
applies its output signal to the microcontroller 62 for storing
temperature data for use in a lookup table 70 or a corresponding
equation for use in determining the operating frequency as
described in connection with FIG. 1. If the lookup table or
equation provides values corresponding to the vibration balancer
spring constant for each sensed temperature, that output is
converted by a frequency adjustment algorithm 72 to determine the
operating frequency. If the lookup table or equation directly
provides frequency, the algorithm 72 may be omitted.
[0052] FIG. 5 illustrates a fourth example of an embodiment of the
invention. It shows the same prior art mechanical apparatus and
control circuit as illustrated in FIG. 4 to which components have
been added for implementing the invention. For implementing the
invention, a vibration sensor 80 is connected to provide a
vibration amplitude input to the microcontroller 82. The
microcontroller 82, under control of software module 84, scans,
dithers or otherwise varies the operating drive frequency within
the limits of the acceptable range of operating frequencies as
described above and implements storage of the least amplitude of
vibration within that range. The microcontroller 82 then selects
and operates the linear motor 86 at the stored operating frequency
that is associated with the least amplitude of vibration. It also
repeats that process at selected intervals or under selected
conditions, such as those described above, so that as operating
conditions may change, the electric linear motor 86 is always
driven at the frequency of least amplitude of vibration.
[0053] FIGS. 6 and 7 are diagrams illustrating the fifth and sixth
examples of embodiments of the invention. Both include the
identical prior art device to which circuitry implementing the
invention is added. Therefore, the common, prior art portion of
both figures will be described first. FIGS. 6 and 7 show a main
machine that is a free piston Stirling engine 100 driving an
electric linear alternator 102 for generating electric power from a
heat source input. They may be of a design known in the prior art
and are typically mounted in the same housing. The reciprocating
power piston of the Stirling engine is mechanically linked to the
reciprocating component of the alternator, usually a series of
permanent magnets supported on a carrier that reciprocates within
an armature winding or coil mounted within the common housing. The
composite mass 104 of these reciprocating structures is
mechanically linked to a vibration balancer 106 and any parasitic
vibration systems 108 through the reaction of the cylinders and
casing of the Stirling engine and alternator. These reaction forces
are transmitted to the cylinder and casing through the usual
mechanical springs, working gas within the Stirling engine and the
electromagnetic coupling from the magnets to the armature coil. The
output of the alternator 102 is connected through a conventional
tuning capacitor 109, used for power factor correction, to supply
power to a useful load 110.
[0054] The frequency of the Stirling engine 100 is controlled using
a principle known in the prior art for connecting a Stirling engine
driven alternator to a power grid to supply power to the grid. The
oscillations of the Stirling engine will synchronize with the AC
power oscillations of the power grid if the Stirling engine driven
alternator is designed to be resonant near the frequency of the
power grid. The applied principle is that the oscillations of a
Stirling engine driven alternator, when connected to an external AC
power supply, will synchronize with the voltage oscillations of the
AC power supply if the AC power supply has a lower internal
impedance than the alternator and if there are only small
variations in frequency. The frequency variations applicable to the
invention and described above are such small variations.
[0055] A variable frequency, variable amplitude power source 111 is
used as an engine output frequency controller and its AC output
terminals are connected to the alternator 102. Such power sources
are commercially available and are therefore not described further.
The output of the variable frequency generator 118 of the
microcontroller 114 is connected to the control input terminal 112
of the variable AC power source 111 which is the input that
controls the frequency of the variable AC power source 111. The
output of the variable frequency generator 118 controls the AC
power source 111. Consequently, within the small frequency
variations used in the present invention, the operating frequency
of the Stirling engine driven linear alternator 102 tracks the
frequency of the variable AC power source 111 and therefore is
controlled by the microcontroller 114.
[0056] The implementation of the invention illustrated in FIG. 6
operates similarly to the embodiment illustrated in FIG. 5. The
controller 114 is programmed to perform the logic and arithmetic
functions for computing the operating frequency of the main machine
that minimizes the amplitude of vibration. A vibration sensor 116,
such as an accelerometer, has an output connected to an input of
the controller 114 for providing a signal representing the
amplitude of the sensed vibration of the prior art, main machine
illustrated in FIG. 6. However, instead of inputting a nominal
operating frequency as a command input to an engine frequency
controller, a variable frequency generator 118 is interposed
between the nominal operating frequency command input and the input
to the variable AC source 111. This permits the controller 114 to
offset the commanded operating frequency from the nominal operating
frequency in order to minimize vibration. The controller 114, under
software control, scans, dithers or otherwise varies the operating
drive frequency within the limits of the acceptable range of
operating frequencies and implements storage of the least amplitude
of vibration within that range. The controller 114 then selects and
operates the Stirling engine 100 at the stored operating frequency
that is associated with the least amplitude of vibration. It also
repeats that process at selected intervals or under selected
conditions, such as those described above, so that, as operating
conditions may change, the free piston Stirling engine 100 is
always driven at the frequency of least amplitude of vibration
within the limits of the acceptable range of operating
frequencies.
[0057] The implementation of the invention illustrated in FIG. 7
operates similarly to the embodiment illustrated in FIG. 2 and
applies the same operating principles for controlling the operating
frequency of the free piston Stirling engine 100 as described in
connection with the embodiment of FIG. 6. The output of a
temperature sensor 120 is connected to an input of a controller 122
to provide a signal representing the temperature of the vibration
balancer 106. The controller 122 then operates the Stirling engine
100 over the range of frequencies of acceptable operation and
stores each temperature in association with an operating frequency
to provide a lookup table 124 by the same process as described
above in connection with FIGS. 2 and 4.
[0058] While certain preferred embodiments of the present invention
have been d in detail, it is to be understood that various
modifications may be adopted departing from the spirit of the
invention or scope of the following claims.
* * * * *