U.S. patent application number 10/420456 was filed with the patent office on 2004-12-30 for shape-memory alloy actuators and control methods.
Invention is credited to MacGregor, Roderick.
Application Number | 20040261411 10/420456 |
Document ID | / |
Family ID | 27495815 |
Filed Date | 2004-12-30 |
United States Patent
Application |
20040261411 |
Kind Code |
A1 |
MacGregor, Roderick |
December 30, 2004 |
Shape-memory alloy actuators and control methods
Abstract
This invention provides stroke-multiplying shape memory alloy
actuators and other actuators using electromechanically active
materials [collectively referred to in this application as SMA
actuators] providing stroke multiplication without significant
force reduction, that are readily miniaturizable and fast acting,
and their design and use; economical and efficient control and
sensing mechanisms for shape memory alloy actuators (including
conventional shape memory alloy actuators as well as the
stroke-multiplying SMA actuators of this invention) for low power
consumption, resistance/obstacle/load sensing, and accurate
positional control; and devices containing these actuators and
control and sensing mechanisms.
Inventors: |
MacGregor, Roderick;
(Antioch, CA) |
Correspondence
Address: |
COOLEY GODWARD, LLP
3000 EL CAMINO REAL
5 PALO ALTO SQUARE
PALO ALTO
CA
94306
US
|
Family ID: |
27495815 |
Appl. No.: |
10/420456 |
Filed: |
April 21, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10420456 |
Apr 21, 2003 |
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09637713 |
Aug 11, 2000 |
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6574958 |
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60148514 |
Aug 12, 1999 |
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60148515 |
Aug 12, 1999 |
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60148516 |
Aug 12, 1999 |
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60148517 |
Aug 12, 1999 |
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Current U.S.
Class: |
60/527 ;
374/E5.031 |
Current CPC
Class: |
G12B 1/00 20130101; F03G
7/065 20130101; G01K 5/483 20130101 |
Class at
Publication: |
060/527 |
International
Class: |
F02G 001/04; F01B
029/10 |
Claims
1. A stroke-multiplying shape memory alloy actuator.
2. The actuator of claim 1 comprising a plurality of parallel rigid
members slidable relative to one another, each connected one to
another by a plurality of shape memory alloy wires in such a way
that the stroke of the actuator is substantially equal to the sum
of the strokes of the individual shape memory alloy wires.
3. The actuator of any of claims 1 and 2 where the rigid members
are a series of concentric tubes.
4. The actuator of any of claims 1 and 2 where the rigid members
are set of parallel plates.
5. The actuator of any of claims 1 through 4 where the shape memory
alloy wires are individually energizable to achieve incremental
stroke of the actuator.
6. The actuator of claim 5 where the shape memory alloy wires are
energizable in sequence to minimize peak power consumption.
7. The actuator of claim 1 comprising a shape memory alloy wire
wrapped around a pair of low-friction non-conductive rods or tubes
having a radius greater than the minimum bending radius of the
shape memory alloy wire, held apart in parallel by a rigid
structure.
8. The actuator of claim 7 where the non-conductive rods or tubes
are made of or coated with a fluorinated polymer, such as PTFE.
9. The actuator of claim 1 comprising a shape memory alloy wire
guided round a curve of greater than the minimum bending radius of
the shape memory alloy wire by passing through an opposed pair of
non-conductive tubes held apart by a rigid structure.
10. The actuator of claim 9 where the non-conductive tubes are made
of or coated with a fluorinated polymer, such as PTFE.
11. The actuator of any one of claims 7 through 10 where the
framework and rods or tubes are shared by multiple actuators.
12. A device comprising a pulse-width modulated power source and a
plurality of shape memory alloy actuators that are resistively
heated using pulse-width modulated signals from the pulse-width
modulated power source, and a resistance measuring circuit and
analog-to-digital converter such that resistance of each actuator
is used to control that actuator, where the resistance measuring
circuit and analog-to-digital converter are connected to the
plurality of shape memory alloy actuators by a multiplexer.
13. The device of claim 12 where selection by the multiplexer of
each of the plurality of shape memory actuators for resistance
measurement and analog-to-digital conversion takes place
sequentially in round-robin fashion.
14. The device of claim 12 where selection by the multiplexer of
each of the plurality of shape memory actuators for resistance
measurement and analog-to-digital conversion takes place in other
than round-robin fashion, such as by non-selection of unenergized
actuators.
15. A method of controlling a shape memory alloy actuator
comprising measuring the change in resistance of the actuator with
time as the actuator is energized and providing control information
for the actuator from the change in resistance with time.
16. The method of claim 15 comprising providing control information
to calibrate the actuator where the physical parameters of the
actuator are not known in advance.
17. The method of claim 15 comprising providing control information
for the actuator by executing a position control function using
resistance as the feedback variable.
18. A method of measuring an applied load on a shape memory alloy
actuator comprising measuring the change in resistance of the
actuator with time as the actuator is energized and determining the
applied load on the actuator from the change in resistance with
time.
19. A method of detecting a collision or mechanical obstruction
encountered by a shape memory alloy actuator comprising measuring
the change in resistance of the actuator with time as the actuator
is energized and detecting the collision or mechanical obstruction
encountered by the actuator from the change in resistance with
time.
20. A method of detecting a system failure in a shape memory alloy
actuator comprising measuring the change in resistance of the
actuator with time as the actuator is energized and detecting the
system failure in the actuator from the change in resistance with
time.
21. A resistive feedback control system for a shape memory alloy
actuator comprising a capacitor connected parallel to the shape
memory alloy actuator and a time measurement circuit within the
control system to measure the charge time or discharge time of the
capacitor, whereby the resistance of the actuator is determined
from the charge time or discharge time of the capacitor.
22. The actuator of any one of claims 1 through 11, further
comprising at least one limit stop to prevent over-extension of the
actuator by application of an excessive external force, limit the
stroke of the actuator to less than its available stroke, or both.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the priority under 35 USC 119(e) of
U.S. Provisional Applications Nos. 60/148,514, entitled "SMA
Actuator Design", 60/148,515, entitled SMA Actuator with Teflon
Guides", 60/148,516, entitled "Time Domain Resistance Analysis for
Realtime SMA Actuator Control", and 60/148,517, entitled "PWM
Multiplexing Controller for SMA Actuator Arrays", all filed on Aug.
12, 1999. These applications are incorporated by reference into
this application.
BACKGROUND OF THE INVENTION
[0002] (a) Field of the Invention
[0003] This invention relates to shape-memory alloy (SMA) actuators
and other actuators using electromechanically active materials
[collectively referred to in this application as SMA actuators] and
to methods for their control. In particular, this invention relates
to SMA actuators that are capable of miniaturization to achieve
fast (sub-second) response, and to control methods for SMA
actuators in general, and also in particular for the miniaturizable
SMA actuators of this invention for low power consumption,
resistance/obstacle sensing, and positional control.
[0004] (b) Description of Related Art
[0005] A class of materials was discovered in the 1950s that
exhibit what is known as the shape memory effect. See, for example,
K. Otsuka, C. M. Wayman, "Shape Memory Materials", Cambridge
University Press, Cambridge, England, 1998; ISBN 0-521-44487X.
These materials exhibit a thermoelastic martensite transformation;
i.e. they are pliable below a certain transition temperature
because the material is in its martensite phase and can be easily
deformed. When their temperature is raised above the transition
temperature the material reverts to its austenite phase and its
previous shape, generating a large force as it does so. Example of
such materials are approximately 50:50 atom percent titanium-nickel
(TiNi) alloys, optionally containing small quantities of other
metals to provide enhanced stability or to alter the
martensite-austenite transition temperatures; and these can be
formulated and treated to exhibit the shape memory effect. Other
such alloys include Cu/Al/Ni and Cu/Al/Zn alloys, sometimes known
as .beta.-brasses. Such alloys are generically referred to as shape
memory alloys (SMA) and are commercially available from a number of
sources in wire form, with diameters from as low as 37 .mu.m to 1
mm or greater. See, for example, Dynalloy Corp., "Technical
Characteristics of Flexinol Actuator Wires", Technical Information
Pamphlet, Dynalloy Corp., 18662 MacArthur Boulevard, Suite 103,
Irvine Calif. 92715, USA.
[0006] SMA wires are wires of shape memory alloy that are treated
such that they can be easily stretched along their longitudinal
axis while in the martensite phase, thus re-arranging their atomic
crystalline structure. Once stretched they remain that way until
they are heated above their austenite transition temperature, at
which point the crystalline structure is restored to its original
(remembered) austenite configuration. This reversion not only
returns the wire to its original length, but also generates a large
force, typically on the order of 50 Kgf/mm.sup.2 cross-sectional
area, depending on the alloy and its treatment. Because of the
large available force per cross-sectional area, SMA wires are
normally produced in small diameters. For example, a 100 .mu.m
diameter wire can deliver about 250 g of force. To obtain more
force, thicker wires or multiple wires are required.
[0007] Although SMAs have been known since 1951, they has found
limited commercial actuator applications due to some inherent
limitations in the physical processes which create the shape memory
properties. This lack of commercial applications is due to a
combination of the following factors:
[0008] (1) Limited Displacement
[0009] A TiNi SMA wire can contract by at most 8% of its length
during the thermoelastic martensite to austenite transition.
However, it can only sustain a few cycles at this strain level
before it fails. For a reasonable cycle life, the maximum strain is
in the 3-5% range. As an example, for an actuator with reasonable
cycle life, it requires over 25 cm of SMA wire to produce 1 cm of
movement.
[0010] (2) Minimum Bend Radius
[0011] An obvious solution to packaging long lengths of SMA into
small spaces is to use some kind of pulley system. Unfortunately
SMA wires can be damaged if they are routed around sharp bends.
Typically an SMA wire should not be bent around a radius less than
fifty times the wire diameter. As an example, a 250 .mu.m diameter
wire has a minimum bending radius of 1.25 cm. It should be noted
that the term "minimum bending radius" as used here means the
minimum radius within which an SMA wire can be bent and still be
capable of repeated austenite-martensite cycling without damage.
The addition of a large number of small pulleys makes the system
mechanically complex, eliminating one of the attractions of using
SMA in the first place. Also the minimum bend radius requirement
places a lower limit on actuator size.
[0012] (3) Cycle Time
[0013] An SMA wire is normally resistively heated by passing an
electric current through it. The wire then has to cool below its
transition temperature before it can be stretched back to its
starting position. If this cooling is achieved by convection in
still air, then it can take many seconds before the actuator can be
used again. The 250 .mu.m wire discussed above has a best cycle
time of about 5 seconds or more. Thus, as an example, Stiquito, an
SMA powered walking insect [J. M. Conrad, J. W. Mills, "Stiquito:
Advanced Experiments with a Simple and Inexpensive Robot", IEEE
Computer Society Press, Los Alamitos Calif., USA, ISBN
0-8186-7408-3] achieves a walking speed of only 3-10 cm/min. Since
the rate of cooling depends on the ratio of the surface area of the
wire to its volume, changes in wire diameter dramatically affect
the cycle time.
[0014] To overcome these limitations designers of SMA based
actuators have typically used long straight wires or coils. See,
for example, M. Hashimoto, M. Takeda, H. Sagawa, I. Chiba, K. Sato,
"Application of Shape Memory Alloy to Robotic Actuators", J.
Robotic Systems, 2(1), 3-25 (1985); K. Kuribayashi, "A New Actuator
of a Joint Mechanism using TiNi Alloy Wire", Int. J Robotics, 4(4),
47-58 (1986); K. Ikuta, "Micro/Miniature Shape Memory Alloy
Actuator", IEEE Robotics and Automation, 3, 2151-2161 (1990); and
K. Ikuta, M. Tsukamoto, S. Hirose, "Shape Memory Alloy Servo
Actuator with Electrical Resistance Feedback and Application for
Active Endoscope", Proc. IEEE Int. Conf on Robotics and
Information, 427-430 (1988). Clearly, in many applications,
especially where miniaturization is desired, it is impractical to
use long straight wires. Coils, although greatly increasing the
stroke delivered, significantly decrease the available force; and,
to compensate for the drop in force, thicker wires are used which
reduce the responsiveness of the resulting actuator, making it
unsuitable for many applications.
[0015] Other mechanisms commonly used to mechanically amplify the
available displacement, such as those disclosed in D. Grant, V.
Hayward, "Variable Control Structure of Shape Memory Alloy
Actuators", IEEE Control Systems, 17(3), 80-88 (1997) and in U.S.
Pat. No. 4,806,815, suffer from the same limitation on available
force, again leading to the requirement for thicker wires and the
attendant problems with cycle time.
[0016] As discussed above, SMA materials can be used as the motive
force for an actuator [See, for example, T. Waram, "Actuator Design
Using Shape Memory Alloys", 1993, ISBN 0-9699428-0-X], whose
position can be controlled by monitoring the electrical resistance
of the alloy. See, for example, K. Ikuta, M. Tsukamoto, S. Hirose,
"Shape Memory Alloy Servo Actuator with Electrical Resistance
Feedback and Application for Active Endoscope", discussed
above.
[0017] A common method of heating SMA actuators to their transition
temperature is pulse width modulation (PWM). In this scheme, a
fixed voltage is applied for a percentage of a pre-set period. As
the percentage on-time to off-time in a single period (referred to
as the duty cycle) is changed, the aggregate amount of power
delivered to the SMA can be controlled. This scheme is popular
because of the ease with which it can be implemented in digital
systems, where a single transistor is all that is required to drive
an actuator, obviating the need for digital-to-analog conversion
and the associated amplifiers.
[0018] In a simple example, a PWM generator supplies PWM pulses to
the SMA element at a duty cycle and period specified by a digital
controller. During the off period of the PWM pulse, a resistance
measuring system measures the resistance of the SMA which is
sampled and then held in a sample-and-hold system. This measurement
is made in the off cycle because the PWM pulse can be quite short
and the controller might not sample the SMA when the pulse is on.
Finally, the analog signal in the sample-and-hold system is
converted to digital form by an analog-to-digital (A-D) converter,
from which it can then be read by the controller. This value is
then used by an algorithm in the controller to vary the duty cycle
of the PWM generator to achieved a desired position of the SMA
element. In systems with more than one SMA element, all of the
systems other than the controller need to be replicated for each
SMA element, which leads to large, complex and expensive control
systems.
[0019] Several schemes have been proposed to avoid this
replication. The most common is to multiplex the A-D converter
across a number of sample & hold circuits, thus only requiring
one A-D converter. Another scheme, described in U.S. Pat. No.
5,763,979, uses electronic switches in a row and column
configuration to isolate a single SMA element and applies a PWM
pulse to each element in turn. This allows for the resistance
measuring, sample and hold and A-D subsystems to be shared across
all actuators, and also has the advantage of reducing the number of
wires required to interconnect the devices. Unfortunately the
scheme also doubles the number of high current switching devices
since each actuator requires two such channels as opposed to only
one in the conventional scheme. These switches are normally the
physically largest element of such control systems because of their
need to dissipate substantial heat due to their high current
operation. So, although this scheme reduces the number of wires, it
actually increases the size and complexity of the controller
subsystem.
[0020] The transition from the martensite (low temperature) phase
to the austenite (high temperature) phase in SMAs does not happen
instantaneously at a specific temperature but rather progresses
incrementally over a temperature range. FIG. 1 shows the
relationship between displacement and temperature, indicating the
austenite start A.sub.s and austenite finish A.sub.f temperatures,
as well as the martensite start and finish temperatures M.sub.s and
M.sub.f, respectively. In the temperature range indicated by
.DELTA.T the alloy consists of a mixture of austenite and
martensite. As can be seen, substantially no change in length
occurs below A.sub.s, and substantially no further change in length
occurs above A.sub.f, as the SMA is heated. Similarly, on cooling
substantially no change in length occurs above M.sub.s, and
substantially no further change in length occurs below M.sub.f;
however, there is typically substantial hysteresis in the
length-temperature curve. As discussed in K. Ikuta, M. Tsukamoto,
S. Hirose, "Shape Memory Alloy Servo Actuator with Electrical
Resistance Feedback and Application for Active Endoscope",
discussed above, and U.S. Pat. No. 4,977,886, there is a
relationship between the electrical resistance of an SMA component
and its temperature, as is shown in FIG. 2, which is shown for an
SMA having an M.sub.f above room temperature. As can be seen,
within the shaded region between Rmin and Rmax the resistance can
be used as an analog for the SMA temperature and hence it is
possible to deduce the percentage transformation between the two
phases based entirely on the resistance value with no direct
measurement of temperature, since the resistance-temperature curve
does not display significant hysteresis. However, due to the large
position-temperature hysteresis illustrated in FIG. 1, knowledge of
the temperature alone is not sufficient to deduce position.
[0021] However, if two actuators are arranged in an antagonistic
fashion, a number of schemes can be used to compensate for the
hysteresis. A common scheme described in Dynalloy Corp., "Technical
Characteristics of Flexinol Actuator Wires" and U.S. Pat. No.
4,977,886 uses the normalized resistance from both actuators in
combination to compensate for the hysteresis. All of these position
control schemes rely upon an a priori knowledge of Rmax and Rmin
(see FIG. 2). These values change over time as the alloy ages, and
also with environmental factors, such that the system has to be
recalibrated before each use for useful position control.
Calibration is achieved either by the attachment of external
sensors to compute Rmax and Rmin at known measured minimum and
maximum displacements or, as in U.S. Pat. No. 4,977,886, by
applying a current large enough and long enough such that the
temperature will exceed A.sub.f and record the minimum and peak
resistances encountered. The former calibration scheme is
impractical for many systems where continuous, low cost operation
is required. The latter scheme relies upon knowledge of the
physical dimensions of the SMA element, and also its current
environment and state (e.g. austenite or martensite) so that the
magnitude and duration of the calibration pulse can be
calculated.
[0022] The disclosures of all documents cited in this section and
elsewhere in this application are incorporated by reference into
this application.
[0023] It would be desirable to develop SMA actuators that are
capable of providing substantially the full force of the SMA wires
comprising them while achieving a greater stroke (contraction) than
is achievable by an SMA wire of the length of the actuator (stroke
multiplication without significant force reduction); SMA actuators
that are miniaturizable and fast acting; and economical and
efficient control and sensing mechanisms for SMA actuators
(including conventional shape memory alloy actuators as well as the
stroke-multiplying SMA actuators of this invention) for low power
consumption, resistance/obstacle/load sensing, and accurate
positional control.
SUMMARY OF THE INVENTION
[0024] This invention provides stroke-multiplying shape memory
alloy actuators and other actuators using electromechanically
active materials [collectively referred to in this application as
SMA actuators] providing stroke multiplication without significant
force reduction, that are readily miniaturizable and fast acting,
and their design and use; economical and efficient control and
sensing mechanisms for shape memory alloy actuators (including
conventional shape memory alloy actuators as well as the
stroke-multiplying SMA actuators of this invention) for low power
consumption, resistance/obstacle/load sensing, and accurate
positional control; and devices containing these actuators and
control and sensing mechanisms.
[0025] In a first aspect, this invention provides a
stroke-multiplying shape memory alloy actuator. In embodiments of
this first aspect of the invention, the actuator comprises multiple
rigid members and shape memory alloy wires.
[0026] In a second aspect, the invention provides a
stroke-multiplying shape memory alloy actuator comprising a single
shape memory alloy wire.
[0027] In a third aspect, this invention provides multiplexed
control and sensing mechanisms for shape memory actuators.
[0028] In a fourth aspect, the invention provides control and
sensing mechanisms for, and methods for controlling, shape memory
alloy actuators using resistive feedback, in which the change in
resistance of the actuator with time as the actuator is energized
is used to generate the control information for the actuator. These
control and sensing mechanisms and methods may be used for
calibration of actuators, executing position control functions,
measuring applied loads on actuators, and detecting collisions or
mechanical obstructions encountered by, or system failures in,
actuators. In a preferred control mechanism, measurement of the
discharge time of a capacitor connected parallel to the actuator is
used to measure the resistance of the actuator.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 shows a length versus temperature graph for a typical
SMA element.
[0030] FIG. 2 shows a resistance versus temperature graph for a
typical SMA element.
[0031] FIG. 3 shows a first embodiment of an SMA actuator of this
invention.
[0032] FIG. 4A shows a second embodiment of an SMA actuator of this
invention.
[0033] FIG. 4B shows an alternative second embodiment of an SMA
actuator of this invention.
[0034] FIG. 5 shows a third embodiment of an SMA actuator of this
invention.
[0035] FIG. 6 shows a rigid element for an alternative third
embodiment of an SMA actuator of this invention.
[0036] FIG. 7 is a perspective view of the alternative third
embodiment of the SMA actuator using the rigid element of FIG.
6.
[0037] FIG. 8A shows the actuator of FIG. 7 in a side view in its
extended configuration.
[0038] FIG. 8B shows the actuator of FIG. 8A in its contracted
configuration.
[0039] FIG. 9 shows schematically an SMA actuator of the second
aspect of this invention, illustrating the use of a single SMA
wire.
[0040] FIG. 10 shows the use of low-friction tubes to guide the SMA
wire of the actuator of FIG. 9.
[0041] FIG. 11 shows the use of a guide tube alone.
[0042] FIG. 12 shows a four-actuator assembly of this second aspect
of the invention.
[0043] FIG. 13 shows a conventional type of PWM controller for an
SMA actuator.
[0044] FIG. 14 shows a multiplexed PWM controller.
[0045] FIG. 15 is a graph showing the variation in conductance and
position with time in a heated SMA element.
[0046] FIG. 16 shows a conventional type of resistive feedback
control for an SMA actuator.
[0047] FIG. 17 shows a capacitive sensing circuit for resistive
feedback control for an SMA actuator.
[0048] FIG. 18 shows the rigid members and SMA wires of a fourth
embodiment of an SMA actuator of this invention.
[0049] FIG. 19 is a side view showing the assembled actuator using
the member/wire assembly of FIG. 18.
[0050] FIG. 20A shows the use of a limit stop for an SMA actuator
in its fully extended configuration.
[0051] FIG. 20B shows the use of a limit stop for the SMA actuator
of FIG. 20A in its fully contracted configuration.
[0052] FIG. 21 shows a walking insect model containing the SMA
actuators of this invention.
[0053] FIG. 22 is a side and above view showing the operation of
one leg of the insect model of FIG. 21.
[0054] FIG. 23 is a rear view showing the operation of the leg of
FIG. 22.
[0055] FIG. 24 is a perspective view showing the operation of the
leg of FIG. 22.
DETAILED DESCRIPTION OF THE INVENTION
[0056] Definitions and General Parameters
[0057] A "shape memory alloy" or "SMA" is an alloy that exhibits a
thermoelastic martensite transformation, such that it can be
deformed while in the martensite phase and the deformation is
recovered when the alloy returns to the austenite phase. SMAs
suitable for room temperature applications of this invention are
those that have an austenite-martensite transition range somewhat
above expected ambient temperature, say a martensite finish
temperature of 30-50.degree. C., so that the SMA will remain in its
martensite phase in the absence of applied heating, and an
austenite finish temperature that is low enough to be compatible
with common engineering plastics, say an austenite finish
temperature of 80-100.degree. C., to minimize the amount of heating
(e.g. electrical energy input to the SMA) required to complete the
martensite-to-austenite transition. Such alloys are readily
commercially available. Alloys with other transition temperature
ranges may be chosen for actuators designed to operate at decreased
(e.g. below 0.degree. C.) or elevated (e.g. above 100.degree. C.)
temperature environments, and a person of ordinary skill in the art
will have no difficulty, having regard to that skill and this
disclosure, in choosing a suitable SMA for a desired purpose. It is
well known that, when an SMA element, such as an SMA wire, is
deformed within the recoverable range of strain below its M.sub.f
temperature, and then heated to above the A.sub.f temperature, it
will revert to its original undeformed shape. However, re-cooling
of the element below the M.sub.f temperature again will not cause
reversion to the deformed shape spontaneously--the shape memory
effect is a one-way effect. Thus a stress, or bias, needs to be
applied to the SMA element for it to revert to the deformed shape
as it re-cools below the M.sub.f temperature. Although it will not
be generally discussed below in relation to the SMA actuators of
this invention, it is to be assumed that a bias is or can be
applied to the actuator to cause reversion to the deformed
martensitic state as the SMA elements of the actuator cool below
the M.sub.f temperature. This bias may be applied either by a
spring (a constant bias application, where the actuator has to
overcome the force of the spring to cause motion of the actuator as
it heats; or by an opposing actuator, where typically one is heated
and the other is unheated, but each may be heated to differing
extents for precise control). The spring bias is economical, but
has the disadvantage that a part of the actuator force is absorbed
by the spring, leaving less available for the actuator to apply to
an external load; the opposed actuator bias offers greater force
availability, since the opposed actuator when unheated takes little
force to move, and greater position sensitivity when both are
differentially energized, but at a cost in complexity of control
and increased power consumption. This is well known in the art; and
the SMA actuators of this invention may be used in either mode.
[0058] SMA "wire" as used in this application refers to SMA
material of elongate form, capable of contraction/elongation along
the long axis. Thus the term "wire" does not imply a circular
cross-section, although that will be the typical cross-section, but
includes cross-sections that may be elliptical, square,
rectangular, or the like.
[0059] The "stroke" of an SMA actuator is the change in distance
between the fully extended length and the fully contracted length
of the actuator. If the actuator contains a limit stop(s) to limit
either contraction and/or extension of the actuator, then the
"stroke" will be the distance between the limit stop(s), which may
be less than the "stroke" if no limit stop(s) were present.
[0060] A "stroke multiplying" SMA actuator is an SMA actuator in
which the stroke of the actuator is greater than the contraction or
extension of an SMA wire of the external length of the actuator in
the direction of its extension or contraction.
[0061] In a first aspect, this invention provides stroke
multiplying SMA actuators, i.e. SMA actuators that are capable of
providing substantially the full force of the SMA wires comprising
them while achieving a greater stroke than is achievable by an SMA
wire of the length of the actuator ("length" being defined as the
length of the actuator in the direction of the axis of the SMA
wires), thereby achieving stroke multiplication without significant
force reduction.
[0062] This stroke multiplication without significant force
reduction enables thin SMA wires to be employed resulting in
greatly increased responsiveness due to the non-linear dependence
of wire cooling speed on wire diameter. As is well-known in the SMA
actuator art, per unit length, the mass of wire to be cooled is
proportional to the cross-sectional area of the wire (a function of
the square of the wire diameter), while the cooling rate is
proportional to the surface area of the wire (a function of the
diameter). In fact, this ratio is further complicated by the
thermal conductivity of the wire itself, but it can be seen that
the rate of cooling of an SMA wire from its A.sub.f temperature to
its M.sub.f temperature decreases substantially with decrease in
wire diameter. This decreases the cycle time of an SMA actuator,
since the heat-up time from the M.sub.f temperature to the A.sub.f
temperature will always be substantially shorter than the cool-down
time provided that sufficient power is applied to achieve a rapid
heating rate. For example, while a 250 .mu.m diameter wire actuator
has a cycle time of 6-7 sec or more, a 50 .mu.m diameter wire
actuator has a cycle time of less than about 1 see, and a 37 .mu.m
diameter wire actuator has a cycle time of about 0.4 sec.
[0063] The basic design of the stroke multiplying SMA actuator
comprises a plurality of parallel, including concentrically
arranged, rigid (i.e. non-SMA) members that are free to slide
relative to one another, each connected one to another by SMA wires
in such a way that the stroke of the actuator is substantially
equal to the sum of the strokes of the individual SMA wires.
[0064] In a first embodiment, as shown in FIG. 3, the slidable
rigid members 301, 302, 303 and the SMA wires 311, 312, 313,
attached at points 321 and 331, 322 and 332, 323 and 333,
respectively, of the actuator shown generally at 30, are arranged
such that, when the SMA wires 311, 312, 313 contract, each pulls on
one end of a rigid member the other end of which is attached to the
next wire. In this way the displacement of one wire is added to the
next one in sequence. One end 341 of rigid member 301 may be
attached by any suitable means to a point, and the end 333 of wire
313 may be attached to another point, the two points being either
both movable or, more usually, one movable and one fixed, so that
the distance between the points is reduced when the wires 311, 312,
313 contract. The double-headed arrow A indicates the direction of
movement, with contraction of the SMA wires causing a contraction
of the distance between points 321 and 333. An arbitrary number of
wires and members can be combined to achieve any desired
displacement.
[0065] For example, if member 301 is fastened rigidly to a
substrate and members 302 and 303 are slidable with respect to each
other and to member 301, when the SMA wires are heated and
contract, and each contracts about 3%, the ultimate contraction of
the assembly, from the attachment point 341 to the attachment point
333, will be about 9%, representing about a three-fold
multiplication of displacement with no diminution of the force
exerted except for the little lost to friction within the actuator.
The design thus delivers the benefits of a long straight wire but
in a more compact form, without reducing available force. In
addition, the fact that all wires are straight means that the issue
of minimum bend radius of the SMA wire is moot, making it possible
to miniaturize the assembly to a scale of a few centimeters, for
example down to 1 cm or less, and potentially down to microscopic
scale.
[0066] The rigidity of each of the members needs to be sufficient
that the actuator will not buckle as the wires contract, but
because the actuator can be encased in a casing preventing buckling
of the members (if such a casing if made from or coated with a
suitable low-friction polymeric material, such as PTFE or another
fluoropolymer, it will also act to provide a low-friction
environment for the sliding of the rigid members), no unusual
strength is required. An actuator of this type (side-by-side
arrangement), as is apparent from the drawing, can be made shallow
compared to its length and width, making it especially useful in a
confined flat space situation.
[0067] Power supply to the SMA wires 311, 312, 313 may be
individual to each wire, allowing maximum control of movement and
spread of peak power draw (if the wires are energized sequentially,
since the current required to heat a wire to its A.sub.f
temperature is greater than the current required to hold the wire
above its A.sub.f temperature once that temperature is reached).
This is an important consideration in battery powered devices where
the life of the battery is dependent to an extent on the rate of
current drain; but there is a cost in the need for numerous leads
and increased control capacity. Power supply may also, more
usually, be from one end of the actuator to the other in a single
run, so that only two leads are required and the control
simplified. In this situation, points 331 and 332 on member 302,
and points 332 and 323 on member 303 need to be electrically
connected so that a current may flow from point 321 to point 333,
thereby causing all three wires 311, 312, 313 to contract
simultaneously when a voltage is applied to points 321 and 333. If
the rigid members 301, 302, 303 are non-conductive, appropriate
electrical paths (jumper wires) must be provided to make the
required electrical connection. If the rigid members are themselves
conductive and the SMA wires fastened to them in an electrically
conductive way, then the rigid members themselves will serve as the
jumpers, but this requires that the SMA wires themselves be
electrically insulated or spaced apart from the rigid members
except at the attachment points 321, 322, 323, 331, 332 to ensure
that current passes completely through the SMA wires.
[0068] In a second embodiment, to further minimize the space
requirements of the actuator the rigid members can be arranged as
concentric tubes with the SMA wires mounted on the outside of the
tube as shown in FIGS. 4A and 4B. In FIG. 4A, the actuator shown
generally at 40 comprises two tubes (or an outer tube and an inner
rod), with the outer tube 401 being anchored to some suitable
structure (not shown) and an end 412 of SMA wire 411 connected in a
tendon-like fashion to the load to be acted upon (also not shown).
Inner tube/rod 402 slides within outer tube 401. The other end of
wire 411 is attached to an attachment point 413 at one end of inner
tube/rod 402, while a second wire 421 is attached to the other end
of inner tube/rod 402 at attachment point 422 and is also attached
to an attachment point 423 rigidly connected to outer tube 401.
Typically the inner tube/rod 402 is electrically conductive thereby
completing an electrical path from point 412 to point 423 through
both SMA wires in series, with the power leads connected to points
412 and 423. The outer tube may be non-conductive; but must be
insulated or electrically separated from the inner tube if it is
conductive. In FIG. 4B, the actuator shown generally at 43 consists
of an outer tube 431 with a pair of opposed inner tubes/rods 432A,
432B, each connected to a different structure (not shown) in order
to exert a force between them. An end of SMA wire 441A is connected
at attachment point 442A to outer tube 431 and the other end is
connected at attachment point 443A to the exposed end of inner
tube/rod 432A. Contraction of wire 441A thus urges inner tube/rod
432A into outer tube 431. Similarly, contraction of wire 441B urges
inner tube/rod 432B into outer tube 431 from the opposite
direction. This creates an opposed actuator, and if the inner ends
433A and 433B were linked to a structure to be moved (not shown)
through an aperture in outer tube 431, that structure could be
moved in one direction or the other along the axis of the outer
tube 431 by applying power to one or other of wires 441A and 441B.
This configuration can be modified into a stroke multiplying SMA
actuator by an actuator of the type shown generally in FIG. 4A for
each of the half-elements of the actuator 43.
[0069] Clearly both configurations of these concentric tube
actuators can be extended by adding additional concentric tubes to
the device to achieve larger displacements. To increase the
available force delivered multiple parallel SMA wires can be used
on the same framework without any penalty in cycle time.
[0070] A prototype set of actuators has been constructed at a scale
suitable for implementing a six-legged Stiquito-like walking robot.
The resulting device can walk at a speed of approximately 1 cm/sec,
which compares very favorably with Stiquito's 3-10 cm/min. In this
embodiment, the concentric tubes used in these actuators were made
from aluminum. The outer tube had a length of 4 cm, an outside
diameter of 2.4 mm, and the actuator produced a stroke of at least
3.2 mm. The SMA wire was anchored to the aluminum tubes using small
brass nuts and bolts of size 00-90. The SMA used was a Flexinol
TiNi alloy (Dyalloy, Inc.), with a diameter of 50 .mu.m, producing
a force of 35 g. The SMA was heated using a 1 KHz PWM signal that
delivered a maximum of 110 mA at an amplitude of 6V. Certain of the
actuators (those responsible for bearing the weight of the robot)
had two parallel SMA wires attached to the tubes and so delivered a
force of 70 g. The cycle time for both types of actuator was
approximately 0.7 sec.
[0071] In a more preferred third embodiment, the actuator consists
of a set of stacked parallel plates electrically insulated from one
another and joined by SMA wires. The construction of such an
actuator is shown in FIGS. 5 through 8.
[0072] FIG. 5 shows conceptually such a stacked plate actuator
shown generally at 50, comprising three rigid conductive plates 511
through 513 connected by two SMA wires 521 and 522. Wire 521 is
connected to plate 511 at attachment point 521A and to plate 512 at
attachment point 521B, while wire 522 is connected to plate 512 at
attachment point 522A and to plate 513 at attachment point 522B.
Plates 521 through 523 are spaced apart and electrically insulated
from one another such as by sheets of low-friction polymeric
material (e.g. PTFE or another fluorinated polymer, or a polyamide
such as a nylon or Kapton) placed between them or by a coating of
low-friction polymeric material applied to the plates, so that the
plates may readily slide with respect to each other. Plate 511 is
provided with an external attachment point shown as aperture 5111
at the end adjacent to wire attachment point 521A, while plate 513
is provided with an external attachment point shown as aperture
5131 at the end adjacent to wire attachment point 522B. When
electric power is applied to the actuator between points on plates
511 and 513, the SMA wires 521 and 522 are heated and contract,
thereby moving external attachment points 5111 and 5131 closer
together. The stroke of the actuator will be approximately the sum
of the contraction of wires 521 and 522, and therefore about twice
the contraction of each wire individually, yet the force exerted
will be not substantially lower than the force exerted by each
wire. It will be evident that an increased stroke for the actuator
can be obtained simply by increasing the number of plates and
wires.
[0073] A variation on the actuator shown in FIG. 5 is shown in
FIGS. 6, 7, 8A, and 8B.
[0074] FIG. 6 shows an "I-beam" or "dogbone" shaped plate for this
actuator. The plate shown generally at 60 has an elongate shaft 61
and ends 62 and 63. External attachment points 62A and 63A, which
may, for example, be apertures into which external tendons or the
like may be connected, may be present at either or both ends of the
plate. Though only one end of the uppermost plate and the other end
of the lowermost plate will typically be externally connected to
convey the force of the actuator to an external load, it may be
convenient for all plates to be made alike. Also present at ends 62
and 63 are wire attachment points 62B and 63B. These are shown at
the sides of the ends for convenience, but may be attached wherever
convenient. It is also possible that similar wire attachment points
may be present on the other sides of the ends, thereby allowing two
wires to be linked between each pair of plates and doubling the
force available from the actuator.
[0075] FIG. 7 is a perspective view showing an actuator, shown
generally at 70, with six stacked plates 71 through 76 and five SMA
wires 711 through 715. In this Figure, the wires are shown slack
and the actuator is shown in its extended position. The plates 71
through 76, which are made of a conductive material such as brass,
are held, spaced apart by insulating layers (not shown), in a case
77, which constrains the plates to move in parallel. The case 77
will typically be made of a thermoplastic polymeric material such
as polycarbonate, polystyrene, or the like. Power may be applied to
the actuator between points 711A (where wire 711 is attached to
plate 71) and 715B (where wire 715 is attached to plate 76), or,
since the plates are electrically conductive, at any places on
plates 711 and 715, and a circuit will be completed through all six
plate and five wires.
[0076] FIGS. 8A and 8B are side views of a similar actuator, in
which FIG. 8A (like FIG. 7) shows the actuator in an extended
position, and FIG. 8B shows it in a contracted position, with the
heavy arrows showing the direction of contraction. Here the
contraction has been shown as being symmetrical, so that the ends
of the plates align, but this is not a requirement. The stroke of
this actuator will be approximately five times the contraction of
any wire individually, while the force that can be exerted by the
actuator will not be substantially lower than the force exerted by
any wire.
[0077] Although the actuator (like all SMA actuators of this
invention) operates by contraction of the SMA wire as it is heated,
so that the actuator decreases in length as shown in FIG. 8B, a
person of ordinary skill in the art will readily understand that is
possible to extend one of the plates, such as plate 76, with an
extension 761 at the opposite end of the plate from the end having
attachment point 715B. By comparing the relative positions of
attachment point 711A and extension 761 in FIGS. 8A and 8B, it can
be seen that extension 761 extends well beyond attachment point
711A when the actuator is contracted. Thus, by appropriate
extension of one of the outermost plates and fastening of the other
outermost plates, an actuator based on contraction may push as well
as pull.
[0078] In this variation of FIGS. 6 through 8B, as mentioned above,
the SMA wires have been shown on one side only of the ends, but it
is possible to have a second set of wires on the other side of the
ends to double the actuating force. Also, as mentioned before, the
number of plates and wires may be increased as desired to increase
the stroke of the actuator.
[0079] A feature that is available to minimize the total voltage
required to drive a multi-plate actuator as the number of plates
increases is to use an odd number of plates (even number of SMA
wires) and, instead of applying power to the actuator between the
outermost plates (where the resistance of the actuator, assuming
the plate resistance to be significantly lower than the wire
resistance, will be the sum of the resistances of all the wires),
electrically linking the outermost plates and applying power
between these two outermost plates and the middle plate (where the
resistance of the actuator will then be one-half the sum of the
resistances of all the wires). This enables use of a lower supply
voltage, although the current draw will be doubled.
[0080] In a second aspect, this invention provides a single-wire
stroke-multiplying SMA actuator that achieves stroke multiplication
without substantial force reduction and without resorting to a
mechanically complex solution like pulleys or to mechanisms that
reduce the available force.
[0081] The basic design of this aspect of the invention, as shown
schematically in FIG. 9, comprises two parallel hollow low-friction
non-conductive tubes or rods 901 and 902 with an SMA wire 910
wrapped around them as if they were pulleys. The tubes/rods, which
have a radius greater than the minimum bending radius of the SMA
wire (as discussed previously, this minimum bending radius for
repeatable austenite-martensite transition is about 50 times the
wire diameter) are made of or covered with a low-friction polymeric
material capable of withstanding the temperature of the SMA wire
when heated. The tubes/rods are mounted on a frame, shown in the
Figure as a pair of plates 921 and 922, that is rigid so that the
tubes/rods are held apart at a constant distance. When the SMA wire
910 is heated (e.g. by passing an electrical current through it),
it contracts and slides over the low-friction tubes/rods, causing
the end of the wire to move in the direction of the arrow. Due to
the low friction of nature of the tubes/rods, the SMA wire slides
over it with no appreciable loss in available force, which enables
thinner SMA wire to be employed resulting in greatly increased
responsiveness due to the non-linear dependence of cooling speed on
wire diameter. Suitable polymers for the tubes/rods or their
coatings are PTFE and other fluorinated polymers. These not only
can withstand high temperatures, but also conduct heat efficiently,
so that the tubes/rods also act as heat sinks for the SMA wire,
further improving the responsiveness of the resulting actuator. The
result is an actuator that is smaller and much more responsive than
could be achieved using conventional methods, but which avoids the
mechanical complexity of pulleys.
[0082] A variation on this aspect is to use a narrow gauge tubing
931, 932, made from or internally coated with a polymer such as
PTFE or other fluoropolymer and having an internal diameter
slightly greater than the diameter of the SMA wire, to encase the
wire as it passes over the tubes/rod 901, as illustrated in FIG. 10
(which shows only one end of the actuator, the other being
similar). This arrangement increases the heat sink effect of the
tubing and also permits the use of conductive tubes/rods. For small
diameter SMA wires, where the force exerted on the tubing 931 by
the SMA wire is low, the tube/rod 901 can be omitted as is shown in
FIG. 11, which illustrates this for just a single bend in the SMA
wire (it being obvious that the other bends in the SMA wire can be
similarly treated).
[0083] An example of how such actuators can be implemented is shown
schematically in FIG. 12. Here four actuators are used in an
antagonistic configuration to implement a two degrees of freedom
joint such as might be used on a robotic hip joint. SMA wires 1201,
1202, 1203, and 1204 (not shown because obscured in this Figure by
limb 1221) pass over low friction tubes/rods 1211, 1212, 1213, 1214
respectively, which are mounted on plate 1200. A similar plate and
set of rods form the "back" of the actuator (shown but not
numbered). Only the ends of the wire extending from the "front" of
the actuator over the tubes/rods are shown, to minimize complexity
and simplify understanding of the Figure, though the arrangement
for each SMA wire is the same as that shown more completely in FIG.
9. By powering the SMA wires, the distal end 1223 of limb 1221,
which limb is articulated from the plate 1200 through a joint 1222
such as a ball-and-socket joint, can be made to move up or down,
left or right, as illustrated by the arrows at the end of the limb.
Powering the SMA wires in combination can produce any desired
diagonal or curved motion.
[0084] When many actuators are required in a small space the
framework and tubing can be shared by multiple actuators; for
example, by an opposed pair of actuators moving over the same rods
but with the SMA wires leading out opposite sides of the framework.
In such a case, the actuators can be independent wires, so that
they are independently activated, thereby reducing the size of the
paired actuator system. It is also possible to use a single wire
for a pair of actuators in such a configuration, where the wire is
fixed at its center. If that center is an electrical contact, each
half of the wire may be independently powered, resulting in a
paired but independent dual actuator system (though of course both
actuators could be activated simultaneously). If that center is not
used as an electrical contact, or if both endpoints of the wire are
electrically linked, so that in either event the whole wire is
powered, then the result is a single actuator moving and exerting a
contractile force on two opposed points. Such configurations are
particularly useful for symmetrical devices; such as a pair of legs
on opposite sides of a multi-legged walking robot, such as an
insect, spider, or centipede; and it will be readily conceivable
that paired actuators can be arranged so that the opposite legs
move simultaneously in one direction, e.g. both forward at once, or
in opposite directions, e.g. one forward and one backward. In such
specialized uses, the number of actuators and the complexity of the
operating circuitry can be reduced by use of paired actuators as
described here and elsewhere in this application.
[0085] In a third aspect, this invention provides a very compact
and simple controller mechanism for arrays of SMA actuators that
are heated using the PWM scheme and which use resistance as the
feedback mechanism (a technique well-known in the art, and
illustrated schematically in FIG. 13). In FIG. 13, the PWM
generator 1302 supplies the PWM pulse to the SMA element 1303 at a
duty cycle and period specified by the controller 1301. During the
off period of the PWM pulse, the resistance measuring system 1306
measures the resistance of the SMA element, which is sampled and
held in the sample-and-hold system 1305. This off-period sampling
avoids the risk of possible mis-sampling during a short on period.
The analog signal in the sample-and-hold system 1305 is passed to
the A-D converter 1304 and converted to digital form, where it can
be read by the controller 1301. This information can then be used
by the controller to vary the duty cycle of the PWM generator to
achieve the desired position of the SMA element. In systems with
more than one SMA element, all of the systems within the dashed box
in FIG. 13 need to be replicated for each actuator, adding
substantially to the cost and complexity of the device containing
them. It is known to multiplex the controller across a number of
sample-and-hold systems, thereby reducing the number of A-D
converters required to just one.
[0086] This invention avoids the duplication of high current
switches required by methods such as those disclosed by U.S. Pat.
No. 5,763,979 discussed above and that illustrated in FIG. 13, and
uses timing control to eliminate the need for sample-and-hold
systems. The resistance measuring system is also greatly reduced in
complexity.
[0087] In this aspect of the invention, as illustrated
schematically in FIG. 14 all SMA actuators have a synchronized duty
cycle. An interrupt is generated in the PWM controller 1401 at the
beginning of each cycle of "power on" from the PWM driver 1402 to
the SMA element 1403, and the controller initiates an
analog-to-digital conversion in the A-D converter 1404, In this way
the PWM pulse is used as both the heating and resistance measuring
voltage. The minimum duty cycle is calculated to be longer than the
time it takes to perform a single A-D conversion plus an allowance
for interrupt latency.
[0088] A second change from conventional design is that the inputs
to the resistance measuring system 1405 are multiplexed so that the
resistance measuring system and A-D converter are shared amongst
all actuators. In this way, only the PWM driver 1402 is replicated
for each SMA element 1403 (as shown in the dashed box). The
interrupt that causes a new A-D conversion to start also selects
the next actuator in turn to be sampled by programming the
multiplexer 1407. Therefore, during each PWM period exactly one
actuator is sampled, and when the controller initiates the
conversion the PWM signal is certain to be "on". In addition, since
the resistance measuring system is behind a multiplexer which is
switched on only when the pulse is high it can be very simple. For
example, it may comprise only a single resistor. This compares
favorably with the complex current source and bridge circuits used
in K. Ikuta, M. Tsukamoto, S. Hirose, "Shape Memory Alloy Servo
Actuator with Electrical Resistance Feedback and Application for
Active Endoscope", discussed above, which measure the resistance
during the off-period of the PWM pulse. The system is also much
more compact than that described in U.S. Pat. No. 5,763,979,
discussed above, because it eliminates one high current switch per
actuator, realizing significant space and cost savings since in
most cases these switches are the largest and most expensive part
of the controller.
[0089] While the multiplexer may select the actuators to be sampled
sequentially in a round-robin fashion, an alternative sampling
scheme increments the multiplexer to the next actuator to be
sampled taking current usage of the system into account. For
example, actuators with a 0% duty cycle (i.e. off) can be skipped.
In many applications only a limited number of the available
actuators are actively heated at a particular time, so this
approach can significantly increase the sample rate of those
actuators being actively controlled.
[0090] PWM control is particularly attractive because many
commercial micro-controllers contain built-in hardware for
generating PWM signals, reducing the computational overhead on the
controller; also, PWM output is often used in sound chips (such as
those used in "talking" greeting cards and the like) as an
inexpensive D-A conversion mechanism, making these low cost chips
suitable as controllers for SMA actuators of this invention. For
example, as two-channel sound chip could be used to generate both
sound and motion in a low cost compact module. In some
applications, full PWM control may not be required, and an
inexpensive timer chip could be used to generate the required
digital signals. Also, PWM control reduces current draw when a
temperature signal (in effect, an R.sub.sma signal) is available,
because no current limiting resistor is needed to prevent
overheating the SMA element. Also, because current flow in an SMA
wire tends (as with all solid conductors) to be concentrated at the
surface of the wire, there is the risk of "hot-spots" and uneven
heat distribution, reducing the life of the wire. Pulsing the
activating voltage allows for thermal conduction in the SMA wire to
lead to more even heat distribution. Further, in a conventional DC
control system, the SMA current is effectively constant and
relatively low, because it is determined by the current-limiting
resistor, the value of which is chosen to avoid overheating of the
SMA element once it is fully contracted. In a PWM or pulsed scheme
with resistance feedback, a high duty cycle can be used to heat the
SMA element initially, leading to rapid initial movement. The duty
cycle can be reduced when the SMA element reaches the desired
position, supplying only enough power to maintain the SMA element
in the desired state.
[0091] In a further aspect, this invention, rather than basing
decisions for position control and calibration on instantaneous or
peak resistance values, uses the memory and processing power of low
cost embedded micro-controllers to analyze the behavior of
resistance over time. This approach results in better performance
for calibration and position control and also allows information to
be extracted that was previously unavailable. In particular, the
system can automatically adjust to SMA configurations within a wide
operating range, can perform continuous calibration and position
control, can detect the applied load on the actuator and can also
detect mechanical obstructions of collisions between the actuator
and some external object.
[0092] Auto-Calibration
[0093] FIG. 15 shows the relationship between conductance and
position of an actuator heated from below A.sub.s to above A.sub.f.
The change in position (extent of contraction) is plotted as a
positive number versus time from the start of voltage application
as a solid line; while the conductance is plotted versus time as a
dashed line. The scales of position and conductance have been
chosen so that the similarity of the curves can most easily be
seen, illustrating the use of conductance as an analog of position.
The time of heating for a typical actuator will be about 0.3-1 sec.
The conductance may be measured (as illustrated in FIG. 16) by
measuring the voltage drop across a sense resistor. For a constant
applied voltage V.sub.sma applied to the SMA element, the drop in
voltage across the sense resistor (which has a resistance
R.sub.sense) is directly proportional to the current flow through
the SMA element and sense resistor. The conductance of the SMA
element (1/R.sub.sma) can therefore readily be calculated from the
current, and hence from the voltage measured by the A-D converter
1602. This digital voltage signal is particularly well adapted to
use in the controller/PWM generator 1601. During initial heating,
the conductance (1/resistance) decreases, and then after a time
changes direction and starts to increase. No motion occurs until
the change in conductance reverses. This point is Cmin (Rmax), and
corresponds to A.sub.s Normally information in this region below
A.sub.s is considered useless and is excluded from position control
schemes. However, analysis and experimentation has shown that the
depth and duration of this reversal is proportional to the applied
load on the actuator. Thus, by electronically "drawing" a
horizontal line across the resistance-position curve from Cstart
(the conductance with no extension) and calculating the area
between the conductance/time curve and that line (shown shaded in
FIG. 15), and multiplying by the appropriate scale factor
(determined experimentally), the applied load against which the
actuator is working can be calculated. These measurements can be
made with actuators having a stroke of only a few millimeters to a
degree of accuracy that that the difference between one and two
one-cent coins as applied loads can be seen.
[0094] Next in FIG. 15 can be seen an area of relatively linear
relation between position and conductance followed by another
reversal in conductance change. The top of conductance peak does
not correspond to Rmin since, as can be seen, the position of the
actuator is still changing at that point, indicating that A.sub.f
has not yet been reached. Systems which use a simple peak detector
for Rmin thus over-estimate Rmin. Better performance is achieved by
recording the value at which the conductance reaches a plateau. At
this point the temperature of the alloy is at or above A.sub.f, and
maximum displacement has been achieved.
[0095] Since the detection of Rmin and Rmax depends entirely on the
time-variance of resistance rather than its absolute value the
system can dynamically adapt to SMA components of varying lengths.
The constraints on the lengths that can be supported are: (1) the
power supply used to heat the SMA must be capable of causing the
SMA to reach the A.sub.f temperature, and (2) the resistance of the
SMA must be large enough that the SMA does not reach A.sub.f and
overheat more quickly than the control system can react. In
practical terms this enables the same control system to adapt to
SMA actuators in a wide range of sizes automatically; which is
particularly attractive because in many applications the control
system will be simultaneously controlling a number of actuators of
different lengths simultaneously.
[0096] Resistive Feedback Control
[0097] FIG. 16 illustrates a first resistance measuring (resistive
feedback) technique. Controller/PWM generator sends a PWM logic
pulse from PWM output 1601A to a power switching transistor 1603
(such as an FET or bipolar transistor), which allows power to flow
through the SMA element. Recognizing that the resistance change
between austenite and martensite phases of an SMA element is small,
typically only about 10% of the resistance of the SMA element,
sensitivity of measurement is required. In FIG. 16, the R.sub.sense
resistor is chosen to have as small a value as possible, e.g. about
1 .OMEGA., so that the maximum current is available to heat the SMA
element R.sub.sma. This in turn means that the voltage change
across R.sub.sense is small, and has to be amplified by an
amplifier 1602 before being fed to the A-D pin of the
controller/PWM generator 1601. For most practical values of the
applied voltage V.sub.sma, the voltage at the A-D converter pin may
exceed the maximum acceptable voltage (typically about 5V) when the
PWM signal is low, so a clamp diode may be used on the output of
amplifier 1602. However, this traditionally-designed circuit
requires not only that the controller 1601 have A-D conversion
capability (or an A-D converter be added), but also that the
circuit include a sense resistor and an amplifier 1602.
[0098] In an improved sensing circuit illustrated schematically in
FIG. 17, both the sense resistor and the amplifier are eliminated,
and the controller requires no A-D conversion capability. When the
PWM signal at output 1701A of the controller/PWM generator 1701 is
at logic "1" ("on"), the switching transistor 1703 permits current
flow through the SMA element denoted by R.sub.sma; and the voltage
at input position 1701B on the controller is a V.sub.sma less the
voltage drop across the switching transistor 1703. The same voltage
is seen at the non-ground terminal of capacitor C.sub.1. Since the
input position 1701B is an input pin and hence of high impedance,
it does not interfere with the current flow through the SMA
element.
[0099] When the PWM signal from position 1701A goes to logic "0"
(during the "off" part of the duty cycle), the switching transistor
1703 switches off current flow through the SMA element, and
capacitor C, immediately begins to discharge through the SMA
element. Eventually the voltage at position 1701B falls below the
transition threshold for that input, and the input switches from
logic "1" to logic "0". By measuring the time taken for the
capacitor C.sub.1 to discharge once the PWM signal goes to logic
"0", the RC constant for R.sub.smaC.sub.1 can be determined. Since
C.sub.1 is constant, the value of F.sub.sma can be determined; and,
as discussed previously, this value will change during the
martensite-austenite transition. A similar method comprises the SMA
element and capacitor being in parallel between the switching
transistor 1703 and V.sub.sma instead of between the transistor
1703 and ground, so that the capacitor charge time rather than the
discharge time would be measured and used to determine the value of
R.sub.sma. Other similar methods employing RC time constant
measurement will be apparent to a person of ordinary skill in the
art in light of this disclosure.
[0100] Position Sensing
[0101] Having identified Rmin and Rmax by the above means during
the first few cycles of heating and cooling, the control system can
accurately report position based upon the output of a simple state
machine that knows whether the wire is in the initial phase of
heating, the linear section, or the top plateau.
[0102] Force Detection
[0103] After a few cycles of heating and cooling, the control
system learns the size and duration of the initial resistance
reversal and assumes that this is the unloaded state. In future
cycles, the system can report the load as a multiple or fraction of
the initial load.
[0104] Collision Detection
[0105] After a few cycles of heating and cooling, the control
system develops an "expectation" of where the upper plateau will
occur. If the plateau occurs at a higher resistance value than
expected, then either the actuator has been mechanically obstructed
or the thermal environmental conditions have changed such that the
power supply can no longer provide enough power to reach A.sub.f
(e.g. the SMA wire may be being cooled by thermal contact with a
cool body or a cooling air stream may be impinging on it). In many
application categories the probability of the change in thermal
environmental conditions is low, and so the system can assume
mechanical obstruction. In fact, the system can even deduce how far
along the stroke the obstruction occurred based upon where in the
linear part of the curve the plateau occurred.
[0106] System Failure Detection
[0107] Finally, sudden spikes in voltage or absence of voltage
during heating can be interpreted by the software as either an open
circuit or short circuit condition and appropriate steps can be
taken to ensure a safe and orderly shutdown of the system. Fine SMA
wires burn when substantially overheated and can be an ignition
source in flammable environments.
[0108] While some prior SMA devices have determined position or
load on a single actuator, an attractive feature of this invention
is that when opposed actuators are used (as is general in
sophisticated applications), measurements may be made using only
the activated actuator. Measurement only on the heated contracting
SMA wire avoids dealing with the inherent hysteresis of the SMA
transition and enables accurate position and load measurement, and
position control, since the unheated wire of the opposed but
unenergized actuator functions purely as a low force constant
spring force.
[0109] Since all of the features being recognized by the software
are simple, very little computation (beyond some basic filtering)
is required. This means that the system can be implemented entirely
in hardware as an application-specific integrated circuit or in
software on a low cost embedded micro-controller.
[0110] Micro-Controller Features
[0111] For systems without feedback, only one micro-controller pin
is required per actuator; however, if several actuators are
required to be driven simultaneously (such as three legs of a
six-legged walking toy), a single output pin can be used to drive
several switching transistors and hence control several actuators.
When spring bias is used, the number of actuators decreases; and
hence a six-legged walker with spring bias requires as few as four
output pins--two each ("lift" and "move forward") for each of two
sets of legs. For systems with feedback, two pins are required per
actuator--one for PWM output and the other for position sensing
input; and if a capacitive sensing scheme is used, a timer input
pin per actuator is also required. Multiplexers may of course be
used if necessary to reduce the number of input pins required. The
number of output pins may also be reduced by using a multi-channel
driver chip, such as an Allegro UDN5832, which contains 32 high
current output drivers and a serial peripheral interface over which
serial bits can be sent and latched into the drivers. In systems
with many SMA actuators, the computational overhead of controlling
the actuators may become significant. By far the most
computationally intensive activity of the controller is the
generation of PWM signals for many channels and servicing the
interrupts for the sensor feedback values. If the PWM signals are
generated entirely in software (no PWM hardware on the chip), then
an 8 MHz Motorola HCO8 microcontroller can only drive about twelve
actuators simultaneously. However, PWM generation can easily be
implemented in hardware, and a custom chip could be developed that
would contain multiplexers, PWM generators, and sensing pins, and
this could communicate with the microcontroller using a serial
peripheral interface, thus increasing the number of actuators that
could be controlled.
EXAMPLE
A Miniature Stroke Multiplying Actuator
[0112] FIG. 18 shows in exploded view the assembly, shown generally
at 181, of the plates and wires of this embodiment. These plates
will be stacked into a parallel array, with plate 1811 being the
lowest, followed in succession by plates 18512 through 1815, and
topped by plate 1816. Each plate is made of a material that is
rigid yet soft enough to permit crimping of the material onto the
SMA wires 1821 through 1825 at crimp joints 1821A and 1821B through
1825A and 1825B respectively without damaging the wires (excessive
compression of the SMA wires causes fragility and change in
transition properties). A suitable material for the plates is a
half hard cartridge brass. Other methods of attachment of the wires
may be used, but crimping is an attractive method for ease,
economy, and not increasing the size of the assembled actuator.
Lowermost plate 1811, which is the plate of the actuator with the
greatest travel relative to uppermost plate 1816, will be provided
with an attachment point 18111 for external connection to an object
to be moved by the actuator, and will typically be provided with a
protrusion 18112 to engage with stops to limit its travel during
extension and contraction of the SMA wires and attachment point(s)
shown as apertures 18113 for attachment of a power lead (not
shown). Uppermost plate 1816 may be provided with indentations or
apertures 18161 to enable location with respect to a case (not
shown in this Figure), and will also be provided with attachment
points shown as apertures 18162 to for attachment of the other
power lead (not shown).
[0113] FIG. 19 shows the assembled actuator in a case 1830 in side
view. Although each of the plates 1811 through 1816 and the crimp
joints 1821A through 1825B are shown, only one SMA wire 1825 is
shown, for clarity. The plate are made of half hard cartridge brass
(CA 260), 0.2 mm thick, to allow adequate crimping of the SMA wires
without damage and yet still give sufficient rigidity of the
plates. An insulation layer (not shown) of 0.08 mm Kapton polyamide
film, type HN, is applied to the undersides of the plates, or a
sheet of similar material may be placed between the plates, to
ensure electrical separation of the plates and provide a low
friction sliding surface. The wires are 50 .mu.m Dynalloy Flexinol
with a 90.degree. C. transition temperature, and are attached under
10 g preload tension to avoid slack that would otherwise result in
lost motion in the actuator. A suitable material for the case is an
engineering thermoplastic such as a polycarbonate or equivalent.
The resulting actuator has a height of 4 mm, a width of 3 mm, an
extended length of 30 mm, and a contracted length of 26 mm, giving
a 4 mm stroke (13% stroke/length ratio). The completed actuator
weighs only 0.7 g. The actuator has a contraction force of 35 g, a
return force of 4 g, and a limit force of 1 Kg, with a contraction
time of 0.5 sec and a cooling time of 0.7 sec. At 6.0 V, the
average current is 50 mA with a peak current of 110 mA.
[0114] FIGS. 20A and 20B illustrate schematically the limit stops
of an actuator, where only the furthest moving plate is shown in
cross-section within a case. Case 2001, within which slides plate
2010, is provided with an attachment point 2002 to attach the
actuator body to the environment of use, and has an open end
defined by defined by stops 2003. The plate 2010 is provided with
an attachment point 2011 to connect to the environment of use,
inner protrusions 2012, and outer protrusions 2013 (the terms
"inner" and "outer" being defined with respect to the case 2001).
When the actuator is in its fully-extended position, as shown in
FIG. 20A, inner protrusions 2012 on plate 2010 engage with stops
2003 to prevent further extension of the actuator. When the
actuator is in its fully-contracted position, as shown in FIG. 20B,
outer protrusions 2013 engage with stops 2003 to limit the
contraction. In this way neither: (1) the application of an
excessive external extension force (much greater than the force
exertable by the actuator) can over-stress the SMA elements, nor
(2) does the actuator contract to the limit of its capability (thus
ensuring that even as the SMA elements age and lose
recoverability--as is well known for SMA elements--the actuator
will still move over the full range between the limit stops.
EXAMPLE
A Walking Insect
[0115] One of the many potential applications of the SMA actuators
of this invention is in small mobile robots or robotic toys. One
toy that has been created is a six legged robotic walking insect.
SMA actuators are used to provide life-like animation and nobility.
FIG. 21 shows a six-legged toy insect shown generally at 2100 with
six SMA actuators 2101 through 2106 (one per leg) visible mounted
above an internal body frame 2110 (11 cm.times.3 cm) that contains
a Motorola MC68HC08 MP16 microcontroller and a 9 V battery. Six
similar actuators 2121 through 2126 (with ends only visible in this
Figure) are mounted below the internal frame. Each actuator is
connected to a corresponding leg of the toy (actuators 2101 and
2121 being connected to leg 2111, and so on). Actuators 2101
through 2016 act to lift the ends of their corresponding legs with
respect to the frame 2110, while actuators 2121 through 2126 act to
move the ends of the legs backward with respect to the frame 2110,
as is discussed with respect to FIGS. 22 through 24. Additional
actuators 2107 and 2108 are connected to pincers 2117 and 2118
respectively, and act to close the pincers. Each of these actuators
is opposed by a bias spring, not numbered, which tends to pull the
end of each leg forward and downward and to open the pincers. FIGS.
22, 23, and 24 show how two SMA actuators are attached to each leg
of the toy. The legs and body of the insect can be made of any
suitable lightweight rigid material, such as a thermoplastic, for
example a polycarbonate such as Lexan.
[0116] FIG. 22 shows a view from above and to the left side of the
insect body frame, illustrating, as a representative example, the
left rear leg 2112 of the insect. Two actuators 2102 and 2122 are
connected to a two degree of freedom joint with axes of rotation
shown by dashed lines 2132 and 2142 (in FIG. 23) where the leg 2112
meets the frame 2110. When both actuators are relaxed (extended),
the spring 2152 urges the end of the leg 2112 downwardly with
respect to the frame, as shown by the arcuate arrow near the spring
2152, and supports the weight of the insect so that it can stand
without consuming power. When the actuator 2102 mounted on top of
the frame 2110 contracts, as shown by the arrow adjacent the
actuator 2102, it pulls via a tendon (not numbered) on the lever
2162, which causes the leg to rotate about horizontal axis 2132,
lifting the leg. When the actuator 2102 relaxes, the spring 2152
causes the leg to return to its original position. By controlling
the amount of contraction of the actuator 2102, the leg 2112 can be
lifted by a specific amount. By varying the length of the lever
2162 and the force of the spring 2152, different step heights and
body weights can be accommodated. When the actuator 2122 is
relaxed, the leg 2112 is held forward by the spring 2172. When the
actuator 2122 contracts, as shown by the arrow adjacent the
actuator 2122, it pulls (via a tendon, not numbered) on L-shaped
extension 2182 of the leg, causing it to rotate about axis 2142,
and swinging the leg 2112 in an arc backward. When the actuator
2122 relaxes, the spring 2172 returns the leg to its original
forward position, as shown by the arrow adjacent spring 2172.
[0117] By controlling the degree of contraction of each of the two
actuators connected to the leg, the foot of the leg can be made to
describe any arbitrary path within a rectangle defined by the range
of motion of the actuators.
[0118] FIG. 23 shows the same leg viewed from the rear of the
frame, more clearly illustrating the action of actuator 2102 and
its opposing spring 2152; while FIG. 24 illustrates both degrees of
freedom.
[0119] The remaining five legs are attached in a similar manner, as
are two pincers (though these require only one actuator per pincer,
as shown in FIG. 21, because they are hinged only for motion in a
horizontal arc). When the insect walks, three of its feet are on
the ground at any one time, and so it must be able to support
itself on just three legs. Therefore the springs must each be
capable of supporting at least one-third of the total weight of the
insect. This in turn means that the actuators on top of the body
used to lift the legs must be strong enough to overcome these
springs. If the length of the lever is chosen to be equal to the
moment arm formed by the joint and the leg as it rotates about the
horizontal axis, then the foot will be able to rise by the same
distance as the actuator contracts. Thus an actuator which
generates a nominal force of 70 grams and contracts 4 mm would be
able to support an insect weighing about 200 g, which lift its feet
4 mm from the ground. In practice, the desired foot lift is usually
larger to accommodate walking over rough terrain, and so the ratio
of the lever to foot moment arm is reduced, resulting in a higher
foot lift but lower load carrying capacity.
[0120] An inexpensive 8 bit micro-controller, such as the Motorola
MC68HC08 MP16 mentioned above can be used to generate the required
PWM signals and to measure the changing resistance values of the
fourteen actuators used. Software can then command each of the six
legs to move to specific positions and to determine when that
position is achieved. In this way a walking algorithm, such as the
tripod gait, which is widely described in the literature [see, for
example, C. Ferrell, A Comparison of Three Insect Inspired
Locomotion Controllers, Massachusetts Institute of Technology
Artificial Intelligence Laboratory Memorandum, Cambridge Mass.,
USA; and M. Binnard, Design of a Small Pneumatic Walking Robot,
Massachusetts Institute of Technology, Cambridge Mass., MS Thesis,
1995] can be easily implemented; and the pincers may be made to
open and close.
[0121] It will be evident to a person of ordinary skill in the art
that numerous modifications may be made to such a robot or toy, for
example, using a single actuator to close both pincers, using
opposed pairs of actuators rather than spring-opposed actuators,
etc., depending on the level of design and engineering
sophistication desired. Similar design techniques may be used to
accomplish any of the many tasks for which SMA actuators may be
useful, and a designer/manufacturer of ordinary skill in the art of
SMA actuators and their use should be able to design/manufacture
SMA actuators and devices containing them without undue
experimentation. It will also be evident that the sensing and
control aspects of this invention are applicable generally to all
SMA actuators and devices containing them, and their use is not
limited to the stroke-multiplying SMA actuators of the first and
second aspects of this invention. Thus the sensing and control
aspects of this invention may also be used in conventional straight
wire, coiled wire, or other SMA actuators such as are already known
to the art and described, for example, in the documents cited in
this application as representative of the knowledge of the art.
[0122] The software needed to implement the various functions,
including sensing and control functions, for the SMA actuators of
this invention will be readily accomplished by a person of ordinary
skill in the art of SMA actuators and their use, having regard to
their skill and the information available to such a person,
including the documents referred to in this application, and the
disclosure herein.
[0123] As discussed in the Summary of the Invention, this invention
also includes other actuators using electromechanically active
materials (materials that expand or contract on the passage of an
electric current or the application of an electric potential to
them other than through the normal processes of thermal expansion
and contraction of the type that are normally linear with
temperature over a wide temperature range). Such materials include,
for example, piezoelectric materials and certain electro-active
polymers. These materials typically exhibit only very limited
expansion or contraction when electrically activated, and devices
employing them as actuators will typically be adaptable to stroke
multiplication of the type discussed in this application in detail
for actuators comprising shape memory alloy elements. Also, the
control and sensing techniques will be equally appropriate for such
other actuators. Thus, when the term "SMA actuator" is used herein,
unless the context makes it apparent that it refers only to an
actuator comprising shape memory alloy elements, it should be
construed as including actuators employing elements comprising
electromechanically active materials, especially shape memory
alloys.
[0124] Various modifications and variations of the present
invention will be apparent to a person of ordinary skill in the art
without departing from the scope and spirit of the invention.
Although the invention has been described in connection with
specific preferred embodiments, it should be understood that the
invention as claimed should not be unduly limited to such specific
embodiments. Indeed, various modifications of the described modes
for carrying out the invention which are obvious to persons of
ordinary skill in the art are intended to be within the scope of
the following claims.
* * * * *