Actuators based on shape memory alloys (SMA) can be small, simple, cheap,
clean, silent, and have high force-to-weight ratios; but they also have
disadvantages, including low efficiency, low speed and low accuracy.
This project aims to improve the speed and accuracy of such actuators, so they
can be used in robots, medical devices, consumer electronics, and so on.
Our actuators employ antagonistic pairs of
FlexinolTM wires, which are made of a
nickel-titanium SMA called nitinol. These wires can be stretched easily
when cool, but contract forcibly to their original length when heated.
The big challenge is to develop control systems that deliver fast and accurate
tracking of force and motion command signals. This is not easy, because
SMAs exhibit highly nonlinear behaviour, including substantial hysteresis,
which makes them very difficult to control.
Results:
We have invented a new rapid heating
algorithm that roughly doubles the speed of SMA actuators, and also
protects them from being overheated. We have two movies showing the
rapid heating algorithm at work:
movie 1 (9MB). This movie shows
the pantograph robot tracking a square at speeds of up to 1Hz, using
Grant's relay controller (see publication 1) augmented with our rapid
heating algorithm. The speed is very good, but the tracking
accuracy is poor.
movie 2 (4.5MB). This movie shows
the step response and tracking response of a new motion control system
(see publication 8) with and without the rapid heating
algorithm.
We have demonstrated that SMA can respond at frequencies of 1kHz and
higher, by building an SMA
loudspeaker. Yes, SMA really can respond at audio frequencies,
and here is the audio to prove
it!
We have obtained a gain/phase model of the high-frequency dynamics of
SMA wires. According to this model, the heating-power to force
transfer function is a first-order low-pass filter in which the gain
varies slightly with mean stress and strain on the wire, but the phase
response is invariant. (See publications 7 and 9.)
We have developed a force control system with a slew rate of 50N/s and
an accuracy better than 1mN in a +/−3N range. (See publications 8
and 9.)
We have implemented an accurate stiffness control system, which makes
the actuator behave like a programmable spring (unpublished).
Publications:
Y. H. Teh 2003. A Control
System for Achieving Rapid Controlled Motions From Shape Memory Alloy (SMA)
Actuator Wires. B.Eng. Honours
Thesis, Dept. Engineering, The Australian National University.
Full Text.
R. Featherstone & Y. H. Teh
2004. Improving the Speed of Shape Memory Alloy Actuators by
Faster Electrical Heating. Int.
Symp. Experimental Robotics (ISER'04), Singapore, 18-21
June 2004. Full Text. Slides.
Y. H. Teh & R. Featherstone
2004. A New Control System for Fast Motion Control of SMA
Actuator Wires. Shape Memory And
Related Technologies (SMART 2004), Singapore, 24-26
November 2004. Full Text.
Slides (powerpoint).
Y. H. Teh & R. Featherstone
2004. Experiments on the Performance of a 2-DOF Pantograph
Robot Actuated by Shape Memory Alloy Wires. Australasian Conf. Robotics and
Automation (ACRA 2004),
Canberra, Australia, 6-8 December 2004. Full
Text.
Y. H. Teh & R. Featherstone
2005. Experiments on the Audio Frequency Response of Shape
Memory Alloy Actuators. Australasian
Conf. Robotics and Automation (ACRA 2005), Sydney,
Australia, 5-7 December 2005. Full
Text.
Y. H. Teh & R. Featherstone
2007. Accurate Force Control and Motion Disturbance Rejection
for Shape Memory Alloy Actuators. IEEE Int. Conf. Robotics and Automation
(ICRA'07), Rome, Italy, 10-14 April,
pp. 4454-4459. (Note: the SMA
model shown in this paper is not correct. The correct model appears in
publications 7 and 9.)
Y. H. Teh & R. Featherstone
2007. Frequency Response Analysis of Shape Memory Alloy
Actuators. Int. Conf. Smart
Materials and Nanotechnology in Engineering (SMN 2007), Harbin, China,
1-4 July. Also in Proc. SPIE vol. 6423, p. J4232,
2007. Full Text.
Y. H. Teh & R. Featherstone
2008. An Architecture for Fast and Accurate Control of Shape
Memory Alloy Actuators. Int. J. Robotics Research, vol. 27, no. 5,
pp. 595-611. Full Text.
Y. H. Teh 2008. Fast,
Accurate Force and Position Control of Shape Memory Alloy Actuators.
Ph.D. Thesis, Dept. Information
Engineering, The Australian National University. Full Text.
