1. SMART 2004 A NEW CONTROL SYSTEM FOR FAST MOTION CONTROL OF SMA ACTUATOR WIRES Yee Harn Teh and Roy Featherstone Department of Information Engineering, Research School of Information Sciences and Engineering, The Australian National University.

2. SMART 2004 Outline In this talk, we describe A rapid heating mechanism that doubles the velocity of SMA antagonistic-pair actuators A simple control system for fast motion control with small limit cycles

3. SMART 2004 Long-Term Objectives To achieve fast and accurate position and force control using SMA actuators To build and experiment with low-inertia robots using SMA actuators

4. SMART 2004 So How Fast Are SMA? Hear this: Hear this:

5. SMART 2004 Rapid Heating Method Measure the electrical resistance of the SMA wire Algorithm: If resistance < threshold then limit current to safe (datasheet) value else allow a larger current Note: supplying the SMA wire with currents beyond the data sheet safe level over a certain time period may overheat and damage the SMA

6. SMART 2004 Resistance vs. Temperature electrical resistance temperature heating cooling threshold resistance safety margin Allow larger currents

7. SMART 2004 Current Limiter Mechanism Maximum allowable current is a function of measured resistance Maximum safe heating current, I max (R) Measured Resistance, R I safe I high R thresh R ramp

8. SMART 2004 Control System Architecture I d, desired heating current, is a function of position error. It does not factor in the state of the SMA element. I h, actual heating current, is the minimum of – –I max (R), the max. safe heating current of the rapid heating mechanism – –I d from the motion controller Motion Sensors Motion Controller Current Limiter Current Regulator SMA element Measured resistance Desired position Actual position IdId IhIh

9. SMART 2004 Experimental Setup dSPACE DS1104 ADCDACEnc PC Current Regulators sensed SMA voltage & current current commands encoder signals

10. SMART 2004 Grant’s Two-Stage Relay Controller Position Error Heating Power Reverse ActuatorForward Actuator

11. SMART 2004 Tracking Response of Two-Stage Relay Controller (No Load) Rapid Heating: ONOFF

12. SMART 2004 Actual Power to Each Actuator OFFON Rapid Heating:

13. SMART 2004 Step Response of Two-Stage Relay Controller (With Load)

14. SMART 2004 Modified Proportional Controller Position Error Heating Power Reverse Actuator Forward Actuator

15. SMART 2004 Step Response of Modified Proportional Controller (With Load)

16. SMART 2004 Conclusion The rapid heating mechanism substantially increases the SMA’s speed of response without changing the cooling regime Motion accuracy could be achieved using better motion controllers A modified proportional controller achieves fast, accurate motion control of inertia-loaded systems, with greatly reduced limit cycles

17. SMART 2004 Future Work Better position control systems for higher motion accuracy New test rig with force sensors Experiment with force control of SMA actuators

18. SMART 2004 Questions