A NEW CONTROL SYSTEM FOR FAST MOTION CONTROL OF SMA ACTUATOR WIRES

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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.

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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

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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

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So How Fast Are SMA?

• Hear this

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Rapid Heating Method

• Measure the electrical resistance of the SMA wire
• Algorithm
• If resistance lt 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

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Resistance vs. Temperature
electrical resistance
heating
cooling
temperature
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Current Limiter Mechanism

• Maximum allowable current is a function of
  measured resistance

Ihigh
Maximum safe heating current, Imax(R)
Isafe
Rthresh
Rramp
Measured Resistance, R
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Control System Architecture
Motion Sensors
Actual position
Id
Ih
Motion Controller
Current Limiter
Current Regulator
SMA element
Desired position
Measured resistance

• Id, desired heating current, is a function of
  position error. It does not factor in the state
  of the SMA element.
• Ih, actual heating current, is the minimum of
• Imax(R), the max. safe heating current of the
  rapid heating mechanism
• Id from the motion controller

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Experimental Setup
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Grants Two-Stage Relay Controller
Heating Power
Reverse Actuator
Forward Actuator
Position Error
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Tracking Response of Two-Stage Relay Controller
(No Load)
Rapid Heating
ON
OFF
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Actual Power to Each Actuator
Rapid Heating
OFF
ON
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Step Response of Two-Stage Relay Controller (With
Load)
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Modified Proportional Controller
Heating Power
Reverse Actuator
Forward Actuator
Position Error
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Step Response of Modified Proportional Controller
(With Load)
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Conclusion

• The rapid heating mechanism substantially
  increases the SMAs 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

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Future Work

• Better position control systems for higher motion
  accuracy
• New test rig with force sensors
• Experiment with force control of SMA actuators

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Questions