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A NEW CONTROL SYSTEM A NEW CONTROL SYSTEM

FOR FAST MOTION CONTROL FOR FAST MOTION CONTROL

OF SMA ACTUATOR WIRESOF SMA ACTUATOR WIRES

Yee Harn Teh and Roy FeatherstoneYee Harn Teh and Roy FeatherstoneDepartment of Information Engineering,Department of Information Engineering,

Research School of Information Sciences and Engineering,Research School of Information Sciences and Engineering,The Australian National University.The Australian National University.

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OutlineOutline

In this talk, we describeIn this talk, we describe

A rapid heating mechanism that doubles the A rapid heating mechanism that doubles the velocity of SMA antagonistic-pair actuatorsvelocity of SMA antagonistic-pair actuators

A simple control system for fast motion control A simple control system for fast motion control with small limit cycleswith small limit cycles

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Long-Term ObjectivesLong-Term Objectives

To achieve fast and accurate position and force To achieve fast and accurate position and force control using SMA actuatorscontrol using SMA actuators

To build and experiment with low-inertia robots To build and experiment with low-inertia robots using SMA actuatorsusing SMA actuators

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So How Fast Are SMA?So How Fast Are SMA?

Hear this: Hear this:

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Rapid Heating MethodRapid Heating Method

Measure the electrical resistance of the SMA Measure the electrical resistance of the SMA wirewire

Algorithm:Algorithm:

IfIf resistance < threshold resistance < threshold

thenthen limit current to safe (datasheet) valuelimit current to safe (datasheet) value

elseelse allow a larger currentallow a larger current

Note: supplying the SMA wire with currents Note: supplying the SMA wire with currents beyond the data sheet safe level over a certain beyond the data sheet safe level over a certain time period may overheat and damage the SMAtime period may overheat and damage the SMA

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Resistance vs. TemperatureResistance vs. Temperature

electrical resistance

temperature

heatingcooling

threshold resistance

safety margin

Allow larger currents

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Current Limiter MechanismCurrent Limiter MechanismMaximum allowable current is a function of measured Maximum allowable current is a function of measured resistanceresistance

Maximum safe heating current,

Imax(R)

Measured Resistance, R

Isafe

Ihigh

Rthresh Rramp

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Control System ArchitectureControl System Architecture

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

Motion Sensors

Motion Controller

Current Limiter

Current Regulator

SMA element

Measured resistance

Desired position

Actual position

Id Ih

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Experimental SetupExperimental Setup

dSPACE

DS1104ADC DAC Enc

PC

Current Regulators

sensed SMA

voltage & current

current commands

encoder signals

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Grant’s Two-Stage Relay ControllerGrant’s Two-Stage Relay Controller

Position Error

Heating Power

Reverse Actuator Forward Actuator

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Tracking Response of Two-Stage Tracking Response of Two-Stage Relay Controller (No Load)Relay Controller (No Load)

Rapid Heating: ONOFF

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Actual Power to Each ActuatorActual Power to Each Actuator

OFF ON

Rapid Heating:

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Step Response of Two-Stage Relay Step Response of Two-Stage Relay Controller (With Load)Controller (With Load)

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Modified Proportional ControllerModified Proportional Controller

Position Error

Heating Power

Reverse Actuator Forward Actuator

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Step Response of Modified Step Response of Modified Proportional Controller (With Load)Proportional Controller (With Load)

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ConclusionConclusion

The rapid heating mechanism substantially The rapid heating mechanism substantially increases the SMA’s speed of response without increases the SMA’s speed of response without changing the cooling regimechanging the cooling regimeMotion accuracy could be achieved using better Motion accuracy could be achieved using better motion controllersmotion controllersA modified proportional controller achieves fast, A modified proportional controller achieves fast, accurate motion control of inertia-loaded accurate motion control of inertia-loaded systems, with greatly reduced limit cyclessystems, with greatly reduced limit cycles

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Future WorkFuture Work

Better position control systems for higher motion Better position control systems for higher motion accuracyaccuracy

New test rig with force sensorsNew test rig with force sensors

Experiment with force control of SMA actuatorsExperiment with force control of SMA actuators

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QuestionsQuestions