(SMA) Actuators - CECS - Australian National University

1

High Performance Force Control forShape Memory Alloy (SMA) Actuators

Dept. Information Engineering,The Australian National University

© 2006 R. Featherstone, Y. H. Teh

http://users.rsise.anu.edu.au/~roy/SMA

(reporting the work of Yee Harn Teh)

Roy Featherstone

© 2008 Roy Featherstone

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An alloy exhibiting the shape memory effect has thefollowing property:

The Shape Memory Effect

it can be deformed easily when cold, but returnsto its original shape when heated.

The shape recovery process is accompanied by largeforces, which can be harnessed to perform mechanicalwork.

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The Shape Memory Effect

The effect is caused by a transformation between twocrystal phases:

a martensite crystal phase, which is stable atlower temperatures.

Austenite crystals are cubic:

Martensite crystals are monoclinic: or

2.

1. an austenite crystal phase, which is stable athigher temperatures, and

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The Shape Memory Effect

hotcooling

cold

deformwarming

5

The Shape Memory Effectm

arte

nsite

ratio

temperature

100%

0%

thermalhysteresis

Af

As

Ms

Mf

6

SMA Actuators

but recover theiroriginal shapewhen heated

SMA wires areeasily stretchedwhen cool

antagonistic−pairactuator

forcesensors

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mechanically simple

easily miniaturized

large force outputs

high force−to−weight ratio

clean

silent

spark−free

cheap

inefficient

slow

hard to controlaccurately

a better controlsystem can fix these

SMA Actuators

Advantages Disadvantages

low strain

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Force Control System

left SMA

right SMA

powerlimit

powerlimit

anti−slack

anti−overload

map

Fcmd

Pmax,LPmax,R

Pcom

PLFL

FR

RL

RR

Fdiff

Fdiff

differentialforce

controller

++

+−

−+

−+

PR

Pdiff

9

Force Control System

left SMA

right SMA

powerlimit

powerlimit

anti−slack

anti−overload

map

Fcmd

Pmax,LPmax,R

Pcom

PLFL

FR

RL

RR

Fdiff

Fdiff

differentialforce

controller

++

+−

−+

−+

PR

Pdiff

PL = Pcom + max(0,Pdiff )

PR = Pcom + max(0,−Pdiff )

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Differential Controller

left SMA

right SMA

powerlimit

powerlimit

anti−slack

anti−overload

map

Fcmd

Pmax,LPmax,R

Pcom

PLFL

FR

RL

RR

Fdiff

Fdiff

differentialforce

controller

++

+−

−+

−+

PR

Pdiff

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Differential Controller

Fcmd

Pmax,L Pmax,R

Fdiff

Pdiff−

+ Kp

−1

Td s minmax

+−

+++

+−

min

max

1Ti s

differential force controller

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Behaviour of the Plant

The small−signal AC response ofnickel−titanium SMA approximatesto a first−order low−pass filter. 7−8 dB

gain

phase

Gain varies with mean stressand strain in a 7−8 dB range

Phase is independent ofstress and strain

Cut−off frequency varies withwire diameter

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What Happened to the Hysteresis?

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A Problem

When FlexinolTM wires are used in an antagonistic−pairactuator, they quickly develop a two−way shape memoryeffect, in which the wires actively lengthen as they cool,even if the tension on the wire is zero.

The wires can become slack as they cool.

Remedy: An anti−slack mechanism that maintains aminimum tension on both wires at all times.

Symptom:

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Anti−Slack Mechanism

left SMA

right SMA

powerlimit

powerlimit

anti−slack

anti−overload

map

Fcmd

Pmax,LPmax,R

Pcom

PLFL

FR

RL

RR

Fdiff

Fdiff

differentialforce

controller

++

+−

−+

−+

PR

Pdiff

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Anti−Slack Mechanism

left SMA

right SMA

powerlimit

powerlimit

anti−slack

anti−overload

map

Fcmd

Pmax,LPmax,R

Pcom

PLFL

FR

RL

RR

Fdiff

Fdiff

differentialforce

controller

++

+−

−+

−+

PR

Pdiff

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Anti−Slack Mechanism

Pcom FL

FR

+−

Fminanti−slack

minKas

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Anti−Slack Mechanism

Pcom FL

FR

+−

Fminout

in

minKas

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0 2 4 6 8 10 12 14 16 18 20−2.5

−2

−1.5

−1

−0.5

0

0.5

1

1.5

2

2.5D

iffer

entia

l For

ce (N

)

Time (s)

ReferenceActual

0 2 4 6 8 10 12 14 16 18 20−0.2

−0.15

−0.1

−0.05

0

0.05

0.1

0.15

0.2

Diff

eren

tial F

orce

Err

or (N

)

Time (s)

0 2 4 6 8 10 12 14 16 18 20−2.5

−2

−1.5

−1

−0.5

0

0.5

1

1.5

2

2.5

Diff

eren

tial F

orce

(N)

Time (s)

ReferenceActual

0 2 4 6 8 10 12 14 16 18 20−5

−4

−3

−2

−1

0

1

2

3

4

5x 10−3

Diff

eren

tial F

orce

Err

or (N

)

Time (s)

without anti−slack with anti−slack

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Another Problem

We want the actuator to be as fast as possible. Thespeed can be increased by means of

a faster heating rate, and/or

a faster cooling rate.

A faster heating rate is more beneficial and easier toimplement.

problem: how to achieve faster heating without risk ofoverheating?

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Why Focus on Heating?

Excerpt from FlexinolTM data sheet:

Diameter(mm)

Current(mA)

ContractionTime (sec)

Off Time70C

Off Time90C

0.050

0.075

0.100

50

100

180

1

1

1

0.3

0.5

0.8

0.1

0.2

0.4

If we use the recommended safe heating currentsthen, for a thin wire, heating takes longer thancooling.

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Rapid Electrical Heating

To obtain a rapid response from an SMA wire, we needa heating strategy that

allows large heating powers when there is no risk ofoverheating, but

allows only a safe heating power when there is a riskof overheating.

This can be accomplished by

measuring the electrical resistance of the wire, and

calculating a heating power limit as a function of themeasured resistance

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Electrical Resistance vs. TemperatureThe electrical resistance (of nitinol) varies with themartensite ratio, and therefore also with temperature,because the resistivity of the martensite phase is about20% higher than the resistivity of the austenite phase.

Res

ista

nce

Temperature

heating

cooling

~20%

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Calculating the Power Limit

operating temperature too hot

Rth

1. Choose a threshold resistance, Rth, which is equal tothe hot resistance of the wire plus a safety margin.

safetymargin

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safe

unsafepowerlevels

Calculating the Power Limit

2. Calculate the power limit, Pmax, as a function of themeasured resistance, Rmeas.

RthPmax

Rmeas

Psafe

Phigh

Rramp

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Rapid Heating Mechanism

left SMA

right SMA

powerlimit

powerlimit

anti−slack

anti−overload

map

Fcmd

Pmax,LPmax,R

Pcom

PLFL

FR

RL

RR

Fdiff

Fdiff

differentialforce

controller

++

+−

−+

−+

PR

Pdiff

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0 1 2 3 4 5

−3

−2

−1

0

1

2

3D

iffer

entia

l For

ce (N

)

Time (s)

ReferenceActual

0 1 2 3 4 50

1

2

3

4

5

6

Forc

e (N

)

Time (s)

Left SMARight SMA

0 1 2 3 4 5

−3

−2

−1

0

1

2

3

Diff

eren

tial F

orce

(N)

Time (s)

ReferenceActual

0 1 2 3 4 50

1

2

3

4

5

6

Forc

e (N

)

Time (s)

Left SMARight SMA

without rapid heating with rapid heating

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Yet Another Problem

Rapid heating can produce excessively high tensions onthe wires, which can cause damage.

remedy: an anti−overload mechanism that cuts theheating power if the tension goes too high.

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Anti−Overload Mechanism

left SMA

right SMA

powerlimit

powerlimit

anti−slack

anti−overload

map

Fcmd

Pmax,LPmax,R

Pcom

PLFL

FR

RL

RR

Fdiff

Fdiff

differentialforce

controller

++

+−

−+

−+

PR

Pdiff

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Anti−Overload Mechanism

left SMA

right SMA

powerlimit

powerlimit

anti−slack

anti−overload

map

Fcmd

Pmax,LPmax,R

Pcom

PLFL

FR

RL

RR

Fdiff

Fdiff

differentialforce

controller

++

+−

−+

−+

PR

Pdiff

−+

−+

Yee Harn’sversion

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Anti−Overload Mechanism

FL

FR

anti−overloadFmax

+− maxKao

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0 1 2 3 4 5 6 7 8 9 10

−3

−2

−1

0

1

2

3D

iffer

entia

l For

ce (N

)

Time (s)

ReferenceActual

0 1 2 3 4 5 6 7 8 9 100

1

2

3

4

5

6

Forc

e (N

)

Time (s)

Left SMARight SMA

0 1 2 3 4 5 6 7 8 9 10

−3

−2

−1

0

1

2

3

Diff

eren

tial F

orce

(N)

Time (s)

ReferenceActual

0 1 2 3 4 5 6 7 8 9 100

1

2

3

4

5

6

Forc

e (N

)

Time (s)

Left SMARight SMA

without anti−overload with anti−overload

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Conclusion

A new architecture for high−performance force control ofantagonistic−pair SMA actuators has been presented,comprising

a PID controller for accurate control of the actuator’soutput force (i.e., the differential force);

an anti−slack mechanism to enforce a minimumtension on both wires;

a rapid−heating mechanism that allows faster heatingrates, but protects the wires from overheating; and

an anti−overload mechanism that protects the wiresfrom mechanical overload.

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EpilogueThis control system has been found to work in thepresence of large motion disturbances, and it has beenused as the inner loop in a position control system thatachieves high setpoint accuracy.

Acknowledgement: The experimental results graphs appearing inthis talk are taken from Yee Harn’s Ph.D. thesis.

For more details, seeYee Harn’s Ph.D. thesisY. H. Teh & R. Featherstone, "An Architecture for Fast andAccurate Control of Shape Memory Alloy Actuators", Int. J.Robotics Research, 27(5):595−611, 2008.

http://users.rsise.anu.edu.au/~roy/SMA/