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![Page 1: Achieving High Performance from Shape Memory Alloy (SMA ...users.cecs.anu.edu.au/~roy/talks/HiPerfSMA.pdf · 3 Force Control System left SMA right SMA power limit power limit anti−](https://reader030.fdocuments.in/reader030/viewer/2022041202/5d4c9ea988c99318328baa84/html5/thumbnails/1.jpg)
1
Achieving High Performance fromShape Memory Alloy (SMA) Actuators
School of Engineering,The Australian National University
© 2006 R. Feathersto ne, Y. H. Teh
this talk was presented at theIEEE ICRA workshop on Biologically−Inspired Actuation,
Shanghai, Friday May 13, 2011
Roy Featherstone(reporting the work of Yee Harn Teh)
© 2011 Roy Featherstone
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2
The Testbed
antagonistic pairof SMA wires
load cells
precision linear stage
lockable pulley withoptional pendulum load
![Page 3: Achieving High Performance from Shape Memory Alloy (SMA ...users.cecs.anu.edu.au/~roy/talks/HiPerfSMA.pdf · 3 Force Control System left SMA right SMA power limit power limit anti−](https://reader030.fdocuments.in/reader030/viewer/2022041202/5d4c9ea988c99318328baa84/html5/thumbnails/3.jpg)
3
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
![Page 4: Achieving High Performance from Shape Memory Alloy (SMA ...users.cecs.anu.edu.au/~roy/talks/HiPerfSMA.pdf · 3 Force Control System left SMA right SMA power limit power limit anti−](https://reader030.fdocuments.in/reader030/viewer/2022041202/5d4c9ea988c99318328baa84/html5/thumbnails/4.jpg)
4
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 )
![Page 5: Achieving High Performance from Shape Memory Alloy (SMA ...users.cecs.anu.edu.au/~roy/talks/HiPerfSMA.pdf · 3 Force Control System left SMA right SMA power limit power limit anti−](https://reader030.fdocuments.in/reader030/viewer/2022041202/5d4c9ea988c99318328baa84/html5/thumbnails/5.jpg)
5
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
![Page 6: Achieving High Performance from Shape Memory Alloy (SMA ...users.cecs.anu.edu.au/~roy/talks/HiPerfSMA.pdf · 3 Force Control System left SMA right SMA power limit power limit anti−](https://reader030.fdocuments.in/reader030/viewer/2022041202/5d4c9ea988c99318328baa84/html5/thumbnails/6.jpg)
6
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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7
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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8
What Happened to the Hysteresis?
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9
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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10
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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11
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
![Page 12: Achieving High Performance from Shape Memory Alloy (SMA ...users.cecs.anu.edu.au/~roy/talks/HiPerfSMA.pdf · 3 Force Control System left SMA right SMA power limit power limit anti−](https://reader030.fdocuments.in/reader030/viewer/2022041202/5d4c9ea988c99318328baa84/html5/thumbnails/12.jpg)
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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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15
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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16
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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17
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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18
Electrical Resistance vs. Temperature
The 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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19
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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20
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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21
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
For
ce (
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
For
ce (
N)
Time (s)
Left SMARight SMA
without rapid heating with rapid heating
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23
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.
![Page 24: Achieving High Performance from Shape Memory Alloy (SMA ...users.cecs.anu.edu.au/~roy/talks/HiPerfSMA.pdf · 3 Force Control System left SMA right SMA power limit power limit anti−](https://reader030.fdocuments.in/reader030/viewer/2022041202/5d4c9ea988c99318328baa84/html5/thumbnails/24.jpg)
24
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
![Page 25: Achieving High Performance from Shape Memory Alloy (SMA ...users.cecs.anu.edu.au/~roy/talks/HiPerfSMA.pdf · 3 Force Control System left SMA right SMA power limit power limit anti−](https://reader030.fdocuments.in/reader030/viewer/2022041202/5d4c9ea988c99318328baa84/html5/thumbnails/25.jpg)
25
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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26
Anti−Overload Mechanism
FL
FR
anti−overloadFmax
+− maxKao
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27
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
For
ce (
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
For
ce (
N)
Time (s)
Left SMARight SMA
without anti−overload with anti−overload
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28
Extension to Position and Stiffness Control
Position
method: close an outerposition loop around theforce controller
result:
Stiffness
method: redefine the errorsignal to be the force error intracking the commandedstiffness
result:
very high accuracylow speed
very high accuracyhigh speed
Recommendation: Use Stiffness Control
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29
Summary
A new architecture for high−performance control of SMAactuators 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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30
Lessons
High performance can be achieved by studying thebehaviour of the plant, discovering how to push theplant safely towards its performance envelope, anddesigning a control architecture accordingly
1.
2. In general, such an architecture has 3 components:
a command−following component,
a performance−optimization component, and
a performance−limiting component with explicitknowledge of the plant’s performance envelope
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The experimental results graphs appearing in this talk aretaken from Yee Harn’s Ph.D. thesis.
For more details, see
Yee Harn’s Ph.D. thesis
Y. 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.cecs.anu.edu.au/~roy/SMA/
Acknowledgements:
Most of the work reported here was done by Yee Harn Teh
The stiffness controller (which has not been published) wasimplemented by Sylvain Toru