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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
2
The Testbed
antagonistic pairof SMA wires
load cells
precision linear stage
lockable pulley withoptional pendulum load
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
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 )
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
6
Differential Controller
Fcmd
Pmax,L Pmax,R
Fdiff
Pdiff−
+ Kp
−1
Td s minmax
+−
+++
+−
min
max
1Ti s
differential force controller
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
8
What Happened to the Hysteresis?
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:
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
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
12
Anti−Slack Mechanism
Pcom FL
FR
+−
Fminanti−slack
minKas
13
Anti−Slack Mechanism
Pcom FL
FR
+−
Fminout
in
minKas
14
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
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?
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.
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
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%
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
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
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
22
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
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.
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
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
26
Anti−Overload Mechanism
FL
FR
anti−overloadFmax
+− maxKao
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
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
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.
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
31
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
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