Shape Memory Alloy Rotary Actuator

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High Torque Motor Actuated by Shape Memory Alloys
Team Members Chris Byers Uri Desai Tim
Guenthner Iris Hill Jonathan McClellan Brad
Taylor Gary Nickel NASA JSC
Mentor Dr. James Boyd Faculty Mentor Reid
Zevenbergen Graduate Mentor
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Outline

• Goal
• Intro to Shape Memory Alloys (SMAs)
• Overall Design Concept
• Characterization of the SMA Wire
• Torque/Rotation Analysis
• Experimental Characterization of Torque/Rotation
• Heat Transfer Analysis
• Thermal Control Circuit
• Summary
• Future Work
• Questions

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Goal

• Design, build and test an electrically powered
  rotary actuator that has a higher torque density
  than conventional gear motors

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Intro to Shape Memory Alloys (SMAs)

• Goal
• Intro to Shape Memory Alloys (SMAs)
• Overall Design Concept
• Characterization of the SMA Wire
• Torque/Rotation Analysis
• Experimental Characterization of Torque/Rotation
• Heat Transfer Analysis
• Thermal Control Circuit
• Summary
• Future Work
• Questions

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What are Shape Memory Alloys?
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Advantages/Disadvantages of SMAs

• Advantages
• High strain actuators
• High force
• High actuation energy density
• Mechanically simple
• Disadvantages
• Low frequency
• Thermodynamically inefficient

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Overall Design Concept

• Goal
• Intro to Shape Memory Alloys (SMAs)
• Overall Design Concept
• Characterization of the SMA Wire
• Torque/Rotation Analysis
• Experimental Characterization of Torque/Rotation
• Heat Transfer Analysis
• Thermal Control Circuit
• Summary
• Future Work
• Questions

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SMA Wire Rotary Actuation

• Rest Position
• Strain introduced by spring
• SMA in martensite phase

• 2. Austenite Transition (heating)
• SMA recovers strain during transition to
  austenite phase

• 3. Martensite Transition (cooling)
• Spring returns SMA to strained state
• Ratchet mechanism prevents regression

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Housing Design

• Most components on single plane
• Output shaft between two rack and pinion
  mechanisms
• One for CW rotation, one for CCW rotation
• One mechanism is active at a time, the other is
  free-spinning on ratchet mechanism
• Housing design to be adapted to developing needs

Preliminary Housing Design
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Housing Design - Prototype
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Ratchet Implementation
Standard Socket
Spur Gear (bored to fit socket)
Palm Ratchet
How it All Fits Together
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Characterization of the SMA Wire

• Goal
• Intro to Shape Memory Alloys (SMAs)
• Overall Design Concept
• Characterization of the SMA Wire
• Torque/Rotation Analysis
• Experimental Characterization of Torque/Rotation
• Heat Transfer Analysis
• Thermal Control Circuit
• Summary
• Future Work
• Questions

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SMA Wire Characterization

• Theory
• The amount of recoverable strain depends on the
  applied stress levels at the time of actuation.
• Negative, linear relationship
• Experiment
• Find actual relationship
• Single Dynalloy Flexinol SMA wire
• Use data to predict rack displacement as a
  function of applied load

Properties of Experimental Wire (Dynalloy, Inc.
Flexinol)
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Experiment Schematic
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Experimental Results
SMA Wire Transformation Strain vs. Stress (Single
Wire)
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Experimental Observations

• Yield Stress
• 500600 MPa
• Permanent loss of transformation strain
• Blocking Stress
• Approx. 830 MPa
• Zero net displacement
• At this point the SMA still underwent a small
  transformation, but the plastic deformation was
  significant enough that at austenite finish, the
  point of measurement was below the detwinned
  martensite reference point.

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Torque/Rotation Analysis

• Goal
• Intro to Shape Memory Alloys (SMAs)
• Overall Design Concept
• Characterization of the SMA Wire
• Torque/Rotation Analysis
• Experimental Characterization of Torque/Rotation
• Heat Transfer Analysis
• Thermal Control Circuit
• Summary
• Future Work
• Questions

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Torque/Rotation Analysis
Goal Find the displacement of the rack as a
function of the spring constant and the force P
applied to the rack by the gear.
Simplified Schematics
KSP
KSMA
x
P
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Model Development
Free Body Diagram
KSP
KSMA
F1
F1
F2
F3
d1
d2
d3
P
Equilibrium
Kinematic constraint
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SMA Analysis
Spring constant and thermal expansion coefficient
SMA displacement and strain
With gear ratios
Combining with FBD analysis
Where P applied force T applied torque d
displacement ? rotation angle
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Model Development
Transformation strain, H, is given by the
relationship previously determined
Where the applied stress, s, is given by
Where the spring constant, KSP, can be
approximated by
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Final Model
Through substitutions and simplifications
Measuring the change in ? between the austenite
phase (CM0) and the martensite phase (CM1)
Using given material properties and gear ratio
for prototype, the following results were
obtained for various values of the applied
torque, t.
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Model Results
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Experimental Characterization of Torque/Rotation

• Goal
• Intro to Shape Memory Alloys (SMAs)
• Overall Design Concept
• Characterization of the SMA Wire
• Torque/Rotation Analysis
• Experimental Characterization of Torque/Rotation
• Heat Transfer Analysis
• Thermal Control Circuit
• Summary
• Future Work
• Questions

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Experimental Results

• Max Rotation/Min Torque
• 24.81 _at_ 64.84 ozin
• Max Torque/Min Rotation
• 120 ozin _at_ 3.81
• Max Torque Density (1 wire)
• Current Design 8.89 oz/in2
• Idealized Design 21.82 oz/in2
• Pittman Motor Comparison 107 ozin 6.83 oz/in2
• Torque Scales with Addition of Multiple Wires

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Comparison of Results
K72.503 N/m
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Comparison of Results
K2806.057 N/m
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Comparison of Results
K3740.709 N/m
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Heat Transfer Analysis

• Goal
• Intro to Shape Memory Alloys (SMAs)
• Overall Design Concept
• Characterization of the SMA Wire
• Torque/Rotation Analysis
• Experimental Characterization of Torque/Rotation
• Heat Transfer Analysis
• Thermal Control Circuit
• Summary
• Future Work
• Questions

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Heat Transfer Analysis

• Goal Find time to heat and cool (actuation cycle
  time) SMA with different coolants
• Air Natural and forced convection
• Water Natural and forced convection

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SMA Temperature Sensitivity

• Temperature controls actuation
• Damaging Temperatures
• Only 10C above austenite finish temperature
• Temperature Response
• Very sensitive to magnitude of applied current
• Must be accurately modeled for environment and
  loading conditions

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Current Heating Model

• Material properties for each phase
• Temperature Response
• Exponential response expected
• Piecewise-continuous (specific heat changes
  drastically during transformation phase)

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Analysis of Ambient Systems

• Current normalized for each condition so the
  final temperature is 100C

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Thermal Control Circuit

• Goal
• Intro to Shape Memory Alloys (SMAs)
• Overall Design Concept
• Characterization of the SMA Wire
• Torque/Rotation Analysis
• Experimental Characterization of Torque/Rotation
• Heat Transfer Analysis
• Thermal Control Circuit
• Summary
• Future Work
• Questions

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Thermal Control Circuit

• Goal
• To electronically control the temperature of the
  SMA wire to provide the maximum mechanical power
  output without damaging (overheating) the wire.
• Method
• ?RSMA?A(?M-?A)Cm
• ?RSMA?Vdrop / ISMA
• ?Vdrop / ISMA ?A(?M-?A)Cm

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Solution to Circuit Requirements Cont.
Voltage increases as wire resistance decreases,
approaching austenite finish.
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Realization
Signal Control
Current Switch
SMA Wire
S
Voltage Sensor
Temperature Sensor
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Realization (Continued)
Signal Control
VCC
VCC
VCC
SMA
TempVoltage
Current Switch
Signal Addition
Temp in
Voltage in
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Benefits/Drawbacks

• Inexpensive
• Accurate with precise components
• Modular design
• Sensitive to extreme temperatures
• Operational amplifier offset voltage

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Summary

• Goal
• Intro to Shape Memory Alloys (SMAs)
• Overall Design Concept
• Characterization of the SMA Wire
• Torque/Rotation Analysis
• Experimental Characterization of Torque/Rotation
• Heat Transfer Analysis
• Thermal Control Circuit
• Summary
• Future Work
• Questions

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Summary

• Developed working prototype that surpasses
  conventional motors
• Developed mathematical and thermal models of SMA
  system behavior
• Developed electrical circuit schematics necessary
  to control thermal system
• High torque, time independent, weight sensitive
  applications

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Future Work

• Goal
• Intro to Shape Memory Alloys (SMAs)
• Overall Design Concept
• Characterization of the SMA Wire
• Torque/Rotation Analysis
• Experimental Characterization of Torque/Rotation
• Heat Transfer Analysis
• Thermal Control Circuit
• Summary
• Future Work
• Questions

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Future Work

• Develop test method that reduces friction losses
• Refine ratchet and brake mechanism
• Optimize prototype design
• Implement multidirectional prototype

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Questions?
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Track and Slide Mechanism

• Simple system purchased online
• Very low friction sliding of rack

Track and Slide Mechanism
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Rack and Gear System
PD Pitch Diameter P Diametrical Pitch (
teeth / PD)

• Gear P increased to minimize backlash
• 1.5 in. PD pinion (96 teeth)
• 1 in. PD output drive gear (64 teeth)
• 4 in. long rack
• Original Gear P 20
• Revised Gear P 64

Revised Gear System (P 64)
Original Gear System (P 20)
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SMA Clamping

• Two designs considered
• Clamp SMA wire between metal plates
• Cylinders reduce localized stresses in wire
• Design 2 has been implemented

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Ratchet Mechanism

• Several alternatives considered
• Standard palm ratchet
• Ratchet from combination ratcheting wrench
• Custom ratchet design
• Palm ratchet alternative chosen due to time and
  cost considerations
• Solution for automatically switching pawls has
  yet to be found

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Ratchet Unit

• Gear fastened to Ratchet
• Ratchet fastened to Socket
• Socket press-fit to Shaft

• Only internal ratchet mechanism is free to
  rotate

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Prototype
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Prototype
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Prototype
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Prototype
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Prototype
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Prototype