Compliant Mechanisms
Presented By:
Ravi Agrawal, Binoy Shah, and Eric Zimney
Northwestern University
Compliant Mechanisms
ME 381 – Fall 2004
Outline
•
•
•
•
•
•
•
Working Principal
Advantages and Disadvantages
Compliance in MEMS devices
Design and Optimization
Analysis: Static and Dynamic
Example Devices
Conclusion
Northwestern University
Compliant Mechanisms
ME 381 – Fall 2004
Working Principle
Compliant Mechanism: A flexible structure that elastically deforms without
joints to produce a desired force or displacement.
• Deflection of flexible
members to store energy in
the form of strain energy
• Strain energy is same as
elastic potential energy in in
a spring
• Since product of force and
displacement is a constant.
There is tradeoff between
force and displacement as
shown in fig on left.
Northwestern University
Compliant Mechanisms
ME 381 – Fall 2004
Macro-scale Examples
Non-compliant crimp
Compliant crimp
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Non-compliant wiper
Compliant wiper
Compliant Mechanisms
ME 381 – Fall 2004
Benefits of Compliant Mechanisms
Advantages
1.
2.
3.
4.
5.
No Joints
No friction or wear
Monolithic
No assembly
Works with piezoelectric, shape-memory
alloy, electro-thermal, electrostatic, fluid
pressure, and electromagnetic actuators
Disadvantages
1.
2.
3.
Small displacements or forces
Limited by fatigue, hysteresis, and creep
Difficult to design
Northwestern University
Compliant Mechanisms
ME 381 – Fall 2004
Compliance for MEMS
Non-Compliant
Actuator - Old
Design
Compliant Actuator – New
design
Features
Impact
Monolithic and Planer
-Suitable for microfabrication
-No assembly (a necessity for MEMS)
-Reduced size
-Reduced cost of production
Joint-less
-No friction or wear
-No lubrication needed
Small displacements or
forces
- Useful in achieving well controlled force or motion at the micro
scale.
Northwestern University
Compliant Mechanisms
ME 381 – Fall 2004
Definitions
• Geometric Advantage:
u out
GA
u in
• Mechanical Advantage:
Fout
MA 
Fin
• Localized Verses Distributed Compliance
Northwestern University
Compliant Mechanisms
ME 381 – Fall 2004
Design of Distributed Compliant
Mechanisms
• Topology Synthesis
– Develop kinematic design to meet input/output
constraints.
– Optimization routine incompatible with stress
analysis.
• Size and Shape Optimization
– Enforce Performance Requirements to determine
optimum dimensions.
Northwestern University
Compliant Mechanisms
ME 381 – Fall 2004
Topology Synthesis
• Energy Efficiency Formulation
– Objective function:
work out

work in
F t u t dt


 F t u t dt
out
out
in
in
– Optimization Problem:
max  
ai ,min  ai  ai ,max
V  Volume
Max Re source
Northwestern University
1
Compliant Mechanisms
ME 381 – Fall 2004
Size and Shape Optimization
• Performance Criteria:
–
–
–
–
Geometric/Mechanical Advantage
Volume/Weight
Avoidance of buckling instabilities
Minimization of stress
concentrations
• Optimization Problem:
max  
ai ,min  ai  ai ,max
V  Volume
 F  1 
h1   out 
 1
F
MA


 in 
FS 
Max Re source
or
i
1
 u  1 
h1   out 
 1
u
GA


 in 
 max  1
Northwestern University
Compliant Mechanisms
ME 381 – Fall 2004
Stress Analysis
• Size and shape refinement
– Same Topology
– Optimized dimensions of the
beams
– Uniformity of strain energy
distribution
• Methods used
– Pseudo rigid-body model
– Beam element model
– Plane stress 2D model
Northwestern University
Compliant Mechanisms
ME 381 – Fall 2004
Dynamic Analysis
• Methods Used
– FEM Tools
• Example of Stroke Amplifier
– First four natural frequencies
are as 3.8 kHz, 124.0 kHz,
155.5 kHz and 182.1 kHz
– Fundamental frequency
dominates
• Dynamic characteristics
– Frequency ratio vs
Displacement Ratio
– Frequency ratio vs GA
Northwestern University
Compliant Mechanisms
ME 381 – Fall 2004
More MEMS applications
Double V-beam
suspension for
Linear Micro
Actuators
HexFlex
Nanomanipulator
(Culpepper, 2003)
(Saggere & Kota 1994)
V-beam
Thermal Actuator
with force
amplification
(Hetrick & Gianchandani, 2001)
The Self
Retracting FullyCompliant
Bistable
Mechanism
(L. Howell, 2003)
http://www.engin.umich.edu/labs/csdl/video02.html
Northwestern University
Compliant Mechanisms
ME 381 – Fall 2004
Contacts
• Universities
Institution
Lab
Faculty
1
Univ. of Michigan
Sridhar. Kota
2
Brigham Young University
Compliant Systems Design
Laboratory
Compliant Mechanism Research
3
Univ. of Illinois at Chicago
Laxman Saggere
4
Univ. of Penn
Micro Systems Mechanisms and
Actuators Laboratory
Computational Design
5
MIT
Precision Compliant Systems Lab
Martin L. Culpepper
6
Technical University of Denmark
Topology optimization
Ole Sigmund
Larry L. Howell
G. Ananthasuresh
• Industry
– FlexSys Inc
– Sandia National Lab
Northwestern University
Compliant Mechanisms
ME 381 – Fall 2004
Conclusion
• Stores potential energy and outputs displacement or
force
• Monolithic – no joints, no assembly, no friction
• Small but controlled forces or displacements
• Can tailor design to performance characteristics.
• Performance dependent on output
• Difficult to design
• Examples: HexFlex Nanomanipulator,
MicroEngine, Force Amplifier
Northwestern University
Compliant Mechanisms
ME 381 – Fall 2004