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 Non-compliant wiper Compliant crimp Compliant wiper Northwestern University 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 Monolithic and Planer Joint-less Small displacements or forces Impact -Suitable for microfabrication -No assembly (a necessity for MEMS) -Reduced size -Reduced cost of production -No friction or wear -No lubrication needed - Useful in achieving well controlled force or motion at the micro scale.

Northwestern University Compliant Mechanisms ME 381 – Fall 2004

Definitions

• Geometric Advantage:

GA



u out u in

• Mechanical Advantage:

MA



F out F in

• 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 work

 

out in

  

F out F in

   

dt out

   

dt in

– Optimization Problem: max

a i

, min 

a i



a i

, max

V



Volume Max

Re

source

 1 Northwestern University 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  

a i

, min 

a i



a i

, max

h

1   

F out F in



V



Volume Max

Re

source

1  

MA

 1 or

h

1  1   

u out u in FS

 

i

 max  1  1  

GA

 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 (Saggere & Kota 1994) (Culpepper, 2003) V-beam Thermal Actuator with force amplification (Hetrick & Gianchandani, 2001) Northwestern University The Self Retracting Fully Compliant Bistable Mechanism (L. Howell, 2003) http://www.engin.umich.edu/labs/csdl/video02.html

Compliant Mechanisms ME 381 – Fall 2004

• Universities 1

Institution

Univ. of Michigan 2 Brigham Young University 3 Univ. of Illinois at Chicago 4 Univ. of Penn 5 MIT 6 Technical University of Denmark

Contacts

Lab

Compliant Systems Design Laboratory Compliant Mechanism Research Micro Systems Mechanisms and Actuators Laboratory Computational Design Precision Compliant Systems Lab Topology optimization

Faculty

Sridhar. Kota Larry L. Howell Laxman Saggere G. Ananthasuresh Martin L. Culpepper Ole Sigmund • 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