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ASME IMECE 2006
Elastomer-Based Micromechanical Energy
Storage System
Sarah BergbreiterProf. Kris Pister
Berkeley Sensor and Actuator CenterUniversity of California, Berkeley
ASME IMECE 2006
Goals and Motivation
• Build a micromechanical system to – Store large amounts of energy (10s of J) in small area
and mass– Integrate easily with MEMS actuators without complex
fabrication
• Motivation– Jumping microrobots– Injector systems– MEMS catapults– Any “high output power
for short time” actuated MEMS system
ASME IMECE 2006
Why Elastomer?
• High Energy Density– Capable of storing large amounts of energy with small
area and volume– 2mm x 50m x 50m rubber band can store up to 45J
• Large Strains– Stress/strain profile suitable for low-power electrostatic
actuators with large displacements– Actuator providing 10mN force over 5mm displacement
would require
Material E (Pa) Maximum Strain (%)
Tensile Strength (Pa)
Energy Density (mJ/mm3)
Silicon 169x109 0.6 1x109 3
Silicone 750x103 350 2.6x106 4.5
Resilin 2x106 190 4x106 4
ASME IMECE 2006
Simplifying Fabrication
• Fabricate elastomer and silicon separately– Simple fabrication– Wider variety of
elastomers available
• Silicon process– Actuators– Assembly points for
elastomers
• Elastomer process– 2 methods to fabricate
micro rubber bands
100 m
+
ASME IMECE 2006
100 m
Fabrication: Silicon
• Two Mask SOI process– Frontside and backside
DRIE etch
• Electrostatic Inchworm Actuators– Many mN force and
several mm displacement in theory
• Hooks– Assembly points for
elastomer25 m
ASME IMECE 2006
Fabrication: Laser-Cut Elastomer
• Simple fabrication– Spin on Sylgard® 186
and cut with VersaLaser™ commercial IR laser cutter
– No cleanroom required
• Poor Quality– 10-20% yield due
to poor precision of laser cutter
– Mean 250% elongation at break
ASME IMECE 2006
Fabrication: Molded Elastomer
• Complex Fabrication– DRIE and passivated
silicon mold– Sylgard® 186 poured
into mold, scraped off and removed with tweezers
• High Quality– Close to 100% yield– Mean 350%
elongation at break
ASME IMECE 2006
Fabrication: Assembly
• Fine-tip tweezers using stereo inspection microscope
• Mobile pieces need to be tethered during assembly
• Yield > 80% and rising 100 m
ASME IMECE 2006
Spring Performance: Laser-Cut
• Using force gauge shown previously, pull with probe tip to load and unload spring
• Trial #1– 165% strain– 7.2 J– 81% recovered
• Trial #2– 183% strain– 8.2 J– 85% recovered
• 8 J would propel a 10mg microrobot 8 cm
ASME IMECE 2006
Spring Performance: Molded
• Using force gauge shown previously, pull with probe tip to load and unload spring
• Trial #1– 200% strain– 10.4 J– 92% recovered
• Trial #2– 220% strain– 19.4 J– 85% recovered
• 20 J would propel a 10mg microrobot 20 cm
ASME IMECE 2006
Quick Release of Energy
• Electrostatic clamps designed to hold leg in place for quick release– Normally-closed
configuration for portability
• Shot a 0.6 mg 0402-sized capacitor 1.5 cm along a glass slide
• Energy released in less than one video frame (66ms)
ASME IMECE 2006
Integrating with Actuator
• Electrostatic inchworm motor translates 30m to store an estimated 4.9nJ of energy and release it quickly
• Motors will be more aggressively designed in the future to substantially increase this number
ASME IMECE 2006
Conclusions and Future Work
• Process developed for integrating elastomer springs with silicon microstructures
• Almost 20 J of energy stored in molded micro rubber bands– Equivalent jump height of 20 cm for 10 mg microrobot
• Build higher force motors to store this energy• Characterize new elastomer materials like latex
and other silicones• Keep the leg in-plane
through integrated staples• Put it all together for an
autonomous jumping microrobot!
Subramaniam Venkatraman, 2006
ASME IMECE 2006
Acknowledgments
Ron Fearing Group for use of VersaLaser™ commercial laser cutter
UC Berkeley Microfabrication Laboratory