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