Elastomer-Based Micromechanical Energy Storage System Sarah Bergbreiter Prof. Kris Pister Berkeley Sensor and Actuator Center University of California, Berkeley

Goals and Motivation Build a micromechanical system to Motivation 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

Energy Density (mJ/mm3) Why Elastomer? High Energy Density Capable of storing large amounts of energy with small area and volume 2mm x 50m x 50m rubber band can store up to 45J 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

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 mm

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 elastomer 100 mm 25 mm

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

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

Fabrication: Assembly Fine-tip tweezers using stereo inspection microscope Mobile pieces need to be tethered during assembly Yield > 80% and rising 100 mm

Spring Performance: Laser-Cut Using force gauge shown previously, pull with probe tip to load and unload spring Trial #1 165% strain 7.2 mJ 81% recovered Trial #2 183% strain 8.2 mJ 85% recovered 8 mJ would propel a 10mg microrobot 8 cm

Spring Performance: Molded Using force gauge shown previously, pull with probe tip to load and unload spring Trial #1 200% strain 10.4 mJ 92% recovered Trial #2 220% strain 19.4 mJ 85% recovered 20 mJ would propel a 10mg microrobot 20 cm

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)

Integrating with Actuator Electrostatic inchworm motor translates 30mm 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

Conclusions and Future Work Process developed for integrating elastomer springs with silicon microstructures Almost 20 mJ 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

Acknowledgments Ron Fearing Group for use of VersaLaser™ commercial laser cutter UC Berkeley Microfabrication Laboratory