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

2. 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

3. 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

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

5. 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

6. 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

7. 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

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

9. 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

10. 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

11. 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)

12. 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

13. 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

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