1. Autonomous Silicon Microrobots
Sarah Bergbreiter
Prof. Kris Pister
Berkeley Sensor and Actuator Center
UC Berkeley
ICAT 47th International Smart Actuator Symposium
October 3, 2006

2. What is an Autonomous Silicon Microrobot?
Size
Total size on order of millimeters
Mobility
Should be able to move around a given environment
Speeds of mm/sec
Manufacture
MEMS techniques used for batch fabrication
Autonomous
Power and control on-board
Communication between robots (?)
Size
Target Space
Input Power
Size ~ mm
Input Power ~ 10s mW
Speed ~ 10 sec / jump
Speed

3. Applications for Autonomous Silicon Microrobots
Mobile Sensor Networks
Search and Surveillance
Cooperative Construction
Planetary Exploration
Key Technologies
Large force, large displacement actuators
Mechanisms (transmission/linkages)
Energy storage for “High output power for short time” actuators

4. Actuator Requirements for Autonomous Microrobots
Low Input Power
Small Size
Force/Displacement
Efficient
Simple Fabrication and Integration
Power Supply Compatibility
Robust
Yeh, 2001
Lindsay, 2001
Kladitis, 2000
Pelrine, 2002
Wood, 2005
Lu, 2003

5. Electrostatic Gap Closing Actuators (GCAs)
High force at low input power and moderate voltage
Fabricated in single mask process l + - V d t k F

6. Electrostatic Inchworm Motor Operation

7. Electrostatic Inchworm Motor Operation

8. Electrostatic Inchworm Motor Operation

9. Electrostatic Inchworm Motor Operation

10. Electrostatic Inchworm Motor Operation

11. Electrostatic Inchworm Motor Operation

12. Electrostatic Inchworm Motor Operation

13. Electrostatic Inchworm Motor Operation

14. Electrostatic Inchworm Motor Operation

15. Electrostatic Inchworm Motor Operation

16. Electrostatic Inchworm Motor Operation

17. Solar Powered 10mg Silicon Robot Inchworm Motors
Designed 400mN 50V in 2.8 mm2 silicon area
Demonstrated 6.8 mm/s shuttle speed
Demonstrated 400 mm shuttle travel
Demonstrated 60 mN force at foot

18. Solar Powered 10mg Silicon Robot

19. Why Was This So Difficult?
Failure in motors due to load forces and/or teeth slipping
Complex process to integrate hinges and motors together
Two 1-DOF legs dragging robot not very robust or adaptable to different surfaces

20. Try Jumping Instead Improve Mobility Improve Efficiency
Obstacles are large!
Improve Efficiency
What time and energy is required to move a microrobot 1 m and what size obstacles can these robots overcome?
Ant (Walking)
Proposed (Jumping)
Hollar (Walking)
Ebefors (Walking)
Mass
11.9 mg
10 mg
80 mg
Time
15 sec
1 min
417 min
2.8 min
Energy
1.5 mJ
5 mJ
130 mJ
180 J
Obstacle Size **
1 cm
50 m
100 m

21. Building a Jumping Microrobot
Spring for energy storage
short legs imply short acceleration times
Higher force, larger displacement motor
Power for motors and control
Controller to direct motors
Landing and resetting for next jump are NOT discussed
From what we’ve already shown, the robot is going to require.
Emphasize that landing and resetting for next jump are not discussed.

22. Demonstrated Energy Storage
20 mJ of energy stored corresponds to 20 cm jump straight up for 10 mg robot

23. Building a Jumping Microrobot
Spring for energy storage
short legs imply short acceleration times
Higher force, larger displacement motor
Power for motors and control
Controller to direct motors
Landing and resetting for next jump are NOT discussed
From what we’ve already shown, the robot is going to require.
Emphasize that landing and resetting for next jump are not discussed.

24. Actuator Challenges for Jumping Microrobots
Motor work  kinetic energy for jump
Drag is not large effect at smaller energies
Actuator requirements
High energy density
Large deflection (5mm)
Large forces (10mN)
Simple process integration
Previous inchworm motor specs could only store 8 nJ

25. Increase Displacements
Remove silicon springs used to keep the shuttle in-plane
Use micro rubber band to return shuttle to original position
Assembled clips demonstrated without complex processing
Subramaniam Venkatraman, 2006

26. Higher Forces Decrease Gap k Increase Voltage V d Increase Area -
Disadvantage: new clutch design and lithography limits
Increase Voltage
Disadvantage: power and electronics
Increase Area
Disadvantage: greater area implies greater mass
Increase dielectric constant
Disadvantage: processing and small displacements l + - V d t k F

27. Reduce Gaps
Use insulating stops integrated in fingers of gap closers to determine final gap
Initial gap = g2
Final gap = g2 – g1
Charging issues minimized if insulator area is kept small
For example: g1 g2 g1
2 mm g2
2.5 mm
Nitride Insulator
Silicon Plate
E. Sarajlic, E. Berenschot, G. Krijnen, and M. Elwenspoek, "Versatile trench isolation technology for the fabrication of microactuators," Microlectronic Engineering, vol , pp , 2003.

28. Reduce Initial Gap 0V Drive Actuator Transmission
Drive force dependent on initial gap of the drive actuator
Add a transmission to reduce initial gap beyond lithographic limits
Provide an additional mechanical stop to limit return motion of drive frame
Force required minimized to just the restoring force of springs on drive frame
Reduces force density of actuator, but effect minimal + - 0V
Drive
Actuator
Transmission

29. Reduce Initial Gap 0V 50V Drive Actuator Transmission
Drive force dependent on initial gap of the drive actuator
Add a transmission to reduce initial gap beyond lithographic limits
Provide an additional mechanical stop to limit return motion of drive frame
Force required minimized to just the restoring force of springs on drive frame
Reduces force density of actuator, but effect minimal + -
50V
Drive
Actuator 0V
Transmission

30. Reduce Initial Gap 50V Drive Actuator Transmission
Drive force dependent on initial gap of the drive actuator
Add a transmission to reduce initial gap beyond lithographic limits
Provide an additional mechanical stop to limit return motion of drive frame
Force required minimized to just the restoring force of springs on drive frame
Reduces force density of actuator, but effect minimal + -
50V
Drive
Actuator
Transmission

31. Reduce Initial Gap 50V 0V Drive Actuator Transmission gnew
Drive force dependent on initial gap of the drive actuator
Add a transmission to reduce initial gap beyond lithographic limits
Provide an additional mechanical stop to limit return motion of drive frame
Force required minimized to just the restoring force of springs on drive frame
Reduces force density of actuator, but effect minimal + - 0V
Drive
Actuator
50V
Transmission
gnew

32. Fabricated Transmission
4mm to 3mm initial gap demonstrated
Suspension problems prevented proper clutch operation

33. New Clutch Design
Need to effectively transmit drive force to the shuttle
If gear teeth are used on the shuttle, reducing step size impacts drive actuators required
If one drive actuator used:
Step size limited to 4 mm
Solution: Remove teeth
Coefficient of friction of ~1 observed
Clutch force dependent on final gap so area does not explode

34. How Much Does This Help? g g g g W All numbers calculated at 50V
Previous Designs (demonstrated)
Current Designs (proposed)
Theory
Force Density (mN/mm2)
0.14
1.6
~50
Power Density (W/g)
0.01
~600 g g g W
All numbers calculated at 50V
Multiply by 10 to get approximate numbers at 150V
Theory assumptions
AR = 25:1, g = 1 m
Spring constant based on given force
Spring constant much higher than could be used in practice due to damping and clocking

35. Conclusions and Future Work
Silicon inchworm motors provide high forces and displacements at low input powers for autonomous silicon microrobots
Assembled clips provide higher linear displacements
Nitride isolation structures and transmission provide smaller gaps for increased forces
Demonstrate a silicon inchworm motor with > 1mN force and > 1 mm displacement (in progress)
Add with energy storage system to build fully functional autonomous jumping microrobot