1. Autonomous Jumping Microrobots Sarah Bergbreiter Ph.D. Qualifying Exam June 23, 2005 Department of Electrical Engineering and Computer Science, UC Berkeley

2. 2/41 Overview Motivation and Previous Work Jumping for Locomotion Robot Design –Actuation –Energy Storage –Power –Control Fabrication and Integration

3. 3/41 Possible Applications Mobile Sensor Networks Flea Transport Planetary Exploration Bi-modal Transportation –Work with flying robots –Work with walking robots for gross adjustments Make Silicon Move! Size Power Speed Target Space Size: mm Power: 100  W Speed: 10 sec / jump

4. 4/41 Sensor Networks and Robots Solar Cell Array CCRs XL CMOS IC Smart Dust (Warneke, et al. Sensors 2002 ) Microrobots (Hollar, Flynn, Pister. MEMS 2002 ) Add Legs Add Robot Body 1mm COTS Dust (Hill, et al. ACM OS Review 2000 ) CotsBots (Bergbreiter, Pister. IROS 2003 )

5. 5/41 Previous Research: CotsBots and Photobeacon Localization Mica Mote MotorBoard Kyosho Mini-Z RC Car Platform (or others…) Part Cost (quantity 50) RC Car/Tank $54.95 Mica Mote $125 MotorBoard $37.12 Parts $14.82 Board $6.30 Assembly $16 Total $217.07 Fisheye Lens High Power LED PhotoBeacon IC ~4mm 256 Photodiodes Multiplexer Blocks 3- wire bus Modified Optical Receiver 1.3mm 1.8mm

6. 6/41 Jumping Insects Froghopper –Mass = 12.3 ± 0.7 mg –Length = 6.1 ± 0.2 mm –Takeoff Angle = 58 ± 2.6 o –Takeoff Velocity = 2.8 ± 0.1 ms -1 –Energy = 49 J –Force = 34 mN –Jump Height = 42.8 ± 2.61 cm –Energy stored in resilin Fruit-fly Larva –Soft-bodied and legless –Mass = 17 mg –Take-off Angle = 60 o –Take-off Velocity = 1.17 ms -1 –Jump Height = 7 cm –Jump Distance = 12 cm –Energy stored in cuticle M. Burrows, "Froghopper insects leap to new heights," Nature, vol. 424, p. 509, 2003. D. P. Maitland, "Locomotion by jumping in the Mediterranean fruit-fly larva Ceratitis capitata," Nature, vol. 355, pp. 159-161, 1992.

7. 7/41 Jumping Robots Burdick and Fiorini, 2003 –Mass = 1.3 kg –Jump height = 0.9 m –Jump distance = 1.8 – 2.0 m –Energy = 125 J / jump –Steel spring for energy storage Scout Robot, 2000 –Mass =.2 kg –Jump height =.3 m –Energy = 25 J / jump –Leaf spring Hopping Robots –Raibert and others –Require dynamic balance J. Burdick and P. Fiorini, "Minimalist Jumping Robots for Celestial Exploration," International Journal of Robotics Research, vol. 22, pp. 653-74, 2003. S. A. Stoeter, I. T. Burt, and N. Papanikolopoulos, "Scout robot motion model," presented at IEEE International Conference on Robotics and Automation, Taipei, Taiwan, 2003.

8. 8/41 Wood, et al, 2003 Microrobots Seiko, 1992Yeh, 1995-2001 Hollar, et al, 2002 Ebefors, et al, 1999 Sandia, 2001

9. 9/41 Overview Motivation and Previous Work Jumping for Microrobot Locomotion Robot Design –Actuation –Energy Storage –Power –Control Fabrication and Integration

10. 10/41 Jumping: Trajectory Muscle/motor work  kinetic energy for jump How high? How far? Can use to jump over obstacles height (cm) distance (cm) Hopping Trajectory, Mass = 15mg, Angle = 60deg

11. 11/41 Jumping: Drag Effects Frontal area to mass ratio increases for smaller objects Low energies translate to small take-off velocities which reduces drag effects Drag coefficient estimate –Bennet-Clark’s projectile experiments showed insects generally have Cd ~ 1.5 with wings Bennet-Clark, H. C., and G. M. Alder. "The Effect of Air Resistance on the Jumping Performance of Insects." The Journal of Experimental Biology 82 (1979): 105-121. Mass = 15 mg, A*C d = 30 mm 2, Angle = 90 o Energy (  J) Velocity (m/s)Height in Vacuum (cm)Height in Air (cm)Efficiency 50.83.43.31.0 101.26.86.30.9 251.817.014.20.8 502.634.024.80.7

12. 12/41 Jumping: Energy Storage Short acceleration times with short legs require energy storage for most actuators For a linear spring, apply force over a distance Force (mN) Distance (mm) Energy Storage in Linear Spring

13. 13/41 Jumping: Energy Release Kinetic energy realized by leg release Assuming a linear spring in tension Burdick and Fiorini reported seeing early lift- off which reduced the kinetic energy delivered to robot by spring Energy (  J) Time (msec) Kinetic Energy v. Time, Mass = 15mg, k = 2 N/m

14. 14/41 Jumping: Microrobot comparison What time and energy is required to move a microrobot 1 m and what size obstacles can these robots overcome? Proposed (Jumping) Hollar (Walking) Ebefors (Walking) Alice (Rolling) Time 1 min417 min2 min, 50 sec25 sec Energy 5 mJ130 mJ180 J300 mJ Obstacle Size 5 cm 50 m100 m 5 mm S. Hollar, "A Solar-Powered, Milligram Prototype Robot from a Three-Chip Process," in Mechanical Engineering: University of California, Berkeley, 2003. T. Ebefors, J. U. Mattsson, E. Kalvesten, and G. Stemme, "A walking silicon microrobot," presented at International Conference on Sensors and Actuators (Transducers '99), Sendai, Japan, 1999. http://asl.epfl.ch/index.html?content=research/systems/Alice/alice.php

15. 15/41 Overview Motivation and Previous Work Jumping for Locomotion Robot Design –Actuation –Energy Storage –Power –Control Fabrication and Integration

16. 16/41 High force, long stroke motor Spring for energy storage Power for motors and control Control to direct motors Landing and resetting for next jump are NOT discussed Robot Design Requirements

17. 17/41 Actuation: Design Considerations Long throw (~ 5 mm) High force (~ 10 mN) Low power and moderate voltage (~50 W, ~50 V) Low mass (~ 5 mg) Simple fabrication and integration Reasonable speed

18. 18/41 Actuation: Inchworm Motor Silicon gap closing actuators provide high force at low power and moderate voltage Inchworm actuation accumulates short displacements for long throw May be fabricated in single mask SOI process Hollar inchworm designed for 500 N of force and 256 m of travel in ~ 2.8 mm 2 + - 0V Clutch Actuator Drive Actuator Shuttle + - + - 0V 50V Clutch Actuator Drive Actuator Shuttle + - + - 50V Clutch Actuator Drive Actuator Shuttle + -

19. 19/41 Actuation: Increase Throw Motor throw previously limited by silicon flexures to constrain the shuttle in the actuator plane and provide restoring force to the shuttle To keep process complexity to a minimum, use assembled “staples” to constrain shuttle These structures will add contact friction

20. 20/41 Actuation: Higher Forces Decrease Gap –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

21. 21/41 Actuation: 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 g1 g2 E. Sarajlic, E. Berenschot, G. Krijnen, and M. Elwenspoek, "Versatile trench isolation technology for the fabrication of microactuators," Microlectronic Engineering, vol. 67-68, pp. 430-7, 2003. For example: g1 2 m g2 2.5 m Nitride Insulator Silicon Plate

22. 22/41 Actuation: Add Nitride to Process (1)(2) (3)(4) (5)(6)

23. 23/41 Actuation: Reduce Initial Gap 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 + - 0V Transmission Actuator + - 50V Drive Actuator + - 0V Transmission Actuator + - 50V Drive Actuator + - 50V Transmission Actuator + - 0V Drive Actuator + - 50V Transmission Actuator g new

24. 24/41 Actuation: Clutch Design Need to effectively transmit drive force to the shuttle If gear teeth are used on the shuttle, reducing step size requires a new clutch –If one drive actuator used: –Step size limited to 4 m Two possible solutions –Simplest design uses friction only to engage –Keep gear teeth, but use multiple sets of teeth to engage

25. 25/41 Actuation: Friction Clutch Design High force required to prevent slipping Clutch force dependent on final gap which reduces area requirements Tas, et al. estimated the friction coefficient of this clamp/shuttle interaction at  = 0.8 ± 0.3 –Stepper motor in single mask 5 m polysilicon –2 m steps, 15 m deflection at 3 N limited by flexures used –Adhesion found low enough to release clamp N. R. Tas, A. H. Sonnenberg, A. F. M. Sander, and M. C. Elwenspoek, "Surface micromachined linear electrostatic stepper motor," presented at IEEE Tenth Annual International Workshop on Micro Electro Mechanical Systems, New York, NY, 1997.

26. 26/41 Motor: Toothed Clutch Design Teeth will require a vernier structure where the full clamp consists of several teeth connected by flexures –Flexures should allow teeth to flex up if not engaged –Should not bend when drive force applied Using gear teeth will also require a well-defined layout and process flow to prevent rounding Rounded teeth New “square” teeth F clutch 3m3m 4m4m

27. 27/41 Actuation: Area Requirements 10 mN drive actuator with initial gap of 1.5 m at 50 V requires ~ 2 mm 2 25 mN clutch actuator with final gap of 0.5 m at 50 V requires ~ 0.6 mm 2 If actuator area approximates surface area, total minimum area required for inchworm ~ 3.2 mm 2 area (mm 2 ) gap (  m) Area v. Gap F = 10mN

28. 28/41 Springs: Design Considerations Support large deflection (5 mm) Withstand large force (10 mN) Low internal viscosity to prevent energy loss Low mass Simple process integration

29. 29/41 Springs: Materials Maximum distance traveled Maximum force which can be applied Energy stored MaterialE (Pa)Strength (Pa)Energy Density (mJ/mm 3 ) Si1.6e113.2e92 Silicone1e62.25e62.5 Polyimide2e9231e613.3 Parylene2e969e61.2 Resilin2e66e69 A For 5 mm travel at 10 mN Si: l = 1 m, A = 12.5  m 2 Polyimide: l = 43 mm, 43  m 2 Silicone: l = 2.2 mm, A = 4400  m 2 l

30. 30/41 Springs: Fabrication Elastomers appear to be a good choice due to high strains available To fabricate micro rubber bands –Use thin elastomer materials already available off-the-shelf (30 m thick latex-like material) –Could also spin on liquid elastomer material (latex, silicone) to desired thickness –Use Nd:YAG laser to cut desired pattern in elastomer –Assemble micro-band into silicon motor M. Schuettler, S. Stiess, B. V. King, and G. J. Suaning, "Fabrication of implantable microelectrode arrays by laser cutting of silicone rubber and platinum foil," Journal of Neural Engineering, vol. 2, pp. 121-128, 2005.

31. 31/41 Springs: Integration Assemble elastomer onto silicon motor Two strategies: –SOI hook for rubber bands –SOI clamp for rubber strips

32. 32/41 Springs: Load Tests (Macro-scale) Top Clamp Bottom Clamp Box Weights (5 hex nuts + wire) ~4.47g Ruler Rubber Latex strip with all dimensions ~ 10x

33. 33/41 Power: Design Considerations Provide power for multiple jumps Minimal additional circuitry to control actuators Small mass and area Simple integration to motors and control element

34. 34/41 Power: Solar Cells Bellew and Hollar used a trench isolation process to stack solar cells for higher voltages (Icarus) Many of these die are still available –1 V, 3 V, and 50 V supplies –8 3V digital input channels connected to high voltage buffers –8 corresponding 50 V output channels Solar cells demonstrated at ~ 10% efficiency Chip area: 3.6 x 1.8 mm 2 Chip mass: 2.3 mg

35. 35/41 Control:Design Considerations Low power (~10 W) Small size Simple integration Programmability Off-the-shelf

36. 36/41 Control: EM6580 Controller EM Microelectronic Power –2 – 5.5 V supply –5.8 A active –3.3 A standby –0.3 A sleep 5 output channels Flash memory (4k x 16 bit) Die package No external components required 32 kHz RC oscillator Small size –2 x 2.7 x 0.28 mm –3.5 mg

37. 37/41 Overview Motivation and Previous Work Jumping for Locomotion Robot Design –Actuation –Energy Storage –Power –Control Fabrication and Integration

38. 38/41 Fabrication Add a 3 rd mask to remove wafer backside and lighten the robot Use clamp techniques developed by Last and Subramaniam for assembly 3 mask + assembly process M. Last, V. Subramaniam, and K. S. J. Pister, "Out of plane motion of assembled microstructures using a single-mask SOI process," presented at International Conference on Solid-State Sensors, Actuators and Microsystems, Seoul, Korea, 2005.

39. 39/41 Integration: In-plane Catapult Test initial integration of high force, long throw motors and elastomers with an in-plane system –Don’t have to worry about addition of “foot”, stability and take-off angle With 25 J of stored energy, can shoot a 1 mm 2 radio IC ~ 7 m –m = 0.7 mg,  = 45 o –Does not include drag or frictional effects M. S. Rodgers, J. J. Allen, K. D. Meeks, B. D. Jensen, and S. L. Miller, "A microelectromechanical high-density energy storage/rapid release system," presented at SPIE, 1999.

40. 40/41 Integration: Full Robot Solar Cells EM6580 Shuttle, Springs, and Motor Wire bonding Mass (mg)Dimensions (mm) Power (  W) Motors @ 500 Hz 8.84 x 8 x 0.330 Spring -2.5 x.03 x.050 Solar Cells + High Voltage Buffers 2.33.6 x 1.8 x 0.15100 EM6580  Controller 3.52 x 2.7 x 0.2811.6 Total Robot 14.64 x 8 x 0.658.4

41. 41/41 Expected Contributions High force, long throw motors + fabrication process Make rubber bands Build an in-plane catapult by assembling rubber bands with high force motors Integrate power and control Put it all together and jump!

42. Backup Slides

43. 43/41 Jumping: Physics 101 Kinetic energy (Work done to jump) Based on takeoff angle, break up velocity into vertical and horizontal components Find height achieved with this velocity Time in downward trajectory Lateral distance traveled

44. 44/41 Jumping: Drag Effects With air resistance as a factor, there will be an optimal mass for the robot –If mass is small, drag forces increase –If mass is large, gravitational forces increase A mass of several mg would be best for these energies height (cm) Mass (mg) Height v. Mass at 60 o

45. 45/41 Jumping: Energy Losses Energy from leg gets left behind Energy of rotation is lost –For rectangular prism rotating about COM –Click beetle loses ~ 40% – 50% in rotation (whole body oscillates) –Locust loses about 0.5% of energy (long thin leg) Viscous losses in spring material Potential early lift-off

46. 46/41 Actuation: Staple Test Structures

47. 47/41 Actuation: Charge Accumulation Three causes of charge accumulation –Contact electrification (identical materials should reduce this) –Breakdown (static and other factors) –RC charging from very small currents resulting from electric field across the insulator Shrinking insulating area recommended to reduce extra charge from breakdown and RC effects K. M. Anderson and J. E. Colgate, "A model of the attachment/detachment cycle of electrostatic micro actuators," presented at ASME Micromechanical Sensors, Actuators, and Systems, DSC-vol 32, Atlanta, GA, 1991. J. Wibbeler, G. Pfeifer, and M. Hietschold, "Parasitic charging of dielectric surfaces in capacitive microelectromechanical systems (MEMS)," Sensors & Actuators A-Physical, vol. 71, pp. 74-80, 1998.

48. 48/41 Actuation: Increasing Friction Monolayer coatings showed changes in coefficient of friction from 0.14 – 1.04 and load independent –Up to 1.5 mN O2 plasma had highest  static M. P. d. Boer, D. L. Luck, W. R. Ashurst, R. Maboudian, A. D. Corwin, J. A. Walraven, and J. M. Redmond, "High-Performance Surface-Micromachined Inchworm Acuator," Journal of Microelectromechanical Systems, vol. 13, pp. 63-74, 2004.

49. 49/41 Actuation: Clutch Design Sample Flexure Design for one group of teeth on clutch

50. 50/41 Actuation: Layout Considerations Spacing for nitride gaps –For clamped-clamped beam with force acting on middle –L max = 200 m y max = 0.2 m, g = 0.1 m, b = 10 m, V = 50 V Cell Size –Back gap determines opposing electrostatic force z = 4 for 16x less force –Mask alignment of nitride stops will determine cell width l t w F g0g0 zg 0

51. 51/41 Actuation: Squeeze Film Damping Squeeze film damping becomes a factor when gaps are small compared to beam size Trying to push air out of the way

52. 52/41 Springs: Examples in Biology Resilin is rubber-like – compliant but weak –Almost perfect cross-links (reduces viscosity) –Used in tension in dragonflies, but generally made short and fat Cuticle is strong and stiff –Crystalline –Often used in tension H. C. Bennet-Clark, "Energy Storage in Jumping Insects," in The Insect Integument, H. R. Hepburn, Ed. Amsterdam: Elsevier Scientific Publishing Company, 1976, pp. 421-443.

53. 53/41 Springs: Fabrication (Molding) Fabricate silicon mold Place liquid elastomer on adhesive film –Polyester film used Press die onto elastomer Place in vacuum to remove bubbles Cure at 100 o C for 1 hour Pry die off film –No problems reported in removing PDMS from silicon die J. I. Hout, J. Scheurer, and V. Casey, "Elastomer microspring arrays for biomedical sensors fabricated using micromachined silicon molds," Journal of Micromechanics and Microengineering, vol. 13, pp. 885-891, 2003.

54. 54/41 Springs: Chemistry Reducing entropy in the system when stretching by ordering polymer chains Release returns these chains to their random state Dissipation factor characterizes losses due to heat while dynamically stretching or compressing elastomer –E’’ is complex modulus (governs viscosity) –E’ is real modulus (governs elasticity) –Smaller tan() means smaller energy loss Silicone < 0.001 at 100 kHz Polyurethane ~ 0.02 at 100 kHz

55. 55/41 Control: Sequencer Jumping does not require dynamic stability, so jumping action may be accomplished through simple FSM Each inchworm requires 3 signals and 4 steps Sequence inchworm motors to stretch spring Release all clutches to jump Delay for flight and reset ABCD Top Clutch0111 Top Drive0011 Bottom Clutch1101

56. 56/41 Localization Ideas for Large Numbers of Robots Fisheye Lens High Power LED PhotoBeacon IC Triangulation v. Trilateration Use light/lens/detector system on each robot to determine relative angles Design an IC with ~1 o resolution and 5-10m ranges with conventional off- the-shelf LEDs IC should be computationally simple Additional benefits of optical communication and obstacle detection    d

57. 57/41 Localization: System Architecture 256x1 Analog Mux....... Programmable Gain Amplifier Photodiodes in circular array 3-wire bus Differential Analog Output 190 o field-of- view lens Lens LED Low divergence, high power LEDs

58. 58/41 Localization: Photobeacon IC ~4mm 256 Photodiodes Multiplexer Blocks 3-wire bus Modified Optical Receiver 1.3mm 1.8mm

59. 59/41 Jumping Robots: Video Burdick and Fiorini, 2003 Scout, 2000