Autonomous Jumping Microrobots

1
Autonomous Jumping Microrobots

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

2
Overview

• Motivation and Previous Work
• Jumping for Locomotion
• Robot Design
• Actuation
• Energy Storage
• Power
• Control
• Fabrication and Integration

3
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
Size mm Power 100 mW Speed 10 sec / jump
Target Space
Power
Speed
4
Sensor Networks and Robots
1mm
Add Legs
Smart Dust (Warneke, et al. Sensors 2002)
Microrobots (Hollar, Flynn, Pister. MEMS 2002)
Add Robot Body
COTS Dust (Hill, et al. ACM OS Review 2000)
CotsBots (Bergbreiter, Pister. IROS 2003)
5
Previous Research CotsBots and Photobeacon
Localization
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
6
Jumping Insects

• Froghopper
• Mass 12.3 0.7 mg
• Length 6.1 0.2 mm
• Takeoff Angle 58 2.6o
• Takeoff Velocity 2.8 0.1 ms-1
• Energy 49 mJ
• 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 60o
• 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
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
Microrobots
9
Overview

• Motivation and Previous Work
• Jumping for Microrobot Locomotion
• Robot Design
• Actuation
• Energy Storage
• Power
• Control
• Fabrication and Integration

10
Jumping Trajectory

• Muscle/motor work ? kinetic energy for jump
• How high?
• How far?
• Can use to jump over obstacles

11
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-Clarks projectile experiments showed
  insects generally have Cd 1.5 with wings

Energy (mJ) Velocity (m/s) Height in Vacuum (cm) Height in Air (cm) Efficiency
5 0.8 3.4 3.3 1.0
10 1.2 6.8 6.3 0.9
25 1.8 17.0 14.2 0.8
50 2.6 34.0 24.8 0.7
Mass 15 mg, ACd 30 mm2, Angle 90o
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.
12
Jumping Energy Storage

• Short acceleration times with short legs require
  energy storage for most actuators
• For a linear spring, apply force over a distance

13
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

Kinetic Energy v. Time, Mass 15mg, k 2 N/m
Energy (mJ)
Time (msec)
14
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 min 417 min 2 min, 50 sec 25 sec
Energy 5 mJ 130 mJ 180 J 300 mJ
Obstacle Size 5 cm 50 mm 100 mm 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?contentresea
rch/systems/Alice/alice.php
15
Overview

• Motivation and Previous Work
• Jumping for Locomotion
• Robot Design
• Actuation
• Energy Storage
• Power
• Control
• Fabrication and Integration

16
Robot Design Requirements

• 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

17
Actuation Design Considerations

• Long throw ( 5 mm)
• High force ( 10 mN)
• Low power and moderate voltage (50 mW, 50 V)
• Low mass ( 5 mg)
• Simple fabrication and integration
• Reasonable speed

18
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 mN of force and
  256 mm of travel in 2.8 mm2

19
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
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

21
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

For example
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.
67-68, pp. 430-7, 2003.
22
Actuation Add Nitride to Process
(1)
(2)
(3)
(4)
(5)
(6)
23
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

24
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 mm
• Two possible solutions
• Simplest design uses friction only to engage
• Keep gear teeth, but use multiple sets of teeth
  to engage

25
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 m 0.8 0.3
• Stepper motor in single mask 5 mm polysilicon
• 2 mm steps, 15 mm deflection at 3 mN 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
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

4mm
27
Actuation Area Requirements

• 10 mN drive actuator with initial gap of 1.5 mm
  at 50 V requires 2 mm2
• 25 mN clutch actuator with final gap of 0.5 mm at
  50 V requires 0.6 mm2
• If actuator area approximates surface area, total
  minimum area required for inchworm 3.2 mm2

28
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
Springs Materials
l

• Maximum distance traveled
• Maximum force which can be applied
• Energy stored

A
Material E (Pa) Strength (Pa) Energy Density (mJ/mm3)
Si 1.6e11 3.2e9 2
Silicone 1e6 2.25e6 2.5
Polyimide 2e9 231e6 13.3
Parylene 2e9 69e6 1.2
Resilin 2e6 6e6 9
For 5 mm travel at 10 mN Si l 1 m, A 12.5
mm2 Polyimide l 43 mm, 43 mm2 Silicone l
2.2 mm, A 4400 mm2
30
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 mm thick latex-like material)
• Could also spin on liquid elastomer material
  (latex, silicone) to desired thickness
• Use NdYAG 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
Springs Integration

• Assemble elastomer onto silicon motor
• Two strategies
• SOI hook for rubber bands
• SOI clamp for
  rubber strips

32
Springs Load Tests (Macro-scale)
Rubber
Top Clamp
Ruler
Bottom Clamp
Box
Weights (5 hex nuts wire) 4.47g
Latex strip with all dimensions 10x
33
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
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 mm2
• Chip mass 2.3 mg

35
ControlDesign Considerations

• Low power (10 mW)
• Small size
• Simple integration
• Programmability
• Off-the-shelf

36
Control EM6580 mController

• EM Microelectronic
• Power
• 2 5.5 V supply
• 5.8 mA active
• 3.3 mA standby
• 0.3 mA 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
Overview

• Motivation and Previous Work
• Jumping for Locomotion
• Robot Design
• Actuation
• Energy Storage
• Power
• Control
• Fabrication and Integration

38
Fabrication

• Add a 3rd 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
Integration In-plane Catapult

• Test initial integration of high force, long
  throw motors and elastomers with an in-plane
  system
• Dont have to worry about addition of foot,
  stability and take-off angle
• With 25 mJ of stored energy, can shoot a 1 mm2
  radio IC 7 m
• m 0.7 mg, q 45o
• Does not include drag or frictional effects

M. S. Rodgers, J. J. Allen, K. D. Meeks, B. D.
Jensen, and S. L. Miller, "A microelectromechanica
l high-density energy storage/rapid release
system," presented at SPIE, 1999.
40
Integration Full Robot
  Mass (mg) Dimensions (mm) Power (mW)
Motors _at_ 500 Hz 8.8 4 x 8 x 0.3 30
Spring - 2.5 x .03 x .05 0
Solar Cells High Voltage Buffers 2.3 3.6 x 1.8 x 0.15 100
EM6580 mController 3.5 2 x 2.7 x 0.28 11.6
Total Robot 14.6 4 x 8 x 0.6 58.4
Wire bonding
Solar Cells
EM6580
Shuttle, Springs, and Motor
41
Expected Contributions
Put it all together and jump!
42
Backup Slides
43
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
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

45
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
Actuation Staple Test Structures
47
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
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 mstatic

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-Micromachine
d Inchworm Acuator," Journal of
Microelectromechanical Systems, vol. 13, pp.
63-74, 2004.
49
Actuation Clutch Design
50
Actuation Layout Considerations

• Spacing for nitride gaps
• For clamped-clamped beam with force acting on
  middle
• Lmax 200 mm
• ymax 0.2 mm, g 0.1 mm, b 10 mm, 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

t
w
F
l
g0
zg0
51
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
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
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 100oC 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
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(d) means smaller energy loss
• Silicone lt 0.001 at 100 kHz
• Polyurethane 0.02 at 100 kHz

55
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

Release all clutches to jump
Sequence inchworm motors to stretch spring
Delay for flight and reset
A B C D
Top Clutch 0 1 1 1
Top Drive 0 0 1 1
Bottom Clutch 1 1 0 1
56
Localization Ideas for Large Numbers of Robots

• Triangulation v. Trilateration
• Use light/lens/detector system on each robot to
  determine relative angles
• Design an IC with 1o resolution and 5-10m
  ranges with conventional off-the-shelf LEDs
• IC should be computationally simple
• Additional benefits of optical communication and
  obstacle detection

57
Localization System Architecture
Low divergence, high power LEDs
190o field-of-view lens
58
Localization Photobeacon IC
3-wire bus
Multiplexer Blocks
1.3mm
4mm
1.8mm
Modified Optical Receiver
256 Photodiodes
59
Jumping Robots Video
Scout, 2000
Burdick and Fiorini, 2003