Autonomous Jumping Microrobots

Sarah BergbreiterPh.D. Qualifying Exam

June 23, 2005

Department of Electrical Engineering and Computer Science, UC Berkeley

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Overview

• Motivation and Previous Work• Jumping for Locomotion• Robot Design

– Actuation– Energy Storage– Power – Control

• Fabrication and Integration

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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

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Sensor Networks and Robots

Solar Cell Array

CCRs

XLCMOS

IC

Smart Dust (Warneke, et al. Sensors 2002) Microrobots (Hollar, Flynn, Pister. MEMS 2002)

Add Legs

Add Robot Body

1mm1mm

COTS Dust (Hill, et al. ACM OS Review 2000) CotsBots (Bergbreiter, Pister. IROS 2003)

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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

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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 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 = 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.

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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.

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Wood, et al, 2003

Microrobots

Seiko, 1992

Yeh, 1995-2001 Hollar, et al, 2002

Ebefors, et al, 1999

Sandia, 2001

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Overview

• Motivation and Previous Work• Jumping for Microrobot Locomotion• Robot Design

– Actuation– Energy Storage– Power – Control

• Fabrication and Integration

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Jumping: Trajectory

• Muscle/motor work kinetic energy for jump

• How high?

• How far?

• Can use to jump over obstacles

mghvm 2sin5.0

cossin2 vgvd

0 10 20 30 40 50-10

-5

0

5

10

15

20

25

30

35

Hopping Trajectory, Mass = 15 mg, Angle = 60 deg

distance (cm)

heig

ht (

cm)

5 uJ10 uJ25 uJ50 uJ

heig

ht (

cm)

distance (cm)

Hopping Trajectory, Mass = 15mg, Angle = 60deg

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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 wingsBennet-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*Cd = 30 mm2, Angle = 90o

Energy (J) 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

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Jumping: Energy Storage

• Short acceleration times with short legs require energy storage for most actuators

• For a linear spring, apply force over a distance

vlt legacc 2

k

FkxFxU strain

2

2 21

21

21 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5

0

5

10

15

20

25

30

35

40Force v. Distance

distance (mm)

forc

e (m

N)

5 uJ10 uJ25 uJ50 uJ

For

ce

(mN

)

Distance (mm)

Energy Storage in Linear Spring

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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

offt

t hR dtVFE0

)cos(1)( wtltx eff

k

mglleff )2cos(1

4)(

2

wtkl

tE eff

0 0.5 1 1.5 2 2.5 3 3.5 4 4.50

5

10

15

20

25Kinetic Energy v. Time, m = 15mg, k = 2 N/m

Time (msec)

Kin

etic

Ene

rgy

(uJ)

Ene

rgy

(J)

Time (msec)

Kinetic Energy v. Time, Mass = 15mg, k = 2 N/m

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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 m 100 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

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Overview

• Motivation and Previous Work• Jumping for Locomotion• Robot Design

– Actuation– Energy Storage– Power – Control

• Fabrication and Integration

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• 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

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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

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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 mm2

+-0V

0V

ClutchActuator

DriveActuator

Shuttle

+-

+-0V

50V

ClutchActuator

DriveActuator

Shuttle

+-

+-50V

50V

ClutchActuator

DriveActuator

Shuttle

+-

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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

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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

22

2

2

1

2

1

Vd

A

d

UF

d

AC

CVU

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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

g1g2

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

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Actuation: Add Nitride to Process

(1) (2)

(3) (4)

(5) (6)

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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

TransmissionActuator

+-50V Drive

Actuator

+-0V

TransmissionActuator

+-50V Drive

Actuator

+-50V

TransmissionActuator

+-0V Drive

Actuator

+-50V

TransmissionActuator

gnew

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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

2stepSize

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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.

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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

Fclutch

3m

4m

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0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.50

1

2

3

4

5

6

Gap in microns

Are

a in

mm

-squ

ared

Area v. gap for F=0.01 N

30 V50 V80 V

Actuation: Area Requirements

• 10 mN drive actuator with initial gap of 1.5 m at 50 V requires ~ 2 mm2

• 25 mN clutch actuator with final gap of 0.5 m at 50 V requires ~ 0.6 mm2

• If actuator area approximates surface area, total minimum area required for inchworm ~ 3.2 mm2

area

(m

m2)

gap (m)

Area v. Gap F = 10mN

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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

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Springs: Materials

• Maximum distance traveled

• Maximum force which can be applied

• Energy stored

E

lx

max

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 9AF max

E

lAxFU

2

maxmaxmax 21

21

A

For 5 mm travel at 10 mN

Si: l = 1 m, A = 12.5 m2

Polyimide: l = 43 mm, 43 m2

Silicone: l = 2.2 mm, A = 4400 m2

l

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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.

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Springs: Integration

• Assemble elastomer onto silicon motor• Two strategies:

– SOI hook for rubber bands

– SOI clamp for rubber strips

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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

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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

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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

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Control:Design Considerations

• Low power (~10 W)

• Small size

• Simple integration

• Programmability

• Off-the-shelf

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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

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Overview

• Motivation and Previous Work• Jumping for Locomotion• Robot Design

– Actuation– Energy Storage– Power – Control

• Fabrication and Integration

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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.

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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 mm2 radio IC ~ 7 m– m = 0.7 mg, = 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 microelectromechanical high-density energy storage/rapid release system," presented at SPIE, 1999.

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Integration: Full Robot

Solar Cells EM6580

Shuttle, Springs, and Motor

Wire bonding  Mass (mg) Dimensions (mm) Power (W)

Motors @ 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 Controller 3.5 2 x 2.7 x 0.28 11.6

Total Robot 14.6 4 x 8 x 0.6 58.4

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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!

Backup Slides

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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

2

2mvKE

cos

sin

vv

vv

horizontal

vertical

mghvm

2

)sin( 2g

vh

2

)sin( 2

2

2gth g

vt

sin

g

vv

g

vd

2sincos

sin2

2

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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

0 2 4 6 8 10 12 14 16 18 200

5

10

15

20

25Height v. Mass

mass (mg)

heig

ht (

cm)

5 uJ10 uJ25 uJ50 uJ

heig

ht (

cm)

Mass (mg)

Height v. Mass at 60o

m

KEACF drag

drag

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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

2)(

12

222

dlm

E

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Actuation: Staple Test Structures

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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.

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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.

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Actuation: Clutch Design

Sample Flexure Design for one group of teeth on clutch

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Actuation: Layout Considerations

• Spacing for nitride gaps– For clamped-clamped beam with force acting

on middle

– Lmax = 200 m • ymax = 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

EI

Fly

192

3

l

t wF

g0

zg0

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

xxg

ltlt

NF filmsqueeze

30

3

)(

)6.01(

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.

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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.

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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

E

Etan)tan(

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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

A B C D

Top Clutch 0 1 1 1

Top Drive 0 0 1 1

Bottom Clutch 1 1 0 1

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Localization Ideas for Large Numbers of Robots

Fisheye LensHigh Power

LED

PhotoBeacon IC

• 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

d

57/41Localization: System Architecture

256x1Analog

Mux

.....

..Programmable Gain Amplifier

Photodiodes in circular array

3-wire bus

Differential Analog Output

190o field-of-view lens

Lens

LED

Low divergence, high power LEDs

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Localization: Photobeacon IC

~4mm

256 Photodiodes

Multiplexer Blocks3-wire bus

Modified Optical Receiver

1.3mm

1.8mm

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Jumping Robots: Video

Burdick and Fiorini, 2003 Scout, 2000