Articulated Mechanisms and Electrostatic Actuators for Autonomous Microrobots by Richard Yeh BS

146 Pages·2001·3.43 MB·English

ArticulatedMechanisms andElectrostaticActuators forAutonomousMicrorobots
by
RichardYeh
B.S.(Universityof California at Berkeley)1993
M.S.(UniversityofCaliforniaat Los Angeles)1995
Adissertationsubmittedinpartial satisfactionof the
requirements forthedegree of
DoctorofPhilosophy
in
Engineering-ElectricalEngineering
andComputerScience
inthe
GRADUATEDIVISION
ofthe
UNIVERSITYOF CALIFORNIA,BERKELEY
Committeeincharge:
ProfessorKristoferS.J.Pister,chair
ProfessorRoger T.Howe
ProfessorDorianLiepmann
Spring2001
ThedissertationofRichardYehis approved:
______________________________________________________
ProfessorKristoferS.J.Pister,Chair Date
______________________________________________________
ProfessorRoger T.Howe Date
______________________________________________________
ProfessorDorianLiepmann Date
UniversityofCalifornia,Berkeley
Spring2001

1
Abstract
ArticulatedMechanisms andElectrostaticActuators forAutonomousMicrorobots
by
RichardYeh
DoctorofPhilosophyinElectrical EngineeringandComputerScience
UniversityofCalifornia,Berkeley
Professor KristoferS.J.Pister,Chair
Enabled by advances in both integrated circuit technology and micro electrome-
chanical systems (MEMS), the continuing miniaturization and integration of electronics,
sensors,actuatorsandmechanismswillmakeitfeasibletocreateinsect-sizedautonomous
microrobots. We propose to create a class of autonomous crawling microrobots the size
on the order of 1cm3 and equipped with a power source, low-power CMOS controller,
sensors, wireless communications devices and motorized articulated legs. The work pre-
sented here demonstrates how articulated insect legs could be created from rigid links,
mechanical couplings andlow-powerelectrostatic micromotors.
Articulatedlegsrequirerigidlinkstosupporttheweightofthemicrorobotandalso
havejointsthatallowout-of-planemotion. Surfacemicromachinedpolysiliconhingesare
utilizedtosatisfybothrequirements. Rigidlinksarecreatedbyfoldingthreehingedpoly-
siliconplatesintoahollowtriangularbeam whichsnapintoplace usingsnaplocks. These
links can be fabricated in series with hinges as revolute joints. Two-link legs with up to
three degrees-of-freedom (DOF)have beendemonstrated.
2
Each link has a mechanical coupling that couples it to its own motor on-chip.
Using hinged lever arms, hinged tendons and sliders, multi-DOF mechanical couplings
can be created to convert linear motion at the motor to angular displacement at the joint.
A 2-link, 2-DOF leg has been demonstrated with two mechanical couplings. The first
mechanical coupling (with 1-DOF) is created from a four-bar linkage (sliding crank) and
coupled tothe first link. The second mechanical coupling (with2-DOF)is created from a
four-bar linkage in series with a five-bar linkage and coupled to the second link. Higher
DOF mechanical couplings could be achieved with higher order n-bar linkages but at a
cost ofhighercomplexity.
The main considerations for actuation are output power density, efficiency, force
density,andintegrationwiththerestofthemicrorobot. OftheMEMSactuationtechnolo-
gies that have emerged over the years, electrostatic gap-closing actuators (GCA) in an
inchworm motor topology is currently best suited for microrobots. Motors fabricated on
silicon-on-insulator (SOI) wafers have been demonstrated with 80µm of travel, stepping
rates of 1000 full steps/second corresponding to 4mm/s shuttle velocity, and ~260µN of
force (~130 times its own weight). In all cases, displacement was limited by contact with
a physical constraint (spring travel limits, nearby structures, etc.) rather than an intrinsic
limit.
In addition to inchworm motors, mechanical digital-to-analog converters (DAC)
havebeendemonstrated. TheseDAC’s convert an-bitdigitalelectricalinputtoananalog
mechanical output (displacement) with 2n position. Based on cascaded lever arms with
high output resistance and gap-stop-limited actuator arrays at the input, the DAC are less
sensitive to loading effects and input noise. These properties are ideal for possible open-
3
loop actuation of microrobots where joint angle feedback is difficult to implement. Four-
bit DAC’s with electrostatic actuators have been demonstrated in SOI technology with a
leastsignificantbit(LSB)of0.6µm,anintegratednon-linearity(INL)of±0.38LSBanda
differentialnon-linearityof±0.35LSB. Surfacemicromachined6-bitDAC’swithhinged
micromirrors have also been demonstrated with an LSB of 90nm, INL of ±3.2 LSB and a
DNLof±0.7LSB.
__________________________________________
ProfessorKristoferS.J.Pister,Chair
4
i
Dedicatedtomymother,
thememoryof my father
and
thememoryof mygrandfather.
GrandfatherHsu,aself-taughtengineer,standingnexttomachines
hebuiltduringtheJapaneseoccupationperiodinTaiwan.
ii
Contents
Listof Figures..................................................................................................................v
Listof Tables..................................................................................................................vii
1 Introduction................................................................................................................1
1.1 TheVision...........................................................................................................1
1.2 OverviewoftheDissertation..............................................................................2
1.3 Previous Work....................................................................................................3
1.4 Concept...............................................................................................................6
1.4.1 Power.........................................................................................................7
1.4.2 Controller...................................................................................................8
1.4.3 ArticulatedLegs andMechanical Couplings...........................................10
1.4.4 Actuation..................................................................................................11
1.4.5 Sensors.....................................................................................................11
1.4.6 Communication........................................................................................12
1.4.7 Gait...........................................................................................................14
2 ArticulatedMechanism...........................................................................................17
2.1 Introduction.......................................................................................................17
2.2 Legs orWheels.................................................................................................17
2.3 RigidLinks.......................................................................................................17
2.4 Stiffness............................................................................................................21
2.5 ArticulatedJoints andMultipleDOF................................................................23
2.6 Mechanical Coupling........................................................................................25
3 Actuation...................................................................................................................31
3.1 Introduction.......................................................................................................31
3.2 Figures ofmerit.................................................................................................31
3.2.1 OutputPower Density..............................................................................31
3.2.2 Forcedensityandforce coefficient..........................................................32
3.2.3 Efficiency................................................................................................32
iii
3.2.4 Integration................................................................................................33
3.3 Comparisonof ActuationMethods...................................................................33
3.3.1 ThermalActuation...................................................................................33
3.3.2 MagneticActuation..................................................................................35
3.3.3 Piezoelectricactuation.............................................................................36
3.3.4 ElectrostaticActuation.............................................................................36
3.3.4.1 Efficiencyofelectrostaticactuators................................................37
3.3.4.2 Forcedensityofelectrostaticactuators...........................................39
3.4 Comparisonof ElectrostaticActuators.............................................................39
3.5 Gap-ClosingActuatorDesign...........................................................................44
3.6 StaticModel......................................................................................................46
3.6.1 Forces.......................................................................................................46
3.6.2 Pull-inVoltageandpull-ingap................................................................46
3.6.3 Failure Voltage........................................................................................48
3.6.4 ForceDensity...........................................................................................49
3.7 DynamicModel................................................................................................50
3.8 Optimization.....................................................................................................50
3.8.1 Initialgap(g)...........................................................................................51
3.8.2 Actuatorstroke (g )..................................................................................52
s
3.8.3 Electrode thickness (t).............................................................................52
3.8.4 Maximum voltage(V).............................................................................52
3.8.5 Electrode length(l)..................................................................................52
3.8.6 Electrode width(w).................................................................................53
3.8.7 Unitcell separation(z).............................................................................53
3.8.8 Optimizationfor highforce density.........................................................53
3.9 Fabrication........................................................................................................54
3.9.1 PolysiliconProcess..................................................................................54
3.9.2 SingleCrytallineSiliconProcess.............................................................56
4 LinearElectrostaticInchwormMotors.................................................................59
4.1 Introduction.......................................................................................................59
4.2 Design...............................................................................................................61
4.2.1 Attachment Force.....................................................................................62
4.2.2 PullingForce............................................................................................63
4.2.3 GearTeeth................................................................................................64
4.2.4 Speed........................................................................................................64
4.2.5 Power.......................................................................................................65
4.3 ScalingEffects..................................................................................................66
4.3.1 ActuatorForce.........................................................................................66
4.3.2 DissipativeForces....................................................................................67
4.3.3 OutputPower Density..............................................................................67
4.4 Results...............................................................................................................68
5 Mechanical Digital-To-AnalogConverters...........................................................73