American Institute of Aeronautics and Astronautics1

2001-1504A DOCKING SYSTEM FOR MICROSATELLITES BASED ON

MEMS ACTUATOR ARRAYS*

David M. MellerJoel Reiter, Mason Terry, Karl F. Böhringer, Ph.D., Mark Campbell, Ph.D

Department of Electrical EngineeringUniversity of WashingtonSeattle, WA 98195-2500

*Copyright � 2001 by the American Institute of Aeronautics andAstronautics, Inc. All rights reserved.

Abstract

Microelectromechanical system (MEMS)technology promises to improve performance of futurespacecraft components while reducing mass, cost, andmanufacture time. Arrays of microcilia actuators offer alightweight alternative to conventional docking systemsfor miniature satellites. Instead of mechanical guidingstructures, such a system uses a surface tiled withMEMS actuators to guide the satellite to its dockingsite.

This report summarizes work on an experimentalsystem for precision docking of a “picosatellite” usingMEMS cilia arrays. Microgravity is simulated with analuminum puck on an airtable. A series of experimentsis performed to characterize the cilia, with the goal tounderstand the influence of normal force, picosat mass,docking velocity, cilia frequency, interface material,and actuation strategy (“gait”) on the performance ofthe MEMS docking system.

We demonstrate a 4cm2 cilia array capable ofdocking a 45g picosat with a 2cm2 contact area withmicrometer precision. It is concluded that currentMEMS cilia arrays are useful to position and alignminiature satellites for docking to a support satellite.

Introduction

A number of MEMS cilia systems have beendeveloped with the common goal of moving andpositioning small objects, so far always under the forceof gravity. Similar to biological cilia, all of thesesystems rely on many actuators working in concert toaccomplish the common goal of transporting an objectof much larger size than each individual cilium. Recenttechniques range from air jets1-3 electromagneticactuators4-6 piezoelectric actuators7 single crystal siliconelectrostatic actuators8,9 thermal-bimorph (bimaterial)actuators10-13 to electrothermal (single-material)

actuators14,15. In parallel, researchers have studied thecontrol of distributed microactuation systems and ciliaarrays16- 22.

Figure 1. Scanning electron microscope (SEM) view of a singlemicrocilia motion cell. The cell is approx. 1mm long and wide.(Image by John Suh, 1997)

The goal of this project was to investigate thefeasibility of a MEMS-based space docking system. Forsuch a system, the docking approach is divided into twophases: (1) free flight and rendezvous, with the goal toachieve physical contact between the two satellites, and(2) precision docking with the goal to reach accuratealignment between the satellites (e.g., to align electricalor optical interconnects). Phase 1 constitutesunconstrained motion with 6 degrees of freedom andlower accuracy for the rendezvous. Phase 2 constitutesplanar motion with 3 degrees of freedom and highaccuracy. The MEMS-based approach for phase 2represents the key innovation in this project. Therefore,the investigation of MEMS cilia as a means to achieveprecise alignment between two satellites represents theprimary focus of this report. To demonstrate acomplete system-level solution, phase 1 wasinvestigated as well.

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Figure 2. Airtable experimental setup to simulate microsatellitedocking. An 8” × 6” perforated aluminum plate with 3 adjustablesupport screws provides levitation support for an aluminum puck(“picosat”). Microcilia chips are mounted on a vertical copper platewith heat sink.

During this project thermally actuated polyimidebased microcilia, as seen in Figure 1 and identical tothose published in Suh, et. al13, are extensivelycharacterized to ascertain their practicality for dockingminiature spacecraft. To this end, experiments wereperformed using an airtable, seen in Figure 2, whichwas designed to support the microcilia in a verticalconfiguration. A rectangular aluminum block, referredto as a puck, which has a mass between 40g and 45g isused to simulate a picosatellite. The airtable can betilted towards the microcilia producing a known normalforce against the faces of the chips. This force can thenbe adjusted independently from the mass of thesimulated picosatellite. To increase the realism of theexperiment and to facilitate data collection, positionsensing and position feedback are incorporated andcomputer controlled. Two position-sensing systems areused: an array of Hall effect sensors and a video capturebased system. These are strictly non-contact techniquescompatible with a space environment.

Next we describe the experiments that wereperformed with the microcilia with the goal to evaluatethe appropriateness of microcilia to spacecraft dockingapplications. The microcilia successfully moved blocksof aluminum in excess of 40g of mass and calculationsindicate that a patch of cilia 25cm in radius would besufficient to position a 40kg satellite. Four differentmaterials, polished ceramic, polystyrene, smoothaluminum and silicon, both rough and polished, areused for the cilia to puck interface surface. Polishedceramic achieved the highest puck velocities of allinterfaces and polished silicon attained higher velocitiesthan rough silicon. Throughout the course of this studymicrocilia were able to provide the speed, robustness,

reliability and strength for use in miniature spacecraftapplications.

Figure 3. Envisioned microsatellite mission

Microsatellite Docking

Figure 3 describes a large, broad purpose satellitesurrounded by a constellation of smaller, missionspecific satellites. The miniature satellites provideinspection, maintenance, assembly and communicationservices for their larger brethren. One important futuretask for the microsatellites is inspecting the largersatellite for damage. Cameras mounted on themicrosatellite provide imagery of the primary platformthat is otherwise unobtainable. From these pictures,damage could be assessed and the mission of the mainsatellite adapted. Due to their simplicity, small size,weight and limited interaction with ground controllersthese specialized satellites are expected to beindispensable during future missions23.

Figure 4. Conceptual cilia picosatellite docking application

As the size of satellites shrink, their ability to carryfuel and power is reduced. It is expected that this willforce microsatellites to dock frequently to replenishtheir resources. Since the time spent docking subtractsfrom the microsatellites’ mission time, this procedureshould be as simple and quick as possible. Whendocking microspacecraft there are two primary tasks:attaching the microsatellite to the larger craft, andorienting the satellite to connect fuel, data and electricalservices. The first docking task is largely the domain ofthe microsatellite and is dependent on how quickly

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velocity adjustments can be made, and on the specificattachment mechanism. The second phase of dockingis dependent on the speed at which the satellite can bepositioned to connect electrical and other services.

Figure 4 shows the conceptual cilia application forspace docking of miniature satellites. A surface on themother satellite is covered with cilia actuators that willposition the picosatellite for refueling and data transfer.

Figure 5. Cross-sectional view of the cilia with two layers ofpolyimide, titanium-tungsten heater loop, silicon-nitride stiffeninglayer, and aluminum electrostatic plate. (Image by John Suh, 1997)

Experimental SetupThe measurements in this paper are performed

using the thermal actuator based microcilia originallydescribed in Suh, et. al13. A cross-section of microciliaarms is shown in Figure 5. The arrayed actuators aredeformable microstructures that curl out of the substrateplane. The curling of the actuators is due to thedifferent CTE of the polyimide layers that make up thebimorph structures. For these devices the top layerCTE is greater than the bottom CTE. The thermalstress from this interface causes the actuator to curlaway from the substrate at low temperatures andtowards it when heated. This stress also aids inreleasing the microcilia arms because theyautomatically rise out of the plane when the sacrificiallayer is etched. The displacement of the microcilia armrelative to its location before release both vertically, δV,and horizontally, δH, is given by:

���

����

���

���

�−=RLRV cos1δ (1)

��

���

�−=RLRLH sinδ (2)

where R (≈ 800µm) is the radius of curvature and L (≈430µm) is the length of the motion actuator whichresults in a horizontal displacement, δH, of 20µm andvertical displacement, δV, of 114µm. A detailed

description of the cilia actuators and their operation waspresented in Suh, et. al12,13.

An array of cilia is configured in an 8 × 8 motioncell layout within a 1cm2 die. Each motion cellcontains four orthogonally-oriented actuators within anarea of 1.1mm × 1.1mm. A control line independentlyactuates each actuator of the motion cell. All actuatorsoriented in the same direction in each motion cell arecontrolled in parallel.

The microcilia arm is placed into motion using atitanium-tungsten heating resistor that is sandwichedbetween the two silicon nitride and two polyimidelayers. When a current is passed through this loop, thetemperature of the actuator increases, and the structuredeflects downward. This produces both horizontal andvertical displacements at the tip of the microcilia.

Objects in contact with the surface of the array aremade to move by coordinating the deflections of manyactuators. For this study both three and four-phase gaitsare used during the interface experiments but only thefour-phase gait is used for the normal forceexperiments. The motions of the microcilia arms for thefour-phase gait are shown in Figure 6. This motion hastwo transitions that produce forward motion.

Figure 6. The four phase cilia motion gait showing all phases ofmotion

The sequence of phases starts with both armsflattened. The west arm is released forcing the objectup and to the east. Next, the east arm is released whichrises to make contact with the object. Then the westarm is actuated leaving the east actuator to support theobject. The final phase and start of a new phase in thesequence results in the object moving down and to theeast. The three phase gait is the same as the four phaseexcept the phase in top of Figure 6 is skipped.

To assess the applicability of microcilia tospacecraft docking this study investigates the effects of:operating frequency, normal force, interface surfaces,microcilia temperature, and, indirectly, microcilia life

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span. Of these variables, frequency and life spandepend directly on the thermal actuation nature of thecilia while the remaining parameters should beapplicable to other types of MEMS microactuatorarrays.

To perform measurements the microcilia are placedvertically, at the end of a tilted airtable as show inFigure 7. The table is first leveled and then the angleadjustment is manipulated to specify a slope runningtowards the microcilia. By adjusting the slope of thetable, the mass of the aluminum airtable puck can varywhile the normal force against the microcilia remainsconstant. Conversely, the mass can remain fixed whilea variable normal force is applied against themicrocilia. Using this parameter independence, theairtable allows for an accurate simulation ofmicrosatellite docking in microgravity.

With this setup, the microcilia can manipulateobjects that would otherwise flatten the actuators if allthe gravitational force were applied as the normal force.This experiment uses four microcilia chips attached to acopper block that both actively cools the microciliausing a Peltier junction and holds them vertical at theend of the airtable. The microcilia chips were gluedinto a grove machined in the copper block, forcing allfour chips to lie in the same plane in a horizontal lineararray.

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Figure 7. LabView interface for the microcilia

Figure 8. Side view of the airtable

During all of the experiments the microcilia werecontrolled with the LabView interface seen in Figure 7and custom circuitry. Each of the microcilia gaits isbroken down into a statemachine describing thesequence of movements for each of the microcilia arms.This statemachine is then loaded into a LSIprogrammable gate array, one per microcilia chip. TheLabView interface instructs the LSI chips which gait touse, the direction to travel and the frequency throughwhich to cycle the cilia gait. LabView also reads theHall effect sensor array and from that data controls thestarting and ending points of the puck.

VideoSignal

Capture

Image Conversionto Black and White

Verticle CenterCalculation

Horizontal CenterCalculation

Display DataAND ofCenter

Calculation

Averageing andVelocity

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Figure 9. Block flow diagram of image capture system

To make displacement measurements two separatesystems are employed. The first is a high-resolutionvideo capture system. This system, equipped with azoom lens, allows for relative measurements on theorder of 5µm and for capturing expanded views of thesystem. A video capture and distance measurementsystem has been developed. A flow diagram, shown inFigure 9, details the specific tasks that are performedupon a video image. The image of the puck is capturedusing a black and white CCD camera mounted on avariable zoom, 1-10, microscope. This image is thenconverted to a black and white image to allowprocessing using Matlab.

Once converted, the center points of both thehorizontal and vertical white regions are calculated.After the center point calculations are complete, Matlabperforms an AND function to determine the center pixelof the captured image. All of the center points can becombined and averaged relative to an internal clock toreduce the error of the capture system.

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The other measurement system is an array of Halleffect sensors. These sensors interact with a magnetmounted atop the puck to provide micrometerresolution. The Hall effect sensor array is integratedinto the LabView controlling software allowing fullyautomated experiments to be performed. Using eitherof these systems it is possible to collect relative puckposition and from this to compute the velocity andacceleration of the puck.

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city

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Figure 10. The velocity of the airtable puck (using the beveled plasticinterface) versus different drive frequencies as a function of normalforces

Experimental Results

The goal of this research is to evaluate theapplicability of microcilia arrays to microsatellitedocking. Thermal microcilia arrays are parameterizedfor operating frequency, normal force, puck mass,interface surfaces, cilia temperature and cilia lifespan.The results for these experiments are presented here.

Effects of Normal ForceFigure 10 shows the velocity of the puck at

different frequencies for four different normal forces.For all of these data points the mass of the puck is41.20gr and the interface surface is a 0.9cm × 2.5cmpiece of polystyrene, beveled on the edges. Each datapoint is an average of four runs over a distance of0.8mm. This setup contains two strong resonantfrequencies between 13 and 16Hz and between 30 and33Hz as illustrated by the graph flattening at thesepoints. For these measurements the video system isused to record the puck velocity.

Outside of these regions the puck velocityincreases linearly which indicates the puck moving inaccordance to the driving period. This characteristicindicates the interface between the puck and microciliaarms is experiencing a fixed slip component. At these

frequencies the puck motion seems largely the result ofa ‘step and carry’ transport. One conclusion from thisgraph is that the overall velocity of the puck increasesas the normal force against the cilia surface decreases toa minimum normal force value. This is an expectedresult because as the normal force increases so does theprecompression of the cilia, reducing their total verticaland horizontal motion. This would indicate that theoptimal normal force is that where the puck exerts justsufficient force to maintain contact with the ciliasurface.

Effects of Interfacing Surfaces

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Figure 11. Influence of interface material on puck velocity using athree-phase gait

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Figure 12. Influence of interface material on puck velocity using afour-phase gait

Differences between thermal conduction andsurface roughness of the puck to microcilia interfaceaffect step size and puck velocity. The five puck-to -microcilia interface materials examined are: polishedceramic, hard polystyrene plastic, aluminum, polished

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and unpolished silicon. Puck velocity versus frequencyfor the differing interface materials is shown in Figure11 and Figure 12 for three and four-phase gaits,respectively. Puck velocity is obtained by averaging aminimum of five trials per frequency with a normalforce of 63µN/mm2. The distance the puck traveled forthese measurements varies between 0.1mm to 0.6mmwith increasing actuation frequency.

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Figure 13. Comparison of both rough and polished silicon interfacefor both three and four-phase gaits

As summarized in Figure 13, the velocity of thepuck is dependent on the material interfaced with themicrocilia. The thermal conduction of the interfacematerial is thought to be the major cause for thevariation in velocity magnitude per material. Surfaceroughness is also observed to have some influence, butto a much lesser extent. Aluminum and silicon have thehighest thermal conduction and this results in the lowestvelocities. Ceramic, an excellent thermal and electricalinsulator, delivers some of the highest velocities. Lowthermal conduction of the ceramic interface allows thecilia to heat and cool in an optimal fashion resulting inhigh actuation amplitudes and high velocities.

Missing data points in the three-phase graph andthe flatter areas of the other graphs are due to the puckoscillating with zero or reduced velocity for multipletrials at that frequency. This effect is distributed overthe entire cilia experimental surface. The regions of17.5Hz and 33Hz show the most pronounced reductionin puck velocity for both gaits and all interfacematerials. The variation of this effect for differentsurface material and puck mass indicates that it isstrongly dependent upon the specific geometry of theexperiment. Regardless of this minor variation, it isthought that this phenomenon can be traced partly tothe puck breaking contact with the microcilia surfaceduring part of the motion cycle. As the normal force is

increased, this effect becomes less pronounced,however, it is still consistently observed. Within thesefrequency bands the puck was observed to move awayfrom the puck surface on the order of 100µm, lendingsupport to this theory.

Thermal EffectsAs the background temperature of the microcilia is

allowed to increase the actuators become less effective.With rising background temperature it takes longer forthe cilia to gain heat during the actuated portion of itsmotion cycle. This results in a lowering of themaximum available driving frequency. In the extremecase the background temperature becomes large enoughthat the heater loop cannot raise the temperature of thecilia higher than the background. At this backgroundtemperature, no heating period would be sufficient toallow the cilia to have a net displacement. At thispoint, objects in contact with the cilia would no longerbe transported.

This scenario was experimentally verified. If thepolarity of the Peltier junction that normally cools themicrocilia is reversed, it provides active heating, ratherthan active cooling. As the background temperature ofthe cilia increases, actuation displacement decreases.Eventually all visible movement halts. Once this pointis reached the heater is turned off and the microcilia areallowed to cool. Subsequent checks of the microcilia,under standard operating conditions, could determineno mechanical or electrical faults. However, prolongedoperation at elevated temperatures will eventuallydamage the actuators. Possible failures includecharring of the polyimide and fusing of the heater wire.

Life SpanOver the course of these experiments the microcilia

are shown to be robust and the results reproducible. Allfour chips, corresponding to 4 × 256 actuators were runfor approximately 150 hours at an average of 20Hz.This corresponds to 10.8 million actuations. Duringthis time only one microcilia actuator leaf was lost dueto a manufacturing defect. This failure was in anindividual heater loop and probably corresponded to alocal thickening of the material or contaminants in thatarea.

Novel Algorithms and Control Software

Rendezvous & DockingNumerous studies have been made in the area of

Rendezvous and Docking (R&D) of small satellites inspace. These studies generally involve small “satellite”robot vehicles moving in 2 nearly frictionless degreesof freedom on an air-bearing surface. Two primaryapproaches have traditionally been pursued. The first

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of these approaches is the use of GPS-like signals andDifferential Carrier Phase GPS techniques as a meansof sensing both the relative position and the relativeattitude of the two satellites. An example can be foundin Zimmerman and Cannon, Jr.25 To simulate the GPSenvironment, several pseudolite transmitters were builtand installed around the perimeter of a laboratory.Algorithms were then developed to extract relativeposition and relative attitude from carrier phasemeasurements.

The second primary approach has been the use ofCCD, or camera-based sensing to determine relativepositions and attitudes between the sensor and thetarget. An example of this approach can be found inHoward and Book26. This study, performed at NASA’sMarshall Space Flight Center, employed a single CCDsensor and laser diodes to illuminate a target. Theimages were then processed to determine navigationdata. This approach has been further developed byNASA and is currently being built as a prototype forflight testing on the Space Shuttle. An important pointto note in this approach is the use of analog CCDsensors and frame-grabber hardware for target sensing.

2-D Puck Experiment

Experimental ApproachThe approach to R&D presented in this section is a

variation of the approach taken by Howard and Book,that is, the use of optical sensing and image processingto determine relative navigation data. However, thisapproach differs significantly from the work citedabove. Sensing is provided by two 512 × 512 Fuga 15ddigital CMOS cameras. The two cameras are mountedparallel to one another, separated by a baseline distance.In this configuration the cameras can determine rangeand attitude with respect to a target through stereoimaging. Another significant difference is the absenceof frame-grabber hardware for image capture, since thecameras feature digital output.

The Fuga 15d Digital CMOS Camera The use of digital, optical, CMOS cameras offers

many advantages over analog CCD cameras. The Fuga15d, from the IMEC Company, is a 512×512 pixel,addressable imaging chip that behaves similar to a 256Kbyte ROM. After reading an X-Y address, the pixelintensity is directly read out and returned in a digitalword of 8 bits, allowing true random access. Directreadout means that the output signal is an instantaneousmeasure of the photocurrent. Camera response islogarithmic, which allows the chip to capture six ormore orders of magnitude of intensities in the sameimage. As a result, the sensor is virtually immune toblooming. The camera can take a picture of a welding

arc without saturating. The Fuga 15d camera is shownin Figure 14.

Figure 14. The Fuga 15d Digital CMOS Camera

Direct readout and random access are a substantialadvantage over analog imagers. Classic CCD and evenaddressable Metal-Oxide-Semiconductor (MOS)imagers of the “integrating” type must be read out aftera defined integration time, and thus cannot be used astrue random access devices. Analog cameras alsorequire frame grabber hardware, whereas the full matrixof pixels, or any desired pixels, on the Fuga 15d can besimply scanned and read directly into a buffer.

Pixel data is read from the camera ROM through 8parallel, digital output lines. Image processingalgorithms must use the intensity value correspondingto each pixel to determine the target location in eachcamera image. A hardware solution was adopted tohandle image processing and data flow using an FPGA.Output lines from the cameras are mapped directly tothe hardware, and all image processing analysis iscarried out by the FPGA processor.

Stereo Imaging

ImageX

w

λλλλ

λλλλ

Z

Z

BImage

Plane of constant Z

(x1, y1)

(x2, y2)

Figure 15. Top view of the stereo imaging process (From Fu,Gonzalez and Lee28.)

Much as in human vision, two cameras imaging thesame target can provide sufficient information about theorientation and location of the target27. The process of

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mapping a 3D scene onto an image plane is a many-to-one operation. This means that a single image pointdoes not uniquely determine the location of acorresponding point in the 3-dimensional world. Moreprecisely, the range to a target cannot be accuratelydetermined from a single camera image. The missingrange information can be determined using stereoimaging techniques.

The objective, stated in another way, is to find thecoordinates (X,Y,Z) of the point w given its imagepoints (x1, y1) and (x2, y2). It is assumed that thecameras are identical and that the coordinate systems ofboth cameras are perfectly aligned, differing only in thelocations of their origins, which are separated by thebaseline distance, B. Range information determined bythe x-coordinate shift of the target location in eachcamera image, or the quantity (x2-x1), using thefollowing equation:

12 xx

BZ−

−= λλ (3)

The most challenging task in implementing thestereo imaging approach is to accurately determine twocorresponding points in two different images of thesame scene. However, with proper target design andimage processing techniques, this can be reliablyaccomplished. For example, the image might consist ofa series of concentric circles against a highlycontrasting background. A simple algorithm could thenbe developed to determine the pixel center of thebinarized image and, therefore, the target center.

The 2-D PuckA 2-dimensional experiment has been developed

that will serve as a test bed for integration of sensorsand for verification of control algorithms. The 2-D“Puck” consists of a series of vertically stacked decks,floating upon a low-friction cushion of air supplied byan on-board air system. The cameras are mounted nearthe outer edge of the second deck and integratedelectronically to a central computer. Control isaccomplished with small solenoid valves that act as airthrusters. Eight thrusters are placed around thecircumference of the second deck, enabling control inthe +x, +y directions and spin control about the verticalaxis. When placed on a sufficiently flat surface the puckcan translate in two dimensions and rotate about its spinaxis with very little friction. The goal of theexperiment is to electronically integrate and close theloop with the CMOS sensors and to demonstraterendezvous with a stationary target.

Figure 16. 2-D Puck Experiment with integrated CMOS camera

All sensor measurements are sent to a TattleTale 8controller board (TT8), which includes a Motorola68332 microprocessor, a 500 Kbaud RS-232 interfaceand a PIC 16C64 microcontroller operating as a super-programmable clock. Data processing software andcontrol algorithms are loaded into the EEPROM of the68332 microprocessor. The TT8 processes incomingsensor signals and sends control signals to the solenoidvalves, thus closing the loop.

Puck Control and Navigation – An AugmentedApproach

While the puck is capable of fully autonomousnavigation to accomplish rendezvous with the ciliaarray, an augmented navigation and control approach ismore desirable. The puck will follow a predeterminedreference trajectory, passing through a waypoint. Thisstrategy is to ensure that the final approach and contactwith the cilia is as perpendicular to the plane of the ciliaas can realistically be achieved with the control systemof the puck. This requirement is especially important toavoid damage to the cilia arrays caused by a rendezvouswith lateral motion components, i.e., in the plane of thecilia array. Such an interaction between the puck andarray could tear and damage the cilia actuators. Anillustration of the waypoint trajectory can be seen belowin Figure 17. The initial navigation phase sends thepuck to the waypoint, which is somewhere removedfrom the final destination. The next phase directs thepuck to slowly approach the cilia array, holding thelateral (X) coordinate constant.

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Cilia

X

Y

Figure 17. Table schematic and puck trajectory

The dynamics of the puck were modeled andincluded in a discrete-time simulation with LQR controlusing the reference trajectories. Both the reference andsimulated paths are shown in Figure 18. In the lowercurve, the ideal x trajectory ramps up to the desiredvalue, corresponding to the waypoint, and subsequentlyremains constant. The y coordinate ramps up to thewaypoint coordinate and from that point follows aslowly increasing parabolic input to slowly approachthe final position at the cilia array. The simulationclosely tracked the reference trajectory and finalposition using LQR control.

Figure 18. Closed loop response to reference trajectory using LQRcontrol.

Conclusions

The results from these experiments indicate that amicrocilia surface can be useful for docking smallspacecraft. These spacecraft, used for inspection,maintenance, assembly and communication services,will see increased use as space missions become moreautonomous and far reaching23. During this scenario,microcilia provide a good match, allowing for simpledocking procedures to be used with these simplesatellites.

Results from the interface experiments indicate thata variety of materials common to spacecraft can be usedas docking surfaces, including aluminum and silicon,thus avoiding the need for special materials on themating surfaces. When studying the performance ofdifferent interface materials, thermal conductiondominates surface roughness to achieve optimal objectvelocity. Surface roughness does affect object velocityas seen in the polished and unpolished silicon. Aninterface material, such as ceramic, with low thermalconduction and little surface roughness should beselected for an optimal docking surface.

Using microcilia to perform the delicate finalorientation and positioning of the satellite will greatlyspeed up the docking operation because the entiresatellite, with its fixed connections, could be mated tofixed connections on the main satellite. This alleviatesthe use of flexible and cumbersome umbilical cords andattendant positioning systems.

A further benefit of using microcilia as a dockingsurface is a reduction in mass compared to otherdocking and alignment techniques. On the host satelliteonly a surface of microcilia is required along withminimal control electronics and sensors. The microciliadocking system could simply replace one of thesatellite’s body panels for maximum weight savings.On the microsatellite side, the additional mass toincorporate docking functionality could be as low aszero. The optimal microcilia interface is a flat plane,which may already be part of the microsatellite chassis,thus requiring minimal integration.

The microcilia themselves have inherentadvantages for this application. Foremost among theseadvantages is their ability to arbitrarily position thesatellite anywhere on the surface and in any orientation.The microcilia can also act as sensors, however, it hasalready been demonstrated that they can positionobjects open loop with little loss of accuracy18. Byusing thousands of microcilia on a single dockingpatch, it is possible to build systems that incorporatemassive redundancy. Thus, if there is some kind ofdocking mishap the entire mission need not be affected.Finally, thermal microcilia have been shown to performbetter in vacuum then air24. This is largely due to a lackof convective cooling which slows the heating cycle.

The scalability of microcilia also enables theconstruction of widely varied systems. While theprimary task envisioned for microcilia is manipulatingpicosatellites (mass <1kg) much greater masses arefeasible. By using additional cilia and a greater contactarea, larger microsatellites can be handled. The current

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generation of microcilia is capable of moving a 41.2gpuck with an interface area of 2cm2. This indicates thata patch only 25cm in radius (100,000 times as large asthe area in the experiment) would be sufficient toposition satellites with more than 40kg mass undermicrogravity conditions.

Throughout the course of this study the microciliaexhibited the speed, robustness, reliability and strengthneeded for this application. These results show thatmicrocilia can be an attractive alternative toconventional docking systems for microsatelliteapplications.

Acknowledgments

This report describes work on “SmartAttachments” performed under AFRL prime contractF29601-98-D-0210, USRA subcontract 9500-20,8/24/99 - 8/23/00. Team members were Dr. Karl F.Böhringer (PI), Department of Electrical Engineering,and Dr. Mark Campbell (co-PI), Department ofAeronautics and Astronautics, University ofWashington, Seattle, WA. Joel Reiter, Mason Terry(UW EE), and David M. Meller (UW AA) weregraduate students working on this project. Dr. R.Bruce Darling (UW EE), Dr. Gregory T. A. Kovacs(Department of Electrical Engineering, StanfordUniversity, CA), and Dr. John W. Suh (Xerox Palo AltoResearch Center, CA) were informal consultants.

The authors wish to thank Mike Sinclair for themanufacture of the airtable setup and for manyinspiring discussions. We are grateful to the membersof the University of Washington MEMS Laboratory,and the members and staff of the WashingtonTechnology Center Microfabrication Laboratory.Fabrication of the microcilia arrays was supported byDARPA and NSF. Development of cilia controlstrategies was supported by NSF.

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