1 of 4 ABSTRACTA tiltable microplatform for adaptive artificial vision applica-tions has been demonstrated that utilizes special bent-beam electro-static actuators to achieve DC tilt angles larger than 10o withactuation voltages less than 20V, and a resonance tilt angle of 19owhen driven to resonance at 33 kHz via a combination of 14V DC,plus 5V AC. This bent-beam actuation mechanism has the furtheradvantage that its physical implementation is out of the way of themicroplatform itself, making it amenable to applications where lightmust pass through the platform and substrate (e.g., adaptive vision).Keywords: MOEMS, artificial vision, tilt angle, low voltageI. INTRODUCTIONIn the wake of rapid advancements in optical MEMS technologyspurred by the telecommunications industry, micromechanical actu-ation mechanisms capable of generating plate tilt angles on theorder of 10o are now commonplace [1]-[3]. Among the various actu-ation strategies, vibromotor [1] and bimorph [2] methods stand outas some of the most voltage efficient, requiring 20V AC and 1-2 V(but with considerable power consumption), respectively, to attaintilt angles greater than 10o in large plates, often for optical scanningapplications. However, actuation methods capable of low power,low voltage tilt actuations of plates that allow light to pass through a3D array of elements are fewer in number [3], and most requireexcessive power or voltage levels.One such 3D arrayed application, for which this work is specifi-cally targeted, is the adaptive artificial vision system depicted inFig. 1, where incoming light is collected, subdivided into a 2Darray, collimated, then directed through an array of tiltable micro-prisms before finally reaching a pixel array. In this system, themicroprisms disperse the color content of the incoming light, and bytilting, modulate the colors from pixel to pixel on the detectionplane—an operation often required by algorithms for color adapta-tion in vision systems [4]. Because the change in the angle of lightpassing through a prism is only a weak function of the prism tilt, tiltangles of 10o or greater are needed for adequate traversal of colorsover adjacent pixels, and such tilts must be accomplished via lowvoltages and with low power consumption.This paper describes a new bent-beam electrostatic (i.e., lowpower) actuation method that achieves DC platform tilts larger than10o with actuation voltages less than 20V and resonance tilts up to19o at 33 kHz, while offering an implementation structure amenableto applications requiring 3D arrayed transmission-based operation.II. MICROPLATFORM OPERATIONFigure 2 presents a perspective-view schematic of the demon-strated tiltable microplatform housing a microprism. As shown, thelow-stress nitride microplatform itself is suspended via rigid poly-silicon suspension beams above a hole in the silicon substrate (as in[3]) that (1) allows light to pass through; and (2) allows unimpededtilting of the microplatform over very large angles. The suspensionbeams are actually not anchored to the substrate directly, but ratherrest upon the substrate via strategically-placed dimples, aroundwhich the suspensions can pivot to affect tilting of the platform.These pivoting suspension beams together with the network ofbeams attached to them then realize the bent-beam actuation mecha-nism that makes possible tilting of the platform at such low volt-ages. As illustrated in Fig. 3, bent-beam actuation is achieved bypulling down a thin doped-polysilicon beam perpendicularlyattached to the suspension beam, and bending this thin beam closeto the suspension dimple so as to pivot the suspension around anangle defined by the degree of bending in the thin beam. In thiswork, the thin beam is pulled down electrostatically by applying asuitable voltage to an underlying, nitride-covered (to prevent short-ing), doped-polysilicon electrode. The voltage required to achieve agiven angle is greatly reduced in this system, because: (1) the pivot-ing dimple eliminates the need to overcome a torsional stiffness; and(2) once pulled down, portions of the beam near the bend are veryclose to the underlying electrode, so the electrostatic beam-bendingforce is larger for a given voltage.III. BENT-BEAM ACTUATOR DESIGNTo obtain an expression for tilt angle as a function of appliedactuation voltage for a bent-beam actuator, an energy method simi-lar to that described in [5] is used, in which the derivative of thetotal potential energy in the system with respect to distance is set toIncomingLightCollectionLens CollimatingLens TiltablePrismArrayArrayArrayDetector ArrayWhite Light ColorDispersedPrismTiltedDispersionAngle SlightlyChangedWhite Light ColorDispersedredgreenblueIncomingLightFig. 1: Schematic of a portion of the adaptive artificial visionapplication targeted by the tiltable microplatform. Beyondthe detector array, vision algorithms are applied to the 2Ddata via transistor DSP circuits, then fed back to controlplatform tilting so as to affect color adaptation.Fig. 2: 3D view of the bent-beam actuated tiltable microplatform ActuationBeamSuspensionBeamPrismPlatformHole on SubstratePivotDimpleActuationElectrodeSpringSupportAABBCCA LOW-VOLTAGE TILTABLE MICROPLATFORM USING BENT-BEAM ACTUATIONYuan Xie and Clark T.-C. NguyenCenter for Integrated MicrosystemsDepartment of Electrical Engineering and Computer ScienceUniversity of MichiganAnn Arbor, Michigan 48109-2122, U.S.A.Y. Xie and C. T.-C. Nguyen, “A low-voltage tiltable microplatform using bent-beam actuation” Tech. Digest, 2002 Solid-State Sensor, Actuator, and MicrosystemsWorkshop, Hilton Head, South Carolina, June 2-6, 2002, pp. 350-353.Travel support has been generously provided by the Transducers Research Foundation and by the DARPA MEMS and DARPA BioFlips programs.2 of 4zero at equilibrium.Using this technique, neglecting fringing electric fields and dim-ple-to-substrate friction, the voltage required to pin the bendingbeam up to the point A in Fig. 3 can be derived and written as(1)where(2)and where Lbb and Wbb are the length and the width, respectively, ofthe bending beam, Le is the length of the actuation electrode, a is theelectrode-to-beam contact length when the bending beam is pinnedto the substrate, hd is the height of the dimple, tnit is the thickness ofthe insulation layer, E is the Young’s modulus of polysilicon, I is thebending moment of the bending beam, εnit is the dielectric constantof the insulation layer, and εo is