Understanding Smart Sensors - Nomads.usp

214 Understanding Smart Sensorshollow nickel-plated polygon reflector on the rotor of an electrostatic drivemicromotor. A microoptomechanical system (MOMS) is based on wafer levelintegration of optical and MEMS components. Microscanners and movableoptical elements have been designed and fabricated using electroless plating ofreflective nickel surfaces on the rotor of the micromotor. The thickness (height)of the nickel is 20 mm, the width 10 mm. Scanners with diameters from 250 to1,000 mm have been produced, but the larger micromotors do not operate reliablyafter they are released. Diffraction grating microscanners were fabricatedwith spatial gratings of 2 and 4 mm using a similar process. The key to MOMSdevices is the ability to fabricate different optical and mechanical structures ona common substrate.An actuator has been proposed that uses optical power to providemechanical energy [19]. An optical actuator has potential advantages of higheroperating speed, lower power consumption, and lower thermal expansion thannonoptical approaches. A silicon cantilever reacts to a photoelectric current byrelaxing, as opposed to an applied electrostatic voltage, which stress the beamfurther.9.3.3 MicrogrippersSeveral researchers have demonstrated microgrippers or microtweezers. Oneresearch team has developed a surface-micromachined polysilicon microgripperthat is activated by an electrostatic comb-drive [20]. The electrostatic combdrive technique provides the force for several microactuators as well as an oscillatingstructure for many sensors. The two movable gripper arms are controlledby a three-element electrostatic comb, as shown in Figure 9.11. The length ofthe drive arms, L dr ,is400mm, and the length of each extension arm, L ext ,is100mm. By using separate open and close drivers, the gripping range for a givenmaximum voltage is doubled. Movement of 5 mm was produced by the gripperswith less than 30V applied to the comb drive.9.3.4 MicroprobesA cantilever beam contacting and scanning across the surface of a sample canmeasure the topography of a surface. The instrument, an atomic force microscope(AFM), requires a mechanical structure with a sharp tip, small springconstant, and high resonant frequency [21]. Batch fabrication yields cantileverswith very reproducible characteristics, and piezoelectric, capacitive, or piezoresistivesensing techniques can be used to sense the deflection of the probe tip.The construction of an AFM probe is illustrated in Figure 9.12(a). The dimensionsof one device are L1 = 175 mm, L2 =75mm, w =20mm, b =90mm, and