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CHAPTER 5: ELECTROSTATIC SENSING AND ACTUATION

CHAPTER 5: ELECTROSTATIC SENSING AND ACTUATION. Introduction . Electrostatic sensing – If the distance & position between two conductor change as a result of applied stimulus, the capacitance value would be changed.

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CHAPTER 5: ELECTROSTATIC SENSING AND ACTUATION

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  1. CHAPTER 5: ELECTROSTATIC SENSING AND ACTUATION

  2. Introduction • Electrostatic sensing – If the distance & position between two conductor change as a result of applied stimulus, the capacitance value would be changed. • Electrostatic actuation- A voltage (or electric field) across two parallel conductors, an electrostatic force would develop between these two objects. • Major advantages of electrostatic sensing and actuation: • Simplicity. Easy to implement. Only two conducting surface- without special functional materials. • Low power. Relies on differential voltage (rather than current). Energy efficient for low frequency applications. • Fast response. High dynamic-response speed. Example: Switching time for Digital micromirror display (DMD) by Texas Instrument= 21 s

  3. A d E Fringe field Parallel plate capacitor A parallel plate capacitor with an overlapping area of A and a spacing of d. A relative electrical permittivity or the dielectric constant denoted as εr and the permittivity of media is ε= εr ε0. ε0 is the permittivity of vacuum (ε0= 8.85x10-14 F/cm)

  4. Movable plate km x0 Anchored plate Equilibrium position of Electrostatic Actuator Under Bias x Fmechanical km Felectrical Anchored plate (b) With electrical bias (a) Without electrical bias Mechanical force has an equal magnitude with Felectric but opposite direction (static position). At starting position At equilibrium position

  5. |F| Contact of plates Spacing at zero bias x V=0 Fixed electrode Movable electrode |F| |Fmechanical|=Kmx Fmechanical Felectrical Magnitude of electric and mechanical force as a function of plate spacing |Felectrical|=εAV2/(d2) V Fmechanical changes linearly with the position for a fixed voltage V Felectrical increases with x in a non-linear fashion. The horizontal point of the interception indicate the equilibrium position of a movable plate.

  6. Pull in Effect of Parallel Plate Actuators

  7. Pull-in voltage

  8. Pull-in voltage

  9. Application of Parallel Plate Capacitors • Inertia Sensor • Pressure Sensor • Flow Sensor • Tactile Sensor • Parallel Plate Actuator

  10. Interdigitated Finger Capacitors Comb drive sensor & actuator

  11. Interdigitated Finger Capacitors; Single fixed finger and two neighboring moving fingers

  12. Interdigitated Finger Capacitors; Transverse comb drive

  13. Interdigitated Finger Capacitors: Longitudinal comb drive

  14. Application of comb drive devices • Inertia Sensor • Actuator

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