Electrostatic Actuators and Sensors

Question 1

Why do electrostatics work well for MEMs devices

Answer 1

SA/Vol ration is higher for micro devices and the electrostatic effect is based on SA, while mass is based on volume

Question 2

Capacitance is governed by ___

Answer 2

• Distance between plates
• overlapping area between plates
• permittivity of the material between the plates

Question 3

Two cases for using electric charge to generate force

Answer 3

Attractive force
Repulsive force

Question 4

Principles of an electrostatic micromotor

Answer 4

• central rotor with one charge
• radial stator poles with opposite charge
• The stator phases are turned off / on in sequence to rotate the rotor

Question 5

Electrostatic comb-drive

Answer 5

• creates a linear motion
• combs are charged push / pulling one another

Question 6

Benefits of Electrostatic devices

Answer 6

• simplicity
• very low power use
• fast response
• deflection can be accurately controlled

Question 7

Disadvantages of electrostatic transducer

Answer 7

• require drive voltages in the 100-150V range
• sensors require carefully designed circuits to measure capacitance change in the presence of noise and interference
• large SA required for comb drive to generate forces equivalent to other microactuators

Question 8

Capacitance is defined as

Answer 8

electric energy stored per volt

Question 9

permittivity of a material can be influenced by

Answer 9

• humidity
• chemical reactions
• temp

Question 10

orientation, overlap or distance between plates can be changed by

Answer 10

• static pressure
• acceleration
• other physical phenomenon

Question 11

Equilibrium of electrostatic devices has two points

Answer 11

1) stable point:
2) meta stable point:

Question 12

Pull in voltage

Answer 12

A critical point of equilibrium at which the mechanical force is equal to the electrical force. This is the last voltage where the actuator can be controlled

Question 13

What happens if V pull in is exceeded?

Answer 13

The device will snap-in and may be damaged, or burnt out

Question 14

When V = Vpull how much displacement is seen?

Answer 14

The plate can be displaced up to 1/3 of the separation distance between the plates

Question 15

Beam stiffness can be added in__

Answer 15

series

Question 16

Solving for pull in displacement yields

Answer 16

Three solutions, only one will be physically realizeable

Question 17

How can comb drive capacitances be calculated?

Answer 17

Fringe capacitance is significant and cannot be ignored, so Electromagnetic finite element analysis can be used to compute the electric fields, then subsequently the C and force

Question 18

Scratch drive actuators work by

Answer 18

• pulling the device down with electrostatic force
• tip deflection moves the device forward when the electrostatic force is released
• electrical connection is not needed as they are capacitively coupled

Question 19

Three types of scratch drive actuators

Answer 19

• tethered SDAs
• Untethered SDAs
• steerable microactuators

Question 20

Operation principle of a mems condenser microphone

Answer 20

• pressure sensor for measuring acoustic pressure fronts created when sound waves travel through air or liquid
  composed of:
• parallel plate capacitor
• solid plate that moves under acoustic deformation
• perforated plate that allows waves through and is barely deformed .