Page 116 - MEMS Mechanical Sensors

5.6 Actuation Techniques 105

1 εε AV 2
2
W = CV = 0 r (5.30)
2 2 g
and the force between the plates is given by

dW εε AV 2
F = = 0 r (5.31)
dg 2 g 2

It is therefore clear that the force is a nonlinear function of both the applied volt-
age and the gap separation. Use of closed loop control techniques can linearize the
response.
An alternative type of electrostatic actuator is the so-called comb-drive, which is
comprised of many interdigitated electrodes (fingers) that are actuated by applying a
voltage between them. The geometry is such that the thickness of the fingers is small
in comparison to their lengths and widths. The attractive forces are therefore mainly
due to the fringing fields rather than the parallel plate fields, as seen in the simple
structure above. The movement generated is in the lateral direction, as shown in
Figure 5.14, and because the capacitance is varied by changing the area of overlap
and the gap remains fixed, the displacement varies as the square of the voltage.
The fixed electrode is rigidly supported to the substrate, and the movable elec-
trode must be held in place by anchoring at a suitable point away from the active
fingers. Additional parasitic capacitances such as those between the fingers and the
substrate and the asymmetry of the fringing fields can lead to out-of-plane forces,
which can be minimized with more sophisticated designs.
Electrostatic actuation techniques have also been used to developed rotary
motor structures. With these devices, a central rotor having surrounding capacitive
plates is made to rotate by the application of voltages of the correct phase to induce
rotation. Such devices have been shown to have a limited lifetime and require lubri-
cation to prevent the rotor from seizing. The practical use has therefore been lim-
ited, but they are, nevertheless, the subject of intensive research.

electrode

Fixed MOTION

electrode

Movable

Figure 5.14 An illustration of the electrostatic comb-drive actuator.