Page 209 - Mechanics of Microelectromechanical Systems

196 Chapter 4
(DC) or alternating-current (AC)‚ and thus actuation is achieved. Conversely‚
when relative motion about one of the three Cartesian directions indicated in
the figure takes place‚ the change in the gap between two adjacent bodies
will translate in a capacitive change that can further be used as a sensing
metric of the mechanical motion in an external circuit. The attraction force
between two oppositely-charged plates is generated by the electric field E‚ as
shown in Fig. 4.17.

Figure 4.16 Potential motion directions of a charged mobile plate when attracted by three
fixed Cartesian wall-plates

Figure 4.17 Attractive nature of the electrostatic force between two charged plates

MEMS devices that use the electrostatic-force principles for transduction
are most of the time designed to be sensitive about one direction of motion.
A simple solution that prevents motion about one direction‚ while enabling
mobility about a different axis‚ is shown in Fig. 4.18. By placing the mobile
central member symmetrically in terms of the y-axis with respect to the fixed