Page 105 - An Introduction to Microelectromechanical Systems Engineering

84 MEM Structures and Systems in Industrial and Automotive Applications

feedback voltage then becomes a measure of the disturbing force. This feature is
integral to the closed-loop operation of many accelerometers and yaw-rate sensors.

Piezoelectric Actuation
Piezoelectric actuation can provide significantly large forces, especially if thick pie-
zoelectric films are used. Commercially available piezoceramic cylinders can provide
up to a few newtons of force with applied potentials on the order of a few hundred
volts. However, thin-film (<5 µm) piezoelectric actuators can only provide a few
millinewtons. Both piezoelectric and electrostatic methods offer the advantage of
low power consumption as the electric current is very small.

Thermal Actuation

Thermal actuation consumes more power than electrostatic or piezoelectric actua-
tion but can provide, despite its gross inefficiencies, actuation forces on the order of
hundreds of millinewtons or higher. At least three distinct approaches have emerged
within the MEMS community. The first capitalizes on the difference in the coeffi-
cients of thermal expansion between two joined layers of dissimilar materials to
cause bending with temperature—the classic case of a bimetallic thermostat studied
by S. Timoshenko in 1925 [4]. One layer expands more than the other as tempera-
ture increases. This results in stresses at the interface and consequently bending of
the stack. The amount of bending depends on the difference in coefficients of ther-
mal expansion and absolute temperature. Unfortunately, the latter dependence
severely limits the operating temperature range—otherwise, the device may actuate
prematurely on a hot day.
In another approach known as thermopneumatic actuation, a liquid is heated
inside a sealed cavity. Pressure from expansion or evaporation exerts a force on the
cavity walls, which can bend if made sufficiently compliant. This method also
depends on the absolute temperature of the actuator. Valves employing this method
will be described later in this chapter.
Yet a third distinct method utilizes a suspended beam of a same homogeneous
material with one end anchored to a supporting frame of the same material [5].
Heating the beam to a temperature above that of the frame causes a differential
elongation of the beam’s free end with respect to the frame. Holding this free end
stationary gives rise to a force proportional to the beam length and temperature dif-
ferential. Such an actuator delivers a maximal force with zero displacement, and
conversely, no force when the displacement is maximal. Designs operating between
these two extremes can provide both force and displacement. A system of mechani-
cal linkages can optimize the output of the actuator by trading off force for displace-
ment, or vice versa. Actuation in this case is independent of fluctuations in ambient
temperature because it relies on the difference in temperature between the beam and
the supporting frame. A plate microvalve utilizing this actuation scheme is described
later.

Magnetic Actuation

Lorentz forces form the dominant mechanism in magnetic actuation on a miniatur-
ized scale [6]. This is largely due to the difficulty in depositing permanently