Page 198 - Mechanics of Microelectromechanical Systems

4. Microtransduction: actuation and sensing 185
annihilate the thermal expansion of the heated bar. As long as the external
force is less than this threshold value‚ usually called bloc force‚ the total
displacement of the tip of the bar is the difference of two opposing
deformations‚ namely:

where F is the external force. Equation (4.3) can be rewritten in the form:

There is a linear relationship between the external force and the total linear
expansion‚ as Eq. (4.4) indicates. For zero axial displacement the
force is maximum and equal to the bloc force:

whereas its minimum value of zero occurs for the free displacement
which is:

Several of the actuators that are going to be presented in this chapter behave
similarly to the thermal actuator‚ and their force-displacement characteristic
can be written in the form:

The linear relationship of Eq. (4.7) is plotted in Fig. 4.2.

Figure 4.2 External force working against constant actuation force in terms of the output
displacement