Driving Mechanisms, Scaling Laws, and Materials

A new type of structure was invented to overcome the limited deflection capability of the parallel plate capacitor the so-called comb drive [302]. An example of this is shown in Fig. 6.111. This structure can easily be im- 

Another very promising approach is the use of piezoelectric poljuner materials such as PVDF and its copol3Tneis that usually are extruded into foils 

The operating range of piezoelectric materials is limited to well below their Curie temperature, which is typically 150... 300°C for ceramics and only 70... 90°C for polymers. Piezoelectric drives generally exhibit hysteresis which can be compensated by sophisticated electronic circuitry. For applications where a purely resonant mode of operation is desired, hysteresis poses no problem. 

All thermal actuators have a reputation of being rather slow, due to thermal time constants typically in the upper millisecond range, particularly for the cooling phase. Large forces can usually be achieved at the expense of considerable power consumption. In small structures, however, it has been shown that substantially higher speeds can be attained, due to reduced thermal time constants. Thus, an accelerometer based on a thermally actuated resonant read-out principle was shown to operate at a frequency of 400 kHz [308], 

For some applications the simultaneous incorporation of two actuation principles in a hybrid way can be of interest. An example for this is the combined use of electromagnetic and electrostatic forces in a microvalve [311]. The electromagnetic force is applied for generating large displacements in opening or closing the valve, whereas the electrostatic force can keep the valve in its closed position with very little power consumption. Similarly, the 

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