DC polarization

AC polarization

Introduction

Applying a pure sinusoidal voltage

What happen if we add an offset voltage to the sinusoidal voltage?

And what about a square, triangular, or patatoidal voltage?

Conclusion

A ±30V voltage amplifier

First of all, electrosatic actuators dislike stray charge that may come from airborne particule, or by improper grounding. To lessen these effects you must ground everything than can be grounded and leave a ground plane just below the actuator. If you forget this you may expect unreliable behaviour and, for example, actuator working for a while and then stopping (build-up of charge in insulating layer that stick the actuator), or actuator operation varying with humidity level (a film of water helps to reduce the static charge by increasing the conductivity) or even actuator not working at all... Actually even with well grounded structure you may already be affected by these problems...

For a comb-drive actuator, where the force does not change with the displacement, and with a linearly increasing restoring force (ie, a spring) it means that the displacement also increases with the square of the voltage. Thus take care not to increase to rapidly your voltage:, when you go from 1 to 10 V, you increase the force by a factor of 100 and at 31.5V it is a factor of 1000!

Moreover, for a gap-closing actuator, where the force increase quadratically with the displacement, associated with a linearly increasing restoring force (ie, a spring), after the actuator has moved 1/3 of the initial gap width, it will snap through and 'close' the gap. This may cause a short-circuit if no insulator or landing point has been designed. However this problem does not always appear and I've noted that for doped polysilicon actuator short-circuit do not appear for moderate voltage, presumably because of the native oxide layer and the large rouhghness of the side that yields a very small contact area.

And finally, if the structure does not move? Hum, hum. Actually this is the most common case :-( Then either your structure is stuck (check your release process, and why not try SAM coating?) or more probably the voltage is not high enough (Actually, and to be honest, it means that your structure is too stiff or the electrodes surface too small :-). Then, you need a DC power supply which can deliver higher voltage or you may try to excite your structure by applying AC signal, which will give you more information on your structure.

F ~ ½V

Then the force is once more proportional to the square of the signal and thus it will have frequency components which will be a multiple of the original signal frequency, and including this frequency. The amplitude of the different harmonics will depend on the signal and may be computed using Fourier analysis. However it is not a simple calculus, as not only square of the signal will appear but also cross-product between harmonics which result in the generation of signal having a frequency corresponding to the difference of the two signal mixed.

Practically you will see your structure being excited in resonance with many different frequencies that you may mistakenly take for higher order mode. However, if you find that they are an integer multiple of the lowest resonnance frequency (which will be

R1 = 18kO; R2 = 1.2kO; R3 = 12k; C = 0.1µF, two BNC plugs for V

We obtained a gain of 11, an output voltage of 58Vpp (V