Current drive methods to extend the range of travel of electrostatic microactuators beyond the voltage pull-in point

When a voltage source drives an electrostatic parallel plate actuator, the well-known pull-in instability limits the range of displacement to 1/3 of the gap. Different strategies have been reported to overcome this limitation. More recently, experimental results [1] have been presented using a capacitor in series with the actuator [2]. Nevertheless, this strategy requires higher voltage than the pull-in voltage value to achieve full range of travel. In order to reduce the operating voltage, a switched-capacitor configuration has been also proposed [3]. In this paper, two different approaches are introduced to control charge in the actuator by means of current driving. Theoretical equations derived for each method show that full range of travel can be achieved without voltage penalty. Both approaches are based on the use of current pulses injecting the required amount of charge to fix the position of the movable plate. However, the first method uses a series capacitance to sense the voltage drop produced by the injected charge [4]. Depending on the sensed voltage more charge is injected, switching on the driving current source, or removed, with the current sink. The second approach operates in open loop configuration [5]. The current source is switched off after injecting the required charge to fix the movable electrode to the desired position. The position shift, due to the leakage currents, is eliminated by a refresh cycle. Experimental measurements, showing that displacement beyond the pull-in point can be achieved, are in good agreement with the theoretical and the predicted simulated behavior.