Abstract
The mechanical stability of the seismic mass (movable electrode) in a PWM electrostatic servo accelerometer which was fabricated by using a silicon micro-machining technique was studied theoretically and experimentally.
The movable electrode is controlled so as to be parallel to the fixed electrodes in the usual servo-operation. From an analysis using an energy model considering electrostatic energy of the electric field between the movable and the fixed electrodes and elastic energy of the cantilever beam which sustains the movable electrode, the occurrence of rotation of the movable electrode around a certain axis was expected at pulse voltages higher than a certain threshold voltage. This threshold voltage VT is proportional to the square root of the rotational spring constant of the cantilever beam when the movable electrode rotates around a certain axis.
This rotation phenomenon of the movable electrode was confirmed by optical observation of the sensor and the measured threshold voltages VT were in close agreement with those calculated by the above energy model.
The maximum measurable acceleration αmax in a PWM electrostatic servo accelerometer is proportional to the square of the pulse voltage which is restricted by the threshold voltage VT and is larger in the sensor structure with larger rotational spring constant of the cantilever beam. for example, αmax for a 2-beam sensor is larger than that for a 1-beam sensor.
