Accelerometers

摘要

A method for closed loop operation of a capacitive accelerometer comprising: a proof mass (102, fig 1a); first and second sets of both fixed (110,112) and moveable capacitive electrode fingers, interdigitated with each other; the method comprising: applying PWM drive signals to the fixed fingers; sensing displacement of the proof mass and changing the mark:space ratio of the PWM drive signals, to provide a restoring force on the proof mass that balances the inertial force of the applied acceleration and maintains the proof mass at a null position; detecting when the mark:space ratio for the null position is beyond a predetermined upper or lower threshold; and further modulating the PWM drive signals by extending or reducing x pulses in every y cycles, where x³1 and y>1, to provide an average mark:space ratio beyond the upper or lower threshold without further increasing or decreasing the mark length of the other pulses. Thus an "over range" condition outside the normal dynamic range of the accelerometer is detected. To be able to respond to high magnitude accelerations without further increasing or decreasing the mark length of all of the PWM pulses, which would otherwise degrade the accuracy of the sensing system, the PWM drive signals are further modulated by extending or reducing some but not all of the pulses. E.g. extending or reducing at least one pulse in every other cycle, or less frequently. The result is an average mark:space ratio, averaged over a number of cycles, that is beyond the predetermined upper or lower threshold, while retaining a maximum and minimum mark length for the other pulses and thus retaining a settling time for the sensing system between at least some of the pulses.