Accelerometer control

An accelerometer closed loop control system comprising:

• a capacitive accelerometer comprising a proof mass moveable relative to first and second fixed capacitor electrodes;
• a pulse width modulation (PWM) generator arranged to generate in-phase and anti-phase PWM drive signals with a drive frequency and an adjustable mark/space ratio, wherein said in-phase and anti-phase PWM drive signals are applied to the first and second fixed capacitor electrodes respectively such that they are charged alternately, wherein the in-phase PWM drive signal has a first amplitude and the anti-phase PWM drive signal has a second amplitude;
• an output signal detector arranged to detect a pick-off signal from the accelerometer representing a displacement of the proof mass from a null position to provide an error signal, wherein the null position is the position of the proof mass relative to the first and second fixed capacitor electrodes when no acceleration is applied;
• a PWM servo operating in closed loop arranged to vary the adjustable mark/space ratio of said in-phase and anti-phase PWM drive signals in response to the error signal so that mechanical inertial forces are balanced by electrostatic forces to maintain the operating point of the proof mass at a null position;
• and
• a differential voltage servo arranged to vary a difference between the first amplitude of the in-phase PWM drive signal and the second amplitude of the anti-phase PWM drive signal in response to the error signal.