Accelerometers

A method for open loop operation of a capacitive accelerometer, the capacitive accelerometer comprising:

• a substantially planar proof mass mounted to a fixed substrate by flexible support legs so as to be linearly moveable in an in-plane sensing direction in response to an applied acceleration;
• first and second sets of moveable capacitive electrode fingers extending from the proof mass, substantially perpendicular to the sensing direction and spaced apart in the sensing direction;
• and
• first and second sets of fixed capacitive electrode fingers extending substantially perpendicular to the sensing direction and spaced apart in the sensing direction;
• wherein the first set of fixed capacitive electrode fingers is arranged to interdigitate with the first set of moveable capacitive electrode fingers and the second set of fixed capacitive electrode fingers is arranged to interdigitate with the second set of moveable capacitive electrode fingers;
• wherein a null position is defined as a spacing of the interdigitated fixed and moveable capacitive electrode fingers when the applied acceleration is zero;
• the method comprising:
• a first mode of operation which comprises electrically measuring a deflection of the proof mass including the moveable capacitive electrode fingers from the null position under an applied acceleration;
• a second mode of operation which comprises applying electrostatic forces to said first and second sets of fixed capacitive electrode fingers, in order to cause said proof mass including the first and second sets of moveable capacitive electrode fingers to deflect from the null position, and electrically measuring the forced deflection so caused, wherein applying said electrostatic forces to said fixed capacitive electrode fingers in said second mode of operation comprises applying an in-phase PWM drive signal to the first set of fixed capacitive electrode fingers and a corresponding anti-phase PWM drive signal to the second set of fixed capacitive electrode fingers;
• and
• applying a PWM drive signal with a mark-to-space ratio that is varied in time during the second mode.