Methods for closed loop operation of capacitive accelerometers

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

• a proof mass moveable along a sensing axis in response to an applied acceleration;
• and
• first and second fixed capacitive electrodes arranged symmetrically either side of the proof mass along the sensing axis with a gap defined between each of the first and second fixed capacitive electrodes and the proof mass under zero applied acceleration;
• the method comprising:
• applying a DC voltage of magnitude VB to the proof mass based on a threshold acceleration value;
• applying a first drive signal V1 to the first fixed capacitive electrode, a second drive signal V2 to the second fixed capacitive electrode, the first and second drive signals each having a periodic waveform varying in amplitude between zero and a maximum value;
• sensing a displacement of the proof mass in response to an applied acceleration;
• determining whether the applied acceleration is greater than the threshold acceleration value;
• in response to determining that the applied acceleration is not greater than the threshold acceleration value, applying pulse width modulation to the first and second drive signals V1, V2 with a variable mark/space ratio so as to provide a net electrostatic restoring force on the proof mass for balancing the inertial force of the applied acceleration and maintaining the proof mass at a null position;
• and
• in response to determining that the applied acceleration is greater than the threshold acceleration value, applying pulse width modulation to the first and second drive signals V1, V2 with a constant mark/space ratio, and changing the magnitude, VB, of the DC voltage applied to the proof mass so as to provide a net electrostatic restoring force on the proof mass for balancing the inertial force of the applied acceleration and maintaining the proof mass at a null position.