Method and apparatus for damping diaphragm vibration in capacitance diaphragm gauges
A system and method mitigate the effects of these external vibrations on a capacitance diaphragm gauge by sensing the motion of the diaphragm at the first natural frequency of the diaphragm of the CDG. The presence of the natural frequency signals superimposed on the pressure signal is determined by sensing variations in the output of a sensor at or near the known natural frequency of the diaphragm and filtering that known low frequency from the output. The filtered signal is used in a feedback circuit to impose electrostatic forces on the diaphragm. The imposed electrostatic forces oppose the motion created by the external vibration to suppress the effects of the vibration on the pressure measured by the CDG.
1. A method for suppressing the effects of vibration on a capacitance diaphragm gauge (CDG) that generates an output signal having an amplitude that varies in accordance with pressure applied to the CDG, the method comprising:
processing the output signal to detect changes in amplitude of at least one frequency corresponding to a vibration frequency to generate a feedback signal responsive to the amplitude of the at least one frequency, wherein processing the output signal comprises:
applying the output signal to a bandpass filter having a band centered approximately at a resonant frequency of the diaphragm of the CDG to generate a band-limited signal responsive to the changes in amplitude of the output signal caused by vibrations;
applying the band-limited signal to a rectifier to generate a rectified signal having an amplitude responsive to the magnitude of the vibrations; and
applying the rectified signal to a feedback control circuit to generate the feedback signal having an amplitude selected to damp the movement of the diaphragm caused by vibration; and
applying the feedback signal to the diaphragm and the fixed electrode to cause the diaphragm to be deflected counter to the deflection caused by vibration to thereby suppress the deflection caused by vibration.
2. A vibration suppression system for a capacitance diaphragm gauge (CDG), the CDG including a diaphragm and at least one fixed electrode wherein the capacitance between the diaphragm and the at least one fixed electrode is responsive to a pressure applied to the CDG that deflects the diaphragm with respect to the at least one fixed electrode, the CDG including a signal source that generates a high frequency voltage that is applied between the diaphragm and the fixed electrode and including a pressure measuring circuit that receives and demodulates a high frequency signal from the diaphragm and the fixed electrode to detect changes in amplitude caused by capacitance changes resulting from pressure changes applied to the CDG, the vibration detection system comprising:
a bandpass filter that receives the high frequency signal from the diaphragm and the fixed electrode and that passes modulation components of the high frequency signal at a range of frequencies selected to encompass a resonant frequency of vibration of the diaphragm;
a rectifier that generates a rectified output signal having amplitudes responsive to the amplitudes of the components of the high frequency signal passed by the bandpass filter; and
a feedback control circuit that receives the rectified output signal and that generates a feedback signal applied between the diaphragm and the fixed electrode to apply an electrostatic force to the diaphragm, the feedback signal responsive to the rectified output signal to apply the electrostatic force to offset movement of the diaphragm caused by vibration.
3. The system as defined in claim 2 , wherein the feedback signal comprises a time-varying DC voltage applied between the diaphragm and the at least one fixed electrode that causes the diaphragm to deflect toward the at least one fixed electrode to offset the deflection of the diaphragm away from the fixed electrode caused by vibration.