Capacitive force-balance system for measuring small forces and pressures

Apparatus for detecting changes in force or pressure, comprising

• a tuned circuit having a capacitor with at least one movable plate for sensing force or pressure changes, said tuned circuit being resonant at a first frequency when there is a zero net force or pressure acting on the movable plate,
• an oscillator tuned to a second frequency which is different from the first frequency.
• means responsive to the output of the oscillator for generating a first output signal at the second frequency for supplying current to the resonant circuit, and including means for generating a second output signal having a phase that is modulated about a prescribed center phase in response to force or pressure changes sensed by the capacitor, and
• means for generating an error signal based on the modulations of phase of the second signal with respect to the phase of the oscillator output for controlling the amplitude of the drive supplied to the tuned circuit.

Apparatus for detecting changes in force or pressure, comprising

• a tuned circuit having a capacitor with at least one movable plate for sensing force or pressure changes on one of its sides, said tuned circuit being resonant at a first frequency when there is a zero force or pressure difference on the two sides of the movable plate,
• an oscillator tuned to a second frequency which is less than the first frequency,
• means responsive to the output of the oscillator for generating a first output signal at the second frequency for supplying current to the resonant circuit, and including means for generating a second output signal phase modulated about a center phase which is in quadrature with the phase of the oscillator output in response to pressure changes sensed by the capacitor, and
• means for generating an error signal based on the difference in phase between the outputs of the oscillator and the generating means for controlling the amplitude of the drive supplied to the tuned circuit.

A force or pressure sensor, which comprises

• a tuned circuit having a capacitor with a movable element adapted for sensing the force or pressure,
• an oscillator driving the tuned circuit and operating at a frequency different from the natural resonant frequency of the tuned circuit,
• means for generating an error signal by comparing the phase of the response from the tuned circuit to the phase of the oscillator output, the error signal being connected to control the amplitude of the oscillator output and to an output terminal of the sensor,
• whereby electrostatic force from voltage appearing across the capacitor deflects the movable element and changes the resonant frequency of the tuned circuit to the frequency of the oscillator and whereby application of the force or pressure to the movable element causes deviations in the error signal related to the magnitude of the force or pressure.