The figure illustrates an improved electronic circuit that generates a signal proportional to either the instantaneous vibrational velocity or the instantaneous position (equivalently, the instantaneous vibrational displacement) of a vibratory capacitive sensor of the type discussed in the three preceding articles. This circuit offers some advantages over circuits designed previously for the same purpose; in particular, there is no need for a reference capacitor, and the circuit is insensitive to capacitive loading of its sensing amplifier.
The vibratory sensor includes a resonator equipped with capacitor electrodes in a manner similar to that of the vibration-regulating scheme described in the second of the preceding articles: Mounted on the moving part of the resonator are two electrodes - the driven and sensing plates- that face corresponding fixed driven and sensing plates. Each pair of electrodes serves as a capacitor; the driven one for electrostatically exciting vibrations, the sensing one for measuring the vibrational displacement or velocity.
Also as in the vibration-regulating scheme, the negative-feedback path of the transconductance amplifier is utilized to maintain the fixed sensing plate at a voltage equal to the dc bias applied to the "+" terminal of the transconductance amplifier. Therefore, in the absence of additional vibratory and/or electronic inputs, this circuit behaves similarly to the corresponding part of the vibration-regulating circuit; namely, the instantaneous output of the transconductance amplifier is a voltage proportional to the instantaneous vibrational velocity.
Additional vibratory/electronic input is necessary to obtain a position signal. In particular, a voltage alternating at a frequency much greater than that of the vibration to be measured is applied to the driven plates to excite a superimposed sinusoidal vibration much smaller than the vibration to be measured. This small, high-frequency vibration gives rise to a velocity signal in the manner described above. The capacitance varies with the position of the sensing plate in a known way, and the magnitude of the velocity signal varies in proportion to the change in capacitance. Thus, the instantaneous magnitude of the velocity signal serves as an indicator of the instantaneous position of the sensing plate.
This work was done by Christopher Stell and Vatché Vorperian of Caltech for NASA's Jet Propulsion Laboratory.