A digital force balanced instrument such as an accelerometer that incorporates a directly digital electrostatic forcer with servo loop plates charged by a digital feedback loop is disclosed. The accelerometer has a proofmass electrode suspended in a cantilever configuration between electrodes on either side thereof, which proofmass is displaced upon acceleration. Each side electrode includes a plurality of electrostatic plates sized in binarily weighted multiples of area. An induced signal in the proofmass electrode is quantized via A/D converter in the feedback loop, which activates the appropriate number of plates to force the proofmass to an initial position.
An acceleration sensor has a mass part movably attached over a substrate with pairs of electrodes arranged relative to the mass part so that one electrode of each pair is under the mass part and the other electrode of that pair is arranged above the mass part. The electrodes are attached immovably to the substrate. Controllable electrical voltages can be applied such via an electronic drive circuit to the electrically conductively doped mass part and to these electrodes so that excursions of the mass part can be electrostatically compensated and, at the same time, the magnitude of inertial forces acting on the mass part, and thus the magnitude of accelerations, can be measured.
A ratiometric circuit (40) includes a differential capacitive sensor (42) for sensing a change in capacitance, a differential capacitive detector (46) operatively connected to the differential capacitive sensor for detecting a change in the capacitance of the differential capacitve sensor, an error voltage generator (48) operatively connected to the differential capacitve detector for generating a corrective voltage in response to the detected change in capacitance, and a bias circuit (50) operatively connected to the differential capacitive sensor for generating a bias voltage inversely proportional to and independent of a supply voltage for maintaining the ratiometricity between an output voltage of the differential capacitive sensor and the supply voltage.
A force balanced instrument system and method for mitigating errors is provided. The system and method mitigate errors in measurement readings caused by charge buildup in force balanced instruments that employ charge pulses to generate an electrostatic force to null an inertial proof mass disposed between opposing electrodes. The system and method mitigate charge buildup by applying positive charge pulses alternately to each opposing electrode for a given charge cycle time period followed by negative charge pulses alternately to each opposing electrode for a second given charge cycle time period. The negative charge pulses remove any residual charge on the electrodes caused by the positive charge pulses. As a result the net residual charge left on the electrodes is reduced on the average.
