The Capacitive Sensor - Pressure and Accelerometer
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Sensors based upon the capacitive sensing technique are strain-based sensors. The typical configuration is shown below where the sensor capacitances are arranged in a push-pull, half-bridge configuration where both capacitors are parameter modulated. Simple media isolation is achieved when one capacitance is parameter-modulated and the other capacitance within the half-bridge circuit is an unmodulated reference capacitance.
Capacitive sensors require a dynamic excitation and all capacitive designs contain an internal oscillator and signal demodulator to provide static capable outputs. In most cases these components will limit the useful operating temperature range from -40ºC to +120ºC.
Capacitive-Based Pressure Sensor
The sensing spring member within the capacitive pressure sensor is conductive or has conductive surfaces deposited upon it and is positioned between two fired-alumina ceramic or glass-compound capacitor support structures.
The capacitor support structures are electrically isolated, where the capacitor plates are screen-printed or vapor-deposited onto the support structures as shown. The push-pull symmetry arrangement results in the capacitance of one side of the sensor module increasing and the other decreasing when unbalanced pressures act upon the spring member.
Non-isolated capacitance designs allow the measurand to contact the sensor spring member, which, in the case of the pressure sensor as shown, deflects towards or away from each of the capacitor support structures as a function of the difference in pressure acting upon the spring member.
Some designs use flexible metal diaphragms and silicone oil-filled cavities to prevent the medium from contacting the active capacitor structures. An alternative capsule design isolates the medium from direct influence on the modulated capacitance sacrificing "push-pull" mechanical symmetry. Only one of the two capacitance structures is parameter-modulated where the other capacitor is present as a reference capacitance for the dual purposes of thermal-zero compensation and to complete the capacitance half-bridge circuit geometry. The sensor configuration shown results in a media-tolerant but lower performance design due to the lack of push-pull symmetry.
The Capacitance-Based Accelerometer
Similar push-pull configurations are used in the design of capacitance based accelerometers as shown below, where the center plate forms the seismic mass and spring assembly. The large physical area of the moving seismic mass assembly, in the capacitance accelerometer, facilitates gas-film damping which provides a mechanical damping factor that is less sensitive to thermal change than the fluidically-damped accelerometer types. One advantage of the half-bridge configuration is that, for slow thermal change, the symmetry of the module is maintained, where thermal growth of the side walls of the module occurs to both capacitance elements.
Under transient thermal conditions some differential expansion of the capacitance structures is to be expected although the very low expansion coefficient of the alumina capacitance modules limits transient outputs. Infrequently a sensor manufacturer blends just the right combination of cost and performance attributes into a sensor design that is useful for a very wide variety of measurements.
The Setra Systems Inc. model 141 capacitive, gas damped accelerometer is such a device. The model 141 accelerometer is a superb instrument for general purpose static as well as dynamic use offering exceptionally low thermal transient outputs.
The mechanical qualities of the fired-alumina spring member are excellent, in possessing high stiffness, low thermal expansion coefficients, high stability, and low hysteretic losses.
The inert nature and zero porosity of the alumina renders it useful in a wide variety of environments.
This article is taken from the Handbook 'The Art of Practical and Precise Strain Based Measurement' by James Pierson
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