Understanding Smart Sensors - Nomads.usp

58 Understanding Smart Sensors3.3.1 Capacitive SensingCapacitive sensors typically have one plate that is fixed and one that moves as aresult of the applied measurand. The nominal capacitance is C = Aε/d, where Ais the area of the plate, ε is the dielectric constant, and d is the distance betweenthe plates. Two common capacitive pressure sensors used in automotive applicationsare based on silicon and ceramic capacitors. A differential capacitorstructure can also be used for acceleration. Figure 3.4 shows the surfacemicromachined capacitor and the resulting output [7].(a)410Accelerometer output (in mV)310Roll-off data for 10-g signal21012310 10 10 10Frequency (in Hz)(b)Figure 3.4 (a) Differential capacitor structure in accelerometer and (b) output.4