Understanding Smart Sensors - Nomads.usp

The Nature of Semiconductor Sensor Output 59Silicon technology combined with surface micromachining has allowedthe capacitance between interdigitated fingers to measure acceleration andother inputs. The value of the nominal capacitance is in the range of 100 fF to1 pF and the variation in capacitance is in femtofarads. Circuitry integrated onthe same silicon chip or in the same package (that takes into account the parasiticcapacitance effect of packaging) can use that level of signal to provide auseful output for a control system [8].3.3.2 Piezoelectric SensingA piezoelectric sensor produces a change in electrical charge when a force isapplied across the face of a crystal, ceramic, or piezoelectric film. The inherentability to sense vibration and the necessity for high-impedance circuitry aretaken into account in the design of modern piezocrystal sensors. Transducersare constructed with rigid multiple plates and a cultured-quartz sensing elementthat contains an integral accelerometer to minimize vibration sensitivityand suppress resonances.Lead zirconate titanate (PZT) ceramic is used to construct biomorphtransducers that sense motion, vibration, or acceleration and can be activatedby an applied voltage. The biomorph consists of two layers of different PZTformulations that are bonded together in rectangular strips or washer shapes.The rectangular shape can be mounted as a cantilever. Motion perpendicular tothe surface generates a voltage [9].More recently, piezofilm sensors that produce an output voltage when theyare deflected provide a method for inexpensive measurements. Figure 3.5(a)shows the construction of a piezoelectric film sensing element [10]. The polymerfilm generates a charge when it is deformed and also exhibits mechanicalmotion when a charge is applied to it. For a vehicle detector, the sensor ismounted in an aluminum channel filled with polyurethane and embedded inthe pavement. The output of the sensor for three different types of vehicles isshown in Figure 3.5(b). In the voltage mode, both the weight and the speed ofthe vehicle passing over it influence the voltage. A positive output occurs whenthe tire compresses the film. The negative output results from expansion afterthe tire has passed over the sensor.Surface micromachining techniques have been combined with piezoelectricthin-film materials, such as zinc oxide, to produce a semiconductor piezoelectricpressure sensor. One design has a sensitivity of 0.36 mV/mbar at 1.4 kHz[11]. Low-level acoustic measurements are a potential application for thetechnology.