Understanding Smart Sensors - Nomads.usp

50 Understanding Smart Sensorsdynamic nature of the signal, and the effect of other inputs on the measurand.The environmental considerations include operating temperature, chemicalexposure, and media compatibility. The measurement systems also apply constraints.Factors such as signal conditioning, signal transmission, data display,operating life, servicing/calibration, impedance of sensor, impedance of system,supply voltage, frequency response, and filtering may make one solution preferableover another. In some cases, the sensor’s design can make more than onesensing technology acceptable. That depends on the design and manufacturingcapability of the sensor supplier. For example, ceramic and semiconductor sensorsusing capacitive or piezoresistive technology frequently compete for a particularapplication. Also, optoelectronic, Hall-effect, magnetoresistive, andeven inductive sensors can measure displacement or velocity. However, once atechnology has been accepted for a particular measurement, displacement by analternative technology is difficult.Determining the proper sensing technology for a particular applicationbegins with understanding the fundamental design principle of the sensor andthe specifications that the manufacturer guarantees. Semiconductor sensorshave defined new terms and allow alternative design methodology for sensorsbased on the micrometer and nanometer scale in which they operate. Thischapter uses piezoresistive pressure sensor examples to explain some key sensorparameters and a nontraditional design approach.3.2.1 Wheatstone BridgeThe change in the resistance of a material when it is mechanically stressed iscalled piezoresistivity. Strain-gauge pressure sensors convert the change in resistancein four (sometimes only one or two) arms of a Wheatstone bridge, asshown in Figure 3.1(a). The output voltage of a four-element Wheatstonebridge, shown in Figure 3.1(b), is given byE RE = ∆ (3.1)oRwhere E o is the output voltage, E is the applied voltage, R is the resistance of allbridge arms, and ∆R is the change in resistance due to an applied pressure [1].Additional variable resistive elements are typically added to adjust the zero offset,calibrate sensitivity, and provide temperature compensation. The Wheatstonebridge can be operated in a constant voltage mode or a constant currentmode. The constant voltage mode is more common because it is easier to generatea controlled voltage source. However, the constant current mode is also