Understanding Smart Sensors - Nomads.usp

6 Understanding Smart Sensorsto provide an output. In addition to the desired input (e.g., pressure and undesiredenvironmental effects, such as temperature, humidity, and vibration) arefactors that affect the performance and accuracy of the transducer, factors thatmust be taken into account during the design of the transducer. Compensationfor those secondary parameters historically has been performed by additionalcircuitry, but with smart sensing technology the compensation can be integratedon the sensor or accomplished in the microcontroller.The output of a micromachined piezoresistive silicon pressure sensor andthe effect of temperature on both the span and the offset are demonstrated inFigure 1.3 [4]. Although the output is quite linear, in this case within 0.1% fullscale (F.S.), the output varies due to the effect of temperature on the span of thesensor by about 0.12 mV/°C. Because that signal level is insufficient to directlyinterface to a control IC, additional amplification and calibration typically areperformed in the next stage of a transducer.In a simple control system, the sensor is only one of three items requiredto implement a control strategy. The sensor provides an input to a controllerwith the desired strategy in its memory, and the controller drives an outputstage to modify or maintain the status of a load, such as a light, a motor, a solenoid,or a display. As shown in Figure 1.4, a signal conditioning interface typicallyexists between the sensor(s) and the controller and between the controller706050Output (in mVdc)403020−40°C+25°CSpanOutput10+125°COffset00102030405060708090100110Pressure differential (in kPa)Figure 1.3 Effect of temperature on piezoresistive pressure sensor output.