Understanding Smart Sensors - Nomads.usp

108 Understanding Smart Sensorslinearization, to provide PWM outputs for control, and to provide autozeroing/autoranging.The operating frequency and switching capability of theMCU must be considered in system design. Finally, the MCU’s computingcapability can be used in place of sensor(s) when sufficient information exists.5.5.1 LinearizationSensor nonlinearity can be improved by the use of table lookup algorithms.The variation in sensor signal caused by temperature can also be improved byusing an integrated temperature sensor and a lookup table to compensate fortemperature effects while linearizing the output, nulling offsets, and settingfull-scale gain from information stored in an EEPROM. Lookup tables can beimplemented in masked ROM, field programmable EPROM, or onboardEEPROM.Compensation for nonlinearity and the number of measurements duringthe test and calibration procedure can be simplified if a nonlinear output correlateswith a sensor design parameter. For example, a strong correlation wasfound between the span and the linearity of a pressure sensor with a thin diaphragm[9]. As Figure 5.7 shows, the nonlinearity increased to almost 5% withthe highest span units.Analytical techniques were investigated to improve the nonlinearity. Apolynomial regression analysis was performed on 139 sensor samples rangingfrom 30- to 70-mV full-scale span to determine the coefficients B 0 , B 1 , and B 2in the formula:Vout2( B0 B1 P B2P B3P )3= V + + ⋅ + ⋅ + + … (5.1)offwhere B 0 , B 1 , B 2 , and B 3 are sensitivity coefficients. The second-order termswere sufficient for calculations to agree with measured data with a worst casevalue for calculated regression coefficient = 0.99999. The relationship of thosevalues to the span allowed a piecewise linearization technique with four windowsto reduce the linearization error of most sensors to less than 0.5%. Thosecalculations could be included in the MCU lookup table for improved accuracyin an application. Others have also investigated linearization in great detail as ageneral means to improve sensor accuracy [10, 11].5.5.2 PWM ControlThe PWM output from the MCU can be used to convert an analog sensor outputto a digital format for signal transmission in remote sensing or noisy