Slide 1'&$%ECE/CE 3720: Embedded System DesignChris J. MyersLecture 22: TransducersSlide 2'&$%Introduction• Many embedded systems are required to collectinformation about the environment.• Such a system is called a data acquisition system (DAS).• Sometimes aquisition of data is system’s fundamentalpurpose: voltmeter, thermometer, camera, etc.• Other times it is only part, such as in control orcommunication systems.1Slide 3'&$%Design Components of a DAS(See Figure 12.1)Slide 4'&$%Signal Paths of a DAS(See Figure 12.2)2Slide 5'&$%Accuracy• Instrument accuracy is absolute error of the entiresystem including transducer, electronics, and software.• Let xmibe measured value and xtibe true value:Average accuracy =1nnXi=1|xti− xmi|Average accuracy of reading =100nnXi=1|xti− xmi|xtiAverage accuracy of full scale =100nnXi=1|xti− xmi|xtmaxMaximum error = max |xti− xmi|Maximum accuracy of reading = 100 max|xti− xmi|xtiMaximum accuracy of full scale = 100 max|xti− xmi|xtmaxSlide 6'&$%Resolution• Instrument resolution is smallest input signal difference,∆, that can be detected by entire system.• Can be limited by noise in either the transducer orelectronics.• Spatial resolution of the transducer is the smallestdistance between two independent measurements.• Determined by size and mechanical properties of thetransducer.3Slide 7'&$%Precision• Precision is number of distinguishable alternatives, nx,from which result is selected.• Can be expressed as bits or decimal digits.• Range is equal to resolution times precision:rx= ∆x· nx(See Table 12.2)Slide 8'&$%Reproducibility• Reproducibility specifies whether the instrument has equaloutputs given identical inputs over some time period.• Specified as full range or standard deviation of outputresults given a fixed input.• This type of error often comes from transducer drift.• Statistical control is similar parameter based onprobabilistic model that defines errors due to noise.4Slide 9'&$%Transducers(See Figure 12.3 and Table 12.3)Slide 10'&$%Transducer Linearity• Linearity is measure of the straightness of the staticcalibration curve.f(ax1+ bx2) = af(x1) + bf(x2)yi= mxi+ bAverage linearity of reading =100nnXi=1|yi− mxi− b|yiAverage linearity of full scale =100nnXi=1|yi− mxi− b|ymax5Slide 11'&$%Transducer Linearity(See Figure 12.4)Slide 12'&$%Transducer Sensitivity and Specificity• Static sensitivity is defined to be:m =∆y∆x• If linear, sensitivity is slope of straight line throughcalibration curve with minimum mean squared error.• Transducers often sensistive to environmental issues (ex.temperature, pressure, vibration, electric fields, etc.).• Specificity is measure of relative sensistivity to desiredsignal compared with sensitivity to other influences.• Signal-to-noise (S/N) ratio is a measure of specificity.6Slide 13'&$%Transducer Impedance• Input impedance is a measure of the effect of the presenseof the transducer on the measurement itself.• Thermocouple has bad input impedance because heat canconduct along the probe inserted in the medium.• An infrared detector has good input impedance becauseit makes no physical contact.Slide 14'&$%Transducer Drift(See Figure 12.6)7Slide 15'&$%Manufacturing Issues• Transducers are often a critical device.• Higher quality transducer produce better signals but atincreased cost.• Availibity of a second source is useful.• Standard cables and connectors simplify construction.• Power requirements, size, weight of device also important.• If calibration necessary, increases manufacturing costs.Slide 16'&$%Dynamic Transducer Specifications(See Figure 12.7 and Table 12.4)8Slide 17'&$%Nonlinear Transducer Responses(See Figure 12.8)Slide 18'&$%Potentiometer-Based Position Sensor(See Figure 12.9)9Slide 19'&$%Linear Variable Differential Transducer (LVDT)(See Figure 12.10)Slide 20'&$%Ultrasonic Pulse-Echo Transducer(See Figure 12.11)10Slide 21'&$%LED-Photosensor Pair(See Figure 12.12)Slide 22'&$%Calculation of Velocity and Acceleration(See Figures 12.13 and 12.14)11Slide 23'&$%Strain Gage(See Figures 12.15 and 12.16)Slide 24'&$%Thermistors(See Figure 12.17 and Table 12.6)12Slide 25'&$%Thermocouples(See Figures 12.18 and 12.19)Slide 26'&$%Thermocouples(See Figures
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