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Instrumentation

Instrumentation. Dr. Xiaofeng Wu. 1. Reference for Sensors. Measurement and instrumentation: theory and application by Morris, Alan S and Langari, Reza. Piezoelectric Sensors. A piezoelectric material produces an electric charge when it is subjected to a force or pressure.

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Instrumentation

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  1. Instrumentation Dr. Xiaofeng Wu 1

  2. Reference for Sensors • Measurement and instrumentation: theory and application by Morris, Alan S and Langari, Reza

  3. Piezoelectric Sensors A piezoelectric material produces an electric charge when it is subjected to a force or pressure.

  4. Charge Sensitivity Model q is the charge generated by the pizeoelectric senor (pC) is the chare sensitivity of the transducer (pC/g, pC/lb, pC/N, pC/psi, pC/pa) a is the mechanical quantity being measured 4

  5. Voltage Follower Circuit for Piezoelectric Sensors 5

  6. Displacement, Velocity and Acceleration Measurement in Inertial Space

  7. Dynamic Response of the Seismic Model

  8. Measurement Errors and Uncertainties Errors are a property of the measurement. Measurement is the process of assigning a value to a physical variable based on a sampling from the population of that variable. Uncertainty is a property of the result. The outcome of a measurement is a result, and the uncertainty quantifies the quality of that result. Errors are effects, and uncertainties are numbers. While errors are the effects that cause a measured value to differ from the true value, the uncertainty is an assigned numerical value that quantifies the probable range of these errors.

  9. Distribution of Errors

  10. Propagation of Uncertainty A general relationship between some dependent variable y and a measured variable x

  11. Quantization Uncertainty and Error

  12. Design Stage Uncertainty The zero-order uncertainty, u0, assumes that the variation expected in the measured values will be only that amount due to instrument resolution and that all other aspects of the measurement are perfectly controlled.

  13. Example 1 • An accelerometer with a charge sensitivity of 100pC/g ±1pC/g (95%) and a capacitance of 200pF ± 2pF (95%) is connected to a voltage follower with an input connector capacitance of 20pF (in parallel with the cable capacitance), a 10,000pF blocking capacitor and a 100MΩ resistance. A 3m long cable with a capacitance of 280pF connects the accelerometer and the voltage follower. Determine: • The schematic of the instrument; • The instrument’s sensitivity in mv/g; • The uncertainty of vo if the acceleration is 1g. • If we use a 5V 10-bit A/D to sample the voltage output, what is the uncertainty in the digital output. 13

  14. Solution

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