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13Acquisition - MAE 591 RANDOM DATA Data Acquisition and...

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MAE 591 RANDOM DATA Data Acquisition and Processing 1 1. Data acquisition: Type of data z z z z z z 1. Continuous (analog) data 2. Originally digital data: stock market, labour statistics, annual rainfall - all physical quantities Problems associated with continuous data 1. Measurement: transducer converts physical quantity into an electrical voltage 2. Transmission/Recording 3. A/D conversion format Problems associated with transducers and signal conditioning: thermocouple, pressure sensor, accelerometer, laser/optical displacement sensor displacement sensor, proximity probe, strain gauge, microphone, hydrophone 1. Linearity: L max 2. Hysteresis 3. Attenuation in gain and phase over frequency range: normally 20% of the fundamental natural frequency of transducer 4. Signal conditioner source impedance: normally lower than 100 Data Transmission 1. Land lines (wiring) – through electrical lines 2. Radio frequency (RF) telemetry – for moving objects such as spacecraft, vehicles, rotors, animals Need for calibration and compensation Ideal calibration: y ) ( ) ( t cx t = y ( t ): output, x ( t ): input, c : constant Dynamic range: assessed in terms of 1. = 2 2 10 log 10 / n s N S ψ ψ : maximum mean square of signal 2 s ψ : mean square of noise floor 2 n ψ 2. = 2 2 max 10 log 10 / n L N PS ψ : peak S/N ratio ) deviations standard (three data random data sinusoidal data static for for for 3 2 max = s s s L ψ ψ ψ 1 J. S. Bendat and A. G. Piersol, Random Data: Analysis and Measurement Procedures , 3rd ed., John Wiley & Sons, 2000, Chapter 10. 1
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MAE 591 RANDOM DATA Watch for manufacturer’s specifications Ex) Accelerometer Voltage Sensitivity, 18 and 100 Hz 0.1 mV/g, nominal C 0 Maximum Sensitivity Change vs Temperature +8% C 0 40 -15% C 0 120 + Transverse Sensitivity 5% maximum Maximum Noise Floor, 3Hz to 40 kHz g’s equivalent, r.m.s. 0.2 g’s Amplitude Linearity ± 1% to 10,000 g’s peak Operating Temperature Range - 40 to C 0 120 + Peak Operating g’s 10,000 Frequency Response 3 Hz to 20 kHz ( 3dB) ± 9 Hz to 12kHz ( ± 5%) Case Grounded, Female Connector, Stainless Steel Case, Taped Hole Mounting, 0.5” Hex x 0.77” High, 12 grams, etc. 2. Data Storage Magnetization-reproduce procedures Nonlinear magnetization process Inherent recording and reproducing errors due to the finite head gap width limits the low frequency response characteristics Modulation-demodulation procedures Frequency Modulation (FM) Pulse Code Modulation (PCM) Pulse Amplitude Modulation (PAM) Pulse Duration (Width) Modulation (PDM, PWM) Digital storage: On-line A/D conversion to digital storage units.
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