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Electronic Instrumentation

Electronic Instrumentation. European PhD – 2009 Transducers and Signal Conditioning Horácio Fernandes. Useful Signal. When converting a signal to a quantity, it is only useful if its representation is kept unchanged within a knew error

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Electronic Instrumentation

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  1. ElectronicInstrumentation European PhD – 2009 Transducers and Signal Conditioning Horácio Fernandes

  2. Useful Signal • When converting a signal to a quantity, it is only useful if its representation is kept unchanged within a knew error • Signal conditioning and transmission is very important in applied physics

  3. Signal paths • Preserve signal quality • DAS less demanding • Preserve and adjust dynamic bandwidth • Resize operational limits • Offset • Amplitude • Bandwith • Linearization • Galvanic isolation • Buffering

  4. Sensors and Transducers Transducers Device capable of changing one form of energy into another Active – External power supply Passive – Internal source (self-generating) Sensors Changing of a characteristic in an electric circuit (R; L, C); Generate an output signal proportional to the stimulus

  5. Sensores

  6. Sensors Transducer Principles Resistive Strain gauges: Force measurements (W. Bridges) Temperature: RTDs, termistors Light: photoelectric cells and photodiodes Position: potentiometers as dividers, grids

  7. Sensors Transducer Principles Capacitive Movement Dielectric constant Geometric configuration Cell chargers Inductive LVDT –Differential Transformer Hall Effect Motors as generator

  8. Sensors Selection Scale: limiting extremes (Worst Case) multiples sensors for scale spanning Threshold Least detected variation (resolution) Behavior Temporal response Dynamic response Accuracy and resolution Stress (consistency) Reproducibility and hysteresis Price

  9. Sensors Operation Environment Dirty Pollution Extreme Temperatures Water presence and moist Chemical corrosion: solvers, acids e bases Environmental protection Susceptibility: eletric/explosion/chash

  10. Sensors Operation Human use Radiation Corrosion/Chemicals manipulation Immersion Erosion/Vibrations Explosion Electric Interference (EMI- high impedance, low current)

  11. Sensors Operation Power Circuit Charger (photocell) Excitation source (noise) Signal Conditioning Physics size

  12. Calibration Measurement Error – Comparison standard should be more exact than sensor resolution Calibration table – Calibration curve Physic model Static and dynamic calibration Bandwidth Impulsive response

  13. Linearization • Transfer function errors • Non-linearity • Sensor • Electronics • Signal path • Compensation • Non-linear electronic circuit • Piecewise interpolation

  14. Buffering • Source/Input isolation • Impedance adaptation • Maximum feed power • Voltage signal • Transducer output • Preserve signal • Next stage charge circuit

  15. Meters and bridges • Differential mode • Common mode

  16. Wheatstone Bridge • Potentiometer divider • Zero Measurement • CMR>100 dB • Sensibility • Thermal immunity

  17. Wheatstone BridgeApplication

  18. Kelvin bridge • Very low resistors (<1R) • Double terminals

  19. Maxwell Bridge

  20. Bridges Configurations

  21. Bridges circuits • AC generators • Current sources • OPAMPs applications

  22. Bridge noise immunity • Pick-up noise • Cable resistance • Signal Bandwidth • 3-wire connection

  23. Noise reduction • …If noise blocking fails in the origin… …Nightmare begins!

  24. What can we filter? • Signal sampling: analog goes digital at what rate? • Nyquist criteria: fs>2fmax • Low-pass filters (cutoff -40 dB) • Guard-band • Sampling band: [fs-fmax, fs+fmax]

  25. Useful Storage Bandwith • Pratical figures • USB=fs/2.5 • Sin Interpolation • USB=fs/10 • Linear Interpolation • USB=fs/25 • No Interpolation • USB=fs/4.6 • Digital correction (factor 1.6) x (1/0.35)

  26. Aliasing

  27. Perceptual aliasing

  28. Image aliasing

  29. Filters • Pass-band • Cut-off • Stop-band • Ripple • Order • Phase and amplitude characteristics

  30. Common Filters • Lowpass • Highpass • Passband • Notch • Digital filtering (made possible with fast ADCs) • Advantages: High order, cutoff frequency, complex transforms • Signal correlation

  31. Other Techniques • Periodic signals • Lock-in amplifiers • Cross correlation • Boxcar integration • Multichannel mean • Overlap of periodic signals • S/N~N1/2

  32. Other Techniques • Pulsed signals • Constant Fraction discriminator

  33. Transimpedance amplifier • Allow very low current sources detection, ex: photodiodes • Tomography • Spectrometers • Line radiation filters • Charge measurements, ex: ion beam • High bandwidth

  34. I(V) probes • Current detection • Ground loop • Safety - galvanic isolation • Sweep waveforms – capacitive coupling and distortion • Fast sweeping – plasma limit operation

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