Electronic instrumentation
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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

ElectronicInstrumentation

European PhD – 2009

Transducers and Signal Conditioning

Horácio Fernandes


Useful signal

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


Signal paths

Signal paths

  • Preserve signal quality

  • DAS less demanding

  • Preserve and adjust dynamic bandwidth

  • Resize operational limits

    • Offset

    • Amplitude

    • Bandwith

  • Linearization

  • Galvanic isolation

  • Buffering


Sensors and transducers

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


Sensores

Sensores


Sensors transducer principles

Sensors Transducer Principles

Resistive

Strain gauges: Force measurements (W. Bridges)

Temperature: RTDs, termistors

Light: photoelectric cells and photodiodes

Position: potentiometers as dividers, grids


Sensors transducer principles1

Sensors Transducer Principles

Capacitive

Movement

Dielectric constant

Geometric configuration

Cell chargers

Inductive

LVDT –Differential Transformer

Hall Effect

Motors as generator


Sensors selection

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


Sensors operation

Sensors Operation

Environment

Dirty

Pollution

Extreme Temperatures

Water presence and moist

Chemical corrosion: solvers, acids e bases

Environmental protection

Susceptibility: eletric/explosion/chash


Sensors operation1

Sensors Operation

Human use

Radiation

Corrosion/Chemicals manipulation

Immersion

Erosion/Vibrations

Explosion

Electric Interference (EMI- high impedance, low current)


Sensors operation2

Sensors Operation

Power

Circuit Charger (photocell)

Excitation source (noise)

Signal Conditioning

Physics size


Calibration

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


Linearization

Linearization

  • Transfer function errors

    • Non-linearity

      • Sensor

      • Electronics

      • Signal path

  • Compensation

    • Non-linear electronic circuit

    • Piecewise interpolation


Buffering

Buffering

  • Source/Input isolation

    • Impedance adaptation

      • Maximum feed power

      • Voltage signal

  • Transducer output

    • Preserve signal

      • Next stage charge circuit


Meters and bridges

Meters and bridges

  • Differential mode

  • Common mode


Wheatstone bridge

Wheatstone Bridge

  • Potentiometer divider

  • Zero Measurement

  • CMR>100 dB

  • Sensibility

  • Thermal immunity


Wheatstone bridge application

Wheatstone BridgeApplication


Kelvin bridge

Kelvin bridge

  • Very low resistors (<1R)

  • Double terminals


Maxwell bridge

Maxwell Bridge


Bridges configurations

Bridges Configurations


Bridges circuits

Bridges circuits

  • AC generators

  • Current sources

  • OPAMPs applications


Bridge noise immunity

Bridge noise immunity

  • Pick-up noise

  • Cable resistance

  • Signal Bandwidth

  • 3-wire connection


Noise reduction

Noise reduction

  • …If noise blocking fails in the origin…

    …Nightmare begins!


What can we filter

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]


Useful storage bandwith

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)


Aliasing

Aliasing


Perceptual aliasing

Perceptual aliasing


Image aliasing

Image aliasing


Filters

Filters

  • Pass-band

  • Cut-off

  • Stop-band

  • Ripple

  • Order

  • Phase and amplitude characteristics


Common filters

Common Filters

  • Lowpass

  • Highpass

  • Passband

  • Notch

  • Digital filtering (made possible with fast ADCs)

    • Advantages: High order, cutoff frequency, complex transforms

    • Signal correlation


Other techniques

Other Techniques

  • Periodic signals

    • Lock-in amplifiers

      • Cross correlation

    • Boxcar integration

    • Multichannel mean

      • Overlap of periodic signals

      • S/N~N1/2


Other techniques1

Other Techniques

  • Pulsed signals

    • Constant Fraction

      discriminator


Transimpedance amplifier

Transimpedance amplifier

  • Allow very low current sources detection, ex: photodiodes

    • Tomography

    • Spectrometers

    • Line radiation filters

  • Charge measurements, ex: ion beam

  • High bandwidth


I v probes

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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