electronic instrumentation

ElectronicInstrumentation

European PhD – 2009Transducers and Signal Conditioning

Horácio Fernandes

Electronic Instrumentation, PhD 2009

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 Preserve signal quality DAS less demanding Preserve and adjust dynamic bandwidth Resize operational limits

Offset Amplitude Bandwith

Linearization Galvanic isolation Buffering


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


SensoresRelated energy Example Comment

Mechanic Flow-meterStrain gauge

Pulse counterMomentum transfer

Thermal ThermocoupleThermal radiation

Junction voltageInfrared sensor

Electromagnetic Antenna Space electromagnetic power converted to electric signals

Magnetic Hall sensorMDH Probes

Voltage derived from Hall EffectInduced EMF

Chemic pH sensor Ionic concentration

Nuclear Ionization chamberScintillators

Current generation induced by free chargesIndirect light proportional to brehmstrallung


Sensors Transducer Principles Resistive

Strain gauges: Force measurements (W. Bridges)

Temperature: RTDs, termistorsLight: photoelectric cells and photodiodesPosition: potentiometers as dividers, grids


Sensors Transducer Principles Capacitive

Movement Dielectric constant Geometric configuration

Cell chargers Inductive

LVDT –Differential Transformer

Hall Effect Motors as generator


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

EnvironmentDirtyPollutionExtreme TemperaturesWater presence and moistChemical corrosion: solvers, acids e basesEnvironmental protectionSusceptibility: eletric/explosion/chash


Sensors Operation

Human useRadiationCorrosion/Chemicals manipulation ImmersionErosion/VibrationsExplosionElectric Interference (EMI- high impedance,

low current)


Sensors Operation

PowerCircuit Charger (photocell)Excitation source (noise)

Signal Conditioning Physics size


Calibration

Measurement Error – Comparison standard should be more exact than sensor resolution

Calibration table – Calibration curvePhysic modelStatic and dynamic calibrationBandwidth Impulsive response


Linearization

Transfer function errors Non-linearity

Sensor Electronics Signal path

Compensation Non-linear electronic

circuit Piecewise interpolation


Buffering

Source/Input isolation Impedance adaptation

Maximum feed power Voltage signal

Transducer outputPreserve signal

Next stage charge circuit


Meters and bridges

Differential mode Common mode


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

c

bax R

RRR


Wheatstone BridgeApplication


Kelvin bridge

Very low resistors (<1R)

Double terminals


Maxwell Bridge


Bridges Configurations


Bridges circuits

AC generators Current sources OPAMPs applications


Bridge noise immunity

Pick-up noise Cable resistance Signal Bandwidth 3-wire connection


Noise reduction

…If noise blocking fails in the origin……Nightmare begins!


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

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


Perceptual aliasing


Image aliasing


Filters

Pass-band Cut-off Stop-band Ripple Order Phase and amplitude

characteristics


Common Filters

Lowpass Highpass Passband Notch Digital filtering (made possible with fast ADCs)

Advantages: High order, cutoff frequency, complex transforms

Signal correlation


W1: TENSILE.1.STRAINRATE

0 20 40 60 80 100 120 140 160 180 200 220 240 260 280-12

-8

-4

0

4



0 20 40 60 80 100 120 140 160 180 200 220 240 260 280-4

-3.5

-3

-2.5

-2

-1.5

W2: Linear Regression

0 20 40 60 80 100 120 140 160 180 200 220 240 260 280-12

-10

-8

-6

-4

-2

0

2

W3: 13th Order Polynomial Fit

0 20 40 60 80 100 120 140 160 180 200 220 240 260 280-12

-8

-4

0

4

W4: Both Fits and Data

0 20 40 60 80 100 120 140 160 180 200 220 240 260 280-8-6

-4-2024

68

W5: Linear Fit Residuals

0 20 40 60 80 100 120 140 160 180 200 220 240 260 280-6

-4

-2

0

2

4

6

W6: Polynomial Fit Residuals


Other Techniques

Periodic signalsLock-in amplifiers

Cross correlationBoxcar integrationMultichannel mean

Overlap of periodic signals S/N~N1/2


Other Techniques

Pulsed signalsConstant Fraction discriminator


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

Current detection Ground loopSafety - galvanic isolationSweep waveforms – capacitive coupling and

distortionFast sweeping – plasma limit operation

electronic instrumentation

Documents

sensors selectionscale

output signal proportional

useful signalwhen

electric circuit r

scale spanningthresholdleast

moistchemical corrosion

force measurements

form of energy