Transcript ppt - IPFN
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
Signal conditioning and transmission is
very important in applied physics
Electronic Instrumentation, PhD 2009
Signal paths
Preserve signal quality
DAS less demanding
Preserve and adjust dynamic bandwidth
Resize operational limits
Offset
Amplitude
Bandwith
Linearization
Galvanic isolation
Buffering
Electronic Instrumentation, PhD 2009
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
Electronic Instrumentation, PhD 2009
Sensores
Related energy
Example
Comment
Mechanic
Flow-meter
Strain gauge
Pulse counter
Momentum transfer
Thermal
Thermocouple
Thermal radiation
Junction voltage
Infrared sensor
Electromagnetic
Antenna
Space electromagnetic power converted to
electric signals
Magnetic
Hall sensor
MDH Probes
Voltage derived from Hall Effect
Induced EMF
Chemic
pH sensor
Ionic concentration
Nuclear
Ionization chamber
Scintillators
Current generation induced by free charges
Indirect light proportional to brehmstrallung
Electronic Instrumentation, PhD 2009
Sensors
Transducer Principles
Resistive
Strain
gauges: Force measurements (W.
Bridges)
Temperature: RTDs, termistors
Light: photoelectric cells and photodiodes
Position: potentiometers as dividers, grids
Electronic Instrumentation, PhD 2009
Sensors
Transducer Principles
Capacitive
Movement
Dielectric constant
Geometric configuration
Cell chargers
Inductive
–Differential
Transformer
Hall Effect
Motors as generator
LVDT
Electronic Instrumentation, PhD 2009
Sensors Selection
Scale: limiting extremes (Worst Case)
multiples
Threshold
Least
sensors for scale spanning
detected variation (resolution)
Behavior
Temporal
response
Dynamic response
Accuracy and resolution
Stress (consistency)
Reproducibility and hysteresis
Price
Electronic Instrumentation, PhD 2009
Sensors Operation
Environment
Dirty
Pollution
Extreme
Temperatures
Water presence and moist
Chemical corrosion: solvers, acids e bases
Environmental protection
Susceptibility: eletric/explosion/chash
Electronic Instrumentation, PhD 2009
Sensors Operation
Human use
Radiation
Corrosion/Chemicals
manipulation
Immersion
Erosion/Vibrations
Explosion
Electric
Interference (EMI- high impedance,
low current)
Electronic Instrumentation, PhD 2009
Sensors Operation
Power
Circuit
Charger (photocell)
Excitation source (noise)
Signal Conditioning
Physics size
Electronic Instrumentation, PhD 2009
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
Electronic Instrumentation, PhD 2009
Linearization
Transfer function
errors
Non-linearity
Sensor
Electronics
Signal path
Compensation
Non-linear
electronic
circuit
Piecewise interpolation
Electronic Instrumentation, PhD 2009
Buffering
Source/Input isolation
Impedance
adaptation
Maximum feed power
Voltage signal
Transducer output
Preserve
signal
Next stage charge circuit
Electronic Instrumentation, PhD 2009
Meters and bridges
Differential mode
Common mode
Electronic Instrumentation, PhD 2009
Wheatstone Bridge
Potentiometer divider
Zero Measurement
CMR>100 dB
Sensibility
Thermal immunity
Ra Rb
Rx
Rc
Electronic Instrumentation, PhD 2009
Wheatstone Bridge
Application
Electronic Instrumentation, PhD 2009
Kelvin bridge
Very low resistors
(<1R)
Double terminals
Electronic Instrumentation, PhD 2009
Maxwell Bridge
Electronic Instrumentation, PhD 2009
Bridges Configurations
Electronic Instrumentation, PhD 2009
Bridges circuits
AC generators
Current sources
OPAMPs applications
Electronic Instrumentation, PhD 2009
Bridge noise immunity
Pick-up noise
Cable resistance
Signal Bandwidth
3-wire connection
Electronic Instrumentation, PhD 2009
Noise reduction
…If noise blocking fails in the origin…
…Nightmare begins!
Electronic Instrumentation, PhD 2009
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]
Electronic Instrumentation, PhD 2009
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)
Electronic Instrumentation, PhD 2009
Aliasing
Electronic Instrumentation, PhD 2009
Perceptual aliasing
Electronic Instrumentation, PhD 2009
Image aliasing
Electronic Instrumentation, PhD 2009
Filters
Pass-band
Cut-off
Stop-band
Ripple
Order
Phase and amplitude
characteristics
Electronic Instrumentation, PhD 2009
Common Filters
Lowpass
Highpass
Passband
Notch
Digital filtering (made possible with fast ADCs)
Advantages:
High order, cutoff frequency, complex
transforms
Signal correlation
Electronic Instrumentation, PhD 2009
W1: TENSILE.1.STRAINRATE
W2: Linear Regression
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W3: 13th Order Polynomial Fit
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W4: Both Fits and Data
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W5: Linear Fit Residuals
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W6: Polynomial Fit Residuals
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Electronic Instrumentation, PhD 2009
Other Techniques
Periodic signals
Lock-in
amplifiers
Cross correlation
Boxcar
integration
Multichannel mean
Overlap of periodic signals
S/N~N1/2
Electronic Instrumentation, PhD 2009
Other Techniques
Pulsed signals
Constant
Fraction
discriminator
Electronic Instrumentation, PhD 2009
Transimpedance amplifier
Allow very low current
sources detection, ex:
photodiodes
Tomography
Spectrometers
Line
radiation filters
Charge measurements, ex:
ion beam
High bandwidth
Electronic Instrumentation, PhD 2009
I(V) probes
Current detection
Ground
loop
Safety - galvanic isolation
Sweep waveforms – capacitive coupling and
distortion
Fast sweeping – plasma limit operation
Electronic Instrumentation, PhD 2009