Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Signal acquisition chain
What are the main elements ?
Amélie Danlos, Florent Ravelet
DynFluid Laboratory, Arts et Métiers ParisTech
January 27, 2014
1
Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Outline
2
1
Acquisition chain elements
2
Sensor
Different signals and sensors
Characteristics of sensors
3
Conditioner
4
Data acquisition and transfer
Multiplexer
Track and hold unit
Analog-to-digital converter
5
Applications
Labview
Measurement chain for visualizations
Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Acquisition chain elements
Data acquisition chain is the set of elements necessary to catch analog or digital data, to
transfer it to the receiver and the user.
measurand = the physical quantity to measure
Designing an acquisition chain is choosing devices and their assembly in order to have
limitations compatible with information saving.
Global characteristics of an acquisition chain depend on each chain link (manufacturer data,
calibration)
3
Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Acquisition chain elements
4
Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Different signals and sensors
Characteristics of sensors
Outline
5
1
Acquisition chain elements
2
Sensor
Different signals and sensors
Characteristics of sensors
3
Conditioner
4
Data acquisition and transfer
Multiplexer
Track and hold unit
Analog-to-digital converter
5
Applications
Labview
Measurement chain for visualizations
Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Different signals and sensors
Characteristics of sensors
Sensor (Transducer)
Interface between physical world and electronic world
It is a component sensitive to a physical quantity which delivers an usable quantity from its
variations (electric signal, electric voltage, mercury height, pointer deflection).
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Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Different signals and sensors
Characteristics of sensors
Morphological classification of signals
Continuous vs. discrete amplitude/time
a analogous
b quantized
c sampled
d digital
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Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Different signals and sensors
Characteristics of sensors
Sensor types
Output signal
Analog: The output is an electric quantity whose value is proportional to the physical
quantity measured by the sensor. The signal amplitude can have infinity values in a given
range. Information is continuous.
Possible signals: output voltage, output current, ...
Examples: thermocouple, strain gauge
Digital: The output is a sequence of logical states.
Possible signals: pulse train, optical encoders, go no-go
8
Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Different signals and sensors
Characteristics of sensors
Sensor types
Example of an analogous transducer
Capacitive sensor
Capacitive sensing is a technology based on capacitive coupling that is used in many different
types of sensors, including those to detect and measure proximity, position or displacement,
humidity, fluid level, and acceleration.
When an object (metallic or not) comes in the sensor detection field, it changes the capacity
between sensor electrodes and induces oscillations detected by the sensor internal electronics.
9
Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Different signals and sensors
Characteristics of sensors
Sensor types
Example of a digital transducer
Optical sensor
A photoelectric sensor is a proximity sensor. It consists of a light emitter associated to a receiver.
An object is detected by the cut or the variation of a light beam. The signal is amplified to be use
by the control part.
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Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Different signals and sensors
Characteristics of sensors
Sensor types
Null and deflection methods
Deflection: The signal produces some physical (deflection) effect closely related to the
measured quantity and transduced to be observable.
Null: The signal produced by the sensor is counteracted to minimize the deflection. That
opposing effect necessary to maintain a zero deflection should be proportional to the signal
of the measurand.
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Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Different signals and sensors
Characteristics of sensors
Characteristics of sensors
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Measuring range: the area in which the sensor characteristics are ensured with respect to
the given specifications.
Resolution: the smallest variation of the measured quantity which induces a perceptible
change.
Sensitivity: slope of the characteristics.
Linearity: zone in which the sensitivity is constant/ departure of the actual characteristics
from a pure linear response.
Response time (to a step function).
Bandwidth: frequency range for which the response amplitude of a system corresponds to a
reference level.
Hysterisis: delay of the effect on the cause.
Usable temperature range
Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Different signals and sensors
Characteristics of sensors
Characteristics of sensors
The sensor and all the processing chain induce errors: noise, delays, non linearity... The
measurement global error can only be estimated. A rigorous design of the measurement chain can
reduce errors and then uncertainty of the result.
Precision: the concept of precision refers to the degree of reproducibility of a measurement.
In other words, if exactly the same value were measured a number of times, an ideal sensor
would output exactly the same value every time. A measure is given by the
root-mean-square of n measurements.
Accuracy: The accuracy of the sensor is the maximum difference that will exist between the
actual value (which must be measured by a primary or good secondary standard) and the
indicated value at the output of the sensor. A measure is given by the mean of n
measurements.
13
Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Outline
14
1
Acquisition chain elements
2
Sensor
Different signals and sensors
Characteristics of sensors
3
Conditioner
4
Data acquisition and transfer
Multiplexer
Track and hold unit
Analog-to-digital converter
5
Applications
Labview
Measurement chain for visualizations
Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Conditioner
Signal conditioning can include amplification, filtering, converting, range matching, isolation
and any other processes required to make sensor output suitable for processing after
conditioning.
In certain cases it also fulfills the functions of galvanic insulation and energization of the
passive sensors.
Signal inputs accepted by signal conditioners include DC voltage and current, AC voltage
and current, frequency and electric charge.
Outputs for signal conditioning equipment can be voltage, current, frequency, timer or
counter, relay, resistance or potentiometer, and other specialized outputs.
15
Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Conditioner
Filtering
Filtering is the most common signal conditioning function, as usually not all the signal
frequency spectrum contains valid data. The common example are 60 Hz AC power lines,
present in most environments, which will produce noise if amplified.
Amplifying
The voltage delivered by the sensor is of the order of a few millivolts. It is then necessary to
amplify the signal for processing downstream.
Signal amplification performs two important functions: increases the resolution of the
inputed signal, and increases its signal-to-noise ratio. For example, the output of an
electronic temperature sensor, which is probably in the millivolts range is probably too low
for an Analog-to-digital converter (ADC) to process directly. In this case it is necessary to
bring the voltage level up to that required by the ADC.
Commonly used amplifiers on signal conditioning include Sample and hold amplifiers, Peak
Detectors, Log amplifiers, Antilog amplifiers, Instrumentation amplifiers or programmable
gain amplifiers
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Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
The quality of an amplifier can be characterized by a number of specifications:
- Gain: The gain of an amplifier is the ratio of output to input power or amplitude, and is
usually measured in decibels.
- Bandwidth: The bandwidth of an amplifier is the range of frequencies for which the
amplifier gives satisfactory performance.
- Efficiency: Efficiency is a measure of how much of the power source is usefully applied to
the amplifier output.
- Linearity: An ideal amplifier would be a totally linear device, but real amplifiers are only
linear within limits. When the signal drive to the amplifier is increased, the output also
increases until a point is reached where some part of the amplifier becomes saturated and
cannot produce any more output; this is called clipping, and results in distortion.
- Noise: This is a measure of how much noise is introduced in the amplification process.
Noise is an undesirable but inevitable product of the electronic devices and components.
The metric for noise performance of a circuit is noise figure or noise factor. Noise figure is a
comparison between the output signal to noise ratio and the thermal noise of the input
signal.
17
Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
- Output dynamic range: Output dynamic range is the range, usually given in dB, between the
smallest and largest useful output levels. The lowest useful level is limited by output noise, while
the largest is limited most often by distortion. The ratio of these two is quoted as the amplifier
dynamic range. More precisely, if S =maximal allowed signal power and N =noise power, the
dynamic range DR is DR = S+N
N .
- Slew rate: Slew rate is the maximum rate of change of the output, usually quoted in volts per
second (or microsecond).
- Rise time: The rise time, tr , of an amplifier is the time taken for the output to change from 10%
to 90% of its final level when driven by a step input.
- Settling time: The time taken for the output to settle to within a certain percentage of the final
value (for instance 0.1%) is called the settling time.
- Stability: Stability is an issue in all amplifiers with feedback, whether that feedback is added
intentionally or results unintentionally. It is especially an issue when applied over multiple
amplifying stages.
18
Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Conditioner
Isolation
Signal isolation must be used in order to pass the signal from the source to the
measurement device without a physical connection: it is often used to isolate possible
sources of signal perturbations.
it is important to isolate the potentially expensive equipment used to process the signal after
conditioning from the sensor.
Magnetic or optic isolation can be used. Magnetic isolation transforms the signal from
voltage to a magnetic field, allowing the signal to be transmitted without a physical
connection (for example, using a transformer). Optic isolation takes an electronic signal and
modulates it to a signal coded by light transmission (optical encoding), which is then used
for input for the next stage of processing.
19
Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Multiplexer
Track and hold unit
Analog-to-digital converter
Outline
20
1
Acquisition chain elements
2
Sensor
Different signals and sensors
Characteristics of sensors
3
Conditioner
4
Data acquisition and transfer
Multiplexer
Track and hold unit
Analog-to-digital converter
5
Applications
Labview
Measurement chain for visualizations
Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Multiplexer
Track and hold unit
Analog-to-digital converter
Data acquisition and transfer
Multiplexer
A multiplexer (or mux) is a device that selects one of several analog or digital input signals
and forwards the selected input into a single line.
A multiplexer of 2n inputs has n select lines, which are used to select which input line to
send to the output.
A multiplexer is also called a data selector.
21
Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Multiplexer
Track and hold unit
Analog-to-digital converter
Data acquisition and transfer
Track and hold unit (Sample and hold)
This device is placed upstream the analog-to-digital converter in order to acquire a given
value of the voltage inlet the converter for a given time and to maintain this value stable for
the conversion duration.
A sampler is an interruptor controled by a digital signal with a frequency Fe . τ is the time
for which the interruptor is closed (a sample duration). We must have: τ << F1e
22
Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Multiplexer
Track and hold unit
Analog-to-digital converter
Data acquisition and transfer
Analog-to-digital converter
An analog-to-digital converter (abbreviated ADC, A/D or A to D) is a device that converts
a continuous quantity to a discrete time digital representation.
Typically, an ADC is an electronic device that converts an input analog voltage or current to
a digital number proportional to the magnitude of the voltage or current.
The digital output may use different coding schemes. Typically the digital output will be a
two’s complement binary number that is proportional to the input, but there are other
possibilities. An encoder, for example, might output a Gray code.
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Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Multiplexer
Track and hold unit
Analog-to-digital converter
Data acquisition and transfer
Analog-to-digital converter
Resolution: The resolution of the converter indicates the number of discrete values it can
produce over the range of analog values.
The values are usually stored electronically in binary form, so the resolution is usually
expressed in bits. In consequence, the number of discrete values available, or ”levels”, is a
power of two. For example, an ADC with a resolution of 8 bits can encode an analog input
to one in 256 different levels.
The values can represent the ranges from 0 to 255 (i.e. unsigned integer) or from −128 to
127 (i.e. signed integer), depending on the application.
Resolution can also be defined electrically, and expressed in volts. The minimum change in
voltage required to guarantee a change in the output code level is called the least significant
bit (LSB) voltage. The resolution Q of the ADC is equal to the LSB voltage. The voltage
resolution of an ADC is equal to its overall voltage measurement range divided by the
number of discrete voltage intervals
In practice, the useful resolution of a converter is limited by the best signal-to-noise ratio
(SNR) that can be achieved for a digitized signal.
24
Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Multiplexer
Track and hold unit
Analog-to-digital converter
Data acquisition and transfer
Analog-to-digital converter
Response type: Most of analog-to-digital converters are linear, so the range of input values
has a linear relationship with the output value.
Accuracy: Quantization error and non-linearity are intrinsic to any analog-to-digital
conversion. There is also a so-called aperture error which is due to a clock jitter and is
revealed when digitizing a time-variant signal (not a constant value).
These errors are measured in a unit called the least significant bit (LSB). In the above
example of an 8-bit ADC, an error of one LSB is 1/256 of the full signal range, or about
0.4%.
Quantization error (or quantization noise) is the difference between the original signal and
the digitized signal. It is due to the finite resolution of the digital representation of the
signal, and is an unavoidable imperfection
All analog-to-converters suffer from non-linearity errors caused by their physical
imperfections, causing their output to deviate from a linear function. Then it decreases the
effective resolution of the ADC. These errors can sometimes be reduced by calibration
25
Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Multiplexer
Track and hold unit
Analog-to-digital converter
Data acquisition and transfer
Analog-to-digital converter
Sampling rate: The analog signal is continuous in time and it is necessary to convert this to
a flow of digital values. It is therefore required to define the rate at which new digital values
are sampled from the analog signal. The rate of new values is called the sampling rate or
sampling frequency of the converter.
The original signal can be exactly reproduced from the discrete-time values by an
interpolation formula. This faithful reproduction is only possible if the sampling rate is
higher than twice the highest frequency of the signal (Shannon-Nyquist sampling theorem).
Many ADC integrated circuits include the sample and hold subsystem internally.
Aliasing: If the input signal is changing much faster than the sample rate, spurious signals
called aliases will be produced at the output of the digital-to-analog converter. The
frequency of the aliased signal is the difference between the signal frequency and the
sampling rate. For example, a 2 kHz sine wave being sampled at 1.5 kHz would be
reconstructed as a 500 Hz sine wave. This problem is called aliasing.
To avoid aliasing, the input to an ADC must be low-pass filtered to remove frequencies
above half the sampling rate. This filter is called an anti-aliasing filter, and is essential for a
practical ADC system that is applied to analog signals with higher frequency content.
26
Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Multiplexer
Track and hold unit
Analog-to-digital converter
Data acquisition and transfer
Software
The driver is a software layer which permits to program easily the card with high level
functions. The quality of the analog-to-digital converter is unefficient if the driver used to
control it has no functions to use all components of the ADC card.
Usable software: Labview, Matlab...
27
Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
28
Amélie Danlos, Florent Ravelet
Multiplexer
Track and hold unit
Analog-to-digital converter
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Labview
Measurement chain for visualizations
Outline
29
1
Acquisition chain elements
2
Sensor
Different signals and sensors
Characteristics of sensors
3
Conditioner
4
Data acquisition and transfer
Multiplexer
Track and hold unit
Analog-to-digital converter
5
Applications
Labview
Measurement chain for visualizations
Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Labview
Measurement chain for visualizations
Movie
Software: LabVIEW (Laboratory Virtual Instrument Engineering
Workbench)
LabVIEW is a development tool based on a programming language called G language.
Application domains: control and command, measurement, instrumentation.
Program library using specialized functions: GPIB, VXI, PXI, DAQ acquisition card, data
processing...
30
Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Labview
Measurement chain for visualizations
Software: LabVIEW
31
Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Labview
Measurement chain for visualizations
Applications
Flow visualizations
32
Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Labview
Measurement chain for visualizations
Flow visualizations
Optic
Parameters:
Focal distance: characterizes the viewing angle
Iris diaphragm (aperture): Adjustment of the amount of light which passes through the lens.
Focusing: Adjustment of the distance between betwen the lens and the principal focus plan.
Quality: resolution, lens number, material, spectral response, geometric and chromatic
aberrations
33
Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Labview
Measurement chain for visualizations
Flow visualizations
Limitations
Vignetting
Flare
Depth of field
34
Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Labview
Measurement chain for visualizations
Flow visualizations
Limitations
Chromatic aberration
Geometric aberration
(Distorsion)
35
Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Labview
Measurement chain for visualizations
Flow visualizations
CCD Sensor (Charged-Coupled Device)
A charge-coupled device (CCD) is a device for the movement of electrical charge, usually
from within the device to an area where the charge can be manipulated, for example
conversion into a digital value.
A CCD image sensor is an analog device. When light strikes the chip it is held as a small
electrical charge in each photo sensor. The charges are converted to voltage one pixel at a
time as they are read from the chip. Additional circuitry in the camera converts the voltage
into digital information.
36
Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Labview
Measurement chain for visualizations
Movie
Movie
Flow visualizations
CMOS Sensor (Complementary metal-oxide-semiconductor)
A CMOS imaging chip is a type of active pixel sensor made using the CMOS semiconductor
process. Extra circuitry next to each photo sensor converts the light energy to a voltage.
Additional circuitry on the chip may be included to convert the voltage to digital data.
37
Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Labview
Measurement chain for visualizations
Flow visualizations
Camera block
It is the sensor case.
It ensures the mechanical positioning of the optic elements and contains the output
electronics
Elements:
- Power supply (12V-2W)
- Video signal output
- Threaded hole to fix the camera
38
Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction
Acquisition chain elements
Sensor
Conditioner
Data acquisition and transfer
Applications
Labview
Measurement chain for visualizations
Flow visualizations
Signal transfer and processing
39
Amélie Danlos, Florent Ravelet
Experimental methods for fluid flows: an introduction