Signal acquisition chain What are the main elements?

Transcription

1 Signal acquisition chain What are the main elements? Amélie Danlos, Florent Ravelet DynFluid Laboratory, Arts et Métiers ParisTech January 27, Amélie Danlos, Florent Ravelet Experimental methods for fluid flows: an introduction

2 Outline Acquisition chain elements 1 Acquisition chain elements 2 Different signals and sensors Characteristics of sensors 3 4 Multiplexer Track and hold unit Analog-to-digital converter 5 Labview Measurement chain for visualizations 2 Amélie Danlos, Florent Ravelet Experimental methods for fluid flows: an introduction

3 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

4 Acquisition chain elements 4 Amélie Danlos, Florent Ravelet Experimental methods for fluid flows: an introduction

5 Outline Acquisition chain elements Different signals and sensors Characteristics of sensors 1 Acquisition chain elements 2 Different signals and sensors Characteristics of sensors 3 4 Multiplexer Track and hold unit Analog-to-digital converter 5 Labview Measurement chain for visualizations 5 Amélie Danlos, Florent Ravelet Experimental methods for fluid flows: an introduction

6 Different signals and sensors Characteristics of sensors (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). 6 Amélie Danlos, Florent Ravelet Experimental methods for fluid flows: an introduction

7 Different signals and sensors Characteristics of sensors Morphological classification of signals Continuous vs. discrete amplitude/time a analogous b quantized c sampled d digital 7 Amélie Danlos, Florent Ravelet Experimental methods for fluid flows: an introduction

8 Different signals and sensors Characteristics of sensors 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

9 Different signals and sensors Characteristics of sensors 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

10 Different signals and sensors Characteristics of sensors 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. 10 Amélie Danlos, Florent Ravelet Experimental methods for fluid flows: an introduction

11 Different signals and sensors Characteristics of sensors 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. 11 Amélie Danlos, Florent Ravelet Experimental methods for fluid flows: an introduction

12 Different signals and sensors Characteristics of sensors Characteristics of sensors 1 Measuring range: the area in which the sensor characteristics are ensured with respect to the given specifications. 2 Resolution: the smallest variation of the measured quantity which induces a perceptible change. 3 Sensitivity: slope of the characteristics. 4 Linearity: zone in which the sensitivity is constant/ departure of the actual characteristics from a pure linear response. 5 Response time (to a step function). 6 Bandwidth: frequency range for which the response amplitude of a system corresponds to a reference level. 7 Hysterisis: delay of the effect on the cause. 8 Usable temperature range 12 Amélie Danlos, Florent Ravelet Experimental methods for fluid flows: an introduction

13 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

14 Outline Acquisition chain elements 1 Acquisition chain elements 2 Different signals and sensors Characteristics of sensors 3 4 Multiplexer Track and hold unit Analog-to-digital converter 5 Labview Measurement chain for visualizations 14 Amélie Danlos, Florent Ravelet Experimental methods for fluid flows: an introduction

15 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

16 Filtering Amplifying 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. 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 16 Amélie Danlos, Florent Ravelet Experimental methods for fluid flows: an introduction

17 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

18 - 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, t r, 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

19 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

20 Outline Acquisition chain elements Multiplexer Track and hold unit Analog-to-digital converter 1 Acquisition chain elements 2 Different signals and sensors Characteristics of sensors 3 4 Multiplexer Track and hold unit Analog-to-digital converter 5 Labview Measurement chain for visualizations 20 Amélie Danlos, Florent Ravelet Experimental methods for fluid flows: an introduction

21 Multiplexer Track and hold unit Analog-to-digital converter 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 2 n 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

22 Multiplexer Track and hold unit Analog-to-digital converter 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 F e. τ is the time for which the interruptor is closed (a sample duration). We must have: τ << 1 Fe 22 Amélie Danlos, Florent Ravelet Experimental methods for fluid flows: an introduction

23 Multiplexer Track and hold unit Analog-to-digital converter 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. 23 Amélie Danlos, Florent Ravelet Experimental methods for fluid flows: an introduction

24 Multiplexer Track and hold unit Analog-to-digital converter 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

25 Multiplexer Track and hold unit Analog-to-digital converter 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

26 Multiplexer Track and hold unit Analog-to-digital converter 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

27 Multiplexer Track and hold unit Analog-to-digital converter 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 Amélie Danlos, Florent Ravelet Experimental methods for fluid flows: an introduction

28 Multiplexer Track and hold unit Analog-to-digital converter 28 Amélie Danlos, Florent Ravelet Experimental methods for fluid flows: an introduction

29 Outline Acquisition chain elements Labview Measurement chain for visualizations 1 Acquisition chain elements 2 Different signals and sensors Characteristics of sensors 3 4 Multiplexer Track and hold unit Analog-to-digital converter 5 Labview Measurement chain for visualizations 29 Amélie Danlos, Florent Ravelet Experimental methods for fluid flows: an introduction

30 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 Amélie Danlos, Florent Ravelet Experimental methods for fluid flows: an introduction

31 Labview Measurement chain for visualizations Software: LabVIEW 31 Amélie Danlos, Florent Ravelet Experimental methods for fluid flows: an introduction

32 Labview Measurement chain for visualizations Flow visualizations 32 Amélie Danlos, Florent Ravelet Experimental methods for fluid flows: an introduction

33 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

34 Flow visualizations Acquisition chain elements Labview Measurement chain for visualizations Limitations Vignetting Flare Depth of field 34 Amélie Danlos, Florent Ravelet Experimental methods for fluid flows: an introduction

35 Flow visualizations Acquisition chain elements Labview Measurement chain for visualizations Limitations Chromatic aberration Geometric aberration (Distorsion) 35 Amélie Danlos, Florent Ravelet Experimental methods for fluid flows: an introduction

36 Labview Measurement chain for visualizations Flow visualizations CCD (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

37 Labview Measurement chain for visualizations Movie Flow visualizations Movie CMOS (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

38 Flow visualizations Acquisition chain elements Labview Measurement chain for 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

39 Labview Measurement chain for visualizations Flow visualizations Signal transfer and processing 39 Amélie Danlos, Florent Ravelet Experimental methods for fluid flows: an introduction