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  • 8/3/2019 data aquisition system and data loggers.. .



    What is data acquisition system?

    You can think of a data acquisitionsystem as a collection of software andhardware that connects you to the

    physical world.

    A typical data acquisition systemconsists of these components.

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    Data acquisition hardware

    At the heart of any data acquisition system lies thedata acquisition hardware. The main function ofthis hardware is to convert analog signals to digitalsignals, and to convert digital signals to analogsignals.

    Sensors and actuators (transducers)

    Sensors and actuators can both be transducers. A

    transducer is a device that converts input energyof one form into output energy of another form.

    For example, a microphone is a sensor thatconverts sound energy (in the form of pressure)

    into electrical energy, while a loudspeaker is anactuator that converts electrical ener into sound

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    4/22/12Signal conditioning hardwareSensor signals are often incompatible with dataacquisition hardware. To overcome this incompatibility, the signal must be

    conditioned. For example, you might need tocondition an input signal by amplifying it or byremoving unwanted frequency components. Outputsignals might need conditioning as well.

    computerThe computer provides a processor, a system

    clock, a bus to transfer data, and memory and diskspace to store data.Software

    Data acquisition software allows you to exchangeinformation between the computer and the hardware. For example, typical software allows you to configurethe sampling rate of your board, and acquire a

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    4/22/12The data acquisition components, and theirrelationship to each other, are shown below.

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    The figure depicts the two important features of a dataacquisition system:Signals are input to a sensor, conditioned, converted intobits that a computer can read, and analyzed to extractmeaningful information.For example, sound level data is acquired from a

    microphone, amplified, digitized by a sound card, andstored in MATLAB workspace for subsequent analysis offrequency content.

    Data from a computer is converted into an analog signal

    and output to an actuator.For example, a vector of data in MATLAB workspace isconverted to an analog signal by a sound card and outputto a loudspeaker.

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    4/22/12Data Acquisition HardwareData acquisition hardware is either internal andinstalled directly into an expansion slot inside yourcomputer, or external and connected to your

    computer through an external cable, which is typicallya USB cable.

    At the simplest level, data acquisition hardware is

    characterized by the subsystems it possesses.

    A subsystem is a component of your data acquisitionhardware that performs a specialized task.

    Common subsystems includeAnalog inputAnalog outputDigital input/outputCounter/timer

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    4/22/12Hardware devices that consist of multiplesubsystems, such as the one depicted below, arecalled multifunction boards.

    Analog Input SubsystemsAnalog input subsystems convert real-world analog input

    signals from a sensor into bits that can be read by yourcomputer. Perhaps the most important of all the subsystemscommonly available, they are typically multichanneldevices offering 12 or 16 bits of resolution.

    Analog input subsystems are also referred to as AI

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    Analog Output Subsystems

    Analog output subsystems convert digital data stored onyour computer to a real-world analog signal.

    These subsystems perform the inverse conversion ofanalog input subsystems.

    Typical acquisition boards offer two output channelswith 12 bits of resolution, with special hardware available

    to support multiple channel analog output operations.

    Analog output subsystems are also referred to as AOsubsystems, D/A converters, or DACs.

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    4/22/12Digital Input/Output Subsystems

    Digital input/output (DIO) subsystems are designed toinput and output digital values (logic levels) to and fromhardware.

    These values are typically handled either as single bits orlines, or as aport, which typically consists of eight lines.

    While most popular data acquisition cards include somedigital I/O capability, it is usually limited to simpleoperations, and specialdedicated hardware is often necessary for performingadvanced digital I/O operations.

    Counter/Timer Subsystems

    Counter/timer (C/T) subsystems are used for eventcountin fre uenc and eriod measurement and ulse

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    A sensor converts the physical phenomena of interestinto a signal that is input into your data acquisition


    There are two main types of sensors based on theoutput they produce: digital sensors and analog sensors.

    Digital sensors produce an output signal that is a digitalrepresentation of the input signal, and has discretevalues of magnitude measured at discrete times.

    A digital sensor must output logic levels that arecompatible with the digital receiver.Some standard logic levels include transistor-transistorlogic (TTL) and emitter-coupled logic (ECL). Examples ofdigital sensors include switches and position encoders.

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    4/22/12Analog sensors

    Analog sensors produce an output signal that isdirectly proportional to the input signal, and is

    continuous in both magnitude and in time.

    Most physical variables such as temperature,pressure, and acceleration are continuous in natureand are readily measured with an analog sensor.

    For example, the temperature of an automobilecooling system and the acceleration produced by achild on a swing all vary continuously.

    The sensor you use depends on the phenomena youare measuring. Some common analog sensors and thephysical variables they measure are listed below.

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    Sensor Physical VariableAccelerometer Acceleration

    Microphone Pressure

    Pressure gauge Pressure

    Resistive temperature device



    Strain gauge Force

    Thermocouple Temperature

    Common AnalogSensors

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    When choosing the best analog sensor to use, you mustmatch the characteristics of the physical variable you aremeasuring with the characteristics of the sensor.The two most important sensor characteristics are:

    The sensor output

    The sensor bandwidth

    Sensor OutputThe output from a sensor can be an analog signal or adigital signal, and the output variable is usually a voltagealthough some sensors output current.

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    Sensor Bandwidth

    The bandwidth is given by the range of frequenciespresent in the signal being measured.

    You can also think of bandwidth as being related to the

    rate of change of the signal.

    A slowly varying signal has a low bandwidth, while arapidly varying signal has a high bandwidth.

    To properly measure the physical phenomena ofinterest, the sensor bandwidth must be compatible withthe measurement bandwidth.

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    You might want to use sensors with the widestpossible bandwidth when making any physicalmeasurement.

    This is the one way to ensure that the basicmeasurement system is capable of respondinglinearly over the full range of interest.

    However, the wider the bandwidth of the sensor,

    the more you must be concerned with eliminatingsensor response to unwanted frequencycomponents.

    i l di i i

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    4/22/12Signal Conditioning

    Sensor signals are often incompatible with data

    acquisition hardware.To overcome this incompatibility, the sensor signal mustbe conditioned.The type of signal conditioning required depends on thesensor you are using.

    For example, a signal might have a small amplitudeand require amplification, or it might contain unwantedfrequency components and require filtering.Common ways to condition signals include

    AmplificationFilteringElectrical isolationMultiplexing

    Excitation source

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    Low-level less than around 100 millivolts usuallyneed to be amplified.

    High-level signals might also require amplificationdepending on the input range of the analog input


    For example, the output signal from a thermocouple issmall and must be amplified before it is digitized.

    Signal amplification allows you to reduce noise and tomake use of the full range of your hardware therebyincreasing the resolution of the measurement.

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    Filtering removes unwanted noise from the signal ofinterest.

    A noise filter is used on slowly varying signals such

    as temperature to attenuate higher frequency signalsthat can reduce the accuracy of your measurement.

    Rapidly varying signals such as vibration oftenrequire a different type of filter known as an

    antialiasing filter.

    An antialiasing filter removes undesirable higherfrequencies that might lead to erroneous


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    Electrical Isolation

    If the signal of interest contains high-voltagetransients that could damage the computer, then thesensor signals should be electrically isolated from the

    computer for safety purposes.

    You can also use electrical isolation to make sure thatthe readings from the data acquisition hardware arenot affected by differences in ground potentials.

    M lti l i

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    A common technique for measuring several signals

    with a single measuring device is multiplexing.Signal conditioning devices for analog signals oftenprovide multiplexing for use with slowly changingsignals such as temperature.This is in addition to any built-in multiplexing on the

    DAQ board. The A/D converter samples one channel,switches to the next channel and samples it, switchesto the next channel, and so on.Because the same A/D converter is sampling many

    channels, the effective sampling rate of eachindividual channel is inversely proportional to thenumber of channels sampled.You must take care when using multiplexers so thatthe switched signal has sufficient time to settle. Refer

    to Noise for more information about settling time.

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    Excitation Source

    Some sensors require an excitation source tooperate.

    For example, strain gauges, and resistive

    temperature devices (RTDs) require externalvoltage or current excitation.

    Signal conditioning modules for these sensorsusually provide the necessary excitation.

    RTD measurements are usually made with acurrent source that converts the variation inresistance to a measurable voltage.

    4/22/12Th C t

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    4/22/12The ComputerThe computer provides a processor, a system clock, abus to transfer data, and memory and disk space to storedata.

    The processor controls how fast data is accepted by theconverter. The system clock provides time informationabout the acquired data.

    Data is transferred from the hardware to system memory viadynamic memory access (DMA) or interrupts.

    DMA is hardware controlled and therefore extremely fast.

    Interrupts might be slow because of the latency time betweenwhen a board requests interrupt servicing and when the computerresponds.

    The maximum acquisition rate is also determined by thecomputer's bus architecture.

    4/22/12S ft

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    Regardless of the hardware you are using, you must

    send information to the hardware and receiveinformation from the hardware. You send configuration information to the hardwaresuch as the sampling rate, and receive informationfrom the hardware such as data, status messages,

    and error messages.You might also need to supply the hardware withinformation so that you can integrate it with otherhardware and with computer resources.

    This information exchange is accomplished withsoftware.There are two kinds of software:

    Driver software

    Application software


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    What is data logging?

    Data logging and recording is a commonmeasurement application.

    In its most basic form, data logging is the measuringand recording of physical or electrical parametersover a period of time.

    The data can be temperature, strain, displacement,flow, pressure, voltage, current, resistance, power,and many other parameters.


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    A wide range of products can be categorized as data

    loggers, from basic devices that perform a singlemeasurement to more complex devices that offeranalysis functions and integrated displays.

    Many applications are more involved than justacquiring and recording signals, sometimes involving acombination of online analysis, offline analysis, display,report generation, and data sharing.

    Moreover, applications are beginning to require theacquisition and storage of other types of data, such asrecording sound and video in conjunction with theother parameters measured during an automobile

    crash test.


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    Figure 1. Data loggers can be both PC-based or stand-alone devices.


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    Data logging is used in a broad spectrum ofapplications. Chemists record data such astemperature, pH, and pressure when performingexperiments in a lab.

    Design engineers log performance parameters suchas vibration, temperature, and battery level to

    evaluate product designs.

    Civil engineers record strain and load on bridges overtime to evaluate safety.

    Geologists use data logging to determine mineralformations when drilling for oil.

    The list of applications for data logging goes on and

    on, but all of these applications have similar common


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    How does data logger works?

    A data logger works with sensors toconvert physical phenomena andstimuli into electronic signals such as

    voltage or current. These electronic signals are then

    converted or digitized into binary data.

    The binary data is then easily analyzedby software and stored on a PC harddrive or on other storage media such

    as memory cards and CDs.


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    Figure 2. How does a datalogger work?

    A few components that every data logger must haveinclude:

    Hardware to digitize what you are logging, includingsensors, signal conditioning, and analog-to-digitalconversion hardwareLong-term data storage, typically onboard memory

    or a PC -


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    Follow these steps when using a data logger:

    Connect sensors like thermocouples, resistance

    temperature detectors (RTDs), thermistors, straingages, and accelerometers to the data logger

    Use your data-logging software to configure your

    data loggerConfigure parameters like sample rate, alarms, andstart or stop conditions for your data-logging taskwith your data-logging software

    Run the data-logging task After your hardwaredigitizes the sensor measurements, analyze andstore your data for future use


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    What can u do with data loggerbesides just log data?

    The ability to take sensormeasurements and store the data forfuture use is, by definition, a

    characteristic of a data logger.

    However, a data-logging applicationrarely requires only data acquisitionand storage. Inevitably, you need theability to analyze and present the data

    to determine results and make


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    Figure 3. Data-LoggingApplication Elements

    Acquire This step includes your sensors and data

    logger hardware as well as the conversion of physicalphenomena into digital signals.


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    Online Analysis This step includes any analysis youwould like to do before storing the data.

    A common example of this is converting the voltagemeasurement to meaningful scientific units, such asdegrees Celsius.You can complete these complex calculations and datacompressions before logging the data.Controlling part of a system based on currentmeasurements for example, a kill switch is also partof online analysis.Every data-logging software application should

    complete the conversion from binary to voltage and theconversion from voltage to scientific units.

    Log This step refers to the storage of analyzed dataincluding any formatting required for the data files.


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    Offline Analysis This step includes any analysisyou would like to do after storing the data.

    A common example is looking for trends inhistorical data or data reduction.

    Displaying, Sharing, Reporting This stepincludes the creation of any reports you need tomake to present your data.

    However, note that the figure above shows you

    can also present data straight from online analysis.

    This represents the ability to monitor and view thedata as you acquire and analyze it in addition to

    simply viewing historical data.


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