Introduction to Data Acquisition Systems


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A brief introduction for data acquisition systems........

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  • This is a block diagram of the steps needed to take a physical variable and make it usable by a computer. The stages of this system will be used in various labs throughout the quarter. Next, we will explain each of these stages in more detail.
  • A data acquisition system consists of many components that are integrated to sense physical variables, by using transducers to convert the physical variable to an electrical signal, then condition that electrical signal to make it readable by an analog-to-digital converter.
  • Transducers are used to sense physical phenomena and translate it into electric signals. Examples of things that can be measured with transducers are temperature, pressure, light, force, displacement, level, electrical signals, and switches.
  • The resolution of a converted signal is the number of bits that are used to store each sample of data. For example, a two bit resolution will allow 2 to the 2nd power number of values for the data, meaning that the data can take 4 possible values. For 3 bit conversion, 8 values are possible, and for 5 bit conversion, 32 values are possible. The higher the resolution, the closer the digitized signal will resemble the original analog waveform.
  • A problem with using too low of a sampling rate is that aliasing might occur. Aliasing is when the acquired signal gets distorted by a sampling rate that is too small. In this example, the original signal is sampled so slowly that the sampled signal looks like a completely different frequency than the original signal.
  • Introduction to Data Acquisition Systems

    1. 1. Introduction to Data Acquisition Systems NIDHIN MANOHAR
    2. 2. Data Acquisition System Is a Generic term for A System that : • acquires measurement data from the process / field instruments • To displays the data / parameters • Process data • Store data • Provide output signals to external Indicator / control devices • makes data available for other functions
    3. 3. DAS : Why do we use them? • Large number of measurement points, • large volume of computations, processing and conversions, • complicated computations, • transients measurement, • improve response to events / alarms, • simultaneous recordings, • reduce / avoid manual data logging • reduce human decision making, • improve safety,
    4. 4. DAS : Why do we use them? • • • • • • • • monitor over long duration, facilitate remote telemetry, connectivity to SCADA or existing MIS systems Strict sequence of operations and controls , Interfacing to sensors, transducers, eliminate indicators, electronics, eliminate T&M Instruments, improve documentation
    5. 5. Data Acquisition System
    6. 6. Data acquisition system, in effect, requires integration of: • Sensor/transducer for measurement of physical variables • Signal-Conditioner/transmission circuitry, that enables conversion of signal outputs from transducers to a readable form for Data Acquisition/interface modules. • The Data Acquisition Hardware comprising Multiplexer, Amplifier, A/D Converter, Buffer Memory, etc. to digitise the analog signals for CPU. • Computer/CPU to process the digital data for data processing, display, outputs (control), storage, transmission, etc.
    7. 7. Process Measurements Temperature, Pressure, Load, Force, Displacement, Speed, Acceleration, Flow, Level, density, viscosity, analytical parameters such as pH, ORP, Conductivity, turbidity, BOD, etc., Light/Dark sensors, gas-composition analyzers / Chromatographs, etc.. Other parameters include gas-leak detectors, fire/smoke detectors, power quality, energy,
    8. 8. PC based Data Acquisition • • • • • PCs are affordable, large variety of interfaces, software and operational flexibility, large storage capabilities, reasonably good data processing capabilities, • user-friendly, • ease of portability to other systems,
    9. 9. PC Interfacing • Add-On Card Plug-in Slots, • COM ports (RS-232C); further add Modem for long distances, • Universal Serial Bus (USB), • Parallel Port (Centronics / Printer), • LAN / Ethernet (TCP/IP) interface, • Add GPIB or IEEE-488 Interface, • Add SCSI or may be built-in, • Add HART, FIELDBUS or other Interfaces
    10. 10. • Analog outputs from transmitters / meters for pressure, temperature, level, strain-gage, load/force, flow, displacement, etc. Eg: 4 to 20mA, mV or V Signal PV
    11. 11. • Digital Pulse outputs : Turbinemeters, Positive displacement meters, Encoders, Tachometers, etc. Hi/Lo or ON/OFF signals: Limit-switches, micro-switches, photosensors, proximity sensors, Opencollector outputs, relays, etc. Serial : RS232C, RS485, USB; HART, MODBUS, FieldBus, Ethernet, etc. Parallel data: IEEE-488, Parallel port, etc.
    12. 12. ON/OFF or OPEN/CLOSE type Signals Pulse or Frequency Signals
    13. 13. Outputs from Data Acquisition & Control Systems • Analog 4 to 20mA control signals for Pneumatic actuated control valves, Controller Set-point , etc. 4 to 20mA or V signal to output flow / other parameters to remote indicators or chart-recorders
    14. 14. • Digital (High speed) Outputs : Pulse / Frequency Output for remote totaliser / counter. Output to speed and motion control systems; Output to Stepper-motor based systems. • Digital (Low-speed) Outputs Control outputs to Relays, Actuators, ON/OFF control for Motors/ Pumps, Valves Status and Alarm Outputs to remote / panel mounted indicators, and annunciators.
    15. 15. When trying to Set Up your own D.A.S Sensor/Transducer selection: • sensors, transmitters and signal conditioners best suited for the application to be chosen. • existing meters may need replacement if there is no suitable output signals.
    16. 16. Performances, Economics? • Mobility (for PC, transducers, etc) • Normal PC? Laptop, Industrial PC? What costs? • Cable-routes to D.A.S? Data Quality, Costs? • Distances involved? No. of cable runs? Type of signals? Wireless options? Costs? • Signal conditioning options, costs?
    17. 17. Main Considerations: D.A.S • • • where to locate DAS? Where are the field devices? How is the Environment? – Dirt, moisture, temperature , etc. – AC Mains power (Voltages fluctuations? Surges/spikes?) – shock and vibration. • Can normal PC be used? whether additional protection necessary? • Can enclosures help? IP required?
    18. 18. Industrial PC • Rugged SMPS (wide ACV range) • redundancy : hot-swappable SMPS • additional cooling fans with dust filters • isolation of plug-in boards, CPUboard, disk drive, etc., from shock • disk-drive bays with covers • membrane-sealed keyboards. • Ingress protection (IP)
    19. 19. Selection of DAS hardware Analog Input Interface: • 12 to 24-bit resolution ADCs • No. of channels : 8, 10,16, 20, 32, or 64, per board. • Speed : 100k, 110k, 330k, 500k, 1.25M, 5M, 20M samples per second. • Resolution Vs Speed.
    20. 20. A/D Converter Resolution
    21. 21. Sampling Frequency • Aliasing – Acquired signal gets distorted if sampling rate is too small.
    22. 22. Selection of hardware Analog Inputs: • Continuous high-speed data acquisition : on-board FIFO RAM • Simultaneous sampling : multiple ADCs • Low-level signal inputs : gains
    23. 23. Selection of hardware Digital to Analog Conversion: • generally one output channel; • Analog output boards may have 2, 4 or more analog outputs
    24. 24. Selection of hardware Digital Input/Output: • usually TTL compatible (0-5V) • direct 12V or 24V Input-Output: if built-in Optical-Isolation • Reed-Relays, SSR: 1 to 3A NO Contacts. • No. of Digital IO: 4, 8, 16, or 24
    25. 25. Selection of hardware Counter-Timer Functions: • event or pulse counting • frequency measurement • pulse output: used in stepper motor control, other control applications. • Single-pulse output • Time measurement (gated) • Internal clock: 10kHz, 100kHz, 1MHz, 10MHz or 20MHz.
    26. 26. PC Specifications: • • • • • • • • Ports, Slots, interfaces Storage. (HDD, Flash) on-line processing. ( RAM) Graphical content / updates. DAS software requirements. Post-process /Analysis needs Operating System needs. Networking, other hardware needs
    27. 27. Software for D.A.S • Pascal, BASIC, C/C++, VB, ….etc. • LabTech-Notebook, LabVIEW, Agilent-VEE, Genie, DasyLab, …etc. • Issues : – GUI s/w costs? Learning / training? – Availability of drivers? – Complete hardware compatibility? – Real-Time? – Software overheads on CPU? • SCADA Software…
    28. 28. Thank you for your Attention. Oh……….. the Wait is OVER!!