Data acquisition system


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Data acquisition system

  1. 1. Data Acquisition system By Shiv chamkure,2011BIN004 1
  2. 2. • What Is Data Acquisition? • What data acquisition system consist of ? • Block diagram of data acquisition system • Transducers(sensors) • Signal conditioning • Ground loops • Data acquisition and control hardware • Functions of a DAQ device • Data Acquisition Software 2
  3. 3. 3 What Is Data Acquisition? Data acquisition (DAQ) is the process of measuring an electrical or physical phenomenon such as voltage, current, temperature, pressure, or sound ,flow rate with a computer.
  4. 4. 4 A data acquisition system consists of : 1. Sense of physical variables ( transducers ) 2. Signal Conditioning for electrical signal to make it readable by an A/D board 3. Convert the signal into a digital format acceptable by a computer(DAQ device) 4. Process, analyze, store, and display the acquired data with the help of software
  5. 5. 5 Data Acquisition System Block Diagram :
  6. 6. 6 What is a Transducer? Transducer converts a physical phenomenon into a measurable electrical signal
  7. 7. 7 Variety of Transducers
  8. 8. 8 Signal Conditioning Improving the quality of signals Amplification Filtering Isolation Noisy, Low-Level Signal Filtered, Amplified Signal Noisy, Low-Level Signal Filtered, Amplified Signal
  9. 9. 9 Amplification : When the input signal is as small as microvolts, electrical noise can drown out the signal itself, leading to meaningless data. For reducing the effects of noise on your signal is to amplify the signal as close to the source as possible. Increases Signal to Noise Ratio (SNR) e.g. A J-type thermocouple outputs a very low-level voltage signal that varies by about 50 µV/°C.
  10. 10. 10 Filtering : To reject unwanted noise within a certain frequency range. Many systems will exhibit 50 Hz periodic noise from sources such as power supplies or machinery. Butterworth Filter Bessel Filter Chebyshev Filter Simple RC Filter Passive & Active Filters e.g
  11. 11. 11 Isolation is one way to eliminate ground loop errors. Isolation breaks ground loops, rejects high common-mode voltages, and protects expensive DAQ boards. Protects from power lines, lightning. . Isolation : Improper grounding of the DAQ system is the most common cause of measurement problems and damaged DAQ boards. There is no physical wiring between the input and output. The input is normally transferred by converting it to an optical or magnetic signal then it is reconstructed on the output.
  12. 12. 12 Grounding Issues To get correct measurements you must properly ground your system. + _ Vs + _ Vs Grounded Floating Signal Source
  13. 13. 13 Signal is referenced to a system ground -earth ground -building ground Examples: -Power supplies -Signal Generators -Anything that plugs into an outlet Grounded Signal Source + _ Vs
  14. 14. 14 Signal is NOT referenced to a system ground – earth ground – building ground Examples: – Batteries – Thermocouples – Transformers – Isolation Amplifiers – building ground + _ Vs Floating Signal Source
  15. 15. 15 Ground Loops :  There are two grounds.  The grounds are at different potentials.  There is a galvanic path between the grounds.
  16. 16. 16 Breaking this galvanic path can be accomplished by isolators optical isolation:- optical isolators, photo-transistors magnetic isolation:- Transformers
  17. 17. 17 Eliminating Ground Loop :
  18. 18. 18 signal transmitters, ground loop isolators, 4-20ma transmitters, 4-20ma converters, 4-20ma isolators, analog signal converters analog signal isolators Signal conditioners are also sometimes referred to as:
  19. 19. 19 Computer Your Signal DAQ Device Q Device Terminal Block Cable Your Signal Computer Data acquisition and control hardware : Terminal Block Cable DAQ Hardware turns your PC into a measurement and automation system
  20. 20. 20 Your Signal Terminal Block Cable 50 pin connector Terminal Block Your Signal Terminal Block and Cable Terminal Block and Cable route your signal to specific pins on your DAQ device
  21. 21. 21 Some things to consider: Connection to the Internet Is there software support? Type of input channels - single-ended input channels differential input channels. Compatible with a variety of bus protocols Number of analog input channels Sampling rate Resolution Accuracy Reduced board size Plug-in data acquisition cards DAQ device
  22. 22. 22 Functions of a DAQ device : - analog input- digital output - digital input- analog output - counter/timer functions
  23. 23. 23 Analog Inputs (A/D) when selecting A/D hardware :  Number of input channels  Single-ended or differential input signals Sampling rate (in samples per second)  Resolution (usually measured in bits of resolution)  Input range (specified in full-scale volts)  Gain
  24. 24. 24 A/D Converter: Sampling Rate  The higher the sampling rate, the better signal we get  Acquired signal gets distorted if sampling rate is too small.
  25. 25. 25 Nyquist sampling theorem tells us that we must sample the signal at more than twice the rate of the maximum frequency component in the analog input signal. According to the Nyquist Theorem, the sampling rate must be at least 2fmax. Nyquist Sampling Theorem Where , highest frequency component = fmax
  26. 26. 26 Minimum and maximum voltages the ADC can digitize Ranges are selectable Pick a range that your signal fits in Smaller range = more precise representation of your signal Range
  27. 27. 27 Number of bits the ADC uses to represent a signal changes can be measured Example: 12-bit resolution 2resolution = 212 = 4,096 levels Larger resolution = more precise representation Resolution
  28. 28. 28 Gain setting amplifies the signal for best fit in ADC range Gain settings are 0.5, 1, 2, 5, 10, 20, 50, or 100 for most devices Proper gain = more precise representation Gain
  29. 29. 29 Code Width  Code Width is the smallest change in the signal your system can detect (determined by resolution, range, and gain) range  code width = gain * 2 resolution  Smaller Code Width = more precise representation
  30. 30. 30 Analog Outputs (D/A)  D/A devices allow the computer to control real- world events.  Analog output signals may directly control process equipment.  The process can give feedback in the form of analog input signals.  This is referred to as a closed loop control system with PID control.
  31. 31. 31 Counter/timer devices provide a variety of measurement solutions, including measuring a number of time-related quantities. Use counter/timers for:  Frequency measurement  Edge or event counting (totalizing)  Pulse-width measurement  Event timestamps  pulse-width modulation (PWM) Counters/Timers
  32. 32. 32 Data Acquisition Software It can be the most critical factor in obtaining reliable, high performance operation Transforms the PC and DAQ hardware into a complete DAQ, analysis, and display system. Different alternatives: – Programmable software. – Data acquisition software packages
  33. 33. 33 Involves the use of a programming language, such as: – C++, Visual C++ – BASIC, Visual Basic + Add-on tools (such as VisuaLab with VTX) • Advantage: flexibility • Disadvantages: complexity and steep learning curve Programmable Software
  34. 34. 34 Data Acquisition Software Packages • Does not require programming. • Enables developers to design the custom instrument best suited to their application. • Examples: TestPoint, SnapMaster, LabView, DADISP, DASYLAB, etc
  35. 35. 35 Computer Timer Digital Control Circuit Trigger Interrupt Parallel/Series Input Port Parallel/Series Output Port A/D D/A Filter + - S/H Sensor Bridge Instrumentation Amplifier Input Strobe Display Control Output Strobe Example of Computer DAQ System
  36. 36. 36 Presented by Shiv Chamkure 2011BIN004