Meeting w9 chapter 3 part 2


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Meeting w9 chapter 3 part 2

  1. 1. Chapter 3 Digital Control System <ul><li>Data Acquisition </li></ul><ul><li>Conversion </li></ul><ul><li>Distribution Systems </li></ul>
  2. 2. 5. Data Acquisition, Conversion and Distribution System <ul><li>Signal conversion in digital control system: </li></ul><ul><li>Multiplexing and demultiplexing </li></ul><ul><li>Sample and hold </li></ul><ul><li>Analog-to-digital conversion (quantizing and encoding) </li></ul><ul><li>Digital-to-analog conversion (decoding) </li></ul>
  3. 3. Cont. Block diagram of data acquisition system Block diagram of a data distribution system
  4. 4. Physical Variable <ul><li>Defined as an input to the system such as position, velocity, acceleration, temperature, pressure, etc. </li></ul><ul><li>This parameter is converted to the voltage or current signal by transducer </li></ul><ul><li>The signal represents the measured value and used in data-acquisition process </li></ul>
  5. 5. Transducer <ul><li>Device that converts input signal (physical variable) into output signal form </li></ul><ul><li>Eg. Pressure sensor that converts pressure signal into voltage output </li></ul><ul><li>Classified as: </li></ul><ul><li>Analog transducer (continuous function of time) </li></ul><ul><li>Sampled-data transducer (periodic discrete time) </li></ul><ul><li>Digital transducer (quantized discrete time) </li></ul>
  6. 6. Exercise 1 <ul><li>Name 4 parameters in process control measurement. </li></ul><ul><li>Level </li></ul><ul><li>Flow </li></ul><ul><li>Pressure </li></ul><ul><li>Temperature </li></ul>
  7. 7. Amplifier <ul><li>Frequently made from operational amplifier </li></ul><ul><li>Amplify the voltage output of the transducer </li></ul><ul><li>Convert current signal into voltage signal </li></ul><ul><li>It also used to buffer the signal </li></ul>
  8. 8. Cont. <ul><li>Figure below illustrates the operational amplifier. </li></ul>Basic op-amp
  9. 9. Low-pass filter <ul><li>Output from amplifier contains noise signal (high-frequency) which may corrupt the data </li></ul><ul><li>Low-pass filter is used to reduce the noise into an analog signal </li></ul><ul><li>However, periodic noise such as in power-line are filtered by notch filter </li></ul>
  10. 10. Cont. <ul><li>Following diagram shows the first-order low-pass filter circuit. </li></ul>First order low-pass filter
  11. 11. Analog multiplexer <ul><li>Used when many signal need to be processed by a single digital controller </li></ul><ul><li>It is actually a multiple switch operate sequentially to provide single output as the following figure </li></ul>Schematic diagram of analog multiplexer
  12. 12. Cont. <ul><li>Only one switch is ‘ON’ in a specific time which allows the input channel connected to the output of multiplexer </li></ul><ul><li>During this short time, S/H circuit samples the analog signal and holds its value and the same time the A/D converter converts the analog signal to digital signal </li></ul>
  13. 13. Sample-and-hold S/H <ul><li>It is actually sample-and-hold amplifier </li></ul><ul><li>The amplifier circuit receives an analog input signal and holds it for a specified time </li></ul><ul><li>The following figure illustrates the sample-and-hold circuit </li></ul>
  14. 14. Cont. <ul><li>Op-amp 1 acts as input buffer with high input impedance </li></ul><ul><li>Op-amp 2 acts as output amplifier that buffer the voltage on the hold capacitor </li></ul>Sample and hold circuit
  15. 15. Cont. <ul><li>Tracking mode (switch close) – input signal is connected </li></ul><ul><li>Hold mode (switch open) – capacitor voltage holds constant for a specified time </li></ul><ul><li>The operation is dictated by a periodic clock </li></ul>
  16. 16. Analog/digital converter (ADC) <ul><li>Converts analog signal in the form of voltage and current into a digital signal which is numerically coded signal or binary number </li></ul><ul><li>Comprises a single IC with some supporting components </li></ul><ul><li>The hardware required some conversion time depends on type of ADC, clock frequency and number of bit </li></ul>
  17. 17. Cont. <ul><li>ADC performs the operations of sample-and-hold, quantizing, and encoding </li></ul><ul><li>amplitude quantization - a process of representing a continuous or analog signal by a finite number of discrete states </li></ul><ul><li>“ Quantizing&quot; means transforming a continuous or analog signal into a set of discrete states </li></ul>
  18. 18. Cont. <ul><li>Encoding is a process of assigning a digital word or code to each discrete state </li></ul><ul><li>The quantization level Q is defined as the range between two adjacent decision points and is given by </li></ul>
  19. 19. 8-bit ADC <ul><li>Below is a block diagram of ADC </li></ul>8-bit Analog-to-Digital Block Diagram
  20. 20. Cont. <ul><li>V in can be any voltage between 0 V and V ref </li></ul><ul><li>When V in is 0 Vdc, the output is 00000000 </li></ul><ul><li>When V in is V ref , the output is 11111111 (255 decimal) </li></ul><ul><li>For input voltages between 0 and V ref , the output increases linearly with V in </li></ul>
  21. 21. Cont. <ul><li>A start-conversion pulse is sent to the ADC </li></ul><ul><li>ADC then samples the analog input and converts it to binary </li></ul><ul><li>When completed, the ADC activates the data-ready output </li></ul><ul><li>This signal can be used to alert the computer to read in the binary data </li></ul>8-bit Analog-to-Digital Block Diagram
  22. 22. Example 1 <ul><li>Calculate the quantization level for a 4 bit ADC with input 0 – 24 mA. </li></ul><ul><li>Solution: </li></ul>
  23. 23. Example 2 <ul><li>Calculate the output of decimal value of 6 bit ADC if the voltage input is 3V with maximum range 12V. </li></ul><ul><li>Solution: </li></ul>
  24. 24. Example 3 <ul><li>Find an output of an 8 bit ADC if the input is 5 V and the reference voltage is 10 V. </li></ul><ul><li>Solution: </li></ul>
  25. 25. ADC method <ul><li>Among the many ADC circuits available, the following types are used most frequently: </li></ul><ul><li>Successive-approximation type </li></ul><ul><li>Integrating type </li></ul><ul><li>Counter type </li></ul><ul><li>Parallel type </li></ul><ul><li>Each type has its own advantages and disadvantages </li></ul><ul><li>Application, the conversion speed, accuracy, size, and cost are the main factors to be considered in choosing the type of ADC </li></ul>
  26. 26. Counter type ADC <ul><li>Simplest type of ADC </li></ul><ul><li>Principle: </li></ul><ul><li>1. Clock pulses applied to the digital counter </li></ul><ul><li>2. This results the output voltage of DAC which is part of feedback loop in ADC stepped up one LSB at a time </li></ul><ul><li>3. At each pulse, the output voltage is compared to the analog input voltage </li></ul><ul><li>4. The clock pulses stopped when output voltage is equal to the magnitude of the input voltage </li></ul><ul><li>5. Therefore, the counter output voltage is the digital output </li></ul>
  27. 27. Successive-approximation type ADC <ul><li>The most frequently used ADC </li></ul><ul><li>Schematic diagram as following figure </li></ul>
  28. 28. Successive-approximation principle: <ul><li>Successive-approximation register turns MSB and compares it with analog input </li></ul><ul><li>The comparator will determine to leave the bit on or off by comparing with analog input voltage </li></ul><ul><li>The MSB is set on if the input voltage is larger </li></ul>