Lecture 10 Measurement, A/D Conversion, Transmission, End Control - PowerPoint PPT Presentation

Lecture 10 Measurement, A/D Conversion, Transmission, End Control Element Process Control Prof. Kannan M. Moudgalya IIT Bombay Monday, 12 August 2013 1/33 Process Control Instrumentation

Outline 1. Signal levels 2. Digital signals and A/D conversion 3. Signal transmission 4. End control element 2/33 Process Control Instrumentation

Components of Process Control Instrumentation System 1. Sensor/transducer 2. A/D and D/A converter 3. Transmitter 4. Feedback controller 5. End control element 3/33 Process Control Instrumentation

Sensor/Transducer, Transmitter ◮ Sensor/Transducer ◮ To sense physical variables: pressure, flow rate, temperature, etc. ◮ Convert into electrical quantity ◮ Transmitter ◮ Raises levels to suitable strength ◮ to prevent loss of information during transmission 4/33 Process Control Instrumentation

1. Signal levels 5/33 Process Control Instrumentation

Popular instrumentation signal levels 1. Pneumatic: 3-15 psig 2. Voltage: 0-5 VDC is most popular 3. Current: 4-20 mA is most popular 6/33 Process Control Instrumentation

Pneumatic vs. Electronic standards for measurement Choose the closest answer: An important reason for using pneumatic transmission method is 1. High sensitivity of pneumatic measurements 2. Fast response time of pneumatic measurements 3. Highly accurate measurements 4. The ambient has high amount of hydrocarbons and hence explosive Answer: 4, explosive atmosphere Example: Purchase of a Gas Chromatograph at NOCIL 7/33 Process Control Instrumentation

Voltage vs. Current standards for measurement Choose the closest answer: Transmission of voltage is preferred over transmission of current when 1. The signal has to travel long distance 2. There is a lot of measurement noise 3. The cost has to be kept low Answer: 3, when the cost has to be kept low An example in the next slide 8/33 Process Control Instrumentation

AD 590 vs. LM 335 for SBHS 1. Both are used to measure temperature 2. Wanted to use LM 335 for SBHS - only Rs. 10 3. Picked up quite a bit of noise 4. Had to go for AD 590, Rs. 500+ 9/33 Process Control Instrumentation

2. Digital signals and A/D conversion 10/33 Process Control Instrumentation

Digital Signals ◮ You must have heard of digital signals, what are they? ◮ Crude definition: binary numbers 0, 1 are used ◮ Rigorous definition later 11/33 Process Control Instrumentation

What is meant by noise in digital signals? 1. Voltage changes from 5 VDC to a different value 2. Current goes outside the range 4-20 mA 3. Transmitted signal is received as a different one Answer: 3. Signal 0 is received as 1 or vice versa 12/33 Process Control Instrumentation

What is meant by 0 or 1 in digital signals? Answer: to range of values 13/33 Process Control Instrumentation

TTL Noise Margin at Low State Considered low if < 0.4V Considered low if < 0 . 8V Guaranteed noise margin = 0.4V at low state! 14/33 Process Control Instrumentation

TTL Noise Margin at High State Considered high if > 2.4V Considered high if > 2V Guaranteed noise margin = 0.4V at high state too! 15/33 Process Control Instrumentation

Benefits of Digital Circuits ◮ Easy to implement/modify circuits - simply change the coefficients! ◮ Margins can handle noise, drift, etc. ◮ Can improve accuracy through more bits ◮ Can implement error checking protocols ◮ Can be reproduced in volumes ◮ Can be fully integrated through VLSI ◮ Through multiplexer, a single processor can handle a large number of digital signals 16/33 Process Control Instrumentation

Benefits of Digital Circuits - Ctd. ◮ So digital devices became popular - impetus for advancement of digital systems ◮ Digital devices have become rugged, compact, flexible and inexpensive ◮ Modern devices (controllers, filters, watches, computers, etc.) are digital 17/33 Process Control Instrumentation

Analog to Digital (A/D) Conversion Actual & Quantized Quantized Sampled Data Data time time ◮ produces binary equivalent - batch process, requires conversion time ◮ digital signal is quantized in value and discrete in time 18/33 Process Control Instrumentation

Analog to Digital (A/D) Conversion - Ctd. ◮ Quantization errors ◮ Finiteness of bits - quantization errors ◮ Increase number of bits to reduce errors ◮ Falling hardware prices help achieve this ◮ Sampling rate ◮ Slow rate ⇒ loss of information ◮ Fast rate ⇒ computational load ◮ Analog’s output is sent to digital through A/D Reverse? 19/33 Process Control Instrumentation

Digital Signals ◮ Digital signals: quantized in value, discrete in time ◮ Binary numbers 0, 1 used ◮ As 0 or 1 refers to a range of voltages, digital signals can be made less noisy ◮ If transmitted signal is received exactly, no noise ◮ Analog circuitry always has noise ◮ Digital devices have good noise margins 20/33 Process Control Instrumentation

Reverse of A/D is D/A We use D/A converter, because 1. A/D converter always comes with a D/A converter also 2. As the price of electronic components are low, D/A converter is available at a low price 3. The end control element needs to know what value to apply in between sampling instants Answer: 3 21/33 Process Control Instrumentation

Digital to Analog (D/A) Conversion ◮ Sampled signal Discrete Signals ◮ Real life systems are analog ◮ Need to know values at all times ◮ Cannot work with binary numbers 22/33 Process Control Instrumentation

Digital to Analog (D/A) Conversion ◮ The easiest way to handle this to use Zero Order Hold (ZOH) � � � � � � � � � � � � Discrete � � � � � � � � � � � � � � � � � � � � ZOH � � � � � � � � � � � � � � � � Signals � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � ◮ ZOH is the most popular ◮ We will consider only ZOH in this course 23/33 Process Control Instrumentation

3. Signal transmission 24/33 Process Control Instrumentation

Protocols for Communication ◮ How to ensure correct information is communicated? ◮ Protocols ◮ Parity check: odd, even ◮ Without these, downloads will be very few! 25/33 Process Control Instrumentation

Transmission lines ◮ Various standards ◮ 4-20mA most popular ◮ Multi pair shielded cables for signal transmission ◮ Power supply in the same loop - no separate wiring ◮ Pneumatic transmission: 100m-200m ◮ Fieldbus protocol - a popular scheme to connect instruments 26/33 Process Control Instrumentation

4. End control element 28/33 Process Control Instrumentation

End Control Elements ◮ Pump ◮ Heater + Silicon Controlled Rectifier ◮ Control valve ◮ Also known as the automatic control valve ◮ Pneumatic control valve ◮ Solenoid control valve (ON-OFF) ◮ Proportional control solenoid valve 29/33 Process Control Instrumentation

Pneumatic control valve 30/33 Process Control Instrumentation

Components of a pneumatic control valve ◮ Valve body ◮ Trim - modulates flow ◮ Ball, plug, disk, gate ◮ Seat protective material ◮ Metal/soft polymer ◮ Protects against corrosive, abrasive material ◮ Actuator: mechanical driver 31/33 Process Control Instrumentation

What we learnt today ◮ Signal levels ◮ Digital signals and A/D conversion ◮ Signal transmission ◮ End control element 32/33 Process Control Instrumentation

Thank you 33/33 Process Control Instrumentation