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

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Outline

• 1. Signal levels
• 2. Digital signals and A/D conversion
• 3. Signal transmission
• 4. End control element

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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

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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

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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

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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

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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

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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+

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• 2. Digital signals and A/D conversion

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Digital Signals

◮ You must have heard of digital signals, what

are they?

◮ Crude definition: binary numbers 0, 1 are used ◮ Rigorous definition later

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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

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What is meant by 0 or 1 in digital signals?

Answer: to range of values

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TTL Noise Margin at Low State

Considered low if < 0.4V Considered low if < 0.8V Guaranteed noise margin = 0.4V at low state!

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TTL Noise Margin at High State

Considered high if > 2.4V Considered high if > 2V Guaranteed noise margin = 0.4V at high state too!

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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

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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

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Analog to Digital (A/D) Conversion

time Actual & Quantized Data Sampled time Data Quantized

◮ produces binary equivalent - batch process,

requires conversion time

◮ digital signal is quantized in value and discrete

in time

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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?

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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

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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

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Digital to Analog (D/A) Conversion

◮ The easiest way to handle this to use Zero

Order Hold (ZOH)

Signals

• Discrete

ZOH

• ◮ ZOH is the most popular

◮ We will consider only ZOH in this course

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• 3. Signal transmission

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Protocols for Communication

◮ How to ensure correct information is

communicated?

◮ Protocols ◮ Parity check: odd, even ◮ Without these, downloads will be very few!

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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

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• 4. End control element

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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

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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

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What we learnt today

◮ Signal levels ◮ Digital signals and A/D conversion ◮ Signal transmission ◮ End control element

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Thank you

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