UDG1606 Process Instrumentation & ControlFaculty of Chemical & Natural Resources Engineering

CHAPTER 7Data Communications

1

Topic Covered

• Media

• Pneumatic tubing

• Electric wires

• Radio waves

• Optic fibers

• Data Representation

• Analog Control

• Digital Control

2

Introduction

• Data communications ~ send data frequently, reliably, safely and in useful format from the point of origin to the point of use.

• Data Communications is the transfer of data or information between a source and a receiver.

• The source transmits the data and the receiver receives it.

• Transmission distance can be a few feet up to certain miles.

• Transmission line can run through a hazardous environment

thus required a moderate amount of power to operate.

3

4

DATA FORMAT/

REPRESENTATION

Analog

Digital

Data represented by a physical quantity that is considered to be continuously variable and has a magnitude directly proportional to the data or to a suitable function of the data.

Data represented in terms of binary digits, also called bits, which takes on values of one(1) or zero (0).

Analog versus Digital Data

5

Analog data Digital data

Continuous value Discrete value

Unstable Stable

Signal Signal

6

Electric

wires

Pneumatic

Tubing

Optic

Fibers

Transmission Medium

Radio

waves

Pneumatic Tubing

• Analog signals are transmitted over pneumatic tubing (PVC/ Copper).

• Usually 3-15 psig or 20-100 kPa.

• The tubing usually ¼’’ O.D, sometimes 3/8 ’’.

• Bundles of tubing sometimes used to transmit multiple signals from a process to a control room for measurement and control.

• Safe medium for use in hazardous environments (eg. Petrochemical).

7

Pneumatic Tubing

• Disadvantages:

– Not used if distance greater than 100 ft ~ slow response

– Not used if complex analysis and display, and digital formats are required

8

Pneumatic Tubing

9

Electric Wires

• Most common medium of communication

• Simple, fairly inexpensive, can handle analog and digitalformats over long distance at high speeds

• Analog signals (usually 4-20 mA DC) and digital signals are transmitted over electric wires.

• Each analog signal usually has its own twisted pair of wires to transmit its signal from the sensor to the computer system

• If a cable of many twisted pairs is used, then a single shield with an insulating outer jacket can be provided around the pairs.

• No significant time delay in an electrical transmission system.However, capacitance and inductance may slow the signal.

10

Electric Wires

• To use a small number of wires for a great many messages between a network of devices, transmission lines have been developed for high speed transfer of serial digital messages.

• High speed transfer rate means line must transit at high frequency.

• Coaxial cables are used for cables television are beginning to be used to carry these messages.

11

Electric Wires

12

A: outer plastic sheathB: copper screenC: inner dielectric insulatorD: copper core

Coaxial Cable

Radio Waves

• SCADA (Supervisory Control and Data Acquisition)systems may use radio waves to transmit signal from RTUs (Remote Terminal Units) to a central station.

• These signals are usually digital and are encoded by microprocessor and modem for transmission.

• Usually several sensors are connected to RTU, and their values are sent as a group to the central station.

**RTU-converting sensor signals to digital data

13

Optic Fibers

• Optic fibers used to send signals at very high rates of data transfer over distances up to several miles in the face of extreme magnetic and electric interference.

• Its offer intrinsic safety where hazards from sparks associated with electric wires exist.

• The messages are usually digital in format and include signals from many sensors and groups of sensors connected in a network.

14

Optic Fibers

• The optic fiber is 100 or 200 microns in diameter, made of glass or plastic that transmits pulsating beams of light rather than electricity (transmit light between the two ends of the fiber).

• Unlike electric signals, light pulses are not affected by random electromagnetic interference in the environment.

• Fiber optic cable has much lower error rate than electric wire, lighter and more durable> It cannot easily be wiretapped, so transmissions are more secure.

15

Optic Fibers

16

Data Representation

17

ANALOG DATA

FORMAT

ELECTRONIC

• The most common form of electronic analog signals uses 4-20 mA equivalents to 0-100% of the measured variables.• This signal is usually transmitted over wires less than 200 ohm resistance.

PNEUMATIC• Varying from 3-15 psig or 20-100kPa• The minimum value or zero value ofthe signal to be transmittedcorresponds to 3 psig, and themaximum value of the signal to betransmitted corresponds to 15 psig.• The signal is usually transmitted in¼’” O.D. tubing.• Existing pneumatic signals are oftenconverted to electric, for computerinputs, using P/I (Pressure-to-current)converters that give 4-20 mAcorresponding to the 3-15 psig.

Analog Data Format

• An analog variable represents a variable

• Represent 0-100% of the possible variation in the variable

• For example:

A pneumatic analog signal varying from 3-15psig may represent the liquid level that varies from 0-2m in height

18

Data Representation

19

DIGITAL DATA FORMAT

• Most new sensor called smart transmitters incorporate microprocessors.

• Therefore, the digital signals can readily used to communicate with a computer.

• Electric wires or optic fibers can be used as transmission media.

• A digital signal may be transmitted using parallel or serial transmission.

Digital Data Format

Parallel

• Digital value may be transmitted instantaneously from the origin to the point of use.

• Parallel transmission moves data quickly but at the expense of many wires.

Serial

• Serial transmission uses only one twisted pair of wires between the point of origin to the point of use.

• It takes longer to send the information than for parallel transmission, but the initial cost of the equipment is less.

20

Data Conversion

• Analog-to-digital converter (ADC) is a device to convert analog output such as voltage into a digital representation.

• In a control system, the sensor often produces an analog output such as voltage.

• Digital-to-analog converter (DAC) convert a digital signal into an analog output

• DAC used to convert the control output of the computer into a suitable form for the final control element

21

ADC

(Digital)

b3

(Analog) b2

b1

b0

Decimal-binary-hex encoding

22

Example

• Base 10 system, N10

• Binary system, 1100112 , n=5

23

01235

4

• Find the base 10 equivalent of the binary whole number 001001112

24

Analog Control

25

• Analog control exists when all variables in the system are analog representation of another variable.

• Q is an analog of the excitation voltage, VQ. Thus, heat can be varied continuously.

• Notice that every signal is analog: VT is analog of T; the error E is an analog of the difference between the reference, Vref and temperature voltage, VT. The reference voltage is simply the voltage that would result from measurement of the specified reference temperature, Tref.

26

Ve = Vref -VT

Vref

VT

VQ

Measurement

(Thermocouple)

Final control

element

(valve)

Controller

Process (Temperature

in the tank)

Block diagram of an analog temperature

control system

Analog Control

Digital Control

• True digital control involves the use of a computer in modern application.

• There are two approaches to using computers for control.

1. Supervisory Control

2. Direct Digital Control (DDC)

i. Smart Sensor

ii. Networked Control Systems

27

Supervisory Control

• Supervisory control emerged as an intermediate step wherein the computer was used to monitor the operation of analog control loops and to determine the appropriate set points.

• A single computer could monitor many control loops and use appropriate software to optimize the set point for the best overall plant operation.

• If the computer failed, the analog loops kept the process running using the last set point until the computer come back on line.

28

Supervisory Control

29

In supervisory control, the computer monitors

measurements and update setpoints, but the loops are still

analog in nature.

Measurement

(Thermocouple)

Ve = Vref -VT

Vref

VT

VQ Final control

element

(valve)

Process (Temperature

in the tank)

Controller

Computer

DAC ADC

Direct Digital Control (DDC)

• As computers have become more reliable and miniaturized, they have taken over the controller function.

• Thus, the analog processing loop is discarded.

• Computer system fully controlled the process operation.

• The operations of the controller have been replaced by software in the computer.

• The ADC and DAC provide interface with the process measurement and control action.

30

Direct Digital Control (DDC)

31

The operation of the controller (error

detection and controller action) have been

replaced by software in the computer.

NQ

VT

VQ

Measurement

(Thermocouple)

Final control

element

(valve)

Process (Temperature

in the tank)Computer

DAC

ADC

NT

Smart Sensor

• Smart sensor ~ integration of a microprocessor based controller computer directly into the sensor assembly.

• Using modern integrated circuit technology, the sensor, signal conditioning, ADC and computer controller are all contained within the sensor housing.

• In one form, the unit also contains a DAC with a 4-20 mA output to be fed to the final control element.

• The set point is programmed by connecting another computer to the unit using a serial interface line.

• The most current technology is to interface the smart sensor to a local area network, which allows the sensors to be connected to other computers and to the final control element over a common digital serial line.

32

Networked Control Systems

• When a plant uses DDC, then it becomes possible to place the computer based controller directly at the site of the plant where the control is needed.

• All the DDC units are placed on a local area network (LAN).

• LAN commonly provides communications as a serial stream of digital data over a variety of carriers such as wires and fiber optics.

33

Networked Control Systems

34