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Technology Training that Works Technology Training that Works www.idc-online.com/slideshare Practical Distributed Control Systems (DCS) for Engineers and technicians

Practical Distributed Control Systems (DCS) for Engineers and Technicians

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This workshop will cover the practical applications of the modern Distributed Control System (DCS). Whilst all control systems are distributed to a certain extent today and there is a definite merging of the concepts of a DCS, Programmable Logic Controller (PLC) and SCADA and despite the rapid growth in the use of PLC’s and SCADA systems, some of the advantages of a DCS can still be said to be Integrity and Engineering time. Abnormal Situation Management and Intelligent Alarm Management is a very important DCS issue that provides significant advantages over PLC and SCADA systems. Few DCSs do justice to the process in terms of controlling for superior performance – most of them merely do the basics and leave the rest to the operators. Operators tend to operate within their comfort zone; they don’t drive the process “like Vettel drives his Renault”. If more than one adverse condition developed at the same time and the system is too basic to act protectively, the operator would probably not be able to react adequately and risk a major deviation. Not only is the process control functionality normally underdeveloped but on-line process and control system performance evaluation is rarely seen and alarm management is often badly done. Operators consequently have little feedback on their own performance and exceptional adverse conditions are often not handled as well as they should be. This workshop gives suggestions on dealing with these issues. The losses in process performance due to the inadequately developed control functionality and the operator’s utilisation of the system are invisible in the conventional plant and process performance evaluation and reporting system; that is why it is so hard to make the case for eliminating these losses. Accounting for the invisible losses due to inferior control is not a simple matter, technically and managerially; so it is rarely attempted. A few suggestions are given in dealing with this. Why are DCS generally so underutilised? Often because the vendor minimises the applications software development costs to be sure of winning the job, or because he does not know enough about the process or if it is a green-field situation, enough could not be known at commissioning time but no allowance was made to add the missing functionality during the ramp-up phase. Often the client does not have the technical skills in-house to realise the desired functionality is missing or to adequately specify the desired functionality. This workshop examines all these issues and gives suggestions in dealing with them and whilst not being by any means exhaustive provides an excellent starting point for you in working with a DCS. MORE INFORMATION: http://www.idc-online.com/content/practical-distributed-control-systems-dcs-engineers-technicians-2

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Page 1: Practical Distributed Control Systems (DCS) for Engineers and Technicians

Technology Training that WorksTechnology Training that Workswww.idc-online.com/slideshare

Practical Distributed Control Systems (DCS) for Engineers and technicians

Page 2: Practical Distributed Control Systems (DCS) for Engineers and Technicians

Technology Training that WorksTechnology Training that Workswww.idc-online.com/slideshare

Learning Objectives

In this chapter we will learn the following:

• Introduction to computer based measurement and control systems

• Role of computers in process control

• Basic components of computer based measurement and control system

• Architecture of computer based control

• Human Machine Interface (HMI)

• Hardware of computer based process control system

• Interfacing computer system with process

• Economics of computer based system for industrial application

Page 3: Practical Distributed Control Systems (DCS) for Engineers and Technicians

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Use of computer for measurement and control (in real-time) application were conceived as early as 1950

Page 4: Practical Distributed Control Systems (DCS) for Engineers and Technicians

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Digital computer application in process industry may be:

Passive or Active• Passive application involves only acquisition

of process data (data acquisition / data logging)

• Active application involves acquisition and manipulation of data and uses it for (real time) process control.

Page 5: Practical Distributed Control Systems (DCS) for Engineers and Technicians

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+v

Controller Eq.(Digital

Algorithm)

Digital -to - Analog

Converter(DAC)

Analog -to - Digital

Converter(ADC)

Set-Point

ue u(t)

Final Control Element (Control Valve)

Processu(t)

y(t)

Controlled Variable

Sensor(Measuring Element)

Measured Variable

_

Computer System

(a) Schematic Diagram

+v(t)

Digital Algorithm

Set-Point

e u(t)Hold Device Process

u(t)

y(t)

_

Digital Computer

(b) Block Diagram

Digital computer used for process control; use of ADC and DAC for

computer to Input and output matching is necessary

Page 6: Practical Distributed Control Systems (DCS) for Engineers and Technicians

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AmplifierHeater Control

Reversible Motor

Control

Air Inlet Position

Fully Close

dFully Open

Variable Air Inlet

Air Flow

Digital Input

Digital OutputADC DAC

COMPUTER

Air Inlet Closed

Air Inlet Opened

Air Inlet Position

SensorDirection On/Off

Control of hot air blower – system interfaced with digital computer for control purpose

Page 7: Practical Distributed Control Systems (DCS) for Engineers and Technicians

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Centralized computer based process control system –

Large computer system with huge space and power consuming type magnetic core memory,

Wired-in arithmetic and logical functions (gate logics)

Expensive due to high cost of core memory and additional electronics used in the system.

Expensive communication system

Single computer system used primarily to justify high cost; popularly known as central or mainframe computer.

Had high electrical noise problems

Sudden computer stoppages led to complete stoppage of plant/process

Page 8: Practical Distributed Control Systems (DCS) for Engineers and Technicians

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Tasks of computer control system

• Monitoring large number of variables operating under a wide range of process dynamics.

• The computer based system develop large number of complex functions which work on a large number of widely scattered actuators of various

• These are based on multiple inputs to the computer as process parameters.

• In conclusion - to meet the production demands while ensuring the quality of the products and safety of the plant’s resources

Page 9: Practical Distributed Control Systems (DCS) for Engineers and Technicians

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Task listing of computer based control system

LEVEL 0; Field Level

LEVEL 1; Control Level

LEVEL 2; Supervisory Level

LEVEL 3; Plant Level

LELEV 4; Management Level

Page 10: Practical Distributed Control Systems (DCS) for Engineers and Technicians

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Human Machine Interface (HMI)

Plant mimic diagram of plant/process overview Alarm overview presenting information on the alarm status of

large areas of the plant

Multiple area displays presenting information on the control system

Loop displays giving extensive information on the details of a particular control loop of group of control loops

Page 11: Practical Distributed Control Systems (DCS) for Engineers and Technicians

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Display unit (CRT)

Keyboard

Input unit

Printing unit

Control Panel/desks, mimic board/panel

Recorders

HMI components

Page 12: Practical Distributed Control Systems (DCS) for Engineers and Technicians

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Hardware for Computer based process control system

• A general purpose digital computer with adequate hardware provisions can be used as an industrial process (real-time) computer control.

• Should have additional features like ability to communicate efficiently and effectively with plant and operating personnel

• Should also be capable of rapid execution of tasks (algorithms) for real time control actions.

Page 13: Practical Distributed Control Systems (DCS) for Engineers and Technicians

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StorageUsed to store data and instructions (programs).

Main storage or immediate access of storage (IAS) Auxiliary or secondary memory storage Cache memory

Picture of High Speed Random Access Memory (RAM) used as main Memory

Page 14: Practical Distributed Control Systems (DCS) for Engineers and Technicians

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The concept of cache memory

L1 Cache; built into chip

L2 Cache; on SRAM memory bank

Local bus

Local bus

(RAM) Main Memory

Page 15: Practical Distributed Control Systems (DCS) for Engineers and Technicians

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The input/output (I/O) interface:

Sub-system through which the CPU communicates with the outside world

Devices like Human Machine Interface (HMI) for communication between the CPU and displays and the CPU and other peripheral devices such as printers, external storage, keyboards, mouse

One of the most complex areas of a computer system because of the wide variation in the rate of data transfer and wide variety of devices which have to be connected

Page 16: Practical Distributed Control Systems (DCS) for Engineers and Technicians

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The I/O devices of process control computers are divided into three types:

Operator IO

Process IO

Computer IO

Page 17: Practical Distributed Control Systems (DCS) for Engineers and Technicians

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Schematic diagram showing interfaces of computers for process control applications system

Page 18: Practical Distributed Control Systems (DCS) for Engineers and Technicians

Bus Interface:an electronic pathway (media) in computer based system that provides a communication path for data to flow between the CPU and its memory and peripherals and amongst the CPUs connected to the computer system

Diagram showing interface (communication) through Bus Interface

Page 19: Practical Distributed Control Systems (DCS) for Engineers and Technicians

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Following are the common expansion buses which were introduced with IBM - compatible PCs

(personal computers):  S-100 bus

ISA (Industry Standard Architecture) bus

ISA-AT (Advanced Technology) bus MCA (micro-channel Architecture) bus

EISA (Extended Industry Standard Architecture) bus

NU-bus

Page 20: Practical Distributed Control Systems (DCS) for Engineers and Technicians

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Interfacing computer system with process:

• Wide variety of instruments and actuators (sensors/ transducers) are connected to process or plant for measurement and control of process parameters like temperature, flow, pressure, level, speed, etc.

• The inputs to and output from computer may be:

Analog Quantities

Digital Quantities

Pulses or pulse rates

Telemetry

Page 21: Practical Distributed Control Systems (DCS) for Engineers and Technicians

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Interface cards which have been developed and added to the computer system to connect to different measurements inputs of process parameters:

Analog interfaces

Digital interfaces

Pulse interfaces

Real-time clock

Standard (bus) interfaces

Page 22: Practical Distributed Control Systems (DCS) for Engineers and Technicians

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Analog interfaces

Analog-to-digital converter (ADC)

Digital -to- analog converter (DAC)

Multiplexing devices

MODEM

Page 23: Practical Distributed Control Systems (DCS) for Engineers and Technicians

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Analog-to-digital converter (ADC)

Bin

ary

Ou

tpu

t C

od

e

Normalized Analog Input Voltage

Fraction of Full Scale

AD

C C

od

e

An 8-level (3-bit) ADC coding scheme

Page 24: Practical Distributed Control Systems (DCS) for Engineers and Technicians

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LSB

MSB Buffer Stage

Buffer Stage

V1V1

V2

V2

8-bit DAC8-bit DAC

D7

D6

D5

D4

D3

D2

D1

D0

Digital-to-Analog converter (DAC)

An 8-bit Digital to Analog Converter (DAC) circuit using R-2R network

Functional diagram of an 8-bit Digital to

Analog Converter (DAC)

Page 25: Practical Distributed Control Systems (DCS) for Engineers and Technicians

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Diagram shows a typical application of ADC and DAC used for interfacing analog signals to a digital computer controlled process

Pla

nt

or

Pro

cess

to

be

con

tro

lled

Pla

nt

or

Pro

cess

to

be

con

tro

lled

Dig

ital

C

om

pu

ter

Dig

ital

C

om

pu

ter

Analog SensorsAnalog Sensors

Analog SensorsAnalog Sensors

Analog SensorsAnalog Sensors

Analog ActuatorAnalog Actuator

Analog ActuatorAnalog Actuator

ADCADC

ADCADC

ADCADC

DACDAC

DACDAC

DACDACAnalog ActuatorAnalog Actuator

Page 26: Practical Distributed Control Systems (DCS) for Engineers and Technicians

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Multiplexing (MUX)

• The process of sending multiple signals or streams of information on a carrier at the same time in the form of a single, complex signal

• Three different methods of multiplexing used for industrial application:

Space Division MultiplexingFrequency Division MultiplexingTime Division Multiplexing

Page 27: Practical Distributed Control Systems (DCS) for Engineers and Technicians

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Space Division Multiplexing (SDM)

• Method of providing multiple fixed bandwidth channels by multiple physical paths (i.e., pairs of wires or optical fibers).

• AS an example, an SDM may use 25-pair cable to carry the information of 25 individual sensors from the field premises to one the local control station of the plant

Page 28: Practical Distributed Control Systems (DCS) for Engineers and Technicians

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Frequency Division Multiplexing (FDM)

The higher bandwidth channel is divided into multiple individual smaller bandwidth channels. Signals on these channels are transmitted at the same time but at different carrier frequencies

FDM, with three signals to three users sharing the same bandwidth

Page 29: Practical Distributed Control Systems (DCS) for Engineers and Technicians

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Time Division Multiplexing (TDM)

• Method of putting multiple data streams in a single signal by separating the signal into many segments, each having a very short duration. Each individual data stream is reassembled at the receiving end based on the timing.

CH1

CH2

CH3

CH1

CH2

CH3

MultiplexerDemultiplexer

MultiplexerDemultiplexer

T1 – Time Slice 1T2 – Time Slice 2

Example of TDM showing three channels multiplexed / demultiplexed and transmitted / received

Page 30: Practical Distributed Control Systems (DCS) for Engineers and Technicians

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MODEM • Device that transmits data between computers, workstations and

other peripheral devices; interconnected by means of conventional communication lines supporting analog transmission

Schematic diagram of modems connecting two remotely placed computers via conventional telephone network.

Page 31: Practical Distributed Control Systems (DCS) for Engineers and Technicians

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DO YOU WANT TO KNOW MORE?

If you are interested in further training or information, please visit:

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