Practical Distributed Control Systems (DCS) for Engineers and Technicians

Practical Distributed Control Systems
(DCS) for Engineers and technicians
www.idc-online.com/slideshare Technology TTrraaiinniinngg tthhaatt WWoorrkkss

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

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

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.

Set-
Point
+v
Digital computer used for process control; use of ADC and DAC for
computer to Input and output matching is necessary
Computer System
Controller Eq.
(Digital
Algorithm)
Digital -to -
Analog
Converter
(DAC)
e u u(t)
Analog -to -
Digital
Converter
(ADC)
Final
Control
Element
(Control
Valve)
Process
u(t)
y(t)
Controlled
Variable
Sensor
(Measuring Element)
Measured
Variable
_
(a) Schematic Diagram
Digital Computer
Set-
Point
+
v(t)
Digital
Algorithm
e u(t)
Hold Device Process
u(t)
y(t)
_
(b) Block Diagram

Control of hot air blower – system interfaced with digital
computer for control purpose
COMPUTER
Digital
Input ADC DAC Digital Output
Air Inlet
Opened
Amplifier
Heater
Control
Direction On/Off
Reversible
Motor
Control
Air Inlet
Position
Fully
Close
d
Fully
Open
Variabl
e Air
Inlet
Air Flow
Air Inlet
Closed
Air Inlet
Position
Sensor

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

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

Task listing of computer based control
system
LELEV 4; Management Level
LEVEL 3; Plant Level
LEVEL 2; Supervisory Level
LEVEL 1; Control Level
LEVEL 0; Field Level

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

HMI components
Display unit (CRT)
Keyboard
Input unit
Printing unit
Control Panel/desks, mimic
board/panel
Recorders

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.

Storage
Used 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

The concept of cache memory
L1 Cache; built into
chip
L2 Cache; on SRAM
memory bank
Local bus
Local bus
(RAM)
Main Memory

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

The I/O devices of process control
computers are divided into three types:
Operator IO
Process IO
Computer IO

Schematic diagram showing
interfaces of computers for process
control applications system

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

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

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

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

Analog interfaces
Analog-to-digital converter (ADC)
Digital -to- analog converter (DAC)
Multiplexing devices
MODEM

Analog-to-digital converter (ADC)
Binary Output Code
Fraction of Full
Scale
An 8-level (3-
bit) ADC
coding scheme
Normalized Analog Input Voltage
ADC Code

LSB
Digital-to-Analog converter (DAC)
An 8-bit Digital to Analog Converter
(DAC) circuit using R-2R network
Buffer
Stage
88--bbiitt DDAACC
D7 D6 D5 D4 D3 D2 D1 D0
MSB Buffer
Stage
V1V1
V2 V2
Functional diagram of
an 8-bit Digital to
Analog Converter (DAC)

Plant or Process
to be controlled
AAnnaalologg S Seennssoorsrs
AADDCC
AADDCC
Diagram shows a typical application of ADC and DAC used for
interfacing analog signals to a digital computer controlled process Plant or Process
to be controlled
Digital
Computer
Digital
Computer
AAnnaalologg A Acctutuaatotorr
AAnnaalologg A Acctutuaatotorr DDAACC
AAnnaalologg A Acctutuaatotorr
AADDCC
DDAACC
DDAACC

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 Multiplexing
Frequency Division Multiplexing
Time Division Multiplexing

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

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

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
Multiplexer
Demultiplexer
Multiplexer
Demultiplexer
T1 – Time Slice 1
T2 – Time Slice 2
Example of TDM showing three channels multiplexed / demultiplexed and
transmitted / received

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.

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Practical Distributed Control Systems (DCS) for Engineers and Technicians

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