Practical Use & Understanding of Foundation FieldBus for Engineers & Technicians
Practical Distributed Control Systems (DCS) for Engineers and Technicians
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Transcript of Practical Distributed Control Systems (DCS) for Engineers and Technicians
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Practical Distributed Control Systems (DCS) for Engineers and technicians
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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
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Use of computer for measurement and control (in real-time) application were conceived as early as 1950
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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.
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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
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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
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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
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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
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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
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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
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Display unit (CRT)
Keyboard
Input unit
Printing unit
Control Panel/desks, mimic board/panel
Recorders
HMI components
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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.
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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
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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
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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
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The I/O devices of process control computers are divided into three types:
Operator IO
Process IO
Computer IO
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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
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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
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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
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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
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Analog interfaces
Analog-to-digital converter (ADC)
Digital -to- analog converter (DAC)
Multiplexing devices
MODEM
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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
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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)
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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
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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
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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
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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
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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
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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.
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