pac unit v _dcs and scada

7/29/2019 Pac Unit v _dcs and Scada

1/10

Distributed Control Systems (DCS) and

Supervisory Control and Data Acquisition (SCADA) Systems

Control systems monitor and control physical processes

SCADA: Supervisory Control and Data Acquisition (WAN)

Oil Pipelines, Trains, Electric Transmission DCS: Distributed Control System (LAN)

Manufacturing, Power Generation, Refinery

Automation System StructureAlthough applications differ widely, there is little difference in the overall architecture of

their control systems. But the biggest difference is the amount of application know-how

embedded in the control system.


2/10

Distributed Control Systems

The DCS was introduced in 1975. Both Honeywell and Japanese electrical engineeringfirm Yokogawa introduced their own independently produced DCSs. Digital

communication between distributed controllers, workstations and other computing

elements (peer to peer access) was one of the primary advantages of the DCS. In the1980s, users began to look at DCSs as more than just basic process control. The 1980s

also witnessed the first PLCs integrated into the DCS infrastructure.

1n 1990s, the traditional DCS suppliers introduced new generation DCS System basedon the latest Communication and IEC Standards, which resulting in a trend of combining

the traditional concepts/functionalities for PLC and DCS into a one for all solution

named "Process Automation System".

A DCS is a .

Collection of hardware and instrumentation necessary for implementing controlsystems

Provide the infrastructure (platform) for implementing advanced control

algorithms

A distributed control system (DCS) refers to a control system usually of amanufacturing system, process or any kind of dynamic system, in which the controller

elements are not central in location (like the brain) but are distributed throughout the

system with each component sub-system controlled by one or more controllers. The entiresystem of controllers is connected by networks for communication and monitoring.

The controllers have extensive computational capabilities and, in addition to proportional,integral, and derivative (PID) control, can generally perform logic and sequential control.

DCSs may employ one or several workstations and can be configured at the workstationor by an off-line personal computer. Local communication is handled by a controlnetwork with transmission over twisted pair, coaxial, or fiber optic cable. A server

and/or applications processor may be included in the system for extra computational, data

collection, and reporting capability.

Advantages of DCS:

Digital computers are more flexible because they are programmable and no

limitation to the complexity of the computations it can carry out. Digital systems are more precise.

Digital system cost less to install and maintain Digital data in electronic files can be printed out, displayed on color terminals,

stored in highly compressed form.

Digital Implementation: Transmission: Digital signals are far less sensitive to noise.

Calculation: The computational devices are digital computers. Access a large amount of current information from the data highway. Monitoring trends of past process conditions.

Readily install new on-line measurements together with local computers.


3/10

Alternate quickly among standard control strategies and readjust controller

parameters in software. A sight full engineer can use the flexibility of the framework to implement his

latest controller design ideas on the host computer.

Digital DCS systems are more flexible. Control algorithms can be changed and

control configuration can be modified without having rewiring the system.

Application:

DCS is a very broad term used in a variety of industries, to monitor and controldistributed equipment. Electrical power grids and electrical generation plants Environmental control systems Traffic signals

Radio signals Water management systems Oil refining plants

Chemical plants

Pharmaceutical manufacturing Sensor networks

Dry cargo and bulk oil carrier ships

Modes of computer Control:

1. Manual2. Automatic

PID with local set point

3. Supervisory

PID with remote set point (supervisory)4. Advanced

FC

signals from digital

computer

Local PID

controller

Supervisory Control mode

Direct digital Control mode

valve setting

from computer

Flow measurement

to computer


4/10

DCS network:

Elements:

A DCS typically uses custom designed processors as controllers and uses both

proprietary interconnections and communications protocol for communication. Input and output modules form component parts of the DCS. The processor

receives information from input modules and sends information to output modules.

The input modules receive information from input instruments in the process

(field) and transmit instructions to the output instruments in the field.

Local Control Unit: This unit can handle 8 to 16 individual PID loops. Data Acquisition Unit: Digital (discrete) and analog I/O can be handle. Batch Sequencing Unit: This unit controls a timing counters, arbitrary function

generators, and internal logic. Local Display: This device provides analog display stations, and video display

for readout. Bulk Memory Unit: This unit is used to store and recall process data. General Purpose Computer : This unit is programmed by a customer or third

party to perform optimization, advance control, expert system, etc Central Operator Display: This unit typically contain several consoles for

operator communication with the system, and multiple video color graphics

display units Data Highway : A serial digital data transmission link connecting all other

components in the system. It allow for redundant data highway to reduce the risk

of data loss Local area Network(LAN)

Operator

Control

Panel

Main

Control

Computer

Operator

Control

Panel

Archival

Data

Storage

Supervisory

(host)

Computer

PROCESS

Local

Computer

Local

Computer

Local

Computer

Local Display Local Display

Data highwayTo other Processes To other Processes

Local data acquisition and

control computers


5/10

DCS for a Power Plant


6/10

Data Quantity & Quality and Hierarchical Level

Higher Level:

When ascending the control hierarchy, data are reduced:higher level data are created (e.g. summary information)

Processing and decisions becomes more complicated (requires using models).

Timing requirements are slackened. Historical data are stored

SCADA Level:Presentation of complex data to the human operator,

aid to decisions (expert system) and maintenance.Requires a knowledge database in addition to the plant's database

Lower LevelsLowest levels (closest to the plant) are most demanding in response time.

Quantity of raw data is very large.

Processing is trivial (was formerly realized in hardware).These levels are today under computer control,

except in emergency situations, for maintenance or commissioning.

The control aspect may include controls to a well pump to increase or decrease output, or

shut down altogether. Pipeline controls may include changing routing, increasing or

reducing the flow of the liquids or gases, etc. More computer power is engineered into

the latest SCADA equipment. More data must be moved to meet the informationdemands of more powerful SCADA systems. And these systems must operate over new

types of networks, including frame relay, Ethernet, and IP.


7/10

Supervisory Control and Data Acquisition (SCADA)

Supervisory Control and Data Acquisition (SCADA) refers to the process of gatheringinformation, often in real time, from remote locations. The data comes from oil wells,

pipelines, electric power grids, traffic signals, manufacturing plants, etc.

This information is used to analyze and control the performance of these systems. For

example, well information might include the volume of liquid pumped over a period oftime and the amount of power used. Pipeline information includes the volume of flow of

liquids or gases over a period of time

Data base


8/10

IO Devices

SCADA Screen


9/10

Data Log and Trend

Report


10/10

pac unit v _dcs and scada

Documents