UDG1606 Process Instrumentation & ControlFaculty of Chemical & Natural Resources Engineering

COMPUTER IN PROCESS CONTROL

CHAPTER 10

TOPIC COVERED

• Data acquisition system

• Distributed control system

Data Acquisition System, DAS

• Data acquisition is the process of sampling of real world physical conditions and conversion of the resulting samples into digital numeric values that can be manipulated by a computer.

• The purpose of any data acquisition system is to gather useful measurement data for characterization, monitoring, or control.

Data Acquisition System, DAS

• The data acquisition systems include:

Devices, boards and equipment which gather data from external instruments and transforms the data into an analog or digital (A/D) form for potential processing or signal conditioning.

Components of DAS

Components Description

Data acquisition hardware The main function of this hardware is to convert analog signals to digital signals, and to convert digital signals to analog signals.

Sensors and actuators (transducers)

Sensors and actuators can both be transducers. A transducer is a device that converts input energy of one form into output energy of another form

Signal conditioning hardware Sensor signals are often incompatible with data acquisition hardware. To overcome this incompatibility, the signal must be conditioned. For example, you might need to condition an input signal by amplifying it or by removing unwanted frequency components.

Computer The computer provides a processor, a system clock, a bus to transfer data, and memory and disk space to store data.

Software Data acquisition software allows you to exchange information between the computer and the hardware.

Data Acquisition System, DAS

• Signals are input to a sensor, conditioned, converted into bits that a computer can read,

and analyzed to extract meaningful information.

•Data from a computer is converted into an analog signal and output to an actuator.

Distributed Control System, DCS

• Development of modern control system

1. Manual control

2. Pneumatic control

3. Electronic control

4. DCS

Distributed Control System - DCS

Distributed Control System, DCS

• Distributed control system (DCS)

– …..is an interactive network of instrumentation , communication and control devices connected by common communication link

– The entire system of controllers is connected by networks for

communication and monitoring.

Working Principle of DCS

Local controller → Interconnecting data link →

Coordinating controller → Information display unit

Local controller:

-mounted in the field near a controlled device.

Interconnecting data link:

-local controllers are connected by data link which

consist of coaxial cables or optical fibre that capable to carry high speed data

Coordinating controller:

-operates at a higher level than a local controller, in a coordinating and supervisory

role

-It has communication via data links with local controllers, with displays and

other human machine interface (HMI)

Information display unit:

consisting of a set of visual display unit, all visible from one location,

Working Principle of DCS

Advantages of DCS

• DCS enabled control modules to be interconnected and grouped, lowered maintenance costs, improved system reliability, and made it easier to configurethe process concept and expand the system

Progressive Trends in DCS Technology

• The fieldbus concept– In large plant, the cabling costs alone run into several thousands of

dollar– Based upon smart valves, smart sensors and controllers installed in the

field– Uses data highway to replace wires from sensor to DCS and to the

control valves– The fieldbus idea (the external bus that connects the system interface

module to the field mounted equipment for example, controller, transmitter and final control element devices) which operates on the same principle as data highway basically requires only two conductors between the system and multidrop junction.

http://kernow.curtin.edu.au/www/Fieldbus/fieldbus.htm

➢ Less expensive installations and better reliability

➢ Can mix different sources (vendors) of sensors, transmitters,

and control valves

➢ Now commercially available and should begin to replace DCS