Key Requirements for Industrial Control Computers

A computer is often used as part of a control system. The input data is typically from the operator’s commands and signals from the plant (flows, pressure, temperatures, limit switches, etc.). The output data comes in form of control actions to the plant and status displays to the operator. The instructions fed into the computer define what action is to be taken as the input data from both the plant and operator changes.

Industrial control has somewhat different requirements than other computer applications.

A conventional computer takes data, often from a keyboard, and outputs data to a screen or printer. The data being manipulated will generally be characters or numbers for example item names and quantities in a stores stock list. On the other hand, an industrial control computer is very different. Its inputs come from a vast number of devices. While some of these will be numeric (temperature, flows, pressures, etc.), the majority will be single bit, ON/OFF, digital signals representing valves, limit switches, motor contactors, etc.

There will also be a similar large amount of digital and analog output signals; a very small-sized control system may have connections to about 15-20 input and output signals; Medium-sized systems can have over 200 connections.

In general, computer control is done in real time, that is, the computer has to respond to random events as they occur.

The following are some of the requirements of industrial control computers:

• They should be designed to survive in an industrial environment.
• They should be capable of dealing with bit form digital input/output signals at the usual voltages encountered in the industry and analog input/output signals. The expansion of the I/O should be simple and straightforward.
• The programming language should be understandable by maintenance staff such as electricians who may have little to no computer training. Programming changes should be easy to perform in a constantly changing plant.
• The system should operate sufficiently fast for real-time control. This may be, for example, in practice, a response time of around 0.01 sec, but this can vary dependent on the application and the controller used.
• It should be possible to monitor the plant operation while it is running to assist in fault finding. Note that, most faults will be in external equipment such as plant mounted limit switches, actuators, and sensors, and it should be possible to observe the action of these from the control computer.

You may also read:

• Basic Features of Distributed Control Systems (DCS)
• Centralised vs. Decentralised Control Systems
• Supervisory Control and Data Acquisition in Power Systems
• Basic Features of Programmable Logic Controllers (PLCs)