PLCs are increasingly being used in industry to control plant and to monitor all site processes. This is because the PLC (Programmable Logic Controller) provide numerous advantages over conventional relay logic which they replace. To change the control philosophy of a piece of plant, which is controlled by conventional relays, timers and trip-amps, could involve lengthy and expensive wiring modifications. With a PLC this can be done with a few keystrokes on a programming computer. This flexibility means that the control of any piece of plant can be changed as often as required. In fact, changing the control of piece of plant can be part of the control program, which could be operated when a tank level change or time of year dictates.

The power of PLCs compared to relay logic is best illustrated by space that a PLC requires compared to convential relay logic. An average PLC, with the capability to control up to and over one hundred pieces of plant equipment would require the space of about twenty five 4pole relays. This is without comparing the huge cost of installing, wiring and building a panel big enough to house all the equivilent relays and timers.

The main langauge of PLCs is ladder logic. Ladder logic is essentially a panel wiring diagram which includes, relays, timers and contacts. Most PLCs allow the software to be divided into sub routines to allow easy viewing and understanding of the software. Most PLCs are 16bit and have huge memories, this means powerful with plenty of space for software expansion. With this mind it is easy to see how plant control has become more complex, to take into account factors which would have been to expensive and would have involved countless hardware components.

PLCs enable plants to run more efficiently with no or little operator intervention. Most PLCs end up removing the need for operators completely. The PLCs can be setup to raise an alarm at a control room on site or raise an alarm via telephone or radio. When an alarm is raised the PLC will automatically perform the necessary tasks to protect the process, like starting standby plant, re-routing, close down lines or shutdown the process completely.

When HMIs and SCADAs are included into the equation it is easy to see why relay logic is fast becoming a thing of the past. SCADA (Supervisory Control And Data Aquasition) and HMI (Human Machine Interface) allow operators to view what the PLC is doing and to view plant status. SCADA and HMI can also be used to view alarms, data, trends, historical logs and to control recipes.

PLCs (Programmable logic Controllers)
HMIs (Human Machine Interfaces)
SCADAs (Supervisory Control And Data Aquasition)
Network Solutions
Programmed Languages