PLCs have been around for many years, and are indispensable in industry where they are used in dedicated small machine and process control. They evolved as industries sought economic ways to automate their production lines and machines. PLCs operate by monitoring input signals from sources such as push-buttons, sensors and limit switches. When changes are detected in signals, the controller system reacts through user programmed internal logic to produce output signals. These signals operate the external loads of the control system, such as relays, motors, indicator lights and alarms.
This type of control system eliminates much of the wiring that was necessary with the conventional relay-based system, as well as the timers, counters and other discrete devices. Instead, the programming logic provides the "wiring network" which can be changed as required by simply reprogramming the PLC. Thus the automated processes of a production line or machine can be controlled and modified at will, for a highly economic adaptability to changing manufacturing circumstances.
A typical programmable controller has three basic components - an input/output section, a central processing unit (CPU), and a programming device.
The smallest PLC available has 8 inputs and 6 outputs with 600 programming steps. The larger PLC has 16 input points and 12 output points with 1194 programme statements. Each of these can be expanded by using add on units.
Eight basic programming steps are required:
A PLC would be ideal in Meccano to automate the movements of a ships coal loader, or a seaside lift, or a sequence of movements on a production line. In industry, they are used in machine tools, packaging machines, printing machines and textile machines to name only a few.
A small PLC costs no more than the total cost of ten relays and 3 timers. It accepts NPN and PNP transistor inputs, and has relay and transistor outputs. It is easily programmed on a small hand-held programme loader, but can also be linked to a personal computer. Application software is available for programming and monitoring on an IBM or compatible personal computer. PLCs have a built in power supply and many units can be linked together. Technicians who have little or no experience with programmable controllers, find them easy to use.
Sensors are needed for inspection, detection and measurement. Sensors for inspection might be for used for reflection, colour, and transparency. Detection of objects over long and short distances can be made. Measurement can be made for size, length, angle and pressure.
Most sensors are of the photo-electric switch type, but others are readily available such as ultrasonic, pyro-sensors, inductive and capacitative proximity switches, red or green beam sensors, rotary encoders, pressure sensors and tilt sensors. All require amplifiers and power supplies.
© Copyright Michael Adler 1996