The programmable logic controller or PLC is a type of computer which has been adapted for the control of manufacturing processes. Examples of their use include assembly lines and robotic devices. Its purpose is to monitor an input or inputs, and make decisions based on a custom program to control the state of an output device.

An example of an input could be some form of sensor or encoder which when triggered sends an electrical signal or message. An output could be a motor being driven by an AC drive at the desired speed to a certain point, similar to how a mechanical cam would work.

PLCs are modular devices which are extremely useful in any industrial application because you can mix the inputs and outputs to meet the users’ needs. This makes them a very useful when a range of sequential steps need to be made on different machines.

To make these steps, the PLC operates in a loop. This is based on three stages, which are:

• Input scan – This is like a snapshot, where the PLC looks at each input and determines whether they are ON or OFF
• Program scan – The PLC executes the written program logic, one instruction at a time
• Output scan – A message is sent from the PLC, based on the programmed logic, which energises or de-energises all output devices
  
  

There are a number of ways to write the program logic of a PLC, these range from modern day programming languages like C++ or C+ to industrial specific ones like structured text, function block diagrams and sequential function charts. Each has their own benefits, depending on the complexity and what the user wants to control. However, the predecessor of all these languages is the ladder diagram. A language which simulated the opening and closing of mechanical switches or relays. And here begins the story of the PLC.

As the industrial revolution advanced, so did the need for greater control of machinery. Relays emerged as a result. These electromagnetic switches allow a user to apply a small amount of current to control a larger appliance. This enabled engineers to manage complex systems without using mechanical switches. Although game changing, there were issues with this technology.

Relays connect together as part of a physical system. Once a system is built, there is no easy way to expand the process. This makes it difficult to improve process inefficiencies or add-in product variation. Secondly, troubleshooting is time consuming, as there’s no device feedback. To find a fault each relay and cable would need to be checked individually. Often these faults would be caused by environmental factors, such as dirt on the contactor of a relay. Tracing unlabelled cables can also cause severe problems, and in some cases may require rewiring the entire system. As you can imagine, excessive downtime incurs a high cost.

The growing demand for integrated control meant that relay systems required a rethink. In 1968, Bill Stone from the Hydromatic Division of General Motors issued a brief which outlined the difficulties he was facing in his factory. GM engineers then drafted a design criteria for a ‘standard machine controller’, which was issued to four companies, they were:

• Allen-Bradley, by way of Michigan-based Information Instruments, Inc.
• Digital Equipment Corporation (DEC)
• Century Detroit
• Bedford Associates

It was not until 1969 when Bedford Associates met the needs of the brief. This was the Modicon 084 which solved dominant algorithms associated with ladder logic.

In 1970 Allen-Bradley released the Bulletin 1774 PLC. This was their second attempt at producing a motion controller, and the first time the term PLC was used. Later, Rockwell Automation acquired Allen-Bradley although the trading name remains intact.

While Allen-Bradley were producing the Bulletin 1774, sales continued to grow for Bedford Associates. And in 1975 they built on their initial success with the release of the Modicon 184. It was designed by engineer Michael Greenberg and marketer, Lee Rousseau. This was the first PLC designed to meet market needs, and was hugely popular. The rapid growth of this device eventually led to demise of the business, dissolving in 1977. Modicon was sold to Gould Electronics, and in 1997 again to Schneider Electronics.

Even though PLCs were a vast improvement over relays, there were still limitations. Critically, they were designed for specific applications in the automotive industry. Engineers found it difficult to accept that one small box could replace 50 cabinets of relays and wiring. Additionally, clean rooms were needed to house the PLC, along with air conditioning and filters. Finally, this was an industry in its infancy, and so PLCs were prone to malfunction.

Things have come a long way since those early days, with PLCs being common in most factories. Huge innovations have been made, and many new companies have appeared to meet the needs of the market. The term PLC is now used to describe any equipment that uses programmable logic control, this includes IPCs and smart variable speed drives.

A lot of thanks should go to the first innovators of PLCs. Those who had the foresight to enable the advancement of modern day industry.