First Programmable Logic Controllers

“General Motors” is among the first who recognized a need to replace the system’s “wired” control board. Increased competition forced auto-makers to improve production quality and productivity.

Flexibility and fast and easy change of automated lines of production became crucial! General Motors’ idea was to use for system logic one of the microcomputers (these microcomputers were as far as their strength beneath today’s eight-bit microcontrollers) instead of wired relays.

Computer could take place of huge, expensive, inflexible wired control boards. If changes were needed in system logic or in order of operations, program in a microcomputer could be changed instead of rewiring of relays. Imagine only what elimination of the entire period needed for changes in wiring meant then. Today, such thinking is but common, then it was revolutionary! Everything was well thought out, but then a new problem came up of how to make electricians accept and use a new device. Systems are often quite complex and require complex programming.

It was out of question to ask electricians to learn and use computer language in addition to other job duties. General Motors Hidromatic Division of this big company recognized a need and wrote out project criteria for first programmable logic controller ( there were companies which sold instruments that performed industrial control, but those were simple sequential controllers û not PLC controllers as we know them today).

Specifications required that a new device be based on electronic instead of mechanical parts, to have flexibility of a computer, to function in industrial environment (vibrations, heat, dust, etc.) and have a capability of being reprogrammed and used for other tasks.

The last criteria was also the most important, and a new device had to be programmed easily and maintained by electricians and technicians. When the specification was done, General Motors looked for interested companies, and encouraged them to develop a device that would meet the specifications for this project. “Gould Modicon” developed a first device which met these specifications.

The key to success with a new device was that for its programming you didn’t have to learn a new programming language. It was programmed so that same language ûa ladder diagram, already known to technicians was used. Electricians and technicians could very easily understand these new devices because the logic looked similar to old logic that they were used to working with. Thus they didn’t have to learn a new programming language which (obviously) proved to be a good move.

PLC controllers were initially called PC controllers (programmable controllers). This caused a small confusion when Personal Computers appeared. To avoid confusion, a designation PC was left to computers, and programmable controllers became programmable logic controllers.

First PLC controllers were simple devices. They connected inputs such as switches, digital sensors, etc., and based on internal logic they turned output devices on or off. When they first came up, they were not quite suitable for complicated controls such as temperature, position, pressure, etc. However, throughout years, makers of PLC controllers added numerous features and improvements.

Today’s PLC controller can handle highly complex tasks such as position control, various regulations and other complex applications. The speed of work and easiness of programming were also improved. Also, modules for special purposes were developed, like communication modules for connecting several PLC controllers to the net. Today it is difficult to imagine a task that could not be handled by a PLC.