PLC generation and defined before the birth PLC, relay control system has been widely used in industrial production, plays an irreplaceable role.
With the gradual expansion of production scale, the relay control system has become increasingly difficult to adapt to the requirements of modern industrial production.
The relay control system is usually designed for a fixed action sequence or production process. Its control functions are also limited to some simple controls such as logic control, timing, and counting.
Once the action sequence or production process changes, it must be repeated. Design, wiring, assembly and commissioning cause serious waste of time and capital.
The relay control system has large volume,high power consumption, poor reliability, short life, short running speed, and poor adaptability.
In order to change this situation, in 1968, General Motors Corporation (GM), the largest automobile manufacturer in the United States, proposed to develop a new type of industry in order to adapt to the needs of constantly updating car models and to have an advantage in the highly competitive automobile industry The control device replaces the relay control device.
To this end, 10 technical requirements for public bidding have been drawn up, namely:
- Simple programming, which can be modified on site
- Convenient maintenance, preferably plug-in type
- Reliability is higher than relay Control cabinet
- the volume is smaller than the relay control cabinet
- the data can be directly sent to the management computer
- the cost can compete with the relay control cabinet
- the input can be AC 115V
- the output can be AC 115V, 2A or more, can Drive solenoid valve, etc.
- When expanding, the original system needs to be changed only a small amount.
- The user program memory capacity can be expanded to at least 4K.
PLC History is Interesting.Let’s Continue
According to the technical requirements of the tender, the second year, the United States Digital Equipment Corporation (DEC) developed the world ’s first PLC. And successfully tried it on GM’s automatic assembly line.
This new type of industrial control device, with its series of advantages such as small size, good variability, high reliability, long service life, simple and easy to understand, and easy operation and maintenance, was quickly promoted and applied in many industries in the United States.
Received great attention from many countries in the world. In 1971, Japan introduced this new technology from the United States and quickly developed their first PLC. In 1973, Western European countries also developed their first PLC
During this period, although PLC adopted the design idea of a computer, in fact PLC can only complete sequential control and only has simple functions such as logic operations, so people call it Programmable Logic Controller (Programmable Logic Controller) for short PLC.
From the late 1970s to the early 1980s, microprocessors became increasingly mature, which greatly increased the processing speed of PLCs and added many functions. In terms of software, in addition to maintaining the original functions of logic calculation, timing, and counting, it also adds functions such as arithmetic operation, data processing, network communication, and self-diagnosis.
In terms of hardware, in addition to maintaining the original switch module, it also adds analog modules, remote I / O modules, and various special function modules, and expands the memory capacity, and also provides a certain number of data registers. To this end, the American Electrical Manufacturing
Association officially named the programmable logic controller as Programmable Controller (PC). But because PC is easy to be confused with personal computer PC (Personal Computer), people are still used to using PLC as the abbreviation of programmable logic controller.
In 1985, the International Electrotechnical Commission (IEC) defined PLC as follows:
Programmable controller is a digital computing operation electronic system, designed for application in industrial environments. It uses a programmable memory to store instructions for performing logical operations, sequence control, timing, counting, and arithmetic operations, and controls various types of machinery through digital and analog inputs and outputs. Or production process.
The programmable controller and its related peripheral equipment should be designed according to the principle that it is easy to form a whole with the industrial control system and to expand its functions.
It can be seen from this definition that PLC is a kind of industrial control computer whose control function is determined by a “program stored in advance”.
PLC is based on the mature and effective relay control concepts and design ideas, using the constantly developing new technologies and new electronic devices, and has gradually formed a series of products with characteristics.
It is a special electronic computer for digital operation. It is to store control programs such as logic operations, sequence control, timing and counting, and arithmetic operations into a memory in the form of a series of instructions, and then according to the stored control content, through analog and digital input and output components, the production equipment and A device that controls the production process.