In the field of automation control, PLC is an important control device. At present, there are over 200 manufacturers in the world producing over 300 varieties of PLC products, which are applied in industries such as automobiles (23%), grain processing (16.4%), chemicals/pharmaceuticals (14.6%), metals/mines (11.5%), pulp/paper (11.3%), etc. In order to make it more convenient for beginners to understand PLC, this article provides an introduction to the development, basic structure, configuration, application, and other basic knowledge of PLC, in the hope of being helpful to netizens.
In industrial production processes, a large number of switch values are sequentially controlled, which act according to logical conditions and control interlocking protection actions according to logical relationships, as well as collect a large amount of discrete data. Traditionally, these functions are achieved through pneumatic or electrical control systems. In 1968, General Motors of the United States proposed to replace electrical control devices. The following year, Digital Corporation of the United States developed a control device based on integrated circuits and electronic technology, which was first applied to electrical control through programmed means. This was the first generation of programmable controllers, known as Programmable Controllers (PCs).
After the development of personal computers (PCs), in order to facilitate and reflect the functional characteristics of programmable controllers, programmable logic controllers were named Programmable Logic Controller (PLC). Nowadays, PLC is still often referred to as PC.
There are many definitions of PLC. The definition of PLC by the International Electrotechnical Commission (IEC) is that a programmable controller is an electronic system operated by digital operations, designed specifically for use in industrial environments. It uses programmable memory to store instructions for performing logical operations, sequential control, timing, counting, and arithmetic operations internally, and controls various types of mechanical or production processes through digital and analog inputs and outputs. Programmable controllers and related equipment should be designed according to the principle of easy integration with industrial control systems and easy expansion of their functions.
From the 1980s to the mid-1990s, PLC was the fastest growing period, with an annual growth rate of 30-40%. During this period, PLC’s ability to handle analog signals, digital operations, human-machine interface, and network capabilities were significantly improved. PLC gradually entered the field of process control and replaced the dominant DCS system in some applications.
PLC has the characteristics of strong universality, convenient use, wide adaptability, high reliability, strong anti-interference ability, and simple programming. The position of PLC in industrial automation control, especially in sequential control, cannot be replaced in the foreseeable future.
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