The English name for programmable logic controller is Programmable Logic Controller, which was abbreviated as PC in the 1970s and 1980s. Due to the development of personal computers, also known as PCs, in the 1990s; In addition, the concept of programmable logic covers a wide range, so AB Company in the United States first named programmable logic controllers (PLCs). For convenience, PLC is still referred to as programmable logic controller. Some people refer to a system composed of programmable controllers as a PCS programmable control system, emphasizing that the manufacturer of programmable controllers has already provided people with a complete system.
The Development History of PLC
In 1968, General Motors of the United States proposed a requirement to replace relay control devices. The following year, Digital Corporation of the United States developed the first generation of programmable controllers, which met the requirements of GM’s assembly line. With the development of integrated circuit technology and computer technology, there are already fifth generation PLC products.
In the manufacturing industry characterized by changing geometric shape and mechanical properties, as well as in the process industry characterized by converting raw materials into products through physical and chemical changes, in addition to feedback control mainly based on continuous variables, there are a large number of open-loop sequential control mainly based on switch variables in the manufacturing industry, which operates in sequence according to logical conditions; In addition, there are controls for chain protection actions that are independent of sequence and timing and follow logical relationships; And a large number of state variables such as switch quantity, pulse quantity, timing, counter, analog quantity exceeding limit alarm are mainly collected and monitored for discrete quantity data. Due to these control and monitoring requirements, PLC has developed into a product that mainly replaces relay circuits and performs sequential control. Over the years of production practice, a tripartite balance has gradually emerged between PLC, DCS, and IPC, as shown in Table 1. Other single circuit intelligent regulators also account for a certain percentage in the market.
The period from the 1980s to the mid-1990s was the fastest growing period for PLC, with an annual growth rate of 30-40%. Due to the advancement of simulation capabilities and network functions of PLC machines, they have squeezed out a portion of the DCS market (process control) and gradually monopolized industries such as wastewater treatment. However, with the emergence of industrial PCs (IPC), especially the development of fieldbus technology, IPC and FCS have also squeezed out a portion of the PLC market. Therefore, the growth rate of PLC is generally slowing down. There are over 200 manufacturers worldwide producing over 300 varieties of PLC products, mainly used in industries such as automotive (23%), grain processing (16.4%), chemical/pharmaceutical (14.6%), metal/mining (11.5%), pulp/paper (11.3%), etc.

Market situation of PLC
There are about 30 domestic PLC production plants, but they have not formed a significant production capacity and branded products. Some of them are produced through imitation, incoming assembly, or “OEM” methods. Therefore, it can be said that PLC has not formed a manufacturing industry in China. As a product with no cutting-edge technical difficulties in terms of principles, technology, and processes, with hard work, it can form a manufacturing industry.
In terms of PLC applications, China is very active, with approximately 100000 new sets of PLC products invested each year and an annual sales revenue of 3 billion yuan. The industry is also widely used. However, compared to other countries, there is still a need to increase investment in the application of mechanical processing and production lines.
The market potential of PLC is enormous, not only in China, but also in industrialized Japan. Surveys have shown that the proportion of electromechanical integrated products matched with PLC accounts for 42%, and 24% are controlled by relays and contactors. So, there are still many occasions where PLC needs to be applied, and this is even more so in China.
From the perspective of technological innovation, China’s large and medium-sized enterprises still need to vigorously develop CIMS (Computer Integrated Manufacturing System), and form FMS flexible manufacturing systems in mechanical manufacturing plants. PLC is the foundation, so the PLC market is broad.
PLC has the advantages of stability, reliability, low price, complete functions, flexible and convenient application, and easy operation and maintenance. This is the fundamental reason why it can sustainably occupy the market. Below, we will focus on several issues and study its development trends.