1. The emergence of PLC

 

 

 

Before the 1960s, automatic control devices in industrial production were basically composed of relay control systems. Its wiring was complex, maintenance and repairs were very troublesome, and it had many inherent shortcomings, as shown in the figure below. After the 1960s, in order to adapt to the needs of continuous updating of production processes, a new type of automatic control device was urgently needed in industrial production.

 

 

 

 

 

In 1969, the American Digital Equipment Corporation (DEC) developed the world's first PLC and successfully applied it on the automobile production line of the American General Motors Company (GM) for the first time, realizing the automation of industrial production.

 

 

 

With the development of electronic technology and computer technology, PLC is also constantly being improved. In recent years, PLC integrates electronic control, electrical instrumentation, and telexing, and its performance has become more superior. It has become the core equipment of automation engineering, as shown in the figure below.

 

 

 

 

 

In 1987, the International Electrotechnical Commission (IEC) promulgated the third draft of the programmable controller (PLC) standard, which defined PLC as follows.

 

 

 

A programmable controller (PLC) is an electronic system that operates on numbers and is designed for use in industrial environments. It uses programmable memory to store instructions for performing operations such as logical operations, sequence control, timing, counting, and arithmetic operations, and controls various types of machinery 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 the industrial control system and easy expansion of its functions.

 

 

 

2. Characteristics of PLC

 

 

 

Nowadays, the functions of PLC have far exceeded its definition scope, and the application fields of PLC are also constantly expanding. At present, PLC has been widely used in various industries such as steel, petroleum, chemical industry, electric power, building materials, machinery manufacturing, automobiles, textiles, transportation, environmental protection, culture and entertainment at home and abroad, as shown in the figure below.

 

 

 

 

 

In order to adapt to the use in various occasions, compared with general control devices, PLC has the following characteristics:

 

High reliability and strong anti-interference ability;

Strong versatility, variable control program, easy to use;

Strong functions and wide adaptability;

Programming is simple and easy to master;

Reduces the workload of control system design and construction;

Small size, light weight, low power consumption and easy maintenance.

 

3. Classification of PLC

 

There are many types of PLC products, and their specifications and performance are also very different. For the classification of PLC, there are usually the following methods:

  

1) Classification according to structural form

 

According to the different structural forms of PLC, PLC can be divided into two categories: integral type and modular type. Small PLCs generally adopt an integral structure, while large and medium-sized PLCs generally adopt a modular structure.

 

(1) Integrated PLC Integrated PLC integrates power supply, CPU, I/O interface and other components into a chassis, as shown in the figure below. It has the characteristics of compact structure, small size and low price.

 

 

 

 

 

(2) Modular PLC Modular PLC is made of each component of the PLC into several separate modules, such as CPU modules, I/O modules, power modules and various functional modules. Modular PLC consists of a frame (or base plate) and various modules. The modules are installed on the sockets of the frame (or base plate), as shown in the figure below.

 

 

 

 

 

(3) Stacked PLC The CPU, power supply, I/O interface, etc. of the stacked PLC are also independent modules, but they are connected by cables, and each module can be stacked layer by layer. As shown below.

 

 

 

 

2) Classification by function

 

According to the different functions of PLC, PLC can be divided into three categories: low-end, mid-range and high-end.

 

(1) Low-end PLC Low-end PLC has basic functions such as logical operations, timing, counting, shifting, self-diagnosis, and monitoring. It also has a small amount of analog input/output, arithmetic operations, data transmission and comparison, communication and other functions. It is mainly used for Single-machine control system with logic control, sequence control or a small amount of analog control.

 

(2) Mid-range PLC In addition to the functions of low-end PLC, mid-range PLC also has strong analog input/output, arithmetic operations, data transmission and comparison, digital conversion, remote I/O, subroutines, communication networking and other functions. , some can also add functions such as interrupt control and PID control, which are suitable for complex control systems.

 

(3) High-end PLC In addition to the functions of mid-range machines, high-end PLC also adds signed arithmetic operations, matrix operations, bit logic operations, square root operations and other special function functions, tabulation and table transmission functions. High-end PLC has stronger communication and networking functions and can be used for large-scale process control or to form a distributed network control system to realize factory automation.

 

3) Classification by I/O points

 

According to the number of I/O points of PLC, PLC can be divided into three categories: small, medium and large.

 

(1) Small PLC The number of I/O points of a small PLC is less than 256 points; it has a single CPU, an 8-bit or 16-bit processor, and the user memory capacity is less than 4KB. The picture below shows a small PLC.

 

 

 

(2) Medium-sized PLC The range of I/O points of medium-sized PLC is 256 to 2048 points; dual CPU, user memory capacity is 2 to 8KB. The figure below shows a medium-sized PLC.

 

 

 

(3) Large PLC The number of I/O points of a large PLC is generally greater than 2048 points; multiple CPUs, 16-bit or 32-bit processors, and user memory capacity of 8 to 16KB. The figure below shows a large PLC.