Comprehensive explanation of the basic structure and working principle of PLC

Definition of PLC

A programmable controller is a digital computing electronic system designed for use in industrial environments. It uses a programmable memory to store instructions for executing logical operations, sequential control, timing, counting and arithmetic operations, and controls various types of machinery or production processes through digital and analog inputs and outputs. The programmable controller and its related peripherals should be designed to be easy to form an integral part of the industrial control system and easy to expand its functions.

Classification of PLC

There are many types of PLC products, and their specifications and performances vary. PLCs are usually roughly classified according to their different structural forms, functional differences, and the number of I/O points. 1. Classification by structural form According to the structural form of PLC, PLC can be divided into two categories: integral type and modular type. (1) Integral type PLC Integral type PLC is a PLC that integrates power supply, CPU, I/O interface and other components in one chassis, as shown in the figure. It has the characteristics of compact structure, small size and low price. Small PLC generally adopts this integral structure. Integral type PLC consists of a basic unit (also called host) with different I/O points and an expansion unit. The basic unit has a CPU, I/O interface, an expansion port connected to the I/O expansion unit, and an interface connected to the programmer or EPROM writer. The expansion unit only has I/O and power supply, but no CPU. The basic unit and the expansion unit are generally connected by a flat cable. Integral type PLC can also be equipped with special function units, such as analog units, position control units, etc., to expand its functions.

(2) Modular PLC Modular PLC makes each component of PLC into several separate modules, such as CPU module, I/O module, power module (some are included in CPU module) and various functional modules. Modular PLC consists of a frame or baseboard and various modules, and the modules are installed on the sockets of the frame or baseboard, as shown in the figure. The characteristics of this modular PLC are flexible configuration, and different scale systems can be selected according to needs. It is also easy to assemble, expand and maintain. Large and medium-sized PLCs generally adopt modular structure.

Some PLCs combine the characteristics of the integral type and the modular type to form the so-called 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 the modules can be stacked layer by layer. In this way, not only can the system be flexibly configured, but it can also be made compact.

2. Classification by function

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

(1) Low-end PLC Low-end PLC has basic functions such as logic operation, timing, counting, shifting, self-diagnosis, monitoring, etc. It may also have a small amount of analog input/output, arithmetic operation, data transmission and comparison, and communication functions. It is mainly used for single-machine control systems with logic control, sequential 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 stronger analog input/output, arithmetic operations, data transmission and comparison, number system conversion, remote I/O, subroutines and communication networking functions; some can also add interrupt control, PID control and other functions, suitable for complex control systems.

(3) High-end PLC In addition to the functions of mid-range PLC, high-end PLC also adds signed arithmetic operations, matrix operations, bit logic operations, square root operations and other special function operations, tabulation and table transmission functions, etc. High-end PLC has stronger communication and networking functions and can be used for large-scale process control or to form distributed network control systems, thereby realizing 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 Small PLC has less than 256 I/O points, a single CPU and an 8-bit or 16-bit processor, and a user memory capacity of less than 4KB. For example: Mitsubishi FX0S series.

(2) Medium-sized PLC The I/O points of medium-sized PLC are between 256 and 2048, it has dual CPUs, and the user memory capacity is between 2 and 8KB.

(3) Large PLC Large PLC has more than 2048 I/O points, multiple CPUs and 16-bit or 32-bit processors, and a user memory capacity of 8 to 16KB. In the world, PLC products can be divided into three major schools by region: one school is American products, one school is European products, and one school is Japanese products. The PLC technology in the United States and Europe was independently researched and developed in isolation from each other, so there are obvious differences between the PLC products in the United States and Europe. Japan's PLC technology was introduced from the United States and has a certain inheritance from the United States' PLC products, but Japan's main product is positioned on small PLCs. The United States and Europe are famous for large and medium-sized PLCs, while Japan is famous for small PLCs. Common PLCs are shown in the table.

Functions and application areas of PLC

PLC is designed, manufactured and developed by integrating the advantages of relay contactor control and the advantages of computer flexibility and convenience, which makes PLC have many characteristics that other controllers cannot match. 1. Function of PLC PLC is a general industrial automatic control device developed with microprocessor as the core, integrating computer technology, automatic control technology and communication technology. It has a series of advantages such as high reliability, small size, strong function, simple program design, flexible and universal, and convenient maintenance. Therefore, it is widely used in metallurgy, energy, chemical industry, transportation, electricity and other fields, and has become one of the three pillars of modern industrial control (PLC, robot and CAD/CAM). According to the characteristics of PLC, its functional forms can be summarized into the following types.

(1) Switching logic control PLC has powerful logic operation ability and can realize various simple and complex logic control. This is the most basic and most widely used application field of PLC, which replaces the traditional relay contactor control.

(2) Analog control PLC is equipped with A/D and D/A conversion modules. The A/D module can convert analog quantities such as temperature, pressure, flow, speed, etc. on site into digital quantities, which are then processed by the microprocessor in the PLC (the microprocessor can only process digital quantities) and then controlled; or converted into analog quantities by the D/A module and then controlled. In this way, the PLC can control analog quantities.

(3) Process control Modern large and medium-sized PLCs are generally equipped with PID control modules, which can perform closed-loop process control. When a variable in the control process deviates, the PLC can calculate the correct output according to the PID algorithm, and then control and adjust the production process to keep the variable at the set value. At present, many small PLCs also have PID control functions.

(4) Timing and counting control PLC has powerful timing and counting functions. It can provide users with dozens, even hundreds or thousands of timers and counters. The timing time and count value can be set by the user when writing the user program, or by the operator through the programmer at the industrial site to achieve timing and counting control. If the user needs to count high-frequency signals, a high-speed counting module can be selected.

(5) Sequential control In industrial control, sequential control can be achieved by using PLC step instruction programming or shift register programming.

(6) Data processing Modern PLCs can not only perform arithmetic operations, data transmission, sorting and table lookup, but also perform data comparison, data conversion, data communication, data display and printing, etc. It has strong data processing capabilities.

(7) Communications and networking

Most modern PLCs use communication and network technologies, and have RS-232 or RS-485 interfaces for remote I/O control. Multiple PLCs can be networked and communicate with each other, and program and data exchange can be achieved between external devices and the signal processing units of one or more programmable controllers, such as program transfer, data file transfer, monitoring and diagnosis. The communication interface or communication processor completes the transfer of programs and data according to standard hardware interfaces or proprietary communication protocols. 2. Application fields of PLC At present, PLC has been widely used in various industries at home and abroad, such as steel, petroleum, chemical industry, electric power, building materials, machinery manufacturing, automobiles, textiles, transportation, environmental protection, culture and entertainment, etc., and its usage can be roughly summarized into the following categories.

(1) Logical control of switching quantities This is the most basic and widely used application area of ​​PLC. It replaces the traditional relay circuit to realize logical control and sequential control. It can be used to control a single device as well as a group of multiple machines and automated assembly lines, such as injection molding machines, printing machines, bookbinding machines, modular machine tools, grinding machines, packaging production lines, and electroplating lines.