Hollysys PLC - Introduction to the functions and application fields of PLC

1. The function of PLC

PLC is a general industrial automatic control device with microprocessor as the core, integrating computer technology, automatic control technology and communication technology, which has a series of advantages such as high reliability, small size, strong function, simple program design, flexible and versatile and convenient maintenance, so it has a wide range of applications in metallurgy, energy, chemical industry, transportation, electric power 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, which can realize a variety of simple and complex logic control. This is the most basic and extensive application field of PLC, which replaces the control of traditional relay contactors.

(2) Analog control

The PLC is equipped with A/D and D/A conversion modules. The A/D module can convert the on-site temperature, pressure, flow, speed and other analog quantities into digital quantities, and then process them by the microprocessor in the PLC (the microprocessor can only process digital quantities), and then control them;

Or it can be converted into analog quantity after the D/A module, and then control the controlled object, so that the PLC can realize the control of the analog quantity.

(3) Process control

Modern large and medium-sized PLCs are generally equipped with PID control modules for closed-loop process control. When there is a deviation of a variable in the control process, 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 function.

(4) Timing and counting control

PLC has a strong timing and counting function, it can provide users with dozens or even hundreds, thousands of timers and counters.

The timing time and counting value can be arbitrarily set by the user when writing the user program, or it can be set by the operator through the programmer at the industrial site, so as to realize the timing and counting control. If the user needs to count the signal with a higher frequency, the high-speed counting module can be selected.

(5) Sequence control

In industrial control, sequential control can be achieved by programming PLC step instructions or by programming with shift registers.

(6) Data Processing

Modern PLC can not only perform arithmetic operations, data transmission, sorting and table lookup, but also data comparison, data conversion, data communication, data display and printing, etc., it has strong data processing capabilities.

(7) Communication and networking

Most modern PLCs use communication and network technology, and have RS-232 or RS-485 interfaces for remote I/O control.

Multiple PLCs can be networked and communicate with each other, and programs and data can be exchanged between external devices and the signal processing unit of one or more programmable controllers, such as program transfer, data document 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 such as steel, petroleum, chemical industry, electric power, building materials, machinery manufacturing, automobiles, light textiles, transportation, environmental protection and culture and entertainment at home and abroad, and the use of PLC can be roughly summarized into the following categories.

(1) Logic control of switching quantity

This is the most basic and most widely used field of PLC, which replaces the traditional relay circuit and realizes logic control and sequence control;

It can be used for the control of a single equipment, but also for multi-machine group control and automatic assembly lines, such as injection molding machines, printing machines, staple machines, combined machine tools, grinding machines, packaging production lines and electroplating lines.

(2) Analog control

In industrial processes, there are many continuously varying quantities, such as temperature, pressure, flow, level and velocity, which are analogue. In order for a PLC to process analog quantities, it is necessary to realize A/D conversion and D/A conversion between analog and digital quantities. PLC manufacturers produce supporting A/D and D/A conversion modules, so that PLC can be used for analog control.

(3) Motion control

PLC can be used for the control of circular or linear motion. In terms of control mechanism configuration, in the early days, it was directly used to connect position sensors and actuators with switching I/O modules, and now it is generally used to use special motion control modules to drive single-axis or multi-axis position control modules of stepper motors or servo motors.

Almost all the products of the world's major PLC manufacturers have motion control functions, which are widely used in various machinery, machine tools, robots, elevators and other occasions.

(4) Process control

Process control refers to the closed-loop control of analog quantities such as temperature, pressure, and flow, and has a wide range of applications in metallurgy, chemical industry, heat treatment, boiler control, etc. As an industrial control computer, PLC can program a variety of control algorithm programs to complete closed-loop control.

PID adjustment is the most used adjustment method in the general closed-loop control system, large and medium-sized PLCs have PID modules, and many small PLCs also have this functional module. PID processing is generally to run a dedicated PID subroutine.

(5) Data processing

Modern PLC has the functions of mathematical operation (including matrix operation, function operation, logic operation), data transmission, data conversion, sorting, table lookup and bit operation, etc., which can complete the collection, analysis and processing of data. These data can be compared with the reference values stored in the memory to complete certain control operations; It is also possible to use the communication function to transmit to other smart devices or to print them for tabulation.

Data processing is generally used in large-scale control systems, such as unmanned flexible manufacturing systems; It can also be used in process control systems, such as some large-scale control systems in the papermaking, metallurgy, and food industries.

(6) Communication and networking

PLC communication includes communication between PLCs and communication between PLCs and other intelligent devices. With the development of computer control, the factory automation network has developed rapidly, and all PLC manufacturers attach great importance to the communication function of PLC, and have launched their own network systems. The newly produced PLCs all have communication interfaces, which are very convenient for communication.

Hollysys PLC - Introduction to the functions and application fields of PLC

Hollysys PLC - Introduction to the functions and application fields of PLC

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