Issues that should be paid attention to when applying and using PLC in switching power supply

　　1. Brief Introduction

　　Over the years, the programmable controller (hereinafter referred to as PLC) has made a leap from wiring logic to storage logic since its creation; its functions have evolved from weak to strong, from logic control to digital control; its application areas have grown from small to large, from simple control of single equipment to competent motion control, process control, distributed control and other tasks. Today's PLC has greatly improved its capabilities in processing analog quantities, digital operations, human-machine interfaces and networks, becoming the mainstream control equipment in the field of industrial control, playing an increasingly important role in all walks of life.

　　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, automobile, textile, transportation, environmental protection, culture and entertainment. The usage is mainly divided into the following categories:

　　1. Switching logic control

　　It replaces the traditional relay circuit to realize logic control and sequence control, and can be used for the control of a single device, as well as for the control of multiple machines and automated production lines, such as injection molding machines, printing machines, bookbinding machines, modular machine tools, grinding machines, packaging production lines, electroplating production lines, etc.

　　2. Industrial process control

　　In the industrial production process, there are some continuously changing quantities (i.e. analog quantities) such as temperature, pressure, flow, liquid level and speed. PLC uses corresponding A/D and D/A conversion modules and various control algorithm programs to process analog quantities and complete closed-loop control. PID regulation is a commonly used regulation method in general closed-loop control systems. Process control is widely used in metallurgy, chemical industry, heat treatment, boiler control and other occasions.

　　3. Motion Control

　　PLC can be used to control circular motion or linear motion. Generally, dedicated motion control modules are used, such as single-axis or multi-axis position control modules that can drive stepper motors or servo motors. They are widely used in various machinery, machine tools, robots, elevators and other occasions.

　　4. Data processing

　　PLC has the functions of mathematical operation (including matrix operation, function operation, logical operation), data transmission, data conversion, sorting, table lookup, bit operation, etc., which can complete data collection, analysis and processing. Data processing is generally used in some large control systems in papermaking, metallurgy and food industries.

　　5. Communications and networking

　　PLC communication includes communication between PLCs and communication between PLCs and other intelligent devices. With the development of factory automation networks, PLCs now have communication interfaces, making communication very convenient.

　　3. Application characteristics of PLC

　　1. High reliability and strong anti-interference ability

　　High reliability is the key performance of electrical control equipment. PLC adopts modern large-scale integrated circuit technology, strict production process, and advanced anti-interference technology in its internal circuit, so it has high reliability. Compared with the relay contactor system of the same scale, the electrical wiring and switch contacts of PLC are reduced to hundreds or even thousands of times, and the failure is greatly reduced. In addition, PLC has the function of hardware fault self-detection, and can send out alarm information in time when a fault occurs. In the application software, the user can also program the fault self-diagnosis program of peripheral devices, so that the circuits and equipment in the system other than PLC can also obtain fault self-diagnosis protection. In this way, the whole system will have extremely high reliability.

　　2. Complete supporting facilities, perfect functions and strong applicability

　　PLC has developed into a series of products of various sizes, which can be used in industrial control occasions of various sizes. In addition to the logic processing function, most PLCs have perfect data computing capabilities and can be used in various digital control fields. A large number of various functional units have emerged, allowing PLC to penetrate into various industrial controls such as position control, temperature control, CNC, etc. In addition, with the enhancement of PLC communication capabilities and the development of human-machine interface technology, it has become very easy to use PLC to form various control systems.

　　3. Easy to learn and use, popular among engineering and technical personnel

　　PLC is an industrial control device for industrial and mining enterprises. It has an easy interface and its programming language is easily accepted by engineers and technicians. The graphic symbols and expressions of the ladder diagram language are quite similar to the relay circuit diagram, which opens the door for people who are not familiar with electronic circuits, computer principles and assembly language to engage in industrial control.

　　4. The system design has small workload, convenient maintenance and easy transformation

　　PLC uses storage logic instead of wiring logic, which greatly reduces the external wiring of the control equipment, greatly shortens the design and construction cycle of the control system, and makes daily maintenance easier. More importantly, it makes it possible to change the production process of the same equipment by changing the program. This is particularly suitable for multi-variety and small-batch production occasions.

　　4. Issues that need attention in PLC applications

　　PLC is a device used for automated control of industrial production. Generally, no measures need to be taken before it can be used directly in an industrial environment. However, despite the high reliability and strong anti-interference ability as mentioned above, when the production environment is too harsh, the electromagnetic interference is particularly strong, or the installation and use are improper, it may cause program errors or calculation errors, resulting in erroneous input and causing erroneous output, which will cause the equipment to lose control and malfunction, and thus cannot guarantee the normal operation of the PLC. To improve the reliability of the PLC control system, on the one hand, PLC manufacturers are required to improve the anti-interference ability of the equipment; on the other hand, it requires that great attention be paid to the design, installation, use and maintenance, and the cooperation of multiple parties can perfectly solve the problem and effectively enhance the anti-interference performance of the system. Therefore, the following issues should be paid attention to during use:

　　1. Work environment

　　(1) Temperature

　　PLC requires an ambient temperature of 0~55oC. When installing, it should not be placed under components that generate a lot of heat, and the space around it should be large enough for ventilation and heat dissipation.

　　(2) Humidity

　　In order to ensure the insulation performance of PLC, the relative humidity of the air should be less than 85% (no condensation).

　　(3) Vibration

　　The PLC should be kept away from strong vibration sources to prevent frequent or continuous vibrations with a frequency of 10 to 55 Hz. When vibration is unavoidable in the operating environment, shock-absorbing measures must be taken, such as using shock-absorbing rubber.

　　(4) Air

　　Avoid corrosive and flammable gases, such as hydrogen chloride, hydrogen sulfide, etc. For environments with a lot of dust or corrosive gases in the air, the PLC can be installed in a well-sealed control room or control cabinet.

　　(5) Power supply

　　PLC has a certain resistance to interference caused by power lines. In environments where reliability requirements are very high or power interference is particularly severe, an isolation transformer with a shielding layer can be installed to reduce interference between the equipment and the ground. Generally, PLCs have a DC 24V output provided to the input end of the metal processing network. When the input end uses an external DC power supply, a DC regulated power supply should be selected. Because ordinary rectifier and filter power supplies are easily affected by ripples, PLCs can easily receive wrong information.