In modern industrial production, the traditional manual operation has been far from achieving good control quality. At present, in the field of electrical control, PLC is widely used at home and abroad. Especially in the field of high temperature, high pressure, inflammable, explosive and high-risk production, PLC is more widely used because of its high reliability and strong anti-interference ability in the harsh industrial environment.
PLC integrates electrical, instrumentation and control into one. It can be easily and flexibly combined into control systems of various sizes and requirements to meet the needs of various industrial controls. PLC is an electronic device specially designed for digital operation in industrial environment. It uses programmable memory to store instructions to perform logical operations, sequential operations, timing, counting and arithmetic operations within it, and to control various types of machinery or production processes through digital or analog inputs and outputs. The programming language such as ladder diagram, logic diagram or statement table is adopted without computer knowledge, so the development cycle of the system is short and the debugging on site is easy. In addition, the program can be modified online to change the control scheme without dismantling the hardware. A small PLC has hundreds of programming elements which can be used by users. It has strong functions and can realize very complex control functions. Compared with the relay system with the same function, it has a high performance-price ratio. PLC can realize decentralized control and centralized management through communication network. Hardware is complete.
Users are easy to use. PLC products have been standardized, serialized, modularized, equipped with a variety of hardware devices for users to choose, users can flexibly and conveniently configure the system to form different functions, different scales of the system. The installation and wiring of PLC is also very convenient. Generally, external wiring is connected by wiring terminals. PLC has strong load capacity and can directly drive general solenoid valves and small AC contactors. After the hardware configuration is determined, the user program can be modified to adapt to the change of process conditions conveniently and quickly. Traditional relay control system uses a large number of intermediate relays and time relays, which are prone to failure due to poor contact. PLC replaces a large number of intermediate relays and time relays with software, leaving only a small number of hardware components related to input and output. The wiring can be reduced to 1/10-1/100 of the relay control system, and the faults caused by poor contact can be greatly reduced. The software function of PLC replaces a large number of intermediate relays, time relays, counters and other devices in the relay control system, which greatly reduces the design, installation and wiring workload of the control cabinet.
The ladder diagram program of PLC is usually designed by sequential control design method.
This programming method is very regular and easy to master. For complex control systems, the design time of ladder diagram is much less than that of relay system circuit diagram with the same function. The failure rate of PLC is very low, and it has perfect self-diagnosis and display function. When the failure of the PLC or the external input device and the actuator occurs, the cause of the failure can be quickly identified according to the information provided by the LED or the programmer on the PLC. The failure can be quickly eliminated by replacing the module. This is the most basic and extensive application field of PLC control technology. It replaces the traditional relay circuit and realizes sequential control and logical control. It can be used not only in the control of single equipment, but also in the automation pipeline, such as production line, modular machine tool, grinder, boring machine and gantry planer. In the actual industrial production process, there will be many continuous changes in the amount, such as temperature, speed, flow, liquid level, pressure and other analog quantities. These analog quantities can be realized by D/A conversion and A/D conversion between digital quantities to ensure that the programmer can process the analog quantities. The centralized control system mainly uses a powerful PLC monitoring system to control many devices, and has formed a “centralized” computer control system.
In this system, the links, contact relations and operation sequence between each device are unified by the central PLC.
In the distributed control system, each control object needs to set up a PLC. Each PLC can produce internal response, thermostatic element issuance or interlocking through signal transmission, or can complete communication tasks by upper computer through data communication bus. The distributed control system adopts the control mode of multiple mechanical production lines, and each production line has data connection. Because each control object is controlled by its own PLC, if one PLC is stopped, it will not affect other PLCs. With the continuous progress of technology, at present, PLC can undertake the bottom control task, through network connection, the combination of PLC and process control. PLC can control circular motion or linear motion. In the configuration of control mechanism, in the past, special motion control module can be used to directly apply to sensors and actuators. For example, multi-axis position control module, servo motor, single-axis, driving stepper motor, etc.
, PLC can be widely used in robots, machinery, elevators, machine tools and other occasions. In the process of data processing, PLC has the functions of data transmission, data conversion, operation, table lookup, sorting and operation, and completes data acquisition, analysis and processing. These data can have reference value as well as the data stored in the memory, and complete the control operation.
In addition, these data can also be transmitted to intelligent devices through the realization of communication functions, or printed into tables. At present, data processing is mostly used in large-scale control systems, such as process control systems, flexible manufacturing systems and so on. Because PLC is specially designed for industrial control, its structure is compact, firm and compact, and it is an ideal control equipment for realizing mechatronics. With the rapid development of microelectronics technology. The production cost of PLC is declining, but its function is greatly enhanced. In the advanced industrial countries, PLC has become the standard equipment of industrial control. The application of PLC covers almost all the industrial enterprises, and it has become the leading position of the three pillars of modern industrial automation (PLC, ROBOT, CAD/CAM).