PLC programmable controller is an electronic device for digital operation designed for use in industrial environments. It uses a programmable memory to store instructions for performing logical operations, sequential operations, timing, counting and arithmetic operations, and can control various types of machinery or production processes through digital or analog input and output. PLC and its related peripherals should be designed according to the principle of easy integration with industrial control systems and easy expansion of their functions.
At present, PLC programmable controllers have been widely used in various industries such as steel, petroleum, chemical industry, electricity, building materials, machinery manufacturing, automobiles, textiles, transportation, environmental protection and culture and entertainment at home and abroad. The use can be roughly summarized into the following categories.
1. Logical control of switching quantity
This is the most basic and most widely used application field of PLC. It replaces the traditional relay circuit to realize logical control and sequential control. It can be used for the control of a single device, as well as for multi-machine group control and automated assembly lines. Such as injection molding machines, printing machines, bookbinding machines, modular machine tools, grinders, packaging production lines, electroplating lines, etc.
2. Analog quantity control
In the industrial production process, there are many continuously changing quantities, such as temperature, pressure, flow, liquid level and speed, which are all analog quantities. In order for the programmable controller to process analog quantities, it is necessary to realize A/D conversion and D/A conversion between analog quantities (Analog) and digital quantities (Digital). PLC manufacturers all produce matching A/D and D/A conversion modules to enable programmable controllers to be used for analog quantity control.
3. Motion control
PLC can be used for circular motion or linear motion control. From the perspective of control mechanism configuration, the switch quantity I/O module was directly used to connect the position sensor and the actuator in the early days, but now a dedicated motion control module is generally used. Such as a single-axis or multi-axis position control module that can drive a stepper motor or servo motor. The products of major PLC manufacturers in the world almost all have motion control functions and 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. As an industrial control computer, PLC can compile a variety of control algorithm programs to complete closed-loop control. PID regulation is a commonly used regulation method in general closed-loop control systems. Large and medium-sized PLCs all have PID modules, and many small PLCs currently also have this function module. PID processing generally runs a dedicated PID subroutine. Process control is widely used in metallurgy, chemical industry, heat treatment, boiler control and other occasions.
5. Data processing
Modern PLCs have functions such as mathematical operations (including matrix operations, function operations, logical operations), data transmission, data conversion, sorting, table lookup, bit operations, etc., which can complete data collection, analysis and processing. These data can be compared with the reference values stored in the memory to complete certain control operations, or they can be transmitted to other intelligent devices using the communication function, or they can be printed and tabulated. Data processing is generally used in large control systems, such as unmanned flexible manufacturing systems; it can also be used in process control systems, such as some large control systems in 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, factory automation networks have developed rapidly. PLC manufacturers attach great importance to the communication function of PLCs and have launched their own network systems. Recently produced PLCs all have communication interfaces, making communication very convenient.
