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Introduction to functions and application fields of PLC
Currently, PLC is widely applied in steel and iron, petroleum, chemical industry, power, building materials, mechanical manufacture, vehicles, light textile, transport, environmental protection and entertainment etc at home and abroad. The usage can be divided into the following types:                                                                                                                                            
1. Logic control of switching value
It is the fundamental and most extensive application field of PLC. It replaced the conventional relay circuits, and realizes logic control and sequence control. It can not only used to control the single device, but also multiple machine 2. Analog control
In the industrial production process, there are many continuously changing values, e.g. temperature, pressure, flow, liquid level and speed (analog). To enable PLC to process the analog, A/D conversion and D/A conversion between analog and digital must be realized. The PLC manufacturers produce the assorted A/D and D/A conversion modules, to make PLC used for analog control.                                   
3. Motion control
PLC can be used to control circular motion or linear motion. In terms of control mechanism configuration, it was directly used in switching I/O continuous position sensors and execution mechanism in earlier stage; now we often use the special motion control module, e.g. single-axial or multi-axial position control module of drivable stepping motors or servo motors. Almost all products of major PLC manufacturers have motion control function. It is widely used in machinery, lathes, robots and elevators etc.
4. Process control
Process control refers to closed-loop control over the analog such as temperature, pressure and flow. As industrial controlling computer, PLC can prepare various control algorithm routines, to complete closed-loop control. PID adjustment is usually the most frequently used adjustment method in closed- loop control system. Large- and medium-sized PLCs have PID module. At present, even many small-sized PLCs also have this functional module. PID process usually uses the special PID subroutine.                            
5. Data processing
Modern PLC has arithmetical operation (including matrix operation, functional operation and logical operation), data transfer, data conversion, sequencing, lookup, bit manipulation and other function. It can complete data collection, analysis and processing. The data can be compared to the reference values stored in the storage, to complete certain control cooperation. It can also use the communication function to transfer the data to other smart devices, or print the data. At present, data processing is usually used in large-sized control system, e.g. no-man controlled FMS, or process control systems.   
6. Communication and networking
PLC communication refers to communication among PLCs and communication between PLC and other smart devices. With the development of computer control, the plant automation network has been developed rapidly. All PLC manufacturers emphasize PLC communication function; they launch their network systems. The newly-produced PLCs have communication interfaces, so communication is very convenient.