On the basis of understanding the program structure and programming method, it is necessary to actually write the PLC program. The PLC control system design needs to go through the following process.
(1) Analyze the accused and propose control requirements
Analyze the technological process and working characteristics of the controlled object in detail, understand the cooperation between the controlled object machine, electricity and hydraulics, put forward the control requirements of the controlled object on the PLC control system, determine the control plan, and draw up the design task book.
(2) Identify input / output devices
According to the control requirements of the system, determine all the input devices (such as buttons, position switches, transfer switches, and various sensors, etc.) and output devices (such as: contactors, solenoid valves, signal indicators, and other actuators) Etc.) to determine the PLC-related input / output devices to determine the PLC's I / O points.
(3) Select PLC
PLC selection includes selection of PLC model, capacity, I / O module, power supply, etc., see section 2 of this chapter for details.
(4) Assign I / O points and design PLC peripheral hardware circuits
1. Assign I / O points
Draw the connection diagram or correspondence table between PLC I / O points and input / output devices. This part can also be carried out in the second step.
2. Design PLC peripheral hardware circuit
Draw the electrical circuit diagram of other parts of the system, including the main circuit and the control circuit that has not entered the programmable controller.
The electrical schematic diagram of the system is composed of PLC I / O connection diagram and PLC peripheral electrical circuit diagram. So far, the hardware electrical circuit of the system has been determined.
(5) Programming
1. Programming
According to the control requirements of the system, a suitable design method is used to design the PLC program. The program must meet the system control requirements as the main line, write programs to realize each control function or each subtask one by one, and gradually improve the functions specified by the system. In addition, the program should usually include the following:
(1) Initialization procedure. After the PLC is powered on, it is generally necessary to perform some initialization operations to make necessary preparations for startup to avoid system malfunctions. The main contents of the initialization program are: clearing some data areas, counters, etc., recovering the data needed in certain data areas, setting or resetting certain relays, displaying certain initial states, etc.
(2) Detection, fault diagnosis and display procedures. These programs are relatively independent, and are generally added when the program design is basically completed.
(3) Protection and chain procedures. Protection and chaining are indispensable parts of the procedure and must be carefully considered. It can avoid the confusion of control logic caused by illegal operation.
2. Program simulation debugging
The basic idea of ​​program simulation debugging is to simulate and produce the actual status of the scene in a convenient form, and to create the necessary environmental conditions for the operation of the program. According to the different ways of generating on-site signals, simulation debugging has two forms: hardware simulation method and software simulation method.
(1) The hardware simulation method is to use some hardware equipment (such as another PLC or some input devices, etc.) to simulate the generation of on-site signals, and connect these signals to the input end of the PLC system in a hard-wired manner, its timeliness Stronger.
(2) The software simulation method is to write another set of simulation programs in the PLC. The simulation provides on-site signals, which is simple and easy to implement, but the timeliness is not easy to guarantee. In the simulation debugging process, the method of segmented debugging can be used, and the monitoring function of the programmer can be used.
(6) Hardware implementation
The hardware implementation is mainly for the design and on-site construction of hardware such as control cabinets (sets). The main contents are:
1) Design the electrical layout and installation wiring diagram of the control cabinet and operation console.
2) Design the electrical interconnection diagram between each part of the system.
3) Conduct on-site wiring according to the construction drawings and conduct detailed inspection.
Because the program design and hardware implementation can be carried out simultaneously, the design cycle of the PLC control system can be greatly shortened.
(7) Online debugging
Online debugging is to conduct further online adjustment through the simulated debugging program. The on-line debugging process should be carried out step by step, starting from the PLC to connect only the input device, then connect the output device, and then connect the actual load for debugging. If it does not meet the requirements, adjust the hardware and procedures. Usually only part of the program needs to be modified.
After all the commissioning is completed, the trial operation is delivered. After a period of operation, if the work is normal and the program does not need to be modified, the program should be solidified into the EPROM to prevent the program from being lost.
(8) Sorting out and writing technical documents
Technical documents include design specifications, hardware schematics, installation wiring diagrams, electrical component schedules, PLC programs, and operating instructions.
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