Based on the understanding of the program structure and programming methods, it is necessary to actually write the PLC program. The PLC control system design needs to go through the following process.
(1) Analyze the controlled object and propose control requirements
Detailed analysis of the process and working characteristics of the controlled object, understanding the cooperation between the controlled object machine, electricity and liquid, proposing the control requirements of the controlled object to the PLC control system, determining the control plan, and drafting the design task book.
(2) Determining input/output devices
Determine all input devices (such as buttons, position switches, transfer switches, and various sensors) and output devices (such as contactors, solenoid valves, signal indicators, and other actuators) required by the system based on system control requirements. Etc.) to determine the input/output devices associated with the PLC to determine the number of I/O points for the PLC.
(three) choose PLC
PLC selection includes selection of PLC model, capacity, I/O module, power supply, etc. See section 2 of this chapter for details.
(4) Allocating I/O points and designing PLC peripheral hardware lines
1. Assign I/O points
Draw the connection diagram or correspondence table of the I/O point of the PLC and the input/output device. This part can also be performed in step 2.
2. Design PLC peripheral hardware circuit
Draw the electrical circuit diagram of the rest of the system, including the main circuit and the control circuit that does not enter the programmable controller.
The electrical schematic of the system is composed of the I/O connection diagram of the PLC and the peripheral electrical circuit diagram of the PLC. The hardware electrical circuit of the system has been determined so far.
(5) Program design
Programming
According to the control requirements of the system, the PLC design is designed using appropriate design methods. The program should meet the system control requirements as the main line, and write the programs that realize the various control functions or sub-tasks one by one, and gradually improve the functions specified by the system. In addition to this, the program should usually include the following:
(1) Initialization procedure. After the PLC is powered on, some initialization operations are generally required to make necessary preparations for startup to avoid system malfunction. The main contents of the initialization program are: clearing certain data areas, counters, etc., restoring the data required in some data areas, setting or resetting certain relays, displaying some initial states, and so on.
(2) Procedures such as detection, fault diagnosis and display. These programs are relatively independent and are usually added when the programming is almost complete.
(3) Protection and chain procedures. Protection and chaining are indispensable parts of the process and must be carefully considered. It can avoid control logic confusion caused by illegal operations.
2. Program simulation debugging
The basic idea of ​​program simulation debugging is to simulate the actual state of the scene in a convenient form and create the necessary environmental conditions for the operation of the program. According to the way of generating the on-site signal, the analog debugging has two forms: hardware simulation method and software simulation method.
(1) The hardware simulation method uses some hardware devices (such as using another PLC or some input devices, etc.) to simulate the signals generated in the field, and these signals are hardwired to the input of the PLC system, and its timeliness Strong.
(2) The software simulation method is to write another set of simulation program in the PLC, and the simulation provides the on-site signal, which is simple and easy, but the timeliness is not easy to guarantee. In the process of simulation debugging, the method of segmentation debugging can be used, and the monitoring function of the programmer can be utilized.
(6) Hardware implementation
The hardware implementation mainly involves the design and on-site construction of hardware such as control cabinets (tables). The main contents are:
1) Design the electrical layout and installation wiring diagram of the control cabinet and console.
2) Design an electrical interconnection diagram between the various parts of the system.
3) Perform on-site wiring according to the construction drawings and conduct detailed inspection.
Since the programming and hardware implementation can be performed simultaneously, the design cycle of the PLC control system can be greatly shortened.
(7) Online debugging
Online debugging is a further online tuning of the program that is debugged through simulation. The online debugging process should be carried out step by step. The PLC is only connected to the input device, connected to the output device, and then connected to the actual load. If the requirements are not met, the hardware and program are adjusted. Usually you only need to modify some programs.
After all debugging is completed, the test run 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 cured into the EPROM to prevent the program from being lost.
(8) Organizing and writing technical documents
Technical documents include design specifications, hardware schematics, installation wiring diagrams, electrical component schedules, PLC programs, and instruction manuals.
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