Having understood the program structure and programming methods, the next step is to actually write the PLC program. The design of a PLC control system involves the following process.
(a) Analyze the controlled object and propose control requirements
A detailed analysis of the technological process and working characteristics of the controlled object is conducted to understand the coordination between the mechanical, electrical, and hydraulic systems of the controlled object. The control requirements of the controlled object for the PLC control system are proposed, the control scheme is determined, and a design task statement is drafted.
(ii) Determine the input/output devices
Based on the system's control requirements, determine all the input devices (such as buttons, position switches, changeover switches, and various sensors) and output devices (such as contactors, solenoid valves, indicator lights, and other actuators) required by the system, thereby determining the input/output devices related to the PLC and thus the number of I/O points for the PLC.
(III) Selecting a PLC
PLC selection includes choosing the PLC model, capacity, I/O modules, power supply, etc. See Section 2 of this chapter for details.
(iv) Allocate I/O points and design PLC peripheral hardware circuitry
1. Allocate I/O points
Draw a connection diagram or correspondence table between the PLC's I/O points and input/output devices. This part can also be done in step 2.
2. Design the PLC peripheral hardware circuit.
Draw the electrical wiring diagrams for the other parts of the system, including the main circuit and the control circuits that are not connected to the programmable controller.
The electrical schematic diagram of the system consists of the PLC's I/O connection diagram and the PLC's peripheral electrical wiring diagram. At this point, the system's hardware electrical wiring has been determined.
(v) Programming
1. Programming
Based on the system's control requirements, a suitable design method should be used to design the PLC program. The program should primarily focus on meeting the system's control requirements, writing the code to implement each control function or subtask step by step, gradually improving the system's specified functions. In addition, the program should typically include the following:
(1) Initialization program. After the PLC is powered on, some initialization operations are usually performed to prepare for startup and avoid system malfunctions. The main contents of the initialization program include: clearing certain data areas, counters, etc., restoring the required data in certain data areas, setting or resetting certain relays, displaying certain initial states, etc.
(2) Detection, fault diagnosis and display programs. These programs are relatively independent and are generally added when the basic program design is completed.
(3) Protection and interlocking procedures. Protection and interlocking are indispensable parts of the procedure and must be carefully considered. They can prevent control logic chaos caused by illegal operations.
2. Program simulation debugging
The basic idea of program simulation debugging is to simulate the actual field conditions in a convenient way, creating the necessary environmental conditions for program operation. Depending on the method of generating field signals, simulation debugging can take two forms: hardware simulation and software simulation.
(1) Hardware simulation method uses some hardware devices (such as another PLC or some input devices) to simulate the generation of field signals, and connects these signals to the input terminal of the PLC system in a hard-wired manner, which has strong timeliness.
(2) The software simulation method involves writing a separate simulation program in the PLC to simulate the field signals. It is simple and easy to implement, but the timeliness is not easy to guarantee. During the simulation debugging process, the segmented debugging method can be adopted, and the monitoring function of the programmer can be used.
(vi) Hardware implementation
The hardware implementation mainly involves the design and on-site construction of hardware such as control cabinets (units). The main contents include:
1) Design the electrical layout diagram and installation wiring diagram for the control cabinet and operating console, etc.
2) Design the electrical interconnection diagram between the various parts of the system.
3) Perform on-site wiring according to the construction drawings and conduct a detailed inspection.
Since program design and hardware implementation can be carried out simultaneously, the design cycle of a PLC control system can be greatly shortened.
(vii) Online debugging
Online debugging involves further integrating and testing the program in real-time after simulation debugging. The online debugging process should be gradual, starting with connecting only input devices to the PLC, then connecting output devices, and finally connecting the actual load. If the requirements are not met, adjustments should be made to the hardware and program. Usually, only a portion of the program needs to be modified.
After all debugging is complete, the program should be put into trial operation. After running for a period of time, if it works normally and the program does not need to be modified, the program should be burned into the EPROM to prevent program loss.
(viii) Organizing and compiling technical documents
Technical documents include design specifications, hardware schematics, installation wiring diagrams, electrical component lists, PLC programs, and user manuals.
