Analysis of PLC control program design

PLC programming is to turn the required control function into a program. Editing is only possible under special circumstances according to the control circuit. The feasible method is to first convert the control function required by the vacuum device into a ladder diagram or flow chart of the control system, and then program according to the PLC programming rules, or use the statement table prepared by the above two diagrams to program, of which the ladder diagram is most commonly used. .

In the relay control circuit diagram, KM1 and HL are used to represent the contactor coils that control the vacuum pump and the signal lamps that characterize the operating state of the vacuum pump; while in the ladder diagram, they are all represented by the output relay Y.

It can be seen from the figure that the two figures are basically similar; the structure of the figure is roughly the same; the symbol of the ladder diagram element is mostly the same as the relay control circuit; the form of signal input stealing and the control function are the same. The difference is that the components are different, the relay control circuit is composed of real relays, and the ladder diagram is composed of triggers (ie soft relays); the working mode is different. In the relay control circuit, when the power is turned on, the circuit The relays in are all in a restricted state, and in the ladder diagram, each soft relay is in a cyclic scan-on state; the number of contacts is different. In the relay control circuit, the number of relay contacts is limited, while in the ladder diagram, The number of contacts of each soft relay is unlimited, and the programming method is also different.

The general rules of programming are:

â‘  The contact should be drawn on the horizontal line, not on the vertical branch line;

â‘¡ Branches that do not contain contacts should be placed in a vertical direction, not on horizontal velvet;

â‘¢ The contact point on the right side of the vertical branch should be connected to the top of the branch;

â‘£ Coils without special functions, the contact area can accommodate up to 7 rows, each row has a maximum of 8 series contacts.

These program design rules are not static, and may be different for different types of PLCs.

Row.

The general steps of programming are:

â‘  According to the control requirements of the vacuum device, the entire vacuum device control system is decomposed into relatively independent control units;

â‘¡Use ladder diagram to show the control process of each control unit;

â‘¢According to the instructions specified by the ladder diagram and PLC products, each logic element (element) writes a statement and writes the program sentence by sentence in the order of logic lines (an output coil forms a logic line) from top to bottom and from left to right . The PLC user program is input to the PLC user program memory through the programmer. The programmer is the implementer. In addition to inputting user programs, it can also read, search, modify, delete, and insert user programs, set constants, and monitor working status during operation.

The programmer is also a microprocessor. According to the function keys on the panel and the display screen, the graphics on the ladder diagram are transformed into machine language and sent to the PLC, which is simple and convenient to use.