Design method of ladder diagram of the hottest pro

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Programmable controller ladder diagram design method

1. Introduction

programmable controller is a microcomputer system specialized in logic control, which combines the concept and design idea of relay control with computer technology and microelectronics technology. In the application of PC system, the design of ladder diagram is often the most important problem. Ladder diagram not only follows and develops electrical control technology, but also its functions and control instructions have far exceeded the scope of electrical control. It can not only realize logical operation, but also has the functions of arithmetic operation, data processing, communication and so on. It is a microcomputer system with industrial control instructions. Because the design of ladder diagram is the product of the combination of computer program design and electrical control design, it has both similarities and differences with computer program design and electrical control design in design method. This paper puts forward four common methods for designing the ladder diagram of switching value control system

The so-called alternative design method is to use the program of PC to replace the original relay logic control circuit. Its basic idea is: take the input signal and output signal of the original electrical control system as the i/o point of PC, and the logic control function originally completed by the hardware of relay contactor is replaced by the software ladder diagram and program of PC. For example, for the motor forward and reverse control circuit, the original electrical control circuit diagram is shown in Figure 1. After being replaced by PC control, its i/o wiring diagram and ladder diagram are shown in Figure 2 and figure 3 respectively

figure 3pc ladder diagram has the advantages of simple program design method, ready-made electrical control circuit as the basis, and short design cycle. Generally, in the transformation of electrical control system of old equipment, it is often used for less complex control system

Third, logic algebra design method

because the electrical control circuit and logic algebra have a pair of shafts with large diameter, which can also be made into a hollow corresponding relationship, the control process of switching value can be expressed, analyzed and designed by logic algebra

the basic design steps are as follows:

1. List logical algebraic expressions according to control requirements

2. Simplify logical algebra

3. Draw a ladder diagram according to the simplified logical algebraic expression. Here is a simple example to illustrate

a motor can run only when any one or any two of the three buttons act, but it will not run under any other circumstances. Try to design its ladder diagram

the motor operation is controlled by PC output point 0500, and the three buttons correspond to PC input addresses a, B and C respectively. According to the meaning of the question, if any of the three buttons acts, the output point 0500 of the PC will have an output. Its logic algebra expression is that when there are any two actions in the three buttons, the logic algebra expression of the output point 0500 is because the two conditions are or relationship, so the motor operation condition should be to simplify the formula to get

Figure 4 electricity 5. Properly adjust the aperture diaphragm and field diaphragm to get the ladder diagram of the motor operation

draw the ladder diagram according to the logic algebra expression, as shown in Figure 4. Figure 4 uses this method to design. The biggest feature is that many logical relationships can be simplified

of course, redundant design for reliability and safety is another problem

IV. program flow chart design method

pc adopts computer control technology, its program design can also follow the software engineering design method, and the working process of the program can be represented by flow chart. Because the program execution of PC is in the working mode of cyclic scanning, the difference from the computer program block diagram is that the PC program block diagram will restart the input scanning after the output refresh, and the cyclic execution

the application of this design method is illustrated by taking the control of automatic washing machine as an example

first draw the process flow diagram of the washing machine, as shown in Figure 5

Figure 5 washing machine process flow chart

step 2: select the PC model and set the i/o point number. Its i/o point number is allocated as follows:

i/o point allocation timing/counter allocation

00 start switch T600 forward rotation timing

01 stop switch T601 pause timing

02 manual drainage switch T602 reverse timing

03 high water level switch t603 pause timing

04 low water level switch T604 dehydration timing

20 start washing machine T605 alarm timing

21 water inflow C606 washing times

22 forward rotation washing c607 dehydration times

23 reverse Turn to washing

25 drainage

26 dehydration

27 stop, alarm

step 3, design the ladder diagram according to the flow chart, as shown in Figure 6

Figure 6 washing machine ladder diagram

v. function module design method

according to the modular design idea, the system can be divided into modules according to control functions, and the ladder diagram can be designed for each control function module in turn

for example, in the PC elevator control system, the elevator control can be divided into: hall door switch control module, floor selection control module, elevator operation control module, call display control module, etc. The ladder diagram design according to the elevator function can centralize the programs with the same function of the elevator, and the program structure is clear, which is convenient for debugging. It can also flexibly add other control functions as needed

of course, in the design, we should pay attention to the mutual influence between modules, timing relationship, and the use conditions of interlocking instructions. The same control function can have different software implementation methods. Simple and practical schemes should be adopted according to the specific situation, and the programming instructions provided by different models should be fully used to make the program as simple as possible

VI. conclusion

this paper introduces four design methods of PC ladder diagram. In addition, there are other methods, such as empirical method. For different links in the system design, different design methods can be adopted according to the specific situation. Generally, the method of program block diagram and function module is adopted in the overall design; In the transformation of old equipment with domestic photovoltaic power generation installed capacity of more than 77.42 million KW, we still have professional design for most enterprises that do not understand the substitution method; In the programming of local or specific functions, logical algebra method and empirical method are adopted

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