In the automatic production line, some production machines need to realize automatic reciprocating motion in a certain order, and some also need to stay at certain positions for a certain time to meet the production process requirements. Using PLC program to realize the automatic round-trip sequence control of the material delivery car is not only simple, convenient and highly reliable, but also has various programming methods, which is convenient for designers at different levels to understand and master. Taking Panasonic FP0 series PLC as an example, five PLC programming methods based on automatic reciprocating sequence control of material conveying trolley are proposed.

2 system control requirements [1>;

Schematic diagram of the automatic round trip of the material delivery trolley to and from sequence control system, as shown in figure 1, shows that the trolley is in the original position A before starting, and the process control requirements of a working cycle are as follows:

1) Press the start button SB 1, and the car will start charging from the original position A. After 10 seconds, the car will move to 1 position, and stop discharging for 8 seconds after reaching 1 position.

2) The car returns to the original position A to continue charging. 10 seconds later, the car moves to the No.2 position for the second time, stops discharging for 8 seconds after reaching the No.2 position, and then returns to the original position A again, and then starts the next round of circulating work;

3) If you press the stop button SB2, you need to complete a work cycle to stop working. Fig. 3 Functional diagram of automatic round-trip of material delivery trolley to and from sequence control system.

4. 1 empirical design method [3];

Empirical design method is based on the process requirements and production process of production machinery, and some modifications and improvements are made on the basis of typical unit programs. The ladder diagram program designed by empirical design method is shown in Figure 4. According to the system control requirements, the car is charged at the original position A(X2) and discharged at the position 1 (X3) and the position 2 (X4) in turn. When the car moves forward twice in a working cycle, it will encounter X3. When it meets it for the first time, it will stop discharging, and when it meets it for the second time, it will move on. Therefore, an internal relay R 1 with memory function should be set to distinguish whether X3 is encountered for the first time or for the second time. Cars should stop moving when they meet X3 and X4 for the first time, so their normally closed contacts are connected in series with Y2' s coil. At the same time, the normally closed contact of X3 is connected in parallel with the normally open contact of internal relay R 1, so that the function of stopping X3 is limited by R 1. The function of R 1 is to remember how many times X3 has been touched, and it only works when the car passes X3 for the second time. Its starting condition and stopping condition are that the car meets X3 and X4 respectively. When the car passes X3 for the first time, the coil of R 1 is turned on, so that the normally open contact of R 1 is short-circuited to the normally closed contact of X2 control circuit. Therefore, when the car passes X3 for the second time, it will not stop moving until it reaches X4. In addition, another pair of normally open contacts of R 1 are connected in parallel with X0 to prepare for the second drive of Y0 charging.

Fig. 5 Ladder diagram of setting/resetting instruction design

4.2 Design Method of Set/Reset Instruction

The ladder diagram program designed by using the set/reset instruction is shown in Figure 5. In the program, each process corresponds to an internal relay, and the normally open contact of the internal relay corresponding to the previous step is connected in series with the contact corresponding to the conversion condition as a condition for setting the internal relay corresponding to the latter step, and the normally open contact of the internal relay corresponding to the latter step is used as a condition for resetting the internal relay corresponding to the previous step. If the car is in the original position A, press SB 1, X0 to start, and set R 1 to drive Y0 to start charging timing. The normally open contact of R 1 and the normally open contact of T0 will be used as the setting condition of R2, and the normally open contact of R2 will be used as the resetting condition of R 1. When the time is right, R2 will drive Y65438. In order to make the system work periodically and circularly, R8(R8 is set to drive Y3 and the car retreats) is connected in series with the normally open contact of R0 and in parallel with X0 as a condition for setting R 1 again. For simple sequence control system, the output relay can also be directly set or reset. This method does not need to add an internal relay to remember the number of times the car passes X3, and the logical sequence conversion relationship is very clear, which is easier for beginners to understand and master when programming.

4.3 Design Method of Hold Instruction

As shown in fig. 6, the ladder program designed with hold instruction is basically similar to the programming technology with set/reset instruction. The difference is that the setting control terminal of the hold command cannot have multiple contacts input in parallel, so an internal relay R9 is added, which will be set during initial startup or cyclic operation, thus setting R1; In addition, the number of program steps compiled with hold instruction is much less than that compiled with set/reset instruction, and the inherent occupation is greatly reduced.

Second, this programming technique is easy to be accepted and mastered by beginners. For experienced engineers, it will also improve the design efficiency. The program is also easy to debug, modify and read, and easy to use. Priority should be given in sequence control design. This method is widely used in industrial automation control.

5 concluding remarks

In this paper, five PLC programming methods based on the automatic round trip of the material trolley to and from sequence control system have their own characteristics. In practical application, we can choose one program according to the actual situation to meet the control requirements of different occasions. Practice shows that these programming methods are easily accepted and mastered by designers, and can be conveniently used to design any complicated sequence control program, thus improving the design efficiency and shortening the production cycle.