Abstract : This paper proposes five PLC programming methods based on the automatic reciprocating sequence control of a material handling trolley. The paper comprehensively elaborates and summarizes the ideas and characteristics of each method and compares them. Keywords: PLC, sequential control, sequence function chart, ladder diagram, instruction. 1 Introduction On automated production lines, some production machinery worktables need to achieve automatic reciprocating motion in a certain sequence, and some also require a certain period of time to stop at certain positions to meet production process requirements. Using PLC programming to implement the automatic reciprocating sequence control of a material handling trolley not only has the advantages of simple, convenient, and reliable programming, but also offers diverse programming methods, making it easy for designers at different levels to understand and master. This paper takes the Panasonic Electric Works FP0 series PLC as an example and proposes five PLC programming methods based on the automatic reciprocating sequence control of a material handling trolley. 2 System Control Requirements [1] The schematic diagram of the automatic reciprocating sequence control system of the material handling trolley is shown in Figure 1. Before starting, the trolley is located at the original position A. The process control requirements for one working cycle are as follows: 1) Press the start button SB1, the trolley loads material from the original position A, and after 10 seconds, the trolley moves forward to position 1. After reaching position 1, it stops for 8 seconds to unload and then moves backward; 2) The trolley moves backward to the original position A to continue loading material. After 10 seconds, the trolley moves forward to position 2 for the second time. After reaching position 2, it stops for 8 seconds to unload and then moves backward back to the original position A, and then starts the next cycle of work; 3) If the stop button SB2 is pressed, the work must be stopped only after one working cycle is completed. [align=center] Figure 1 Schematic diagram of automatic reciprocating sequence control system for material handling trolley[/align] 3 Program design scheme According to the system control requirements, the system inputs include: start and stop button signals; original position, position 1, and position 2 limit switch signals; the system outputs include: loading and unloading control solenoid valve drive signals; forward and backward control motor contactor drive signals. A total of 5 actual input points and 4 output points are required. The I/O wiring diagram of the PLC for the automatic reciprocating sequence control system of material handling trolley is shown in Figure 2: [align=center] Figure 2 PLC I/O wiring diagram[/align] The above control process can be represented by the PLC sequential function chart [2] (also known as the control system flowchart), which is a graphical language above other programming languages, used to compile sequential control programs. As shown in Figure 3: the entire program is programmed directly according to the order of actions, which is intuitive, simple, and clear, and is very suitable for sequential control. 4 System Program Design [align=center] Figure 3 Sequential Function Diagram of Automatic Reciprocating Sequential Control System for Material Handling Trolley [/align] 4.1 Experience Design Method [3] The experience design method is based on the process requirements of the production machinery and the production process, and makes certain modifications and improvements to the typical unit program. The ladder diagram program designed using the experience design method is shown in Figure 4. According to the system control requirements, the trolley loads material at the original position A (X2) and unloads material alternately at positions 1 (X3) and 2 (X4). The trolley will encounter X3 twice in one working cycle. When it encounters X3 for the first time, it stops to unload. When it encounters X3 for the second time, it continues to move forward. Therefore, an internal relay R1 with memory function should be set to distinguish whether it is the first or the second time X3 is encountered. The trolley should stop moving forward upon first encountering X3 and X4. Therefore, their normally closed contacts are connected in series with the coil of Y2. Simultaneously, the normally closed contact of X3 is connected in parallel with the normally open contact of the internal relay R1, making the stopping action of X3 constrained by R1. R1's function is to remember how many times X3 has been encountered; it only activates when the trolley passes X3 for the second time. Its starting and stopping conditions are the trolley encountering X3 and X4, respectively. When the trolley first passes X3, the coil of R1 is energized, causing the normally open contact of R1 to short-circuit the normally closed contact of X3 in the Y2 control circuit. Therefore, the trolley will not stop moving forward when it passes X3 for the second time, and R1 only resets when it reaches X4. Furthermore, another pair of normally open contacts of R1 is connected in parallel with X0 to prepare for the second drive of Y0 for loading. [align=center]Figure 4 Ladder diagram designed using the empirical design method[/align] To achieve two unloading points, the contacts of X3 and X4 are connected in parallel to drive Y1 and T1. To avoid a brief unloading action when the trolley passes X3, the normally closed contacts of Y2 and Y3 are connected in series with the coil of Y1. The trolley starts to reverse from X4. To prevent R1 from being set again when passing X3, which would cause the trolley to fail to stop when it first reaches X3 in the next working cycle, the normally closed contact of Y3 is connected in series in the starting circuit of R1. To achieve periodic operation, an internal relay R0 is set in the program. Its normally open contact is connected in parallel with X0 to drive Y0. When X0 is turned on, R0 is set, preparing for the start of the next cycle. When the stop button SB2 is pressed, X1 is disconnected, R0 is reset, and the system stops working after completing the current cycle. The quality and time consumed by programs designed using the experience-based design method are highly dependent on the designer's experience. It is effective for simpler programs, providing a quick and easy result. However, the resulting programs often lack rigor and are prone to omissions, leading to poor readability of the ladder diagrams. It is only suitable for simple programs and is difficult for beginners to understand and master. [align=center] Figure 5 Ladder diagram designed using set/reset instructions[/align] 4.2 Set/Reset Instruction Design Method A ladder diagram program designed using set/reset instructions is shown in Figure 5. In the program, each process corresponds to an internal relay. The normally open contact of the internal relay corresponding to the previous step is connected in series with the contact corresponding to the switching condition, serving as the condition for setting the internal relay corresponding to the subsequent step. The normally open contact of the internal relay corresponding to the subsequent step serves as the condition for resetting the internal relay corresponding to the previous step. If the trolley is at position A, pressing SB1 activates X0, sets R1, and drives Y0 to begin loading and timing. The normally open contact of R1 is connected in series with the normally open contact of T0 as the setting condition for R2, and the normally open contact of R2 is used as the reset condition for R1. When the timing expires, R2 sets and drives Y1, the trolley moves forward, and R1 resets. To enable the system to work periodically, R8 (R8 sets and drives Y3, the trolley moves backward) and the normally open contact of R0 are connected in series, and in parallel with X0 as the condition for setting R1 again. For simple sequential control systems, the output relays can also be set or reset directly. This method eliminates the need for additional internal relays to remember the number of times the trolley passes X3, and the logical sequence conversion relationship is very clear, making it easier for beginners to understand and master. 4.3 Hold Instruction Design Method: The ladder diagram program designed using hold instructions is shown in Figure 6. This programming technique is basically similar to the programming technique using set/reset instructions. The difference is that the set control terminal of the hold instruction cannot have multiple contacts connected in parallel, so an internal relay R9 is added. When starting up or working in a cycle, R9 is set, which in turn sets R1. In addition, the number of program steps written using the hold instruction is much less than the number of program steps written using the set/reset instruction, and the internal memory occupied is greatly reduced. [align=center] Figure 6 Ladder diagram of hold instruction design[/align] 4.4 Left shift register instruction design method [align=center] Figure 7 Ladder diagram of left shift register instruction design[/align] The function of the SR left shift register instruction can only shift the 16-bit data of the internal relay WR left by 1 bit. This instruction mainly processes the data input, shift pulse input, and reset input signal. The data is shifted up bit by bit on the rising edge of the shift pulse input. The highest bit overflows. When the reset signal arrives, all contents of the register are cleared[4]. The ladder diagram designed using the SR left shift register instruction is shown in Figure 7. The data input control terminal of the SR instruction is the normally open contact of R1, the shift pulse input control terminal is the normally open contact of R2, and the reset signal input control terminal is composed of the normally open contacts of X2 and R37 (R37 sets to drive Y3, causing the trolley to move backward) and the normally closed contact of R0 connected in series. Initially, at the home position A, since all bits of WR3 are 0, R1 is set. When X0 is turned on, R0 is set, and R2 is turned on for one cycle, 1 is shifted to the last bit, R30 is set to drive Y0, starting loading and timing, while R1 is reset. When the timing period is up, R2 is turned on for another cycle, R31 is set to drive Y1, and the trolley moves backward. Whenever R2 receives a signal, the data in each bit of WR3 in the internal register is shifted to the left once in sequence, so that R30 to R37 are energized in sequence. The system works in this order until one cycle is completed, R1 is reset, and the system starts the next cycle. When X1 is turned on, R0 is reset. After the system completes the work of this cycle, all contents of WR3 are cleared, and the system stops working. The ladder diagram designed by this method looks simple, and the design efficiency is further improved, making it easy for beginners to understand and accept. This design method can be used not only in the automatic reciprocating sequence control circuit of the feeding trolley, but also widely in the sequential control circuit of colored lights. 4.5 Step Instruction Design Method Step instructions are instructions specifically provided for sequential control design. Step instructions execute each program segment in a strict order. Each step program segment is relatively independent. Only after the previous program segment is executed can the next program segment be activated. Before executing the next program segment, the PLC must reset the previous step process to prepare for the execution of the next program segment. The output relays, internal relays, timers, counters, etc. used in each program segment are not allowed to have the same number; otherwise, it will be treated as an error. The ladder diagram program designed using step instructions is shown in Figure 8. X0 and X2 are connected in series as the start step signal, X2 and the normally closed contact of R0 are connected in series as the end step signal, and X2 and the normally open contact of R0 are connected in series as the start signal for periodic cyclic step operation. T0, X3, T1, X2, T0, X4, and T1 are the transition control signals between processes 0 and 7, respectively. [align=center]Figure 8 Ladder diagram designed using step instructions[/align] This programming technique is easily accepted and mastered by beginners. For experienced engineers, it also improves design efficiency. The program is easy to debug, modify, and read, and convenient to use. It should be given priority in sequential control design. This method is widely used in industrial automation control. 5 Conclusion This paper proposes five PLC programming methods for an automatic reciprocating sequential control system for a material handling trolley, each with its own characteristics. In practical applications, one method can be selected to design the program according to the actual situation to adapt to the control requirements of different occasions. Practice shows that these programming methods are easy for designers to accept and master. They can be used to design arbitrarily complex sequential control programs, thereby improving design efficiency and shortening the production cycle. References : [1] Chang Dounan, ed. Programmable Logic Controller Principles, Applications and Experiments [M]. Beijing: Machinery Industry Press, 1998. [2] Dai Minghong, ed. Electrical Control and PLC Applications [M]. Beijing: Beijing University of Aeronautics and Astronautics Press, 2007. [3] Huang Jing, ed. Electrical Control and Programmable Logic Controllers [M]. Beijing: Machinery Industry Press, 2004. [4] Li Xiangdong, ed. Electrical Control and PLC [M]. Beijing: Machinery Industry Press, 2007. About the author: Sun Keli, male, born in December 1968, Han nationality, from Taizhou, Jiangsu Province, works at Taizhou Electromechanical Higher Vocational Technical School, senior teacher, maintenance electrician technician, mainly engaged in teaching and research work in mechatronics, and has published many papers in provincial and above journals. Contact number: 13852863593, Postcode: 225300