Abstract Smooth stopping is one of the important process requirements of layer winding machines. The accuracy and stability of stopping directly affect the quality and efficiency of layer winding. A stopping control system that combines the material level of the wire storage floating roller with the wire feeding brake has achieved smooth stopping. The implementation method of PLC and frequency converter serial communication control technology in Modbus RTU communication mode and the implementation process of fixed-length stopping and emergency stopping are presented. The debugging results show that the system can meet the design requirements, and the fixed-length stopping error is less than 5%. Keywords Layer winding machine stopping system; Modbus RTU; R4S58 serial communication; PLC; frequency converter As an important mechanical equipment for winding and turning welding wire in the production of finished welding wire, the layer winding machine has been widely used in various welding wire manufacturers. It completes its production process through four units: wire feeding, wire storage, wire arrangement, and wire take-up. Because welding wire requires uniform wire feeding, uniform resistance, and stable arc during welding, the process and winding accuracy of the layer winding machine are highly demanding. It requires high-precision winding of the welding wire and neat arrangement. The process control requirements generally include seven aspects: (1) The design of the variable frequency speed control system for the wire opening and storage system requires constant tension and adaptive linear speed; (2) The servo drive for the left and right movement of the wire guide requires smooth movement and accurate tracking and positioning; (3) The angle adaptive control requires linkage with the host and accurate follow-up; (4) High-precision angle detection and control; (5) Variable angle movement and positioning of the wire guide equipment, and automatic phase switching; (6) Accurate photoelectric detection of fixed length and weight, and automatic and smooth control of stopping through serial communication between the host and the frequency converter; (7) The human-machine interface of the system is designed for easy operation; The frequency converter is controlled by digital communication, which is not only stable and reliable but also highly accurate. Through the serial port of the PLC, the frequency converter communicates with the PLC. In Modbus RTU mode, the smooth stopping control of the layer winding machine is realized. 1.1 System Composition and Principle of Layer Winding Machine Parking Control System The parking control system has functions such as fixed-length parking and emergency parking. Fixed-length parking refers to a parking method when the weight of the wound welding wire reaches the preset weight required by the user. A pre-deceleration parking weight is set. Once the wound weight reaches this value, the take-up motor begins to decelerate. With the cooperation of the braking unit, the equipment stops smoothly when the weight of the welding wire reaches the preset weight, requiring speed and smoothness. Emergency parking refers to the rapid stopping of the main unit when an emergency failure occurs and the emergency stop button is pressed. Emergency parking requires speed and smoothness, and the wire arrangement must not be tangled. The difference between fixed-length parking and emergency parking is that fixed-length parking requires smoothness, speed, and accurate fixed length; emergency parking only requires speed and smoothness. The layer winding machine control system is shown in Figure 1. It consists of a human-machine interface, PLC, frequency converter, take-up motor, wire arrangement mechanism, wire storage mechanism, and wire feeding device. The welding wire is fed out by the wire feeding device, passes through the wire storage mechanism, and is then sent to the wire arrangement mechanism. Finally, it is layer-wound into a reel by the take-up motor. The human-machine interface (HMI) is responsible for setting and displaying various system parameters, including user parameters and debugging parameters. User parameters are used to set the wire diameter, specific gravity, and preset weight of the produced welding wire. These parameters are crucial for fixed-length stopping. Debugging parameters are important parameters set by the debugging personnel during the debugging of the winding machine to optimize some variables affecting the stopping effect and achieve the best stopping effect. These parameters mainly include the winding frequency increase speed, reversing frequency increase speed, winding frequency decrease speed, reversing frequency decrease speed, deceleration material level lower limit, deceleration percentage, braking maximum material level limit, remaining weight g2 during fixed-length braking, trip weight g0, and trip frequency. These parameters play a key role in the stopping effect. The wire storage mechanism is shown in Figure 2. The main components are the tension frame, floating roller, and five proximity switches distributed from top to bottom. These five proximity switches can be designed to provide nine material levels. The floating roller has a sensing plate. When the floating roller reaches a certain proximity switch, the sensing plate can cause the proximity switch to send a signal. The signal is sent to the PLC, which can then detect the material level of the floating roller according to the set material level rules. Figure 2 shows the material level switches from top to bottom as follows: High-High, High-Medium, Medium, Low-Medium, and Low-Low. Material level 2 represents the state when the floating roller leaves the Low-Medium or Low-Low switch, and material level 8 represents the state when the floating roller leaves the High-Medium or High-High switch. Parameters are set via the human-machine interface, the PLC detects the material level in the wire storage mechanism, adjusts the circuit output, and controls the frequency converter and the wire feeding brake mechanism. The control signal sent to the frequency converter is implemented through RS485 serial communication using the MODBUS-RTU protocol. The frequency converter controls the running speed of the main motor, and the brake signal sent by the PLC controls the wire feeding brake device, completing the control of fixed length and emergency stop. 1.2 Control Principle of Parking System The control ideas of fixed-length parking and emergency parking are both based on the detection of material level. According to a certain control algorithm, the deceleration of the main motor and the braking of the wire feeding device are controlled. The control principle is shown in Figure 3. The lower limit of the deceleration material level is first set on the human-machine interface. This lower limit is used as the given parameter of the system. The detected material level signal is compared with the set signal to obtain the deviation signal. After processing by the Pl' controller, the relationship between the output value (M) of the regulator and the deviation value () within one serial communication cycle is: MV = eX KX f (1) Where f is the given frequency of the PLC inverter, and K is the deceleration percentage, then the input signal of the inverter is: The input signal of the MVO inverter is transmitted from the PLC to the inverter through the RS485 serial communication interface. The given frequency is based on the serial communication cycle. The given frequency signal is sent to the inverter after each serial cycle until the frequency sent to the inverter is 0, that is, the take-up motor stops running. Smoothness is an important indicator of stopping control. Only when the material level of the floating roller is stable can the tension of the winding be stable. If the deceleration of the take-up motor is too fast, the material level of the floating roller will drop, and the take-up tension will be too low; if the deceleration of the take-up motor is too slow, the material level of the floating roller will rise, and the take-up tension will be too high. Therefore, the smooth control of the floating roller material level is an important control indicator of the braking process. 2 Software Design 2.1 Setting the Inverter Communication Format VFD The 1B frequency converter has serial data communication capabilities, employing the Modbus Networks communication protocol. The Modbus protocol has two communication modes: ASCII mode and RTU (Remote Terminal Unit) mode. ASCII mode sends a byte as two ASCII characters, while RTU mode transmits data in hexadecimal, with one byte constituting one frame. Therefore, RTU mode is more efficient than ASCII mode. ASCII encoding converts the data to be transmitted into its corresponding ASCII code before transmission, while RTU transmits data directly without conversion. Parameter settings are shown in Table 1. 2.2 Communication between PLC and Frequency Converter The PLC's communication parameter settings must be consistent with the frequency converter's. Communication parameters must be set before the PLC starts running the program to ensure serial communication between the PLC and the frequency converter. Serial communication is implemented through the MODWR instruction in MODBUSRTU mode, as shown in Table 2. The MODWR instruction is one of the dedicated instructions for communication between the PLC and the frequency converter, greatly facilitating software design. The control signals transmitted from the PLC to the frequency converter must correspond to parameter addresses. Different parameter addresses correspond to different functions, as shown in Table 3. Table 3 shows that the parameter address for controlling the start and stop of the frequency converter is H2000, and the given frequency value of the frequency converter is sent to H2001. 2.3 The implementation of fixed-length stop and emergency stop in the layer winding process requires the layer winding machine to achieve automatic and smooth stop when the wound wire reaches the preset weight. The process flow is shown in Figure 4. When the weight of the wound wire is greater than or equal to the starting weight for fixed-length stop, the PLC executes the fixed-length stop program. The stopping process involves controlling the main motor to decelerate according to the material level algorithm, coupled with the electric brake of the wire feeding mechanism. The electric brake is limited by the maximum material level; once the material level exceeds this maximum limit, the electric brake is released. As the brake is released, the wire storage level drops again. At this point, the PLC determines whether further deceleration and stopping are needed, and whether the electric brake is required. This cyclical judgment continues until the wound weight reaches the preset weight and the given frequency is reached, ultimately achieving a stable fixed-length stop for the layer winding machine. Emergency stop is a parking method in an emergency. It requires that the automatic and smooth stop without loss of fixed length and weight can be achieved in an emergency. When the emergency stop switch is pressed, the PLC executes the material level control algorithm program to realize the deceleration of the main motor, the frequency of the inverter is rapidly reduced to 0, and with the cooperation of the electric brake of the wire feeding mechanism, the entire layer winding machine stops running. 3 Conclusion The layer winding machine parking control system based on Modbus RTU communication realizes the emergency stop of the layer winding machine in an emergency, and also achieves the requirement of fixed length and weight smooth stop. The inverter in the system has Modbus RTU communication protocol, which can form a master-slave system with the PLC to realize real-time control of motor speed. The control system using this digital communication method is not only stable and reliable, but also has high precision. After on-site debugging and operation, the weight error of the winding wire after fixed length stop is 5%, and the system runs stably. 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