The main characteristics of the traction control system for maglev trains are: based on the motion principle of linear synchronous motors, and considering efficiency and power requirements, it is a large-scale, complex, and highly real-time distributed electric drive control system using a long stator linear field motor with stator segment configuration. To avoid the interference problem of thrust loss during the typical transition between two stator segments, two stator windings parallel to the train track are set up, and the segment transition method can solve this problem [LI]. In the actual maglev train operation control system, a frequency converter remote control system based on a PROFIBUS-DP fieldbus network is built to realize data exchange between the master station and slave station in the network and the monitoring of the entire system. This is of great significance for studying and simulating the power switching logic sequence based on the traction power supply system and the traction control of maglev trains using the long stator commutation method. 1. PROFIBUS-DP Network Structure for Analog Control Systems PROFIBUS-DP bus control characteristics with frequency converters: PROFIBUS-DP is a high-speed, byte-level device bus communication system for transmitting digital and analog signals in complex equipment. It typically operates as a periodic data exchange between a master and slave station in an RS485 network. The slave stations in this master-slave polling network use a deterministic bus access technology. Its characteristics are: data exchange between the master and slave stations is initiated by the master device; slave stations have no bus access rights, and all slave stations have the same priority. PROFIBUS-DP offers significant advantages in multi-frequency converter systems: 1) Different devices can use various transmission media in different applications, achieving interconnection of all devices with a single transmission medium; 2) The use of digital signal technology reduces signal interference and enables error detection, greatly improving the reliability and accuracy of signal transmission; 3) The improved intelligence and reliability of frequency converters enhance their on-site data processing capabilities, simplifying data exchange between devices. The combination of the decentralized nature of this type of fieldbus and the inherent intelligence of the frequency converter enhances the reliability of the entire control system; 4) The implementation of the frequency converter control algorithm is achieved by the master station controller. For different applications, only the control software needs to be redesigned, improving the system's flexibility; 5) The open network allows different frequency converter products to be connected to the same network and jointly achieve control. Simulation System Network Control Structure The maglev train system mainly consists of the following subsystems: 1) The operation control subsystem, which completes train safety monitoring, generates train given information and collects actual information, and generates corresponding operation control commands to issue to the traction control subsystem; 2) The traction control subsystem, which receives information and commands issued by the operation control subsystem to complete the control tasks based on the linear synchronous motor as the basic control object, and controls the commutation control of the long stator according to the current operating state; 3) The communication subsystem connecting the various subsystems, functions, and equipment. The maglev train system is a type of network system with a decentralized structure and layered functions. To simulate the long stator commutation process, it is divided into three functional layers: upper computer monitoring, field controllers, and various field execution devices, which are connected through two layers of networks. The industrial Ethernet SIMATIC control system is connected to the industrial Ethernet via CP343-1 and the host PC through a network interface card (NIC). The host computer (safety computer, monitoring computer) uses the functions provided by the Ethernet software SOFTNET provided by SIEMENS and is embedded in the Windows application to complete the data exchange with the SIMATIC system in order to complete the monitoring and safety control functions of the host computer. The PROFIBUS-DP fieldbus network PROFBUS-DP is the backbone network of this system, and the network topology is a linear bus type. The system uses CPU315-2DP as the field master controller in the PROFIBUS-DP network, and at the same time, the digital I/O display system itself is connected to the backplane in the main slot for network communication and program control operation status. The remote I/O slave station of the system adopts the ET200M distributed I/O system, which simulates the switching logic relationship of the power switch station and remote monitoring [2]. The system control object adopts the frequency converter with the UD73 optional module of CT company with PROFIBUS-DP network interface. This module also features an RS485/232 serial communication interface and an operating system for the software control of the frequency converter drive, along with the corresponding drive programming language DPL, enabling local self-control of the frequency converter. To visualize the variable values ​​and statuses of the program execution in each software layer (the STEP 7 control program of the master station in PROFIBUS-DP, and the DPL program for local control of the CT frequency converter group) and facilitate corresponding program debugging, the master station is connected to the program monitoring machine via an MPI interface, and the frequency converter is connected to the frequency converter monitoring machine via the serial port provided by UD73. Implementation of Communication and Monitoring in the PROFIBUS-DP Network Based on the system's hardware and communication requirements, the hardware and network are configured using STEP 7 software. Control and communication are achieved based on the slave station PROFIBUs_DP addresses and central slot I/O addresses assigned according to the hardware configuration. I/O Definitions in the Simulation System Based on the required system functions and the I/O addresses defined during configuration, the definitions of two types of I/O functions are briefly described as follows: 1) The distributed I/O system composed of ET200M completes the frequency converter I/O control. Main I/O functions include: input control such as disable operation, normal operation preparation, start-up, restore previous speed value, restore initial speed, acceleration, and deceleration; output control and monitoring such as operation fault, operation preparation (float ready), normal operation, network device status, and power supply status. For situations with multiple points simultaneously receiving signals, the priority of each control input is implemented in the software according to the simulation system requirements. 2) I/O monitoring port definitions in the central slot: controller's own status (whether control functions can be implemented); Ethernet operation normal; PROFIBUS-DP operation normal, etc. Output monitoring. Data Exchange between the Master Station and the Frequency Converter The optional UD73 module of the frequency converter provides data channels for implementing network control. Table 1 describes the meaning of their data channels and mapping parameters. By setting parameter #20.05 (setting the PROFIBUS-DP address parameter of the frequency converter) in the UD73 module to the address assigned in the configuration, the connection with the master station is achieved. Since the MOVE instruction can only read no more than 4 bytes of continuous data at a time when accessing I/O or process mirror input/output tables, to ensure the continuity of communication data (this continuity ensures that data is sent continuously as a whole without interruption, preventing potentially serious control consequences due to interruption), the master station and slave station typically exchange data in consecutive 8-byte increments. This requires the application of SFC14 (DPRD-DAT) and SFC15 (DPWR-DAT). Below is an example of the application of these two system functions in a practical application. //... CALL "DPRD_DAT" // The master station reads 8 bytes (4 words) from the inverter. LADDR :=W #1#11O // The starting address configured in the input area for reading data from the inverter; RET_VAL :=MW20 // Returns a fault code if an error occurs during function execution; RECORD :=P#M 30.0 BYTE 8 // The destination area for the user data read. Must be the same as the data length configured in the module selected in STEP 7. Only BYTE data types are allowed. //... CALL "DPWR_DAT" // The master station sends 8 bytes (4 words) to the inverter. LADDR :=W #16#100 // The starting address configured in the output address of the module to be written data; RECORD :=P#M 6O. O BYTE 8 // The parameter meaning is the same as "DPRD_DAT", but RET_VAL:=MW22 // Here RECORD is declared as "INPUT"; //... As seen in the table, IN Word3 is not used in the cyclic data channel. If it is mapped to =1 in inverter parameter #2O.04, the data read in is meaningless and should be masked. The PROFIBUS-DP network treats general "distributed peripheral devices" as I/ O modules in the central slot where the master station is located. However, the data types of the I/O modules actually located in the central slot and the remote I/O modules in the STEP 7 software system are different, but the access methods are the same; that is, both can use the MOVE command in STEP 7 to complete data exchange. In actual application, read remote I/O instance: //... L "ET200M_IN" // Remote I/O input symbol variable corresponds to PIW0, T MW 140 is defined in the variable table // Remote I/O input is loaded into internal intermediate relay Mw14O by MOVE; //... Implementation of system monitoring function STEP 7 provides a large number of system monitoring function organization blocks. For the application of this system, organization blocks such as OB86 (subnet, rack or station failure in distributed I/O system), OB87 (communication failure), OB122 (I/O access error), OBI21 (program error) are adopted [6]. Since the local variables in these organization blocks provide information such as error type, code, interrupt priority and interrupt call time of all the faults they handle, in the application, a data structure block (DB) defined for each type of fault is used to record these fault information, such as fault type, code and fault time, etc., and these DBs are scanned in the main application at a certain time period to complete the monitoring of the entire system operation. Meanwhile, it is precisely by calling these organizational blocks that the system control program can still operate under certain fault conditions, thereby ensuring the safety and stability of the entire system. Application of PROFIBUS-DP-based Inverter Control in Analog Control Systems Algorithm Description Long stator commutation technology describes a method for the logical sequential operation of related stator windings based on the current train operating state in time. This process can be considered as a time-based coordinated control problem for multiple motors. Research on long stator winding commutation technology can be simulated through a multi-motor coordinated control system. Different operating conditions require different traction forces, leading to different commutation methods. The corresponding number of associated stator windings, the number of control devices, and the control complexity will all differ. Existing commutation methods mainly include: two-step method (train traction within the station); three-step method (train traction at high speed), etc. Taking the two-step method as an example, the control method of two adjacent long stator windings on one side of a maglev train will be simulated. The two-step process is as follows: When the train is at a certain position in the current stator segment, the current in this stator segment begins to decrease (it must be ensured that the current is zero when the train enters the next stator segment). When the current is zero, the switch is turned off, disconnecting the stator segment from the power supply, and connecting the next stator segment to the power supply, completing the switching. Thrust loss is minimized throughout the process. Preliminary Control Implementation Through analysis of the PROFIBUS-DP-based multi-motor control system, a hierarchical network structure was adopted to simulate the commutation process of a maglev train. Its control structure is shown in Figure 2. Two motors are considered as adjacent stator segments in a two-step method, and their speeds are used as a quantitative description of the thrust change during the switching process. The time interrupt organization block OB35 (time base of 100 ms) is used as the time base for the control software to periodically query and update the status. Counter1 is designed as the time period for the master station to obtain control status information from remote I/O and output control to the frequency converter, while Counter2 is designed as the time period for monitoring the status of each device in the network during non-updating periods. The network control flowchart based on PROFIBUS-DP is shown in Figure 3. Conclusion This paper describes a remote frequency converter control system based on PROFIBUS-DP fieldbus, outlining the system's hardware and software. It realizes communication between the master station and various slave stations, as well as system monitoring tasks. The basic functions of a maglev train motion control simulation system are completed: simulation of various train operation states and long stator commutation methods. The reliability, intelligence, and flexibility of the multi-frequency converter speed control system based on PROFIBUS-DP fieldbus are discussed, providing a reference for solving complex multi-motor coordinated control problems.