1 Introduction Rolling double-sided shear unit is one of the key equipment in the finishing workshop of steel rolling mill. It is mainly used for conveying steel plates, cutting longitudinal edges, fixing width, cutting broken edges and collecting broken edges[1]. The double-sided shear unit mainly includes the pre-shear roller table, the post-shear roller table, the main shear and the pinch roll, etc. Among them, the pinch roll is a very important piece of equipment. In the past, most double-sided shear pinch roll control systems in steel rolling mills adopted analog control. This system has disadvantages such as low control accuracy, complex wiring and difficult system maintenance. In addition, the instruments and speed controllers do not have communication interfaces, which makes the control system form an "information island" and the information cannot be comprehensively utilized and integrated. With the development of fieldbus technology, technical support has been provided for the development of a new generation of pinch roll control system. For this reason, a fully digital double-sided shear pinch roll control system based on profibus-dp bus was developed. This system uses a DC speed controller with profibus-dp communication interface and distributed I/O. It has the advantages of high control accuracy, open communication structure, simple wiring and convenient maintenance, which meets the needs of production and management. 2. System Process Flow The double-sided shear with rollers consists of 4 sets of pinch rollers, 2 units per set, for a total of 8 units. These are located on the moving and fixed sides of the inlet and outlet ends of the double-sided shear, respectively called the inlet fixed side (upper/lower rollers), inlet moving side (upper/lower rollers), outlet fixed side (upper/lower rollers), and outlet moving side (upper/lower rollers). The lower pinch rollers on each side are connected by a synchronous shaft. The four pairs of pinch rollers continuously adjust their speed and perform lifting/lowering actions according to the requirements of the steel plate feeding. They are the main transmission component of the double-sided shear unit and the primary control object in the renovation project. The spatial location diagram of the pinch rollers is shown in Figure 1. [align=center] Figure 1 Spatial Location Diagram of Pinch Rollers[/align] The process and operation flow of the double-sided shear control system are mainly achieved by the pinch roller unit and the shear unit through a series of closely coordinated and mutually integrated actions. According to the process requirements, the system has three shearing speeds: 24 cuts/min, 18 cuts/min, and 12 cuts/min, and three step lengths: 1100mm, 1200mm, and 1300mm. When shearing steel plates, different shearing step lengths and shearing speeds must be selected according to the plate thickness and length, and the shearing step lengths for the head and tail of the plate must be adjusted in real time according to the actual detected position of the steel plate. To simplify the explanation, the commonly used shearing speed of 24 cuts/min is used as an example to analyze the system. The specific process of one shearing cut is shown in Figure 2. [align=center] Figure 2 Process flow of shearing steel plates[/align] In Figure 2, θ is the crankshaft angle value during the shearing process. All the above processes are completed according to the control logic within one revolution of the crankshaft (360°). The shearing speed is 24 cuts per minute, which means that the above steps must be completed within 2.5 seconds, of which the plate feeding time is about 1 second. The steel plate conveying is accomplished by eight pinch rollers, divided into inlet and outlet pinch rollers. The upper roller of each group can be raised or lowered according to the position of the steel plate on the production line. Each pinch roller is driven by a DC motor, thus requiring synchronous control of the eight pinch roller drive motors. Other processes are controlled by the main control PLC in the original system. The plate feeding process must closely coordinate with the actions of other equipment within one revolution of the crankshaft, requiring timely feeding, rapid stopping, and delivery to the correct position within a specified time. Therefore, the system must ensure that various measurement and control signals and control signals of the eight drive motors can be transmitted in real time. For such a system with high real-time requirements and multiple distributed control objects, an effective solution is to use PROFIBUS-DP to construct a fieldbus control system. 3 PROFIBUS-DP Fieldbus Technology PROFIBUS is the fieldbus standard of the German national standard DIN19245 and the European standard EN50170, and is also one of the international fieldbus standards. The reference model is the ISO/OSI model. There are three series: PROFIBUS-DP, PROFIBUS-FMS, and PROFIBUS-PA. The DP series is used for high-speed transmission between distributed peripherals and is suitable for applications in the field of manufacturing automation. 3.1 Bus Protocol PROFIBUS-DP uses layers 1 and 2 of the OSI model and the user interface; layers 3 to 7 are not described. This optimized, high-speed, and inexpensive communication protocol is mainly designed for communication between automatic control systems and device-level distributed I/O. It can replace 24V and 0-20mA signals to achieve high-speed data transmission in distributed fully digital control systems. 3.2 Bus Access Methods The bus adopts two access methods: token passing between masters and master-slave mode. The main application of PROFIBUS-DP is the periodic data exchange between master and slave stations in the master-slave mode. The working characteristics of the master-slave mode are that one master station can control multiple slave stations on the bus. The master station establishes a logical link with each slave station. The master station issues commands, and the slave station responds. The slave station can continuously send multiple frames until there is no more information to send, the sending limit is reached, or it is stopped by the master station. 3.3 Transmission Technology PROFIBUS-DP uses RS-485 and fiber optic transmission technology. Fiber optic is used in cases of severe electromagnetic interference. The transmission rate is 9.6kbps to 12mbps. 3.4 System Configuration Each PROFIBUS-DP system may include three different types of devices. (1) Type I DP master stations (DPM1) are some central stations (such as PLCs, PCs) that exchange information with slave stations according to a specified communication cycle; (2) Type II DP master stations (DPM2) are mainly some programming and monitoring stations, mainly used for configuring, starting and running the monitoring system; (3) Slave stations are some peripheral devices that are directly connected to I/O signals, such as inputs, outputs, drivers, etc. 4 System Hardware Composition and Functions of Each Part As shown in Figure 3, the control system is a fieldbus control system based on PROFIBUS-DP, adopting a single master-slave bus topology. All hardware devices are Siemens products. [align=center] Figure 3 System Hardware Structure Diagram[/align] 4.1 S7-400 PLC Master Station System The S7-400 PLC station is the most important part of the entire system. It includes a power supply module, a CPU module, a digital input module (DI), a digital output module (DO), and a counting module (FM). These modules are installed on the rack (UR) in a certain order. It mainly performs the following functions: (1) Completes information exchange with the main shear control PLC. The S7-400 PLC receives signals such as start, stop, and pressure plate pressing of the pinch rollers from the main shear control PLC. It feeds back signals such as driver ready, driver fault, motor and fan ready, and motor and fan fault to the main shear control PLC. (2) Controls the operation of each pinch roller according to the process requirements of the double-sided shear unit and the signals from the main shear control PLC. (3) To ensure that the steel plate does not deviate during the shearing process, the 8 pinch roller motors are synchronously controlled. The control method is as follows: When the steel plate head enters the inlet end of the double-sided shear, the two upper pinch rollers press down, and the steel plate moves forward. The lower pinch roller on the fixed side of the inlet end is speed controlled, and the other three inlet-side pinch rollers follow the torque. At this time, the inlet-side pinch rollers are controlled in a "1 master 3 slave" mode. When the steel plate head enters the outlet end of the double-sided shear, the two upper pinch rollers on the outlet end press down. The lower pinch roller on the fixed side of the outlet end is speed controlled, and the other seven pinch rollers follow the torque. The eight pinch rollers are controlled in a "1 master 7 slave" mode. When the tail of the plate leaves the inlet end, the two upper inlet rollers lift up, the lower pinch roller on the fixed side of the outlet end is speed controlled, and the other three outlet-side pinch rollers follow the torque. The outlet-side pinch rollers are controlled in a "1 master 3 slave" mode. 4.2 6ra70 Fully Digital DC Speed ​​Regulator The 6ra70 fully digital DC speed regulator is a drive device for a DC motor and is one of the core units of the system. The main circuit adopts a three-phase bridge anti-parallel circuit, and the excitation circuit is a single-phase semi-controlled bridge circuit. This system is a logic-free, reversible system, capable of frequent forward and reverse operations, and has the mechanical characteristic of operating in four quadrants. In this system, it acts as an intelligent slave station connected to the PROFIBUS-DP network. Its main responsibilities include receiving control commands from the PLC, feeding back necessary information to the PLC, and controlling the pinch roller motor and fan. The master PLC establishes PROFIBUS-DP master-slave communication with the speed controller via the CBP2 communication module. 4.3 Distributed I/O Since each speed controller is installed in a different control cabinet, for ease of installation and maintenance, each speed controller is equipped with a distributed I/O module—ET200M. The ET200M intelligent slave station uses the IM153 as the interface module to connect to the PROFIBUS-DP bus and is responsible for processing some switch signals from the speed controller cabinet and control panel. 5. Implementation of the Control System 5.1 PROFIBUS-DP Communication between 6RA70 and PLC (1) Network Data Structure In the PROFIBUS-DP network, the master station uses a master-slave mode and usually exchanges data with the transmission device periodically. In the PROFIBUS-DP data structure, the useful data of the periodic channel of the 6RA70 speed control device is defined as a parameter process data object (PPO). The parameter process data object is up to 14 words long and is divided into two areas: parameter data (PKW) and process data (PZD). They transmit data in their respective messages, as shown in Figure 4. The parameter area is used to read and write the parameters of the speed control device, such as reading faults and parameter characteristic information. The process data is used to coordinate the work of the transmission device and other units in the automation system, mainly including control words, set values, status words, and actual values. [align=center] Figure 4 Parameter Process Data Object (PPO1 type)[/align] There are 5 types of PPO: the first category is the parameterless area, with process data of two words (PPO3) or six words (PPO4); the second category is the parameter area, with process data of two words (PPO1), six words (PPO2) or ten words (PPO5). (2) Setting of PPO type When the bus system is started, the PPO used for communication between the PROFIBUS-DP master station and the speed control device can be configured by the master station. The choice of which type of PPO depends on the task of the transmission device in the automation network. Since it is necessary to read and write necessary parameters in the pinch roller control system, such as ramp rise time and fall time parameters, speed/current closed loop selection parameters, etc.; and also to give the speed value through the bus. Therefore, we choose to use the PPO1 type. (3) Implementation of communication After the interconnection between the parameters of the DC speed control device and the process data in the PPO type is set, bus communication can be realized by programming the master station PLC. Because accessing I/O or the input image area using load instructions can only read a maximum of 4 consecutive bytes, while the PPO1 data structure used in this system contains 6 words, the system functions provided by the PLC must be used to implement bus communication. System function SFC14 is used to read continuous data from the DP standard slave, and SFC15 is used to write continuous data to the DP standard slave. In the periodic data exchange mode, PPO data can only be accessed through system functions SFC14 and SFC15. 5.2 Design of Control Program The system program software development environment is Siemens STEP 7 v5.3. The entire system adopts modular program design, that is, the program is decomposed into individual, self-contained program parts. This simplifies program organization, makes large-scale programs easier to understand, and makes program modification and system debugging easier. The overall program structure is shown in Figure 5. [align=center] Figure 5 System Program Module Structure Diagram[/align] In Figure 5, OB1 is the program loop organization block, which will be executed periodically after the S7 CPU operating system is powered on. Because ob1 executes in a loop, the main program of the control program is always placed in ob1. During CPU operation, under normal circumstances, functional blocks such as the process module, the pinch roller motor control module, and the counter control module are called cyclically in ob1 according to process requirements. When a loop interruption or fault interruption occurs, the system will exit the currently running program to handle the corresponding interrupt module. ob1 has the lowest priority and can respond to any interrupt. The functions of each functional block are as follows: (1) The process module is responsible for the process requirements of the double-sided shear and coordinates the operation of each mechanism of the double-sided shear according to the process requirements; Figure 6 is the main program structure diagram of the process module to realize the shearing requirements of the double-sided shear; (2) The inlet roller motor control module consists of four functions (fc), each function completes the basic control of a pinch roller motor. The function of the inlet roller motor control module is to control the start and stop of the four motors of the inlet pinch roller in real time according to the requirements of the process module; (3) The outlet roller motor control module is similar in function to the inlet roller motor control module. Its function is to control the start and stop of the four motors of the outlet pinch roller in real time according to the requirements of the process module; (4) The counter control module completes the control of the inlet lower roller encoder and the outlet lower roller encoder; (5) The cycle interrupt module is used for the acquisition of current and voltage data of each pinch roller motor; (6) The fault interrupt module completes the functions of system function self-check, fault self-diagnosis and fault self-processing. [align=center]Figure 6 Flowchart of the Software Main Control Program[/align] 6 Conclusion The use of PROFIBUS-DP fieldbus network control to replace the traditional analog control method realizes a fully digital distributed control system, representing a qualitative leap in the control system. This system has been in operation for over a year at the Shaogang No. 2 Rolling Mill's double-sided shearing unit without any failures. Practical application shows that this system has the characteristics of simple structure, convenient maintenance, high reliability, and high performance-price ratio, making it highly valuable for widespread application.