introduction
After being cooled by cooling water, the surface gloss and shape of hot-rolled steel plates often fail to meet user requirements, making them unsuitable for direct machining. Especially with increasingly stringent quality requirements for steel plates in recent years, leveling is generally necessary. The steel plates are leveled by hydraulic positive and negative bending roll devices on the rolling mill to reduce or eliminate undesirable shape. The key to leveling lies in the hydraulic bending roll control system. During leveling, the positive and negative hydraulic bending rolls need to switch rapidly, requiring the entire hydraulic control system and the main rolling mill speed control system to have strong real-time performance, quickly adapting to changes in plate shape. Therefore, the hardware for the Nanjing Iron & Steel Group's rolling mill hydraulic bending roll control system and main rolling mill speed control system uses CPU414-2DP and FM458-1DP functional modules.
1. Hydraulic bending roll control system
1.1 Classification of Hydraulic Bending Rollers
The effects of hydraulic bending roll force can be mainly divided into two categories: one is that the bending roll force is in the same direction as the AGC rolling force, which reduces the edge roll gap of the mill, i.e., negative bending, as shown in Figure 1, which is beneficial to reduce or eliminate edge waviness of the strip; the other is that the bending roll force is in the opposite direction to the AGC rolling force, which increases the edge roll gap of the mill, i.e. positive bending, as shown in Figure 1, which is beneficial to reduce or eliminate center waviness of the strip.
Figure 1 Schematic diagram of hydraulic bending roller
1.2 Hydraulic Bending Roller Control Method
The hydraulic bending roll control system utilizes an electro-hydraulic servo valve and controls the hydraulic cylinder's oil pressure through negative feedback from pressure sensors. This operation is not only simple and reliable but also improves the control accuracy of the bending roll force. Currently, hydraulic bending roll control systems can be mainly divided into two categories: manual adjustment and automatic adjustment systems. The control block diagram of the Nanjing Iron & Steel rolling mill's hydraulic bending roll control system is shown in Figure 2. The optimal bending roll force is pre-calculated by the host computer based on the plate shape. During the leveling rolling process, the bending roll force is adjusted in real-time based on the rolling force fluctuation feedback calculated from the rolling force adjustment value. Manual intervention to increase or decrease the bending roll force is also possible via pressure sensors.
Figure 2. Control block diagram of hydraulic bending roll
The actual output pressure is measured, and the measured actual pressure signal is fed back to the control system as a negative feedback quantity to ultimately form a closed-loop control, thereby achieving precise control of the rolling force.
Figure 3 Schematic diagram of the hydraulic control system for positive and negative bending rollers
Figure 3 shows the schematic diagram of the hydraulic control system for the bending rolls of the Nanjing Iron & Steel rolling mill. This hydraulic control system is a typical electro-hydraulic servo control system, characterized by fast response, high power, and high precision. The positive and negative bending rolls and the support roll balancing hydraulic cylinders are mainly controlled by electro-hydraulic servo valves and electro-hydraulic relief valves. The entire system is supplied with hydraulic oil by high-pressure and low-pressure oil pumps. For safety and smooth operation, high-pressure or low-pressure storage tanks are connected before the servo valves to prevent oil circuit fluctuations or sudden interruptions from impacting the entire hydraulic servo system during operation. The electro-hydraulic relief valve provides maximum pressure protection to prevent overpressure. Positive bending is controlled by electro-hydraulic servo valves 1 and 24, negative bending by electro-hydraulic servo valves 2 and 24, and the support roll balancing hydraulic cylinder is controlled by electro-hydraulic servo valves 4 and 12. Electro-hydraulic servo valve 3 and 2.5 electro-hydraulic relief valve provide a back pressure of 2.5 to the piston rod chambers of the positive and negative bending roll hydraulic cylinders and the support roll hydraulic cylinder.
1.3 Hydraulic Bending Roller Servo Valve Integrated Electronic Controller
The control of the electro-hydraulic servo valve is the core of the entire hydraulic bending roll control system. Its function is to convert low-power electrical signals into high-power hydraulic outputs, which are then used by the hydraulic actuator to automate the bending roll control. Figure 4 shows the control block diagram of the integrated circuit of the hydraulic bending roll servo valve board. The integrated circuit of the hydraulic bending roll servo valve board is controlled by two pins, D and E, of the differential amplifier. The ramp generator causes the magnet current to decrease or increase with a delay according to the step of the set value (0 to 10V or 10 to 0V). The time required for the electromagnet current to increase can be adjusted by potentiometer R1, and the time required to decrease can be adjusted by potentiometer R2. By adjusting potentiometers R1 and R2, the impact on the equipment caused by the instantaneous opening or closing of the hydraulic servo valve can be prevented. When the input set value is at its maximum value, the ramp generation time can reach a maximum value of 5 seconds. If the set value is reduced, the ramp generation time will also decrease accordingly. The characteristic curve generator is used to adjust the set value so that the characteristic curve of the servo valve electromagnet current reaches the required value. This can compensate for the nonlinear factors of hydraulics and obtain a linear set value-pressure characteristic curve. The current controller can make the servo valve electromagnet current unaffected by the coil resistance. Potentiometer R3 can be used to change the gain of the setpoint-current characteristic curve and the setpoint-pressure characteristic curve of the servo valve. Potentiometer R4 is used to set the bias current without changing this setpoint. If necessary, the zero point of the valve seat setpoint-pressure characteristic curve can be set. A switching amplifier is used to form the power stage required to control the proportional electromagnet, using pulse width modulation at a pulse frequency of 300Hz. The electromagnet current can be detected by measuring points MP1 and MP2; a 0.352V decrease in voltage across the measuring resistor corresponds to a 1.6A change in electromagnet current. By setting the above control valve parameters, the performance of the servo valve can be optimized, thereby making the electro-hydraulic servo valve more compatible with the hydraulic bending roller control system.
Figure 4. Circuit block diagram of the integrated electronic controller for the hydraulic bending roll servo valve plate.
2. Hardware diagram of the automatic control system for hydraulic bending rollers
Figure 5 shows the hardware configuration block diagram of the automatic control system for hydraulic bending rolls of Nanjing Iron & Steel rolling mill. To achieve accurate control of the hydraulic bending roll control system and accurate measurement of the hydraulic bending roll force, the entire hydraulic bending roll control system is required to have strong real-time performance and be able to respond quickly according to changes in the shape of the plate. For this reason, the hydraulic bending roll control system of Nanjing Iron & Steel rolling mill selects the function blocks FM458-1DP and CPU416-2DP with very fast response speed.
The hydraulic bending roll servo control system mainly consists of two parts: the control section and the data acquisition and processing section. The control section uses a CPU416-2DP as the master station, connected to the monitoring software HMI via MPI, and utilizes an FM458 as a high-speed dynamic response controller. The PLC416-2DP is connected to the ET200M distributed I/O via a PROFIBUS bus. Field data from electro-hydraulic servo valves, relief valves, etc., are acquired through the ET200M and sent to the PLC. The monitoring software HMI realizes the monitoring, data input, and display functions of the entire hydraulic bending roll control system.
Figure 5 Hardware configuration diagram of the automatic control system for hydraulic bending rolls
The FM458 integrates the P-bus, LE-bus, and K-bus, belonging to the D7 series and possessing very fast computing capabilities. It can be used for closed-loop and open-loop calculations, as well as position and speed control. It can be used with the expansion modules EXM438 and EXM448, and they can communicate with each other via LE. The EXM438 is an I/O expansion module that integrates three peripheral interfaces: X1, X2, and X3, used to read and output time-sensitive signals, such as encoder signals, positive and negative bending servo sensors in hydraulic servo systems, and pressure sensor signals. The EXM448 integrates an SLB communication board. Two communication boards, SLB and CBP, are installed on the CUVC expansion slot of the 6SE70 frequency converter for communication. The SLB communication board on the frequency converter and the EXM448 communication expansion module form a series closed-loop circuit to establish communication between the frequency converter and the PLC. To ensure the accuracy of the work roller (hydraulic bending roller) drive control, a control method combining Simolink and PROFIBUS buses can be used. PROFIBUS is used to send speed and torque control commands, while the fiber optic Simolink is used to transmit IGBT trigger and control signals. This further improves the system's response time and control accuracy to the load.
The FM458 and PLC416 are two independent, parallel CPUs that communicate via a P-bus. The FM458 reads and writes data to the CPU416 using function blocks CTR and CTV, while the CPU416 reads data from the FM458 using system function blocks SFC59 and SFC58. Through the combined operation of the FM458 and PLC416, the hydraulic and electrical drives in the bending roll control system can be precisely and quickly controlled in real time based on the plate shape.
3. Summary
The servo valve hydraulic bending roll control system has the dual advantages of hydraulic and electrical systems. It features high power, low inertia, good stability, convenient and fast signal processing, high precision, easy remote control, convenient operation and easy maintenance. It is the core part of Nanjing Iron & Steel's rolling mill, ensuring the quality of strip and creating huge economic benefits for Nanjing Iron & Steel.
