Introduction 1 The hydraulic screw down control system (hydraulic AGC system) of 2500 mm four high reversing rolling mill has been reformed.The control system was designed and debugged by the State Key Laboratory of Northeast University and put into use in the same year.After the transformation of AGC steel plate and the poor quality of the hydraulic plate will be further improved. 2. Composition of hydraulic AGC system The system is composed of two levels of computer control system. The first level is the basic automation system of HAGC (hydraulic automation control) and the second level is the HAGC process control system.The process computer of AGC process control system of rolling mill adopts one American Compaq server system with mirror disk array of 3 * 40Gb; the process computer is connected with the basic automation level PLC through industrial Ethernet, and two terminals are connected at the same time, which are respectively installed in the rolling mill and the process machine room for process monitoring and control. The main functions of basic automation in rolling area are AGC and rolling control; Siemens S7-400 / fm458 PLC and PROFIBUS-DP remote I / oet200 system are adopted.The top cap sensor and hydraulic cylinder displacement sensor used in the system are magnetostrictive digital absolute displacement sensors made by MTS Systems Corporation of the United States. They have the characteristics of high precision (resolution 2 μ m), high temperature resistance, high protection level, non-contact, no wear, calibration free, SSI interface output.The top cap sensor is used to measure the displacement of the screw, one on the operating side (OS) and one on the drive side (DS).The sensor is installed on the top of the screw, and a hole is opened in the center of the screw, which can directly measure the static and dynamic displacement of the screw. Each hydraulic cylinder is equipped with two MTS displacement sensors. The external diagonal installation can not only measure the displacement of the hydraulic cylinder, but also measure the deflection of the hydraulic cylinder. This external installation method is convenient for inspection, maintenance and replacement.This system adopts German HYDAC oil pressure sensor with high precision and stable output.The displacement sensor of hydraulic cylinder is used as the position feedback of hydraulic screw down system. If the position feedback is not accurate, it will directly affect the roll gap in the rolling process, resulting in abnormal roll gap, which will directly affect the production of medium plate mill. 3. Problems and Solutions 3.1 inaccurate roll gap display On March 25, 2003, the factory pulled out the hydraulic cylinder at the operation side to replace the spherical pad. On the afternoon of March 26, the mill operator reported that the roll gap was not displayed correctly and there was roll gap running. Through PLC on-line monitoring and analysis, we found that the MTS displacement sensor reading at the outlet of the hydraulic cylinder at the operation side began to change when the reading of MTS displacement sensor at the outlet of the hydraulic cylinder at the operation side did not change. The typical value of the jump was from 70However, when it jumps to 60, sometimes it can jump back immediately, and sometimes it will be stable for a period of time in this position. Moreover, this kind of jump has no rule. It can occur in the process of rolling, in the gap of rolling, at the moment of steel biting, at the moment of throwing steel, when the rolling mill is rotating, or when the screw down motor is running, this kind of liquid has no ruleThe instability of pressing position leads to the instability of roll gap, which seriously affects the normal production. We have to stop the use of hydraulic screwdown system and only adopt electric rolling.By tracking, we get the trend chart of sensor readings, as shown in Figure 2.We take the following several methods for troubleshooting. 3.2 troubleshooting (1) The clock signals and data signals of four displacement sensors of hydraulic cylinders are measured. The clock signals are about 2.8V DC and 0.123v AC; the data signals are about 2.8V DC and 0.245v AC.It can be judged that PLC clock and MTs data bit receive and receive pulse circuit are normal. (2) Check that the mounting screws of the outlet sensor on the operation side are in good condition.It indicates that there is no problem in the installation of the sensor. Figure 2MTS displacement sensor reading trend (3) Check the external cable of the sensor, there is no short circuit and the insulation to the ground is excellent.Check the wiring of the hydraulic cylinder junction box and the terminal box on the top of the archway. The wiring condition is good, and the shielding layer of all signal lines is reliably grounded, but the phenomenon is still. (4) When the displacement sensor at the outlet of the operation side is replaced, the reading of the new displacement sensor still changes randomly. (5) It is found that the MTS at the outlet of the operation side still has random jump when the wiring of the displacement sensor at the operation side is connected in the junction box.It indicates that there is no problem with the cable between the terminal box on the top of the archway and the junction box of the hydraulic cylinder. (6) It is found that the MTS at the outlet of the operation side still jumps randomly, and the inlet sensor is normal.The drive side sensor is also normal.The MTS displacement sensor is good. (7) In the PLC cabinet, the channels connected to the two displacement sensors on the operation side were exchanged. It was found that the outlet MTS still had random jump and the inlet sensor was normal.Indicates that the template channel is OK. (8) The special line between the outlet MTS sensor on the operation side was replaced from the top of the archway to the junction box of the hydraulic cylinder. It was found that the MTS at the outlet of the operation side still changed randomly, and the inlet sensor was normal.Again, there is no problem with this section of cable. (9) In the PLC cabinet, the channels connecting the displacement sensors on the operation side and the transmission side were adjusted as a whole. It was found that the MTS at the outlet of the hydraulic cylinder on the operation side still showed random jump, and other sensors were normal.There is no problem with the template. (10) We carefully analyzed each test process and the installation position of the sensor, and found that the environmental temperature of the displacement sensor at the outlet of the hydraulic cylinder on the operation side is the worst.Therefore, when the outlet sensor on the operation side is filled with cooling water, the phenomenon of violent jump will disappear immediately. If the hydraulic system continues to be put into operation, the sensor at the operation side will start to jump again after four or five days. (11) The fm458 and exm438 templates were replaced with new templates, and the MTS at the operation side outlet still showed random jump phenomenon, which further showed that there was no problem with the PLC template. (12) A new MTS displacement sensor is taken out to fix its position, and it is directly connected to the terminal box on the top of the mill housing. After the system is powered on, we find that the reading of the sensor with the fixed position should remain unchanged, but it will jump randomly with the rolling process.So we took the sensor to the back of PLC electrical cabinet and directly connected to the PLC template channel. After continuous tracking and observation, we found that the sensor reading was still and very stable. From this, we concluded that only the cable between the PLC electrical cabinet and the terminal box on the top of the rolling mill housing did have unmeasurable interference. 4 treatment method The connection cables of MTS displacement sensor are all re laid with separate bridge, all cables are replaced, bridge cover is covered, all shielding layers are reliably grounded, and interference signal is completely eliminated.It has been proved that after the improvement measures are completed, the similar phenomenon has never happened again. Conclusion During the installation, the hardware should be strictly controlled to prevent the hidden danger and the legacy of the hardware.After the fault occurs, it should be recorded in detail, so as to prevent the occurrence of false signals or phenomena, resulting in numbness and carelessness, and affecting the real troubleshooting. Only in this way can we maintain each equipment well
