Abstract: This paper describes various detection and testing methods used to investigate the fault phenomena of displacement sensors and their impact on the hydraulic AGC system, ultimately identifying the problem and restoring the normal operation of the hydraulic AGC system. 1 Introduction Nanjing Iron & Steel Group's Medium Plate Plant was built in 1976 and officially put into operation in 1986. In September 1994, a 2500mm four-high reversible rolling mill was added. Its control system was designed, debugged, and put into use by the Design Institute of the First Heavy Machinery Group Corporation, forming a medium plate production line with a three-high Lauter mill for roughing and a four-high mill for finishing. By the end of 2002, it had reached an annual output of 800,000 tons. The product specifications are: 5-25×1500-2200×6000-12000mm, with a maximum rough-edge plate length of 23m. Main varieties include: plain carbon steel plates, shipbuilding plates (Grade A, Grade B, A32, A36), container plates, boiler plates, low-alloy structural plates, and bridge plates (16Mnq, Q345qC, Q235qC), etc. To meet the requirements of variety development and improve product quality, the hydraulic pressing control system (hereinafter referred to as the hydraulic AGC system) of the 2500mm four-roll reversible rolling mill was upgraded in December 2002. This control system was designed, debugged, and put into use that year by the National Key Laboratory of Northeastern University. The upgraded hydraulic AGC system will further improve the difference between steel plates of the same and different grades, better realizing the improvement of steel plate quality. 2. Hydraulic AGC System Composition This system consists of a two-level computer control system. Level 1 is the basic automation system (HAGC) for the rolling mill; Level 2 is the process control system for the rolling mill's HAGC. The process computer of the rolling mill AGC process control system uses one Compaq server system from the USA, with a mirrored disk array of 3*40GB. The process computer is connected to the basic automation level PLC via an industrial Ethernet network, and also has two terminals connected, one installed in the rolling mill and one in the process room, for process monitoring and control. The main functions of the basic automation in the rolling zone are AGC and rolling control; it uses a Siemens S7-400/FM458 PLC and a PROFIBUS-DP remote I/O ET200 system. The block diagram of the computer control system is shown in Figure 1. The cap sensor and hydraulic cylinder displacement sensor used in this system are magnetostrictive digital absolute displacement sensors from MTS Systems Corporat, USA, which feature high precision (2μm resolution), high temperature resistance, high protection level, non-contact, wear-free, calibration-free, and SSI interface output. The top cap sensor is used to measure the displacement of the lead screw, with one on the operating side (OS) and one on the transmission side (DS). The sensor is installed at the top of the lead screw, with a hole in the center, allowing direct measurement of the lead screw's static and dynamic displacement; this is a non-contact measurement method. Figure 1 shows the block diagram of the S7-400/FM458 lead screw control system. Two MTS displacement sensors are installed on each hydraulic cylinder, externally and diagonally. This external installation method measures not only the displacement of the hydraulic cylinder but also its runout, facilitating inspection, maintenance, and replacement. The system uses German HYDAC hydraulic sensors, characterized by high accuracy and stable output. The hydraulic cylinder displacement sensors serve as position feedback for the hydraulic lead screw system. Inaccurate position feedback directly affects the roll gap during the rolling process, leading to abnormal roll gaps that directly impact the production of the medium plate mill. 3. Problems and Troubleshooting Methods 3.1 Inaccurate Roll Gap Display On March 25, 2003, our factory removed the hydraulic cylinder on the operating side to replace the spherical pad. On the afternoon of March 26, the mill operator reported that the roll gap display was inaccurate, showing a phenomenon of roll gap slippage. After online monitoring and analysis using PLC, we found that the MTS displacement sensor reading at the outlet of the operating side hydraulic cylinder began to jump even though the displacement had not changed. The typical jump value was a sudden drop from 70 to 60, sometimes it would jump back immediately, and sometimes it would remain stable at that position for a period of time. Moreover, this jump had no regularity; it could occur during the rolling process, during the rolling gap, at the moment of steel biting, or at the moment of steel ejection. It could occur while the mill was rotating or when the pressing motor was running. This instability of the hydraulic position led to instability in the roll gap, seriously affecting normal production, forcing us to stop using the hydraulic pressing system and switch to electric rolling. By tracking, we obtained the trend graph of the sensor readings, as shown in Figure 2. We adopted the following methods to troubleshoot the problem. 3.2 Troubleshooting (1) Measure the clock signal and data signal of the four hydraulic cylinder displacement sensors. The clock signal is about 2.8V DC and about 0.123V AC; the data signal is about 2.8V DC and about 0.245V AC. It can be determined that the PLC clock and MTS data bit transmit and receive pulse circuit are normal. (2) Check the mounting screws of the operating side outlet sensor. They are all intact. This indicates that there is no problem with the sensor installation. Figure 2 MTS displacement sensor reading trend (3) Check the external cable of the sensor. There is no short circuit and the insulation to ground is excellent. Check the wiring in the hydraulic cylinder junction box and the top sub-box of the archway. The wiring is good. The shielding layer of all signal lines is reliably grounded, but the problem persists. (4) Replace the operating side outlet displacement sensor. The reading of the new displacement sensor still shows random jumps. (5) Swap the wiring of the operating side inlet and outlet displacement sensors in the junction box. It is found that the outlet MTS still shows random jumps. This indicates that there is no problem with the cable between the top sub-box of the archway and the hydraulic cylinder junction box. (6) After swapping the outlet displacement sensor on the operating side with the inlet displacement sensor on the transmission side, it was found that the outlet MTS on the operating side still exhibited random fluctuations, while the inlet sensor was normal. The transmission side sensor was also normal. This indicates that the MTS displacement sensor is good. (7) In the PLC cabinet, the channels connected to the two displacement sensors on the operating side were swapped, and it was found that the outlet MTS still exhibited random fluctuations, while the inlet sensor was normal. This indicates that there is no problem with the template channel. (8) The dedicated cable between the outlet MTS sensor on the operating side and the hydraulic cylinder junction box was replaced, and it was found that the outlet MTS on the operating side still exhibited random fluctuations, while the inlet sensor was normal. This again indicates that there is no problem with this section of cable. (9) In the PLC cabinet, the channels connected to the displacement sensors on the operating side and the transmission side were swapped as a whole, and it was found that the outlet MTS on the operating side hydraulic cylinder still exhibited random fluctuations, while other sensors were normal. This indicates that there is no problem with the template. (10) We carefully analyzed each test process and the installation position of the sensors and found that the ambient temperature of the outlet displacement sensor of the operating side hydraulic cylinder was the worst. Therefore, cooling water was passed through the operating side outlet sensor, and the phenomenon of violent fluctuations immediately disappeared. The hydraulic system was put back into operation, but after four or five days, the operating side outlet sensor started to fluctuate again. (11) The FM458 and EXM438 templates were all replaced with new templates, but the operating side outlet MTS still showed random fluctuations, further indicating that there was no problem with the PLC template. (12) A new MTS displacement sensor was taken out and its position was fixed. It was directly connected to the terminal box on the top of the rolling mill stand. After the system was powered on, we observed that the reading of the sensor with its position fixed should remain unchanged, but it fluctuated randomly with the rolling process. So we took the sensor to the back of the PLC electrical cabinet and connected it directly to the PLC template channel. After continuous tracking and observation, we found that the reading of the sensor did not move at all and was very stable. From this, we concluded that there was indeed an immeasurable interference phenomenon in the cable between the PLC electrical cabinet and the terminal box on the top of the rolling mill stand. 4. Solution All connecting cables for the MTS displacement sensors were re-laid on separate cable trays, all cables were replaced, the cable tray covers were properly installed, and all shielding layers were reliably grounded to completely eliminate interference signals. This improvement proved effective, as no similar issues have occurred since. 5. Conclusion As electrical maintenance personnel, we must continuously learn new knowledge and apply it to our work. We must be courageous in exploring and constantly practicing to better solve various difficult problems encountered in our work. During installation, hardware must be strictly controlled to prevent hidden dangers and future problems. After a fault occurs, it should be recorded in detail to prevent false signals or phenomena from causing complacency and affecting the actual troubleshooting. Only in this way can we maintain every piece of equipment effectively.