Abstract: This article takes the 1 # auxiliary roll of the 1580 strip steel mill coiling machine in a certain steel company as an example to introduce the analysis of the causes of hydraulic servo control system failures, and proposes corresponding preventive measures to ensure the stability and accuracy of the auxiliary roll hydraulic servo control system. It plays a certain role in preventing the failure of the auxiliary roll hydraulic servo control system and improving the service life of the hydraulic cylinder and connected mechanical equipment.
Keywords: servo system; Fault; analysis; prevention
1. Overview of Hydraulic Servo Control System and Winder Auxiliary Roll
The hydraulic servo control system is widely used in the production of strip steel in the metallurgical industry, AWC、AGC、 Servo systems are used in control systems such as loop, bending roller, side guide plate, pinch roller, and auxiliary roll, and servo valves determine the performance and strip quality of the servo system. The hydraulic servo control system, as a high-precision system combining mechanical, electrical automation, hydraulic, and production process technology, integrates multiple characteristics and has the advantages of high control accuracy, fast response speed, flexible signal processing, high output power, compact structure, and difficult maintenance. How to improve the reliability of servo control systems is crucial, and ensuring the accurate, fast, and stable operation of the control system is of great significance.
During the process of coiling steel billets in the 1580 strip steel mill, when its head bites into the finishing mill units F1, F2, or F3, the coiling machine is already in a ready working state. At this time, the auxiliary roll wraps around the drum, and under the control of the hydraulic servo control system mechanism, the biting thickness of the strip steel between the auxiliary roll and the drum is maintained at 1.5 times. When the strip steel enters the coiler, a closed path is formed between the auxiliary roll and the drum with the help of a guide plate device, allowing the strip steel to be wound onto the drum in sequence. After the first lap of the strip steel is rolled into the drum, the auxiliary roll is controlled by stepping. After the strip steel is rolled 3-5 laps, all the auxiliary rolls are opened and enter the normal winding state. The strip steel forms a stable tension between the drum and the finishing mill. When the tail of the strip steel leaves the F7 precision rolling mill, the auxiliary roll closes and presses against the outer coil until it opens during unwinding. Refer to the schematic diagram (Figure 1).
Figure 1 Schematic diagram of coiling equipment for 1580 strip steel mill
2. Working principle of the hydraulic servo control system for the auxiliary roll of the coiler
The main equipment of the coiling servo hydraulic station is five (four in use and one backup) inclined disc constant pressure variable piston pumps. When the system is working normally, the pressure oil discharged by the main pump is divided into two paths through the three-way outlet of the pump: one is the main oil path with a pressure of 21Mpa; The other circuit is the pressure reduction control circuit, which reduces the pressure to 12Mpa and supplies oil specifically for the pilot part of the servo valve. When the rolling mill is working normally, the pressure oil in the main oil circuit is controlled by a servo valve to operate the hydraulic cylinder, in order to achieve the purpose of positioning the auxiliary roll of the coiler and controlling the roll shape of the slab; When the rolling mill is shut down for maintenance, the servo valve is closed, the bypass circuit is opened, and the pressure oil in the main oil circuit is controlled to the maintenance position through the electro-hydraulic directional valve; When rolling steel normally, open according to the position requirements of the auxiliary roll to the maximum position or maintain the roll gap position 1.5 times the thickness of the strip steel and wait for the steel to be rolled. At this time, the servo valve opens, and the two ends of the bypass circuit electro-hydraulic reversing valve lose power, and the valve core stops at the middle position. Refer to the schematic diagram (Figure 2).
Figure 2 Hydraulic schematic diagram of the 1 # auxiliary roll of the 1580 strip steel mill coiler
3. Analysis of faults and causes in the hydraulic servo control system of the auxiliary roll of the coiler
3.1 Hydraulic cylinder oscillation of the auxiliary roll and inability or prolonged pressing of the auxiliary roll
3.1.1 Analysis of the causes of hydraulic cylinder oscillation in the auxiliary roll (see Figure 3)
Figure 3 Vibration Curve of Hydraulic Cylinder for No.1 Auxiliary Roll of 1580 Strip Steel Mill Winder
When the auxiliary roll is waiting for the steel strip to be rolled, the automatic positioning control of the hydraulic cylinder is inaccurate and oscillates or shakes within a certain range, causing interference between the drum and the auxiliary roll. The main reason for this phenomenon is:
(1) The servo valve is controlled by a pilot valve to regulate the movement of the main valve core, and the auxiliary roll needs to maintain high-frequency sine wave oscillation to ensure fast response. The servo valve of 1580 steel mill is generally in a sinusoidal oscillation of 2000HZ when not specified. Therefore, when the edge of the core sleeve of the pilot valve core is damaged due to wear or other reasons, the valve body will experience internal leakage, and the servo valve will experience periodic abnormal oscillation.
(2) Due to the tight fitting structure between the pilot valve core and valve body of the servo valve, the servo system has extremely high requirements for oil products. When small particles in the hydraulic system get stuck in the gap, it can affect the normal operation of the valve body and cause jamming and other phenomena. According to the principle of position control analysis, oscillations usually occur at the stuck area. So the cleanliness of hydraulic oil is also an important factor in generating system oscillations.
(3) When the servo valve is put into use, the integral gains of the inner and outer loops have been set. The precise gain setting value can ensure the response speed of the servo valve and also prevent (or minimize) overshoot when reaching the given reference value. However, during the use of servo valves, the response characteristics of servo valves may change due to various reasons. Generally speaking, over time, the response speed of servo valves will decrease. In this way, the gain value set by the servo valve amplifier will gradually increase relative to the initial set value. At this point, overshoot will occur, and when the gain is too large, system oscillation will occur.
(4) The pressure difference △ P between the inlet and outlet of the servo valve is too high; The mechanical connections of the hydraulic cylinder pin shaft, cylinder base, and other parts are not firm, and the rubber hose from the valve station to the cylinder is too long and elastic. The distance between the control valve station and the cylinder is too far; The ramp time was inserted into the TDC closed-loop control program, and the differential and integral gain settings were too low.
3.1.2 Analysis of the reasons why the auxiliary hydraulic cylinder cannot be pressed against (see Figure 4)
Figure 4: State curve of the auxiliary hydraulic cylinder when the pressure does not reach the set value
When the hydraulic cylinder of the auxiliary roll presses against the roll to a roll gap of 1.5 times the thickness of the strip steel, it often occurs that the roll gap cannot reach the set value. The main reason for this phenomenon is:
(1) The hydraulic system oil pressure has not reached the required pressure for driving the load, and there is severe internal leakage in the valve body and cylinder, resulting in the rodless chamber of the cylinder not reaching the required pressure for driving the load. The pilot control pressure of the servo valve cannot meet the system requirements.
(2) The integral gain setting value of the TDC control system program is too high, the gain and bias direction are incorrect, and the proportional and integral setting values are too low.
3.1.3 Analysis of the Reasons for the Excessive Compression Time of the Auxiliary Hydraulic Cylinder (see Figure 5)
Figure 5: The auxiliary hydraulic cylinder has been pressed for too long
The main reason for this phenomenon is that when the auxiliary roll hydraulic cylinder presses against the drum to maintain a gap of 1.5 times the thickness of the strip steel with the roll gap, it takes too long to reach the set value
(1) The pressure sensitivity of the control valve is too low, and the signal collection time of detection devices such as magnetic rulers and pressure sensors is too long.
(2) The proportional gain setting value of the TDC control system program is too low, and the bias current is incorrect.
3.2 Analysis of Hydraulic Impact Caused by Hydraulic Servo System of Auxiliary Roll
When hydraulic shock occurs in the system, the peak pressure of the oil can sometimes reach 3-4 times the normal pressure, causing damage to hydraulic components such as control valves and even pipelines in the system, and causing abnormal signals from pressure sensors and cylinder magnetic rulers, resulting in the system not working properly and affecting the quality of strip steel products; Hydraulic impact can also cause strong vibration and impact noise, causing the oil temperature to rise rapidly and easily causing system leakage, seriously affecting the stability and reliability of hydraulic system performance. The main reasons for this phenomenon are:
(1) During the steel rolling process, the assisting force acts on the assisting roller and is transmitted to the servo cylinder, causing it to be impacted, which in turn causes vibration in the valve frame and pipeline, resulting in poor stability of the hydraulic system and oil leakage in the pipeline, affecting the smooth progress of production.
(2) The zero drift of the servo valve is too large, which can easily cause system vibration, resulting in roller gaps and pressure fluctuations, leading to poor system stability.
(3) The servo control valve operates too quickly, the differential gain value is too high, and the step response speed is too fast.
3.2 Analysis of Reasons for Temperature Rise in Hydraulic System
Excessive temperature in hydraulic systems can reduce the performance of hydraulic equipment, shorten the lifespan of components, increase the number of failures, and lead to the aging and failure of sealing rings, affecting system performance. The main reasons for the high rise in hydraulic system oil temperature are:
(1) The cooling effect of the hydraulic station's circulating cooling system is poor and the hydraulic oil cannot flow normally. There are significant throttling or damping on the pipelines.
(2) The working pressure adjustment of the main oil pump in the system is inconsistent, and the auxiliary roll hydraulic system supplies oil to the hydraulic system in parallel with four variable plunger pumps. If the working pressure output value of one of the pumps is greater than that of the other three pumps, only this pump will supply oil to the system normally, while the other three pumps will supply oil abnormally or continuously circulate in the pump body, causing the pump body to continuously heat up and the system oil temperature to continuously rise.
(3) The internal leakage of pumps or valves is also one of the important reasons for the increase in oil temperature. Whether it is a pump or a valve, when internal leakage occurs, a large amount of heat is generated due to friction, which is transferred to the oil and will inevitably cause system temperature rise.
4. Preventive measures for faults in the hydraulic servo control system of the auxiliary roll of the coiler
4.1 Countermeasures and measures for the hydraulic cylinder oscillation of the auxiliary roll and the inability or prolonged pressing of the auxiliary roll
When the hydraulic cylinder of the auxiliary roll vibrates, it poses a great threat to the hydraulic system. How to prevent vibration and how to eliminate it after it occurs? According to the maintenance routine of equipment, prevention should be the main focus, and elimination should be secondary. Based on the actual experience of the 1580 strip steel plant, the main preventive measures for the oscillation of the hydraulic cylinder of the auxiliary roll and the inability or prolonged pressing of the auxiliary roll are as follows:
(1) When the service life of the servo valve reaches one year or the zero bias current value reaches 1mA or more, the servo valve should be cleaned or repaired in a timely manner; Regularly conduct step response tests on servo valves. When conducting response tests on a daily basis, it is important to promptly detect any wear on the valve core of the servo valve. If any issues are found, the servo valve should be cleaned or repaired promptly.
(2) Regularly test the oil of the servo system. If the cleanliness level of the oil is lower than NAS5, the filter of the hydraulic system should be replaced immediately to prevent large particles from entering the pilot valve core of the servo valve, causing blockage and vibration of the servo valve.
(3) Regularly inspect and tighten the hydraulic cylinder and transmission mechanism of the auxiliary roll, and promptly address any issues such as jamming, looseness, or excessive clearance; Regularly calibrate the roll diameter, levelness of the auxiliary roll, and radial clearance between the auxiliary roll and the roll. Avoid vibration caused by mechanical assembly.
(4) Regularly inspect the cables of the servo valve feedback system and take measures to prevent interference and oscillation caused by external strong magnetic and electric fields on the feedback system.
(5) Regularly tighten the wiring and magnetic tape of the system to avoid abnormal high resistance caused by loose screws, resulting in feedback signal errors or output signal errors and vibration.
If the hydraulic servo control system of the auxiliary roll experiences oscillation or inability to press, based on the on-site experience of 1580 strip steel plant for many years, combined with the given value and feedback value curve of the auxiliary roll gap on the operation screen, the actual measured pressure and feedback pressure of the hydraulic cylinder, the zero deviation and zero drift degree curve of the servo valve, the fault diagnosis and handling should be carried out in the following order:
(1) Observe whether there is any obvious overshoot during the pressing process of the auxiliary roll, as this is the easiest phenomenon to observe, and adjust the gain of the servo system downwards within the range of no overshoot. Simultaneously conducting response testing on the system can eliminate oscillations caused by improper gain.
(2) Check for any looseness in the bolt connections of the given and feedback lines, and tighten them appropriately to eliminate oscillations caused by signal interference.
(3) Check the alarm. If there is a servo system alarm, first analyze the cause. If there is a problem with the feedback cable, it should be replaced in a timely manner to eliminate the oscillation caused by the feedback system.
(4) If there is no alarm and the feedback is normal, it is necessary to conduct a response test on the servo valve. If there is a jumping action on the test curve during the test, it indicates that the pilot valve or main valve core is stuck, and the servo valve should be cleaned or repaired. This can eliminate system oscillations caused by servo valve failures.
(5) If the vibration is caused by mechanical reasons such as low system pressure or horizontal alignment of the two ends of the auxiliary roll, it will manifest as irregular vibration, which is sometimes difficult to capture. It is necessary to measure and calibrate the vertical and horizontal alignment of the two ends of the auxiliary roll, and check all related mechanical equipment for loose or large gaps to eliminate hidden dangers one by one.
4.2 Countermeasures and measures to reduce hydraulic impact of auxiliary rollers
(1) On the premise of ensuring the quality of the steel coil shape, during the process of coiling 2-5 turns of the product, try to minimize the step distance. After coiling for 5 turns, adjust the proportional gain setting value during the opening process of the auxiliary roller to slowly open it.
(2) Reasonably plan to minimize the length of hydraulic pipelines and reduce the distance of shock wave propagation. Without affecting the step response speed, add rubber hose connections between the valve station and the oil cylinder to alleviate the impact.
(3) Regularly calibrate the pressure of the rodless chamber overflow valve of the oil cylinder to be 23MPa; Install an accumulator in front of the hydraulic shock source valve station.
(4) Reasonably adjust the zero drift range of the servo valve to reduce the harm caused by hydraulic shock. When the oil supply pressure varies within the set range of 18.9Mpa to 23.2Mpa, the zero drift should be less than 0.62Mpa; When the return oil pressure varies within the range of 0-6.3Mpa, its zero drift should be less than 0.62Mpa; When the control current varies within the range of 0-10mA, the zero drift should be less than 0.4mA.
4.3 Countermeasures for High Temperature in Hydraulic System and Its Impact on System Performance
(1) Clean the filter screen of the cooler during maintenance and other times, and disassemble the cooler for descaling and cleaning treatment. Properly increase the pressure and flow rate of cooling water during work to enhance the cooling effect.
(2) Adjust the working pressure of the main hydraulic pump to be consistent during maintenance time. When adjusting the pump pressure, the pressure of the four working pumps should be adjusted within the specified pressure difference range, and the relative pressure difference should not exceed 2%.
(3) Regularly calibrate the set values of the overflow valve on the pump or the overflow valve and pressure reducing valve on the valve platform, check whether the control valve has internal leakage, and replace hydraulic components that cannot be adjusted or are damaged.
(4) Use a thermometer to measure the temperature of the pipeline, check and eliminate abnormal overflow, throttling, and damping phenomena in the pipeline and valve group.
5. Conclusion
After maintenance and regular upkeep, the hydraulic servo control system for the auxiliary roll of the 1580 strip steel mill in a certain steel limited company has achieved a focus on prevention. Regular testing and calibration of the system's overflow valve, servo valve, auxiliary roll gap, and drum diameter expansion have been carried out, resulting in a significant improvement in the stability performance of the hydraulic system during operation. Malfunctions such as system temperature rise and vibration impact have been well controlled. Through the maintenance of the hydraulic servo control system for the auxiliary roll in recent years, the operating cycle of the equipment has been greatly improved, and the failure time of the auxiliary roll machine has been significantly reduced. This has provided strong support for improving our company's strip steel production and yield rate, bringing considerable economic benefits to the company.
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