Abstract: This paper summarizes the technical experience in improving and optimizing the pressure control method of the Morgan bearing lubrication station's finishing mill lubrication system on the Handan Iron and Steel Group's 2250 hot rolling production line. The original control method did not consider compensating for factors that interfered with system pressure fluctuations, and the variable-speed pump in the Morgan bearing lubrication station had limited ability to adjust for pressure fluctuations. This resulted in significant pressure fluctuations in the system during the rolling process. By improving the control method, the problem of excessive pressure fluctuations during the rolling process was fundamentally solved, ensuring stable production operation.
Abstract: This thesis summarized the experience on the pressure control methed improvement and optimization of FM Morgoil bearing lubricating system in the 2250 Hot Strip Mill rolling line,Hansteel company.In the previous condition,the system didn't take the pressure disturbance factors compensation into consideration in control and the speed adjusting ablility of the pumps was not sufficient.The system pressure had very big fluctuation during rolling process. By improving the control pattern,the problem that the pressure fluctuation is too much during the rolling process was essentially solved, which satisfied the production requirement.
The finishing mill's Morgan bearing lubrication system provides lubricating oil to the Morgan bearings, lubricating and cooling them while also providing oil film support. This requires the system pressure to remain stable during the rolling process. However, during commissioning, observations revealed significant pressure fluctuations in the system during the finishing mill's biting, speed increases, speed decreases, and steel ejection processes. This is extremely detrimental to the normal operation and protection of the bearings. Without improvement, it will negatively impact the bearings' normal operation and lifespan, and also adversely affect the precise control of the sheet shape. Our analysis of the factors affecting system pressure and the control methods led to the optimization and improvement of the entire system's pressure control. We added feedforward compensation for interference factors and improved the control method of the variable frequency speed-regulating pump, resolving the problem of excessive system pressure fluctuations during rolling and ensuring the normal operation of the mill's Morgan bearings.
1. The original control method and existing problems of Morgan bearing lubrication system
The pressure in Morgan's bearing lubrication system is adjusted by a variable-speed pump (variable-frequency motor). Under normal circumstances, three of the four pumps in the finishing mill's Morgan lubrication system are in operation, with one on standby. In cases of low pressure, the fourth standby pump automatically starts to increase pressure adjustment capability. A pressure sensor is installed after the cooling unit in the Morgan lubrication system piping. During system operation, the deviation between the online pressure value fed back by the pressure sensor and the set pressure value is used to adjust the output speed of the variable-speed pump according to a certain functional relationship, thereby achieving the purpose of controlling the system pressure. Morgan pressure control is a typical example of pressure feedback control.
The Morgan bearing lubrication system's variable frequency drive (VFD) pumps start in two acceleration phases. Pressure control activates when the user's pressure reaches the set pressure. Of the four VFD pumps, one is the master pump, and the other two are slave pumps. The master pump is selected based on which pump first reaches the set speed for the second phase. During operation, the master pump adjusts for real-time pressure fluctuations, changing its speed accordingly, while the slave pumps maintain a constant speed. If the master pump's adjustment capability is insufficient, it will be switched over.
The problem with Morgan bearing lubrication systems is that during the production process, there are many impact load factors, such as steel biting, mill speed-up, steel throwing, and roll gap adjustment during AGC control. Because the Morgan system's speed-regulating pumps respond slowly to these impact loads, and the adjustment force of a single pump is insufficient, large pressure fluctuations occur during rolling. This is detrimental to the protection of Morgan bearings and the precise control of the plate shape. Therefore, certain measures need to be taken to solve this problem.
2. PI control
PI control (proportional and integral control) involves two main components: immediate proportional regulation and tracking compensation integral regulation. The control process is illustrated in the diagram below.
In PI control, the proportional term acts like a lever, adjusting the control deviation in real time, while the integral term provides tracking compensation for the deviation. When the deviation is positive, the integral term continuously increases the adjustment of the proportional term; when the deviation is negative, the integral term continuously decreases the adjustment of the proportional term. The control parameters achieve a balance between the control target value and the system through the combined effect of the real-time adjustment of the proportional term and the tracking compensation of the integral term.
PI control is a typical feedback control, mainly consisting of proportional and integral elements. This type of control is widely used in the TMEIC control program. However, for the Morgan pressure control system, the drawback of this control method is that it does not incorporate the main factors affecting pressure fluctuations during the rolling process as feedforward compensation to the system. Instead, it simply uses feedback control for adjustment. When the system response speed is slow, this control method is relatively passive and has poor resistance to system fluctuations.
Morgan's lubrication system pressure control uses a PI control method, and mainly includes the following parameters:
• System set pressure P = 4.0 bar
• System feedback pressure Pi
• Detect pressure deviation ∆; P = P - Pi
• Control dead band (DB) = 0.1 bar
• The upper limit of the PI control speed adjustment range is n1 = 400 r/min
• The lower limit of the PI control speed adjustment range is n2 = -1150 r/min
• Total gain for detecting pressure deviation Ktotal = 1
• Integral gain KI
Proportional gain KP
• PLC scan time parameter T = 0.05 s
In addition, with the PI control dead zone set to DB=0.05, the pressure deviation value will only be transmitted to the PI control unit for pressure adjustment when the pressure deviation exceeds this value.
3. Optimization of pressure control
Through analysis of the pressure control system using ODG (online data gathering) software, we found two defects in the previous control system: firstly, it did not compensate for factors affecting pressure fluctuations in the control system; secondly, the speed regulation capability of the control method used by the variable speed pump was insufficient.
3.1 Compensation factors: rolling force and rolling speed
Our analysis of the ODG curves revealed that the difference between the maximum and minimum pressure fluctuations exceeded 2 bar. The minimum reached 2.7 bar, while the maximum rose to 4.7 bar. The pressure fluctuations would be even greater when rolling thinner strips. Therefore, this is highly detrimental to the protection and normal operation of Morgan bearings.
We know that during the biting and throwing processes of the rolling mill, the rolling force suddenly increases, causing an increase in the flow rate of Morgan lubricating oil and resulting in fluctuations in system pressure. Based on this factor, we observed the rolling force and pressure fluctuation curves during biting and throwing, confirming the above viewpoint. For pressure compensation caused by the rolling force, we adopted the simplest fixed-value compensation method. For F1-F7, a fixed value was assigned to each value based on the different degrees of pressure impact during rolling and compensated to the speed reference value of the variable-speed pump. After repeated trials and modifications, we compensated the following set of parameters for the finishing mill Morgan lubrication system:
Similarly, increases or decreases in mill speed will also cause changes in the Morgan bearing oil flow rate, thus affecting system pressure. Because the rotational speeds of different mills are not the same, we multiply the average speed of each stand by a suitable coefficient and compensate for it in the output reference value of the Morgan bearing lubrication pump speed. After practical trials and modifications, we determined that a compensation coefficient of K=26 yielded good results. The total compensation parameter K' for the mill rolling force and speed is:
1) Change in the control method of Morgan lubrication variable speed pump
As shown in Figure 2, the three speed-regulating pumps operate as follows during the rolling process: one master pump and three slave pumps. When pressure fluctuates during rolling, the master pump's speed is adjusted in real-time, while the three slave pumps maintain a constant output speed of 1000 r/min. When the pressure fluctuation becomes too large, the master pump's speed regulation capability reaches its limit (rated speed 1800 r/min). At this point, the two slave pumps increase their output speed to 1800 r/min to raise the starting point for the master pump's speed regulation. This control method has significant drawbacks. First, its adjustment capability is limited, as only one master pump performs real-time adjustments. Second, when a slave pump experiences a speed increase, it itself becomes a factor in system pressure fluctuations, causing large fluctuations in system pressure. Therefore, this method needs to be changed.
In previous control methods, the speed compensation value of PI control only modified the real-time output speed of the main pump for speed adjustment, while other slave pumps operated at constant speeds. When the speed adjustment capability of the main pump reached its limit, the slave pumps stepped up to their rated speed, raising the starting point for the main pump's speed adjustment. After analysis, we decided to change this control method: eliminate the distinction between the main and slave pumps, allowing all three pumps (with the fourth pump automatically starting when the speed adjustment reaches its limit) to perform real-time adjustments simultaneously. The real-time feedback compensation from PI control and the compensation for disturbance factors are simultaneously applied to all three (or four) pumps. This significantly improves the pressure adjustment capability and speed of the lubrication system, increasing the system's response speed. The relationship between the proportional element and disturbance factor compensation is: the more comprehensive and accurate the system's disturbance factor compensation, the smaller the adjustment amplitude of the proportional element, and the more stable the system. Furthermore, when system pressure adjustment requires the start of the fourth pump, we considered the impact of pump startup on system pressure and adjusted the pump's acceleration to ensure the impact of the fourth pump's startup and shutdown on the system is minimized. Therefore, the improved Morgan bearing lubrication system fundamentally eliminates the problem of excessive pressure fluctuations, which is of great practical significance for ensuring the normal operation and protection of Morgan bearings. Figure 5 shows the pressure fluctuation curve of the improved Morgan system. During the rolling process, the pressure fluctuation range is less than 0.11 bar, which greatly meets the production needs.
Conclusion:
Morgan bearings are widely used in industry, and their lubrication system plays a crucial role in bearing operation. The operating conditions of a precision-rolled Morgan bearing system are complex, with numerous impact loads and frequent speed changes. Therefore, pressure control of the system requires consideration of many factors, necessitating appropriate control methods. This paper, completed during actual production on a hot rolling line, objectively summarizes the technical experience in improving the pressure control of the Morgan lubrication system, providing valuable reference for the modification and optimization of other similar systems.
