Film thickness uniformity is a key indicator in the production of blown film. Vertical thickness uniformity can be controlled by extrusion and traction speed stability, while film transverse thickness uniformity is generally based on precision molding of the die. In order to improve the lateral thickness uniformity of the film, an automatic lateral thickness control system must be introduced. The common control methods include automatic die (thermal expansion screw control) and automatic air ring. Here, the automatic air ring is mainly introduced. Principles and applications.
Fundamental:
The automatic air ring structure adopts a double air outlet method, wherein the air volume of the lower air outlet is kept constant, and the air outlet is divided into a plurality of air passages, each air passage is composed of a wind chamber, a valve, a motor, etc., and the motor drives the valve to adjust the air passage opening. Degree, control the air volume of each air duct.
During the control process, the film thickness signal is detected by the thickness measuring probe and transmitted to the computer. The computer compares the thickness signal with the current set average thickness, and calculates the thickness according to the thickness deviation and the curve change trend, and controls the motor to drive the valve to move when the film is biased. When thick, the motor moves forward and the air outlet is closed; on the contrary, the motor moves in the opposite direction and the air outlet increases. By changing the air volume at each point on the circumference of the air ring, the cooling speed of each point is adjusted, so that the lateral thickness deviation of the film is controlled within the target range. .
Control scheme design
The automatic wind ring is an online real-time control system. The system is controlled by several motors distributed on the wind ring. The cooling airflow sent by the fan is distributed to each air channel through the constant pressure of the air ring air chamber, and the motor drives the valve to open and close to adjust the size of the air outlet and the air volume, and change the cooling effect of the film blank at the die discharge. Therefore, the thickness of the film is controlled. From the control process, there is no clear relationship between the thickness variation of the film and the amount of motor control. The thickness and thickness of the film and the different positions of the valve vary nonlinearly with the control amount, and each valve is adjusted. The influence of adjacent points is very large, and the adjustment has hysteresis, which makes the different moments related to each other. For this highly nonlinear, strong coupling, time-varying and control uncertainty system, the exact mathematical model is almost impossible. Establishing, even if the mathematical model can be established, it is very complicated and difficult to solve, so that there is no practical value, and the traditional control has better control effect on the determined control model, but the control effect is very poor for highly nonlinear, uncertain, and complex feedback information. Even powerless. In view of this we have chosen the fuzzy control algorithm. At the same time, the method of changing the fuzzy quantization factor is adopted to better adapt to the change of system parameters.
Principle and implementation of fuzzy control structure
How fuzzy control works:
In the fuzzy control process, according to the deviation amount e of the thickness set value s and the feedback value y and the rate of change e' of the deviation amount, the fuzzy control algorithm is used to calculate the output control quantity u, and then converted into a motor control pulse to drive the valve. The amount of opening, the amount of air is adjusted, and the thickness of the film is controlled.
In the actual control process, the deviation amount e and the rate of change e' of the deviation amount are multiplied by the quantization factor Ke, Ke' to obtain the fuzzy amount E, E', and then call the table lookup program to find the corresponding fuzzy control amount from the control table. The value u is output and multiplied by the quantization factor Ku to derive the actual control amount.
The fuzzy control scale is a control rule summarized based on the experience of on-site debugging. Using the membership function table of each input and output, the fuzzy relations corresponding to different E, E' and U are calculated, and their relationships are merged and then combined. Finding the membership function vector of the control decision corresponding to various input states, and then judging by the maximum principle of the membership function, and obtaining the corresponding control quantity fuzzy value U, when obtaining the output value of the fuzzy control quantity corresponding to various input states , you can get the fuzzy control scale.
In order to obtain better control effect under different process parameters of the film, the quantization factors Ke, Ke' and Ku are automatically adjusted according to the change of system parameters. During the debugging process, the influence factors of the quantization factors Ke, Ke' and Ku on the performance index of the control system as follows:
a) The larger Ke, the smaller the error in steady state, the faster the system response, but the overshoot will increase, even the thick and thin vibration phenomenon, the smaller Ke is the opposite;
b) The larger the Ke', the faster the system's performance is, the slower the response, the more the overshoot will decrease, the sensitivity of the rate of change of the deviation increases, and the smaller the Ke', the opposite;
c) The larger the Ku, the faster the system responds, the more the overshoot will increase, and the most obvious response to the output.
According to the above rules, in order to take into account the rapid dynamic performance and static accuracy of the system, the quantization factors Ke, Ke', and Ku are dynamically modified according to the magnitude of the deviation of the controlled object during the operation. The modification method is to quantify the different deviation values ​​in advance. The factors Ke, Ke', and Ku set the corresponding segmentation curve. In the control process, Ke, Ke', and Ku are found by the deviation values. Each curve is obtained according to experience adjustment in the production process.
Another difficulty in the control process is to find the point automatically. Whether the thickness detection feedback signal corresponds to the upper air duct of the wind ring will affect the whole control. Before each input, the automatic search must be done once. The control is to select and detect the change in the curve. The section that is gentle and more reflective of the control effect is used as the detection object, and the air volume of the adjacent three air ducts is reduced. The middle air duct is fully closed, and the left and right opening degrees are 50%. After several weeks of scanning, the measurement is stable. After that, the thinnest point in the area is detected, which is the air passage corresponding to the fully closed state, and the control software compensates according to the detected offset. In addition, the automatic finding point is also related to the rotating traction rotation speed, the rotation direction, and The thickness of the thickness sensor is related to the direction of rotation, and the software compensates for these.
System composition
The electrical control system consists of a rotary scanning thickness sensor, a computer, an industrial controller (PCC), a display, a driver, a controller and a motor, and a tuyere position detecting sensor.
Hardware introduction
The rotary scanning thickness measurement sensor adopts a capacitive thickness gauge to measure according to different dielectric constants of different thickness films. The measurement range is 0~200um. The system adopts computer and PCC controller. The cpu adopts CP380, CP476, step control adopts high-speed I/O module DO135, switch input and output adopts DM465 module, the system adopts multi-cpuCAN network bus structure, Ethernet connection between computer and PCC, PCC and scanning thickness sensor The PCD-LINK communication method is used to transmit data. The CP380 mainly performs data acquisition and calculation processing. The CP476 and DO135 mainly complete the stepper motor drive control. The computer mainly completes the thickness measurement sensor data acquisition, calculation, statistics, historical data recording, and operation status monitoring. Wait.
After many tests and commissioning, the automatic wind ring research and development has been successful. From the use situation, the thickness of the blown film can be increased by 3 to 5%, and the average limit deviation is 4%, achieving better control effect.