In the process of blown film production, the uniformity […]
In the process of blown film production, the uniformity of film thickness is a very key indicator. The uniformity of the longitudinal thickness can be controlled by the stability of the extrusion and traction speed, while the uniformity of the transverse thickness of the film generally depends on the precision manufacturing of the die. , And change with the change of production process parameters. In order to improve the film thickness uniformity in the transverse direction, an automatic transverse thickness control system must be introduced. Common control methods include automatic die head (thermal expansion screw control) and automatic air ring. Here we mainly introduce automatic air ring. Principle and application.
1. Basic principles:
The structure of the automatic air ring adopts a double air outlet method, in which the air volume of the lower air outlet remains constant, and the upper air outlet is divided into several air ducts. Each air duct is composed of air chambers, valves, motors, etc., and the motor drives the valve to adjust the opening of the air duct. Control the air volume of each air duct.
During the control process, the film thickness signal detected by the thickness measuring probe is sent to the computer. The computer compares the thickness signal with the current set average thickness, performs calculations based on the thickness deviation and the curve change trend, and controls the motor to drive the valve to move. When it is thick, the motor moves forward and the tuyere is closed; on the contrary, the motor moves in the reverse direction, and the tuyere increases. By changing the air volume of each point on the circumference of the wind ring, adjust the cooling speed of each point, so that the lateral thickness deviation of the film is controlled within the target range .
2. Control scheme design
The automatic wind ring is an online real-time control system. The controlled objects of the system are several motors distributed on the wind ring. The cooling air flow sent by the fan is distributed to each air duct after constant pressure in the air ring air chamber, and the motor drives the valve to open and close to adjust the size of the tuyere and air volume, and change the cooling effect of the film blank at the die discharge. In order to control the film thickness, from the perspective of the control process, there is no clear relationship between the film thickness change and the motor control value. The thickness of the film and the valve position of the valve changes nonlinearly and irregularly between the thickness change and the control value. Every time a valve is adjusted Time has great influence on neighboring points, and the adjustment has hysteresis, so that different moments are related to each other. For this kind of highly nonlinear, strong coupling, time-varying and control uncertain system, its precise mathematical model is almost impossible Established, even if a mathematical model can be established, it is very complicated and difficult to solve, so that it has no practical value. Traditional control has a better control effect on a relatively definite control model, but it has a poor control effect on high nonlinearity, uncertainty, and complex feedback information. Even powerless. In view of this, we chose the fuzzy control algorithm. At the same time, the method of changing the fuzzy quantization factor is adopted to better adapt to the changes of the system parameters.
3. The principle and implementation of fuzzy control structure
The working principle of fuzzy control:
During the fuzzy control process, according to the deviation e of the thickness setting value s and the feedback value y and the deviation change rate e', the output control value u is calculated according to the fuzzy control algorithm, and then converted into motor control pulse to drive the valve The amount of opening, adjust the air volume, and control the thickness of the film.
In the actual control process, the deviation amount e and the deviation amount change rate e'are respectively multiplied by the quantization factor Ke, Ke' to obtain the fuzzy amount E, E'then call the look-up program to find out the corresponding fuzzy control amount from the control table The output value u is multiplied by the quantization factor Ku to obtain the actual control value.
Fuzzy control scale is a control law summed up based on on-site debugging experience. Using the membership function table of input and output quantities, the fuzzy relationship corresponding to different E, E'and U is calculated, and their relationship is merged, and then combined operation , Calculate the membership function vector of the control decision corresponding to various input states, and then judge by the principle of maximum membership function to obtain the corresponding control variable fuzzy value U. When the fuzzy control variable output value corresponding to various input states is calculated , You can get the fuzzy control scale.Extrusion Blow Molding Machines Factory
In order to obtain a better control effect under different process parameters of the film, the quantization factors Ke, Ke′ and Ku are automatically adjusted according to the changes of the system parameters. During the commissioning process, the quantization factors Ke, Ke′ and Ku have an influence on the performance index of the control system. as follows:
a) The larger the Ke, the smaller the error in the steady state, and the faster the system response, but the overshoot will increase, and even thickness vibration will occur. The smaller the Ke, the opposite will be the case;
b) The larger Ke’ is, the faster performance of the system will be reduced, the response will be slower, the overshoot will be reduced, and the sensitivity to the rate of change of the deviation will increase, and the smaller Ke’ is, the opposite will happen;
c) The larger the Ku, the faster the system response, 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 deviation value of the controlled object during operation. The modification method is to quantify the different deviation values in advance. The factors Ke, Ke', and Ku set the corresponding segmented curves. In the control process, Ke, Ke', and Ku are correspondingly found out through the deviation value. Each curve is obtained after adjusting according to experience in the production process.