Principle and application of automatic air ring of

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Principle and application of automatic air ring of blow molding machine

in the production process of blown film, film thickness uniformity is a key index, in which the longitudinal thickness uniformity can be controlled by the stability of extrusion and traction speed, while the transverse thickness uniformity of film generally depends on the precision manufacturing of die head, and changes with the change of production process parameters. In order to improve the transverse thickness uniformity of film, Automatic horizontal thickness control system must be introduced. The common control methods include automatic die head (thermal expansion screw control) and automatic air ring. Here, the principle and application of automatic air ring are mainly introduced

1. Basic principle:

the automatic air ring structure adopts the double tuyere mode, in which the air volume of the lower tuyere remains constant, and the circumference of the upper tuyere is divided into several air ducts, each air duct is composed of air chamber, valve, motor, etc., and the motor drives the valve to adjust the opening of the air duct and control the air volume of each air duct

during the control process, the film thickness signal detected by the thickness measuring probe is transmitted to the computer. The computer compares the thickness signal with the current set average thickness, calculates according to the thickness deviation and curve change trend, and controls the motor to drive the valve to move. When the film is thicker, the motor moves forward and the air vent is turned down; 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 and adjusting the cooling speed at each point, the horizontal thickness deviation of the film is controlled within the target range

2. When selecting a microcomputer controlled pressure testing machine, you can refer to the above standards for considering the quality of equipment to determine the control scheme design. The automatic air loop is a real-time control system, and the controlled objects of the system are several motors distributed on the air loop. The cooling air flow sent by the fan is distributed to each air duct after the constant pressure of the air ring air chamber. The motor drives the valve to open and close to adjust the size of the air outlet and air volume, and change the cooling effect of the film blank at the discharge of the die head, so as 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 quantity, There are nonlinear and irregular changes between the thickness changes of different thickness films and different positions of valves and the control quantity. Each time a valve is adjusted, it has a great impact on the adjacent points, and the adjustment has hysteresis, so that different times are interrelated. For this highly nonlinear, strong coupling, time-varying and control uncertainty system, its accurate mathematical model can hardly be established, and even if the mathematical model can be established, it is very complex, It is difficult to solve, so it has no practical value, while the traditional control has a better control effect on the more certain control model, while the control effect is very poor or even powerless for the highly nonlinear, uncertain and complex feedback information. In view of this, we choose the fuzzy control algorithm. At the same time, the method of changing the fuzzy quantization factor is better adapted to the change of system parameters

3. The structure principle and implementation of fuzzy control

the working principle of fuzzy control is shown in the figure:

it can be seen from the figure that in the process of fuzzy control, according to the deviation e between the thickness setting value s and the feedback value y and the change rate e 'of the deviation, the output control value u is calculated according to the fuzzy control algorithm, and then converted into motor control pulse to drive the opening of the valve, adjust the air volume, and control the thickness of the film

in the actual control process, the deviation E and the change rate of the deviation e 'are multiplied by the quantization factor Ke, Ke' to obtain the fuzzy quantity e, e 'and then call the table lookup program to find out the output value u of the corresponding fuzzy control quantity from the control table, and then multiply by the quantization factor Ku to obtain the actual control quantity

fuzzy control scale is based on the control law summarized from the on-site debugging experience. Using the membership function table of input and output quantities, it calculates the fuzzy relations corresponding to different e, e 'and u, merges them, and then calculates the membership function vector of the control decision corresponding to various input states through the merging operation, and then judges by the principle of maximum membership function to obtain the fuzzy value u of the corresponding control quantity, When the output value of fuzzy control quantity corresponding to various input states is obtained, the fuzzy control quantity table can be obtained

in order to get better control effect under different process parameters of the film, the quantization factors Ke, Ke ', Ku are automatically adjusted according to the change of system parameters. During the debugging process, the influence law of quantization factors Ke, Ke', Ku on the performance index of the control system is as follows:

a) the larger the Ke is, the smaller the error is in the steady state, the faster the system response is, but the overshoot will increase, and even thickness vibration will occur. The smaller the Ke is, the opposite is true

b) the larger the Ke ', the faster the system performance will be reduced, the slower the response will be, the overshoot will be reduced, and the sensitivity to the change rate of the deviation will be increased. The smaller the Ke', the opposite is true

c) the larger Ku, the faster the system response, and the overshoot will increase, which is 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 quantitative factors Ke, Ke 'and Ku are dynamically modified according to the deviation value of the controlled object during the operation. The modification method is expected to ship to customers from July 2016. The corresponding segmented curves are set for each quantitative factor Ke, Ke' and Ku with different deviation values in advance. In the control process, Ke, Ke 'and Ku are found through the corresponding deviation values, Each curve is obtained after adjustment according to experience in the production process

another difficulty in the control process is automatic point finding. Whether the thickness detection feedback signal corresponds to the air duct on the air ring will affect the whole control. Automatic point finding must be done every time before putting into automatic operation. The control is to select the section of the curve that changes gently and can reflect the control effect as the detection object, and reduce the air volume of the adjacent three air ducts, of which the middle air duct is fully closed, The left and right opening degree 11. All relevant historical records can be automatically queried according to the conditions of the sample, the experimental date and the experimental data. After several weeks of scanning and measurement, the thinnest point in the area is detected, which is the air duct corresponding to the fully closed state. The control software then compensates according to the detected offset. In addition, the automatic point finding is also related to the rotation traction, rotation speed and rotation direction, As well as the thickness measurement sensor rotation speed, rotation direction, the software should compensate for these

4. System composition

the electrical control system is composed of rotary scanning thickness measurement sensor, computer, industrial controller (PCC), display, driver, controller, motor, air duct and air outlet position detection sensor, etc

5. introduction to hardware

the rotary scanning thickness sensor adopts a capacitive thickness gauge to measure the dielectric constant of different thickness films according to different principles, with a measurement range of 0~200um. The system adopts a computer and PCC controller. Among them, cp380 and cp476 are used for CPU, high-speed i/o module do135 is used for stepping control, dm465 module is used for switch input and output, the system adopts multi CPU can network bus structure, Ethernet connection is used between computer and PCC, PCD link communication mode is used between PCC and scanning thickness sensor to transmit data, cp380 is mainly used for data acquisition and calculation processing, cp476 and do135 are mainly used to complete stepping motor drive control, The computer mainly completes the data acquisition, calculation, statistics, historical data recording, operation status monitoring, etc. of the thickness measurement sensor

after many tests and debugging, the research and development of automatic air ring has been successful. In terms of application, the thickness and uniformity of blown film can be increased by 3~5%, and the average limit deviation can reach 4%, achieving good control effect

source: China Industrial Equipment

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