Plastic hollow molding machine is one of the leading products of plastic machinery. Although its development started late in China, after nearly 30 years of development, domestically produced machines have basically been able to meet the molding needs of various ordinary plastic hollow products. Among them, the hollow molding machine series developed by Qinchuan Machinery Development Co., Ltd. (hereinafter referred to as “Qinchuan Companyâ€) has a market share of over 80% in the Chinese market. With the rapid development of the packaging industry and the automotive industry, the application of plastic hollow containers and hollow molded products has become more and more extensive, and its structure has also become more diversified. For extrusion blow molding of hollow containers, parison wall thickness control is one of the most critical technologies, and it is also a difficult point in the design of blow molding machines. Qinchuan Company successfully designed the wall thickness control system through technical breakthroughs. Through the production practice of many users, the system is superior to the current imported products in terms of operation and control, and has achieved great success in the market.
Parison wall thickness control system structure
As the geometry of hollow blow molded articles becomes more complex, good preform designs are particularly important for articles that achieve the desired wall thickness distribution and are structurally stable with minimal raw material consumption. Therefore, in the parison forming stage, the wall thickness distribution of the parison can be adjusted according to the inflation ratio of each portion when the parison is inflated. At the same time, considering the changes in extrusion expansion, sagging and rebound during the formation of the parison, a reasonable parison thickness distribution curve is edited, and the electro-hydraulic servo system is adjusted to make the die gap between the inner mold and the outer mold. The position of the blank changes, thereby inflating the product into a thick and uniform product. The impact resistance test of the product shows that the uniform wall thickness makes the product greatly improved in strength; at the same time, the raw material is saved, the cooling time of the finished product is shortened, the defective rate is reduced, and the production efficiency and product quality are greatly improved. .
The parison wall thickness control system is mainly composed of display device and control and electro-hydraulic servo (proportional servo valve and servo oil source) system. The working principle is shown in Figure 1.
Figure 1 Structure diagram of the parison wall thickness control system
1. Control principle of wall thickness control system
The 64-point axial wall thickness controller (AWDS) is one of the core technologies of Qinchuan's plastic hollow molding machine, which has a decisive influence on the performance of the whole machine. In recent years, through the continuous optimization of the hardware structure of the wall thickness control system, the radial parison controller (PWDS), software program structure, motion program, control algorithm and mathematical model have been added to develop the axial wall thickness controller and A wall thickness control system in combination with a radial wall thickness controller. The system has been successfully applied in production and is at the leading level in China.
The PLC and touch screen control system is one of the design features of Qinchuan Plastic Blowing Machine. The touch panel (ie, man-machine interface) is used to adjust the parison thickness curve and related process parameters, so that after receiving the curve data and parameter information set by the man-machine interface, the PLC controller controls the servo proportional valve to achieve the predetermined die. gap. The whole process requires the control system to respond in milliseconds, which means that not only the hardware is computationally fast, but also a new breakthrough in software and digital control theory, in order to meet the rapid response and precise control of the parison wall thickness control system. Stable and reliable requirements.
For PLC and touch screen control systems, Qinchuan uses a real-time multitasking operating system to define the priority and cycle time of each task, and the cycle time is as short as 1ms, which optimizes the real-time performance of the entire system. Among them, PWS3261-TFT touch industrial-grade human-machine interface is produced by Taiwan Delta Electronics Co., Ltd. The touch screen is a programmable display panel that integrates display and touch switch. It has compact structure, convenient connection and intuitive operation. Features. It has multi-channel serial communication function, which can display device status, operation indication, parameter setting, action flow, statistics, alarm information and simple report in real time. In the plastic electrical control system, the setting of various parameters, the use of the button switch, the display of the wall thickness control curve, and the alarm information can all be completed by the touch screen. At the same time, various function curves can be set on the touch screen, which provides a very favorable working platform for the editing of wall thickness graphics.
In addition, Qinchuan also uses the CJIG-CPU43H programmable controller manufactured by OMRON of Japan. The product has a wealth of arithmetic instructions, sequential instructions, data processing instructions and high-speed processing functions to achieve instructions, and high reliability, adaptability, and can be applied in a variety of complex and harsh environments. In the electrical control of plastic machinery, the controller can well perform functions such as precise control of motion, analog conversion operation, data processing, and I/O signal processing.
2. Electro-hydraulic servo system
Electro-hydraulic servo systems have many advantages, the most prominent of which are high response speed, high output power and high control accuracy, which are the basic requirements of the parison wall thickness control system. In the electro-hydraulic servo system, the servo valve is one of the most important components, and it is the key link to realize electro-hydraulic conversion, which directly affects the response speed and stability of the servo system. In the electro-hydraulic servo system, Qinchuan Company adopted the CSDY3 type jet electro-hydraulic servo valve. The valve's signal changes from 0 to 100%, response time ≤ 10ms, the response is extremely fast, the accuracy is also high, but the oil quality requirements are also very high. The structure of the electro-hydraulic servo system is shown in Figure 2.
Figure 2 Electro-hydraulic servo system structure
Software design of parison wall thickness control system
The design of the parison wall thickness control system software is divided into two parts: human-machine interface software design and PLC software design.
1. Human-machine interface software design
The algorithm used in the design of human-machine interface software is mainly interpolation, that is, for the specified function f(x), as long as it knows the function value of several points in a certain interval, to make appropriate specific functions to make these Take the known value at the point and use the value taken by this particular function at other points in the interval as an approximation of the function f(x). If the particular function is a polynomial, it is called an "interpolation polynomial". In general, the interpolation algorithm is distinguished based on the class of the interpolation polynomial. In the imported blow molding machine, there are several different wall thickness control graphics software, which are typically 30 points, 100 points and 200 points of the US moog. They use linear interpolation, difference quotient interpolation and spline interpolation respectively. law.
The wall thickness control requires continuous conduction between the set points of the wall thickness pattern. Otherwise, for the axial wall thickness distribution system (AWDS), a ring pattern will appear on the parison, and the radial wall thickness distribution system (PWDS) , axial streaks will appear on the parison. The linear interpolation algorithm is simple and straightforward, and it is very easy to calculate the sum between two points, and the amount of calculation is also small. However, since the algorithm does not consider the smooth transition between the endpoints of the interval, basically every point has to be adjusted. Therefore, the wall thickness graph with fewer points can be used for this algorithm, and when the interval is large, the operation is cumbersome, and the algorithm is not suitable. In comparison, the spline interpolation method guarantees the continuous conductivity between all the set points, but since all the set points can be smoothly transitioned, when a certain point is stretched beyond a certain degree, not only near this point The curve changes, and the curve that is farther away from it changes. This violates the original intention of the design, so that the original adjusted wall thickness curve area is invalidated due to deformation, and the efficiency of the operator to adjust the curve will be greatly reduced.
Linear interpolation, difference quotient interpolation, and spline interpolation cannot fully meet the requirements of wall thickness graphics. Qinchuan's wall thickness control system proposes an interpolation method combining B-spline and Bezier curve in consideration of the smooth transition and mutual influence, so that it can meet the requirements of the smooth transition of the curve end point, and Overcome the shortcomings of the spline interpolation method that the deformation of the far curve is caused by the excessive stretching of a certain point. Even with few control points, you can draw any wall thickness curve of 30~300 points, and it is convenient to operate. The wall thickness curve operation obtained by this algorithm has the following characteristics:
(1) The curve is used to fit the two set points;
(2) There are two ways of fine-tune each point stretching and area curve;
(3) There are three ways to adjust the weight: overall adjustment, high-end mode and low-end mode;
(4) The wall thickness pattern is enlarged and reduced with the adjustment range, which is easy to adjust and observe;
(5) Marker output can be set at each point and displayed on the interface;
(6) The wall thickness pattern modification has a confirmation link to avoid misoperation.
The wall thickness graphical interface is shown in Figure 3.
Figure 3 parison wall thickness curve screen
2.PLC control software
PID control is one of the earliest developed control strategies because it involves simple design algorithms and control structures, and is well suited for engineering applications. In addition, the PID control scheme does not require a precise mathematical model of the controlled object, and the PID control effect is generally satisfactory, so the application of the PID control is extensive.
The standard PID controller model is:
Where U(t) is the control variable entering the control object, and e(t) is the error signal.
Discretize equation (1) to:
Order, k=kp/T1
Typical characteristics of electro-hydraulic servo systems are low damping and time-varying, nonlinear, large inertia and hysteresis, making it difficult to obtain a good mathematical model. Conventional linear PID algorithms, under large inertia and hysteresis, tend to result in large overshoots and longer fluctuations. Under the linear proportional term, the robustness of the system is poor, and it is difficult to achieve the requirements of fast and accurate response. Therefore, when debugging the control system, an intelligent nonlinear PID control method is introduced for the PLC digital controller.
The intelligent nonlinear PID is based on the traditional PID control, transforming kpe(k)+ki∑e(j)T into a nonlinear function f(e(k))∑e(j) about e(k). ), plus an adjustable gain kp. The schematic diagram of the intelligent nonlinear PID is shown in Fig. 4.
Figure 4 Schematic diagram of intelligent nonlinear PID
In Figure 4, Continuous is the continuous wall thickness control setting, and Accumulator is the storage wall thickness control setting. Co is the filter constant, which is used for the D part of the PID controller. Act is the feedback of the linear displacement sensor of the servo cylinder and is filtered into the PID controller. The PID controller scan cycle is 2ms, and the Io scan also refreshes the completion output. The intelligent nonlinear PID system has strong influence ability, good robustness and certain adaptability, effectively overcomes the large inertia and hysteresis of electro-hydraulic servo, and accurately realizes the storage cylinder or continuous blow molding. Wall thickness control requirements for wall thickness of machine blanks from 30 to 300 points.
The combination of the axial controller and the radial controller will continue to be the future direction of the parison wall thickness control system and will require higher control accuracy. At present, the development of the domestic plastic hollow molding machine wall thickness control system, in addition to the need to continue to adopt more advanced control methods, in keeping with the international development in terms of technical level, it is also necessary to accelerate the development of its production and scale and specialization. . In this way, the domestic parison wall thickness control system can be used as an independent product to enter the international market as soon as possible.
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