Design of blank wall thickness control system for

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Design of blank wall thickness control system for hollow molding machine

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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, domestic 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 company") has a market share of more than 80% in China. With the rapid development of packaging industry and automobile industry, plastic hollow containers and hollow shaped products are more and more widely used, and their structures tend to be diversified. For the extrusion blow molding of hollow containers, the control of parison wall thickness is one of the most critical technologies, and it is also the difficulty in the design of hollow molding machine. Qinchuan company successfully designed the wall thickness control system through technical breakthrough. The production practice of many users has proved that the system is superior to the current imported products in both operation and control, and has achieved great success in the market

structure of parison wall thickness control system

with the increasingly complex geometry of hollow blow molded products, good preform design is particularly important for parts that obtain the required wall thickness distribution with minimal raw material consumption and stable structure. Therefore, in the parison forming stage, the wall thickness distribution of the parison can be adjusted according to the expansion ratio of each part during parison expansion. At the same time, considering the changes of extrusion expansion, sag and rebound during the formation of the parison, a reasonable parison thickness distribution curve is obtained, and then the electro-hydraulic servo system is adjusted to make the die gap between the inner mold and the outer mold change with the change of the parison position, so that the products with uniform thickness can be blown out. The impact resistance test of the products shows that the uniform wall thickness makes the strength of the products greatly improved; At the same time, it also saves raw materials, shortens the cooling time of finished products, reduces the defective rate, and greatly improves the production efficiency and product quality

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. Its working principle is shown in Figure 1

Figure 1 structure diagram of parison wall thickness control system

1 The control principle of the wall thickness control system

64 point axial wall thickness controller (AWDs) is one of the core technologies of the plastic hollow molding machine of Qinchuan company, which plays a decisive role in the performance of the whole machine. In recent years, through continuously optimizing the hardware structure of the wall thickness control system and adding the radial parison controller (PWDs), software program structure, action program, control algorithm and mathematical model, a wall thickness control system with the combined action of axial wall thickness controller and radial wall thickness controller has been developed. The system has been successfully applied in production and is at the leading level in China

The control system of PLC and touch screen is one of the design features of plastic hollow molding machine of Qinchuan company. Use the touch screen (i.e. human-machine interface) to adjust the parison thickness curve and related process parameters, so that the PLC controller can control the servo proportional valve to achieve the predetermined die gap after receiving the curve data and parameter information set by the human-machine interface. The whole process requires the control system to respond in milliseconds, which means that not only the calculation speed of hardware should be high, but also there should be new breakthroughs in software and digital control theory, so as to meet the requirements of rapid response, accurate control, stability and reliability of the parison wall thickness control system

for the control system of PLC and touch screen, Qinchuan company adopts a real-time multi task operating system, so the priority and cycle time of each task can be defined, and the cycle time can be as short as 1ms, which optimizes the real-time performance of the whole system. Among them, pws3261-tft touch industrial man-machine interface is produced by Taiwan Delta Electronics Company. The touch screen is a programmable display panel integrating display and touch switch. It has the characteristics of compact structure, convenient connection and intuitive and concise operation. It has the function of multi-channel serial port communication, and can display the equipment status, operation instructions, parameter settings, action flow, statistical data, alarm information and simple reports in real time. In the plastic electrical control system, the setting of various parameters, the use of button switches, the display of wall thickness control curve and alarm information can 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 wall thickness graphics

in addition, Qinchuan company also adopts cjig-cpu43h programmable controller produced by OMRON in Japan. The product has rich operation instructions, sequence instructions, data processing instructions and high-speed processing functions to realize instructions, and has high reliability and strong adaptability. It can be applied in various complex and harsh environments. In the high temperature requirement of the electrical control of plastic machinery, the controller can well realize the functions of precise control of action, conversion and operation of analog quantity, data processing and i/o signal processing

2. Electrohydraulic servo system

electrohydraulic servo system has many advantages, among which the most prominent ones are high response speed, large output power and high control accuracy, which are exactly the basic requirements of parison wall thickness control system. In the electro-hydraulic servo system, servo valve is one of the most important components and the key link to realize electro-hydraulic conversion. It directly affects the response speed and stability of the servo system. In the electro-hydraulic servo system, Qinchuan company adopts csdy3 jet electro-hydraulic servo valve. The signal of the valve changes from 0 to 100%, the response time is ≤ 10ms, the response is extremely fast, the accuracy is also very high, but the requirements for oil quality are also very high. The structure of the electro-hydraulic servo system is shown in Figure 2

Figure 2 structure diagram of electro-hydraulic servo system

software design of parison wall thickness control system

software design of parison wall thickness control system 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 we know the function values of several points in a certain interval, we can make an appropriate specific function to take the known values at these points, and use the values taken by this specific function at other points in this interval as the approximate values of function f (x). If the specific function is polynomial, it is called "interpolation polynomial". Generally, the interpolation algorithm is identified according to the category of interpolation polynomial. In imported blow molding machines, there are several different wall thickness control graphics software, such as 30 points, 100 points and 200 points of American Moog, which adopt linear interpolation, difference quotient interpolation and spline interpolation respectively

wall thickness control requires that the set points of the wall thickness graph should be continuous and derivable, otherwise, for the axial wall thickness distribution system (AWDs), there will be rings on the parison, and for the radial wall thickness distribution system (PWDs), there will be axial stripes on the parison. The linear interpolation algorithm is simple and straightforward. It is very easy to fit between two points, and the amount of calculation is also very small. However, because the algorithm does not consider the smooth transition between the end points of the interval, basically every point needs to be adjusted. Therefore, this algorithm can be used for wall thickness graphics with fewer points, and when there are more intervals, the operation is more cumbersome, so this algorithm is not applicable. In comparison, although spline interpolation ensures the continuous derivability between all set points, due to the smooth transition of all set points, when a point is adjusted and stretched beyond a certain degree, not only the curve near the point will change, but also the curve far away from it will change. This violates the original intention of the design, making the originally adjusted wall thickness curve area invalid 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 completely meet the requirements of wall thickness graphics. Considering the smooth transition and mutual influence, the wall thickness control system of Qinchuan company puts forward the interpolation method combining B-spline and Bezier curve, which can not only meet the requirements of smooth transition at the end of the curve, but also overcome the shortcomings of spline interpolation method that the stretching amplitude of a point is too large and leads to the deformation of distant curves. Even through a few control points, any wall thickness curve of 30~300 points can be drawn, and the operation is more convenient. The wall thickness curve obtained by this algorithm has the following characteristics:

(1) curve fitting between two set points

(2) there are two ways: stretching at each point and fine-tuning the regional curve

(3) there are three ways to adjust the weight: overall adjustment, high-end mode and low-end mode

(4) the wall thickness pattern enlarges and shrinks with the change of adjustment range, which is easy to adjust and observe

(5) mark output can be set at each point and displayed on the interface

(6) there is a confirmation link for the modification of wall thickness graphics, which can avoid misoperation

the graphical interface of wall thickness is shown in Figure 3

Figure 3 blank wall thickness curve picture

c control software

pid control is one of the earliest developed control strategies, because the design algorithm and control structure involved are very simple, and it is very suitable for engineering application background. In addition, the PID control scheme does not require an accurate mathematical model of the controlled object, and the effect of using PID control is generally satisfactory, so the application of PID control is relatively wide, but it is difficult for everyone to use it

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 obtain:

let, k=kp/t1

the typical characteristics of electrohydraulic servo system are low damping, time-varying, nonlinear, large inertia and hysteresis, so it is difficult to get a good mathematical model. The integral term of conventional linear PID algorithm often leads to large overshoot and long-time fluctuation under large inertia and lag. Under the action of proportional term, the robustness of the system is poor, and it is difficult to meet the requirements of rapid and accurate response. Therefore, when debugging the control system, the intelligent nonlinear PID control method is introduced into the PLC digital controller

intelligent nonlinear PID is to transform KPE (k) + ki Σ e (J) t on the basis of traditional PID control, making it a nonlinear function f (E (k)) ∑ e (J) about e (k), plus an adjustable gain KP. The schematic diagram of intelligent nonlinear PID is shown in Figure 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 by Part D of the PID controller. Act is the feedback of the linear displacement sensor of the servo cylinder, which is filtered into the PID controller. The scanning cycle of PID controller is 2ms, and IO scanning also refreshes and completes the output at the same time. The intelligent nonlinear PID system has strong influence ability, good robustness, and certain adaptability, which effectively overcomes the large inertia and hysteresis of the electro-hydraulic servo, and accurately realizes the wall thickness control requirements of 30~300 points of the blank wall thickness of the storage cylinder or continuous blow molding machine


among many plastic hollow molding machines in Qinchuan company, the above billet wall thickness control system project has been widely used. After years of practice, it has been proved that the wall thickness control effect of the system is good, easy to operate, and has been highly praised by users

the combination of axial controller and radial controller will

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