A temperature control implementation method optimized based on the electrofusion temperature characteristics of PE electrofusion pipe fittings

Abstract

The present invention belongs to the technical field of PE electrofusion fitting welding, and discloses a temperature control implementation method optimized based on the electrofusion temperature characteristics of PE electrofusion fittings. Based on finite element simulation, this method determines the adjustment rule of the power supply output parameters that can obtain good electrofusion temperature characteristics by optimizing the electrofusion temperature characteristics in different time periods, and then uses this adjustment rule as a control rule to control the output parameters of the power supply during the fusion process of electrofusion fittings, so as to realize the temperature control during the fusion process of electrofusion fittings. The method of the present invention successfully realizes the effective control of the temperature during the fusion process of electrofusion fittings in the case where non-destructive detection of the internal temperature of PE electrofusion fittings cannot be achieved. At the same time, the concept of closed-loop temperature control fusion is introduced to make the temperature control implementation method of the present invention have better adaptability.

Description

Technical Field

[0001] The present invention belongs to the technical field of welding of PE electrofusion fittings, and particularly relates to the technology of controlling the heating temperature of a resistance wire to a PE material during the welding process of PE electrofusion fittings. Background Art

[0002] A high-density polyethylene (PE) electrofusion fitting is a pipe connector with resistance wires for electric heating fusion buried inside. A group of resistance wires are distributed in two annular zones at both ends inside the fitting. After the two ends are inserted into the butt pipes, the resistance wires of the PE electrofusion fitting are energized to heat up, reaching the melting temperature range of the PE material and maintaining for a period of time, so that the PE materials of the PE fitting and the butt pipe section both have a certain depth of molten layer. At the same time, the molten slurry fills the gaps in the annular zone of the butt pipe section, and then after power-off and natural cooling, reliable molecular entanglement and curing are achieved to complete the entire welding process.

[0003] Currently, the commonly used method for welding PE fittings is to supply power to the resistance wires of the PE fittings with a constant voltage output from the power supply and a fixed time length. The control of the energization time length generally depends on the empirical parameters under specific fitting structures and butt conditions. For a certain specification of fitting, a set of electrofusion parameters (usually a fixed 39.5V - time length t) corresponds. This electrofusion method actually does not have a strict concept of controlling the welding temperature of the butt pipe section, and may be affected by multiple factors such as the gap of the butt pipe, the environmental temperature and air convection conditions, and the matching rationality of the fitting structure parameters and electrofusion parameters, resulting in over-welding (coking), under-welding and other phenomena, and then leading to welding quality problems of the butt pipe, leaving serious safety hazards for the construction of gas pipelines and other pipe networks.

[0004] During the electrofusion process, effective control of the temperature is an effective means to ensure the welding quality and efficiency. The control of the temperature during the electrofusion process includes: control of over / under-fusion at the interface, control of the molten zone range, control of the welding efficiency, and control of the cooling time. Theoretically, by monitoring the temperature at the boundaries of the interface and the molten zone in real time (for example, opening holes on the fitting and implanting thermocouples into the parts that need to monitor the temperature such as the boundaries of the interface and the molten zone), and immediately adjusting the power output parameters (voltage / current) according to the monitored values, the above control purposes can be achieved. However, there is currently no effective solution for non-destructively detecting the internal temperature of PE electrofusion fittings. Whether there is a way to achieve non-destructive detection of the internal temperature of PE electrofusion fittings, or, in the case where non-destructive detection of the internal temperature of PE electrofusion fittings cannot be achieved, whether there is a way to control the temperature during the electrofusion process, is a problem that has not been solved yet. Summary of the Invention

[0005] The object of the present invention is to address the reality that the temperature control during the electrofusion process of PE electrofusion fittings has not been achieved in the prior art. In the case where non-destructive detection of the internal temperature of PE electrofusion fittings cannot be realized, the electrofusion temperature control is achieved through the optimization of the electrofusion temperature characteristics of PE electrofusion fittings.

[0006] To achieve the above object, the technical solution provided by the present invention is: a temperature control implementation method based on the optimization of the electrofusion temperature characteristics of PE electrofusion fittings, characterized by including:

[0007] Step 1, Optimization of the electrofusion temperature characteristics of PE electrofusion fittings based on simulation analysis. This step is based on the thermal analysis of finite element simulation. By adjusting the output parameters of the simulated power supply, the electrofusion temperature characteristics of PE electrofusion fittings are optimized, specifically including:

[0008] Step 1-1, Establish a simulation model of the target PE electrofusion fitting;

[0009] Step 1-2, Conduct a constant-parameter simulation (finite element thermal analysis with constant power supply output parameters) on the simulation model under different power supply output parameters, select the optimal one from the simulation results, and use its electrofusion temperature characteristics as the basis for optimizing the electrofusion temperature characteristics for the next step;

[0010] Step 1-3, Optimization simulation of electrofusion temperature characteristics in stages: Divide the entire welding process into three stages. In the first stage, use the power supply output parameter value corresponding to the optimal electrofusion temperature characteristics in Step 1-2 as the basic power supply output parameter value, and add a preset initial upward adjustment parameter value (for example, 50% of the power supply output parameter value corresponding to the optimal electrofusion temperature characteristics) as the power supply output parameter value for the first stage to start the simulation process; when the interface (that is, the center of the target melting zone) reaches the set upper limit temperature, lower the power supply output parameter value to the basic power supply output parameter value. At this time, start the second stage from this downward adjustment. In the second stage, set the power supply output parameter adjustment rule to adjust the power supply output parameter once when the condition is met to make the interface temperature fluctuate within the set range; when the boundary of the target melting zone reaches the target temperature, the simulation process enters the third stage, stop the power supply output, and until the welding interface drops to the target temperature, the optimization simulation process ends, thus obtaining the optimized electrofusion temperature characteristics (it is not difficult to see that by continuously adjusting the power supply output parameters, the electrofusion temperature characteristics achieved satisfy the conditions that the interface is not over-melted and the boundary is not under-melted); during the entire welding process, every time the power supply output parameter value is adjusted, the welding process enters a new time period;

[0011] Step 2, Implementation of temperature control during the electrofusion process based on the adjustment of power supply output parameters, including:

[0012] Step 2-1: Use the power output parameter time-division adjustment rule corresponding to the optimized electrofusion temperature characteristics in Step 1 as the control rule for the output parameters of the electrofusion power supply.

[0013] Step 2-2: Optimization of the temperature control method based on the ambient temperature: Before the electrofusion process starts, detect the ambient temperature, calculate the ambient temperature optimization parameter for optimizing the power output parameters according to the formula t = a * t0, and optimize each power output parameter in the power output parameter time-division adjustment rule according to this ambient temperature optimization parameter; where a is the ambient temperature coefficient, t0 is the set duration, and t is the corrected duration.

[0014] Step 2-3: Optimization of the temperature control method based on resistance value monitoring: During the electrofusion process, monitor the resistance value of the heating wire, calculate the real-time temperature of the heating wire according to the resistance value by the formula T = (R / R0 - 1) / a + T0, and when the real-time temperature of the heating wire deviates from the optimized temperature characteristic curve, adjust the output parameters of the electrofusion power supply in real time; where a is the resistance temperature coefficient; T is the real-time temperature value; T0 is the initial temperature value, that is, the ambient temperature, and the standard value is 20°C; R0 is the initial resistance value of the heating wire; R is the real-time resistance value of the heating wire.

[0015] Further, the power output parameter adjustment rule set in the second stage is: when the interface reaches the set lower limit temperature, the power output parameter value is increased by a preset upward adjustment amplitude, and when the interface reaches the set upper limit temperature, the power output parameter value is decreased by a preset downward adjustment amplitude.

[0016] Further, in the preset amplitudes of the power output parameter adjustment rule set in the second stage, the preset upward adjustment amplitude is 10% of the basic power output parameter value, and the preset downward adjustment amplitude is 10% of the basic power output parameter value; the preferred setting range of the interface temperature is: the upper limit temperature is 230 degrees, and the lower limit temperature is 190 degrees.

[0017] Further, in Step 1-3, the initial upward adjustment parameter value in the first stage is: 50% of the basic power output parameter value.

[0018] Further, the electrofusion power supply preferably outputs a power supply with adjustable voltage, and the power output parameter refers to the output voltage of the electrofusion power supply.

[0019] Further, Step 1-1 specifically includes:

[0020] Step 1-1-1: For different types of pipe fittings, establish a three-dimensional model through three-dimensional modeling software based on the wall thickness of the inner and outer pipes of the PE electrofusion pipe fitting, the diameters of the inner and outer pipes, the diameter of the heating wire, the wire pitch of the heating wire, and the number of turns.

[0021] Step 1-1-2: Using a finite element preprocessor software, perform meshing on the three-dimensional model established in Step 1-1-1. The mesh around the resistance wire is denser and gradually becomes sparser outward to establish a meshed model;

[0022] Step 1-1-3: Using a non-linear finite element simulation software, add parameters to the meshed model established in Step 1-1-2, including the material, thermal conductivity, temperature coefficient, and mass density of the resistance wire, and the material, thermal conductivity, and mass density parameters of the pipe fitting to improve the model. And establish a simulation model by applying boundary conditions such as ambient temperature; then use existing empirical values as the output parameters of the simulated power supply, and perform thermodynamic simulation through the non-linear finite element simulation software; select the mesh nodes of the model to obtain the temperature characteristics obtained by the simulation; and by drilling holes in the pipe wall and inserting thermocouples into the corresponding positions, the actual temperature characteristics can be obtained. Compare the simulation results with the measured values to analyze and verify the reliability of the model, thus completing the establishment of the simulation model.

[0023] Further, in Step 1-2 and Step 1-3, during the simulation, take the axial section of the fusion ring surface (median position) of the PE electrofusion pipe fitting and the butt pipe section to show the temperature change during the entire electrofusion process, and obtain the temperature field distribution map of this section at different moments after the start of the fusion process and the radial temperature gradient distribution data. By synthesizing the temperature field distribution maps at different time points on the same section, the process of the fusion pipe section dynamically expanding (thickening) the molecular entanglement temperature zone upward and downward starting from the surface of the resistance wire can be obtained.

[0024] Further, in Step 1-2, the judgment criterion for the quality of the electrofusion temperature characteristics is: the closer the interface temperature and the boundary temperature of the molten zone are to their respective target temperatures and target durations, the better the electrofusion temperature characteristics are judged.

[0025] Advantages of the present invention:

[0026] Based on finite element simulation, the present invention determines the adjustment rule of the power supply output parameters that can obtain good electrofusion temperature characteristics by optimizing the electrofusion temperature characteristics method, and then uses this adjustment rule as a control rule to control the output parameters of the power supply during the fusion process of the electrofusion pipe fitting. This method successfully realizes the effective control of the temperature during the fusion process of the electrofusion pipe fitting in the case where non-destructive detection of the internal temperature of the PE electrofusion pipe fitting cannot be achieved.

[0027] The present invention divides the entire welding process into three stages: the first stage is rapid heating, the second stage is temperature maintenance, and the third stage is natural cooling. In the first stage, by using power output parameters much higher than the traditional empirical values (such as adjusting 39.5 to 60), according to q = i²rt (where q is the heat absorbed by the resistance wire, i is the power output current value, r is the resistance value of the resistance wire, and t is the heating duration), more heat is generated by the resistance wire in the same time, achieving the effect of rapid heating. In the second stage, the power output parameters are repeatedly adjusted up or down with the upper and lower limits of the interface temperature as nodes, so as to achieve rapid welding while ensuring that the interface does not undergo over-welding or under-welding. Through the implementation method of the present invention, the temperature control in the electrofusion fitting welding process can achieve a very high welding quality.

[0028] In addition, in the implementation link of temperature control of the present invention, the concept of closed-loop temperature-controlled welding is also introduced. Not only an optimization mechanism based on the ambient temperature is designed, but also a method for indirectly monitoring the actual temperature of the interface by monitoring the resistance value of the resistance wire is designed. Based on the monitoring results of the interface temperature, the power output parameters are adjusted in real time, so that the temperature control implementation method of the present invention has better adaptability.

[0029] In summary, in the welding process under the electrofusion temperature control implemented based on the method of the present invention, compared with the constant-pressure and constant-duration welding process, it has significant technical effects. Brief Description of the Drawings

[0030] Figure 1 is the conventional electrofusion temperature characteristic curve (represented by the interface temperature) with constant voltage output.

[0031] Figure 2 is the ideal electrofusion temperature characteristic curve (represented by the interface temperature).

[0032] Figure 3 is the schematic diagram of the three-dimensional model of the electrofusion fitting.

[0033] Figure 4 is the schematic diagram of the one-eighth three-dimensional model.

[0034] Figure 5 is the schematic diagram of the meshed model.

[0035] Figure 6 is the schematic diagram of the simulation model.

[0036] Figure 7 is the distribution diagram of the heating temperature field at 70s.

[0037] Figure 8 is the distribution diagram of the cooling temperature field at 600s.

[0038] Figure 9 is the temperature characteristic curve of the interface and at 2mm radially in the single-period simulation.

[0039] Figure 10 It is the temperature characteristic curve at the radial 2 mm and the multi-period simulation sub-interface.

[0040] Figure 11 It is a schematic diagram of the power system control principle. Specific implementation manners

[0041] The following further explains the temperature control implementation method of the present invention in conjunction with the accompanying drawings.

[0042] The electrofusion temperature characteristic of the present invention refers to the temperature-time change situation inside the pipe fitting during the electrofusion process. The temperature-time change situation at a certain point can be expressed by a temperature characteristic curve.

[0043] According to the present invention, in order to achieve electrofusion temperature control through the optimization of the electrofusion temperature characteristic of PE electrofusion pipe fittings, generally the following three aspects of work are involved: (1) analysis of the electrofusion temperature characteristic and determination of the electrofusion temperature characteristic that conforms to the fusion mechanism and meets the fusion quality requirements; (2) temperature-controlled fusion implemented relying on an intelligent electrofusion power supply; (3) optimization of the fusion control based on the environment and real-time detection.

[0044] The starting temperature of the PE pipe fitting material melting is about 130 °C. When the pipe fittings are fused, it is desired to reach the material molecular entanglement temperature of 180 - 240 °C. The material will carbonize after being higher than 240 °C. The conventional electrofusion process of PE electrofusion pipe fittings is to apply a fixed voltage (such as 39.5 V) to the resistance wire of the electrofusion pipe fitting, and continue a pre-determined matching energization time for a specific specification of the pipe fitting, and then complete the heating and fusion process. Its fusion temperature characteristic is as Figure 1 shown. From the perspective of temperature control, this process is a temperature open-loop control implemented with the pre-calibrated time parameter as the core for the butt joint pipe fitting object. Obviously, because the control object may be affected by many butt joint conditions, this method of controlling the fusion temperature has defects.

[0045] For the actual electrofusion butt joint situation in the application of PE electrofusion pipe fittings, the electrofusion temperature field of the butt joint pipe section can be modeled and analyzed. The simulation model is established based on the specifications of PE electrofusion pipe fittings, the distribution of resistance wires and their basic parameters, the matching conditions of the butt joint pipes, the ambient temperature, etc. Taking the axial section of the fusion pipe section (median) of the PE electrofusion pipe fitting and the butt joint pipe fitting to show the temperature change during the entire electrofusion process, the temperature field distribution map of this section at different moments after the start of the fusion process and the temperature gradient distribution data along the radial direction can be obtained. By synthesizing the temperature field distribution maps at different time points on the same section, the process of the molecular entanglement temperature zone dynamically expanding (thickening) upward and downward from the surface of the resistance wire of the fusion pipe section can be obtained.

[0046] If the molten layer of the pipe fitting is too thin, the welding reliability will be reduced. The thicker it is, the more energy is consumed, and the risk of deformation of the overall structure of the pipe fitting also increases. Therefore, in order to ensure welding reliability, prevent the deformation of the structure of the welded pipe fitting and control the electrofusion energy consumption, it is necessary to control the thickness of the molten layer of the PE electrofusion pipe fitting within a reasonable range. According to experience, it is preferably to control the PE electrofusion pipe fitting at a molecular entanglement molten layer thickness of 2-3 mm. From the perspective of welding construction efficiency, it is also hoped that the time to achieve reliable welding can be as short as possible. The control of the molecular entanglement layer thickness is a process of reasonable matching between the heating rate of converting electrical energy into heat energy and the heat conduction rate of the material. If the heating is too fast to keep up with the conduction, local high temperature exceeding the limit and carbonization may occur. If the heating is too slow, not only will the heat conduction loss increase, but more importantly, the heat energy will have the opportunity to conduct more to the parts outside the target molten layer, increasing the risk of pipe fitting deformation. If the existing open-loop temperature control of constant voltage and constant duration welding is improved to closed-loop temperature control of variable voltage and variable duration, an optimized electrofusion temperature characteristic may be obtained.

[0047] To sum up, combining the practical experience of electrofusion pipe fitting construction welding, the optimization objectives of the welding temperature characteristics can be summarized as follows: ① The temperature of the welded pipe section rises to the control temperature range ≥180°C relatively quickly; ② The temperature of the molten layer is controlled within the molecular entanglement temperature range of the material (180-240°C), and the extension and thickening of the entangled molten layer reach the standard (2-3 mm); ③ Less power consumption of the power supply and the shortest overall power-on welding time.

[0048] According to the above objectives, the ideal electrofusion temperature characteristics should be as Figure 2 shown. During the entire electrofusion process, according to the temperature change law, it is divided into three stages in sequence: the first stage is the rapid heating stage by applying electricity; the second stage is the temperature control and protection stage (the molecular entanglement molten layer gradually extends); the third stage is the natural cooling and temperature reduction stage after power-off.

[0049] The overall temperature control duration (t1 + t2) is related to the temperature characteristics of the first and second sections. The temperature control optimization is mainly carried out for the output (u-t) characteristics of the electrofusion power supply; the characteristics of the third section correspond to the natural cooling of the molten zone temperature after power-off, which is related to the subsequent quality control links. It should be particularly pointed out that continuous temperature control in the range of 180-240°C is to achieve a certain thickness of the entangled molten zone, and the extension speed of this entangled molten zone mainly depends on the temperature conduction speed.

[0050] Figure 2The ideal electrofusion temperature characteristics shown, compared with the temperature characteristics under traditional constant voltage and constant time control, have significant advantages: ① From room temperature to the melting temperature, the characteristics can be programmed for the heating rate according to the requirement of rapid heating (depending on the output control of the electrofusion power supply), which is beneficial to the control of the melting layer thickness, energy conservation, and time saving; ② After heating to the melting temperature range, the characteristics have obvious out-of-tolerance control limits, and there will be no over-melting or under-melting caused by too high or too low temperature; ③ The holding time of the melting temperature corresponds to the thickness of the entangled melting layer, and the corresponding layer thickness control can be achieved. For the temperature characteristics that the actual expected electrofusion power supply is relatively to achieve for the fusion pipe fittings, if the continuous temperature control section (section II) is close to the high position of 240°C, the time (t1 + t2) is relatively short, and if the continuous temperature control section is close to the low position of 180°C, the time (t1 + t2) is relatively long.

[0051] After determining the ideal electrofusion temperature characteristics, the next thing to solve is how to achieve this ideal electrofusion temperature characteristics through the control of the power supply, which is the biggest technical difficulty of the present invention.

[0052] For this reason, the present invention designs an electrofusion temperature characteristics optimization based on the electrofusion temperature characteristics under conventional constant voltage. In the simulation environment, by adjusting the output parameters of the power supply (simulated), the electrofusion temperature characteristics are optimized until the electrofusion temperature characteristics that meet the optimization objectives are obtained. Then, the power supply output parameter adjustment rules for the electrofusion temperature characteristics that meet the optimization objectives are used as the power supply control rules in the actual electrofusion process, and finally, relatively ideal electrofusion temperature characteristics are achieved in practice.

[0053] Taking the adjustment of the voltage of the power supply as an example below, a specific description is made of the above-mentioned electrofusion temperature characteristics optimization process.

[0054] In this example, a three-dimensional model of the electrofusion pipe fitting is established for finite element simulation to simulate the change of the internal temperature field during the actual pipe fitting fusion process. And based on the temperature characteristics of the PE material that the heating section rises rapidly; the holding section remains in the optimal melting zone; and the cooling section cools naturally, the existing process parameters are optimized. By comparing and analyzing the simulation results, the feasibility of temperature control is confirmed.

[0055] The first step is to establish a three-dimensional model diagram of the electrofusion pipe fitting through the drawing software Solidworks. Taking the DN63 pipe fitting as an example, the established model is as Figure 3 shown.

[0056] Among them, 1 is the outer pipe, 2 is the inner pipe, and 3 is the resistance wire. The dimensions of the inner and outer pipes are shown in the following table:

[0057]

[0058] The parameters of the resistance wire are shown in the following table:

[0059]

[0060] For the convenience of calculation, one-eighth of the model is intercepted as the simulation model without affecting the simulation results, as shown in Figure 4 shown.

[0061] In the third step, the established three-dimensional model is imported into the CAE software Hypermesh. Considering that the resistance wire is the heat source and the molecular entanglement melting zone 2 mm away from the contact surface of the pipe fitting is relatively critical, the mesh near the resistance wire is denser. The mesh division result is as shown in Figure 5 shown.

[0062] After the mesh division is completed, the inner and outer pipe and resistance wire models are respectively imported into the finite element software Msc.Marc for thermal analysis. The simulation model nodes are as shown in Figure 6 shown.

[0063] Perform preprocessing on the imported model. It is known that the material of the resistance wire is H65 brass and the material of the pipe fitting is PE100. According to the material manual, the relevant parameters are confirmed as shown in the following table:

[0064] Table 1 Physical property parameters of H65 brass

[0065]

[0066] Table 2 Physical property parameters of PE100 varying with temperature

[0067]

[0068] Since the heat source is the resistance wire that generates heat due to the applied voltage, its heat is obtained from the formula Q = I²Rt. The resistance value of the resistance wire changes according to the formula R = R0[1 + a(T - T0)] during the temperature rise process, where a is the temperature coefficient of resistance; T is the real-time temperature value; T0 is the initial temperature value, that is, the ambient temperature, and the standard value is 20°C; R0 is the initial resistance value of the resistance wire; and R is the real-time resistance value of the resistance wire.

[0069] Currently, electrothermal fusion welders with a constant voltage output of 39.5 V are mostly used in the market for welding. The hot melt welding process parameters of DN63 pipe fittings are shown in the following table:

[0070] Hot melt welding process parameters of DN63 PE electrofusion pipe fittings

[0071]

[0072] Determine the volumetric heat flux of the heat source as P = Q / V according to the above parameters. Among them, Q is the generated heat and V is the volume of the resistance wire.

[0073] After the pre-treatment work is completed, the simulation of the model begins. Since a constant voltage of 39.5 V is applied for 70 s, the temperature of the heating wire at the center of the pipe fitting reaches the highest at 70 s. At this time, the temperature field of the entire model is as Figure 7 shown.

[0074] After heating for 70 s, the power supply stops, the heating wire no longer generates heat, and its temperature begins to drop. However, heat transfer is a process, and the PE material that conducts heat radially from the heating wire will still show a rising trend in temperature for a period of time before dropping. The schematic diagram of the temperature field after 600 s of natural cooling is as Figure 8 shown.

[0075] Select the nodes at the interface and 2 mm radially from the interface, and observe the temperature changes during the whole process, as Figure 9 shown. The upper curve in the figure is the temperature characteristic curve of the interface, and the lower curve is the temperature characteristic curve at 2 mm radially from the interface.

[0076] As can be seen from the figure, during the single-segment simulation with a constant voltage of 39.5 V, the temperature of the interface reaches the melting point at 18 s, reaches the optimal melting zone of 180 °C - 240 °C at 35 s, and reaches the highest point of 257.2 °C at 70 s; the temperature at 2 mm radially from the interface reaches the melting point at 63 s and the highest point of 145.1 °C at 78 s.

[0077] Optimize the single-period simulation and use a multi-segment voltage for simulation. The set parameters are 60 V for 10 s, 40 V for 5 s, 35 V for 5 s, 40 V for 5 s, the heating duration is 70 s, and the cooling is 600 s. Select the nodes at the interface and 2 mm radially from the interface, and observe the temperature changes during the whole process, as Figure 10 shown. The upper curve in the figure is the temperature characteristic curve of the interface, and the lower curve is the temperature characteristic curve at 2 mm radially from the interface.

[0078] As can be seen from the figure, during the initial heating process at 60 V, the interface reaches the melting point of 130 °C only at 6 s, reaches the optimal melting zone at 10 s, and reaches the highest point of 237.2 °C at 65 s without over-melting; at 2 mm, it reaches the melting point at 61 s and the highest is 143.3 °C at 81 s.

[0079] From the simulation results, the optimized segmented fusion welding has better temperature field data than single-segment fusion welding both in the heating stage and the holding stage, indicating that the optimized temperature control is indeed effective.

[0080] After optimizing the electrofusion temperature characteristics, to finally achieve the control of the temperature during the electrofusion process, it is necessary to further construct a controllable power supply system, and its control principle is as Figure 11 shown;

[0081] When the resistance wire of PE electric fusion pipe fitting is energized, the heat generated is:

[0082] Q = I²Rt = (U² / R)t…………(1)

[0083] Where t is the duration of heating, U is the output voltage of the power supply, and R is the resistance of the resistor;

[0084] When the resistance wire starts to heat up from the ambient temperature, the resistance value of the resistance wire will change from R0 to:

[0085] Rt = R0(1+AT+BT²) … …(2)

[0086] Where A and B are constants corresponding to the resistance wire material;

[0087] When U and I at both ends of the resistance wire can be accurately measured, the Rt after heating can be calculated by Ohm's theorem, and then the average temperature T of the resistance wire at the corresponding moment can be solved from equation (2). As an internal heat source, the heat generated by the resistance wire in the power-on state is always transferred to the surrounding area of ​​the resistance wire, and the surface temperature of the wire should also be higher than the temperature of the surrounding material. According to the temperature field simulation or experimental measurement data of the butt-joint pipe section structure, the heat conduction relationship between the heat source and the material can be basically determined, the temperature gradient value of radial conduction can be summarized, and the thickness value and time consumed for the formation and gradual increase of a certain molecular entanglement melting layer can be evaluated.

[0088] In order to realize the optimized temperature characteristics based on the output control of the electric melting power supply, it is also necessary to design a power supply control strategy or method. Figure 2 Taking the idealized temperature characteristic (single-valued function in the temperature range of 180~240℃) as a given input, a dual-input fuzzy control strategy (not limited to this) of △T and its rate of change with the optimization goal of controlling the thickness of the molten layer and shortening the power-on time can be established inside the electric fusion power supply. The controller output will dynamically adjust the u(t) characteristics on the load and the resistance wire to achieve the electric fusion temperature control target. When the output control accuracy of the temperature closed-loop system is high, raising the temperature characteristics of the continuous temperature control section of the closed-loop system output to close to 240℃ will inevitably help shorten the electric fusion time.

[0089] The temperature control closed-loop system uses the estimated value of the resistance wire temperature as feedback to estimate and evaluate the thickness of the entangled and melted layer starting from the surface of the resistance wire. Based on the simulation or experimental empirical values under certain conditions, in the presence of several uncertain factors, it is impossible to ensure the accuracy of the estimation of the melted layer thickness. This uncertainty is equivalent to the interference outside the temperature control closed-loop and can only be suppressed by prior prediction and by appropriately adjusting the control strategy or correcting the control parameters. In implementation, it is necessary to combine changes in the structural parameters and material parameters of the PE electrofusion fitting (such as the wall thickness of the electrofusion fitting, the position of the resistance wire, the wire loop pitch / unit ring surface electric power, the wire material and resistivity, the wire burial depth, the quality control tolerance), etc., to adjust the control strategy or correct the control parameters. For example, appropriately adjust the temperature control range and duration considering the following factors: (a) The average resistance Rp (Ω / mm2) per unit area of the wire winding ring area; (b) The wire burial depth h (mm); (c) The error ±△b of the wall thickness relative to the standard fitting; (d) The standard deviation of the butt joint gap ±δ; (e) The ambient temperature.

Claims

1. A temperature control implementation method optimized based on the electrofusion temperature characteristics of PE electrofusion fittings, characterized in that, Including: Step 1, Optimization of the electrofusion temperature characteristics of PE electrofusion fittings based on simulation analysis. This step is based on the thermal analysis of finite element simulation. By adjusting the simulated power supply output parameters, the electrofusion temperature characteristics of PE electrofusion fittings are optimized, specifically including: Step 1-1, Establish a simulation model of the target PE electrofusion fitting; Step 1-2, Conduct a constant parameter simulation on the simulation model under different power supply output parameters, select the optimal one among the simulation results, and use its electrofusion temperature characteristics as the basis for optimizing the electrofusion temperature characteristics for the next step; Step 1-3, Optimize the simulation of electrofusion temperature characteristics in stages: Divide the entire welding process into three stages. In the first stage, use the power supply output parameter value corresponding to the optimal electrofusion temperature characteristics in Step 1-2 as the basic power supply output parameter value, and add a preset initial increase parameter value as the power supply output parameter value in the first stage to start the simulation process; When the interface reaches the set upper limit temperature, lower the power supply output parameter value to the basic power supply output parameter value. At this time, enter the second stage from this downward adjustment. In the second stage, set the power supply output parameter adjustment rule to adjust the power supply output parameter once when the condition is met, so that the interface temperature fluctuates within the set range; When the boundary of the target molten zone reaches the target temperature, the simulation process enters the third stage, stop the power supply output until the welding interface drops to the target temperature, and the optimization simulation process ends, thus obtaining the optimized electrofusion temperature characteristics; During the entire welding process, every time the power supply output parameter value is adjusted, the welding process enters a new time period; Step 2, Implementation of electrofusion process temperature control based on power supply output parameter adjustment, including: Step 2-1, Use the power supply output parameter adjustment rule in stages corresponding to the optimized electrofusion temperature characteristics in Step 1 as the control rule for the power supply output parameters of the electrofusion power supply; Step 2-2, Optimization of the temperature control method based on the ambient temperature: Before the electrofusion process starts, detect the ambient temperature, calculate the ambient temperature optimization parameter for power supply output parameter optimization according to the formula t = a * t0, and optimize each power supply output parameter in the power supply output parameter adjustment rule in stages according to this ambient temperature optimization parameter; where a is the resistance temperature coefficient, t0 is the set duration, and t is the correction duration; Step 2-3, Optimization of the temperature control method based on resistance value monitoring: During the electrofusion process, monitor the resistance value of the resistance wire, calculate the real-time temperature of the resistance wire according to the resistance value by the formula T = (R / R0 - 1) / a + T0. When the real-time temperature of the resistance wire deviates from the optimized temperature characteristic curve, adjust the power supply output parameters of the electrofusion power supply in real time, where a is the resistance temperature coefficient; T is the real-time temperature value; T0 is the initial temperature value, that is, the ambient temperature, and the standard value is 20°C; R0 is the initial resistance value of the resistance wire; R is the real-time resistance value of the resistance wire.

2. The temperature control implementation method according to claim 1, characterized in that, The power supply output parameter adjustment rule set in the second stage is: When the interface reaches the set lower limit temperature, increase the power supply output parameter value by a preset increase amplitude, and when the interface reaches the set upper limit temperature, lower the power supply output parameter value by a preset decrease amplitude.

3. The temperature control implementation method according to claim 2, wherein Among the preset amplitudes of the power output parameter adjustment rules set in the second stage, the preset upward adjustment amplitude is 10% of the basic power output parameter value, and the preset downward adjustment amplitude is 10% of the basic power output parameter value; the setting range of the interface temperature: the upper limit temperature is 230 degrees, and the lower limit temperature is 190 degrees.

4. The temperature control implementation method according to claim 1, wherein In the steps 1-3, the initial upward adjustment parameter value in the first stage is: 50% of the basic power output parameter value.

5. The temperature control implementation method according to claim 1, characterized in that The electrofusion power supply is a power supply with adjustable output voltage, and the power output parameter refers to the output voltage of the electrofusion power supply.

6. The temperature control implementation method according to claim 1, wherein The step 1-1 specifically includes: Step 1-1-1, for different types of pipe fittings, based on the wall thickness of the inner and outer pipes of the PE electrofusion pipe fittings, the diameters of the inner and outer pipes, the diameter of the resistance wire, the wire spacing and the number of turns of the resistance wire, establish a three-dimensional model through three-dimensional modeling software; Step 1-1-2, use finite element preprocessor software to perform meshing on the three-dimensional model established in step 1-1-1. The mesh around the resistance wire is dense and gradually becomes sparse outward to establish a meshed model; Step 1-1-3 uses nonlinear finite element simulation software to add parameters to the meshed model established in step 1-1-2, including the material, thermal conductivity, temperature coefficient, and mass density of the resistance wire, and the material, thermal conductivity, and mass density parameters of the pipe fittings to improve the model, and establish a simulation model by applying boundary conditions; then use the existing empirical values as the output parameters of the simulated power supply, and perform thermodynamic simulation through nonlinear finite element simulation software; select the model mesh nodes to obtain the temperature characteristics obtained by the simulation; then drill holes in the pipe wall and insert thermocouples into the corresponding positions to obtain the actual temperature characteristics; compare the simulation results with the measured values to analyze and verify the reliability of the model, thereby completing the establishment of the simulation model.

7. The temperature control implementation method according to claim 1, characterized in that, In the steps 1-2 and 1-3, during the simulation, take the axial section of the fusion ring surface of the PE electrofusion pipe fitting and the butt joint pipe section to show the temperature change during the entire electrofusion process, and obtain the temperature field distribution map and the radial temperature gradient distribution data of this section at different moments after the start of the fusion process; comprehensively combine the temperature field distribution maps at different time points on the same section to obtain the process of the fusion pipe section dynamically expanding the molecular entanglement temperature zone upward and downward from the surface of the resistance wire.

8. The temperature control implementation method according to claim 1, wherein In the step 1-2, the judgment standard for the quality of the electrofusion temperature characteristics is: the higher the degree that the interface temperature and the boundary temperature of the molten zone are simultaneously close to their respective target temperatures and target durations, the better the electrofusion temperature characteristics are judged.

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