阅读说明：本技术 基于电阻丝的电阻温度系数自动测量的电熔管件焊接方法 (Electric melting pipe welding method based on resistance wire resistance temperature coefficient automatic measurement ) 是由 施建峰 任亦心 郑津洋 项光特 胡裕锐 葛周天 于 2021-08-06 设计创作，主要内容包括：本发明涉及电熔焊接技术,旨在提供一种基于电阻丝的电阻温度系数自动测量的电熔管件焊接方法。该方法包括：在电熔管件焊接前,通过脉冲电压激励电熔管件,自动测算电熔管件内部电阻丝的电阻温度系数；在焊接过程中,实时获取电路中的电压电流用于计算电阻丝的阻值变化情况；通过控制焊接电路的电压,将基于电阻温度系数计算的电阻温度控制在恒定的最优值,实现焊接过程中电熔管件内部温度的精准控制。本发明不需要分离已内嵌于电熔管件的电阻丝或使用成本较高、流程较复杂的水浴油浴调温等测量方式,仅借助焊机本身以及相对较短的时间即完成电阻温度系数的测算。可便捷获取电阻丝的电阻温度系数,支持电熔焊机的熔区温度监测以及智能焊接控制。(The invention relates to an electric fusion welding technology, and aims to provide an electric fusion pipe welding method based on resistance wire resistance temperature coefficient automatic measurement. The method comprises the following steps: before the electric melting pipe fittings are welded, the electric melting pipe fittings are excited through pulse voltage, and the resistance temperature coefficient of resistance wires inside the electric melting pipe fittings is automatically measured and calculated; in the welding process, voltage and current in a circuit are acquired in real time and are used for calculating the resistance value change condition of the resistance wire; by controlling the voltage of the welding circuit, the resistance temperature calculated based on the resistance temperature coefficient is controlled at a constant optimal value, and the accurate control of the internal temperature of the electric melting pipe fitting in the welding process is realized. The invention can measure and calculate the resistance temperature coefficient only by means of the welding machine and in a relatively short time without separating resistance wires embedded in electric melting pipe fittings or measuring modes such as water bath oil bath temperature regulation with higher use cost and more complex flow. The resistance temperature coefficient of the resistance wire can be conveniently acquired, and the melting zone temperature monitoring and intelligent welding control of the electric melting welding machine are supported.)

1. A resistance wire-based electric melting pipe welding method for automatically measuring resistance temperature coefficient is characterized by comprising the following steps:

the method comprises the following steps: measuring the ambient temperature;

step two: measuring the temperature coefficient of resistance, comprising: applying voltage excitation of a rectangular waveform to the electric melting pipe fitting, collecting voltage and current in a circuit after excitation, and calculating a resistance temperature coefficient measurement value of a resistance wire of the electric melting pipe fitting;

step three, repeating the measurement and calculation, comprising: after the temperature of the resistance wire is cooled to the environment temperature, repeatedly executing the step for two n times to obtain n resistance temperature coefficient measurement values, and calculating the average value of the n resistance temperature coefficient measurement values as a resistance temperature coefficient;

step four: starting welding, detecting voltage and current in a welding circuit, and calculating the current resistance temperature according to the resistance temperature coefficient calculated in the step three; and controlling the resistance temperature at a constant value during the welding process according to the voltage execution of the welding circuit.

2. The method according to claim 1, wherein the second step specifically comprises:

(2.1) applying a voltage excitation of rectangular waveform to the electrofused pipe, t₀Is the total duration of a single voltage excitation; collecting voltage U in welding output circuit_iAnd current I_iI is the number of sampling cycles, which is between 1 and N, N is t₀Total number of sampling cycles in time, U_iIs the voltage value of the ith sampling period, I_iThe current value of the ith sampling period; calculating real-time resistance R of resistance wire_i(ii) a Calculating resistance R of 1 st and Nth sampling period₁、R_NRespectively recorded as initial resistance values R₀And a final resistance value R;

(2.2) calculating the effective volume of the resistance wire according to the following formula:

wherein, V_mIs the effective volume of the resistance wire of the electric melting pipe fitting, and the unit is m³；d_rIs the diameter of the resistance wire and has the unit of m; d_nThe nominal diameter of the electric melting pipe and the pipe fitting is m; l is_dThe distance between the center of the resistance wire and the inner surface of the electric melting pipe fitting is m; n is_cThe actual number of resistance wires in the melting zone range;

(2.3) calculating the temperature coefficient of resistance α after completion of voltage excitation according to the following formula:

wherein R is_jIs the contact thermal resistance between the resistance wire and the polyethylene and has the unit of m²DEG c/W; s is the contact area between the resistance wire and the polyethylene, and the unit is m²；C_mThe specific heat capacity of the resistance wire is expressed in J/kg DEG C; v_mIs the volume of the resistance wire, and the unit is m³；ρ_mIs the density of resistance wire, and the unit is kg/m³。

3. The method of claim 2, wherein in step (2.1), the voltage and current in the welding output circuit are collected at a sampling frequency greater than 10 Hz.

4. The method according to claim 2, wherein in step (2.1), a single voltage stimulus is applied for a time period t₀The value of (c) is between 100ms and 1000 ms.

5. The method according to claim 1, wherein in the third step, n is an integer not less than 2 according to the test precision requirement.

6. The method according to claim 1, wherein the fourth step specifically comprises:

(4.1) in the welding process, acquiring voltage and current in a circuit in real time, and calculating the real-time resistance value R of the resistance wire_t；

(4.2) calculating the real-time temperature of the resistance wire according to the following formula:

in the formula, T_tThe temperature of the resistance wire at t is given in unit; t is the time of welding; r_tThe resistance value of the resistance wire is t, and the unit is omega; r₀Is an initial resistance value; alpha is the resistance temperature coefficient of the resistance wire; t is₀Is an ambient temperature value;

(4.3) regulating and controlling the voltage of the welding circuit to regulate the resistance temperature T_tThe control is at the constant value to realize the accurate control of the inside temperature of electric smelting pipe fitting in the welding process.

7. An electric melting pipe fitting welding device based on resistance temperature coefficient automatic measurement of resistance wires comprises an electric melting welding machine; the electric fusion welding machine is characterized in that a welding circuit of the electric fusion welding machine comprises the following modules: the device comprises a power supply module, a welding voltage output module, a voltage and current detection module, a welding controller module and an automatic resistance temperature coefficient measuring and calculating module;

the power supply module is used for processing an external power supply and modulating the external power supply into direct current;

the welding voltage output module is used for outputting real-time welding voltage calculated by the welding controller module;

the voltage and current detection module is used for detecting real-time voltage and current in the welding circuit;

the automatic resistance temperature coefficient measuring and calculating module is used for measuring and calculating the resistance temperature coefficient of a resistance wire in the to-be-welded electric melting pipe fitting;

the welding controller module is used for implementing a constant temperature control algorithm according to the real-time voltage and current detected by the voltage and current detection module and the resistance temperature coefficient obtained by the automatic measuring and calculating module of the resistance temperature coefficient, and controlling the real-time voltage in the welding process;

wherein, the automatic measuring and calculating module of the temperature coefficient of resistance includes:

the pulse voltage output submodule is used for applying voltage with a rectangular waveform to the electric melting pipe fitting;

the voltage and current detection submodule is used for collecting voltage and current in the welding output circuit;

the resistance temperature coefficient calculation submodule is used for calculating the resistance temperature coefficient of the resistance wire;

and the environment temperature measuring submodule is used for measuring the environment temperature around the electric fusion welding machine.

Technical Field

The invention relates to a plastic pipe welding technology, in particular to a resistance wire-based electric melting pipe welding method and device for automatically measuring resistance temperature coefficient.

Background

Compared with the traditional metal pipeline, the polyethylene pipeline has the advantages of light weight, good flexibility, large specific strength, corrosion resistance, long service life, economy, environmental protection and the like, and is gradually replacing the metal pipeline to become the mainstream of the buried gas pipeline in recent years. When the polyethylene pipeline is arranged for a long distance and applied to engineering, the pipe materials need to be connected. The electrofusion welding technology is the most widely used plastic pipeline connection mode in engineering because of low equipment cost, simple operation, high automation degree and high construction efficiency. The operation process of the electric melting welding is that two sections of pipes to be connected are oppositely inserted into an electric melting pipe fitting embedded with spiral conductive copper wires, so that the outer surface of the electric melting pipe fitting is attached to the inner surface of the pipe fitting. When in use, a fixed welding voltage is applied to the electric melting pipe fitting. After the resistance wire is electrified, the heat energy is generated due to the Joule effect, so that the temperature of the solid polyethylene around the resistance wire is increased and melted. The melting area expands to a welding interface along with the welding process until the pipe and the pipe fitting are integrated, and the electric melting joint with certain strength is formed after cooling.

With the increasingly wide application of polyethylene pipeline systems in the fields of gas, water supply and the like, the safety problem of the electrofusion welding technology is receiving more and more attention. According to the PPDC plastic tubing database committee report of 2020, 57% of tubing failures occurred at the tubing joints. The dangerous defect rate of the electric melting joint reaches 7.1 percent as further shown by the literature (application case analysis of polyethylene gas pipeline ultrasonic phased array detection engineering [ J ]. pressure vessel, 2020, 37 (12): 54-62). The electric melting joint is used as the weakest part of the pipeline system, the service life of the whole pipeline system is influenced, and therefore the key for guaranteeing the safety of the polyethylene pipeline system is to improve the quality of electric melting welding.

In the electric fusion welding, the internal fuse zone temperature is a main influence factor affecting the quality of the electric fusion joint. If the temperature of the welding interface is too low or does not stay above the melting temperature for a long enough time, cold welding defects can be generated; if the temperature of the melting zone is too high, the polyethylene material may be subjected to a risk of pyrolysis, resulting in an overweld defect. In view of the fact that the conventional thermocouple temperature measurement mode cannot be practically applied to internal temperature measurement of the electric melting pipe fitting, researchers utilize the linear change relation of resistance values of resistance wires embedded in the pipe fitting along with the temperature, and provide a method for indirectly measuring and calculating the internal welding temperature through measuring the resistance values. This method is also applied to a number of patents for welding temperature control: for example, in the chinese invention patent "electrofusion welding method and electrofusion welder capable of preventing cold welding and overwelding defects" (CN201110428347.2), the inventor obtains the resistance change condition of the resistance wire in the electrofusion pipe fitting in real time by measuring voltage and current, and then indirectly measures and calculates the temperature of the polyethylene material around the resistance wire; the Chinese invention patent 'an intelligent welding method for electric melting pipe fittings' (CN104816467A) adopts control algorithms such as PID and the like, and controls the resistance value of the resistance wire of the electric melting pipe fittings by adjusting voltage so as to maintain the temperature of a melting zone at a constant temperature.

However, in the implementation of these methods, the most critical characteristic parameter, the temperature coefficient of resistance, is utilized as a known parameter. However, such a method cannot be applied to an actual welding scene, and the following three main reasons exist:

(1) different manufacturers, pipe fitting specification even the pipe fitting under batch, the resistance wire material is all different.

At present, the national standard of polyethylene pipe fittings only standardizes the basic parameters such as pipe fitting size, appearance, fit clearance and the like, and does not clearly standardize the material and the components of resistance wire materials. Taking domestic products as an example, electric melting pipe fittings produced by brands such as Yuhua, Asia and Dada, Cangzhou Mingzhu, Jilin Songjiang and Qingfa are different in resistance wire material and components. Even different specifications and different batches have different temperature coefficient of resistance.

(2) The temperature coefficients of resistance wires made of different materials have a large range.

For reasons of heat conductivity, electrical conductivity, flexibility and cost, the resistance wire material of the electrofused pipe is usually pure copper or copper alloy (brass, constantan, cupronickel, etc. are common choices). These materials also have a wide temperature coefficient of resistance range due to differences in copper content. For example, pure copper is generally an order of magnitude higher in temperature coefficient of resistance than copper alloys. In brass, the temperature coefficient of resistance decreases nearly by a factor of two as the copper content decreases from 57% to 65%. Therefore, the temperature coefficients of resistance wires made of different materials have great difference, and great influence is generated on the final result.

(3) The resistance temperature coefficient of the electric melting pipe fitting is not subjected to unified regulation or test

Because the common welding machine does not apply the resistance temperature coefficient, the pipe fitting manufacturer can not carry out uniform measurement and marking on the resistance temperature coefficient when carrying out test and calibration.

Based on the three reasons, under the condition that the real resistance temperature coefficient of the resistance wire of the pipe fitting cannot be determined, the internal temperature monitoring methods or the internal temperature control methods for improving the quality of electric fusion welding cannot be applied to a real welding scene, so that the method can be well popularized. Therefore, if the characteristics of the resistance wire of the electric melting pipe fitting are used for monitoring and controlling the temperature of the melting zone, the temperature coefficient of resistance needs to be accurately measured before welding.

At present, the measurement of the resistance temperature coefficient mainly refers to the national standard GB/T6148-. The method mainly comprises the steps of cutting a standard test section from a resistance wire to be detected, placing the standard test section in constant temperature tanks at normal temperature and different preset temperatures for heating and resistance measurement, obtaining an R-T relation curve, and solving a resistance temperature coefficient.

The method needs a thermostatic bath, a thermometer, a resistance measuring instrument and other test devices, and is only suitable for measuring and giving out resistance temperature coefficient parameters in the process design stage of the electric melting pipe fittings by manufacturers. When the electric melting pipe fittings are actually welded, the parameter also needs to be input into the electric melting welding machine. On one hand, the welding construction procedure is increased, the problems of wrong parameter input of electric melting pipe fittings of different brands in the same project and the like can be caused, and on the other hand, the difference between individuals and batches is inevitable. If the test link and test equipment specified in GB/T6148 and 2005 can be omitted, the resistance temperature coefficient of the resistance wire of the electric melting pipe fitting can be rapidly measured, the early-stage complex flow of resistance temperature coefficient measurement can be greatly simplified, and the implementation difficulty of electric melting welding temperature monitoring is reduced.

The invention aims to provide a method and a device for welding an electric melting pipe fitting based on resistance wire resistance temperature coefficient measurement. The resistance temperature coefficient of the resistance wire embedded in the electric melting pipe fitting is measured on line on the welding site without other sensing detection equipment except a welding machine, so that the electric melting welding based on the resistance temperature coefficient measurement of the resistance wire is completed.

Disclosure of Invention

The invention aims to solve the problem of overcoming the defects in the prior art and provides a method and a device for welding an electric melting pipe fitting based on resistance temperature coefficient measurement of a resistance wire.

In order to solve the technical problem, the technical scheme adopted by the invention is as follows:

the method for welding the electric melting pipe fittings based on the automatic resistance temperature coefficient measurement of the resistance wire comprises the following steps:

the method comprises the following steps: measuring the ambient temperature;

step two: measuring the temperature coefficient of resistance, comprising: applying voltage excitation of a rectangular waveform to the electric melting pipe fitting, collecting voltage and current in a circuit after excitation, and calculating a resistance temperature coefficient measurement value of a resistance wire of the electric melting pipe fitting;

step three, repeating the measurement and calculation, comprising: after the temperature of the resistance wire is cooled to the environment temperature, repeatedly executing the step for two n times to obtain n resistance temperature coefficient measurement values, and calculating the average value of the n resistance temperature coefficient measurement values as a resistance temperature coefficient;

step four: and starting welding, detecting the voltage and the current in the welding circuit, calculating the current resistance temperature according to the resistance temperature coefficient calculated in the step three, regulating and controlling the voltage execution of the welding circuit, and controlling the resistance temperature at a constant value in the welding process.

In the present invention, the first step specifically includes:

(1.1) measuring ambient temperature T around electric fusion welder₀；

The second step specifically comprises:

(2.1) applying a voltage excitation of rectangular waveform to the electrofused pipe, t₀For the total duration of the voltage excitation, the voltage U in the welding output circuit is detected_iCurrent I_iI is the number of sampling cycles, which is between 1 and N, N is t₀Total number of sampling cycles of time, U_iIs the voltage value of the ith sampling period, I_iThe current value of the ith sampling period; calculating real-time resistance R of resistance wire_i(ii) a Calculating resistance R of 1 st and Nth sampling period₁，R_NRespectively recorded as initial resistance values R₀And a final resistance value R;

(2.2) calculating the effective volume of the resistance wire according to the following formula:

wherein, V_mIs the effective volume of the resistance wire of the electric melting pipe fitting, and the unit is m³；d_rIs the diameter of the resistance wire and has the unit of m; d_nThe nominal diameter of the electric melting pipe and the pipe fitting is m; l is_dThe distance between the center of the resistance wire and the inner surface of the electric melting pipe fitting is m; n is_cOf resistance wires in the region of the melting zoneThe actual number.

(2.3) calculating the temperature coefficient of resistance α after completion of voltage excitation according to the following formula:

wherein R is_jIs the contact thermal resistance between the resistance wire and the polyethylene and has the unit of m²DEG c/W; s is the contact area between the resistance wire and the polyethylene, and the unit is m²；C_mThe specific heat capacity of the resistance wire is expressed in J/kg DEG C; v_mIs the volume of the resistance wire, and the unit is m³；ρ_mIs the density of resistance wire, and the unit is kg/m³。

In the invention, in the step (2.1), when the voltage and the current in the welding output circuit are collected, the sampling frequency is more than 10 Hz.

In the invention, in the step (2.1), a single voltage excitation application time t₀The value of (c) is between 100ms and 1000 ms.

In the third step, n is an integer not less than 2 according to the requirement of test precision.

The fourth step specifically comprises:

(4.1) in the welding process, acquiring voltage and current in a circuit in real time, and calculating the real-time resistance value R of the resistance wire_t；

(4.2) calculating the real-time temperature of the resistance wire according to the following formula:

in the formula, T_tThe temperature of the resistance wire at t is given in unit; t is the time of welding; r_tThe resistance value of the resistance wire is t, and the unit is omega; r₀Is an initial resistance value; alpha is the resistance temperature coefficient of the resistance wire; t is₀Is an ambient temperature value;

(4.3) regulating and controlling the voltage of the welding circuit to regulate the resistance temperature T_tControlled at a constant value to effect electricity during weldingAnd (4) accurately controlling the internal temperature of the melting pipe fitting.

The invention further provides an electric melting pipe fitting welding device based on resistance temperature coefficient automatic measurement of the resistance wire, which comprises an electric melting welding machine; the welding circuit of the electric melting welding machine comprises the following modules: the device comprises a power supply module, a welding voltage output module, a voltage and current detection module, a welding controller module and an automatic resistance temperature coefficient measuring and calculating module;

the power supply module is used for processing an external power supply and modulating the external power supply into direct current;

the welding voltage output module is used for outputting real-time welding voltage calculated by the welding controller module;

the voltage and current detection module is used for detecting real-time voltage and current in the welding circuit;

the automatic resistance temperature coefficient measuring and calculating module is used for measuring and calculating the resistance temperature coefficient of a resistance wire in the to-be-welded electric melting pipe fitting;

the welding controller module is used for implementing a constant temperature control algorithm according to the real-time voltage and current detected by the voltage and current detection module and the resistance temperature coefficient obtained by the automatic measuring and calculating module of the resistance temperature coefficient, and controlling the real-time voltage in the welding process;

wherein, the automatic measuring and calculating module of the temperature coefficient of resistance includes:

the pulse voltage output submodule is used for applying voltage with a rectangular waveform to the electric melting pipe fitting;

the voltage and current detection submodule is used for collecting voltage and current in the welding output circuit;

the resistance temperature coefficient calculation submodule is used for calculating the resistance temperature coefficient of the resistance wire;

and the environment temperature measuring submodule is used for measuring the environment temperature around the electric fusion welding machine.

Compared with the prior art, the invention has the technical effects that:

(1) the electric melting pipe fitting welding method and device based on resistance wire resistance temperature coefficient measurement provided by the invention have the advantage that the resistance wire temperature control in electric melting welding can be practically applied. The automatic resistance temperature coefficient measuring module in the device provides an on-line resistance temperature coefficient measuring method. The calculation is simplified according to the heat transfer characteristics of a welding system, the measurement of the resistance temperature coefficient can be completed only by means of the welding machine and relatively short time (30s) without separating resistance wires embedded in electric melting pipe fittings or using measurement modes such as water bath oil bath temperature regulation with higher cost and more complex flow.

(2) By the method, the resistance temperature coefficient of the resistance wire can be conveniently and automatically acquired. On one hand, the method can be used for a pipe fitting manufacturer to carry out factory (automatic) inspection on the material quality of the electric melting pipe fitting resistance wire; on the other hand, welding personnel can carry out the process design of electric fusion welding according to the resistance wire temperature, and the melting zone temperature monitoring and the intelligent welding control of the electric fusion welding machine are supported.

(3) The control method can be realized by relatively simple modification on the existing digital welding machine, and the electric melting pipe fitting welding device based on resistance wire resistance temperature coefficient measurement is formed. The welding device is suitable for the existing electric melting pipe fittings with all specifications, and does not need to improve the electric melting pipe fittings in any structure or circuit. The welding device can accurately sense the real-time temperature of the resistance wire of the pipe fitting according to the measured resistance temperature coefficient, so that high-quality electric fusion welding is realized through temperature control.

Drawings

FIG. 1 is a flow chart of a method for measuring resistance temperature coefficient of an electric melting pipe fitting resistance wire based on a welding machine;

FIG. 2 is a schematic diagram of a module of an electric melting pipe welding device based on resistance wire resistance temperature coefficient measurement;

FIG. 3 is a graph of voltage-resistance change before welding in example 1;

FIG. 4 is a comparison graph of the temperature coefficient of resistance test result obtained in the embodiment 1 and the result interval obtained by the detection according to the requirement of the national standard GB/T6148-;

fig. 5 is a graph of resistance temperature change during welding.

Detailed Description

First, it should be noted that the present invention relates to computer technology, which is an application of computer technology in the field of engineering control. In the implementation process of the invention, the application of a plurality of software functional modules is involved. The applicant believes that it is fully possible for one skilled in the art to utilize the software programming skills in his or her own practice to implement the invention, as well as to properly understand the principles and objectives of the invention, in conjunction with the prior art, after a perusal of this application. The aforementioned software functional modules include but are not limited to: the welding controller module, the resistance temperature coefficient calculation submodule and the like all belong to the scope mentioned in the application document of the invention, and the applicant does not enumerate one by one.

The invention provides an electric melting pipe fitting welding device based on resistance wire resistance temperature coefficient measurement, wherein a welding circuit comprises the following modules: the device comprises a power supply module, a welding voltage output module, a voltage and current detection module, a welding controller module and an automatic resistance temperature coefficient measuring and calculating module; the automatic resistance temperature coefficient measuring and calculating module is a main working module in automatic resistance temperature coefficient measurement before welding. The module also comprises four sub-modules, which are respectively: the device comprises a pulse voltage output submodule, a voltage current detection submodule, a resistance temperature coefficient calculation submodule and an environment temperature measurement submodule.

Based on the welding device, the invention provides an electric melting pipe fitting welding method based on resistance wire resistance temperature coefficient measurement. The method comprises the following steps: before welding, the electric melting pipe fitting is excited by pulse voltage, resistance temperature coefficients of resistance wires of the electric melting pipe fitting are automatically measured, and the resistance temperature coefficients are used for welding control after being obtained. In the welding process, the voltage and the current in the circuit are acquired in real time, the resistance value change condition of the resistance wire is calculated, and the resistance temperature calculated based on the resistance temperature coefficient is controlled to be a constant value by controlling the voltage of the welding circuit so as to realize the accurate control of the internal temperature of the electric fusion welding.

As shown in fig. 1, the user 1 applies the method to a welding apparatus including an electric fusion welding machine 2 and an electric fusion joint 3. The specific implementation steps of the electric melting pipe fitting welding method based on resistance wire resistance temperature coefficient measurement are described as follows:

the method comprises the following steps: and after removing oxide skin at the welding part of the electric melting pipe fitting to be welded, the electric melting pipe fitting and the electric melting pipe fitting are installed together to form the electric melting joint to be welded. Two input and output lines of the electric fusion welding machine are connected with the electric fusion pipe fitting.

Firstly, the ambient temperature is detected by an ambient temperature measuring submodule arranged in the automatic resistance temperature coefficient measuring module and recorded as T₀。

Applying a voltage of a rectangular waveform using the pulse voltage output submodule, the expression of which is expressed by the following expression (1):

in the formula (1), t is the time of applying the voltage, U (t) -the output voltage at the time of t, U₀The amplitude of the output voltage can be set to be 39.5V, t which is the maximum voltage capable of being output by the welding machine₀-the duration of application of a single voltage stimulus. t is t₀The value of (c) is between 100ms and 1000 ms.

Step two: the voltage and current detection submodule collects voltage and current in the welding output circuit at a sampling frequency greater than 10Hz, such as 100 Hz. Let us assume at t₀In time, N sampling periods are total, and the resistance of each sampling period is calculated as the following equation (2):

where i is the sampling period, the value of which is between 1 and N, R_i-real time resistance value of ith sampling period, U_i-voltage value of ith sampling period, I_i-current value of the ith sampling period; in N sampling periods, calculating the real-time resistance value R of the 1 st and the Nth sampling periods₁，R_NRespectively recorded as initial resistance values R₀And a final resistance value R;

step three: and the resistance temperature coefficient calculation submodule calculates the resistance temperature coefficient of the resistance wire.

The heat balance analysis is carried out on the structure of the resistance wire, and the specific steps are as follows:

the thermal loading of the resistance wire includes: joule heat W generated by electrifying resistance wire_p(ii) a The heat leakage of the resistance wire comprises: heat conduction and leakage W to the surrounding polyethylene material₂(ii) a In addition, the resistance wire also needs to consume heat W when being heated_m. The expression of the thermal load is as follows (3) to (5).

According to the law of conservation of energy, the energy balance relations of the resistance wire can be obtained as the following formulas (6) to (7):

W_p＝W₂+W_m (6)

wherein, T_mTemperature of the resistance wire in degrees Celsius, T_pTemperature of polyethylene around the resistance wire in degrees Celsius, S-contact area between the resistance wire and the polyethylene in units of m²，R_jThermal contact resistance between resistance wire and polyethylene, in m²·℃/W，C_mThe specific heat capacity of the resistance wire is expressed in J/kg DEG C_mResistance wire volume in m³，ρ_mResistance wire density in kg/m³。

Effective volume V of resistance wire_mIs calculated as in equation (8):

wherein d is_rIs the diameter of the resistance wire and has the unit of m; d_nThe nominal diameter of the electric melting pipe and the pipe fitting is m; l is_dThe distance between the center of the resistance wire and the inner surface of the electric melting pipe fitting is m; n is_cThe actual number of resistance wires in the melting zone range.

The resistance wire of the electric melting pipe fitting is mainly pure copper or copper alloy, such as copper-based alloy cupronickel taking nickel as a main additive element, copper-based alloy manganin taking manganese as a main additive element, copper-nickel-manganese alloy constantan and copper-based alloy brass taking zinc as a main additive element. The specific heat capacity and density of copper and various copper alloys are relatively close to each other at normal temperature, and can be regarded as constants. The specific heat capacity is within the range of 380-400J/kg DEG C, and 380J/kg DEG C is taken; the density is 8500-8800 kg/m³8800kg/m³。

The thermal contact resistance between the resistance wire and the polyethylene is related to the micro roughness and deformation of the contact surface. At t₀In the time, the polyethylene is not melted, the resistance wire is not in close contact with the polyethylene, a certain air gap exists, and the value is about 2-3 m²C ℃/kW, R can be taken in the actual calculation_j＝2.5m²·℃/kW。

Due to t₀→0，Because the contact thermal resistance between the resistance wire and the polyethylene is larger in the earlier stage of the system, the heat transfer efficiency is lower, and the resistance wire has better heat-conducting property and high heat transfer efficiency. At a relatively short t₀In time, the resistance wire is heated up rapidly, and the temperature of the polyethylene around the resistance wire can be still regarded as normal temperature.

The plausibility of this assumption and acceptable interval of parameters can be verified efficiently by simulation software. Building a heat transfer model of a polyethylene electric melting pipe fitting welding process in COMSOL Multiphysics, and placing probes at the position of a resistance wire and the position of peripheral polyethyleneA prediction of the temperature change is made. Recording parametersThe larger K represents the faster the relative temperature rise speed of the resistance wire, the more reasonable the assumption that the polyethylene can be regarded as constant temperature relative to the resistance wire.

Table 1 shows the simulation results and T₀Results for K value at 20.0 ℃:

TABLE 1

Based on this assumption, formula (7) is simplified to yield formula (9):

the formula (9) is merged and simplified to obtain the estimated resistance wire temperature T_mThe following formula (10):

step four: calculating the temperature coefficient of resistance:

wherein alpha is the temperature coefficient of resistance, R₀Is the initial resistance value, and R is the final resistance value; at t₀Within time, R₀And R are the real-time resistance values of the 1 st and Nth sampling periods, respectively. The temperature of the resistance wire is raised to the formula (10), and the resistance wire is substituted into the formula (11) to obtain the formula (12):

step five: and cooling the temperature of the system, and repeating the steps.

In order to avoid welding machine hardware and other errors possibly existing objectively as far as possible, after each pulse voltage excitation and measurement, cooling for 3-10 seconds, waiting for the resistance wire to recover to the room temperature, and repeating the steps again; in order to ensure the measurement accuracy, the measurement is repeated for 5 times or more, and the temperature coefficient of resistance obtained by each group is averaged.

Step six: welding can be started after the temperature coefficient of resistance α is obtained.

In the welding process, the voltage and current detection submodule measures the voltage and the current of the welding circuit in real time so as to obtain the resistance change condition of the resistance wire in the electric melting pipe fitting.

During welding, the resistance temperature can be accurately calculated by the formula (13):

where t-the moment at which welding is carried out, R_tThe resistance of the resistance wire at T is given by omega and T_t-t time resistance wire temperature in units of ℃.

The welding controller module regulates and controls the voltage of the welding circuit to regulate the resistance temperature T_tAnd controlling at a constant value to realize the control of the internal temperature of the electric fusion welding.

The optimal setting of the constant value can be based on research papers (Zheng J, Zhong S, Shi J, et al. study on the Allowable Temperature for the preceding overview Over Welding During Welding of Polyethylene Pipe [ J ]. Journal of Pressure Vessel Technology, 2015, 137 (2): 021401.). The thermal degradation behavior of PE100 after welding at different temperatures was investigated by thermogravimetric analysis and gel permeation chromatographic analysis. The composition of the residue after thermal degradation was analyzed by molecular weight and Molecular Weight Distribution (MWD) measurements to conclude that the maximum allowable weld temperature for typical industrial grade PE100 materials is around 270 ℃.

However, the method of carrying out the present patent is not limited to the case where the maximum welding temperature is 270 ℃. The method described in this patent can also be used to limit the maximum temperature allowed for welding if other materials have better or lower temperature resistance.

The specific implementation example is as follows:

according to the resistance wire resistance temperature coefficient measuring method of the electric melting pipe fitting based on the welding machine, the resistance temperature coefficient of a certain brand of pipe fitting is subjected to self-measurement.

The nominal diameter DN of the pipe fitting is 63mm, the standard size ratio SDR is 11, the total number of resistance wires is 36 (the single side is 18), and the wire embedding depth of the resistance wires is 0.5 mm.

Fig. 2 is a schematic diagram of a module of the electric melting pipe welding device based on resistance wire resistance temperature coefficient measurement. According to the method of the invention, the first step to the fifth step, the implementation process and the data transmission relationship in the welding device are as follows:

1. the field ambient temperature was measured by the ambient temperature measurement submodule to be 20 ℃.

2. A voltage of 39.5V was applied to the connected weld joints for 0.5s by the pulse voltage output submodule.

3. The voltage and current detection submodule records voltage and current at a sampling frequency of 20HZ and calculates real-time resistance.

Take three time nodes of 0.05s, 0.20s, 0.30s and 0.50s as an example:

at 0.05s, the welding voltage measured by the welder was 38.51v, the current was 17.42A, and the resistance value was 2.211 Ω.

At 0.20s, the welding voltage measured by the welder was 38.63V, the current was 17.41A, and the resistance value was 2.219 Ω.

At 0.30s, the welding voltage measured by the welder was 38.71V, the current was 17.19A, and the resistance value was 2.223 Ω.

At 0.50s, the welding voltage measured by the welding machine is 38.73V, the current is 17.09A, and the resistance value is 2.266 omega.

4. Ambient temperature measurement submodule and voltage meterThe flow detection submodule transmits the obtained data to the resistance temperature coefficient calculation submodule, and the structure of the resistance wire is subjected to heat balance analysis. The thermal loading of the resistance wire includes: joule heat W generated by electrifying resistance wire_p(ii) a The heat leakage of the resistance wire comprises: heat conduction and leakage W to the surrounding polyethylene material₂(ii) a In addition, the resistance wire also needs to consume heat W when being heated_m。

Due to t₀→0，Because the contact thermal resistance between the resistance wire and the polyethylene is larger in the earlier stage of the system, the heat transfer efficiency is lower, and the resistance wire has better heat-conducting property and high heat transfer efficiency. At a relatively short t₀Within the time, the resistance wire is heated up rapidly, and the temperature of the polyethylene around the resistance wire can be still regarded as the normal temperature, T_p＝T₀。

At 0.50s, the resistance wire generates joule heat W by electrification_p666.9W, heat conduction and leakage W to the polyethylene material around the heat conduction and leakage₂Is composed ofThe resistance wire also needs to consume heat W when rising temperature_mIs composed of

According to the law of conservation of energy, the energy balance relation w of the resistance wire can be obtained_p＝W₂+W_m

Can be obtained at normal temperature of 25 ℃ and T_mThe temperature was 43.47 ℃.

The resistance temperature coefficient calculation submodule calculates the resistance temperature coefficient:

5. cooling for 4.5s, recovering the system temperature to the initial state, and repeating the steps. Measurements were performed in 5 replicates. Fig. 3 is a test graph of this process. The temperature coefficients of the resistance of the test measured in the last four times are 0.00132, 0.00156, 0.00160 and 0.00146 respectively. The average of the temperature coefficients of resistance obtained for the five groups was 0.001462.

FIG. 4 is a comparison graph of the temperature coefficient of resistance test result obtained by the method of the present invention and the result interval obtained by the detection according to the requirements of the national standard GB/T6148-. As can be seen from the figure, each result obtained by the calculation method of the present invention is within the real interval [0.0013, 0.0017 ]. Therefore, the error between the calculation method result and the real numerical value of the invention can not influence the calculation and control of the resistance wire temperature.

6. The welding is started. And transmitting the measured resistance temperature coefficient to a welding controller through an automatic resistance temperature coefficient measuring module. In the welding process, the voltage and current detection module measures the current and the voltage of the welding circuit in real time to obtain the resistance change condition of the resistance wire in the electric melting pipe fitting and transmit data to the welding controller. The controller calculates the resistance temperature according to the resistance temperature coefficient and acts on the welding voltage output module for regulation and control.

At 10s, the resistance temperature measured and calculated by a welding machine is 135.2 ℃, and the control voltage is 39.50V;

at 20s, the resistance temperature measured by a welding machine is 186.9 ℃, and the control voltage is 39.50V;

at 30s, the resistance temperature measured by the welding machine is 224.6 ℃, and the control voltage is 39.50V;

at 60s, the resistance temperature measured by the welding machine is 270.4 ℃, and the control voltage is 32.72V:

at 80s, the resistance temperature measured and calculated by a welding machine is 270.2 ℃, and the control voltage is 29.55V;

fig. 5 is a test graph of this process. The upper half part of the graph is a voltage change curve, the welding is firstly carried out at 39.5V, and the voltage is gradually reduced as the resistance temperature approaches a target value; the lower half of the graph is a resistance temperature variation curve which first rises rapidly and finally approaches the set target value 270 c due to the voltage drop.