Temperature measurement method based on eddy current reflection resistor

Abstract

Aiming at the application that the temperature of a heated metal body needs to be measured in the eddy current principle heating process of an induction cooker, an industrial intermediate frequency furnace / high frequency furnace and the like, according to the electromagnetic induction principle and the Kirchhoff's law, equivalent resistance and equivalent inductance of an eddy current loop are equivalent to reflection resistance and reflection inductance; in the heating process, the change of equivalent resistance in an eddy current loop is related to the temperature, the change of reflection resistance of an excitation loop is caused by coupling, when an excitation power supply is given, the change relation of the reflection resistance along with the temperature is obtained by calculating and utilizing active power and current effective values through voltage and current sampling values of the excitation loop, and temperature measurement is achieved. According to the method, non-direct contact temperature measurement of the heated metal body is realized by using the eddy current reflection resistor in the eddy current principle heating process, the structure of an exciting coil does not need to be changed, the circuit is simple, and the influence of surface oxide, surface temperature and external environment of the heated metal on the measured temperature is reduced.

Description

Technical Field

[0001] The present invention relates to a method for measuring temperature using the reflected resistance of eddy current. This method is applied in the heating process using the eddy current principle, such as induction cookers, industrial intermediate frequency furnaces / high frequency furnaces, etc., where it is necessary to measure the temperature of the heated metal body. However, due to the constraints of the usage conditions, it is difficult to directly measure the temperature of the heating object. The present invention provides a non-direct contact temperature measurement method using the reflected resistance of eddy current. Background Art

[0002] To ensure the safe use and temperature control of heating equipment such as induction cookers, industrial intermediate frequency furnaces / high frequency furnaces, etc., it is necessary to measure the temperature of the heated metal body (referred to as the load). Currently, the temperature measurement methods can generally be divided into direct contact temperature measurement and non-direct contact temperature measurement. In the direct contact temperature measurement method, the temperature measurement device is in direct contact with the object to be measured. Due to the different structures, materials, heating objects, etc. of the heated metal furnace bodies of induction cookers, industrial intermediate frequency furnaces / high frequency furnaces, etc., it is difficult to directly measure the temperature. Non-contact temperature measurement usually measures other physical quantities and then calculates the temperature through this physical quantity to achieve the purpose of measuring the temperature. Currently, mainly temperature measurement using electrical signal data, infrared temperature measurement, etc. are used. The infrared temperature measurement method is easily affected by factors such as the surface oxide of the temperature measurement object and the surrounding environment. The measurement method of electrical signal data is relatively convenient, and has high accuracy and high real-time performance. Currently, the most commonly used is an internal temperature sensor, which measures the temperature using the principle of coil electromagnetic coupling. There has been research on detecting the eddy current intensity and measuring the temperature by increasing the auxiliary coil to change the structure of the eddy current excitation coil. The present invention provides a non-direct contact temperature measurement method using the electrical signal of the reflected resistance of eddy current without changing the structure of the excitation coil, reducing the influence of surface oxide, surface temperature, and external environment on the measured temperature. Summary of the Invention

[0003] The present invention provides a method for measuring temperature using the reflected resistance of eddy current. This method is based on the eddy current heating equivalent circuit as shown in Figure 1 . The excitation circuit mainly consists of an excitation source and an excitation coil. The inductance of the excitation coil is 𝐿1, the resistance is 𝑅1, the excitation source is voltage , the angular frequency is , and the current flowing through the excitation coil is ; the eddy current circuit is composed of the equivalent resistance 𝑅2 and the equivalent single-turn inductance 𝐿2 of the eddy current circuit of the heated metal body in series; the mutual inductance coefficient between the excitation coil and the equivalent inductance 𝐿2 is 𝑀. During the heating process, the change of the equivalent circuit 𝑅2 in the eddy current circuit is related to the temperature. Due to the coupling between the excitation circuit and the eddy current circuit, the part of the resistance change in the excitation circuit caused by the eddy current circuit is called the reflected resistance 𝑅 r , and the part of the inductance change in the excitation circuit caused by the eddy current circuit is called the reflected inductance 𝐿r , due to the existence of reflection resistance and reflection inductance, the equivalent impedance of the excitation circuit changes, where 𝑅 e is the equivalent resistance of the excitation circuit, and 𝐿 e is called the equivalent inductance of the excitation circuit. Figure 2 is the equivalent circuit including reflection resistance and reflection inductance. The reflection resistance is a quantity related to the temperature of the metal body that generates eddy currents, and the change in the reflection resistance 𝑅 r can be used to measure the temperature of the heated metal body. According to Kirchhoff's law, formula (1) can be obtained, and through derivation, the relationship expressions of the impedance z1 of the excitation circuit, the equivalent resistance 𝑅 e of the excitation circuit, the equivalent inductance 𝐿 e of the excitation circuit, the reflection resistance 𝑅 r of the eddy current circuit, and the reflection inductance 𝐿 r of the eddy current circuit are obtained.

[0004]

[0005] When the eddy current heating device has a metal body load (the excitation coil is close to the metal body and generates eddy currents in the metal body to heat the metal body), the electric power coupled to the eddy current circuit is mainly the active power on the equivalent load R2 of the eddy current circuit, that is, R2 >> ωL2 in the eddy current circuit. Therefore , the reflection resistance can be simplified to obtain formula (II).

[0006]

[0007] The load of the heated metal body is generally composed of iron and its alloys. The change of the equivalent resistance R2 with temperature can be described as R2(T) as shown in formula (3), where is the initial resistance at the ambient temperature of 0 °C, and R 20 can be obtained in advance through experiments, so it can be considered as a constant. α is the temperature coefficient constant of the metal resistance. Then, the reflection resistance and equivalent resistance of the excitation circuit also change with temperature, which are represented by R r (T) and R e (T) respectively, and their description is shown in formula (4).

[0008]

[0009]

[0010] When the excitation source of the excitation circuit is the voltage u(t) and the current i(t) flowing through the excitation coil as shown in formula (5), where U is the effective value of the excitation voltage, I is the effective value of the current, the frequency is f, and the angular frequency ω = 2πf. There is a phase difference between the current and the voltage.

[0011]

[0012] For any t x (t x =m·T c +t j ,m = 1, 2, 3, …, n, 0 ≤ t j <T c ,T c = 1 / f is the period of the sine wave), the average power within the time is as shown in Equation (6).

[0013]

[0014] The first term of Equation (6) is the active power Pr consumed by the equivalent resistance 𝑅 e . When t x is a full cycle (i.e., t j = 0), . When t x is an incomplete full cycle (i.e., 0 < t j <Ts), when m is large enough (m ≥ 150), is very small. Therefore, for any t x the average power within the time can be approximated as the active power (when m ≥ 150, its relative error is not greater than 1%).

[0015] During the heating process, voltage-current isolation detectors with a bandwidth of 50 kHz to 250 kHz (such as Hall voltage sensors and Hall current sensors) can be used to detect the instantaneous values of u(t) and i(t) in real time. Using Formulas (4) and (7), the relationship between the reflection resistance Rr and the temperature T, i.e., Equation (8), can be obtained, and then the temperature T(t) can be obtained as shown in Equation (9).

[0016]

[0017]

[0018]

[0019] The present invention is implemented as follows:

[0020] The main feature of this method is that based on the characteristic of the eddy current reflection resistance changing with temperature, without changing the basic structure of the excitation coil, a reflection resistance temperature measurement system is constructed as Figure 3 shown. Figure 3 In it, L1 and R1 are the resistance and inductance of the excitation coil, R2 is the equivalent resistance of the eddy current loop, and L2 is the equivalent inductance of the eddy current loop; Figure 3④ in it is the original controller; Figure 3 ① in it is the current isolation detection circuit. This current isolation detection circuit is mainly composed of an isolation current detection sensor with a bandwidth of 50 kHz to 250 kHz, a current of 10 A to 50 A, and a main circuit resistance not greater than 5 mΩ, and a self-stabilizing zero operational amplifier. The main circuit of the isolation current detection sensor is connected in series in the L1-R1 circuit. Due to its extremely small resistance, its insertion does not affect the working characteristics of the original circuit. The current isolation detection circuit converts the current i(t) in the excitation circuit into a current detection signal isolated from the excitation circuit , where s i is the sensitivity of the current isolation detection circuit; Figure 3 ② in it is the voltage isolation detection circuit. The voltage isolation detection circuit is mainly composed of an isolation current detection sensor with a bandwidth of 50 kHz to 250 kHz and a current of 0.5 A to 5 A, a self-stabilizing zero operational amplifier, and a precision resistor R V with a precision not less than 0.5%. Through the resistor R V , the excitation voltage u(t) on the excitation winding is converted into a current . Using an isolation current detection sensor with a bandwidth of 50 kHz to 250 kHz and a current range of 0.5 A to 5 A, the current i u (t) is converted into a voltage detection signal isolated from the excitation circuit , where s u is the sensitivity of the voltage isolation detection circuit; Figure 3 ③ in it is the reflection resistance and temperature detection module, which is composed of hardware and software. The hardware is mainly composed of an embedded processor. The current isolation detection signal i s (t) and the voltage isolation detection signal u s (t) are input into the reflection resistance and temperature detection module. Through the AD conversion circuit, the analog signals i s (t) and u s (t) are converted into digital signals, and the detection value (unit: Ω) of the reflection resistance R r and the temperature measurement value T (unit: °C) of the measured metal body are obtained using software algorithms.

[0021] (1) The input excitation voltage is , the effective voltage value is U, the frequency is f = 1 / T C , the angular frequency ω = 2πf, L1, R1, α are stored in the reflection resistance and temperature detection system as known constants, and M, L2, R 20 which are experimental constants that can be obtained through pre-experiments are also stored in the reflection resistance and temperature detection system. The voltage and current of the excitation circuit are alternately collected at an equal-period sampling time T S , m is t xThe number of cycles of the measured signal contained in the time, where n is the number of sampling points per cycle of the measured signal. According to m = t x / T C ,n = T C / T S calculate m and n, where m ≥ 150. As long as it is close to a complete cycle, it is not required to be exactly equal to a complete cycle. u(k) is the sampling value of the voltage u(t) in the excitation circuit, and i(k) is the sampling value of the current i(t) flowing through the excitation coil in the excitation circuit.

[0022] (2)The flow chart of the reflection resistance and temperature measurement is as Figure 4 shown. After the eddy current heating device is powered on and running, the reflection resistance and temperature detection system initializes and starts the AD converter and timer, sets the initial time t0 = 0, and calls the data acquisition and reconstruction program.

[0023] (3)In the data acquisition and reconstruction program, the signals u s (t) and i s (t) with a frequency f ≤ 50 kHz are sampled for m cycles of the measured signal at a sampling rate of ≥ 500 kHz. The reflection resistance and temperature detection system realizes the detection of u s (t) and i s (t) and reconstructs the functions of u s (t) and i s (t), and the sampling periods of u s (t) and i s (t) signals are the same, and they are carried out continuously and alternately. Through the detection of the output signal u Figure 3 (t) of the sensor ② as shown, the excitation voltage on the excitation coil can be obtained s , and through the detection of the output signal i of the sensor ① as shown Figure 3 , the current flowing through the excitation coil can be obtained s (t). Calculate the sampling value u(k) of the voltage u(t) and the sampling value i(k) of the current i(t) in the excitation circuit, obtain the sampling data of m cycles and save them. After the data sampling and reconstruction program is executed, enter the load detection. ,calculate the sampling value u(k) of the voltage u(t) in the excitation circuit and the sampling value i(k) of the current i(t), obtain the sampling data of m cycles and save them. After the data sampling and reconstruction program is executed, enter the load detection.

[0024] (4)After entering the load detection, use the voltage sampling value u(k) and current sampling value i(k) of the excitation circuit, and combine the constants L1 and R1 to judge whether the eddy current heating device has a load. Until it is detected that the eddy current heating device has a load, enter the temperature measurement.

[0025] (5) After entering the temperature measurement, set the initial time t0 = 0, call the data acquisition and reconstruction program. Using the sampled value u(k) of the voltage and the sampled value i(k) of the current in the excitation circuit, the reflection resistance Rr and the temperature T of the metal body to be measured can be obtained through the following formula, completing one temperature measurement. Then set the initial time t0 = 0 again, call the data acquisition and reconstruction again, and perform load detection and temperature measurement until shutdown.

[0026]

[0027] This method is for measuring the temperature of the metal body to be heated during the heating process using the eddy current principle in induction cookers, industrial intermediate frequency furnaces / high frequency furnaces, etc. According to the electromagnetic induction principle and Kirchhoff's law, the equivalent resistance and equivalent inductance of the eddy current circuit are equivalent to the reflection resistance and reflection inductance. During the heating process, the change of the equivalent resistance in the eddy current circuit is related to the temperature, causing the change of the reflection resistance in the excitation circuit. When a given excitation power supply is provided, the relationship between the reflection resistance and the temperature is obtained by calculating the active power and the effective value of the current using the sampled values of the voltage and current in the excitation circuit to achieve temperature measurement. The present invention provides a method for measuring temperature using the eddy current reflection resistance, realizing non-contact temperature measurement of the metal body to be heated, with a simple circuit, reducing the influence of surface oxides, surface temperature, and external environment of the metal body to be heated on the measured temperature. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is the equivalent circuit of eddy current heating of the present invention.

[0029] Figure 2 is the reflection equivalent circuit of eddy current heating of the present invention.

[0030] Figure 3 is the block diagram of the reflection resistance temperature measurement system for eddy current heating of the present invention.

[0031] Figure 4 is the circuit diagram of the reflection resistance temperature measurement system for eddy current heating of the present invention.

[0032] Figure 5 is the flow chart of the reflection resistance and temperature measurement for eddy current heating of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0033] Example:

[0034] S1: As Figure 4 shown, L1 and R1 are the resistance and inductance of the excitation coil, R2 is the equivalent resistance of the eddy current circuit, L2 is the equivalent inductance of the eddy current circuit, and the precision resistor R V = 10KΩ, Figure 3① in it is mainly composed of a Hall current sensor CC6920 - 10A (250kHz bandwidth, sensitivity s1 = 200mV / A, main circuit resistance ≯2mΩ) and an OPA365 operational amplifier (amplification factor K1 = 1.2); Figure 3 ② in it is mainly composed of a Hall current sensor CC6920 - 2.5A (250kHz bandwidth, sensitivity s2 = 800mV / A) and an OPA365 operational amplifier (amplification factor K2 = 100); Figure 3 ③ in the reflection resistance detection system mainly uses an STM32H743VIT6 processor to form the hardware system, and the output signal u s (t) of the voltage isolation detection circuit ② and the output signal i s (t) of the current isolation detection circuit ① are input into the AD converter inside the STM32H743VIT6 processor, and the STM32H743VIT6 processor uses software to process and obtain the detection value of the reflection resistance R r and the measured value of the temperature T of the metal body to be measured in real - time.

[0035] S2: The input excitation voltage is , the effective voltage value is U = 150~250V, f = 30khz, the period T C = 1 / f, the angular frequency is ω = 2πf, L1, R1, α as known constants and M, L2, R 20 The experimental constants obtained through pre - experiments have been stored in the Flash of the STM32H743VIT6 processor. Taking the number of sampling points per cycle of the measured signal n = 20, then the sampling time T S of the voltage and current of the excitation circuit sampled alternately at equal intervals is 1.66us. Taking m = 300, then t x = 10ms, and the detection value of the reflection resistance Rr and the measured value of the temperature of the metal body to be measured are calculated every 20ms.

[0036] S3: As Figure 5 shown, after the eddy - current heating device is powered on and running, the STM32H743VIT6 processor in the reflection resistance and temperature detection system completes initialization, turns on the AD converter and the timer, and sets the initial time t0 = 0, and calls the data acquisition and reconstruction program.

[0037] S4: In the data acquisition and reconstruction program, the STM32H743VIT6 processor samples at an equal - period sampling time T S = 1.66us, alternately samples the signal after passing through the OPA365 operational amplifier through the AD, and realizes the reconstruction of the detection of u s (t), i s (t) through an algorithm, and u s (t) and is (t) The sampling periods of the signals are the same and are continuously alternated. According to the voltage of the excitation circuit , the current flowing through the excitation coil , the voltage sampling values u(k) of the excitation circuit and the current sampling values i(k) for m periods are calculated and saved. After the data acquisition and reconstruction processing are completed, the load detection is entered.

[0038] S5: After entering the load detection, the STM32H743VIT6 processor uses the voltage sampling values u(k) and current sampling values i(k) of the excitation circuit obtained from the data acquisition and reconstruction processing, and combines the constants L1 and R1 to determine whether there is a load in the eddy current heating device. Until it is detected that there is a load in the eddy current heating device, the temperature measurement process is entered.

[0039] S6: After entering the temperature measurement, set the initial time t0 = 0. The STM32H743VIT6 processor calls the data acquisition and reconstruction program. The detection value of the reflection resistance Rr and the measured value of the temperature T of the metal body to be measured are obtained through the voltage sampling values u(k) and current sampling values i(k) of the excitation circuit using the following formula, and one temperature measurement is completed. Then set the initial time t0 = 0, call the data acquisition and reconstruction processing program again, and perform load detection and temperature measurement until shutdown.

[0040]

Claims

1. A method for non-contact temperature measurement using eddy current reflection resistance, characterized in that, The method includes: Composition of the reflected resistance temperature measurement system; Process and steps of reflected resistance and temperature measurement.

2. The non-contact temperature measurement method using the eddy current reflection resistance according to claim 1, characterized in that, The composition of the reflected resistance temperature measurement system includes the following features: The main feature of this method is that based on the characteristic of the eddy current reflected resistance changing with temperature, without changing the basic structure of the excitation coil, a reflected resistance temperature measurement system is constructed; (1) As shown in Figure 3, the reflected resistance temperature measurement system mainly consists of a current isolation detection circuit connected in series with the excitation winding, a voltage isolation detection circuit connected in parallel with the excitation winding, a reflected resistance and temperature detection module; (2) The current isolation detection circuit is mainly composed of an isolation current detection sensor with a bandwidth of 50 kHz to 250 kHz, a current range of 10 A to 50 A, and a main circuit resistance not exceeding 5 mΩ, and a self-stabilizing zero operational amplifier; the current isolation detection circuit converts the current i(t) in the excitation circuit into a current detection signal isolated from the excitation circuit , where s i is the sensitivity of the current isolation detection circuit; (3) The voltage isolation detection circuit is mainly composed of a precision resistor R with an accuracy of not less than 0.5%, V an isolation current detection sensor with a bandwidth of 50 kHz to 250 kHz and a current range of 0.5 A to 5 A, and a self-stabilizing zero operational amplifier; through the resistor R V the excitation voltage u(t) on the excitation winding is converted into a current , using an isolation current detection sensor with a bandwidth of 50 kHz to 250 kHz and a current range of 0.5 A to 5 A, the current i u (t) is converted into a voltage detection signal isolated from the excitation circuit , where s u is the sensitivity of the voltage isolation detection circuit; (4)The reflection resistance and temperature detection module consists of hardware and software, with the hardware centered around an embedded processor; the current isolation detection signal i s (t) and the voltage isolation detection signal u s (t) are input into the reflection resistance and temperature detection module, and the detected value of the reflection resistance R r and the measured value of the temperature of the metal object to be measured T are obtained through signal processing and software algorithms. The unit of the reflection resistance R r is Ω, and the unit of the temperature measured value T is °C.

3. A method for non-contact temperature measurement using eddy current reflection resistance according to claim 1, characterized in that, The process and steps of the reflected resistance and temperature measurement include the following features and steps: After the eddy current heating equipment is powered on and running, the reflection resistance and temperature detection system are initialized and the AD converter and timer are started. According to m = t x / T C , n = T C / T S Set the values of m and n, where m is the number of cycles of the measured signal within t x time, m ≥ 150, n is the number of sampling points per cycle of the measured signal, and T S is the equal-period sampling time of the measured signal, and T C is the period of the sinusoidal wave of the excitation source. Set the initial time t0 and call the data acquisition and reconstruction program; (2) Data acquisition and reconstruction program, which samples the signals u s (t) and i s (t) for m periods of the signals to be measured, where m ≥ 150. The reflection resistance and temperature detection system realizes the detection of u s (t) and i s (t), and reconstructs the functions of u s (t) and i s (t). Moreover, the sampling periods of u s (t) and i s (t) are the same, and they are carried out continuously and alternately. Through the formulas and the sampling values u(k) of the voltage u(t) of the excitation circuit and the sampling values i(k) of the current i(t) in the excitation circuit are calculated until the sampling data of m periods of the signals to be measured are obtained and saved. After the data acquisition and reconstruction program is executed, the load detection is entered; (3) After entering the load detection, the voltage sampling value u(k) and current sampling value i(k) of the excitation circuit are used, and combined with the constants L1 and R1 to judge whether there is a load in the eddy current heating device. When a load in the eddy current heating device is detected, enter the temperature measurement; After entering the temperature measurement, set the initial time t0, call the data acquisition and reconstruction processing, and the reflected resistance R can be obtained by using the voltage sampling value u(k) and the current sampling value i(k) of the excitation circuit through the following formula r and the temperature T of the metal body to be measured: , complete one temperature measurement, then set the initial time, call the data acquisition and reconstruction processing program again, perform load detection and temperature measurement until shutdown, where R1, R 20 , M, ω, α are constants for a specific object, and they can be obtained in advance through experiments and stored in the system.

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