CN112485506B - Low-frequency alternating current amplitude detection method based on infrared thermal imaging - Google Patents
Low-frequency alternating current amplitude detection method based on infrared thermal imaging Download PDFInfo
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Abstract
The invention discloses a low-frequency alternating current amplitude detection method based on infrared thermal imaging, which comprises the steps of extracting a temperature distribution curve of an infrared thermal image of a standard metal wire by utilizing an envelope algorithm, extracting an amplitude at an alternating frequency doubling position in a frequency domain by utilizing FFT (fast Fourier transform) transformation to serve as an actual temperature fluctuation amplitude, calculating a theoretical temperature fluctuation amplitude by utilizing a mathematical model from a thermal field to an electric field, and fitting the actual temperature fluctuation amplitude and the theoretical temperature fluctuation amplitude to obtain a proportionality coefficient; and finally, during the real-time measurement of the current amplitude, the theoretical temperature fluctuation amplitude of the metal wire to be measured is obtained by multiplying the actual temperature fluctuation amplitude by the proportionality coefficient, so that the current amplitude of the metal wire to be measured after the low-frequency alternating current is introduced is inversely calculated.
Description
Technical Field
The invention belongs to the technical field of current detection, and particularly relates to a low-frequency alternating current amplitude detection method based on infrared thermal imaging.
Background
In recent years, in various practical applications such as measurement and control in industrial production, along with improvement of various equipment performances, the significance of accurate measurement of alternating current is becoming more and more important. At present, the detection mode of alternating current mainly comprises a contact type and a non-contact type. The contact detection mode has the advantages of high detection precision, but has the disadvantages that a detection circuit needs to be connected into a circuit to be detected, the two circuits usually have mutual influence, and the position of the detection circuit is difficult to move. The traditional non-contact current detection mode is mainly magnetic detection, and the magnetic detection sensor comprises a current transformer, a Rogowski coil, a Hall current sensor, a reluctance current sensor and the like. These traditional non-contact current detection means highlight a number of insurmountable drawbacks: for the current transformer, the measuring range is limited during measurement, the sensitivity of the current transformer is in direct proportion to the turns of the secondary winding of the current transformer, although the sensitivity can be improved by increasing the turns of the induction coil, the lead perforation area is reduced, the volume of the current transformer is increased, and the coil resistance is greatly influenced by the external environment, so that the measuring accuracy is influenced. The current transformer with the iron core in the magnetic detection mode has the defects of magnetic leakage and poor linearity, and the same electromagnetic current transformer cannot meet the requirements of large-area detection and precision at the same time.
In many conventional non-contact current detection systems, attention is paid to a magnetic detection method and the characteristics of a sensor are studied and improved, and the improvement of a detection means and a detection principle is less considered. Infrared thermal imaging has been widely applied to various industrial scenes as a common nondestructive testing means, and the thermal wave theory contained in infrared thermal imaging has the potential of detecting current density, but has never been used for detecting the leakage current problem of a power system.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provides a low-frequency alternating current amplitude detection method based on infrared thermal imaging.
In order to achieve the above object, the present invention provides a low-frequency ac current amplitude detection method based on infrared thermal imaging, which is characterized by comprising the following steps:
(1) acquiring an infrared thermal image sequence;
(1.1) setting detection points x on K standard metal wires with the length of l, then introducing low-frequency alternating current to each standard metal wire at room temperature, collecting infrared thermal image videos with the detection points x by using a thermal infrared imager, and collecting K infrared thermal image videos in total;
(1.2) randomly extracting P infrared thermal images in each infrared thermal image video according to the acquisition sequence to form an infrared thermal image time sequence TP,k(x, t), wherein K is 1,2, …, K, the size of each infrared thermal image frame is M × N, and M, N is the length and width of the infrared thermal image respectively;
(2) extracting upper envelope lines and lower envelope lines;
respectively solving T frame by using packet access algorithm in matlab toolP,kObtaining the peak and the trough of each frame by the extreme value in (x, T), and forming an upper envelope line T by connecting the peaks in all the framesk,a(x, T), and connecting troughs in all frames to form a lower envelope line Tk,b(x, t); wherein a and b represent upper and lower envelopes, respectively;
(3) extracting the actual temperature fluctuation amplitude of each standard metal wire;
(3.1) extracting a temperature curve T between the central axes of the upper envelope line and the lower envelope linek(x,t)；
(3.2) to Tk(x, t) performing fast Fourier transform to obtain frequency domain data Fk(s)＝FFT(Tk(x, t)), and adding Fk(s) doubling frequency of the medium and low frequency alternating current 2fkThe amplitude of the reference signal is taken as the actual temperature fluctuation amplitude of each standard metal wire and is recorded as Ak；
(4) Calculating the theoretical temperature fluctuation amplitude of each standard metal wire;
(4.1) constructing a spatial transient function of the temperature distribution of each standard metal wire according to a Fourier heat conduction formula;
where κ is thermal conductivity, ρ is density, c is specific heat capacity, and P iskIs an alternating current heat source;
(4.2) calculating the thermal power generated by the alternating current on each standard metal wire;
setting AC current I introduced to each standard metal wirekComprises the following steps:
wherein,
showing the amplitude of the alternating current passing through the k-th standard metal wire, fkRepresenting the frequency of alternating current led into the kth standard metal wire;
thus, an alternating current IkGenerated thermal power PkComprises the following steps:
wherein R is0Resistance per unit length of standard metal line;
(4.3) setting initial conditions and boundary conditions
The initial conditions were: t isk(x,0)＝Fk,0＜x＜l；
The boundary conditions are as follows: t isk(0,t)＝Tk(l,t)＝E,t＞0；
Wherein E is ambient temperature, FkIs the initial temperature, T, of the kth standard metal linek(0, T) represents the temperature of the left end of the k standard metal line, Tk(l, T) represents the temperature at the right end of the kth standard metal line, Tk(x,0) represents the initial temperature at detection point x;
(4.4) solving the space transient function according to the separation variable method
Heating power PkAnd substituting the initial condition and the boundary condition into the spatial transient function in the step (4.1) to obtain:
then obtaining the temperature distribution T according to a separation variable methodkThe analytical solution of (x, t) is expressed as:
(4.5) according to the temperature distribution T of each standard metal wirekThe analytic solution of (x, t) is used for calculating the theoretical temperature fluctuation amplitude
(5) Fitting the theoretical temperature fluctuation amplitude and the actual temperature fluctuation amplitude to obtain a proportionality coefficient alpha;
theoretical temperature fluctuation amplitude for each standard metal wire
Amplitude A of fluctuation of actual temperaturekCarrying out normalization and fitting a relational expression:
wherein,
respectively representing the normalized actual temperature fluctuation amplitude and the normalized theoretical temperature fluctuation amplitude of the kth standard metal wire, Akmax、AkminRespectively representing the maximum value and the minimum value in the actual temperature fluctuation amplitude of the kth standard metal wire,
representing the maximum value and the minimum value in the k standard metal wire theoretical temperature fluctuation amplitude;
and obtaining a proportionality coefficient alpha according to the fitted relation:
(6) measurement of the actual current amplitude
Obtaining the actual temperature fluctuation amplitude of the metal wire to be measured according to the method in the steps (1) to (3), and multiplying the actual temperature fluctuation amplitude by a proportionality coefficient alpha to obtain the theoretical temperature fluctuation amplitude of the metal wire to be measured;
finally, inversely calculating the current amplitude I of the metal wire to be measured after the low-frequency alternating current is introduced according to the theoretical temperature fluctuation amplitude*
Wherein f is0For the frequency of the low-frequency alternating current, p0For the resistivity of the metal line to be measured, A0Is the theoretical temperature fluctuation amplitude, S, of the metal wire to be measured0The cross-sectional area of the metal wire to be measured.
The invention aims to realize the following steps:
the invention relates to a low-frequency alternating current amplitude detection method based on infrared thermal imaging, which comprises the steps of extracting a temperature distribution curve of an infrared thermal image of a standard metal wire by utilizing an envelope algorithm, extracting an amplitude at an alternating frequency doubling frequency position in a frequency domain by utilizing FFT (fast Fourier transform) transformation to serve as an actual temperature fluctuation amplitude, calculating a theoretical temperature fluctuation amplitude by utilizing a mathematical model from a thermal field to an electric field, and fitting the actual temperature fluctuation amplitude and the theoretical temperature fluctuation amplitude to obtain a proportionality coefficient; and finally, during the real-time measurement of the current amplitude, the theoretical temperature fluctuation amplitude of the metal wire to be measured is obtained by multiplying the actual temperature fluctuation amplitude by the proportionality coefficient, so that the current amplitude of the metal wire to be measured after the low-frequency alternating current is introduced is inversely calculated.
Meanwhile, the low-frequency alternating current amplitude detection method based on infrared thermal imaging also has the following beneficial effects:
(1) the invention adopts a mathematical model from the thermal field to the electric field, thus fully utilizing the energy conversion between the thermal field and the electric field and realizing the measurement of the low-frequency alternating current based on the infrared thermal imaging;
(2) the invention provides an electric heating double-field detection system of a current amplitude detection method, which is a novel non-contact alternating current detection method based on a thermal wave theory and kirchhoff current law, overcomes the defect that a bypass is introduced in the traditional contact current detection method, avoids the influence of a traditional contact measurement circuit on a to-be-detected circuit, improves the condition that the measurement range and the detection precision of a non-contact magnetic detection mode cannot be obtained simultaneously, and has the advantages of large-area detection and high resolution under the condition of keeping the advantage of remote detection of an infrared thermal imaging technology;
(3) the thermal infrared imager is used as a sensor, so that the direct contact with the conductor to be detected is avoided, a special magnetic core material is not needed during detection, the conductor to be detected does not need to pass through the detection coil, and the limitation on the detection position and the area of the conductor to be detected is relaxed;
(4) the invention adopts the combined mode of the envelope method, the Fourier transform and the normalization method to better extract the temperature response signal, improves the signal-to-noise ratio from the system method and has wide application prospect.
Drawings
FIG. 1 is a flow chart of a low frequency AC current amplitude detection method based on infrared thermal imaging according to the present invention;
FIG. 2 is a schematic diagram of a low-frequency AC current amplitude detection method based on infrared thermal imaging;
FIG. 3 is an infrared thermal imaging graph of a standard metal line and an exemplary diagram of a detection point;
FIG. 4 is an exemplary graph of envelope algorithm extraction temperature fluctuations;
FIG. 5 is an explanatory diagram of an analysis of a temperature distribution T (x, T);
FIG. 6 is a diagram showing an example of a process of changing the surface temperature of a wire sample;
FIG. 7 is a normalized fit of the theoretical temperature fluctuation amplitude to the actual temperature fluctuation amplitude at 5 Hz.
Detailed Description
The following description of the embodiments of the present invention is provided in order to better understand the present invention for those skilled in the art with reference to the accompanying drawings. It is to be expressly noted that in the following description, a detailed description of known functions and designs will be omitted when it may obscure the subject matter of the present invention.
Examples
FIG. 1 is a flow chart of a low-frequency AC current amplitude detection method based on infrared thermal imaging.
In this embodiment, as shown in fig. 1, the method for detecting a low-frequency alternating current amplitude based on infrared thermal imaging of the present invention includes the following steps:
s1, acquiring an infrared thermal image sequence;
s1.1, as shown in figure 2, arranging detection points x on K standard metal wires with the length of l, then introducing low-frequency alternating current into each standard metal wire at room temperature, acquiring infrared thermal image videos with the detection points x by using an infrared thermal imager, and acquiring K infrared thermal image videos in total;
in this embodiment, as shown in fig. 3, a low-frequency sinusoidal alternating current is generated using a function generator and amplified by a power amplifier. The current was limited to 2A-3.5A by connecting a standard metal wire in series with a resistor and real-time monitoring of the current was performed using an oscilloscope. The surface temperature of the metal wire is collected by using an infrared thermal imager with the resolution of 640 x 120, wherein a silicon chip is covered on the standard metal wire to reduce the influence of environmental factors.
S1.2, randomly extracting P infrared thermal images in each infrared thermal image video according to the acquisition sequence to form an infrared thermal image time sequence TP,k(x, t), wherein K is 1,2, …, K, the size of each infrared thermal image frame is M × N, and M, N is the length and width of the infrared thermal image respectively;
s2, extracting upper and lower envelope curves;
respectively solving T frame by using packet access algorithm in matlab toolP,kObtaining the peak and the trough of each frame by the extreme value in (x, T), and forming an upper envelope line T by connecting the peaks in all the framesk,a(x, T), and connecting troughs in all frames to form a lower envelope line Tk,b(x, t); at this time, the temperature variation curve is wrapped between the upper envelope line and the lower envelope line Tk,a(t)＜TP,k(x,t)＜Tk,b(T) a portion between the central axes of the upper envelope and the lower envelope reflects the temperature fluctuation signal TP,k(x, t), a, b representing the upper and lower envelopes, respectively;
s3, extracting the actual temperature fluctuation amplitude of each standard metal wire;
s3.1, as shown in FIG. 4, extracting a temperature curve T between the central axes of the upper envelope line and the lower envelope linek(x,t)；
S3.2, to Tk(x, t) performing fast Fourier transform to obtain frequency domain data Fk(s)＝FFT(Tk(x, t)), and adding Fk(s) doubling frequency of the medium and low frequency alternating current 2fkThe amplitude of the reference signal is taken as the actual temperature fluctuation amplitude of each standard metal wire and is recorded as Ak；
S4, calculating the theoretical temperature fluctuation amplitude of each standard metal wire;
s4.1, constructing a spatial transient function of the temperature distribution of each standard metal wire according to a Fourier heat conduction formula;
where κ is thermal conductivity, ρ is density, c is specific heat capacity, and P iskIs an alternating current heat source;
s4.2, calculating thermal power generated by alternating current on each standard metal wire;
setting AC current I introduced to each standard metal wirekComprises the following steps:
wherein,
showing the amplitude of the alternating current passing through the k-th standard metal wire, fkRepresenting the frequency of alternating current led into the kth standard metal wire;
thus, an alternating current IkGenerated thermal power PkComprises the following steps:
wherein R is0Resistance per unit length of standard metal line;
s4.3, setting initial conditions and boundary conditions
The initial conditions were: t isk(x,0)＝Fk,0＜x＜l；
The boundary conditions are as follows: t isk(0,t)＝Tk(l,t)＝E,t＞0；
Wherein E is ambient temperature, FkIs the initial temperature, T, of the kth standard metal linek(0, T) represents the temperature of the left end of the k standard metal line, Tk(l, T) represents the temperature at the right end of the kth standard metal line, Tk(x,0) represents the initial temperature at detection point x;
s4.4, solving the space transient function according to the separation variable method
Heating power PkSubstituting the initial condition and the boundary condition into the spatial transient function in the step S4.1 to obtain:
then, according to the separation variation method, as shown in FIG. 5, a temperature distribution T was obtainedkThe analytical solution of (x, t) is expressed as:
wherein, TxRepresenting the temperature distribution, T, along a standard metal line caused by a constant heat sourcetShowing the change in temperature over time, T, due to an alternating heat sourceeRepresents the transient temperature change process from the initial state to the steady state, shown in FIG. 6, TeWill eventually converge to ambient temperature E, n representing a natural number;
in the present embodiment, TxTemperature distribution along the gauge wire caused by a constant heat source, T, when the position of the detection point on the wire is determinedxTo a known amount; at low frequencies, the fluctuation term T is passed since the fluctuation characteristics of the temperature signal are significanttObtaining the current amplitude value according to the amplitude value of the temperature distribution T and the temperature distribution T according to a separation variable methodkThe analytical solution approximation of (x, t) can be expressed as:
s4.5, according to the temperature distribution T of each standard metal wirekThe analytic solution of (x, t) is used for calculating the theoretical temperature fluctuation amplitude
S5, fitting the theoretical temperature fluctuation amplitude and the actual temperature fluctuation amplitude to obtain a proportionality coefficient alpha as shown in figure 7;
theoretical temperature fluctuation amplitude for each standard metal wire
Amplitude A of fluctuation of actual temperaturekCarrying out normalization and fitting a relational expression:
wherein,
respectively representing the normalized actual temperature fluctuation amplitude and the normalized theoretical temperature fluctuation amplitude of the kth standard metal wire, Ak max、Ak minRespectively representing the maximum value and the minimum value in the actual temperature fluctuation amplitude of the kth standard metal wire,
representing the maximum value and the minimum value in the k standard metal wire theoretical temperature fluctuation amplitude;
and obtaining a proportionality coefficient alpha according to the fitted relation:
s6, measurement of actual current amplitude
According to the method of the steps S1-S3, obtaining the actual temperature fluctuation amplitude of the metal wire to be measured, and multiplying the actual temperature fluctuation amplitude by the proportionality coefficient alpha to obtain the theoretical temperature fluctuation amplitude of the metal wire to be measured;
finally, inversely calculating the current amplitude I of the metal wire to be measured after the low-frequency alternating current is introduced according to the theoretical temperature fluctuation amplitude*
Wherein f is0For the frequency of the low-frequency alternating current, p0For the resistivity of the metal line to be measured, A0Is the theoretical temperature fluctuation amplitude, S, of the metal wire to be measured0The cross-sectional area of the metal wire to be measured.
Although illustrative embodiments of the present invention have been described above to facilitate the understanding of the present invention by those skilled in the art, it should be understood that the present invention is not limited to the scope of the embodiments, and various changes may be made apparent to those skilled in the art as long as they are within the spirit and scope of the present invention as defined and defined by the appended claims, and all matters of the invention which utilize the inventive concepts are protected.
Claims (1)
1. A low-frequency alternating current amplitude detection method based on infrared thermal imaging is characterized by comprising the following steps:
(1) acquiring an infrared thermal image sequence;
(1.1) setting detection points x on K standard metal wires with the length of l, then introducing low-frequency alternating current to each standard metal wire at room temperature, collecting infrared thermal image videos with the detection points x by using a thermal infrared imager, and collecting K infrared thermal image videos in total;
(1.2) randomly extracting P infrared thermal images in each infrared thermal image video according to the acquisition sequence to form an infrared thermal image time sequence TP,k(x, t), wherein K is 1,2, …, K, the size of each infrared thermal image frame is M × N, and M, N is the length and width of the infrared thermal image respectively;
(2) extracting upper envelope lines and lower envelope lines;
respectively solving T frame by using packet access algorithm in matlab toolP,kObtaining the peak and the trough of each frame by the extreme value in (x, T), and forming an upper envelope line T by connecting the peaks in all the framesk,a(x, T), and connecting troughs in all frames to form a lower envelope line Tk,b(x, t); wherein a and b represent upper and lower envelopes, respectively;
(3) extracting the actual temperature fluctuation amplitude of each standard metal wire;
(3.1) extracting a temperature curve T between the central axes of the upper envelope line and the lower envelope linek(x,t)；
(3.2) to Tk(x, t) is subjected to fast Fourier transformConverting to obtain frequency domain data Fk(s)＝FFT(Tk(x, t)), and adding Fk(s) doubling frequency of the medium and low frequency alternating current 2fkThe amplitude of the reference signal is taken as the actual temperature fluctuation amplitude of each standard metal wire and is recorded as Ak；
(4) Calculating the theoretical temperature fluctuation amplitude of each standard metal wire;
(4.1) constructing a spatial transient function of the temperature distribution of each standard metal wire according to a Fourier heat conduction formula;
where κ is thermal conductivity, ρ is density, c is specific heat capacity, and P iskIs an alternating current heat source;
(4.2) calculating the thermal power generated by the alternating current on each standard metal wire;
setting AC current I introduced to each standard metal wirekComprises the following steps:
wherein,
showing the amplitude of the alternating current passing through the k-th standard metal wire, fkRepresenting the frequency of alternating current led into the kth standard metal wire;
thus, an alternating current IkGenerated thermal power PkComprises the following steps:
wherein R is0Resistance per unit length of standard metal line;
(4.3) setting initial conditions and boundary conditions
The initial conditions were: t isk(x,0)＝Fk,0＜x＜l；
The boundary conditions are as follows: t isk(0,t)＝Tk(l,t)＝E,t＞0；
Wherein E is ambient temperature, FkIs the initial temperature, T, of the kth standard metal linek(0, T) represents the temperature of the left end of the k standard metal line, Tk(l, T) represents the temperature at the right end of the kth standard metal line, Tk(x,0) represents the initial temperature at detection point x;
(4.4) solving the space transient function according to the separation variable method
Heating power PkAnd substituting the initial condition and the boundary condition into the spatial transient function in the step (4.1) to obtain:
then obtaining the temperature distribution T according to a separation variable methodkThe analytical solution of (x, t) is expressed as:
(4.5) according to the temperature distribution T of each standard metal wirekThe analytic solution of (x, t) is used for calculating the theoretical temperature fluctuation amplitude
(5) Fitting the theoretical temperature fluctuation amplitude and the actual temperature fluctuation amplitude to obtain a proportionality coefficient alpha;
theoretical temperature fluctuation amplitude for each standard metal wire
Amplitude A of fluctuation of actual temperaturekCarrying out normalization and fitting a relational expression:
wherein,
respectively representing the normalized actual temperature fluctuation amplitude and the normalized theoretical temperature fluctuation amplitude of the kth standard metal wire, Akmax、AkminRespectively representing the maximum value and the minimum value in the actual temperature fluctuation amplitude of the kth standard metal wire,
representing the maximum value and the minimum value in the k standard metal wire theoretical temperature fluctuation amplitude;
and obtaining a proportionality coefficient alpha according to the fitted relation:
(6) measurement of the actual current amplitude
Obtaining the actual temperature fluctuation amplitude of the metal wire to be measured according to the method in the steps (1) to (3), and multiplying the actual temperature fluctuation amplitude by a proportionality coefficient alpha to obtain the theoretical temperature fluctuation amplitude of the metal wire to be measured;
finally, inversely calculating the current amplitude I of the metal wire to be measured after the low-frequency alternating current is introduced according to the theoretical temperature fluctuation amplitude*
Wherein f is0For the frequency of the low-frequency alternating current, p0For the metal wire to be testedResistivity of A0Is the theoretical temperature fluctuation amplitude, S, of the metal wire to be measured0The cross-sectional area of the metal wire to be measured.
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| US5869986A (en) * | 1997-06-13 | 1999-02-09 | Maxim Integrated Products, Inc. | Power level sense circuit |
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| US9194893B2 (en) * | 2013-02-07 | 2015-11-24 | Texas Instruments Incorporated | Bi-directional input, bi-directional output, lossless current sensing scheme with temperature compensation |
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