Weak current detection circuit and sensor based on second harmonic amplitude detection

Abstract

The invention discloses a weak current detection circuit and sensor based on secondary harmonic amplitude detection, relates to the technical field of current measurement, and solves the problem that low complexity, high sensitivity and high measurement frequency band are difficult to consider in existing even harmonic-based tiny direct current signal detection. Performing frequency division to obtain homologous excitation signals and double demodulation signals; the driving circuit generates a driving signal according to the excitation signal; the signal processing circuit is used for driving the excitation coil to generate an alternating magnetic field according to the driving signal to detect the current on the circuit to be detected, driving the analog switch to separate a second harmonic signal from a signal induced by the feedback coil according to the double modulation signal, and performing filtering and integration processing on the second harmonic signal; obtaining a voltage signal proportional to the current on the line to be measured, adjusting the current in the feedback coil according to the voltage signal, and compensating the magnetic core to a zero magnetic flux state; the problem is solved by extracting a second harmonic component for current measurement through a simulation scheme.

Description

Technical Field

[0001] The invention relates to the technical field of current measurement, and more particularly to a weak current detection circuit and a sensor based on second harmonic amplitude detection. Background Art

[0002] Fluxgate technology has unique advantages in non-contact weak current detection due to its high stability, high resolution, and wide range. With the advancement of AC / DC hybrid power grid construction, power devices pursue faster switching speeds. In addition to industrial frequency AC, electrical equipment also generates high-frequency AC, pulsating DC, and even smooth DC and other residual currents. The residual current sensors widely used in China can no longer meet the requirements of weak leakage current protection in these special cases due to their low resolution, accuracy, and frequency.

[0003] Most of the leakage current sensors on the market are designed based on the fluxgate principle. In order to take into account the cost requirements, whether it is an open-loop or closed-loop magnetic modulation principle sensor, it is based on the single-loop fluxgate effect (the multi-loop solution has a complex process and high cost). The fluxgate detection principle utilizes the symmetrical nonlinearity of high-permeability ferromagnetic materials. The alternating symmetrical excitation source excites the nonlinear magnetic core to saturation. Due to the symmetrical nonlinearity of the magnetization curve, the induced voltage waveform on the detection winding is symmetrical, and there are only odd harmonics in the induced voltage spectrum. If a small DC signal is passed through the signal winding, a DC bias magnetic field will be generated in the magnetic core, thereby destroying the symmetry of the excitation flux, causing the induced voltage waveform on the detection winding to be distorted, thereby generating even harmonics proportional to the DC signal, among which the second harmonic is the main one. Therefore, the detection of small DC signals can be achieved by detecting even harmonics or second harmonics.

[0004] The commonly used low-cost solution on the market is to use self-excited oscillation fluxgate technology to convert the second harmonic detection into the average value detection of the excitation current or the duty cycle change of the self-excited oscillation waveform, that is, the average current model and the duty cycle model. The principles of these two models are based on the self-excited oscillation fluxgate technology. The advantages are low cost, simple structure, and easy circuit implementation; the disadvantages are that the sensitivity is not high enough, the stability is not high enough due to the influence of the self-excited circuit, and the product measurement band is low. Another solution is to use digital sampling and software calculation for the demodulation of the second harmonic. This solution can be implemented, but in order to ensure the accuracy and bandwidth of the product, the processor speed and AD accuracy are required to be high, resulting in complex circuits and high costs, and cannot meet the needs of high-precision detection of low-cost weak currents.

[0005] Therefore, the present application provides a weak current detection circuit and sensor based on second harmonic amplitude detection to solve the above problems. Summary of the invention

[0006] The purpose of this application is to provide a weak current detection circuit and sensor based on second harmonic amplitude detection, so as to solve the problem that the existing detection of tiny DC signals based on even harmonics or second harmonics is difficult to achieve low complexity, high sensitivity and high measurement frequency band. This application extracts the second harmonic component through an analog scheme for current measurement, thereby achieving current detection with low cost, high sensitivity and high measurement frequency band.

[0007] The present application first provides a weak current detection circuit based on second harmonic amplitude detection, including: a waveform generating circuit, used for frequency division to obtain a homologous excitation signal and a double demodulation signal, the frequency of the double demodulation signal is twice the frequency of the excitation signal; a driving circuit, used to generate a driving signal according to the excitation signal; a signal processing circuit, including an excitation coil and a feedback coil wound on the same magnetic core, used to drive the excitation coil to generate an alternating magnetic field to detect the current on the circuit to be measured according to the driving signal, drive an analog switch according to the double modulation signal to separate the second harmonic signal from the signal induced by the feedback coil, filter and integrate the second harmonic signal to obtain a voltage signal proportional to the current on the circuit to be measured, and adjust the current in the feedback coil according to the voltage signal to compensate the magnetic core to a zero flux state.

[0008] By adopting the above technical scheme, when the magnetic field generated by the current on the circuit to be measured is superimposed on the alternating magnetic field generated by the coil, the second harmonic component induced by the coil is extracted, and the signed amplitude of the extracted second harmonic component is proportional to the current on the circuit to be measured, thereby realizing current measurement; the magnetic field signal generated by the current on the circuit to be measured is output in the form of analog voltage, which can detect extremely weak leakage current on the circuit and improve the sensitivity of detection; the size of the external magnetic field is calculated by integrating the output analog voltage, and the output bandwidth of the detection circuit depends on the integration time, which can effectively broaden the detection bandwidth; the detection circuit adopts a full analog circuit, and the circuit implementation structure is simple.

[0009] In one possible implementation, a waveform generating circuit includes: a frequency source and a frequency division counter connected to each other, the frequency source is used to generate an alternating square wave, the frequency division counter is used to divide the alternating square wave into an excitation signal and a double demodulation signal, the frequency of the excitation source signal is f / 2, the frequency of the double demodulation signal is f, and f is the frequency of the frequency source.

[0010] In a possible implementation, the frequency source is a ceramic or temperature compensated crystal oscillator.

[0011] In a possible implementation, the driving circuit includes: a pair of complementary MOS transistors: a PMOS transistor Q1B and an NMOS transistor Q1A, wherein the source of the PMOS transistor Q1B is connected to a power supply VCC, the gate is connected to an excitation signal output by a waveform generating circuit, the drain is connected to a drain of the NMOS transistor Q1A, the gate of the NMOS transistor Q1A is connected to the excitation signal output by the waveform generating circuit, the source is grounded, the drain connection point of the PMOS transistor Q1B and the NMOS transistor Q1A is connected to a resistor R1, the resistor R1 is connected to one end of a capacitor C1, and the other end of the capacitor C1 serves as an output of the driving circuit to output a driving signal.

[0012] In a possible implementation, the same-name ends of the excitation coil and the feedback coil in the signal processing circuit are connected to ground to form a center-tapped transformer.

[0013] In a possible implementation, the signal processing circuit further includes: a double-throw switch, a filter circuit, an integration circuit and a feedback resistor; the input of the double-throw switch is connected to the double demodulated signal output by the waveform generating circuit, the output is connected to the opposite end of the feedback coil, the output is also connected to the integration circuit through the filter circuit, the output of the integration circuit serves as the output of the weak current detection circuit, and the output of the weak current detection circuit is connected to the opposite end of the feedback coil through the feedback resistor.

[0014] In a possible implementation manner, the excitation coil and the feedback coil are formed by winding a conventional annular magnetic core or by processing based on a MEMS process.

[0015] The present application also provides a weak current detection sensor based on second harmonic amplitude detection, including: the above-mentioned weak current detection circuit based on second harmonic amplitude detection, wherein the excitation coil, feedback coil and magnetic core form a detection head for approaching the circuit to be tested.

[0016] In a possible implementation manner, the excitation coil and the feedback coil are wound on a magnetic core to form a ring-shaped detection head.

[0017] In a possible implementation manner, the excitation coil and the feedback coil are wound on a magnetic core to form a cylindrical detection head.

[0018] Compared with the prior art, the present application has the following beneficial effects: the weak current detection circuit based on second harmonic amplitude detection is implemented by a fully analog circuit with a simple structure; the output bandwidth of the detection circuit depends on the integration time, and a fast response can be achieved; by selecting appropriate magnetic core coils and parameters, the weak current resolution can be increased from mA level to μA level, and the detection bandwidth can be increased from less than 1KHz to 10KHz, effectively broadening the measurement sensitivity and measurement range. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The drawings described herein are used to provide a further understanding of the embodiments of the present invention, constitute a part of this application, and do not constitute a limitation of the embodiments of the present invention. In the drawings:

[0020] Figure 1 It is the principle diagram of the weak current detection circuit based on second harmonic amplitude detection;

[0021] Figure 2 This is a schematic diagram of a weak current detection circuit based on second harmonic amplitude detection. DETAILED DESCRIPTION

[0022] Hereinafter, the term "include" or "may include" used in various embodiments of the present application indicates the presence of the function, operation or element applied for, and does not limit the increase of one or more functions, operations or elements. In addition, as used in various embodiments of the present application, the terms "include", "have" and their cognates are intended only to indicate specific features, numbers, steps, operations, elements, components or a combination of the foregoing items, and should not be understood as first excluding the presence of one or more other features, numbers, steps, operations, elements, components or a combination of the foregoing items or the possibility of increasing one or more features, numbers, steps, operations, elements, components or a combination of the foregoing items.

[0023] It should be noted that if it is described that one component element is “connected” to another component element or is “connected to” another component element, the first component element may be directly connected to the second component element, and a third component element may be “connected” between the first component element and the second component element. Conversely, when one component element is “directly connected” to another component element or is “directly connected to” another component element, it can be understood that there is no third component element between the first component element and the second component element.

[0024] The terms used in the various embodiments of the present application are only used for the purpose of describing specific embodiments and are not intended to limit the various embodiments of the present application. Unless otherwise specified, all terms (including technical terms and scientific terms) used herein have the same meaning as the meanings commonly understood by ordinary technicians in the field to which the various embodiments of the present application belong. Terms (such as terms defined in generally used dictionaries) will be interpreted as having the same meaning as the contextual meaning in the relevant technical field and will not be interpreted as having an idealized meaning or an overly formal meaning, unless clearly defined in the various embodiments of the present application.

[0025] In order to make the objectives, technical solutions and advantages of the present application more clearly understood, the present application is further described in detail below in conjunction with examples and drawings. The illustrative implementation scheme of the present application and its description are only used to explain the present application and are not intended to limit the present application.

[0026] First, some of the terms and related technologies involved in this application are explained to facilitate understanding by those skilled in the art.

[0027] Residual current, also known as leakage current, refers to the current that does not flow through the normal path in the circuit. It is usually caused by insulation failure of electrical equipment, grounding failure or electric shock to the human body. Ideally, the current should all return to the power source after it passes through the electrical equipment. However, in actual use, due to insulation damage, line aging and other reasons, part of the current may not flow along the intended path, which is the so-called residual current.

[0028] See also Figure 1-2 As shown, the present application first provides a weak current detection circuit based on second harmonic amplitude detection, including: a waveform generating circuit, used for frequency division to obtain a homologous excitation signal and a double demodulation signal, the frequency of the double demodulation signal is twice the frequency of the excitation signal; a driving circuit, used to generate a driving signal according to the excitation signal; a signal processing circuit, including an excitation coil and a feedback coil wound on the same magnetic core, used to drive the excitation coil to generate an alternating magnetic field according to the driving signal to detect the current on the circuit to be measured, drive the analog switch according to the double modulation signal to separate the second harmonic signal from the signal induced by the feedback coil, filter and integrate the second harmonic signal to obtain a voltage signal proportional to the current on the circuit to be measured, and adjust the current in the feedback coil according to the voltage signal to compensate the magnetic core to a zero flux state.

[0029] Specifically, the waveform generating circuit is used to generate a homologous excitation signal and a double demodulation signal. The excitation signal is used to drive the excitation coil, and the double demodulation signal is used for subsequent signal processing to ensure the synchronization of signal processing. The driving circuit is used to receive the excitation signal generated by the waveform generating circuit and generate a driving signal according to the excitation signal. The excitation coil in the signal processing circuit generates an alternating magnetic field under the action of the driving signal. The alternating magnetic field interacts with the magnetic field generated by the current on the circuit to be measured. The feedback coil senses the change in the magnetic field to generate an induction signal containing the second harmonic; the double demodulation signal controls the conduction frequency of the analog switch, separates the second harmonic signal from the induction signal of the feedback coil, and filters and integrates the second harmonic signal to output a voltage signal proportional to the current on the circuit to be measured, so as to realize the current detection of the circuit to be measured. At the same time, the current in the feedback coil is reversely adjusted according to the voltage signal to generate a compensation magnetic field to offset the magnetic field generated by the current on the circuit to be measured until the magnetic flux of the magnetic core is 0, thereby improving the linearity and stability of the measurement.

[0030] The principle of this scheme is that when the magnetic field generated by the current on the circuit to be measured is superimposed on the alternating magnetic field generated by the coil, the second harmonic component induced by the coil is extracted. The signed amplitude of the extracted second harmonic component is proportional to the current on the circuit to be measured, thereby achieving the purpose of measuring the current.

[0031] The improvement of this scheme lies in that the magnetic field signal generated by the current on the circuit to be tested is output in the form of analog voltage, which can detect extremely weak leakage current on the circuit and improve the sensitivity of detection; the size of the external magnetic field is calculated by integrating the output analog voltage, and the output bandwidth of the detection circuit depends on the integration time, which can effectively broaden the detection bandwidth; the detection circuit adopts a fully analog circuit, and the circuit implementation structure is simple.

[0032] In one possible implementation, a waveform generating circuit includes: a frequency source and a frequency division counter connected to each other, the frequency source is used to generate an alternating square wave, the frequency division counter is used to divide the alternating square wave into an excitation signal and a double demodulation signal, the frequency of the excitation source signal is f / 2, the frequency of the double demodulation signal is f, and f is the frequency of the frequency source.

[0033] Furthermore, the frequency source adopts a ceramic or temperature compensated crystal oscillator.

[0034] Specifically, the sensitivity of the detection circuit is closely related to the excitation source in addition to the cross-sectional area of ​​the magnetic core, the magnetic permeability μ of the magnetic core and the winding. Increasing the excitation frequency can improve the sensitivity of the circuit. According to the different excitation methods, it can be divided into voltage excitation and current excitation. Voltage excitation is easier to implement in the circuit. Considering the comprehensive cost and performance, the square wave excitation method is preferred. The stability of the excitation waveform cannot be ignored on the entire circuit, and it cannot be distorted, otherwise there will be secondary harmonics when there is no input, affecting the measurement accuracy. Therefore, it is preferred to use a high-frequency, high-stability frequency source such as ceramics or temperature-compensated crystal oscillators to obtain a homologous excitation source signal and a corresponding double demodulation signal through frequency division. The oscillation waveform generated by the frequency source is divided by a frequency division counter to provide a standard alternating square wave drive signal, wherein the alternating square wave drive signal with an output frequency of f / 2 is used as an excitation signal, and the alternating square wave drive signal with an output frequency of f is used as a double demodulation signal.

[0035] In a possible implementation, the driving circuit includes: a pair of complementary MOS transistors: a PMOS transistor Q1B and an NMOS transistor Q1A, wherein the source of the PMOS transistor Q1B is connected to a power supply VCC, the gate is connected to an excitation signal output by a waveform generating circuit, the drain is connected to a drain of the NMOS transistor Q1A, the gate of the NMOS transistor Q1A is connected to the excitation signal output by the waveform generating circuit, the source is grounded, the drain connection point of the PMOS transistor Q1B and the NMOS transistor Q1A is connected to a resistor R1, the resistor R1 is connected to one end of a capacitor C1, and the other end of the capacitor C1 serves as an output of the driving circuit to output a driving signal.

[0036] Specifically, an alternating square wave driving signal with an output frequency of f / 2 is used as an excitation signal of the driving circuit. The driving circuit uses a pair of complementary MOS tubes to provide driving capability for the excitation coil, so that the circuit works more stably.

[0037] In one possible implementation, the like-name ends of the excitation coil and the feedback coil in the signal processing circuit are connected to ground to form a center-tapped transformer; further, the signal processing circuit also includes: a double-throw switch, a filter circuit, an integration circuit and a feedback resistor; the input of the double-throw switch is connected to the double demodulated signal output by the waveform generating circuit, the output is connected to the opposite-name end of the feedback coil, and the output is also connected to the integration circuit through the filter circuit, the output of the integration circuit serves as the output of the weak current detection circuit, and the output of the weak current detection circuit is connected to the opposite-name end of the feedback coil through the feedback resistor.

[0038] Specifically, in order to separate the second harmonic from the primary (input) current, the excitation coil and the compensation coil are connected in series in reverse order to form a center-tapped transformer. The transformer coil consists of two parts, which are connected in such a way that I + and I - The magnetic flux generated has opposite polarity. When there is no primary (input) current, the entire system is magnetically symmetrical and the current I + and I - The magnitudes are equal and the directions are opposite, and the entire magnetic flux is zero; when the primary (input) current increases, the current generates a superposition of magnetic fields, causing the magnetic flux of the coil to change, thereby causing the current I + and I - The "AC" characteristics of the two are different. One increases (in the same direction) and the other decreases (in the opposite direction). The waveform is asymmetric, which generates harmonic signals. In this case, all the odd harmonics (1, 3, 5...) of the current I generate a total magnetic flux of zero, while the even harmonics (2, 4...) generate AC flux, so that the signal enters the secondary winding of the center-tapped transformer. Among the harmonic content of the transformer output signal, only the second harmonic is detected by the circuit. The signal processing circuit uses a signal with twice the excitation frequency f (the signal frequency is exactly the same as the frequency of the second harmonic signal) to drive the analog switch to ensure the "synchronous detection" of the second harmonic signal. Then a low-pass filter circuit is used to filter the output of the synchronous detection. The filtered output signal is integrated through an integration circuit. The voltage of the integrated output is fed to the feedback coil through a feedback circuit formed by resistor R2. The current in the feedback coil will generate a compensation magnetic field in the opposite direction of the external magnetic field until the magnetic field inside the magnetic core is zero. At this time, when the external magnetic field is constant, the amplitude of the second harmonic signal is zero, the integration circuit no longer works, and the output voltage remains constant. When the external magnetic field changes, the balance is broken until it is restored to balance. Therefore, the magnitude of the external magnetic field can be calculated by the output voltage of the integration circuit.

[0039] In a possible implementation, the coils of the excitation coil and the feedback coil are formed by conventional toroidal core winding or MEMS-based processing. By selecting a magnetic material processing method with high magnetic permeability and low saturation magnetic induction intensity, high sensitivity and high bandwidth of detection are ensured.

[0040] It can be understood that the weak current detection circuit based on second harmonic amplitude detection provided by this solution is implemented by a full analog circuit with a simple structure; the output bandwidth of the detection circuit depends on the integration time, which can achieve fast response. In addition, by selecting appropriate magnetic core coils and parameters, the weak current resolution can be increased from mA level to μA level, and the detection bandwidth can be increased from less than 1KHz to 10KHz, effectively broadening the measurement sensitivity and measurement range.

[0041] The present application secondly provides a weak current detection sensor based on second harmonic amplitude detection, including a weak current detection circuit based on second harmonic amplitude detection as described above, wherein the excitation coil, feedback coil and magnetic core form a detection head for approaching the circuit to be tested.

[0042] When in use, the detection head of the weak current detection sensor is close to the circuit to be tested. When the current on the circuit to be tested is zero, the weak current detection sensor has no output; when there is a weak leakage current on the circuit to be tested, the weak current detection sensor generates an output voltage, and the leakage current size on the circuit to be tested can be determined based on the output voltage.

[0043] In a possible implementation manner, the excitation coil and the feedback coil are wound on a magnetic core to form a ring-shaped detection head.

[0044] When in use, the annular hollow of the annular detection head surrounds the circuit to be tested to detect leakage current on the circuit to be tested.

[0045] In a possible implementation manner, the excitation coil and the feedback coil are wound on a magnetic core to form a cylindrical detection head.

[0046] When in use, the cylindrical detection head is placed close to the circuit to be tested to detect leakage current on the circuit to be tested.

[0047] It should be noted that the weak current detection sensor based on second harmonic amplitude detection includes the above-mentioned weak current detection circuit based on second harmonic amplitude detection, and has corresponding technical effects, so it will not be elaborated on here.

[0048] The above specific implementation methods further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above are only specific implementation methods of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A weak current detection circuit based on second harmonic amplitude detection, characterized in that: include: A waveform generating circuit, used for frequency division to obtain a homologous excitation signal and a double demodulation signal, wherein the frequency of the double demodulation signal is twice the frequency of the excitation signal; A driving circuit, used for generating a driving signal according to the excitation signal; The signal processing circuit includes an excitation coil and a feedback coil wound on the same magnetic core, and is used to drive the excitation coil to generate an alternating magnetic field according to the driving signal to detect the current on the circuit to be measured, drive the analog switch according to the double modulation signal to separate the second harmonic signal from the signal induced by the feedback coil, filter and integrate the second harmonic signal to obtain a voltage signal proportional to the current on the circuit to be measured, and adjust the current in the feedback coil according to the voltage signal to compensate the magnetic core to a zero flux state.

2. A weak current detection circuit based on second harmonic amplitude detection according to claim 1, characterized in that: The waveform generating circuit comprises: a frequency source and a frequency division counter which are connected to each other, wherein the frequency source is used to generate an alternating square wave, and the frequency division counter is used to divide the alternating square wave into an excitation signal and a double demodulation signal, wherein the frequency of the excitation source signal is f / 2, and the frequency of the double demodulation signal is f, where f is the frequency of the frequency source.

3. A weak current detection circuit based on second harmonic amplitude detection according to claim 2, characterized in that: The frequency source adopts ceramic or temperature compensated crystal oscillator.

4. A weak current detection circuit based on second harmonic amplitude detection according to claim 1, characterized in that: The driving circuit includes: a pair of complementary MOS tubes: a PMOS tube Q1B and an NMOS tube Q1A, wherein the source of the PMOS tube Q1B is connected to a power supply VCC, the gate is connected to an excitation signal output by a waveform generating circuit, the drain is connected to the drain of the NMOS tube Q1A, the gate of the NMOS tube Q1A is connected to the excitation signal output by the waveform generating circuit, the source is grounded, the drain connection point of the PMOS tube Q1B and the NMOS tube Q1A is connected to a resistor R1, the resistor R1 is connected to one end of a capacitor C1, and the other end of the capacitor C1 serves as an output of the driving circuit to output a driving signal.

5. A weak current detection circuit based on second harmonic amplitude detection according to claim 1, characterized in that: The same-name ends of the excitation coil and the feedback coil in the signal processing circuit are connected to ground to form a center-tapped transformer.

6. A weak current detection circuit based on second harmonic amplitude detection according to claim 5, characterized in that: The signal processing circuit further includes: a double-throw switch, a filter circuit, an integration circuit and a feedback resistor; The input of the double-throw switch is connected to the double demodulated signal output by the waveform generating circuit, and the output is connected to the opposite-name end of the feedback coil. The output is also connected to the integration circuit through a filter circuit. The output of the integration circuit serves as the output of the weak current detection circuit. The output of the weak current detection circuit is connected to the opposite-name end of the feedback coil through a feedback resistor.

7. A weak current detection circuit based on second harmonic amplitude detection according to claim 1, characterized in that: The coils of the excitation coil and the feedback coil are formed by winding a conventional annular magnetic core or by processing based on a MEMS process.

8. A weak current detection sensor based on second harmonic amplitude detection, characterized in that: include: A weak current detection circuit based on second harmonic amplitude detection as described in any one of claims 1-7, wherein the excitation coil, the feedback coil and the magnetic core form a detection head for approaching the circuit to be tested.

9. A weak current detection sensor based on second harmonic amplitude detection according to claim 8, characterized in that: The excitation coil and the feedback coil are wound on the magnetic core to form a ring-shaped detection head.

10. A weak current detection sensor based on second harmonic amplitude detection according to claim 8, characterized in that: The excitation coil and the feedback coil are wound on the magnetic core to form a columnar detection head.

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