Anti-electromagnetic interference high-precision current sensing circuit and current transformer

Abstract

The invention discloses an anti-electromagnetic interference high-precision current sensing circuit and a current sensor, and relates to the technical field of current measurement. The system comprises a signal conditioning circuit used for carrying out voltage division on power supply voltage to obtain direct current bias voltage, superposing the direct current bias voltage with a sensor signal collected by a TMR sensor, carrying out amplification processing on the superposed sensor signal, and transmitting the amplified sensor signal to a signal filtering circuit; the signal filter circuit is used for filtering interference signals and clutters of the amplified sensor signals, and the filtered sensor signals are sent to the MCU for signal processing. The anti-electromagnetic interference high-precision current sensing circuit has the beneficial effects that firstly, the anti-electromagnetic interference high-precision current sensing circuit analyzes related interference sources and propagation paths of electromagnetic interference in a power grid operation environment, optimizes and selects types of electronic components and device layout, and effectively reduces the interference of the electromagnetic interference on the current sensing circuit; secondly, a hierarchical protection system structure is followed, and the influence of electromagnetic interference on a measurement circuit is reduced;

Description

Technical Field

[0001] The present invention relates to the technical field of current measurement, in particular to an electromagnetic interference resistant high-precision current sensing circuit and a current transformer. Background Art

[0002] At present, most traditional current measurement technologies use direct contact, so the insulation layer must be stripped off and the power supply must be disconnected before measurement can be performed. In contrast, non-contact measurement technology can more conveniently utilize information from the surrounding environment to more accurately reflect the electrical parameters of the cable without damaging the insulation layer. It is also more convenient to operate and can complete the measurement task faster and more accurately. In addition, this technology is also applicable to a wide range of three-phase four-wire transmission and distribution lines and equipment, which can better meet the needs of changing environments and applications. Due to the lack of effective real-time current data monitoring, it is difficult to detect and solve system problems early.

[0003] In addition to the current measurement method that can improve the reliability of current measurement, environmental electromagnetic interference is also a big problem. In the field of power grid operation, networked and automated control technology has been unprecedentedly developed and applied, greatly improving the efficiency of power grid operation, reducing the labor intensity of workers, and bringing about gratifying changes in the power transmission operation and maintenance system. However, intelligence and digitization require many parameters of the power grid to support, which leads to more and more monitoring equipment installed in the power grid. While these monitoring equipment bring convenience, they also make the interference factors related to the production site more and more, and the electromagnetic environment is becoming more and more complex. In recent years, it has become the main failure factor of various control products in industrial sites. Therefore, the anti-electromagnetic interference performance of the product is increasingly required and valued. From the current operation analysis of the technology, the electromagnetic interference at the industrial production site will be more and more complex. These electromagnetic interferences greatly affect the reliability and accuracy of the system, and ensuring the reliable operation of the product is the basis for its value creation. At present, the research on anti-interference technology of industrial control products has become a research hotspot worldwide. Therefore, the research on anti-electromagnetic interference of industrial control products is particularly important and urgent. At the same time, since the product runs in an outdoor operating box, it is a process-layer device in a smart substation and also an intelligent subsystem of a primary device. The working and interference environments it faces are extremely harsh, and strict requirements are placed on its product design and implementation of anti-interference technology.

[0004] Common direct current measurement, such as shunts, is based on a simple resistor divider, which is directly connected to the measured circuit, has requirements for the upper limit of measurement, and has no isolation between input and output. However, when measuring pulsed large current, the shunt will be interfered by the high-frequency signal of the pulsed large current, resulting in increased inductive reactance. At the same time, due to the skin effect and self-inductance of the resistor itself, the measurement accuracy is not high. The skin effect must be taken into account in the actual test of the shunt. When the frequency of the measured current changes, the resistance of the shunt is affected, the test result will produce errors, and it is difficult to reproduce the original waveform. Common indirect measurement methods include electromagnetic current transformers, Hall sensors, Rogowski coils, etc. Electromagnetic current transformers are the most commonly used current measurement equipment. This type of sensor technology is mature and has high accuracy, but there are many limitations in the use process. When measuring high voltages above 500kV, the size of the equipment increases and the cost increases. At the same time, because the magnetic core is prone to magnetic saturation under a large current impact, its measurement dynamic range is small and the frequency band is narrow. The output signal of the transformer needs to be converted into a digital quantity twice, and cannot be directly connected to the control device, and cannot adapt to the development of digitalization and automation. Hall effect current sensors can be used to measure DC currents of hundreds of amperes, but their bandwidth is usually less than 1MHz, they are easily disturbed by external magnetic fields, they are large in size and expensive. When measuring large currents, since the Hall current sensor is a closed loop, a high-power compensation circuit is required, which limits the amplitude range of the measured current and is generally not used to measure large current signals. Summary of the invention

[0005] The purpose of this section is to summarize some aspects of embodiments of the present invention and briefly introduce some preferred embodiments. Some simplifications or omissions may be made in this section and the specification abstract and the invention title of this application to avoid blurring the purpose of this section, the specification abstract and the invention title, and such simplifications or omissions cannot be used to limit the scope of the present invention.

[0006] In view of the problems existing in the prior art, the present invention is proposed.

[0007] Therefore, the technical problem to be solved by the present invention is how to improve the anti-electromagnetic interference capability of the existing current measurement circuit.

[0008] To solve the above technical problems, the present invention provides the following technical solutions: an electromagnetic interference resistant high-precision current sensing circuit, comprising a signal conditioning circuit, which is used to divide the power supply voltage to obtain a DC bias voltage, and superimpose it with the sensor signal collected by the TMR sensor, and amplify the superimposed sensor signal and transmit it to the signal filtering circuit; the signal filtering circuit is used to filter out interference signals and clutter on the amplified sensor signal, and the filtered sensor signal is sent to the MCU for signal processing; the signal conditioning circuit resists electromagnetic interference through an interference suppression module; the signal filtering circuit blocks low-frequency and high-frequency interference and noise through a power decoupling module.

[0009] As a preferred solution of the electromagnetic interference resistant high-precision current sensing circuit of the present invention, wherein: the signal conditioning circuit includes a first voltage-dividing resistor, a second voltage-dividing resistor, a first-stage operational amplifier circuit and a second-stage operational amplifier circuit;

[0010] The first voltage-dividing resistor and the second voltage-dividing resistor divide the supply voltage, and take half of the supply voltage as a DC bias voltage.

[0011] As a preferred solution of the electromagnetic interference resistant high-precision current sensing circuit described in the present invention, wherein: the first-stage operational amplifier circuit is a COMOS process structure, including a first operational amplifier, a second capacitor, a fourth resistor and a fifth resistor; the first operational amplifier amplifies the positive end output signal of the TMR sensor, and the amplification factor is adjusted by the fourth resistor and the fifth resistor.

[0012] As a preferred solution of the electromagnetic interference resistant high-precision current sensing circuit described in the present invention, wherein: the second-stage operational amplifier circuit includes a second operational amplifier and a second capacitor; the second-stage operational amplifier circuit amplifies the negative end signal of the TMR sensor, and subtracts the amplified result from the output result of the first-stage operational amplifier circuit, amplifies the TMR differential output signal and then superimposes a DC bias level to make the TMR differential output signal away from the zero level position.

[0013] As a preferred solution of the electromagnetic interference resistant high-precision current sensing circuit described in the present invention, the signal filtering circuit obtains a second-order active bandpass filter by combining a low-pass filter and a high-pass filter, the upper cutoff frequency of the low-pass filter is 70Hz, and the lower cutoff frequency of the high-pass filter is 30Hz.

[0014] As a preferred solution of the electromagnetic interference resistant high-precision current sensing circuit described in the present invention, the interference suppression module is used to resist electromagnetic interference of the first-stage operational amplifier circuit, including a TVS tube connected in parallel at the power input end and the sensor leads and both ends of the communication line, which is used to discharge high voltage caused by static electricity and strong pulses.

[0015] As a preferred solution of the electromagnetic interference resistant high-precision current sensing circuit of the present invention, the interference suppression module further includes low-frequency magnetic beads connected in series in each input terminal to block spike pulse interference.

[0016] As a preferred solution of the electromagnetic interference resistant high-precision current sensing circuit described in the present invention, the power decoupling module includes 10μF and 100nF capacitors connected in parallel at both ends of each chip power supply, which are respectively used to filter out low-frequency and high-frequency interference; the power decoupling module also includes an LC filtering network for isolating the analog power supply from the digital power supply.

[0017] To solve the above technical problems, the present invention also provides the following technical solutions: a current transformer, comprising the above-mentioned electromagnetic interference resistant high-precision current sensing circuit, and also comprising a radio frequency microwave energy harvesting module and a current waveform transmission module integrated on the same PCB board as the electromagnetic interference high-precision current sensing circuit.

[0018] As a preferred solution of the current transformer of the present invention, a metal shielding box is installed on the outer side of the anti-electromagnetic interference high-precision current sensing circuit.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] First, the electromagnetic interference-resistant high-precision current sensing circuit of the present invention analyzes the relevant interference sources and propagation paths of electromagnetic interference in the power grid operation environment, optimizes the selection of electronic components and device layout, and effectively reduces the interference of electromagnetic interference to the current sensing circuit; second, it follows a hierarchical protection system structure to reduce the impact of electromagnetic interference on the measurement circuit. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative labor. Among them:

[0022] Figure 1 The schematic diagram of the electromagnetic interference resistant high-precision current sensing circuit used in the present invention;

[0023] Figure 2 This is a simulation result diagram of the electromagnetic interference resistant high-precision current sensing circuit used in the present invention;

[0024] Figure 3 It is the system block diagram of the signal conditioning circuit;

[0025] Figure 4It is the system block diagram of the signal filtering circuit;

[0026] Figure 5 This is the block diagram of the electrostatic pulse protection circuit;

[0027] Figure 6 This is the principle block diagram of the magnetic bead anti-interference processing of the power supply;

[0028] Figure 7 This is the block diagram of the power supply decoupling circuit;

[0029] Figure 8 It is a structural schematic diagram of a current transformer;

[0030] Fig. 9 Schematic diagram of electromagnetic wave transmission in a single-layer shell. DETAILED DESCRIPTION

[0031] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below in conjunction with the accompanying drawings.

[0032] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein, and those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0033] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The term "in one embodiment" that appears in different places in this specification does not necessarily refer to the same embodiment, nor does it refer to a separate or selective embodiment that is mutually exclusive with other embodiments.

[0034] Example 1, reference Figure 1 , which is the first embodiment of the present invention, and provides an electromagnetic interference resistant high-precision current sensing circuit, the circuit comprising a tunnel magnetoresistance sensor (TMR sensor), a signal conditioning circuit 100, a signal filtering circuit 200, an interference suppression module 103, a power supply decoupling module and an MCU;

[0035] The signal conditioning circuit 100 divides the supply voltage to obtain a DC bias voltage, which is added to the sensor signal collected by the TMR sensor, so that the sensor signal is far away from the zero level, and then the collected sensor signal is amplified through a two-stage amplifier circuit;

[0036] The signal filtering circuit 200 filters out interference signals and clutter on the amplified sensor signal, and the filtered sensor signal is sent to the MCU for signal processing;

[0037] The interference suppression module 103 and the power supply decoupling module 201 are used to increase the anti-interference capability of the circuit.

[0038] Tunnel magnetoresistance effect (TMR): TMR sensor is the fourth generation of magnetic sensor application technology. TMR sensor has the advantages of high precision, high sensitivity, low power consumption, small size, good temperature stability and wide operating temperature range.

[0039] The schematic diagram of the electromagnetic interference resistant high-precision current sensing circuit used in the present invention is as follows: Figure 1 As shown in the figure, the topology of the instrument amplifier circuit is adopted, and the amplification and filtering device model is selected as LPV821. LPV821 is a single-channel, nanowatt-level power, zero-drift operational amplifier with a power consumption of only 650nA. LPV821 uses a proprietary automatic calibration technology to provide low offset voltage (maximum 10μV) while achieving minimal drift over time and temperature changes. In addition to low offset voltage and ultra-low quiescent current, it also has a picoampere bias current. Different operational amplifiers have different susceptibility to EMI. If conducted EMI enters the operational amplifier, the DC offset value in the amplifier output may deviate from its nominal value when there is EMI. This deviation is caused by signal correction related to the internal semiconductor junction. Although all operational amplifier pin functions are affected by EMI, the input pins may be the most susceptible. The LPV821 operational amplifier contains an internal input low-pass filter that reduces the amplifier's response to EMI. This input filter provides common-mode and differential-mode filtering.

[0040] Example 2, reference Figure 2 , which is the second embodiment of the present invention, and provides the feasibility of circuit simulation. The high-precision, low-power, anti-interference sensor circuit is simulated. Since there is no LPV821 in Multisim, LM358AD is used instead. The simulation results are as follows: Figure 2 . The input voltage first outputs a bias voltage of Vcc / 2 through a voltage follower, then amplifies the weak signal through a two-stage amplifier circuit, and then filters out interference signals and some clutter through an active first-order low-pass filter. At the same time, adding bypass capacitors and decoupling capacitors can further increase the circuit's anti-interference ability. Then the signal is processed by the MCU. According to actual measurements, the power consumption of the entire signal amplification and filtering circuit is only about 3μA. In summary, the high-precision, low-power, anti-interference sensing circuit proposed in this article can be effectively used in signal processing and conditioning applications. At the same time, in response to voltage fluctuations and external electromagnetic interference, the voltage and various external electromagnetic interferences can be suppressed by adding a metal shielding box.

[0041] Example 3, reference Figure 3, which is the third embodiment of the present invention, and this embodiment provides a signal tuning circuit 100.

[0042] Voltage changes in sensor circuits are a common phenomenon. This change is not limited to the ups and downs of voltage values, but also generates complex electromagnetic field fluctuations in the surrounding space, which in turn causes a series of electromagnetic compatibility problems and affects the normal operation of sensors and other sensitive electronic devices. The sources of voltage changes are varied, including but not limited to the normal operation of sensor circuits, sensor circuit failures, and external electromagnetic interference. In daily operation, voltage fluctuations are inevitable due to frequent changes in loads. For example, the start and stop of industrial equipment and the use of high-power electrical appliances will generate transient voltage fluctuations on the power grid. Although this fluctuation is within the design specification, it may still cause interference problems for some sensitive electronic equipment and sensors. The voltage fluctuations caused by these faults are usually more severe than those caused by normal operation and load changes, and are accompanied by strong electromagnetic pulses. This electromagnetic pulse has high energy and a wide spectrum, which strongly interferes with surrounding electronic equipment and may even cause permanent damage. In addition, external electromagnetic interference is also an important source of voltage fluctuations. External electromagnetic interference can come from various sources, including lightning, radio transmission equipment, high-frequency switching power supplies, etc. The electromagnetic waves generated by these interference sources affect the normal operation of electronic equipment and sensors through various channels such as electromagnetic induction, electrostatic induction, and direct coupling.

[0043] In order to further determine the current electromagnetic interference of the current transformer, the signal at the local installation of the CT port was collected, and the parameters related to the macro pulse and micro pulse of the waveform were obtained and displayed using an algorithm. The electromagnetic interference involved includes rainbow pulse: peak-to-peak value, maximum value, number of micro pulses, and duration of macro pulse; micro pulse: actual rise time, oscillation frequency, duration, interval time and spectrum characteristics.

[0044] In these complex electromagnetic interference environments, sensors are particularly vulnerable. As an important part of electronic systems, sensors convert physical quantities into electrical signals. However, these electrical signals are very susceptible to electromagnetic interference during transmission, so these interference effects need to be considered in the sensor circuit design process.

[0045] On this basis, this embodiment proposes a sub-component design diagram of an electromagnetic interference resistant high-precision current sensing circuit.

[0046] The signal conditioning circuit 100 mainly includes a three-stage operational amplifier with chopping, a ripple suppression loop and a low-noise two-stage operational amplifier.

[0047] Figure 3This is the system block diagram of the signal conditioning circuit 100. Since the TMR sensor outputs a full-bridge differential circuit and the signal is too weak to be directly collected, it is necessary to design a differential amplifier circuit that can amplify the sensor signal. The sensor signal itself is very weak, and there are various noises in the circuit at the zero level. Therefore, it is necessary to add a bias level to the sensor signal to ensure that the sensor signal is far away from the zero level. This project directly takes half of the power supply voltage of the sensor circuit as the DC bias level. This level is obtained by dividing the voltage by two resistors R1 and R2, and the voltage is approximately Vcc / 2. The obtained level often has poor load capacity, so a voltage follower is designed to improve the load capacity of the level. The voltage follower circuit is implemented by the U22 op amp. Since the signal from the TMR sensor is weak, a two-stage op amp cascade is designed to amplify the signal. The two-stage op amp circuit is as follows Figure 3 shown.

[0048] The first stage op amp is mainly implemented by the second operational amplifier U23.1. This op amp is made with COMOS technology and has a low bias current error. It is a rail-to-rail output op amp with no output inversion to ensure unidirectional signal transmission. The unit gain bandwidth product of this op amp can reach about 6kHz, and the unit gain is very stable, which is conducive to high-precision current measurement. This op amp has a wide operating voltage range and can operate between 1.7v and 5.5v, which meets the low-power operating voltage range of wireless microwave energy extraction. The op amp also has a built-in electromagnetic interference protection function to reduce the electromagnetic interference effect of the device in harsh environments such as power grids and ensure measurement reliability. The first stage op amp only amplifies the output signal of the positive end of the TMR sensor, and the amplification multiple can be adjusted by R4 and R5. In addition, the op amp circuit is also a subtraction circuit to achieve the subtraction of the reference reference level and the positive end level of the TMR.

[0049] The second stage output of the instrumentation amplifier is realized through the second operational amplifier U23, as shown in Figure 3 As shown. This op amp is the same model as the first stage op amp, and is also a high-performance op amp with small offset current. This stage op amp mainly amplifies the negative end signal of the TMR sensor. At the same time, it performs a subtraction operation with the output result of the first stage op amp, thus amplifying the TMR differential output signal, and at the same time superimposing a DC bias level to ensure that the TMR differential output signal is at the zero level position, thereby improving the anti-interference ability of the system.

[0050] Example 4, reference Figure 4 , which is a fourth embodiment of the present invention, and this embodiment provides a signal filtering circuit 200 such as Figure 4As shown, the signal filtering circuit (200) used in the present invention is a second-order active bandpass filter. Bandpass filtering is a filtering method, the rule is that the signal within the frequency band can pass normally, while the high and low frequency signals exceeding the set critical value are blocked and weakened. However, the blocking and weakening amplitude will change according to different frequencies and different filtering procedures (purposes). It is sometimes also called out-of-band suppression. The essence of bandpass filtering is a combination of high-pass filtering and low-pass filtering.

[0051] Low-pass filtering can be simply considered as: setting a frequency point, when the signal frequency is higher than this frequency, it cannot pass. In digital signals, this frequency point is also the cutoff frequency. When the frequency domain is higher than this cutoff frequency, all values ​​are assigned to 0. Because in this processing process, all low-frequency signals are allowed to pass, it is called low-pass filtering. The process of high-pass filtering is just the opposite of low-pass filtering.

[0052] The meaning of 'low' and 'high' - such as the cutoff frequency - depends on the characteristics of the filter. (The term "low-pass filter" refers only to the shape of the filter response. A high-pass filter can be designed to have a lower cutoff frequency than any low-pass filter. Different frequency responses are the basis for distinguishing them.) Electronic filters can be designed to have any desired frequency range - up to microwave frequencies (over 1000MHz) or even higher. The bandpass filter designed in the present invention is realized by combining low-pass and high-pass filtering. The upper cutoff frequency of the low-pass filter is 70Hz, and the lower cutoff frequency of the high-pass filter is 30Hz, which can just filter out most of the electromagnetic interference signals measured.

[0053] Example 5, reference Figure 5 and Figure 6 , which is the fifth embodiment of the present invention, and this embodiment provides an interference suppression module 103.

[0054] There are various electrical equipment and monitoring equipment with different levels of voltage in the power grid, which are all potential strong sources of static electricity. When operating the instrument by hand, static electricity discharge may occur and enter the current sensor through the route. The voltage can reach tens of volts. If no protective measures are taken, it is possible to damage the electronic components in the tag sensing circuit that integrates microwave energy collection, power management, low-power communication and current sensing. Strong pulse interference from space coupled into the sensor circuit may cause erroneous current measurement results and even damage the sensor circuit, so it needs to be protected. The method to protect electronic devices is to connect a TVS tube in parallel at the power input end and the sensor leads and both ends of the communication line. Since the operating voltage is lower than the breakdown voltage of the TVS tube, the TVS tube is usually in an almost open circuit state and does not affect the operation of the circuit. When static electricity and strong pulses enter the line terminal and high voltage appears, the TVS tube breaks down and conducts, allowing most of the energy to be discharged from the TVS tube bypass, thereby effectively protecting other devices. The design schematic is shown in the figure below. Figure 5 .

[0055] At the same time, connect low-frequency magnetic beads in series in each input terminal, such as Figure 6 As shown in the figure, since the magnetic beads have a large impedance to high-frequency components, they can effectively block spike pulse interference from entering the circuit. At the same time, using isolated power supply in communication circuits is also an effective protection method.

[0056] Example 6, reference Figure 7 , which is the sixth embodiment of the present invention, and this embodiment provides a power decoupling module 201.

[0057] Since the power supply of the tag sensor circuit is obtained by rectifying and doubling the microwaves in the environment, there is a boost circuit in the power management circuit module. They will generate corresponding high-frequency noise when working. The digital circuit in the circuit is also in a switching state when working, which will also generate noise. Although the current is not large, its impact on high-precision current sensors cannot be ignored. In addition, operational amplifiers inevitably have various nonlinearities, various offset voltages and currents, as well as thermal noise and shot noise of the circuit. When these noises are added, they will be multiplied by the differential sensing signal of TMR to produce a DC bias error, thereby affecting the current measurement results of this project. For this reason, in this project, 10μF and 100nF capacitors are connected in parallel at both ends of each chip power supply to filter out low-frequency and high-frequency interference, respectively. Figure 7 shown.

[0058] The analog power supply and the digital power supply are isolated from the LC filter network to minimize the switching noise of the digital circuit from entering the analog circuit. Since the inductor has a large impedance to high frequencies and the capacitor has a large impedance to low frequencies, the effect of connecting them in series is more significant than that of a single capacitor or resistor. Therefore, the LC filter network can effectively block high-frequency noise, thereby reducing current measurement errors and stability.

[0059] Example 7, reference Figure 8 and 9 , which is the seventh embodiment of the present invention, and provides a current transformer, specifically a passive wireless tag current transformer, including a radio frequency microwave energy acquisition module, a current waveform transmission module and the above-mentioned electromagnetic interference-resistant high-precision current sensing circuit integrated on the same PCB board, such as Figure 8 As shown, the radio frequency microwave energy collection module, the current waveform data low-power transmission module and the anti-electromagnetic interference high-precision current sensing circuit are integrated together to complete the design of the passive wireless tag current transformer circuit. The designed measurement circuit board card size is 57mm×50mm. The distance between the microwave wireless power source antenna and the tag antenna is 1 meter. At this time, the power management module on the tag can provide a stable 2.2v voltage, and under long-term measurement, the voltage basically remains stable. The experimental test results show that the anti-electromagnetic interference current waveform sensing circuit of the present invention can work in a low-power mode and work stably.

[0060] In addition to interfering with the tag electronic mutual inductor through conduction in the circuit, electromagnetic interference can also interfere with the tag electronic mutual inductor through radiation. Electromagnetic induction is one of the main interference mechanisms. When the voltage changes, a changing magnetic field will be generated in the surrounding space. This changing magnetic field will induce current (i.e. induced current) in the conductive part of the sensor. After these induced currents enter the sensor circuit, they will cause signal distortion or increase noise, affecting the measurement accuracy.

[0061] Electrostatic induction is an important interference pathway. When voltage changes cause electric field changes, the sensor is in this changing electric field, and the internal charge distribution will be affected, causing interference to its normal operation. Especially for capacitive sensors, this electrostatic induction effect will cause changes in capacitance, resulting in measurement errors.

[0062] Direct coupling is another common interference mode. High-frequency voltage fluctuations directly enter the input circuit of the sensor through electromagnetic coupling, affecting its normal operation. In a high-frequency environment, the sensor's leads may pick up external electromagnetic signals like an antenna, causing signal distortion and noise problems.

[0063] In order to deal with the electromagnetic field anomalies and the interference caused by these voltage changes, metal shielding boxes are usually used to protect the sensors. The design of the metal shielding box is based on the principle of Faraday cage, which effectively blocks the interference of external electromagnetic fields by providing a closed conductive shielding layer. The metal shielding box generates induced current on the surface of the shielding layer through its conductivity and reflection properties, forming a reverse electric field to offset the influence of the external electromagnetic field, thereby protecting the internal sensor.

[0064] Assume that an electromagnetic wave is projected onto a metal shell with a thickness of t (in mm). When the electromagnetic wave reaches the surface of the shell, part of the electromagnetic wave is reflected by the shell, and the remaining part of the electromagnetic wave passes through the first surface of the shell into the metal conductor, attenuates and transmits in the metal conductor of the shell, and reaches the second surface of the shell after a distance of t. At this time, part of the electromagnetic wave is reflected into the metal conductor of the shell, and part of the electromagnetic wave passes through the shell into the sensor circuit. The part of the electromagnetic wave reflected back into the metal conductor on the second surface of the metal shell continues to attenuate and transmit in the opposite direction in the metal conductor. When it reaches the first surface of the metal shell after a distance of t, part of the electromagnetic wave passes through the first surface of the shell in the opposite direction and enters the area where the electromagnetic wave starts to be projected. The other part of the electromagnetic wave is still reflected back into the metal conductor of the product shell and continues to transmit. The above process is repeated. Fig. 9 As shown. R1 is the reflection loss of the first surface, R2 is the reflection loss of the second surface, A is the absorption loss, and B is the multiple reflection loss.

[0065] When the areas on both sides of the shell are free space, its electromagnetic shielding effectiveness SE is the sum of absorption loss A, reflection loss R, and multiple reflection loss B, that is, SE = A + R1 + R2 + B = A + R + B.

[0066] Loss calculation:

[0067] 1. Reflection loss

[0068] The reflection loss of electromagnetic waves at the interface between two media (shell metal conductor and free space (air)) is related to the characteristic impedance of the medium. The reason for reflection is the impedance mismatch between the metal conductor and free space. Generally, the wave impedance of free space is much larger than the wave impedance of the metal shield. The reflection loss equation is

[0069]

[0070] Where: C is a constant, which is 168 in plane wave, 321.7 in electric field, and 14.6 in magnetic field; μ r is the relative magnetic permeability of the conductor material; f is the frequency, Hz; σ r is the relative conductivity of the conductor material.

[0071] The higher the conductivity of the shielding material, the lower the magnetic permeability, and the greater the reflection loss. In order to achieve the shielding effect, the shell should be designed as an electrical continuum. Magnetic shielding is not only achieved by cutting off the coupling, but also by shunting a magnetic bypass because the magnetic lines are closed.

[0072] For the same shielding material, different field source characteristics result in different reflection losses. Generally, the magnetic field reflection loss is less than the plane wave reflection loss and the electric field reflection loss. Let the plane wave reflection loss in the far field region (r >> λ / 2π, where r is the distance from the measurement point to the radiation source and λ is the electromagnetic wave wavelength) be RP, the electric field reflection loss in the near field region be Re, and the magnetic field reflection loss be Rm, i.e., Rm < RP < Re. Therefore, considering reliability, when calculating the total shielding effectiveness, the field type should be considered as the magnetic field.

[0073] 2. Absorption Loss

[0074] When electromagnetic waves pass through the metal conductor of the outer shell, eddy currents are induced in the metal conductor, generating ohmic losses and converting them into heat energy. At the same time, the eddy current reverse magnetic field cancels the incident electromagnetic waves, forming absorption losses. In engineering, for the convenience of calculation, the relative conductivity and relative permeability of the metal shielding material are often used to represent the absorption losses. The absorption loss equation is:

[0075]

[0076] As can be seen from the above equation, the absorption loss increases with the increase of the thickness t and frequency f of the shielding body, and also increases with the increase of the relative conductivity σ r and relative permeability μ r of the shielding material.

[0077] 3. Multiple Reflection Loss

[0078] The multiple reflection loss equation is:

[0079]

[0080] In the formula: K is the ratio of the air wave impedance to the metal wave impedance.

[0081] When the outer shell of the product is relatively thick or the frequency of the incident electromagnetic wave is relatively high, the absorption loss of the outer shell of the product is large (A > 10 dB), and the multiple reflection loss can be ignored. However, when the outer shell of the product is relatively thin or the frequency of the incident electromagnetic wave is relatively low, the absorption loss of the outer shell of the product is very small (A < 10 dB), and the multiple reflection loss must be considered.

[0082] 4. Shielding of Gaps and Openings

[0083] In the actual application process, there are factors such as seams and openings in the outer shell. Therefore, any actually enclosed metal shell is not a complete ideal shielding body. Various inevitable discontinuous gaps and openings destroy the integrity of the shielding body, resulting in electromagnetic energy leakage and reducing the shielding effectiveness of the metal outer shell.

[0084] The shielding effectiveness of the gap consists of the reflection loss Rd at the gap opening and the transmission loss Ad of the gap. The gap shielding effectiveness equation is:

[0085]

[0086] In the formula: Kc is the ratio of the wave impedance at the space opening to the wave impedance of the incident wave in the space, Kc=j6.69*10-5fg, where f is the frequency, MHz; d is the depth of the gap, cm; g is the length of the gap, cm.

[0087] The shielding effectiveness equation for openings is:

[0088] SE a =A a +R a +B a +K1+K2+K3

[0089] Where: Aa is the transmission attenuation in the hole. Ra is the single reflection loss of the hole. Ba is the multiple reflection correction term. K1 is the correction term related to the number of holes. K2 is the low-frequency correction term caused by different depths. K3 is the correction term caused by the mutual coupling between adjacent holes.

[0090] The calculation formulas for each variable are:

[0091] A a =36d / D

[0092]

[0093] K1=-10lg(an)

[0094] K2=-20lg(1+35p -2.3 )

[0095]

[0096] In the formula: d is the depth of the hole, D is the diameter of the hole, Kh is the ratio of the wave impedance at the space opening to the wave impedance of the incident wave in the space, Kh = j5.79*10-5fD; a is the surface area of ​​each hole, n is the number of holes per square centimeter; p is the conductor width / depth between holes. The theoretical analysis and actual calculation results of the electromagnetic shielding effectiveness of metal shells show that the better the conductivity and magnetic permeability of the metal shell material, the higher the shielding effectiveness; the shielding effectiveness of the metal shell is significantly affected by its material, size, thickness, the size and number of holes on the shell, and the frequency of the interference source. Among them, when the material is stainless steel and the thickness of the metal shell is 5mm, the shielding performance is the best, the electric field shielding performance has been improved by nearly 30dB, and the magnetic field shielding performance has been improved by nearly 25dB.

[0097] Therefore, the present invention electromagnetically shields the tag-type electronic mutual inductor by designing a low-power filter circuit combined with a metal shielding shell. A bandpass circuit is realized by designing active second-order low-pass and high-pass circuits, and the upper cutoff frequency is designed to be 70Hz and the lower cutoff frequency is 30Hz, ensuring that the 50Hz current waveform signal can be collected, while suppressing most of the various conducted interferences caused by electromagnetic interference outside the frequency band. In addition, by selecting stainless steel materials with a material thickness of more than 5mm, most of the electromagnetic interference caused by radiation can be suppressed, ensuring the working stability of the tag sensing current.

[0098] Importantly, it should be noted that the construction and arrangement of the present application shown in a number of different exemplary embodiments are only exemplary. Although only a few embodiments are described in detail in this disclosure, it should be readily understood by those who refer to this disclosure that many modifications are possible without substantially departing from the novel teachings and advantages of the subject matter described in the application (e.g., mounting arrangements, use of materials, color, changes in orientation, etc.). For example, the element shown as integrally formed may be composed of multiple parts or elements, the position of the element may be inverted or otherwise changed, and the nature or number or position of the discrete element may be changed or changed. Therefore, all such modifications are intended to be included within the scope of the present invention. The order or sequence of any process or method steps may be changed or reordered according to alternative embodiments. In the claims, any "device plus function" clause is intended to cover the structure of the execution function described herein, and is not only structurally equivalent but also equivalent structure. Without departing from the scope of the present invention, other replacements, modifications, changes and omissions may be made in the design, operating conditions and arrangement of the exemplary embodiments. Therefore, the present invention is not limited to a specific embodiment, but extends to a variety of modifications that still fall within the scope of the appended claims.

[0099] Additionally, in an effort to provide a concise description of example embodiments, all features of an actual implementation may not be described.

[0100] It should be understood that in the development of any actual implementation, as in any engineering or design project, numerous implementation-specific decisions may be made. Such a development effort may be complex and time-consuming, but for those of ordinary skill having the benefit of this disclosure, the development effort will be a routine task of design, fabrication, and production without undue experimentation.

[0101] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. An electromagnetic interference resistant high-precision current sensing circuit, characterized in that: include, A signal conditioning circuit (100) is used to divide the supply voltage to obtain a DC bias voltage, and to superimpose the voltage with the sensor signal collected by the TMR sensor, and to amplify the superimposed sensor signal and transmit the amplified sensor signal to the signal filtering circuit (200); The signal filtering circuit (200) is used to filter out interference signals and clutter from the amplified sensor signal, and the filtered sensor signal is sent to the MCU for signal processing; The signal conditioning circuit (100) resists electromagnetic interference through an interference suppression module (103); The signal filtering circuit (200) blocks low-frequency and high-frequency interference and noise through a power decoupling module (201).

2. The electromagnetic interference resistant high-precision current sensing circuit according to claim 1, characterized in that: The signal conditioning circuit (100) comprises a first voltage-dividing resistor (R1), a second voltage-dividing resistor (R2), a first-stage operational amplifier circuit (101) and a second-stage operational amplifier circuit (102); The first voltage-dividing resistor (R1) and the second voltage-dividing resistor (R2) divide the supply voltage, and take half of the supply voltage as a DC bias voltage.

3. The electromagnetic interference resistant high-precision current sensing circuit according to claim 2, characterized in that: The first-stage operational amplifier circuit (101) is a COMOS process structure, comprising a first operational amplifier (U22), a second capacitor (C2), a fourth resistor (R4) and a fifth resistor (R5); The first operational amplifier (U22) amplifies the positive end output signal of the TMR sensor, and the amplification factor is adjusted by the fourth resistor (R4) and the fifth resistor (R5).

4. The electromagnetic interference resistant high-precision current sensing circuit according to claim 3 is characterized in that: The second-stage operational amplifier circuit (102) comprises a second operational amplifier (U23) and a second capacitor (C2); The second stage operational amplifier circuit (102) amplifies the negative end signal of the TMR sensor, and performs a subtraction operation on the amplified result and the output result of the first stage operational amplifier circuit (101), amplifies the TMR differential output signal and then superimposes a DC bias level, so that the TMR differential output signal is away from a zero level position.

5. The electromagnetic interference resistant high-precision current sensing circuit according to claim 4, characterized in that: The signal filtering circuit (200) obtains a second-order active bandpass filter by combining a low-pass filter and a high-pass filter, the upper cutoff frequency of the low-pass filter is 70 Hz, and the lower cutoff frequency of the high-pass filter is 30 Hz.

6. The electromagnetic interference resistant high-precision current sensing circuit according to claim 5, characterized in that: The interference suppression module (103) is used for the first-stage operational amplifier circuit (101) to resist electromagnetic interference, and comprises a TVS tube connected in parallel to the power input terminal and the two ends of the sensor lead and the communication line, and is used for discharging high voltage caused by static electricity and strong pulses.

7. The electromagnetic interference resistant high-precision current sensing circuit according to claim 6, characterized in that: The interference suppression module (103) also includes low-frequency magnetic beads connected in series in each input terminal, which are used to block peak pulse interference.

8. The electromagnetic interference resistant high-precision current sensing circuit according to claim 7, characterized in that: The power decoupling module (201) comprises 10μF and 100nF capacitors connected in parallel at both ends of each chip power supply, respectively used to filter out low-frequency and high-frequency interference; The power supply decoupling module (201) also includes an LC filter network, which is used to isolate the analog power supply from the digital power supply.

9. A current transformer, characterized in that: It comprises the electromagnetic interference resistant high-precision current sensing circuit as claimed in any one of claims 1 to 8, and also comprises a radio frequency microwave energy harvesting module (300) and a current waveform transmission module (400) integrated on the same PCB board as the electromagnetic interference high-precision current sensing circuit.

10. The current transformer according to claim 9, characterized in that: A metal shielding box is installed on the outer side of the anti-electromagnetic interference high-precision current sensing circuit.

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