Electromagnetic detection multi-level current transmission system for highland environment and control method

Abstract

The present application belongs to the technical field of electromagnetic detection, and relates to an electromagnetic detection multi-level current emission system and control method for highland environment. The control method comprises calculating a load model impedance; analyzing an MFPWM control process to determine optimal control parameters for an emission spectrum intensity maximization optimization target; and MFPWM optimization control: by adjusting the frequency and amplitude of each modulation signal component, directional power distribution, independent regulation and control of each frequency component are realized. The present application adopts a multi-level topology structure, which realizes high output voltage while reducing switching voltage stress, avoids the cost increase and high system complexity caused by the dependence of traditional H-bridge inverters on high power supply voltage; in combination with multi-frequency pulse width modulation optimization control, the spectrum energy can be flexibly configured, the high-frequency component of the emission signal is enhanced, and the spectrum energy utilization rate and signal-to-noise ratio are improved; the high dynamic range ground impedance changes can be accurately tracked, and the needs of deep geological electromagnetic detection in extreme environments are met.

Description

Electromagnetic Detection Multilevel Current Emission System and Control Method for High-Altitude Environments Technical Field

[0001] This invention belongs to the field of electromagnetic detection technology, specifically relating to a novel current transmitter and control method for high-altitude environments, and particularly to a multi-level current transmission system for electromagnetic detection in high-altitude environments and its control method. Background Technology

[0002] Currently, electromagnetic exploration technology is widely used in mineral exploration, geological surveys, and other fields. However, when conducting exploration in high-altitude and frigid regions, high-resistivity anomalies and permafrost significantly increase the regional grounding impedance, causing a sharp attenuation of the transmitted current. Simultaneously, complex terrain and deep target burial can lead to dynamic jumps in grounding resistance, compounded by impedance time-varying drift caused by electrode heating, resulting in load mismatch and severely affecting the stability of the electromagnetic field signal. Existing high-power electromagnetic transmitters are mostly designed for low to medium impedance, unable to provide sufficient output power in high-impedance environments, resulting in blind spots in deep geological exploration in high-altitude areas and failing to meet the needs of extreme environment exploration.

[0003] The principle of electromagnetic detection in plateau regions is as follows: The electromagnetic transmission system uses a grounded electrode as the transmitting carrier, and a transmission circuit is formed through a long grounded conductor and the earth. During exploration, the transmission system provides a high-power excitation signal to the ground load through the grounding cable, and then conducts exploration by radiating an electromagnetic field in space. Under the excitation of the primary field, anomalies located in the target area will generate a secondary field. By collecting data through the receiving system and performing post-processing, geological information of the target area can be obtained.

[0004] During the exploration process, a higher source signal amplitude helps reduce erroneous results caused by noise overwhelming the received data, and improves the accuracy of detection results when the noise level is constant, thereby improving the measurement accuracy and clarity of anomaly identification and imaging. Furthermore, due to wave impedance, electromagnetic waves attenuate rapidly in the strata, and the signal energy weakens with distance. Therefore, stronger excitation signal energy is usually required to ensure a sufficient signal-to-noise ratio at the target depth. In summary, high-power source emission signals help improve exploration depth, clarity, and accuracy. However, existing high-power electrical source transmitters are mostly designed for low to medium impedance conditions and cannot provide sufficient output power in high impedance environments, failing to meet the needs of deep exploration in extreme environments. This has become a major obstacle to effective electromagnetic exploration in plateau regions.

[0005] Therefore, there is an urgent need to develop an electromagnetic detection multi-level current emission system and its control method for use in high-altitude environments, in order to effectively solve the above problems. Summary of the Invention

[0006] The purpose of this invention is to provide a multi-level current transmitter and MFPWM optimized control system for high ground impedance environments, and to provide a control method for the above system, so as to solve the problem of blind spots in deep geological exploration caused by transmission current attenuation and load mismatch in traditional transmission systems in high ground impedance environments such as plateaus.

[0007] This invention is achieved through the following technical solution:

[0008] An electromagnetic detection multilevel current transmission system for high-altitude environments consists of a DC power supply, a multilevel transmitter, and an equivalent load.

[0009] The multilevel transmitter is connected in series between the DC power supply and the equivalent load; a mobile power supply vehicle is used as the DC voltage. The multilevel transmitter includes a capacitor. capacitors Switching devices - Switching devices and clamping diodes - Clamping diode In electromagnetic detection scenarios, the sensing carrier for electromagnetic detection is a long grounded cable and the ground, which together constitute a linear electromagnetic load, equivalent to an inductor. With resistance The series model.

[0010] A control method for an electromagnetic detection multilevel current emission system for high-altitude environments includes the following steps:

[0011] A. Treat the multilevel transmitter as an equivalent voltage source with adjustable output voltage. Based on its working state in the positive and negative half-cycles of transmission, establish the load model of the multilevel transmitter under different output voltage levels and calculate the impedance of the load model.

[0012] B. Analyze the MFPWM control process, optimize the transmission spectrum intensity to determine the optimal control parameters, and improve transmission performance under the condition of limited DC power supply voltage.

[0013] C. Obtain the initial system impedance through offline identification combined with Fast Fourier Transform (FFT). ;

[0014] D. Based on the set transmission current Obtain the initial voltage of the multilevel transmitter. ;

[0015] E. Perform real-time online identification of load parameters, using the initial voltage. To provide the excitation input, the load current is collected. The real-time system harmonic impedance was obtained through FFT analysis. ;

[0016] F. By real-time load current and the set emission current The difference between the real-time impedance and the actual impedance Obtain the voltage difference This is used to superimpose the voltage onto the initial voltage, calculate the voltage correction value, and obtain the actual transmission voltage. This ensures a stable output of the transmitting current;

[0017] G. Generate a hybrid modulation wave to obtain a driving signal, drive the multi-level transmitter to output current, and achieve directional power distribution and independent control of each frequency component by adjusting the frequency and amplitude of each modulation signal component, thus realizing MFPWM control.

[0018] Further, in step A, during the positive half-cycle of transmission, when the switch... , , , When the circuit is turned on, the voltages of the two capacitors at the DC power supply terminal are simultaneously applied to the load terminal, and the circuit state satisfies the equation:

[0019] (1)

[0020] In the formula, The on-state voltage for the IGBT module. This is the emission current; if the IGBT turn-on voltage is ignored... Equivalent load voltage It can be approximated as the DC power supply voltage. ;

[0021] With only a single capacitor voltage applied to the load terminal at the DC power supply terminal, the circuit state satisfies the following equation:

[0022] (2)

[0023] In the formula, This represents the diode voltage drop.

[0024] Furthermore, in step A, the impedance of the load model is mainly composed of the long conductor, the grounding electrode, and the electrical parameters of the ground layer between them, specifically:

[0025] (3)

[0026] In the formula, It is a linear time-varying impedance. For formation impedance, , For line inductance, , For line resistance, , It is a parallel capacitor. The imaginary unit, .

[0027] Further, step B specifically includes:

[0028] B1. Based on the definition of transmitted spectrum energy, the objective of maximizing transmitted spectrum intensity can be expressed as:

[0029] (twenty one)

[0030] By combining the relationship between amplitude and frequency, the theoretical value of the modulation amplitude parameter can be approximately solved:

[0031] (twenty two)

[0032] By rationally allocating the limited spectrum of energy, we ensure that each detection frequency has a sufficient signal-to-noise ratio;

[0033] B2. Define the optimization objective as the degree of deviation between the modulation amplitude and the root mean square value. By minimizing the calculated result of this deviation, determine the main frequency amplitude that is closest to the theoretical value.

[0034] (twenty three)

[0035] The optimization objective set from the perspective of phase parameters is:

[0036] (twenty four)

[0037] In the formula, For the first The phase of the current at a detection frequency;

[0038] B3. Transform the solution of the nonlinear equation for the optimal control parameters into nonlinear optimization, and set the optimization objective:

[0039] (25)

[0040] In the formula, , and This is a weighting factor, with a value range of [0,1].

[0041] B4. By minimizing To determine the optimal parameter solution, the solution variables are defined as follows: Introducing a penalty function The optimal control parameters are determined by solving the problem.

[0042] Furthermore, step B4, the solution process is as follows:

[0043] (26)

[0044] First, give the initial parameter values ​​and the initial solution. Subsequently, a loop algorithm is used to iteratively update the solution until the convergence condition is met; in each loop, the following equation is solved iteratively:

[0045] (27)

[0046] In the formula, To update the step size, a very small value is usually taken. For update rate, The gradient of the penalty function is used; the loop terminates when either of the following convergence conditions is met: a) the penalty value is less than the set convergence value. b. The number of iterations reaches the set value. The optimal control parameters can then be determined.

[0047] Further, step C specifically includes:

[0048] Offline identification of load parameters is performed. Due to the low transmission frequency, the influence of capacitive parasitic parameters is ignored. At this time, the system load impedance can be simplified to:

[0049] (4)

[0050] An AC square wave voltage source containing multiple spectral components, as shown in the following formula, is directly applied to the load. :

[0051] (5)

[0052] In the formula, The harmonic order is... for Step source voltage;

[0053] FFT analysis was performed on the sampling results of the transmitted voltage and current signals to obtain the load resistance and reactance at different frequencies as shown in formula (6):

[0054] (6)

[0055] In the formula, for First-order signal source voltage and load current, and Let n be the load impedance and reactance.

[0056] Furthermore, step E, online identification of load parameters, specifically involves: first, setting the dominant frequency and amplitude of the multi-frequency synthesized wave; then, superimposing the DC signal onto the stable initial stage of the electromagnetic transmission waveform; next, measuring the transmission current in the initial stage; and finally, performing spectrum analysis on the transmission current in the initial stage to obtain the DC current. Fundamental current Second harmonic current Third harmonic current And the phases 1, 2, and 3 of the AC component; finally, the parameters of each part are calculated by formula (4) to determine the lumped parameters of the final load impedance.

[0057] Further, step G specifically includes the following steps:

[0058] G1. Determine the appropriate detection frequency group based on the target detection depth range. Modulation amplitude values ​​corresponding to each detection frequency Differentiated designs are implemented based on the media attenuation characteristics at different exploration depths to ensure that the detection signals at each depth have sufficient signal-to-noise ratio;

[0059] G2, modulating signal components of different frequencies and amplitudes. Superposition generates hybrid modulated waves ; using cascaded triangular carriers and For the mixed modulated wave respectively and inverted modulation wave Modulation is performed to generate control pulses. - and - These signals serve as the drive signals for the front and rear IGBTs of the multilevel transmitter, respectively, and output a high-frequency mixed pulse drive signal containing the target detection frequency component to achieve precise control of the multilevel transmitter.

[0060] G3. Based on the principle of electromagnetic induction, the driving signal drives the multi-level transmitter to output excitation current. After being converted by the electromagnetic transducer, the radiated spatial magnetic field contains preset detection frequency components, realizing multi-frequency collaborative detection.

[0061] Compared with the prior art, the beneficial effects of the present invention are:

[0062] 1. This invention addresses the problems of transmission current attenuation and load mismatch in traditional transmission systems in high-grounding-impedance environments such as plateaus. By innovatively adopting a multi-level topology, it achieves high output voltage while reducing switching voltage stress, thus avoiding the cost increase and excessive system complexity caused by the reliance on high supply voltage in traditional H-bridge inverters.

[0063] 2. With multi-frequency pulse width modulation (MFPWM) optimized control, the spectrum energy can be flexibly configured to enhance the high-frequency components of the transmitted signal and improve the spectrum energy utilization and signal-to-noise ratio;

[0064] 3. This invention combines an offline identification + real-time identification load impedance modeling scheme, which can accurately track changes in ground impedance with a high dynamic range. By setting the feedforward voltage and coordinating closed-loop dynamic compensation, it ensures stable output of the transmission current. Ultimately, it effectively meets the needs of deep geological electromagnetic exploration in extreme environments and solves the pain point of insufficient transmission performance of existing technologies under high impedance conditions. Attached Figure Description

[0065] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0066] Figure 1 shows the circuit topology of a multi-level transmitting system.

[0067] Figure 2 shows a simplified model of a multilevel transmitter;

[0068] Figure 3 is , , and Operating model of a multilevel transmitter circuit with input voltage level enabled;

[0069] Figure 4 is , , and Operating model of a multilevel transmitter circuit with input voltage level enabled;

[0070] Figure 5 is , , and Operating model of a multilevel transmitter circuit with input voltage level enabled;

[0071] Figure 6 shows the load impedance model of the electrical source;

[0072] Figure 7 is a schematic diagram of the synthetic multi-frequency transmission principle used for online identification;

[0073] Figure 8 shows the equivalent parasitic parameter model of the multi-level transmitting circuit;

[0074] Figure 9 shows the equivalent parasitic parameter model of the positive half-cycle multi-level transmitter circuit;

[0075] Figure 10 is a schematic diagram of the MFPWM control circuit;

[0076] Figure 11 is a schematic diagram of the rule-based sampling method;

[0077] Figure 12 is a flowchart of the parameter optimization solution;

[0078] Figure 13 shows the overall control strategy diagram of the three-level transmitter;

[0079] Figure 14 is a flowchart of the control method for an electromagnetic detection multilevel current emission system used in high-altitude environments. Detailed Implementation

[0080] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and not intended to limit it. Furthermore, it should be noted that, for ease of description, the accompanying drawings show only the parts relevant to the present invention, and not all of the structures.

[0081] It should be noted that similar reference numerals and letters in the following figures indicate similar items; therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures. Furthermore, in the description of this invention, terms such as "first," "second," etc., are used only to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0082] This invention provides an electromagnetic detection multilevel current transmission system for high-altitude environments. The system topology is shown in Figure 1. The electromagnetic detection multilevel transmission system consists of a DC power supply, a multilevel transmitter, and an equivalent load. The multilevel transmitter is connected in series between the DC power supply and the equivalent load. In Figure 1, the part shown in the dashed box (1) is the DC power supply, the part shown in the dashed box (2) is the multilevel transmitter, and the part shown in the dashed box (3) is the equivalent load. In field experiments, a mobile power supply vehicle is usually used to provide a stable DC voltage for the system. The core components of a multilevel transmitter include capacitors. , Switching devices - Switching devices and clamping diodes - In electromagnetic detection scenarios, the sensing carrier is a long grounded cable connected to the ground, which together constitute a linear electromagnetic load, equivalent to an inductor. With resistance The cascade model. Among them, , These are the output voltages of the front and rear branches of the multilevel transmitter, respectively. This is the output voltage on the load side. This is the output current on the load side.

[0083] The specific operating modes of the multilevel transmitter are shown in Table 1. In the table, "1" represents the switching device being on, and "0" represents the switching device being off. By selecting the corresponding switching state combination in the table, the load-side output voltage can be... Precise control in , ,0, Five discrete voltage levels. Based on this operating characteristic, the multilevel transmitter can be equivalent to a voltage source with adjustable output voltage, and its simplified circuit model is shown in Figure 2. In this invention, a three-level multilevel topology is used as an example to analyze the working principle of the multilevel transmitter. Referring to Table 1, and taking the positive half-cycle as the analysis object, the circuit operating models of the multilevel transmitter under different output voltage levels can be obtained, as shown in Figures 3-5.

[0084] Table 1

[0085]

[0086] As shown in Figure 14, the control method of the electromagnetic detection multi-level current emission system for plateau environments of the present invention includes the following steps:

[0087] A. Treat the multilevel transmitter as an equivalent voltage source with adjustable output voltage. Based on its working state in the positive and negative half-cycles of transmission, establish the load model of the multilevel transmitter under different output voltage levels and calculate the impedance of the load model.

[0088] B. Analyze the MFPWM control process, optimize the transmission spectrum intensity to determine the optimal control parameters, and improve transmission performance under the condition of limited DC power supply voltage.

[0089] C. Obtain the initial system impedance through offline identification combined with Fast Fourier Transform (FFT). ;

[0090] D. Based on the set transmission current Obtain the initial voltage of the multilevel transmitter. ;

[0091] E. Perform real-time online identification of load parameters, using the initial voltage. To provide the excitation input, the load current is collected. The real-time system harmonic impedance was obtained through FFT analysis. ;

[0092] F. By real-time load current and the set emission current The difference between the real-time impedance and the actual impedance Obtain the voltage difference This is used to superimpose the voltage onto the initial voltage, calculate the voltage correction value, and obtain the actual transmission voltage. This ensures a stable output of the transmitting current;

[0093] G. Generate a hybrid modulation wave to obtain a driving signal, drive the multi-level transmitter to output current, and achieve directional power distribution and independent control of each frequency component by adjusting the frequency and amplitude of each modulation signal component, thus realizing MFPWM control.

[0094] Specifically, the operating status of the multilevel transmitter within one working cycle is analyzed as follows:

[0095] As shown in Figure 3, during the positive half-cycle of transmission, when the switch... , , , When the circuit is turned on, the voltages of the two capacitors at the DC power supply terminal are simultaneously applied to the load terminal. At this time, the circuit state satisfies the equation:

[0096] (1)

[0097] In the formula, The on-state voltage for the IGBT module. This is the emission current; if the IGBT turn-on voltage is ignored... Equivalent load voltage It can be approximated as the DC power supply voltage. .

[0098] As shown in Figures 4 and 5, only a single capacitor voltage is applied to the load terminal from the DC power supply terminal. At this time, the circuit state satisfies the following equation:

[0099] (2)

[0100] in, This represents the diode voltage drop.

[0101] The working process of the negative half-cycle of the multilevel transmitter is similar to that of the positive half-cycle, so it will not be analyzed in detail here.

[0102] To address the issue of dynamic changes in load parameters under high grounding impedance conditions, a load impedance modeling and identification scheme combining "offline identification + online identification" is proposed, as detailed below:

[0103] The load impedance of an electrical source emission system is mainly composed of the electrical parameters of the long conductor, the grounding electrode, and the ground layer in between. Among these, the parameters of the long conductor vary relatively little and can be characterized using a lumped parameter model, as shown in Figure 6. This model includes the line inductance. , Line resistance , With parallel capacitor , The contact resistance between the grounding electrode and the earth exhibits time-varying characteristics under continuous high current conditions, and can be modeled as a linear time-varying impedance. Formation impedance This reflects the electrical response of the underground medium between the two grounding electrodes, where, The transmission angular frequency, For time. In Figure 6, the part shown in the dashed box (1) is the transmission cable, the part shown in the dashed box (2) is the grounding electrode, and the part shown in the dashed box (3) is the ground impedance.

[0104] The overall impedance of the load model is:

[0105] (3)

[0106] In the formula, It is a linear time-varying impedance. For formation impedance, , For line inductance, , For line resistance, , It is a parallel capacitor. The imaginary unit, .

[0107] In field experiments, impedance is often measured in advance. Load parameters are first identified offline, and since the transmission frequency is low, the influence of capacitive parasitic parameters is ignored. At this point, the system load impedance can be simplified to:

[0108] (4)

[0109] An AC square wave voltage source containing multiple spectral components, as shown in the following formula, is directly applied to the load. :

[0110] (5)

[0111] In the formula, The harmonic order is... for Step source voltage.

[0112] FFT analysis was performed on the sampling results of the transmitted voltage and current signals to obtain the load resistance and reactance at different frequencies as shown in formula (6):

[0113] (6)

[0114] In the formula, for First-order signal source voltage and load current, and Let n be the load impedance and reactance.

[0115] Subsequently, online identification of real-time impedance is performed based on the results of offline identification. As shown in Figure 7, the transmitted DC signal is superimposed in the initial stage of multi-frequency transmit current stabilization to avoid interference with the excitation electromagnetic field, thereby ensuring the overall stability of the transmitted waveform.

[0116] The online identification process of load parameters can be divided into the following five steps:

[0117] 1. Set the dominant frequency and amplitude of the multi-frequency synthesized wave;

[0118] 2. Superimpose a DC signal onto the stable initial stage of the electromagnetic emission waveform;

[0119] 3. Measure the emission current in the initial stage;

[0120] 4. Perform spectrum analysis on the initial stage emission current to obtain the DC current. Fundamental current Second harmonic current Third harmonic current and the phases 1, 2, and 3 of the AC components;

[0121] 5. Calculate the parameters of each part using formula (4) to determine the lumped parameters of the final load impedance.

[0122] Various parasitic parameters inevitably exist in multilevel transmitting circuits, mainly including conductor parasitic parameters, IGBT parasitic parameters, and load parasitic parameters. These parasitic parameters can adversely affect the stability of the transmit current and waveform quality; therefore, conducting parasitic parameter analysis is crucial for optimizing transmit performance.

[0123] Combining the IGBT equivalent parasitic parameter model and the load equivalent parasitic parameter model, the equivalent parasitic parameter model of the multi-level transmitter circuit is constructed as shown in Figure 8. In the figure, The equivalent parasitic inductance of the conductor, Equivalent stray inductance of IGBT collector Equivalent stray inductance of emitter The sum of For the output capacitor of the IGBT module, This is the equivalent parasitic capacitance of the load.

[0124] Regarding the circuit operating state shown in Figure 3 (during the positive half-cycle) , , , Turn on, output voltage A detailed analysis will be conducted. To simplify the analysis process, the following reasonable assumptions are proposed:

[0125] (1) Assume that the parasitic parameters of all IGBTs (S1-S8) have the same value to simplify the derivation process;

[0126] (2) During the conduction period of the switching device, the influence of inductive parasitic parameters on the emission current is dominant, so the influence of capacitive parasitic parameters is ignored in the analysis process.

[0127] Based on the above assumptions, the equivalent parasitic parameter model of the positive half-cycle of the multi-level transmitting circuit is further obtained, as shown in Figure 9. Combining this equivalent model, the equivalent parasitic inductance of the circuit can be derived. The expression is as follows:

[0128] (7)

[0129] The expression for the emission current under the influence of parasitic parameters is then obtained as follows:

[0130] (8)

[0131] In the formula, and The respective The frequency and phase of the modulated wave signal.

[0132] It is evident that the presence of circuit parasitic parameters introduces additional inductive impedance components, which in turn significantly suppresses the rise rate and peak amplitude of the emission current.

[0133] To reduce switching losses in multilevel transmitters and avoid waveform distortion, this invention proposes an optimized MFPWM control strategy, which allows for the flexible synthesis of multiple frequency components. The basic principle of the control strategy is shown in Figure 10.

[0134] Specifically, through cascaded carriers and Modulation of two multi-frequency hybrid modulation waves with the same amplitude and opposite polarity and For the corresponding IGBT in the multilevel transmitter - Generate drive signal - carrier wave and It is a triangular wave with a frequency of The ranges are [0,1] and [-1,0], respectively. The modulated wave is a hybrid modulated wave composed of multiple sinusoidal components with adjustable amplitude and frequency. , can be represented as:

[0135] (9)

[0136] In the formula, For the first The amplitude of the modulated wave signal. The value range is [0,1].

[0137] To avoid overmodulation and ensure the effective operating range of PWM modulation, the amplitude of the synthesized modulated wave must be strictly limited to the carrier amplitude range. Furthermore, the switching frequency of the multilevel inverter must be consistent with the carrier frequency. Maintain consistency. Modulation wave frequency. With carrier frequency The carrier ratio N satisfies The relationship is as follows. To suppress the generation of additional harmonic interference, the carrier ratio N should be set to an integer. Therefore, in practical engineering applications, it is necessary to determine the carrier ratio based on multiple preset transmission frequencies. carrier frequency Set it to the least common multiple of the transmission frequency.

[0138] The specific implementation logic of the MFPWM control process is as follows:

[0139] 1. Determine the appropriate detection frequency group based on the target detection depth range. Modulation amplitude values ​​corresponding to each detection frequency Differentiated designs are implemented based on the media attenuation characteristics at different exploration depths to ensure that the detection signals at each depth have a sufficient signal-to-noise ratio.

[0140] 2. Modulate signal components of different frequencies and amplitudes. , ,... Superposition generates hybrid modulated waves ; using cascaded triangular carriers and For the mixed modulated wave respectively and inverted modulation wave Modulation is performed to generate control pulses. - and - These signals serve as the drive signals for the front and rear IGBTs of the multilevel transmitter, respectively. Ultimately, a high-frequency mixed pulse drive signal containing the target detection frequency component is output, enabling precise control of the multilevel transmitter.

[0141] 3. Based on the principle of electromagnetic induction, the driving signal drives the multi-level transmitter to output excitation current. After being converted by the electromagnetic transducer, the radiated spatial magnetic field contains preset detection frequency components, realizing multi-frequency collaborative detection.

[0142] The MFPWM control proposed in this invention can achieve directional power allocation and independent control of each frequency component by flexibly adjusting the frequency and amplitude of each modulation signal component, thus adapting to the needs of different detection scenarios. In specific engineering implementation, the above control objectives can be achieved by precisely configuring the carrier ratio and modulation depth of each frequency component.

[0143] The output voltage harmonics under MFPWM control were analyzed using Fourier series expansion to evaluate its transmit spectrum performance. The transmit voltage can be expanded as follows:

[0144] (10)

[0145] Among them, a n and b n For n th The Fourier coefficients of harmonics can be expressed as:

[0146] (11)

[0147] Combining the regular sampling method shown in Figure 11, the transmission voltage can be further derived as follows:

[0148] (12)

[0149] In the formula, H represents the high-frequency harmonics introduced by the high-frequency carrier wave, which can be specifically expressed as:

[0150] (13)

[0151] As can be seen from formula (13), high-order harmonics in the high-frequency range are mainly distributed in the range of 2×m× At (m=1,2,3,...), where, Let be the carrier frequency. The adverse effects of higher harmonics are minimal because they can be largely eliminated by the low-pass filter of the receiver system. Therefore, ignoring the effects of high-frequency harmonics, the output voltage expression can be simplified to formula (14). The inverter output voltage can be considered as the superposition of sinusoidal voltages of different frequencies.

[0152] (14)

[0153] As can be seen from formula (14), the transmit voltage contains the required frequency component. Furthermore, according to Arrival theory, the parameters should follow formula (15).

[0154] (15)

[0155] The load impedance at different transmission frequencies is:

[0156] (16)

[0157] Combining formulas (15) and (16), the emission current is:

[0158] (17)

[0159] In the formula and (i = 1, 2) are respectively The specific expressions for detecting the frequency current amplitude and phase are as follows:

[0160] (18)

[0161] (19)

[0162] In the formula, and These are the load inductance and resistance, respectively.

[0163] Since the multi-frequency transmission power conforms to the superposition theorem, the total transmission power is a linear superposition of the power components at different frequencies.

[0164] (20)

[0165] In summary, the proposed MFPWM control method can achieve power transmission of the desired frequency components. By controlling the parameters of the multi-frequency modulation signal, the transmission frequency distribution, amplitude, and phase can be configured.

[0166] To improve transmission performance under the condition of limited DC power supply voltage, the following analysis will be conducted from the following optimization perspectives.

[0167] First, with maximizing the transmitted signal energy as the core objective, the optimization direction of maximizing the transmitted spectrum intensity is established. Based on the definition of transmitted spectrum energy in formula (20), this optimization objective is expressed as:

[0168] (twenty one)

[0169] From the perspective of modulation amplitude parameters, inductive impedance has an adverse effect on the intensity of the dominant frequency current; and existing segmented control strategies, lacking spectral energy regulation capabilities, often exhibit low amplitude characteristics at higher frequencies. To ensure a uniform distribution of signal spectral energy across dominant frequencies with approximate amplitudes, this influencing factor needs to be carefully considered. The theoretical approximate solution method for the modulation amplitude parameters is as follows:

[0170] (twenty two)

[0171] Under this premise, by rationally allocating the limited spectrum energy, it is possible to ensure that each detection frequency has a sufficient signal-to-noise ratio.

[0172] Secondly, from the perspective of amplitude, the optimization objective is defined as the degree of deviation between the modulation amplitude and the root mean square value, as shown in formula (23). By minimizing this deviation, i.e., minimizing the calculation result of formula (23), the main frequency amplitude closest to the theoretical value can be determined:

[0173] (twenty three)

[0174] The dominant frequency phase of the modulation signal affects the peak-to-peak value of the current, i.e., the uniformity of the time-domain current amplitude within a duty cycle. A more reasonable dominant frequency phase configuration can improve DC voltage utilization under DC voltage constraints. Based on this, the optimization objective set from the perspective of phase parameters is:

[0175] (twenty four)

[0176] In the formula, For the first The phase of the current at a detection frequency.

[0177] Subsequently, to address the aforementioned optimization objectives, the solution of the nonlinear equations for the optimal control parameters can be transformed into a nonlinear optimization problem. The core optimization objective is to maximize the spectral energy of the transmitted signal while ensuring that each detection frequency possesses approximately sufficient signal strength, given a fixed DC power supply voltage.

[0178] Based on the above optimization perspectives, the final comprehensive optimization objective is set as follows:

[0179] (25)

[0180] In the formula, , and is a weighting factor, with a value range of [0,1]. Different weighting coefficients represent the degree of preference for different optimization objectives.

[0181] Furthermore, it can be achieved by minimizing To determine the optimal parameter solution, the solution variables are defined as follows: Introducing a penalty function As shown in equation (26), this optimization problem is solved. The specific solution process is shown in Figure 12, and is explained as follows:

[0182] (26)

[0183] First, the initial parameter values ​​and the initial solution are given. Then, a loop algorithm is used to iteratively update the solution until the convergence condition is met. In each loop, the following equation is solved iteratively:

[0184] (27)

[0185] In the formula, To update the step size, a very small value is usually taken. For update rate, This is the gradient of the penalty function. The loop terminates when either of the following convergence conditions is met: 1. The penalty value is less than the set convergence value. 2. The number of iterations reaches the set value. At this point, the optimal control parameters can be determined.

[0186] Based on the above analysis, the overall control strategy flow of the three-level transmitter, as shown in Figure 13, can be systematically summarized as follows:

[0187] 1. Set the dominant frequency and amplitude parameters of the multi-frequency synthesized wave;

[0188] 2. The initial system impedance is obtained through offline identification combined with Fast Fourier Transform (FFT). ;

[0189] 3. According to the set transmission current Obtain the initial voltage of the multilevel transmitter. ;

[0190] 4. Perform real-time load parameter identification, based on the initial voltage. To provide the excitation input, the load current is collected. The real-time system harmonic impedance was obtained through FFT analysis. ;

[0191] 5. By real-time load current and the set emission current The difference between the real-time impedance and the actual impedance Obtain the voltage difference This voltage is then superimposed on the initial voltage for correction, yielding the actual transmit voltage. This ensures a stable output of the transmitting current.

[0192] This strategy employs a composite control architecture based on multi-frequency signal impedance identification: offline identification provides feedforward voltage settings for the system, while real-time identification is used for closed-loop dynamic compensation. The two work together to achieve accurate tracking of time-varying impedance, ultimately achieving high-precision and high-stability control of the transmit current.

[0193] It will be understood by those skilled in the art that the present invention is not limited to the specific embodiments described herein, and that various obvious changes, readjustments, and substitutions can be made by those skilled in the art without departing from the scope of protection of the present invention. Therefore, although the present invention has been described in detail through the above embodiments, the present invention is not limited to the above embodiments, and may include more other equivalent embodiments without departing from the concept of the present invention, and the scope of the present invention is determined by the scope of the appended claims.

Claims

1. A control method for an electromagnetic detection multilevel current transmission system for high-altitude environments, comprising a DC power supply, a multilevel transmitter, and an equivalent load; the multilevel transmitter is connected in series between the DC power supply and the equivalent load; a mobile power supply vehicle is used as the DC voltage source. The multilevel transmitter includes a capacitor. capacitors Switching devices - Switching devices and clamping diodes - Clamping diode In electromagnetic detection scenarios, the sensing carrier for electromagnetic detection is a long grounded cable and the ground, which together constitute a linear electromagnetic load, equivalent to an inductor. With resistance The series model is characterized by, The control method includes the following steps: A. Equivalently representing the multilevel transmitter as a voltage source with adjustable output voltage, establishing load models of the multilevel transmitter at different output voltage levels based on its operating states during the positive and negative half-cycles of transmission, and calculating the impedance of the load models; B. Analyzing the MFPWM control process, determining the optimal control parameters with the goal of maximizing the transmission spectrum intensity, to improve transmission performance under the condition of limited DC power supply voltage; C. Obtaining the initial system impedance through offline identification combined with Fast Fourier Transform (FFT). D. According to the set transmission current Obtain the initial voltage of the multilevel transmitter. E. Perform real-time online identification of load parameters, using the initial voltage. To provide the excitation input, the load current is collected. The real-time system harmonic impedance was obtained through FFT analysis. F. Through real-time load current and the set emission current The difference between the real-time impedance and the actual impedance Obtain the voltage difference This is used to superimpose the voltage onto the initial voltage, calculate the voltage correction value, and obtain the actual transmission voltage. To ensure stable output of the transmitting current; G, generate a hybrid modulation wave to obtain a driving signal, drive the multi-level transmitter to output current, and achieve directional power distribution and independent control of each frequency component by adjusting the frequency and amplitude of each modulation signal component, thereby realizing MFPWM control.

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