A signal acquisition device of a frequency domain electromagnetic detection system based on multi-frequency LC filtering

Abstract

This invention relates to a signal acquisition device for a frequency domain electromagnetic detection system based on multi-frequency LC filtering. The device includes a battery, a receiving coil, an analog conditioning module, a digital control module, and a host computer. The battery is connected to the analog conditioning module and the digital control module, serving as the power supply for the overall signal acquisition system. The receiving coil is connected to the analog conditioning module, which in turn is connected to the digital control module, which is connected to the host computer. This invention provides a parameter design flow guided by frequency band loss (BW). The multi-frequency LC filter circuit can retain multiple target frequency components in the measured signal while suppressing noise. The filtered signal has a 23.35 dB higher signal-to-noise ratio, which helps improve the detection effect of FDEM (Frequency-Dependent Electromagnetic Detection). This invention uses the calibration method of the FDEM receiving system to eliminate errors caused by the insertion loss of the filter, providing a new technical means for ground-to-space frequency domain electromagnetic detection and improving the ability of FDEM to detect underground structures.

Description

Technical Field

[0001] This invention relates to the field of metal mining technology, and in particular to a signal acquisition device for a ground-to-air frequency domain electromagnetic detection system based on multi-frequency LC filtering. Background Technology

[0002] Frequency domain electromagnetic detectors (FDEMs) are effective tools for detecting underground electrical anomalies. However, in practical applications, because the signals acquired by the receiving system are complete voltage time series, the acquired signals are not only induced signals generated by the target anomaly, but also signals coupled from multiple sources in space, including signals at the target frequency and various noises. The noise is mainly divided into two parts. The first part is noise at the same frequency as the target. To reduce the influence of this type of noise, the transmission and reception frequencies (target frequencies) selected in the FDEM do not coincide with the main noise frequencies. The second part is noise at frequencies other than the target frequency (non-target frequency noise), such as industrial harmonic noise and human noise. This noise reduces the signal-to-noise ratio of electromagnetic data acquisition, adversely affecting the detection accuracy and exploration accuracy of frequency domain electromagnetic detection. This is the main type of noise considered in this invention. The digital acquisition module of the frequency domain electromagnetic receiving system has a range limitation. When the noise in the measured signal is too large, the peak value of the measured signal exceeds the range of the digital acquisition module, leading to system saturation and spectral distortion. To reduce or eliminate the impact of noise on detection, many scholars have proposed various methods. Li Gang, Zhang Chunfeng, and others proposed applying quadrature lock-in amplification (QLAB) technology to the design of FDEM receiving systems. This technology can directly obtain the amplitude and phase of a single frequency component in the measured signal. However, in current FDEM systems, to improve detection efficiency, it is usually necessary to simultaneously transmit and acquire signals at least three frequencies. Therefore, QLAB technology cannot meet this requirement, and it also has high requirements for the orthogonality of the reference signal and the accuracy of the low-pass filter. Kumngern et al. proposed a digitally programmable gain amplifier, which has the advantages of adjustable gain and adjustable passband. However, this circuit cannot have multiple passbands simultaneously, and the active components (power supply, chips, etc.) in the circuit usually introduce additional noise, which is a common drawback of active filtering circuits. Passive filtering is also a commonly used method in noise suppression. Ahn, Hyo Min, and others proposed LCL filters with passive damping circuits for three-phase grid-connected inverters. This circuit has a simple structure and uses current signals for transmission. Since interference in the environment and within the instrument system often has high voltage but low energy, using current signals for transmission provides strong anti-interference capabilities. However, this circuit also cannot filter multiple frequency bands. Changsheng Liu et al. proposed a multi-frequency resonant circuit for electromagnetic transmission systems. This circuit simultaneously amplifies multiple target frequency components in the signal and suppresses non-target frequency components; however, this circuit cannot control the gain at each frequency, and its noise suppression effect is uncontrollable.Qiulin Tan et al. proposed a sensor design based on dual LC resonance. This sensor extracts two frequencies from the input signal as the sensor's temperature and pressure outputs, indirectly realizing multi-band filtering. However, this filtering circuit requires the fabrication of a PCB circuit board, which has high requirements for process and precision.

[0003] Therefore, considering the various problems mentioned above, it is necessary to propose a universally applicable optimized design scheme for the analog acquisition module of the traditional frequency domain electromagnetic receiving system. This scheme combines passive filtering circuits and the strong interference capability of current signals to suppress noise at non-target frequencies, avoid saturation of the receiving system, and achieve high signal-to-noise ratio electromagnetic acquisition at multiple target frequencies. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a signal acquisition device for a frequency domain electromagnetic detection system based on multi-frequency LC filtering, comprising a battery, a receiving coil, an analog conditioning module, a digital control module, and a host computer. The battery is connected to the analog conditioning module and the digital control module, serving as the power supply for the overall signal acquisition system. The receiving coil is connected to the analog conditioning module, the analog conditioning module is connected to the digital control module, and the digital control module is connected to the host computer.

[0005] The analog conditioning module includes a multi-frequency LC filter circuit, an ADC acquisition circuit, and a magnetic coupling isolation circuit, which are connected in sequence.

[0006] The digital control module includes an FPGA chip, a PSRAM, a configuration circuit, a crystal oscillator, and a GPS module. The PSRAM is connected to the FPGA chip through the configuration circuit, and the crystal oscillator and GPS module are connected to the FPGA chip.

[0007] The FPGA chip provides the data transmission protocol for ADC signal acquisition and the clock signal required for the ADC acquisition circuit to operate. The frequency of this signal is obtained by multiplying and dividing the output frequency of the crystal oscillator using the digital phase-locked loop of the FPGA clock management module. Internally, the FPGA chip incorporates a FIFO as a relay station for bus data distribution and a PSRAM as a signal data relay station between the FPGA and the host computer, thereby achieving data transmission matching between different chips.

[0008] The coil skeleton of the receiving coil has an indented structure with two layers of wire wound around it. Each layer is wound with a certain number of turns of silver-plated copper wire, and the winding directions of the two layers of coil are opposite. The lead wire of each layer is connected to the input terminal of the receiver's analog conditioning module.

[0009] The circuit structure of the multi-frequency LC filter circuit is as follows: several LC branches are connected in parallel, and a sampling resistor R is connected in series. s ;

[0010] The multi-frequency LC filter circuit is equipped with a power amplifier at its front end; a voltage divider resistor R0 is connected in series in the multi-frequency LC filter circuit to counteract the amplification effect of the power amplifier, and the resistance value of R0 is determined by equations (1) and (2):

[0011]

[0012] U = A U U s (2)

[0013] Where U is the total voltage signal of the filter circuit; U s For R s Voltage signals at both ends; A U R is the amplification factor of the power amplifier; s The value of the sampling resistor;

[0014] Because a power amplifier is added at the front end of the multi-frequency LC filter circuit, the output impedance of the power amplifier will affect the frequency characteristics of the circuit, requiring impedance matching.

[0015] U i R represents the voltage signal source after amplification by the power amplifier. i The output impedance of the power amplifier is given by equation (3).

[0016]

[0017] In the formula, X is the imaginary part of the impedance, ω is the angular frequency of the target frequency component in the measured signal; / / is the parallel operator of the circuit, calculated as follows: a / / b=(ab) / (a+b); when the imaginary part of the impedance is X=0, the input impedance of the circuit is equal to R0+R s In order to eliminate R i The effect of R on the frequency characteristics of the filter circuit s +R0 should satisfy the condition R s +R0>>R i Under this condition, when the imaginary part of the impedance is 0, R s +R O The voltage across the two ends is closest to U i At this time R i The effect on the filter circuit is negligible, and at this time we can obtain equation (4):

[0018]

[0019] When the inductor and capacitor parameters are determined, solving equation (4) yields n positive real solutions for the angular frequency ω. By selecting appropriate inductor and capacitor parameters, simultaneous acquisition of any number of target frequencies in the signal and suppression of noise outside the target frequency can be achieved.

[0020] In actual detection, the target frequency is determined by the specific detection depth and the transmission subsystem, and is a known variable; according to the calculation method of the / / operator, if either of the two ends of the operator is 0, the calculation result is 0. Therefore, the solution process of equation (4) is transformed into equation (5):

[0021]

[0022] In the formula, k = 1, 2...n, which correspond to different LC branches; by substituting the target frequency into formula (5), the inductance and capacitance values ​​obtained are the design parameters of the LC branch corresponding to the target frequency.

[0023] The beneficial effects of this invention are:

[0024] This invention provides a signal acquisition device for a frequency-domain electromagnetic detection system based on multi-frequency LC filtering to acquire components of any number of target frequencies in the measured signal and suppress noise. By analyzing the relationship between the 20dB bandwidth (BW) and the noise suppression effect, and the relationship between capacitor and inductor parameters and BW, this invention presents a parameter design process guided by BW. The multi-frequency LC filter circuit can retain multiple target frequency components in the measured signal while suppressing noise, reducing the noise amplitude by more than 90%. In experiments, the signal-to-noise ratio of the filtered signal increased by 23.35dB, which helps improve the detection effect of FDEM. Through repeated experiments and measurement uncertainty analysis, it was determined that the calibration method using the FDEM receiving system can eliminate the difference between simulation and experiment caused by the insertion loss of the filter, providing a new technical means for ground-to-air frequency-domain electromagnetic detection and helping to improve the detection capability of FDEM for underground structures. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the overall structure of the signal acquisition system of the present invention;

[0026] Figure 2 This is a schematic diagram of the receiving coil structure of the present invention;

[0027] Figure 3 This is a schematic diagram of the multi-frequency LC filter circuit of the present invention;

[0028] Figure 4 This is a schematic diagram of the equivalent circuit of the overall circuit of the present invention;

[0029] Figure 5This is a schematic diagram showing the comparison of the spectrum analysis of signal I, signal II, and signal III in the experiment of this embodiment of the invention. Detailed Implementation

[0030] See Figure 1 As shown, the present invention provides a signal acquisition device for a frequency domain electromagnetic detection system based on multi-frequency LC filtering, comprising a 12V lithium battery, a receiving coil, an analog conditioning module, a digital control module, and a host computer. The 12V lithium battery is connected to the analog conditioning module and the digital control module, serving as the power supply for the overall signal acquisition system. The receiving coil is connected to the analog conditioning module, the analog conditioning module is connected to the digital control module, and the digital control module is connected to the host computer.

[0031] The analog conditioning module includes a multi-frequency LC filter circuit, an ADC acquisition circuit, and a magnetic coupling isolation circuit, which are connected in sequence.

[0032] The digital control module includes an FPGA chip, a PSRAM, a configuration circuit, a crystal oscillator, and a GPS module. The PSRAM is connected to the FPGA chip through the configuration circuit, and the crystal oscillator and GPS module are connected to the FPGA chip.

[0033] A multi-frequency LC filter circuit suppresses signals of non-target frequencies, thereby eliminating interference and noise and improving the signal-to-noise ratio of the measured signal. The ADC acquisition circuit converts analog signals into digital signals, and the magnetic coupling isolation circuit reduces interference in the digital signals, improving signal quality and stability. The FPGA chip provides the data transmission protocol for ADC signal acquisition and a 40MHz clock signal required for the ADC acquisition circuit to operate. This signal frequency is obtained by multiplying and dividing the output frequency of the crystal oscillator using the digital phase-locked loop of the FPGA clock management module. The FPGA chip internally incorporates a FIFO as a relay station for bus data distribution and a PSRAM as a signal data relay station between the FPGA and the host computer, thus achieving data transmission matching between different chips. The FPGA and the host computer communicate via Gigabit Ethernet using the Ethernet UDP communication protocol. The FPGA program implements ARP UDP PING functionality. The FPGA communicates with the PHY chip via the GMII bus, and the PHY chip sends data to the host computer, thus realizing data transmission between the FPGA and the host computer. The host computer performs subsequent data processing to complete the signal acquisition of the multi-frequency LC-filtered electromagnetic detection system.

[0034] like Figure 2As shown, the coil skeleton of the receiving coil has a two-layered indented structure with a certain number of turns of silver-plated copper wire wound in each layer, and the two layers of coil are wound in opposite directions (the first layer is wound clockwise and the second layer is wound counterclockwise); the lead wire of each layer is connected to the input terminal of the receiver analog conditioning module.

[0035] like Figure 3 The schematic diagram of the multi-frequency LC filter circuit shows that the circuit structure consists of several LC branches connected in parallel and a sampling resistor R connected in series. s ;

[0036] Since the measured signal is a voltage signal provided by the receiving coil, it is a weak signal with very low power. The passive filter circuit has little effect on small signals, so directly passing the measured signal into the filter circuit will not work. Therefore, the multi-frequency LC filter circuit is equipped with a power amplifier at the front end. The power amplifier converts the voltage signal output by the sensor into a current signal and amplifies it, thereby increasing the signal power and enabling the passive filter circuit to function. It also improves the circuit's anti-interference capability.

[0037] Figure 3 Chinese R s A 1Ω sampling resistor is used, and the voltage across it is the output signal of the filter circuit. This is to prevent the measured signal voltage from exceeding the system range, causing system saturation and distortion of all frequencies; and to ensure that the acquired voltage accurately reflects the target frequency component of the measured signal, i.e., R... s The voltage across the two ends is equal to the voltage value of the measured signal. Therefore, a voltage divider resistor R0 is connected in series in the multi-frequency LC filter circuit to counteract the amplification effect of the power amplifier. The resistance value of R0 is determined by equations (1) and (2):

[0038]

[0039] U = A U U s (2)

[0040] Where U is the total voltage signal of the filter circuit; U s For R s Voltage signals at both ends; A U R is the amplification factor of the power amplifier; s The value of the sampling resistor;

[0041] Since a power amplifier is added to the front end of the multi-frequency LC filter circuit, the output impedance of the power amplifier will affect the frequency characteristics of the circuit, so impedance matching of the circuit is required. Figure 4 This is the equivalent circuit diagram of the entire circuit.

[0042] Ui R represents the voltage signal source after amplification by the power amplifier. i The output impedance of the power amplifier is given by equation (3).

[0043]

[0044] In the formula, X is the imaginary part of the impedance, ω is the angular frequency of the target frequency component in the measured signal; / / is the parallel operator of the circuit, calculated as follows: a / / b=(ab) / (a+b); when the imaginary part of the impedance is X=0, the input impedance of the circuit is equal to R0+R s In order to eliminate R i The effect of R on the frequency characteristics of the filter circuit s +R0 should satisfy the condition R s +R0>>R i Under this condition, when the imaginary part of the impedance is 0, R s +R O The voltage across the two ends is closest to U i At this time R i The effect on the filter circuit is negligible, and at this time we can obtain equation (4):

[0045]

[0046] When the inductance and capacitance parameters are determined, solving equation (4) yields n positive real solutions for the angular frequency ω; this means that the impedance of the actual circuit at the n target frequencies is only affected by R. s The effect of +R0 is that at other frequencies, the inductance and capacitance introduce additional impedance. Therefore, by selecting appropriate inductance and capacitance parameters, it is possible to simultaneously acquire multiple target frequencies in the signal and suppress noise outside the target frequencies.

[0047] The main objective of circuit design is to determine the parameters of the inductor and capacitor based on the given target frequency. In actual detection, the target frequency is determined by the specific detection depth and the transmitting subsystem, and is a known variable. As can be seen from the calculation method of the / / operator, if any one of the two ends of the operator is 0, the calculation result is 0. Therefore, the solution process of equation (4) can be transformed into equation (5):

[0048]

[0049] In the formula, k = 1, 2...n, which correspond to different LC branches; by substituting the target frequency into formula (5), the inductance and capacitance values ​​obtained are the design parameters of the LC channel corresponding to the target frequency.

[0050] This embodiment uses experimental verification, and the experimental results are as follows: Figure 5As shown, in the experiment, the measured signal was a mixture of a three-frequency pseudo-random signal with a fundamental frequency of 1024Hz and a 50Hz sine wave signal, with both signals having an amplitude of 1V; the power amplifier voltage amplification factor was 20; the target frequencies were 1024Hz, 2048Hz, and 4096Hz, respectively; the ADC acquisition circuit acquired data for signal I, signal II, and signal III, which were the original measured signal, the measured signal with ±1V cutoff (the oscilloscope range was set to achieve the cutoff effect), and the measured signal after passing through a multi-frequency LC filter circuit, respectively. Figure 5 The spectra of signals I, II, and III are given.

[0051] As can be seen from the figure, compared with the target frequency amplitude of signal I, the target frequency amplitude of signal II is severely distorted, indicating that acquisition saturation does indeed have a detrimental effect on the accuracy of signal acquisition; in signal III, the noise amplitude is significantly reduced. To clearly demonstrate the noise suppression effect of the circuit, a signal-to-noise ratio calculation formula is defined, as shown in equation (6):

[0052]

[0053] In the formula V s and V n , representing the target frequency amplitude and noise amplitude in the signal, respectively; m and n, representing the number of target frequencies and the number of other frequencies in the graph, respectively. This represents the sum of squares of the amplitudes at each target frequency. This represents the sum of squares of the amplitudes of other known frequencies (noise) in the figure. According to equation (6), the signal-to-noise ratios (SNRs) of signal I and signal III are 4.31 dB and 27.66 dB, respectively. The SNR of the filtered signal is increased by 23.35 dB, which differs from the simulation, and the amplitude of the target frequency of signal III is attenuated compared to signal I. This is because the actual values ​​of the components used in the circuit construction in the experiment deviate from their nominal values ​​to varying degrees. The deviation of the inductor and capacitor will cause the reactance of the circuit at the target frequency to be greater than expected (R0 + R). sThis causes a deviation in signal amplitude. In addition, even though it operates at a relatively low frequency (below 10kHz), the signal will still experience some loss after passing through the multi-frequency LC filter circuit due to the skin effect. The noise of the power amplifier and other devices, as well as the thermal noise of the resistor, will also affect the signal. All of these reasons can be collectively referred to as the insertion loss of the device. In a traditional FDEM receiving system, if the measured value deviates from the true value and remains constant, the system can be calibrated to eliminate the influence of this deviation on the measurement. Therefore, for a multi-channel LC filter circuit, if the loss of the target frequency amplitude of the filtered signal relative to the target frequency amplitude of the original measured signal is constant in the experiment, the circuit can be calibrated to eliminate the measurement distortion caused by the insertion loss of the filter. Based on the above analysis, the amplitude of the measured signal is changed, and multiple experiments are conducted. The insertion loss of the filter at the target frequency is calculated using equation (7):

[0054]

[0055] Among them IL f Let V be the insertion loss of the filter, f be the target frequency, V1 be the target frequency amplitude of the original measured signal, and V2 be the target frequency amplitude of the filtered signal. The calculation results are shown in Table 1.

[0056] Table 1

[0057]

[0058] Where A represents the amplitude of the three-frequency pseudo-random signal, and B represents the amplitude of the sine wave signal. The table shows that the fluctuation in insertion loss at the target frequency caused by changing the amplitude of the measured signal does not exceed 0.03 dB. Therefore, it can be approximately assumed that for the same multi-frequency LC resonant circuit, the filter insertion loss at the target frequency remains constant. The calibration method of the FDEM receiver subsystem can be used to eliminate the influence of insertion loss on the measurement, further improving the signal-to-noise ratio.

[0059] To demonstrate the reliability of the experimental results, the measurement uncertainty of the target frequency amplitude during the experiment was analyzed based on the uncertainty calculation method. The main sources of uncertainty are: 1. Standard uncertainty component introduced by measurement repeatability; 2. Standard uncertainty component introduced by oscilloscope accuracy; 3. Standard uncertainty component introduced by oscilloscope resolution. Table 2 shows the calculation results of the measurement uncertainty.

[0060] Table 2

[0061]

[0062]

[0063] Wherein, I1, I2, and I3 are the optimal estimates of the target frequency amplitudes of signals I, II, and III, respectively, and U1, U2, and U3 are their expanded uncertainties. As can be seen from the table, the measurement uncertainty is very small compared to the optimal estimates. The ratio of the optimal estimate to the measurement uncertainty does not exceed 3%. These results demonstrate that the experimental results are reliable and indirectly reflect the stability of the multi-frequency LC filter circuit of this invention.

Claims

1. A signal acquisition device for a frequency domain electromagnetic detection system based on multi-frequency LC filtering, characterized in that: The system includes a battery, a receiving coil, an analog conditioning module, a digital control module, and a host computer. The battery is connected to the analog conditioning module and the digital control module, serving as the power source for the overall signal acquisition system. The receiving coil is connected to the analog conditioning module, the analog conditioning module is connected to the digital control module, and the digital control module is connected to the host computer. The analog conditioning module includes a multi-frequency LC filter circuit, an ADC acquisition circuit, and a magnetic coupling isolation circuit, which are connected in sequence. The digital control module includes an FPGA chip, a PSRAM, a configuration circuit, a crystal oscillator, and a GPS module. The PSRAM is connected to the FPGA chip through the configuration circuit, and the crystal oscillator and GPS module are connected to the FPGA chip. The circuit structure of the multi-frequency LC filter circuit is as follows: several LC branches are connected in parallel, and a sampling resistor R is connected in series. s ; The multi-frequency LC filter circuit is equipped with a power amplifier at its front end; a voltage divider resistor R0 is connected in series in the multi-frequency LC filter circuit to counteract the amplification effect of the power amplifier, and the resistance value of R0 is determined by equations (1) and (2): (1) (2) Where U is the total voltage signal of the filter circuit; U s For R s Voltage signals at both ends; A U R is the amplification factor of the power amplifier; s The value of the sampling resistor; Impedance matching of the circuit: R i The output impedance of the power amplifier is given by equation (3). (3) In the formula, X is the imaginary part of the impedance. Let be the angular frequency of the target frequency component in the measured signal; / / is the parallel operator of the circuit, calculated as follows: a / / b=(ab) / (a+b); when the imaginary part of the impedance is X=0, the input impedance of the circuit is equal to R0+R s R s +R0 should satisfy the condition R s +R0>>R i Equation (4) is obtained: （4） When the inductor and capacitor parameters are determined, equation (4) is solved to obtain the angular frequency. The n positive real solutions; by selecting appropriate inductor and capacitor parameters, simultaneous acquisition of any number of target frequencies in the signal and suppression of noise outside the target frequencies can be achieved: In actual detection, the detection depth and transmission subsystem of the target frequency are known. According to the calculation method of the / / operator, if any number on either side of the operator is 0, the calculation result is 0. The solution process of equation (4) is transformed into equation (5): （5） In the formula, k=1,2…n, which correspond to different LC branches; by substituting the target frequency into formula (5), the inductance and capacitance values ​​obtained are the design parameters of the LC branch corresponding to the target frequency.

2. The signal acquisition device for a frequency domain electromagnetic detection system based on multi-frequency LC filtering according to claim 1, characterized in that: The FPGA chip provides the data transmission protocol for ADC signal acquisition, as well as the clock signal required for the ADC acquisition circuit to operate. The frequency of this signal is obtained by multiplying and dividing the output frequency of the crystal oscillator by the digital phase-locked loop of the FPGA clock management module. The FPGA chip is internally designed with a FIFO as a relay station for bus data distribution, and uses PSRAM as a signal data relay station between the FPGA and the host computer to achieve data transmission matching between different chips.

3. The signal acquisition device for a frequency domain electromagnetic detection system based on multi-frequency LC filtering according to claim 1, characterized in that: The coil skeleton of the receiving coil has an indented structure with two layers of winding wire, each layer having a certain number of turns of copper wire wound around it, and the winding directions of the two layers of coil are opposite; the lead wire of each layer is connected to the input terminal of the receiver analog conditioning module.

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