Semi-aviation transient electromagnetic signal adaptive gain method and system

Abstract

The invention discloses a semi-aviation transient electromagnetic signal adaptive gain method and system, and relates to the technical field of geophysical exploration, and the method comprises the steps: obtaining a differential signal of a receiving coil, and carrying out the chopping self-stabilization zero amplification processing of the differential signal; the method comprises the following steps: dividing a total time window of a differential signal into a plurality of time channels under a set time reference, adjusting the length of each time channel according to a signal attenuation characteristic, calculating the time length and data point number of each time channel according to a sampling rate and the total time window, and calculating the time length and data point number of each time channel; the data points are distributed to an early time channel, a middle time channel and a late time channel, so that different gain multiples are switched, and adaptive gain amplification is carried out; and the differential signal subjected to adaptive gain amplification is transmitted to a receiver after being collected by an ADC (Analog to Digital Converter). By dynamically adjusting the gain coefficient in different time channels, the technical problems of large dynamic range of transient electromagnetic signals and low signal-to-noise ratio of weak signals are solved, and high-fidelity acquisition of full-time-domain signals is realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of geophysical prospecting, in particular to a semi-airborne transient electromagnetic signal adaptive gain method and system. BACKGROUND

[0002] In semi-airborne transient electromagnetic prospecting, the amplitude of the signal induced by the receiving coil rapidly decays from the millivolt level of the early time channel to the microvolt level of the late time channel, and the dynamic range exceeds 100dB.

[0003] The traditional fixed gain amplification circuit has the following defects: early signal saturation, leading to signal distortion; low signal-to-noise ratio of late signal, leading to signal being submerged in noise; limited dynamic range, limiting the detection depth and resolution. Although the chopping self-stable zero and low noise operational amplifier in the prior art can suppress noise, it still cannot solve the adaptation problem of large dynamic range signals. SUMMARY

[0004] To solve the above problems, the present application provides a semi-airborne transient electromagnetic signal adaptive gain method and system, which dynamically adjusts the gain coefficient by time channel to solve the technical problems of large dynamic range of transient electromagnetic signals and low signal-to-noise ratio of weak signals, and realizes high-fidelity acquisition of full-time domain signals.

[0005] To achieve the above purpose, the present application adopts the following technical scheme: In a first aspect, the present application provides a semi-airborne transient electromagnetic signal adaptive gain method, comprising: obtaining a differential signal of a receiving coil and performing chopping self-stable zero amplification processing on the differential signal; under a set time reference, dividing the total time window of the differential signal after chopping self-stable zero amplification processing into multiple time channels, adjusting the length of each time channel according to the signal decay characteristic, calculating the time length and data points of each time channel according to the sampling rate and the total time window, distributing the data points to the early time channel, the middle time channel and the late time channel, and switching different gain multiples to perform adaptive gain amplification; The differential signal after adaptive gain amplification is transmitted to a receiver after being collected by an ADC.

[0006] As an optional implementation, the design of the receiving coil comprises: Optimization of internal resistance: calculating the overall winding resistance of the coil: ; wherein, N is the number of turns of the inner conductor of the hollow coil, is the resistivity of the conductor, is the cross-sectional area of the conductor, is the diameter of the coil; Adjustment of matching resistance: changing the damping coefficient and system bandwidth of the second-order transmission network by changing the matching resistance; in particular: If the receiving coil works in the critical damping state, the matching resistance and the system bandwidth are: When the receiving coil works in the over-damping state, i.e. K >1, or the receiving coil works in the under-damping state, i.e. 0< K <1, the matching resistance is: wherein, K is the working state of the receiving coil; is the winding resistance; is the coil distributed inductance, is the coil distributed capacitance, is the frequency.

[0007] As an optional embodiment, the design of the receiving coil also includes: Adjustment of natural resonant frequency: the natural resonant frequency of the differential inductive air-core coil is: The distributed inductance is: The distributed capacitance is: wherein, d is the wire diameter, is the space permeability, is the equivalent radius of the coil cross section; is the number of coil segments, is the total number of coil winding layers, is the relative dielectric constant of the wire, delta is the distance between each layer, is the relative dielectric constant of the skeleton, is the height of the skeleton, e is the width of the skeleton slot, and N is the number of turns of the coil.

[0008] As an optional embodiment, the chopping self-stable zero amplification process includes: Multiplying the differential signal with the modulation signal , modulating from low frequency to the center frequency band, and in the band, the amplifier background noise is white noise. ​​​​​​The modulated signal is amplified by a low-noise amplifier, and the amplified signal is down-converted, that is, the amplified signal is multiplied again with the modulation signal The noise signal is moved to a high frequency band after being modulated, and finally low-pass filtered to realize low-noise amplification of the low-frequency signal.

[0009] As an alternative embodiment, the length of each time channel is adjusted according to the signal attenuation characteristics. The transient electromagnetic data attenuation conforms to the e-exponential attenuation law, so the early, middle and late time channels are divided by using exponential time channel division: Early time channel: the first 0-50 time channels, the signal attenuates fast, the time window is narrow, and the amplification factor G1=1; Middle time channel: 51-1586 time channels, the amplification factor is G2, G2 is set to 10, 20, 30, 40 and 50-100, as a transition zone; Late time channel: 1587-5119 time channels, the signal attenuates slowly, the time window is long, and the amplification factor is increased to G3, G3 is set to 200, 300, 400 and 500.

[0010] As an alternative embodiment, the differential signal amplified by the adaptive gain is processed by low-pass filtering and then collected by ADC; the low-pass filtering processing includes: The low-pass filter single cutoff frequency is lower than the lowest frequency band of the unmanned aerial vehicle high-frequency noise, that is, 14.2 kHz is selected as the low-pass filter cutoff frequency; A fourth-order Butterworth low-pass filter chip is selected for low-pass filtering processing, and the cutoff frequency is set by an external resistor, with a maximum support of 256 kHz cutoff frequency.

[0011] In a second aspect, the present application provides a semi-airborne transient electromagnetic signal adaptive gain system, comprising: A front-end conditioning module configured to obtain a differential signal of a receiving coil and perform chopper self-stable zero amplification processing on the differential signal; An adaptive gain module configured to divide the total time window of the differential signal after chopper self-stable zero amplification processing into multiple time channels under a set time reference, adjust the length of each time channel according to the signal attenuation characteristics, calculate the time length and data points of each time channel according to the sampling rate and total time window, and distribute the data points to early, middle and late time channels to switch different gain multiples, thereby performing adaptive gain amplification; A sending module configured to transmit the differential signal after adaptive gain amplification to a receiver after being collected by ADC.

[0012] In a third aspect, the present application provides an electronic device comprising a memory and a processor, and computer instructions stored in the memory and running on the processor, when the computer instructions are run by the processor, the method of the first aspect is completed.

[0013] In a fourth aspect, the present application provides a computer readable storage medium for storing computer instructions, when the computer instructions are executed by the processor, the method of the first aspect is completed.

[0014] In a fifth aspect, the present application provides a computer program product comprising a computer program, when the computer program is executed by the processor, the method of the first aspect is completed.

[0015] Compared with the prior art, the present application has the following beneficial effects: (1) The present application effectively reduces thermal noise by optimizing internal resistance and matching resistance, thereby significantly improving signal-to-noise ratio. This improvement not only enhances the clarity of the signal, but also works in cooperation with the subsequent adaptive gain circuit, so that the system can process signals from the early time channel of millivolt level to the late time channel of microvolt level, meeting the acquisition requirements of full-time signals. In addition, by adjusting the inherent resonant frequency and distributed parameters, the coil can more accurately induce the semi-airborne transient electromagnetic signal, thereby improving the detection accuracy. In terms of structural design, the present application adopts a hollow coil with a diameter of 50 cm. This design not only ensures the portability of the structure, but also makes it very suitable for unmanned aerial vehicle load, thereby ensuring the efficiency and long endurance of the detection process.

[0016] (2) The present application effectively suppresses 1 / f noise and DC offset by using chopper self-stabilizing zero technology, while avoiding the increase of the background noise, thereby significantly improving the signal-to-noise ratio of the signal and realizing low noise performance. In addition, the selected AD8629 operational amplifier has low offset voltage, low temperature drift characteristics and high common mode rejection ratio, which ensures high precision amplification of the signal. The low power design of AD8629 makes it very suitable for devices such as unmanned aerial vehicles with strict power requirements, effectively prolonging the endurance time of the device. At the same time, the wide working temperature range of AD8629 (-55℃ to 125℃) makes it suitable for various harsh environmental conditions, ensuring the stability and reliability of the system.

[0017] (3) The application realizes adaptive dynamic range expansion by adjusting the gain coefficient in time channels, enabling the system to process signals from the millivolt level of early time channels to the microvolt level of late time channels, effectively solving the problem of low signal-to-noise ratio caused by early signal saturation and late signal amplification deficiency when the traditional fixed gain amplification circuit processes signals with a large dynamic range. The adaptive gain amplification circuit can dynamically adjust the gain according to the time characteristics of the signal, ensuring high-precision acquisition of millivolt to microvolt full-time domain signals, significantly improving the detection precision and data quality. In addition, the control logic circuit based on FPGA (Field Programmable Gate Array) has high flexibility and adaptability, and can adjust the gain switching strategy according to different detection requirements and signal characteristics, further improving the overall performance of the system.

[0018] (4) The application effectively removes high-frequency noise above 20kHz through the designed low-pass filter unit, significantly improving the purity and stability of the signal. By accurately setting the cutoff frequency of the filter, it ensures that the useful signal components will not be attenuated too much due to filtering, thereby optimizing the signal quality. In addition, the optimized circuit design enhances the anti-interference ability of the filter, enabling it to maintain stable operation in complex and variable environments, providing reliable protection for high-precision acquisition of semi-airborne transient electromagnetic signals.

[0019] The advantages of the additional aspects of the application will be partially given in the following description, partially will become obvious from the following description, or will be known by the practice of the application. BRIEF DESCRIPTION OF DRAWINGS

[0020] In order to more clearly illustrate the technical solutions in the embodiments of the application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiment or prior art description. Obviously, the drawings in the following description are only embodiments of the application, and for those skilled in the art, other drawings can be obtained without creative labor on the basis of the provided drawings.

[0021] Figure 1 The adaptive gain method flowchart of the semi-airborne transient electromagnetic signal provided for the embodiment 1 of the application; Figure 2 The equivalent circuit schematic diagram of the differential induction type air core coil provided for the embodiment 1 of the application; Figure 3 The adaptive gain amplification schematic diagram provided for the embodiment 1 of the application; Figure 4 The primary amplification circuit diagram provided for the embodiment 1 of the application; Figure 5 The distribution time channel schematic diagram provided for the embodiment 1 of the application; Figure 6 An amplification circuit diagram of a variable gain stage provided for Embodiment 1 of the present application; Figure 7 A simulation result diagram of adaptive gain amplification provided for Embodiment 1 of the present application. DETAILED DESCRIPTION

[0022] The present application will be further described below in conjunction with the accompanying drawings and embodiments.

[0023] It should be noted that the following detailed description is exemplary in nature and is intended to provide further description of the present application. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.

[0024] It is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments in accordance with the present application. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and / or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof.

[0025] The embodiments in the present application and the features in the embodiments can be combined with each other without conflict.

[0026] Embodiment 1 The present embodiment provides a method for adaptive gain of semi-airborne transient electromagnetic signals, as shown in the accompanying drawings, comprising: Figure 1 obtaining a differential signal of a receiving coil and performing chopper self-stabilized zero amplification processing on the differential signal; dividing a total time window of the differential signal after the chopper self-stabilized zero amplification processing into a plurality of time channels under a set time reference, adjusting the length of each time channel according to signal attenuation characteristics, calculating the time length and data points of each time channel according to the sampling rate and the total time window, and distributing the data points into early time channels, middle time channels and late time channels to switch different gain multiples, thereby performing adaptive gain amplification; The differential signal after the adaptive gain amplification is transmitted to a receiver after being collected by an ADC.

[0027] In the present embodiment, the receiving coil adopts an inductive air-core coil and adopts a layered differential winding structure to match the resistance for optimization to reduce thermal noise.

[0028] ​The specific design of the receiving coil is described in detail below.

[0029] (1) Structural design and material selection.

[0030] The layered differential winding structure is adopted. The layered winding reduces the distributed capacitance and inductance, and provides a physical basis for dynamic gain switching. The adaptive gain compensates for the inherent defects of the coil in early / late signal processing, forming a closed-loop optimization mechanism. Specifically, it includes the inner skeleton, outer skeleton and circuit cabin of the coil.

[0031] The coil diameter is 50 cm. The 50 cm coil diameter is the result of a comprehensive trade-off between detection performance, unmanned aerial vehicle load capacity, signal processing requirements and engineering implementation cost, which can effectively meet the needs of semi-airborne transient electromagnetic detection, and improve the detection accuracy and efficiency.

[0032] The coil height is 10-15 cm, and the overall weight is light, which is suitable for unmanned aerial vehicle load.

[0033] The tail is the circuit cabin, which is built-in low-noise signal conditioning circuit, power supply circuit and attitude positioning module circuit, and is layered wound with multi-turn air core structure. The interlayer spacing δ = 0.1 mm, and the skeleton slot width e = 2 mm.

[0034] Among them, the low-noise signal conditioning circuit adopts the chopping self-stable zero technology, which suppresses the 1 / f noise of the differential signal from the receiving coil, and reduces the offset voltage and time / temperature drift.

[0035] The power module adopts a double 8.4V lithium ion battery, constructs a ±8.4V bipolar voltage input architecture, generates stable ±5V power through DC-DC power module and symmetric LDO (low-dropout regulator), to realize high-stable power supply for operational amplifier and reduce signal distortion. The power module includes a low-noise LDO with adjustable voltage to stabilize the input positive and negative power to ±5V, and is equipped with an LED indicator light to display the battery power state. The performance parameters of the positive and negative LDO chips are consistent, respectively providing about 200mA and 300mA of output current, meeting the total current demand of about 50mA of the latter operational amplifier and differential output circuit.

[0036] The attitude positioning module has the function of outputting longitude and latitude, attitude angle and time information of the electromagnetic induction sensor. The module outputs longitude and latitude and time parameters by integrating GPS and Beidou positioning systems using NEMA0183 protocol. Specifically, the nine-axis attitude angle sensor is used as the attitude module in the embodiment, the sensor integrates high-precision gyroscopes, accelerometers and geomagnetic field sensors, and uses high-performance microprocessors and advanced dynamic solution and Kalman dynamic filtering algorithm, which can quickly and accurately solve the real-time motion attitude of the module. The theoretical positioning accuracy of the positioning module is ±2.5 meters. The attitude positioning module integrates the positioning information and the attitude information, and outputs the information through the attitude module, including time, longitude and latitude, height, angle (including navigation angle, pitch angle and roll angle) and temperature.

[0037] The differential input can adopt a symmetrical circuit structure, and the values of the basic parameters in the upper and lower circuit structures are equal, as shown in Figure 2 , including equivalent internal resistance, equivalent inductance and equivalent capacitance.

[0038] Specifically, in the upper part of the circuit structure, one end of the inductor L1 is connected to one end of the resistor R1, the other end of the resistor R1 is connected to one end of the capacitor C1, the other end of the capacitor C1 is connected to the other end of the inductor L1, and the capacitor and the resistor R T1 are connected in parallel, and one end of the resistor R T1 is connected to the positive electrode, and the other end is grounded. In the lower part of the circuit structure, one end of the inductor L2 is connected to one end of the resistor R2, the other end of the resistor R2 is connected to one end of the capacitor C2, the other end of the capacitor C2 is connected to the other end of the inductor L2, and the capacitor and the resistor R T2 are connected in parallel, and one end of the resistor R T2 is connected to the negative electrode, and the other end is grounded.

[0039] Wire material: The wire material is selected to be enameled wire with a diameter of <0.25mm, and the material is copper, gold, silver or other materials with low resistivity, which can effectively reduce the internal resistance of the coil. Thus, the resistance thermal noise is reduced, and the signal-to-noise ratio is improved; after each layer of winding is completed, the insulating tape is used for fixing to ensure the stability of the coil; during the winding process, attention should be paid to maintaining the uniformity and symmetry of the coil to reduce the non-uniformity of the distributed parameters; the insulating material of the enameled wire is high-quality polyester paint, which has good electrical insulation performance and mechanical strength, and can effectively prevent short circuit and electromagnetic interference between wires.

[0040] Framework material: The inner framework and the outer framework are made of lightweight and high-strength engineering plastics (such as polycarbonate or nylon) as the inner framework and outer framework materials of the coil. These materials not only meet the load requirements of the unmanned aerial vehicle, but also have good mechanical properties and electrical insulation properties.

[0041] Circuit compartment shell: The circuit compartment shell is also made of lightweight engineering plastic, with good protective performance, which can protect the internal circuit from the influence of the external environment.

[0042] Other materials: Matching resistance: Choose high-precision, low-noise metal film resistance, resistance value is kΩ level, which can be 1kΩ-10kΩ, used to optimize the matching characteristics of the coil, and carry out damping matching.

[0043] Connection line: Use high-quality shielded signal line to reduce electromagnetic interference and ensure the stability of signal transmission.

[0044] (2) Internal resistance optimization: Choose low-resistivity wire type to reduce noise.

[0045] The overall winding resistance of the coil is estimated by formula (1): (1); Where, N is the number of turns of the inner conductor of the hollow coil, is the resistivity of the conductor, is the cross-sectional area of the conductor, is the diameter of the coil.

[0046] (3) Intrinsic resonant frequency adjustment: determined according to the physical structure of the coil (such as winding method, wire diameter, insulation material, etc.). The intrinsic resonant frequency of the differential inductive hollow coil Mathematical expression as follows: (2); Where, is the distributed inductance of the coil, is the distributed capacitance of the coil.

[0047] (4) Distributed inductance calculation: The distributed inductance can be approximately calculated by simplifying the coil structure.

[0048] The total inductance expression of the hollow coil can be simplified as: (3); Where, d is the wire diameter, is the space permeability, which is 4π× H / m, is the number of layers of the inner conductor of the hollow coil, is the equivalent radius of the coil cross-section.

[0049] (5) Distributed capacitance reduction: Distributed capacitance can be divided into interlayer distributed capacitance and intersegment distributed capacitance. Estimated by formula (4): (4); Where, is the number of coil segments, is the total number of coil winding layers, represents the relative dielectric constant of the wire, delta Indicates the spacing between each layer, represents the relative dielectric constant of the skeleton, Indicates the height of the skeleton, e It represents the width between the skeleton slots, and N is the number of coil turns.

[0050] (6) Matching resistor adjustment: The matching resistor value ranges from several thousand ohms to tens of thousands of ohms. The matching resistor is connected in parallel at the output end to increase system stability and adjust the system damping ratio. The damping coefficient and system bandwidth of the second-order transmission network can be changed by modifying the matching resistor.

[0051] Matching resistor and the system cutoff frequency The expression is: (5); (6).

[0052] By changing the matching resistor value, the receiving coil of the semi-aerospace transient electromagnetic will work in different states. When working in the over-damped state, that is, K>1, the amplitude-frequency response is too flat, and the high-frequency signal cannot be highlighted, and only the low-frequency signal can be reflected. The sensitivity of the coil is low, and the local anomaly of the geological body is not prominent, and only the general structure can be reflected. When working in the under-damped state, that is, 0 <K<1,响应信号幅值大,信号衰减小,线圈的灵敏度高,可以突出地质体的局部异常,信号会存在振荡现象;而在临界阻尼状态时,响应信号幅值中等,信号衰减也较小,线圈的灵敏度适中,总体指标介于另外两种状态之间。

[0053] If the receiving coil works in the critical damping state, that is, K ≈1, matching resistor and system bandwidth The expression is: (7); (8); in, is the frequency.

[0054] when K >1 or 0< K <1, (9); That is, the basic parameters of the coil are kept unchanged, and the damping ratio can be changed by changing the matching resistance.

[0055] like Figure 3The adaptive gain amplification principle diagram is shown, and specifically comprises: (1) The differential signal of the receiving coil is first subjected to chopper self-bias zero amplification processing, wherein an AD8629 operational amplifier is selected to realize the chopper self-bias zero amplification function.

[0056] (2) The differential signal subjected to the chopper self-bias zero amplification processing is input into a primary amplification circuit for processing, wherein an AD8429 low-noise primary operational amplifier is selected to realize 10 times fixed gain amplification.

[0057] (3) At the FPGA control end, the signal output by the primary amplification circuit is subjected to time channel identification, wherein a TTC timer is adopted to divide multiple time channels (early (0-t1), middle (t1-t2), late (t2-5120), and real-time signal (on-time)) using a 1PPS (Pulses Per Second) pulse as a time reference to determine the time channel in which the signal is located; then, in the gain state machine, the GPIO pin of the FPGA control end is used to control the analog switch ADG1419 to realize real-time control of the switching of the gain resistor network.

[0058] (4) In the variable gain stage composed of an OPA2188 operational amplifier and an ADG1419 analog switch, 1 / G1 / G2 / G3 times gain control is realized by switching the gain resistor network to switch different gain multiples, thereby realizing adaptive gain amplification and outputting the amplified signal; thus, the differential signal subjected to the adaptive gain amplification is collected by an ADC (Analog to Digital Converter) and then transmitted to the receiver.

[0059] The following will be described in detail.

[0060] In this embodiment, the preamplifier circuit design based on the chopper self-bias zero technology specifically comprises the following contents.

[0061] In a semi-airborne transient electromagnetic SATEM signal acquisition system, the performance of the amplification circuit plays a crucial role in the quality and detection accuracy of the signal. In traditional amplification circuits, 1 / f noise, DC offset voltage, time drift, and temperature drift effects are common, which can seriously affect the accuracy of the operational amplifier in transmitting signals. The chopper self-bias zero technology can suppress 1 / f noise and DC offset without increasing the background noise, and can also be used for continuous amplification of signals in the time domain.

[0062] In the front-end conditioning circuit, the chopper self-bias zero technology is used to suppress 1 / f noise, reduce the offset voltage and time / temperature drift of the differential signal from the receiving coil.

[0063] (1) The core principle of chopper-zero technology: By modulating the input differential signal from low frequency to high frequency and then demodulating the high frequency signal back to low frequency, 1 / f noise and DC offset can be effectively suppressed while avoiding increasing the background noise.

[0064] (1-1) Modulation process: differential signal of receiving coil With the modulation signal Multiply, Modulate from low frequency to center frequency frequency band, in At this frequency band, the noise floor of the amplifier is white noise.

[0065] The core of chopping technology is to move the low-frequency signal (and the 1 / f noise in the same frequency band) to a higher frequency band at the input end, that is, The amplifier faces white noise in this frequency band instead of 1 / f noise, thus reducing the overall low-frequency noise of the system. The amplifier's own 1 / f noise will be at a certain inflection point frequency. The following rises rapidly, select , it can ensure that the amplifier operates in the white noise area. Do not choose too high to avoid exceeding the bandwidth of the amplifier or post-stage filter, otherwise the modulated signal cannot be fully amplified or restored. For example.

[0066] (1-2) Low-noise amplification: The modulated signal is amplified by a low-noise amplifier to ensure that no additional noise is introduced during the amplification of the signal in the high-frequency band.

[0067] (1-3) Demodulation process: The amplified signal is down-converted, that is, the amplified signal is again mixed with the modulated signal Multiply again, demodulate the signal back to the low frequency band, and the noise signal is moved to the high frequency band after modulation, and finally through low-pass filtering, low-noise amplification of the low-frequency signal can be achieved, avoiding 1 / f Noise impact.

[0068] (2) Selection and application of high-performance operational amplifiers: The AD8629 operational amplifier is selected to implement the chopper-stabilized zero-amplification function. The AD8629 device effectively ensures the low offset voltage and low temperature drift characteristics of the data acquisition system. The low noise characteristics of the AD8629 are relatively wide bandwidth (0~10kHz), and the voltage noise density is as low as 22.1nV at 1kHz. , the current noise density is about 5fA at 10Hz The AD8629 can provide 16-bit accuracy, wider operating temperature, ultra-low offset, drift and bias current characteristics, and its ultra-low low-frequency noise is a better choice for high-resolution detection equipment such as the half-airborne transient electromagnetic receiving system.

[0069] (3) Optimal selection of analog switch: The modulator and demodulator of the chopping self-biased zero amplifier are generally composed of analog switches. In the design process of the chopping amplifier, it is crucial to select an analog switch with a low charge injection amount. The combination of the AD8629 operational amplifier and the ADG1419 analog switch solves the residual offset problem of the traditional chopping self-biased zero circuit. Charge injection effects and clock feedthrough effects can cause the generation of injected charges, which not only form residual offset voltages on the load, but also form noise voltages on the source, significantly increasing the equivalent input noise of the chopping amplifier. Therefore, particular attention is paid to the selection of the analog switch during the design process to minimize the charge injection amount, thereby further reducing noise and offset voltage.

[0070] In the embodiment, the adaptive gain amplification circuit specifically includes the following contents.

[0071] (1) Circuit architecture.

[0072] The adaptive gain amplification circuit is arranged before the ADC acquisition circuit and after the chopping self-biased zero amplification circuit, and is built-in in the receiving coil signal conditioning circuit. The control logic is realized by the FPGA of the receiver, the gain control decision is made by the gain state machine, and the gain resistance network is controlled by the GPIO (general input / output port) port to realize adaptive amplification.

[0073] Specifically: the differential signal processed by the chopping self-biased zero amplification is first amplified by the primary amplification circuit, then the PPS pulse synchronization is used as the time reference at the FPGA control end, or the time stamp is used as the time reference, the signal arrival time is detected, and the time channel (early, middle, late) of the signal is judged. According to the judgment result of the time channel, the gain state machine controls the switching of different gain multiples to realize adaptive gain based on time judgment. Finally, the signal amplified by the adaptive gain is output to the ADC for acquisition and transmitted to the FPGA for storage in the SD card.

[0074] (2) Primary amplification circuit: the 10-fold fixed gain amplification is performed by the AD8429 low-noise primary operational amplifier to ensure low noise and high bandwidth. The intrinsic noise density of the AD8429 is low, the equivalent input noise can still be kept at a low level after the signal is amplified by 10 times, and the high common-mode rejection ratio can effectively prevent interference signals from damaging data acquisition, which is suitable for μV-level weak signal amplification.

[0075] As Figure 4As shown, in particular: The pin 1 and pin 3 of the coil input chip P15 are connected to the pin 4 and pin 1 of the AD8429 through the resistor R120 (1k±1%) and the resistor R117 (1k±1%) respectively, and the pin 2 of the coil input chip P15 is grounded; The pin 2 and pin 3 of the AD8429 are connected through the resistor R118 (665R±1%); The pin 1 of the AD8429 is connected with the resistor R115 (2k±1%) and the capacitor C126 (10nF 50V) in parallel, and the parallel end of the resistor R115 and the capacitor C126 is connected to the pin 1 of the AD8429 and the resistor R117, and the other end is grounded; The pin 4 of the AD8429 is connected with the resistor R122 (2k±1%) and the capacitor C132 (10nF 50V) in parallel, and the parallel end of the resistor R122 and the capacitor C132 is connected to the pin 4 of the AD8429 and the resistor R120, and the other end is grounded; The pin 5 of the AD8429 is connected to the-5V power supply and grounded through the capacitor C130 (100nF 50V); The pin 8 of the AD8429 is connected to the+5V power supply and grounded through the capacitor C125 (100nF 50V); The pin 6 of the AD8429 is grounded, and the pin 7 of the AD8429 outputs the processed signal.

[0076] (3) Time channel division and gain control.

[0077] Time channel division: (3-1) Determine the total time window: determine the total time window according to the characteristics of the signal, usually the time period from the rising edge to the decay to the weak signal.

[0078] (3-2) Divide the time channel: divide the total time window into multiple time channels (≤3), and the length of each time channel is adjusted according to the decay characteristics of the signal. The early time channel has less data, the middle time channel is moderate, and the late time channel is longer.

[0079] The transient electromagnetic data decay conforms to the e exponential decay law, so the early, middle and late time channel division adopts exponential time channel division. For example, the early signal decays fast, the time window is narrow, and the first 0~50 time channels (about 97us) are selected, and the general electrical source emission off time is about 50-100us; the amplification factor in the middle time channel design is used as a transition zone, and the middle time channel is 51~1586 (about 3ms); the late signal decays slowly and the window can be extended to 1587~5120 points (about 7ms).

[0080] (3-3) Calculate the number of time channels: according to the sampling rate and the total time window, calculate the specific time length and data points of each time channel.

[0081] For example, the early signal decays quickly, the time window is narrow, and the first 0-50 time channels (about 97 s) are selected. The general electrical source emission off time is about 50-100 s. The amplification multiple in the middle time channel design is used as a transition zone. The middle time channel is 51-1586 (about 3 ms). The late signal decays slowly, and the window can be extended to 1587-5120 points (about 7 ms).

[0082] (3-4) Assign time channels: distribute the total data points to early, middle and late time channels.

[0083] Taking the emission of a 25Hz bipolar pulse signal as an example, after detecting the rising edge of the 1PPS signal, the early, middle and late time channels are judged based on the data points every second.

[0084] The first 10ms is the emission rising edge stage, which contains 5120 data points (sampling points = sampling rate x time, i.e. 512000 x 0.01 = 5120). The amplification multiple in this stage is 1.

[0085] The second 10ms is the off falling edge stage, which is used to detect the secondary field induction signal of interest. According to the data points corresponding to different time channels, the 5120 data points are divided into three parts.

[0086] As shown in Figure 5 , 25Hz indicates that one period is 40ms, and only the secondary induction signal at the off time is concerned. The three parts are only divided at the off time, i.e. the second and fourth 10ms.

[0087] Since the transient electromagnetic data decay conforms to the e exponential decay law, the early, middle and late time channels are divided by exponential time channel division: Early time channel: the first 0-50 time channels (about 97 s), the signal decays quickly, the time window is narrow, and the amplification multiple G1 = 1.

[0088] Middle time channel: 51-1586 time channels (about 3 ms), the amplification multiple is G2, and G2 can be set to 10, 20, 30, 40, 50-100, which is used as a transition zone.

[0089] Late time channel: 1587-5119 time channels (about 7 ms), the signal decays slowly, the time window is long, and the amplification multiple increases to G3, which can be set to 200, 300, 400, 500.

[0090] (3-5) Gain control: using FPGA GPIO pin control analog switch ADG1419, create AXI GPIO IP core, through hardware time trigger timer and AXI GPIO real-time control gain resistance network switching, taking into account the early signal anti-saturation and late weak signal enhancement.

[0091] (4) Variable gain stage: through switching gain resistance network to realize 1 / G1 / G2 / G3 times gain control. The switching speed of ADG1419 is about 140ns-190ns, and the data acquisition rate is 512ksps (about 1950ns), which is much larger than the analog switch speed, ensuring that the analog switch is in the closed state every time data sampling is performed, and the data sampling will not be caused by the switching of the analog switch. Jump point.

[0092] As shown in Figure 6 , specifically comprising: Pin 7 of chip U29B is connected to an input signal, and is connected to pin 6 of chip U29B through resistor R113 (4.7k±1%); Pin 5 of chip U29B is connected to parallel resistor R119 and capacitor C128, one end of the parallel connection of resistor R119 (4.7k±1%) and capacitor C128 (10nF 50V) is connected to pin 5 of chip U29B and RF radio frequency signal, and the other end is grounded; The parallel connection of resistor R119 and capacitor C128 is also connected to pin 1 of chip U29A through resistor R116 (100±1%); Pin 4 of chip U29B is connected to-5V power supply, and is grounded through capacitor C131 (10nF 50V); Pin 8 of chip U29B is connected to+5V power supply, and is grounded through capacitor C127 (10nF 50V); Pin 2 of chip U29B is connected to gain interface of G1, G10 and G100 through resistors R114 (0R), R112 (1k±1%) and R111 (10k±1%) respectively, and is grounded through resistor R110 (100±1%); Pin 3 of chip U29B outputs the processed signal, and is grounded through connecting parallel resistor R121 (10k±1%) and capacitor C129 (10nF 50V); Among them, chip U29B and chip U29A are both OP A2188 operational amplifiers, and ADG1419 analog switch (not shown) is used.

[0093] (5) Simulation verification: A simulation circuit is built with a maximum signal amplitude of 100 mV. A two-stage resistance network is constructed to achieve signal segmentation amplification: the early signal amplification factor is 1, while the late signal channel is applied with a 10-fold gain coefficient. To simplify the amplification process, the number of early and late time channels is designed to be equal here. The simulation output results show that the late signal amplitude is increased to 540 mV after amplification, which realizes a 10-fold gain compared to the original signal (54 mV), consistent with the designed gain coefficient, and the designed amplification circuit can effectively amplify the late signal. As shown in FIG. 8, it is a simulation result of adaptive gain amplification (three-stage amplification). Figure 7

[0094] In this embodiment, the low-pass filter unit design specifically includes the following contents.

[0095] (1) Filter design target.

[0096] Noise characteristics: The noise of the unmanned aerial vehicle is mainly concentrated in the high frequency band greater than 20 kHz. In order to effectively remove these high-frequency noise, the cutoff frequency of the low-pass filter unit should be lower than the lowest frequency band of the high-frequency noise.

[0097] Cutoff frequency selection: According to the characteristics of the SATEM signal and the distribution of the unmanned aerial vehicle noise, 14.2 kHz is selected as the cutoff frequency of the low-pass filter unit. This frequency can effectively remove high-frequency noise without causing excessive attenuation to useful signal components.

[0098] (2) Filter chip selection.

[0099] Chip model: LTC1563-2IGN four-order Butterworth low-pass filter chip is selected. This chip has active RC rail-to-rail low-pass filter characteristics and can provide good filtering effect.

[0100] External resistance configuration: The cutoff frequency is set by external resistors R1-R6, supporting a maximum cutoff frequency of 256 kHz. By accurately calculating and configuring these external resistors, the required 14.2 kHz cutoff frequency can be achieved.

[0101] (3) Filter performance optimization.

[0102] Through simulation verification, the bandwidth of the low-pass filter unit at -3dB is 14.5 kHz, which can effectively remove high-frequency noise above 20 kHz and ensure the purity of the signal.

[0103] Embodiment 2 The embodiment provides a semi-airborne transient electromagnetic signal adaptive gain system, which comprises: A front-end conditioning module configured to obtain a differential signal of a receiving coil and perform chopper self-stabilized zero amplification processing on the differential signal. ​An adaptive gain module is configured to divide a total time window of the chopped self-holding zero amplified differential signal into a plurality of time channels at a set time reference, adjust a length of each time channel according to signal attenuation characteristics, calculate a time length and a data point number of each time channel according to a sampling rate and the total time window, and distribute the data points into early time channels, middle time channels and late time channels to switch different gain multiples, so as to perform adaptive gain amplification. A sending module is configured to transmit the differential signal amplified by the adaptive gain to a receiver after being collected by an ADC.

[0104] It should be noted that the above modules correspond to the steps described in Embodiment 1, and the above modules have the same examples and application scenarios as the corresponding steps, but are not limited to the content disclosed in Embodiment 1. It should be noted that the above modules can be executed in a computer system such as a set of computer executable instructions as part of a system.

[0105] In more embodiments, there are also provided: An electronic device includes a memory and a processor, and computer instructions stored in the memory and executed on the processor, when executed by the processor, complete the method described in Embodiment 1. For brevity, it will not be repeated here.

[0106] It should be understood that in the embodiments, the processor can be a central processing unit CPU, and the processor can also be other general-purpose processors, digital signal processors DSP, application-specific integrated circuits ASIC, ready-to-program gate arrays FPGA or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. The general-purpose processor can be a microprocessor or the processor can also be any conventional processor, etc.

[0107] The memory can include read-only memory and random access memory, and provide instructions and data to the processor, and a part of the memory can also include non-volatile random access memory. For example, the memory can also store device type information.

[0108] A computer readable storage medium for storing computer instructions, when executed by a processor, completes the method described in Embodiment 1.

[0109] The method in embodiment 1 can be directly embodied as being completed by a hardware processor or being completed by a combination of hardware and software modules in the processor. The software modules can be located in a storage medium mature in the art, such as a random access memory, a flash memory, a read-only memory, a programmable read-only memory, an electrically erasable programmable memory, a register, or the like. The storage medium is located in a memory, and a processor reads information in the memory and completes the steps of the above method in combination with hardware. To avoid repetition, no longer detailed description is given herein.

[0110] A computer program product comprises a computer program, which, when executed by a processor, implements the method described in embodiment 1.

[0111] The present application also provides at least one computer program product tangibly stored on a non-transitory computer-readable storage medium. The computer program product includes computer-executable instructions, such as instructions embodied in program modules, executed by devices in the target real or virtual processor to perform processes / methods as described above. Generally, program modules include routines, programs, libraries, objects, classes, components, data structures, etc. that perform particular tasks or implement particular abstract data types. In various embodiments, the functionality of program modules can be combined or split between program modules as desired. Machine-executable instructions for program modules can be executed within a local or distributed device. In a distributed device, program modules can be located in local and remote storage media.

[0112] Computer program code for carrying out operations of the present application can be written in one or more programming languages. These computer program code can be provided to a processor of a general purpose computer, a special purpose computer, or other programmable data processing apparatus to produce a machine, such that the program code, when executed by the computer or other programmable data processing apparatus, causes the functions / operations specified in the flowcharts and / or block diagrams to be implemented. The program code can be entirely on the computer, partially on the computer, as a stand-alone software package, partially on the computer and partially on a remote computer, or entirely on a remote computer or server.

[0113] In the context of the present application, computer program code or related data can be carried by any suitable carrier to enable a device, apparatus or processor to perform various processes and operations described above. Examples of carriers include signals, computer-readable media, and the like. Examples of signals can include electrical, optical, radio, sound or other forms of propagated signals, such as carrier waves, infrared signals, and the like.

[0114] Those skilled in the art can understand that the units and algorithm steps of each example described in combination with the present embodiment can be realized in electronic hardware or a combination of computer software and electronic hardware. Whether the functions are realized in hardware or software manner depends on the specific application and design constraints of the technical solution. The skilled person can use different methods to realize the described functions for each specific application, but such implementation should not be considered beyond the scope of the present application.

[0115] Although the specific embodiments of the present application are described above in combination with the drawings, it is not a limitation on the scope of protection of the present application. Those skilled in the art should understand that various modifications or variations made by those skilled in the art on the basis of the technical solutions of the present application without creative labor are still within the scope of protection of the present application.

Claims

1. A semi-aeronautical transient electromagnetic signal adaptive gain method, characterized in that: include: Obtaining the differential signal of the receiving coil and performing chopping and zero-stabilization amplification processing on the differential signal; Under a set time base, the total time window of the differential signal after chopper-zero amplification is divided into multiple time channels. The length of each time channel is adjusted according to the signal attenuation characteristics. The time length and number of data points of each time channel are calculated based on the sampling rate and the total time window. The data points are then allocated to the early, mid, and late time channels to switch between different gain multiples, thereby performing adaptive gain amplification. The differential signal after adaptive gain amplification is collected by the ADC and transmitted to the receiver.

2. A semi-aeronautical transient electromagnetic signal adaptive gain method according to claim 1, characterized in that: The design of the receiving coil includes: Optimization of internal resistance: Calculate the winding resistance of the entire coil: ;in, N is the number of wire turns in the hollow coil, is the conductor resistivity, is the cross-sectional area of ​​the conductor, is the coil diameter; Adjustment of matching resistance: By changing the matching resistance, the damping coefficient and system bandwidth of the second-order transmission network can be changed; specifically: If the receiving coil works in the critical damping state, the matching resistance and system bandwidth for: ； ； When the receiving coil works in the overdamped state, that is, K >1, or the receiving coil is working in the underdamped state, that is, 0< K <1, matching resistance for: ； in, K The receiving coil is in working state; is the wire-wound resistor; is the distributed inductance of the coil, is the coil distributed capacitance, is the frequency.

3. The method for adaptive gain of semi-aeronautical transient electromagnetic signals according to claim 1, wherein: The design of the receiving coil also includes: Adjustment of the natural resonant frequency: Natural resonant frequency of differential induction air-core coil for: ; The distributed inductance is: ; The distributed capacitance is: ; in, d is the wire diameter, is the spatial magnetic permeability, is the equivalent radius of the coil cross section; is the number of coil segments, is the total number of coil winding layers, is the relative dielectric constant of the wire, δ is the spacing between each layer, is the relative dielectric constant of the skeleton, is the height of the skeleton, e is the width between the skeleton slots, and N is the number of coil turns.

4. The method for adaptive gain of semi-aeronautical transient electromagnetic signals according to claim 1, wherein: The chopper-zero processing includes: The differential signal With the modulation signal Multiply, Modulate from low frequency to center frequency frequency band, in At the frequency band, the amplifier noise floor is white noise; The modulated signal is amplified by a low noise amplifier and down-converted, that is, the amplified signal is mixed with the modulated signal again. The signal is demodulated and returned to the low frequency band after multiplication, and the noise signal is modulated and moved to the high frequency band, and finally passes through low-pass filtering to achieve low-noise amplification of the low-frequency signal.

5. The method for adaptive gain of semi-aeronautical transient electromagnetic signals according to claim 1, wherein: The length of each time channel is adjusted according to the signal attenuation characteristics. The attenuation of transient electromagnetic data conforms to the exponential decay law. Therefore, the division of early, middle and late time channels adopts the exponential time channel division: Early time channel: the first 0-50 time channels, with fast signal decay, narrow time window, and amplification factor G1=1; Mid-term time channels: 51-1586 time channels, magnification is G2, G2 is set to 10, 20, 30, 40 and 50-100 as the transition zone; Late time channels: 1587-5119 time channels, slow signal decay, long time window, magnification increased to G3, G3 settings are 200, 300, 400 and 500.

6. The method for adaptive gain of semi-aeronautical transient electromagnetic signals according to claim 1, wherein: The differential signal after adaptive gain amplification is processed by low-pass filtering and then collected by ADC. The low-pass filtering process includes: The low-pass filter cutoff frequency is lower than the lowest frequency band of the drone's high-frequency noise, that is, 14.2kHz is selected as the low-pass filter cutoff frequency; A fourth-order Butterworth low-pass filter chip is used for low-pass filtering, and the cutoff frequency is set by an external resistor, supporting a maximum cutoff frequency of 256kHz.

7. A semi-aeronautical transient electromagnetic signal adaptive gain system, characterized in that: include: The front-end conditioning module is configured to obtain the differential signal of the receiving coil and perform chopping and zero-stabilization amplification processing on the differential signal; The adaptive gain module is configured to divide the total time window of the differential signal after chopper-zero amplification processing into multiple time channels under a set time base, adjust the length of each time channel according to the signal attenuation characteristics, calculate the time length and number of data points of each time channel based on the sampling rate and the total time window, and distribute the data points to the early time channel, the middle time channel, and the late time channel, thereby switching different gain multiples to perform adaptive gain amplification; The sending module is configured to collect the differential signal after adaptive gain amplification through the ADC and transmit it to the receiver.

8. An electronic device, characterized in that: The method comprises a memory and a processor, and computer instructions stored in the memory and executed on the processor, wherein when the computer instructions are executed by the processor, the method according to any one of claims 1 to 6 is completed.

9. A computer-readable storage medium, characterized in that Used to store computer instructions, which, when executed by a processor, complete the method according to any one of claims 1 to 6.

10. A computer program product, characterized in that The invention comprises a computer program, which is used to implement the method according to any one of claims 1 to 6 when the computer program is executed by a processor.

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