Current waveform and electromagnetic response acquisition system with variable sampling rate and control method

Abstract

The invention discloses a variable-sampling-rate current waveform and electromagnetic response acquisition system and a control method. The variable-sampling-rate current waveform and electromagnetic response acquisition system comprises nine parts, namely a sensor circuit, a signal conditioning circuit, an analog-to-digital conversion circuit, an analog-to-digital isolation circuit, a main control circuit, a synchronous circuit, a storage circuit, a man-machine interaction circuit and a power management module. The voltage signal is firstly processed by the signal conditioning circuit to meet the sampling requirement of the 24-bit ADC module, and then the ADC module converts an analog voltage signal into a binary digital signal based on a reference voltage; and under the action of a clock signal of the synchronization module, the converted digital signal is firstly transmitted to an external SRAM (Static Random Access Memory) through DMA (Direct Memory Access) to be cached and stored. Interference of digital circuits such as a single-chip microcomputer and crosstalk of analog signals among different channels are effectively reduced, and the problems that an acquisition system is single in sampling rate, high in short-circuit noise level, large in shallow detection blind area and low in urban underground space complex target electromagnetic detection precision are solved.

Description

Technical Field

[0001] The present invention relates to the field of geological exploration, and in particular to a current waveform and electromagnetic response acquisition system with a variable sampling rate and a control method. Background Art

[0002] Transient Electromagnetic Method (TEM) has the advantages of simple operation, large detection depth and low cost. In recent years, it has been widely used in mineral exploration, hydrogeological survey, urban underground space detection and tunnel prediction. Usually, the TEM detection system consists of a bipolar current pulse transmitter, a multi-channel receiver, a transmitting coil and a low-noise induction coil. In order to improve the resolution of the signal, the superconducting quantum interference device (SQUID) sensor and readout circuit can be used instead of the low-noise induction coil. When the transmitting coil uses an ungrounded loop, it is called the magnetic source TEM method; the pulse current in the ungrounded loop will generate a time-varying primary field, which will cause eddy currents to flow in the underground conductor. When the pulse current is turned off, these eddy currents will generate a secondary field. The induction coil, amplifier and receiver record the sum of the primary field and the secondary field in the form of the time derivative of the magnetic field; the SQUID sensor, readout circuit and receiver record the total field of the underground medium in the form of a magnetic field. The data recorded by the receivers are inverted to obtain the electromagnetic response of the underground medium in order to interpret and identify underground anomalies in the area of ​​interest.

[0003] In recent years, scholars at home and abroad have actively carried out research on numerical simulation, detection system development, and data interpretation of combined waveform detection methods. The sampling rate and noise level of the electromagnetic response acquisition device determine the quality of the acquired signal to a certain extent. The existing receiving systems generally have the problem of single sampling rate and large short-circuit noise. In order to record the emission current waveform and electromagnetic response with high precision, it is necessary to develop a low-noise receiving system. In order to improve the accuracy of joint data interpretation of multi-waveform responses, it is first necessary to record the emission current waveform and electromagnetic response with high precision, and then obtain the pure secondary induced electromotive force of different current waveforms based on the primary field removal method of the measured current waveform, and finally perform normalized data processing and joint resistivity imaging on the multi-waveform responses.

[0004] In order to meet the needs of fine-grained detection of shallow targets in urban underground space, the traditional single sampling rate acquisition system is no longer applicable, because the noise level of the acquisition system is related to the sampling rate. The higher the sampling rate, the greater the noise level. In order to obtain the first effective signal as early as possible and obtain near-surface geological information, the sampling rate of the acquisition system must be high enough. At the same time, in order to extend the effective signal length and increase the effective detection range, the short-circuit noise level of the acquisition system must be low enough. In order to meet the requirements of high sampling rate and low noise at the same time, it is necessary to develop a variable sampling rate acquisition system with high and low sampling rates to record the electromagnetic response with high precision and ensure the acquisition of high-quality and long-time window electromagnetic response data. In order to process the electromagnetic response data with high precision, the emission current waveform must be accurately recorded first to achieve effective removal of the primary field. Therefore, the development of a current waveform and electromagnetic response acquisition system with a variable sampling rate can reduce the blind spot of shallow detection and realize fine-grained detection of urban underground space.

[0005] In order to meet the needs of shallow detection and obtain early electromagnetic response, it is necessary to emit a small-amplitude, fast-off triangular wave emission current; at the same time, in order to improve the quality of the electromagnetic response signal, it is necessary to emit a large-amplitude, fast-off trapezoidal wave emission current. Therefore, a low-power triangular wave-trapezoidal wave combined detection system can achieve refined detection of shallow targets.

[0006] In order to jointly interpret the pure secondary field response of triangular waves and trapezoidal waves, the attenuation curves of triangular waves and trapezoidal waves were spliced ​​together with reference to the splicing method of trapezoidal wave data with different amplitudes by Auken et al., and then the spliced ​​data was used for resistivity imaging. In the splicing process, the early response data was responded with triangular waves, and the late response data was responded with trapezoidal waves. At this time, the start time of the spliced ​​data was earlier, the signal-to-noise ratio of the late data was higher, and the quality of the overall attenuation curve was higher. The attenuation curve splicing method realized the joint data interpretation of triangular wave and trapezoidal wave responses and improved the interpretation accuracy.

[0007] The prior art discloses an underwater multi-channel distortion-free data acquisition and storage system under high sampling rate. By adopting a double buffer structure, fast data storage under high sampling rate state is achieved. In addition, the storage efficiency is improved and the loss of original data is avoided through a data accumulation storage method. This provides ideas for the acquisition and storage of large amounts of data under high sampling rate state.

[0008] The prior art discloses a data acquisition and storage system and method based on dual sampling rates, which converts part of the high sampling rate data into low sampling rate data through data extraction processing, thereby indirectly realizing the storage of dual sampling rate data. This method can extend the sampling time as long as possible while still maintaining a high time resolution of the details of interest, and can balance the contradiction between the sampling time and the sampling rate.

[0009] The above-mentioned methods disclose data acquisition and storage strategies for reducing distortion under high sampling rate conditions, as well as methods for converting part of high sampling rate data into low sampling rate data to indirectly realize high and low sampling rate acquisition. All of them are researched for a single high sampling rate storage system. However, for the shallow and refined detection of urban underground space using transient electromagnetic method, it is difficult for existing acquisition systems to meet the requirements; and most electromagnetic acquisition systems have a fixed sampling rate, and the short-circuit noise level is generally large, and shallow geological information is seriously lost, which cannot meet the electromagnetic response recording needs of complex geological conditions in urban underground space; in addition, it is difficult for existing acquisition systems to record the emission current waveform with high precision, which is not conducive to the removal of the electromagnetic response primary field. How to accurately record the emission current waveform in multi-waveform combined time-domain electromagnetic detection, obtain high-quality and long-time window electromagnetic response data, reduce the blind spot of shallow detection and realize the refined detection of complex targets in urban underground space is a technical problem that technicians in this field are eager to solve. Summary of the invention

[0010] In view of the above-mentioned deficiencies in the prior art, the present invention provides a current waveform and electromagnetic response acquisition system with a variable sampling rate and a control method.

[0011] In order to achieve the above-mentioned object of the invention, the technical solution adopted by the present invention is:

[0012] A current waveform and electromagnetic response acquisition system with variable sampling rate includes a sensor circuit, a signal conditioning circuit, an analog-to-digital conversion circuit, an analog-to-digital isolation circuit, a main control circuit, a synchronization circuit, a storage circuit, a human-computer interaction circuit and a power management module, wherein:

[0013] The sensor circuit converts the emission current and electromagnetic response into a voltage signal. The voltage signal is first processed by a signal conditioning circuit, and the analog voltage signal is converted into a binary digital signal based on a reference voltage. Under the action of the clock signal of the synchronization module, the converted digital signal is transmitted to the external SRAM cache through DMA and stored in the SD card; the analog-to-digital isolation circuit is used to isolate the analog circuit part from the digital circuit part.

[0014] Furthermore, the main control circuit includes a controller minimum system, a human-computer interaction module, a synchronization module, and an acquisition and storage module, which are used to receive acquisition instructions, display acquisition status, send clock signals and control signals to the acquisition circuit, receive collected data signals and transmit and store them, receive GPS second pulses, time, longitude and latitude information, and achieve synchronization between the acquisition system and the transmission system.

[0015] Furthermore, the sensor circuit includes a receiving coil sensor and a Hall current sensor, wherein the Hall sensor is used to sample the transmitting current and convert the current signal into a voltage signal for collection; the receiving coil sensor is used to convert the changing magnetic field into an induced voltage, adjust the damping state of the circuit through matching resistors, and amplify the signal in one stage through a preamplifier before inputting it into a signal conditioning circuit.

[0016] Furthermore, the preamplifier circuit includes operational amplifiers U1 and U3, voltage followers U2 and U4, sliding rheostats R3 and R4, resistors R1, R2, R5, R6 and R7, capacitors C1 and C2, wherein the operational amplifiers U1 and U3 are symmetrically distributed to form a differential amplifier circuit, and the voltage followers U2 and U4 are symmetrically distributed to form a differential voltage follower; the input ends of the differential amplifier circuit are respectively connected to the voltage signals obtained by the sensor circuit, the output end of the differential amplifier circuit is connected to the differential voltage follower, and the voltage signal is output through the differential voltage follower; the two ends of the sliding rheostat R3 are connected between the two input ends of the differential amplifier circuit , used as a matching resistor for the sensor circuit; a series circuit of a sliding rheostat R4 and a resistor R5 is connected between the two input terminals of the differential amplifier circuit to adjust the gain of the amplifier; two ends of a parallel circuit of a resistor R6 and a capacitor C1 are respectively connected to the input terminal of the operational amplifier U1 and the output terminal of the voltage follower U2; two ends of a parallel circuit of a resistor R7 and a capacitor C2 are respectively connected to the input terminal of the operational amplifier U3 and the output terminal of the voltage follower U4; one end of the resistor R1 is connected to the input terminal of the operational amplifier, and the other end is grounded; one end of the resistor R2 is connected to the input terminal of the operational amplifier U3, and the other end is grounded.

[0017] Furthermore, the signal conditioning circuit includes operational amplifiers U5 and U6, voltage stabilizing diodes D1, D2, D3 and D4, capacitors C3, C4, C5, resistors R8, R9, R10, R11, R12 and R13, wherein the operational amplifiers U5 and U6 are symmetrically distributed, one of the input ends is connected to the output end of the preamplifier circuit through resistors R10 and R11 respectively, and the other input end is connected to the signal conditioning circuit reference voltage V bias, the output end outputs the conditioned signal through the resistor R14 and the resistor R15 respectively; the voltage zener diodes D1 and D2 are reversely connected, and one end thereof is connected to an output end of the preamplifier circuit, and the other end is grounded; the voltage zener diodes D3 and D4 are reversely connected, and one end thereof is connected to another output end of the preamplifier circuit, and the other end is grounded; the two ends of the capacitor C3 are respectively connected to the input end and the output end of the operational amplifier U5; one end of the resistor R12 is connected to the input end of the operational amplifier U5, and the other end is connected to an output end of the signal conditioning circuit; the two ends of the capacitor C4 are respectively connected to the input end and the output end of the operational amplifier U6; one end of the resistor R13 is connected to the input end of the operational amplifier U6, and the other end is connected to an output end of the signal conditioning circuit; the two ends of the capacitor C5 are respectively connected between the two output ends of the signal conditioning circuit.

[0018] Furthermore, the analog-to-digital conversion circuit includes multiple sampling rates, and the minimum difference between the high sampling rate and the low sampling rate is 4 times. The sampling rate of the analog-to-digital conversion circuit is controlled by multiple pins, and the sampling rate is set through the input and output ports of the main control circuit.

[0019] A control method for a current waveform and electromagnetic response acquisition system with a variable sampling rate comprises the following steps:

[0020] S1, system initialization and configuration of acquisition parameters;

[0021] S2, the main control circuit configures the GPS baud rate and serial port baud rate of the synchronization circuit through serial port communication, creates a data file name, sends an acquisition control signal to the analog-to-digital conversion circuit, and simultaneously turns on the DMA transmission of the storage circuit and enters the GPS second pulse interrupt; at this time, the analog-to-digital isolation circuit also starts working;

[0022] S3, the main control circuit receives the GPS second pulse from the synchronization circuit and detects the DMA transmission completion interrupt. If the transmission is completed, the main control circuit writes the data into the SD card of the storage circuit. The main control circuit determines whether the one-second acquisition is completed. If so, the serial port interrupt is turned on, otherwise the DMA continues to transmit;

[0023] S4, the main control circuit enters the serial port interrupt to extract the GPS coordinate information of the synchronization circuit, and writes the time, longitude and latitude and other information into the SD card of the storage circuit to determine whether the acquisition is completed. If so, the acquisition is exited, otherwise it will continue to start the acquisition of the next second.

[0024] The present invention has the following beneficial effects:

[0025] Under the control of the main control circuit, the acquisition system can select the sampling rate and the number of sampling points to achieve high and low sampling rate acquisition, which can not only obtain the first effective signal in advance, but also extend the length of the late signal, significantly improving the acquired response time window. By applying a low-noise 24-bit ADC module and a signal conditioning circuit, the short-circuit noise level of the acquisition system is reduced, and the signal-to-noise ratio of the acquired electromagnetic response is significantly improved. By simultaneously applying a Hall current sensor and a receiving coil for dual-channel acquisition, the simultaneous recording of the transmitting current waveform and the electromagnetic response is achieved, and a high-quality field removal can be performed, and the quality of the acquired electromagnetic response signal is significantly improved. The variable sampling rate acquisition system provides hardware support for the multi-waveform combination detection method, reduces the blind area of ​​shallow detection, and realizes the refined electromagnetic detection of complex targets in urban underground space. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is the overall design block diagram of the variable sampling rate current waveform and electromagnetic response acquisition system of the present invention;

[0027] Figure 2 It is a software design flow chart of the variable sampling rate acquisition system control method of the present invention;

[0028] Figure 3 It is the equivalent circuit model of the receiving coil and the relationship between the damping coefficient and the matching resistance;

[0029] Figure 4 It is the circuit design diagram of the preamplifier circuit;

[0030] Figure 5 This is the connection circuit diagram of the Hall current sensor;

[0031] Figure 6 It is the design diagram of the signal conditioning circuit and its Bode diagram;

[0032] Figure 7 It is the block diagram of the variable sampling rate acquisition circuit;

[0033] Figure 8 It is the sampling rate and short-circuit noise test results of the variable sampling rate acquisition system;

[0034] Fig. 9 It is the multi-waveform emission current acquisition result of the variable sampling rate acquisition system;

[0035] Fig.10 It is the response comparison result of the variable sampling rate acquisition system and the 30kHz sampling rate receiver;

[0036] Fig.11 It is the resistivity imaging comparison result between the variable sampling rate acquisition system and the 30kHz sampling rate receiver. DETAILED DESCRIPTION

[0037] The specific implementation modes of the present invention are described below so that those skilled in the art can understand the present invention. However, it should be clear that the present invention is not limited to the scope of the specific implementation modes. For those of ordinary skill in the art, as long as various changes are within the spirit and scope of the present invention as defined and determined by the attached claims, these changes are obvious, and all inventions and creations utilizing the concept of the present invention are protected.

[0038] See also Figure 1 As shown, the variable sampling rate current waveform and electromagnetic response acquisition system provided by the present invention mainly consists of nine parts: sensor circuit, signal conditioning circuit, analog-to-digital conversion circuit, analog-to-digital isolation circuit, main control circuit, synchronization circuit, storage circuit, human-computer interaction circuit and power management module; based on Figure 1 The overall design of the hardware circuit of the variable sampling rate acquisition system has been completed.

[0039] The keypad and display module form a human-computer interaction interface, which sets the working parameters of the acquisition system, including the sampling rate and number of sampling points of the 24-bit ADC module. The sensor circuit converts the emission current and electromagnetic response into a voltage signal. The voltage signal is first processed by the signal conditioning circuit to meet the sampling requirements of the 24-bit ADC module. Then, the ADC module converts the analog voltage signal into a binary digital signal based on the reference voltage. Under the clock signal of the synchronization module, the converted digital signal is first transferred to the external SRAM cache through DMA, and finally stored in the SD card. In this process, the analog-to-digital isolator is responsible for isolating the analog circuit part from the digital circuit part, effectively reducing the interference of digital circuits such as single-chip microcomputers and the crosstalk of analog signals between different channels. The power supply voltage required by each module in the circuit is provided by the power management module.

[0040] The specific functions of the nine parts of the variable sampling rate acquisition system are:

[0041] 1) Sensor circuit: The sensor consists of a receiving coil sensor and a Hall current sensor. The Hall sensor is responsible for sampling the transmitting current and converting the current signal into a voltage signal for collection. The coil sensor converts the changing magnetic field into an induced voltage, adjusts the damping state of the circuit through a matching resistor, and then amplifies the signal through a preamplifier and inputs it into the signal conditioning circuit.

[0042] 2) Signal conditioning circuit: The signal conditioning circuit is mainly responsible for attenuating, filtering and level-raising the voltage signal output by the sensor so that the conditioned signal can meet the input voltage range requirements of the ADC. The signal conditioning circuit uses two low-noise operational amplifiers as core components;

[0043] 3) Analog-to-digital conversion circuit: The analog-to-digital conversion circuit includes the analog-to-digital converter and its peripheral circuits. The analog-to-digital converter is the core device of the entire system and determines the accuracy and sampling rate of data acquisition. It is mainly responsible for sampling the conditioned analog voltage signal and converting the continuous analog signal into a discrete digital signal. According to the numerical simulation results, in order to achieve shallow and refined detection, the time to obtain the first valid signal is required to be no more than 4μs, that is, the sampling rate is required to be higher than 250kHz. In addition, in order to increase the data length of the effective signal and reduce the noise level of the acquisition system, the acquisition system is required to include a low sampling rate. Therefore, the selected ADC chip must be able to acquire at a variable sampling rate and have a low overall noise level.

[0044] 4) Analog-to-digital isolation circuit: The analog-to-digital isolation circuit mainly includes the analog-to-digital isolator and its peripheral circuits, which are responsible for isolating the analog circuit part from the digital circuit part. The connection between the analog part and the digital part is mainly on the ADC. It is necessary to isolate important signals such as SDATA, LRCK, SCLK, MCLK, FSYNC, DFS, HPFE, etc. Therefore, it is necessary to place multiple four-channel isolators to effectively reduce the interference of digital circuits such as single-chip microcomputers and the crosstalk of analog signals between different channels;

[0045] 5) Main control circuit: The main control circuit is mainly responsible for generating system clock and control signal, controlling the system's variable sampling rate acquisition, transmission and reception synchronization, data transmission and storage, etc.;

[0046] 6) Synchronization circuit: The synchronization circuit mainly includes the GPS chip and its peripheral circuits. It realizes mutual synchronization between the transmitting system and the receiving system through the GPS second pulse signal, and can record time and longitude and latitude information in real time. The commonly used synchronization methods are wired synchronization and GPS synchronization.

[0047] 7) Storage circuit: The storage circuit includes two parts: data transmission and data storage. The digital signal output by the analog-to-digital converter is first transferred to the external SRAM for cache through DMA, and then the microcontroller controls it to be transferred to the SD card for storage;

[0048] 8) Human-computer interaction circuit: The human-computer interaction circuit mainly includes buttons and display screens. The system acquisition parameters are selected by buttons, and the current acquisition status can be displayed in real time on the display screen;

[0049] 9) Power management module: The power management module converts the power supply battery voltage into different voltage levels through a low voltage difference linear regulator to power various circuits of the system.

[0050] The main control circuit mainly includes the controller minimum system, human-computer interaction module, synchronization module, acquisition and storage module, which realizes human-computer interaction functions such as acquisition instruction reception and acquisition status display; sends clock signals and control signals to the acquisition circuit, receives the collected data signals and transmits and stores them; receives GPS second pulses, time, longitude and latitude and other information to achieve synchronization between the acquisition system and the transmission system; the control method software process based on the main control circuit includes six parts: initialization, SD card detection, parameter setting, start acquisition, synchronization setting and obtain GPS information.

[0051] See also Figure 2 As shown, the present invention provides a control method for a variable sampling rate acquisition system, wherein the main control circuit mainly includes a controller minimum system, a human-computer interaction module, a synchronization module, and an acquisition and storage module, which realizes human-computer interaction functions such as receiving acquisition instructions and displaying acquisition status; sends a clock signal and a control signal to the acquisition circuit, receives the collected data signal and transmits and stores it; receives the second pulse, time, longitude and latitude and other information of GPS, and realizes synchronization between the acquisition system and the transmission system; based on Figure 2 The software flow chart design of the variable sampling rate acquisition system control method has been completed, including:

[0052] S1, system initialization and configuration of acquisition parameters;

[0053] The power management module supplies power to the main control circuit, which drives the human-computer interaction circuit to work and performs various initialization tests, including delay function initialization, setting the system's interrupt priority grouping, OLED screen initialization, external SRAM cache initialization, GPIO port initialization configuration, button initialization, etc. After the initialization is completed, the sensor circuit and signal conditioning circuit are always in working state, waiting for the analog-to-digital conversion circuit to work

[0054] After initialization is completed, the main control circuit will create a new file and write a random string of numbers to the SD card through the data pin to determine whether the SD card is available. If the write fails, the SD card error flag will be returned to the main control circuit and output to the OLED display.

[0055] Input working instructions to the main control circuit through the external buttons and OLED screen of the human-computer interaction circuit, and configure system sampling rate, sampling frequency, superposition times and other parameters.

[0056] S2, the main control circuit configures the GPS baud rate and serial port baud rate of the synchronization circuit through serial port communication, creates a data file name, sends an acquisition control signal to the analog-to-digital conversion circuit, and simultaneously turns on the DMA transmission of the storage circuit and enters the GPS second pulse interrupt; at this time, the analog-to-digital isolation circuit also starts working;

[0057] S3, the main control circuit receives the GPS second pulse from the synchronization circuit and detects the DMA transmission completion interrupt. If the transmission is completed, the main control circuit writes the data into the SD card of the storage circuit. The main control circuit determines whether the one-second acquisition is completed. If so, the serial port interrupt is turned on, otherwise the DMA continues to transmit;

[0058] S4, the main control circuit enters the serial port interrupt to extract the GPS coordinate information of the synchronization circuit, and writes the time, longitude and latitude and other information into the SD card of the storage circuit to determine whether the acquisition is completed. If so, the acquisition is exited, otherwise it will continue to start the acquisition of the next second.

[0059] See also Figure 3 , the induction coil converts the change in magnetic flux into an induced voltage signal. Since the induction coil has many turns and contains capacitors and inductors, an adjustable matching resistor needs to be connected in parallel at both ends of the induction coil to make the induction coil work in a critical damping state and improve the fidelity of the induced voltage signal; in order to meet the needs of different transmission parameters, the amplification factor of the preamplifier is adjustable. Under the premise of meeting the range of the acquisition system, the amplification factor should be as large as possible to improve the signal-to-noise ratio of the induced voltage; the voltage signal is first processed by the signal conditioning circuit. The bandwidth of the analog signal conditioning circuit must be greater than the sampling bandwidth of the ADC module and the passband must be smooth to meet the sampling requirements of the 24-bit ADC module. The formula derivation process of the matching resistor under the critical damping state is:

[0060] Assume r s is the internal resistance of the receiving coil, L s is the receiving coil inductance, C s is the parasitic capacitance of the receiving coil, R m is a matching resistor used to adjust the damping coefficient; U i Represents the input voltage, i.e., the induced electromotive force, U o represents the output voltage, that is, the voltage signal actually output by the receiving coil, i(t) is the transmitting current waveform; based on Figure 3 The equivalent circuit model of the receiving coil can list the relationship between the current and voltage in the circuit as:

[0061]

[0062] Substituting the second term in formula (1) into the first term yields:

[0063]

[0064] Simplifying formula (2) we can get:

[0065]

[0066] Where ε is the input voltage U i , U is the output voltage U o ,and

[0067]

[0068] Where ω 0 Denotes the natural resonant frequency of the receiving coil, δ and ω p is an intermediate parameter. In order to analyze the working state of the receiving coil, the damping coefficient ζ is introduced here:

[0069]

[0070] According to the principle of automatic control, the characteristic equation of formula (3) is established as:

[0071]

[0072] The characteristic root can be obtained by the root-finding formula:

[0073]

[0074] When ζ>1, the characteristic equation has two unequal negative real roots, and the system is in an overdamped state; when ζ=1, the characteristic equation has two equal negative real roots, and the system is in a critical damped state; when ζ<1, the characteristic equation has two conjugate negative roots with negative real parts, and the system is in an underdamped state; after the receiving coil is wound, the internal resistance r of the receiving coil is s and inductor L s Basically unchanged, parasitic capacitance C s It does not change under certain conditions, so it can only be adjusted by adjusting the matching resistor R m To change the damping state of the receiving coil; matching resistor R m The relationship between and the damping coefficient ζ is:

[0075]

[0076] According to the principle of automatic control, the system can achieve the best performance when it is in the critical damping state. Let ζ = 1 in equation (7), and the matching resistance in the critical damping state is:

[0077] or

[0078] Normally, the parasitic capacitance C s is very small, resulting in R m1 It is usually negative, so it is considered that R m2is the actual critical damping matching resistor; in order for the receiving coil to work properly, the matching resistor needs to be adjusted to the critical damping or over-damping state; when the receiving coil is in the critical damping state, the amplitude of the electromagnetic response signal is the largest and the fidelity is the highest; in order to meet the needs of receiving coils with different parameters, a sliding rheostat is selected as the matching resistor, and by adjusting the resistance of the sliding rheostat, the receiving coil can work normally and the damping coefficient is close to 1.

[0079] See also Figure 4 , the voltage signals Vin1 and Vin2 obtained by the induction coil or Hall current sensor are connected to the input ends of the differential amplifiers U1 and U3 composed of two LT1028 to achieve voltage amplification of the input signal; the signal output by the differential amplifier is then connected to the differential voltage followers U2 and U4 composed of two ELH0002H to achieve signal buffering and isolation, and output voltage signals Vout1 and Vout2. In order to meet the needs of different transmission parameters, the amplification factor of the preamplifier is adjustable. Under the premise of meeting the range of the acquisition system, the amplification factor should be as large as possible to improve the signal-to-noise ratio of the induced voltage; the resistor R 6 and R 7 The resistance is 100kΩ, R 5 The resistance is 1kΩ, R 4 The resistor is a sliding resistor with a resistance range of 0 to 100 kΩ; the gain of the preamplifier is about 3 to 200, which can meet the design requirements. Figure 4 As shown, the sliding resistor R 3 is the matching resistance of the induction coil, the sliding rheostat R 4 Used to adjust the gain of the amplifier. U1 and U3 are operational amplifiers, whose model is LT1028, and they form a differential amplifier through symmetrical distribution. U2 and U4 are voltage followers, whose model is ELH0002H / 883B, which are used to modulate the output voltage after amplification. Using the "virtual short" and "virtual break" principles of the amplifier, the gain expression of the preamplifier can be obtained as:

[0080]

[0081] The resistor R 6 and R 7 The resistance is 100kΩ, R 5 The resistance is 1kΩ, R 4 It is a sliding rheostat with a resistance range of 0 to 100 kΩ; therefore, the amplification factor of the preamplifier is approximately 3 to 200, which can meet the design requirements.

[0082] See also Figure 5, an open-loop Hall current sensor CS-100B is selected to sample the emission current. The connection circuit between the sensor and the acquisition system is shown in the figure. The rated input current of the CS-100B Hall current sensor is ±100A, and the response time is less than 3μs.

[0083] See also Figure 6 , the two signals Vout1 and Vout2 output by the voltage follower are connected to Vin3 and Vin4 respectively, and the two signals are connected to the input ends of the signal conditioning circuits U5 and U6 composed of two LT1028s, and the voltage signals finally output to the ADC module are Vout3 and Vout4. The bandwidth of the analog signal conditioning circuit must be greater than the sampling bandwidth of the ADC module and the passband must be smooth to meet the sampling requirements of the 24-bit ADC module; the signal conditioning circuit is designed based on two LT1028 operational amplifiers, and the bandwidth of attenuation of 3dB is about 158.5kHz, which is higher than the ADC sampling bandwidth and the passband is smooth, which can meet the design requirements. The signal protection function is realized by four reverse-connected Zener diodes D1~D4, whose model is MMSZ4681T1G. In the circuit, let R 10 =R 11 =R a =20kΩ, R 12 =R 13 =R b =10kΩ, C 3 =C 4 =C o =100pF, the transfer function of the analog signal conditioning circuit is:

[0084]

[0085] Where ω is the angular frequency, V bias is the reference voltage of the signal conditioning circuit, and f is the low-pass filter cutoff frequency. 2 is the gain of the analog signal conditioning circuit, and its Bode diagram is as follows Figure 6 As shown, the bandwidth of 3dB attenuation is about 158.5kHz, which is higher than the ADC sampling bandwidth and the passband is smooth, which can meet the design requirements.

[0086] See also Figure 7Under the control signal of the variable sampling rate of the microcontroller, the ADC works at a sampling rate of 64kHz or 256kHz. ADR431 provides a +2.5V reference voltage for the ADC module, and the 16.384MHz external clock provides a time reference for the ADC. During the acquisition process, the DRDY pin is used as an identifier to send the converted data to the microcontroller memory through the serial peripheral interface (SPI), and the data in the memory is imported into the SD card for storage through the SDIO interface. The 24-bit ADC module has multiple sampling rates, and the difference between the high sampling rate and the low sampling rate is greater than or equal to 4 times. In order to obtain the first effective signal as early as possible, the shallowest detection depth is less than 4 meters, and the high sampling rate is selected as 256kHz and the low sampling rate is 64kHz; the sampling rate of the 24-bit ADC module is controlled by multiple pins, and the sampling rate is set through the I / O port of the main control circuit; in order to achieve low-noise acquisition, the root mean square error of the short-circuit noise level of the high and low variable sampling rate acquisition systems is required to be less than 3μV and 1μV respectively; in addition, the acquisition system has two channels, the emission current waveform and the electromagnetic response need to be recorded simultaneously, and the pure secondary induced electromotive force is obtained by the primary field removal method based on the measured current waveform; the electromagnetic response of the same measuring point needs to be collected once with a high sampling rate and a low sampling rate respectively. After obtaining the data, the two attenuation curves of the high and low sampling rates are put together, the early response uses the high sampling rate data, the mid-term overlap part response uses the high sampling rate data, and the late response uses the low sampling rate data, thereby improving the data quality of the electromagnetic response, reducing the shallow detection blind area, and realizing the refined detection of the shallow target body.

[0087] See also Figure 8 In order to verify the receiving device's ability to collect standard signals and test the receiving device's sampling rate, a signal generator is used to generate a standard sine wave signal with a peak value of 1V, a frequency of 1kHz, and a voltage offset of 0. The number of sampling points of the acquisition system is fixed to 512, and the acquisition system is used to collect standard signals. The standard signal results recorded by the low sampling rate receiving device are as follows: Figure 8 As shown on the left, the data length is 8ms, the sampling result is consistent with the standard sine wave signal and the sampling rate is 64kHz. The standard signal result recorded by the high sampling rate receiving device is as follows Figure 8 As shown on the right, the data length is 2ms, the sampling result can restore the standard sine wave signal and the sampling rate is 256kHz. The two input terminals of the receiving device are short-circuited, and the number of superpositions is 256. The short-circuit noise level at a sampling rate of 64kHz is as follows Figure 8 As shown on the left, the noise amplitude is about 3μV and the RMS error is about 0.907μV. The short-circuit noise level at a sampling rate of 256kHz is as follows: Figure 8 As shown on the right, the noise amplitude is approximately 8μV and the RMS error is approximately 2.417μV.

[0088] See also Fig. 9 ,A dual-waveform combination transmitter is used to output trapezoidal wave and triangle wave transmission current with different off-times to test the current acquisition function of the acquisition system. Fig. 9 The peak current of the trapezoidal wave on the right is 29A, and the turn-off times are 430μs, 220μs, and 140μs respectively. Fig. 9 The peak current of the left triangle wave is 4.8A, the rise time is 80μs, and the turn-off time is 60μs, 36μs, and 24μs respectively. The test results show that the acquisition system can record the emission current with high precision, which provides a basis for the primary field removal of the electromagnetic response signal.

[0089] See also Fig.10 and Fig.11 , using a trapezoidal-triangular combination wave as the excitation source, the transmitting coil has a side length of 4m and a number of turns of 4. The measured responses of the dual waveforms recorded by the two acquisition systems are as follows: Fig.10 shown. Fig.10 The left side shows the electromagnetic response recorded by the 30kHz sampling rate receiver. The first valid signal appears at about 33μs. The electromagnetic response recorded by the 64kHz and 256kHz variable sampling rate acquisition systems is shown in Figure 1. Fig.10 As shown on the right, the first effective signal appears at about 4μs. The triangular wave response and the trapezoidal wave response are normalized to the spliced ​​response. The spliced ​​response resistivity imaging results of different acquisition devices are shown in Fig.11 As shown; Fig.11 The above figure shows the measurement results of the ground penetrating radar on this survey line. The results show that there are two obvious reflections of the radar wave at a depth of 4m underground, indicating that there is a high-resistance anomaly on the left and right sides of the underground. Fig.11 The left side is the resistivity imaging result of the stitched response of the receiver with a sampling rate of 30kHz. A high-resistance anomaly appeared at a depth of 12m underground. The anomaly was distributed on the right side of the survey line, which is basically consistent with the distribution position of the anomaly on the right side of the ground penetrating radar, but the depth of the anomaly was obviously larger. In addition, the stitched response recorded by the receiver with a sampling rate of 30kHz did not detect the high-resistance anomaly on the left. Fig.11 The right side shows the resistivity imaging result of the spliced ​​response of the variable sampling rate acquisition system. Two high-resistance anomalies appeared at a depth of 4m underground. The anomalies are distributed on the left and right sides of the survey line, which is basically consistent with the distribution position and depth of the anomalies in the ground penetrating radar imaging results. Fig.11 It can be seen from the left and right sides that the variable sampling rate acquisition system obtains the first valid signal earlier, so the shallow layer information is richer and the detection accuracy is higher.

[0090] The present invention uses specific embodiments to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only used to help understand the method of the present invention and its core idea. At the same time, for those skilled in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as a limitation on the present invention.

[0091] Those skilled in the art will appreciate that the embodiments described herein are intended to help readers understand the principles of the present invention, and should be understood that the protection scope of the present invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific variations and combinations that do not deviate from the essence of the present invention based on the technical revelations disclosed by the present invention, and these variations and combinations are still within the protection scope of the present invention.

Claims

1. A current waveform and electromagnetic response acquisition system with variable sampling rate, characterized in that: It includes sensor circuit, signal conditioning circuit, analog-to-digital conversion circuit, analog-to-digital isolation circuit, main control circuit, synchronization circuit, storage circuit, human-computer interaction circuit and power management module, among which, The sensor circuit converts the emission current and electromagnetic response into a voltage signal. The voltage signal is first processed by a signal conditioning circuit, and the analog voltage signal is converted into a binary digital signal based on a reference voltage. Under the action of the clock signal of the synchronization module, the converted digital signal is transmitted to the external SRAM cache through DMA and stored in the SD card; the analog-to-digital isolation circuit is used to isolate the analog circuit part from the digital circuit part.

2. The current waveform and electromagnetic response acquisition system with variable sampling rate according to claim 1, characterized in that: The main control circuit includes a minimum controller system, a human-computer interaction module, a synchronization module, and an acquisition and storage module, which are used to receive acquisition instructions, display acquisition status, send clock signals and control signals to the acquisition circuit, receive acquired data signals and transmit and store them, receive GPS second pulses, time, longitude and latitude information, and achieve synchronization between the acquisition system and the transmission system.

3. The current waveform and electromagnetic response acquisition system with variable sampling rate according to claim 1, characterized in that: The sensor circuit includes a receiving coil sensor and a Hall current sensor, wherein the Hall sensor is used to sample the transmitting current and convert the current signal into a voltage signal for collection; the receiving coil sensor is used to convert the changing magnetic field into an induced voltage, adjust the damping state of the circuit through a matching resistor, and amplify the signal in one stage through a preamplifier before inputting it into a signal conditioning circuit.

4. The current waveform and electromagnetic response acquisition system with variable sampling rate according to claim 3, characterized in that: The preamplifier circuit includes operational amplifiers U1 and U3, voltage followers U2 and U4, sliding rheostats R3 and R4, resistors R1, R2, R5, R6 and R7, capacitors C1 and C2, wherein the operational amplifiers U1 and U3 are symmetrically distributed to form a differential amplifier circuit, and the voltage followers U2 and U4 are symmetrically distributed to form a differential voltage follower; the input ends of the differential amplifier circuit are respectively connected to the voltage signals obtained by the sensor circuit, the output end of the differential amplifier circuit is connected to the differential voltage follower, and the voltage signal is output through the differential voltage follower; the two ends of the sliding rheostat R3 are connected between the two input ends of the differential amplifier circuit, and the differential voltage follower is used to output the voltage signal ... As the matching resistor of the sensor circuit; the series circuit of the sliding rheostat R4 and the resistor R5 is connected between the two input terminals of the differential amplifier circuit to adjust the amplification factor of the amplifier; the two ends of the parallel circuit of the resistor R6 and the capacitor C1 are respectively connected to the input terminal of the operational amplifier U1 and the output terminal of the voltage follower U2; the two ends of the parallel circuit of the resistor R7 and the capacitor C2 are respectively connected to the input terminal of the operational amplifier U3 and the output terminal of the voltage follower U4; one end of the resistor R1 is connected to the input terminal of the operational amplifier, and the other end is grounded; one end of the resistor R2 is connected to the input terminal of the operational amplifier U3, and the other end is grounded.

5. The current waveform and electromagnetic response acquisition system with variable sampling rate according to claim 4, characterized in that: The signal conditioning circuit includes operational amplifiers U5 and U6, voltage stabilizing diodes D1, D2, D3 and D4, capacitors C3, C4, C5, resistors R8, R9, R10, R11, R12 and R13, wherein the operational amplifiers U5 and U6 are symmetrically distributed, one of the input ends is connected to the output end of the preamplifier circuit through resistors R10 and R11 respectively, and the other input end is connected to the signal conditioning circuit reference voltage V bias , the output end outputs the conditioned signal through the resistor R14 and the resistor R15 respectively; the voltage zener diodes D1 and D2 are reversely connected, and one end thereof is connected to an output end of the preamplifier circuit, and the other end is grounded; the voltage zener diodes D3 and D4 are reversely connected, and one end thereof is connected to another output end of the preamplifier circuit, and the other end is grounded; the two ends of the capacitor C3 are respectively connected to the input end and the output end of the operational amplifier U5; one end of the resistor R12 is connected to the input end of the operational amplifier U5, and the other end is connected to an output end of the signal conditioning circuit; the two ends of the capacitor C4 are respectively connected to the input end and the output end of the operational amplifier U6; one end of the resistor R13 is connected to the input end of the operational amplifier U6, and the other end is connected to an output end of the signal conditioning circuit; the two ends of the capacitor C5 are respectively connected between the two output ends of the signal conditioning circuit.

6. The variable sampling rate current waveform and electromagnetic response acquisition system according to claim 1, characterized in that: The analog-to-digital conversion circuit includes multiple sampling rates, and the minimum difference between the high sampling rate and the low sampling rate is 4 times. The sampling rate of the analog-to-digital conversion circuit is controlled by multiple pins, and the sampling rate is set through the input and output ports of the main control circuit.

7. A control method for a current waveform and electromagnetic response acquisition system with a variable sampling rate according to any one of claims 1 to 6, characterized in that: The steps include: S1, system initialization and configuration of acquisition parameters; S2, the main control circuit configures the GPS baud rate and serial port baud rate of the synchronization circuit through serial port communication, creates a data file name, sends an acquisition control signal to the analog-to-digital conversion circuit, and simultaneously turns on the DMA transmission of the storage circuit and enters the GPS second pulse interrupt; at this time, the analog-to-digital isolation circuit also starts working; S3, the main control circuit receives the GPS second pulse from the synchronization circuit and detects the DMA transmission completion interrupt. If the transmission is completed, the main control circuit writes the data into the SD card of the storage circuit. The main control circuit determines whether the one-second acquisition is completed. If so, the serial port interrupt is turned on, otherwise the DMA continues to transmit; S4, the main control circuit enters the serial port interrupt to extract the GPS coordinate information of the synchronization circuit, and writes the time, longitude and latitude and other information into the SD card of the storage circuit to determine whether the acquisition is completed. If so, the acquisition is exited, otherwise it will continue to start the acquisition of the next second.

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