A digital low-frequency seismic sensor

Abstract

The present invention provides a digital low-frequency seismic sensor to solve the problems that the existing seismic detection system cannot directly output digital signals, or has high cost, complex structure, high power consumption, and cannot detect low-frequency seismic signals deep in the earth's crust. The sensor includes a pendulum system, an electromagnetic transducer, a detection stage circuit, a multi-stage integrator, a quantizer, a drive stage circuit, a PID controller, and a DAC; the pendulum system senses vibration to generate a vibration signal, which is converted into a voltage signal by the electromagnetic transducer; the voltage signal is reversely amplified by the detection stage circuit, enters the multi-stage integrator, and is sampled, held, and integrated and filtered; the filtered voltage signal is converted into an analog-to-digital conversion by the quantizer; the DAC is used to generate a pulse width modulated voltage from the digital bit stream output by the quantizer, which is conditioned into a control voltage by the PID controller, and converted into a coil current of the electromagnetic transducer by the drive stage circuit, and the coil current is converted into an electromagnetic force by the magnetic field transducer, which is used to balance the movement trend of the pendulum system.

Description

Technical Field

[0001] The invention relates to the field of sensors, and in particular to a digital low-frequency seismic sensor. Background Art

[0002] At present, the exploration seismic detection system is mainly composed of seismic detectors and data acquisition devices. Its structural characteristics are that the seismic detector, as a sensing unit, picks up seismic signals and outputs analog voltage, and the data acquisition device converts the voltage signal into a digital quantity. This type of detection system cannot directly output digital signals, and its digital acquisition relies on analog-to-digital conversion chips, which have problems such as high cost, complex structure and high power consumption; in addition, the commonly used moving coil seismic detector adopts the principle of velocity-type electromagnetic transduction, which is a non-feedback open-loop structure, limiting its lower cutoff frequency to 4Hz~50Hz, and cannot detect low-frequency seismic signals reflected and transmitted from the deep crust, affecting the detection accuracy and depth of underground medium structure and oil and gas resources. Summary of the invention

[0003] In order to solve the technical problems that the existing seismic detection systems cannot directly output digital signals, or the analog-to-digital conversion chips used are high in cost, complex in structure, and high in power consumption, and cannot detect low-frequency seismic signals reflected and transmitted deep in the earth's crust, thus affecting the detection accuracy and depth of underground medium structures and oil and gas resources, the present invention provides a digital low-frequency seismic sensor.

[0004] To achieve the above purpose, the technical solution provided by the present invention is:

[0005] A digital low-frequency seismic sensor, which is special in that it includes a signal pickup unit and a digital feedback control unit; the signal pickup unit includes a pendulum system, an electromagnetic transducer, a detection stage circuit, a multi-stage integrator, and a quantizer;

[0006] The pendulum system senses vibration and generates a vibration signal, which is converted into a voltage signal through an electromagnetic transducer; the voltage signal is reversely amplified by a detection circuit, enters a multi-stage integrator, and is sampled, held, and integrated and filtered; the filtered voltage signal is converted into digital by a quantizer, and if the voltage signal is greater than a threshold, the quantizer outputs a digital high level +1; if the voltage signal is less than or equal to the threshold, the quantizer outputs a digital low level -1;

[0007] The digital feedback control unit includes a driving stage circuit, a PID controller, and a DAC.

[0008] The DAC is connected to the output of the quantizer and is used to generate a pulse width modulated voltage from the digital bit stream output by the quantizer; the pulse width modulated voltage is conditioned into a control voltage by a PID controller and converted into a coil current of an electromagnetic transducer by a drive-level circuit; the coil current is converted into an electromagnetic force by the electromagnetic transducer and is used to balance the movement trend of the pendulum system.

[0009] Further, the electromagnetic transducer includes an induced voltage Vcoil and a coil resistance Rcoil, and one end of the induced voltage Vcoil is connected to one end of the coil resistance Rcoil;

[0010] The detection stage circuit includes a detection stage operational amplifier and a feedback resistor R1;

[0011] The other end of the induced voltage Vcoil is connected to the negative input end of the detection-level operational amplifier via the input resistor R2, the two ends of the feedback resistor R1 are respectively connected to the negative input end and the output end of the detection-level operational amplifier, the positive input end of the detection-level operational amplifier is connected to the other end of the coil resistor Rcoil, the other end of the coil resistor Rcoil is connected to one end of the matching resistor R3, and the other end of the matching resistor R3 is grounded.

[0012] Furthermore, the matching resistor R3 is equal to the coil resistance Rcoil, and the feedback resistor R1 is equal to the input resistor R2.

[0013] Furthermore, the multi-stage integrator adopts a 3rd order distributed feedback structure or a 2nd order distributed feedback structure.

[0014] Further, the third-order distributed feedback structure includes a first-order filter, a second-order filter and a third-order filter;

[0015] The first-stage filter includes a first subtractor, a feedforward amplifier K1, and a first integrator; the second-stage filter includes a second subtractor, a feedforward amplifier K2, and a second integrator; and the third-stage filter includes a third subtractor, a feedforward amplifier K3, and a third integrator;

[0016] The first subtractor, the feedforward amplifier K1, the first integrator, the second subtractor, the feedforward amplifier K2, the second integrator, the third subtractor, the feedforward amplifier K3, and the third integrator are connected in sequence, and the positive input end of the first subtractor is connected to the output end of the detection stage operational amplifier, the output end of the third integrator is connected to the input end of the quantizer, and the negative input end of the first subtractor, the negative input end of the second subtractor, and the negative input end of the third subtractor are respectively connected to the output end of the quantizer.

[0017] Further, the second-order distributed feedback structure includes a first-order filter and a second-order filter;

[0018] The first-stage filter includes a first subtractor, a feedforward amplifier K1, and a first integrator, and the second-stage filter includes a second subtractor, a feedforward amplifier K2, and a second integrator;

[0019] The first subtractor, the feedforward amplifier K1, the first integrator, the second subtractor, the feedforward amplifier K2, and the second integrator are connected in sequence, and the positive input end of the first subtractor is connected to the output end of the detection stage operational amplifier, the output end of the second integrator is connected to the input end of the quantizer, and the negative input end of the first subtractor and the negative input end of the second subtractor are respectively connected to the output end of the quantizer.

[0020] Further, the DAC includes an analog switch, and an input end of the analog switch is connected to an output end of the quantizer;

[0021] When the input digital signal is +1, the analog switch connected to the positive control voltage +Vfb is turned on; when the input digital signal is -1, the analog switch connected to the negative control voltage -Vfb is turned on.

[0022] Furthermore, the PID controller includes an adding circuit and three parallel amplification branches, the three amplification branches are a proportional amplifier, an integral amplifier, and a differential amplifier respectively;

[0023] One end of the proportional amplifier, the integral amplifier and the differential amplifier are all connected to the output end of the analog switch, and the other ends thereof are all connected to the three positive input ends of the adding circuit.

[0024] Further, the driving stage circuit includes a control stage operational amplifier;

[0025] The positive input terminal of the control stage operational amplifier is connected to the output terminal of the adding circuit, the negative input terminal of the control stage operational amplifier is connected to the other end of the coil resistor Rcoil, and the output terminal of the control stage operational amplifier is connected to the other end of the induced voltage Vcoil.

[0026] Furthermore, the quantizer is a 1-bit quantizer or a low-bit quantizer; the DAC and the quantizer correspond to a 1-bit DAC or a low-bit DAC.

[0027] Compared with the prior art, the advantages of the present invention are:

[0028] 1. The digital low-frequency seismic sensor of the present invention has the characteristics of directly outputting digital signals, adopting an analog-digital hybrid circuit system for sampling and quantization, and does not require an analog-digital conversion chip. It has the characteristics of good low-frequency response, compact structure, low power consumption, large dynamic range, and good linearity;

[0029] The pendulum system and electromagnetic transducer are connected to the detection-level circuit and the drive-level circuit, so that the pendulum relies on its original coil to detect the pendulum movement, convert the ground velocity into induced electromotive force, and apply feedback electromagnetic force to balance the movement trend of the pendulum, expand the low-frequency bandwidth of the mechanical system, and realize closed-loop feedback.

[0030] 2. The digital low-frequency seismic sensor of the present invention has good low-frequency response and adopts force balance technology to expand the low-frequency band of the pendulum system;

[0031] 3. The digital low-frequency seismic sensor of the present invention has strong compatibility, and the circuit system can be directly connected to the existing moving coil seismic detector or similar pendulum and transducer system to expand the low-frequency response and output digital signals.

[0032] 4. The multi-stage integrator in the sensor of the present invention can adopt a 3rd-order distributed feedback structure. The 3rd-order structure has the advantages of high-order noise shaping characteristics and low quantization noise;

[0033] The multi-stage integrator can also adopt a 2nd-order distributed feedback structure, which discards the third-pole filter in the 3rd-order distributed feedback structure. The structure is simpler, the power consumption is lower, and the system is easy to stabilize. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is a principle schematic diagram of the digital low-frequency seismic sensor of the present invention;

[0035] Figure 2 It is a circuit schematic diagram of a detection stage circuit, a driving stage circuit, and a PID controller in a digital low-frequency seismic sensor of the present invention;

[0036] Figure 3 It is a circuit schematic diagram of a multi-stage integrator, a quantizer, and a DAC in Embodiment 1 of the digital low-frequency seismic sensor of the present invention;

[0037] Figure 4 It is a circuit schematic diagram of a multi-stage integrator, a quantizer, and a DAC in Embodiment 2 of the digital low-frequency seismic sensor of the present invention;

[0038] The reference numerals are as follows:

[0039] 1-electromagnetic transducer, 2-PID controller, 21-adding circuit, 22-amplifying branch, 3-operational amplifier, 31-negative input terminal of the detection-level operational amplifier, 32-positive input terminal of the detection-level operational amplifier, 33-output terminal of the detection-level operational amplifier, 4-control-level operational amplifier, 41-negative input terminal of the control-level operational amplifier, 42-positive input terminal of the control-level operational amplifier, 43-output terminal of the control-level operational amplifier, 5-circuit reference point, 6-first-stage filter, 61-first subtractor, 62-first integrator, 7-second-stage filter, 71-second subtractor, 72-second integrator, 8-third-stage filter, 81-third subtractor, 82-third integrator, 9-quantizer, 10-analog switch. DETAILED DESCRIPTION

[0040] The present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0041] Embodiment 1

[0042] like Figure 1 As shown, a digital low-frequency seismic sensor includes a signal pickup unit and a digital feedback control unit; the signal pickup unit includes a pendulum system, an electromagnetic transducer detection stage circuit, a multi-stage integrator, and a quantizer 9, and the digital feedback control unit includes a drive stage circuit, a PID controller 2, and a DAC; a conventional moving coil seismic detector can be used as a pendulum system and an electromagnetic transducer 1, connected to the detection stage circuit and the drive stage circuit, so that the pendulum relies on its original single coil to detect the movement of the pendulum, and converts the ground velocity into an induced electromotive force, and applies feedback electromagnetic force to balance the movement trend of the pendulum and expand the low-frequency bandwidth of the mechanical system; in addition, the digital feedback control unit changes the analog circuit system used in the prior art into an analog-hybrid circuit system, and the operating frequency of the quantizer 9 in the sensor is much higher than the Nyquist frequency, so as to realize oversampling quantization of low-speed seismic signals, shape the quantization noise to outside the frequency band of the seismic signal, and complete the high-precision digital acquisition of the seismic signal.

[0043] like Figure 2 As shown, the electromagnetic transducer 1 is equivalent to an induced voltage Vcoil and a coil resistance Rcoil, and one end of the induced voltage Vcoil is connected to one end of the coil resistance Rcoil;

[0044] The detection level circuit is composed of an operational amplifier 3 which constitutes an inverting amplifier, and the output voltage is the negative value of the coil induced voltage, including the detection level operational amplifier 3 and the feedback resistor R1; the other end of the induced voltage Vcoil is connected to the negative input terminal 31 of the detection level operational amplifier through the input resistor R2, the two ends of the feedback resistor R1 are respectively connected to the negative input terminal 31 of the detection level operational amplifier and the output terminal 33 of the detection level operational amplifier, the positive input terminal 32 of the detection level operational amplifier is connected to the other end of the coil resistor Rcoil, the other end of the coil resistor Rcoil is connected to one end of the matching resistor R3, and the other end of the matching resistor R3 is grounded.

[0045] The working principle of the detection stage circuit is as follows: the electromagnetic transducer 1 is equivalent to an induced voltage Vcoil connected in series with a coil resistance Rcoil, which is connected to the negative input terminal 31 of the detection stage operational amplifier through an input resistor R2, and a feedback resistor R1 is connected across the negative input terminal and the output terminal to form a reverse amplification circuit. The reverse circuit reference point 5 is connected to the positive input terminal of the operational amplifier 33 and is connected to the matching resistor R3, and the matching resistor R3 is grounded GND, wherein the circuit reference point 5 indicates that the two operational amplifiers and the transducer can be used as the circuit reference point during circuit calculation; let the matching resistor R3 take a value equal to the coil resistance Rcoil, and the feedback resistor R1 equal to the input resistance R2. According to the characteristics of the operational amplifier 3, the output resistance of the detection stage circuit is the negative value of the induced voltage, that is, Vout = -Vcoil.

[0046] like Figure 3 As shown, the multi-stage integrator adopts a 3rd-order distributed feedback structure, and the 3rd-order distributed feedback structure includes a first-stage filter 6, a second-stage filter 7 and a third-stage filter 8; the first-stage filter 6 includes a first subtractor 61, a feedforward amplifier K1, and a first integrator 62, the second-stage filter 7 includes a second subtractor 71, a feedforward amplifier K2, and a second integrator 72, and the third-stage filter 8 includes a third subtractor 81, a feedforward amplifier K3, and a third integrator 82;

[0047] The first subtractor 61, the feedforward amplifier K1, the first integrator 62, the second subtractor 71, the feedforward amplifier K2, the second integrator 72, the third subtractor 81, the feedforward amplifier K3, and the third integrator 82 are connected in sequence, and the positive input of the first subtractor 61 is connected to the output 33 of the detection stage operational amplifier, the output of the third integrator 82 is connected to the input of the quantizer 9, and the negative input of the first subtractor 61, the negative input of the second subtractor 71, and the negative input of the third subtractor 81 are respectively connected to the output of the quantizer 9.

[0048] The working principle of the multi-stage integrator is as follows: the sample-and-hold device converts the voltage Vout output by the detection stage circuit into a discrete time signal under the synchronization of the clock, and the discrete time signal enters the first subtractor 61, the feedforward amplifier K1, the first integrator 62, the second subtractor 71, the feedforward amplifier K2, the second integrator 72, the third subtractor 81, the feedforward amplifier K3, and the third integrator 82 in sequence. The cascade output outputs a digital bit stream +1 or -1 through the 1-bit quantizer 9, and the digital bit stream is fed back at the first subtractor 61, the second subtractor 71, and the third subtractor 81 through the distributed feedback branch. The 3-order structure has high-order noise shaping characteristics and has the advantages of low quantization noise, but has high power consumption and requires special adjustment of system parameters to ensure the closed-loop stability of the system.

[0049] The quantizer 9 can be implemented using a 1-bit quantizer or a low-bit quantizer. This embodiment adopts a 1-bit quantizer implementation method. The 1-bit quantizer can be implemented by a clock-synchronized comparator circuit. When the input analog signal is greater than the reference voltage, it outputs a digital high level +1, otherwise it outputs a digital low level -1, which is the digital bit stream output by the seismic sensor of this application.

[0050] DAC is a digital-to-analog conversion circuit that completes the conversion of input digital quantity to pulse modulated analog voltage signal. The selection of the DAC should be matched with the quantizer 9. Suitable for the above-mentioned 1-bit quantizer, this embodiment provides an embodiment using a 1-bit DAC, the quantizer is implemented by an analog switch 10, when the input digital signal is +1, the analog switch 10 connected to the positive feedback voltage +Vfb is turned on and outputs the positive feedback voltage, when the input digital signal is -1, the analog switch 10 connected to the negative control voltage -Vfb is turned on and outputs the negative feedback voltage, and its output is a pulse width modulated voltage signal proportional to the digital bit stream.

[0051] The PID controller 2 is a proportional-integral-differential controller, including a control branch and an adding circuit 21 formed by three amplifying branches 22 in parallel. The three amplifying branches 22 are a proportional amplifier, an integral amplifier, and a differential amplifier respectively; one end of the proportional amplifier, the integral amplifier, and the differential amplifier are connected to the output end of the analog switch 10, and the other end thereof are connected to the three positive input ends of the adding circuit 21.

[0052] The pulse width modulated voltage generated by the digital bit stream passing through the 1-bit DAC is the input signal of the PID controller 2. The PID controller 2 is composed of three parallel signals, namely proportional, differential and integral. The three signals respectively calculate the pulse width modulated voltage and become the control voltage Vctrl after being summed up by the adding circuit 21.

[0053] The driving stage circuit uses the transducer coil as the feedback resistor of the control stage operational amplifier 4, driving the coil current, which is proportional to the input voltage of the driving stage. The driving stage circuit includes the control stage operational amplifier 4;

[0054] The control voltage Vctrl is connected to the positive input terminal 42 of the control-stage operational amplifier, the negative input terminal 41 of the control-stage operational amplifier is connected to the other end of the coil resistor Rcoil (circuit reference point 5), and the output terminal 43 of the control-stage operational amplifier is connected to the input resistor R2 of the detection-stage circuit and is connected to the electromagnetic transducer 1; according to the characteristics of the operational amplifier 3, the voltage at the circuit reference point 5 is equal to the control voltage Vctrl, the current Icoil flowing through the coil resistor Rcoil is Vctrl / R3, and the electromagnetic force is proportional to the coil current Icoil and the control voltage.

[0055] Principle of the digital low-frequency seismic sensor in this embodiment:

[0056] When the external environment applies vibration to the pendulum system, the pendulum deviates from the equilibrium position due to inertia and is converted into a voltage signal by the electromagnetic transducer 1; the voltage signal is reversely amplified by the detection stage circuit and enters the multi-stage integrator for sampling, holding and integral filtering. The filtered signal is compared by a 1-bit quantizer 9, and if it is greater than the threshold, +1 is output (if the voltage signal is greater than the threshold, the quantizer 9 outputs a digital high level +1), otherwise -1 is output (if the voltage signal is less than or equal to the threshold, the quantizer 9 outputs a digital low level -1). The binary bit stream is a digital output, synchronized with the sampling clock, and serves as the input signal of the feedback control part.

[0057] The 1-bit DAC is connected to the output of the quantizer 9, and is used to generate a pulse width modulated voltage from the digital bit stream output by the quantizer 9. Specifically, when the bit stream is +1, the 1-bit DAC outputs a positive voltage, and vice versa. The digital pulse width modulated signal is conditioned into a control voltage by the PID controller 2, and converted into a coil current of the electromagnetic transducer 1 by the drive stage circuit. The coil current is converted into an electromagnetic force through a magnetic field, which is used to balance the movement trend of the pendulum, so that the movement of the pendulum relative to the stationary reference system is extremely small. Under the action of this feedback force, the mechanical characteristics of the pendulum system are changed, the lower cutoff frequency is expanded, the low-frequency response of the system is improved, and the maximum detectable vibration amplitude of the system is increased.

[0058] The signal pickup part and the digital feedback control part of the present application are implemented by using an analog-digital hybrid high-order sum-delta modulator circuit system, which has a digital circuit in addition to the prior art using an analog circuit system. Under this structure, the sensor of the present application has the advantage of directly outputting digital signals: no data acquisition device is required for analog-to-digital conversion, low power consumption, easy storage of digital signals, anti-interference and crosstalk between channels, and low attenuation suitable for long-distance transmission.

[0059] Embodiment 2

[0060] The difference from the first embodiment is that Figure 4As shown, the multi-stage integrator adopts a 2nd-order distributed feedback structure;

[0061] The second-order distributed feedback structure includes a first-stage filter 6 and a second-stage filter 7; the first-stage filter 6 includes a first subtractor 61, a feedforward amplifier K1, and a first integrator 62, and the second-stage filter 7 includes a second subtractor 71, a feedforward amplifier K2, and a second integrator 72;

[0062] The first subtractor 61, the feedforward amplifier K1, the first integrator 62, the second subtractor 71, the feedforward amplifier K2, and the second integrator 72 are connected in sequence, and the positive input of the first subtractor 61 is connected to the output 33 of the detection stage operational amplifier, the output of the second integrator 72 is connected to the input of the quantizer 9, and the negative input of the first subtractor 61 and the negative input of the second subtractor 71 are respectively connected to the output of the quantizer 9.

[0063] The working principle of the multi-stage integrator is as follows: the sample-and-hold device converts the voltage Vout output by the detection stage circuit into a discrete time signal under the synchronization of the clock, and the discrete time signal enters the first subtractor 61, the feedforward amplifier K1, the first integrator 62, the second subtractor 71, the feedforward amplifier K2, and the second integrator 72 in sequence. The cascade output outputs a digital bit stream +1 or -1 through the 1-bit quantizer 9, and the digital bit stream is fed back at the first subtractor 61 and the second subtractor 71 through the distributed feedback branch. The third-order filter in the third-order structure is discarded, so the second-order structure is relatively simple, the power consumption is low, and the system is easy to stabilize.

[0064] The above is only a description of the preferred implementation mode of the present invention, and the technical solution of the present invention is not limited thereto. Any known deformation made by those skilled in the art on the basis of the main technical concept of the present invention belongs to the technical scope to be protected by the present invention.

Claims

1. A digital low-frequency seismic sensor, characterized in that: It includes a signal pickup unit and a digital feedback control unit; The signal pickup unit comprises a pendulum system, an electromagnetic transducer (1), a detection stage circuit, a multi-stage integrator, and a quantizer (9); The pendulum system senses vibration and generates a vibration signal, which is converted into a voltage signal by an electromagnetic transducer (1); the voltage signal is reversely amplified by a detection circuit, enters a multi-stage integrator, and is sampled, held, integrated, and filtered; The filtered voltage signal is converted into digital form by a quantizer (9). If the voltage signal is greater than a threshold value, the quantizer (9) outputs a digital high level +1; if the voltage signal is less than or equal to the threshold value, the quantizer (9) outputs a digital low level -1. The digital feedback control unit comprises a drive stage circuit, a PID controller (2), and a DAC; The DAC is connected to the output of the quantizer (9) and is used to generate a pulse width modulated voltage from the digital bit stream output by the quantizer (9); the pulse width modulated voltage is conditioned into a control voltage by the PID controller (2), and is converted into a coil current of the electromagnetic transducer (1) by the drive stage circuit; the coil current is converted into an electromagnetic force by the electromagnetic transducer (1) and is used to balance the movement trend of the pendulum system.

2. The digital low-frequency seismic sensor according to claim 1, characterized in that: The electromagnetic transducer (1) comprises an induced voltage Vcoil and a coil resistance Rcoil, one end of the induced voltage Vcoil is connected to one end of the coil resistance Rcoil; The detection stage circuit comprises a detection stage operational amplifier (3) and a feedback resistor R1; The other end of the induced voltage Vcoil is connected to the negative input terminal (31) of the detection stage operational amplifier via the input resistor R2, the two ends of the feedback resistor R1 are respectively connected to the negative input terminal (31) of the detection stage operational amplifier and the output terminal (33) of the detection stage operational amplifier, the positive input terminal (32) of the detection stage operational amplifier is connected to the other end of the coil resistor Rcoil, the other end of the coil resistor Rcoil is connected to one end of the matching resistor R3, and the other end of the matching resistor R3 is grounded.

3. The digital low-frequency seismic sensor according to claim 2, characterized in that: The matching resistor R3 is equal to the coil resistance Rcoil, and the feedback resistor R1 is equal to the input resistor R2.

4. The digital low-frequency seismic sensor according to claim 3, characterized in that: The multi-stage integrator adopts a 3rd order distributed feedback structure or a 2nd order distributed feedback structure.

5. The digital low-frequency seismic sensor according to claim 4, characterized in that: The third-order distributed feedback structure comprises a first-order filter (6), a second-order filter (7) and a third-order filter (8); The first-stage filter (6) comprises a first subtractor (61), a feedforward amplifier K1, and a first integrator (62); the second-stage filter (7) comprises a second subtractor (71), a feedforward amplifier K2, and a second integrator (72); and the third-stage filter (8) comprises a third subtractor (81), a feedforward amplifier K3, and a third integrator (82); The first subtractor (61), the feedforward amplifier K1, the first integrator (62), the second subtractor (71), the feedforward amplifier K2, the second integrator (72), the third subtractor (81), the feedforward amplifier K3, and the third integrator (82) are connected in sequence, and the positive input end of the first subtractor (61) is connected to the output end (33) of the detection stage operational amplifier, the output end of the third integrator (82) is connected to the input end of the quantizer (9), and the negative input end of the first subtractor (61), the negative input end of the second subtractor (71), and the negative input end of the third subtractor (81) are respectively connected to the output end of the quantizer (9).

6. The digital low-frequency seismic sensor according to claim 4, characterized in that: The second-order distributed feedback structure comprises a first-order filter (6) and a second-order filter (7); The first-stage filter (6) comprises a first subtractor (61), a feedforward amplifier K1, and a first integrator (62); the second-stage filter (7) comprises a second subtractor (71), a feedforward amplifier K2, and a second integrator (72); The first subtractor (61), the feedforward amplifier K1, the first integrator (62), the second subtractor (71), the feedforward amplifier K2, and the second integrator (72) are connected in sequence, and the positive input end of the first subtractor (61) is connected to the output end (33) of the detection stage operational amplifier, the output end of the second integrator (72) is connected to the input end of the quantizer (9), and the negative input end of the first subtractor (61) and the negative input end of the second subtractor (71) are respectively connected to the output end of the quantizer (9).

7. The digital low-frequency seismic sensor according to claim 5 or 6, characterized in that: The DAC comprises an analog switch (10), wherein an input end of the analog switch (10) is connected to an output end of the quantizer (9); When the input digital signal is +1, the analog switch (10) connected to the positive control voltage +Vfb is turned on; when the input digital signal is -1, the analog switch (10) connected to the negative control voltage -Vfb is turned on.

8. The digital low-frequency seismic sensor according to claim 7, characterized in that: The PID controller (2) comprises an adding circuit (21) and three parallel-connected amplifying branches (22), wherein the three amplifying branches (22) are respectively a proportional amplifier, an integral amplifier, and a differential amplifier; One end of the proportional amplifier, the integral amplifier, and the differential amplifier are all connected to the output end of the analog switch (10), and the other ends thereof are all connected to the three positive input ends of the adding circuit (21).

9. The digital low-frequency seismic sensor according to claim 8, characterized in that: The driving stage circuit comprises a control stage operational amplifier (4); The positive input terminal (42) of the control-stage operational amplifier is connected to the output terminal of the adding circuit (21), the negative input terminal (41) of the control-stage operational amplifier is connected to the other end of the coil resistor Rcoil, and the output terminal (43) of the control-stage operational amplifier is connected to the other end of the induced voltage Vcoil.

10. The digital low-frequency seismic sensor according to claim 1, characterized in that: The quantizer is a low-bit quantizer; DAC corresponds to quantizer as low-bit DAC.

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