Outer well lid state detection device and method

Abstract

The invention belongs to the technical field of well lid detection, and particularly relates to an outer well lid state detection device and method, and the device comprises an MCU microprocessor, an electromagnetic signal transmitting coil, an electromagnetic signal receiving coil, and a signal conditioning circuit. The MCU microprocessor is connected with the electromagnetic signal transmitting coil and is used for generating a PWM (Pulse Width Modulation) driving signal and controlling the electromagnetic signal transmitting coil to transmit an electromagnetic wave with a set frequency; the electromagnetic signal receiving coil is connected with the signal conditioning circuit and is used for receiving the electromagnetic signal modulated by the manhole cover metal body; the signal conditioning circuit is connected with the MCU microprocessor and is used for conditioning the received signal and sending the signal to the MCU microprocessor for analog-to-digital conversion and analysis; the MCU microprocessor is configured to analyze abnormal frequency and net amplitude caused by eddy current generated by a metal body of the well lid based on the frequency spectrum characteristics of the received signals, judge the in-place, loose or lost state of the well lid according to the pre-calibrated corresponding relation between the net amplitude of the abnormal frequency and the distance of the metal body, and detect the in-place state of the well lid in a non-contact mode.

Description

Technical Field

[0001] This invention belongs to the field of manhole cover detection technology, specifically a device and method for detecting the condition of external manhole covers. Background Technology

[0002] The statements in this section merely refer to the background art related to this invention and do not necessarily constitute prior art.

[0003] Municipal manhole covers are an important part of urban infrastructure and are widely distributed in public areas such as roads and sidewalks. Common types of manhole covers include power cable manholes, telecommunications maintenance manholes, water supply and drainage manholes, gas valve manholes, and heating pipeline manholes. Their function is to enclose underground utility tunnels and ensure the safe operation of internal facilities and public passage safety.

[0004] When manhole covers are stolen, damaged by heavy vehicles, or displaced or loosened due to foundation settlement, the resulting "traps" pose a safety threat to pedestrians and vehicles. Furthermore, damaged manhole covers expose underground facilities to rainwater, dust, and even human-caused damage, accelerating equipment aging. For specialized facilities such as cable wells and communication wells, this can lead to short circuits, signal interruptions, or even safety accidents due to foreign objects falling in or unauthorized personnel entering.

[0005] In order to detect the damage to manhole covers in a timely manner, existing technologies attempt to integrate various sensors on the manhole covers and use the signals emitted to determine the condition of the manhole covers.

[0006] For example: A mechanical trigger switch is installed under the manhole cover. When the manhole cover is removed, the switch changes state and triggers an alarm. This method is simple in structure, but it suffers from problems such as mechanical wear, susceptibility to corrosion, and false alarms caused by vibration or slight displacement, resulting in poor long-term reliability.

[0007] Installing tilt / accelerometer sensors on manhole covers to detect their posture or movement triggers an alarm. This method requires an independent power supply and communication module for each cover, making it costly, and the sensors themselves are exposed to harsh environments, raising concerns about durability.

[0008] Passive Radio Frequency Identification (RFID) technology is used: electronic tags are installed on manhole covers, and inspection personnel use handheld devices to read them to confirm their presence. This method cannot achieve real-time monitoring and is a reactive inspection method with limited detection effectiveness.

[0009] In summary, existing sensors are typically directly connected to manhole covers, exposing them to extreme mechanical and climatic stresses along with the covers, making it difficult to guarantee their lifespan and reliability. The harsh environment underground, characterized by dampness, dust, and corrosive gases, along with continuous vibrations and impacts from the road surface, negatively impacts the long-term stable operation of most electronic sensors.

[0010] Secondly, most existing solutions can only detect obvious changes in condition such as "in place / missing" or "overturning," and lack the sensitive and reliable early identification capability for risks that gradually change, such as loosening or settling of manhole covers. Summary of the Invention

[0011] This invention provides a device and method for detecting the status of manhole covers, achieving low-power, high-reliability status monitoring of manhole covers. By installing miniature sensors underground, the on-site status of the manhole cover is detected in a non-contact manner.

[0012] The first aspect of the present invention discloses an external manhole cover status detection device, including an MCU microprocessor, an electromagnetic signal transmitting coil, an electromagnetic signal receiving coil, and a signal conditioning circuit; The MCU microprocessor is connected to the electromagnetic signal transmitting coil to generate a PWM drive signal, which controls the electromagnetic signal transmitting coil to emit electromagnetic waves at a set frequency. The electromagnetic signal receiving coil is connected to the signal conditioning circuit to receive the electromagnetic signal modulated by the metal body of the manhole cover; the signal conditioning circuit is connected to the MCU microprocessor to condition the received signal and send it to the MCU microprocessor for analog-to-digital conversion and analysis. The MCU microprocessor is configured to: analyze the abnormal frequency and its net amplitude caused by the eddy current generated by the metal body of the manhole cover based on the spectral characteristics of the received signal, and determine the status of the manhole cover being in place, loose, or missing according to the pre-calibrated correspondence between the net amplitude of the abnormal frequency and the distance to the metal body.

[0013] Furthermore, both the electromagnetic signal transmitting coil and the electromagnetic signal receiving coil adopt standardized miniature patch antenna coils.

[0014] Furthermore, the MCU microprocessor drives the electromagnetic signal transmitting coil through the electromagnetic wave transmitting circuit; The electromagnetic wave transmitting circuit includes a switching transistor, a current sampling resistor, a gate drive resistor, a pull-up resistor, and a decoupling capacitor; One end of the electromagnetic signal transmitting coil is connected to the power supply, and the other end is connected to the main current input terminal of the switching transistor via a current sampling resistor; The main current output terminal of the switching transistor is grounded, and its control terminal is connected to the PWM output pin of the MCU microprocessor via the gate drive resistor; The pull-up resistor is connected between the control terminal of the switching transistor and ground, and the decoupling capacitor is connected between the control terminal of the switching transistor and ground.

[0015] Furthermore, the signal conditioning circuit is an instrumentation amplifier; The electromagnetic signal receiving coil is connected to the differential input terminal of the instrumentation amplifier, and a gain resistor is connected between the gain setting pins of the instrumentation amplifier to set the amplification factor. The output of the instrumentation amplifier is connected to the output node via a coupling capacitor. The output node is grounded via a pull-down resistor and connected to the ADC input pin of the MCU microprocessor.

[0016] Furthermore, the positive power supply pin of the instrumentation amplifier is connected to the power supply and grounded via the first decoupling capacitor, while the negative power supply pin is directly grounded. The reference voltage pin of the instrumentation amplifier is connected between the power supply and ground through a symmetrical resistor divider network to set the reference voltage to half of the power supply voltage.

[0017] Furthermore, the duty cycle of the PWM drive signal output by the MCU microprocessor is not greater than a set value, which is set to 1:100 in this scheme.

[0018] Furthermore, the MCU microprocessor is also configured to perform the following steps to obtain the net amplitude of the abnormal frequency: S1: Perform analog-to-digital conversion and digital bandpass filtering on the conditioned received signal to obtain the target frequency band signal; S2: Perform a fast Fourier transform on the target frequency band signal to obtain its spectrum; S3: Estimated background noise floor amplitude in the spectrum; S4: Within the preset anomaly detection frequency band, search for the peak frequency of the spectrum whose amplitude is significantly higher than the noise floor amplitude, and record its frequency as the anomaly frequency; S5: Calculate the difference between the original amplitude at the peak of the spectrum and the amplitude of the noise floor to obtain the net amplitude of the abnormal frequency.

[0019] Furthermore, the MCU microprocessor is also configured to perform the following status determination steps: The calculated net amplitude of the abnormal frequency is compared with the preset threshold range; If the net amplitude is within the first preset range, the manhole cover is determined to be in a normal position. If the net amplitude is within the second preset range, the manhole cover is determined to be in a loose or tilted warning state based on the direction and duration of its deviation from the first preset range. If the net amplitude is lower than the third preset threshold, the manhole cover is determined to be in a lost or stolen alarm state.

[0020] Furthermore, the MCU microprocessor uses a continuous confirmation mechanism of "N samplings, M confirmations" to determine the final state, where N and M are preset positive integers, and M ≤ N.

[0021] A second aspect of the present invention discloses a method for detecting the condition of an external manhole cover, comprising the following steps: Drive the transmitting coil to emit electromagnetic waves of a specific frequency; Receive and modulate electromagnetic signals modulated by the manhole cover; Spectral analysis of the received signal was performed to extract the abnormal frequencies and their net amplitudes generated by the eddy current effect of the manhole cover's metal body. Based on the pre-defined correlation between the net amplitude of abnormal frequencies and the distance to the manhole cover, it can be determined whether the manhole cover is currently in a normal position, loose and tilted, or lost or stolen.

[0022] Compared with existing technologies, one or more of the above technical solutions have the following beneficial effects: 1. This solution, which is non-contact, resistant to harsh environments, and easily deployed on a large scale, solves the problems of sensors and manhole covers being damaged together and high deployment costs. The device is independently installed in the manhole wall or well, without direct physical connection to the manhole cover, avoiding the impact of crushing, shock, and corrosion on the sensor along with the cover, and overcoming the lifespan and reliability issues caused by sensor exposure to extreme environments. By employing a miniature antenna coil and a pulse drive method with an extremely low duty cycle, it is small in size, consumes very little power, supports long-term battery power, and eliminates the need to lay cables for each manhole cover, enabling large-scale, low-cost deployment with a "plug and play" approach.

[0023] 2. The device achieves precise sensing of manhole cover status, moving beyond simply detecting "presence / absence" to assessing "distance changes," thus addressing the insufficient early identification capability for progressive risks such as loosening and subsidence. Utilizing the eddy current effect, the device extracts characteristic signals reflecting the distance to the metal manhole cover through spectral analysis and makes judgments based on their calibration relationship with distance. This allows detection to move beyond simple binary "present / missing" judgments, enabling sensitive detection of millimeter-level distance changes caused by loosening, tilting, or subsidence. This provides early warnings before the manhole cover completely fails, achieving a leap from "post-event alarm" to "pre-event warning."

[0024] 3. In signal processing, environmental noise is removed by calculating the "net amplitude," and a continuous confirmation mechanism of "N samplings, M confirmations" combined with signal trend analysis is employed. This effectively distinguishes between transient interference caused by vehicles passing by momentarily and continuous signal characteristic changes resulting from actual theft or loosening of manhole covers, thus greatly reducing the false alarm rate. Anti-interference judgment logic ensures high reliability of alarm information and reduces unnecessary maintenance verification. Attached Figure Description

[0025] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0026] Figure 1 This is a schematic diagram of the principle of an external manhole cover detection device provided in one or more embodiments of the present invention; Figure 2 A schematic diagram of the electromagnetic wave transmitting circuit of the external manhole cover detection device provided in one or more embodiments of the present invention; Figure 3 A schematic diagram of the electromagnetic wave receiving circuit of an external manhole cover detection device provided in one or more embodiments of the present invention; Figure 4 A top view of the transmitting coil / receiving coil provided in one or more embodiments of the present invention; Figure 5 A side view of the transmitting coil / receiving coil provided in one or more embodiments of the present invention; Figure 6 This is a bottom view of the transmitting / receiving coil structure provided in one or more embodiments of the present invention. Detailed Implementation

[0027] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0028] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, 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 invention pertains.

[0029] As described in the background section, existing sensors are typically directly connected to manhole covers, exposing them to extreme mechanical and climatic stresses, making it difficult to guarantee their lifespan and reliability. The harsh environment underground, characterized by dampness, dust, and corrosive gases, along with continuous vibrations and impacts from the road surface, negatively affect the long-term stable operation of most electronic sensors.

[0030] Secondly, most existing solutions can only detect obvious changes in condition such as "in place / missing" or "overturning," and lack the sensitive and reliable early identification capability for risks that gradually change, such as loosening or settling of manhole covers.

[0031] To improve the working environment of sensors, a detection scheme based on the eddy current effect can be explored, thereby avoiding direct connection between the electronic sensor and the manhole cover. This type of scheme utilizes the principle of electromagnetic induction, installing a detection coil underground, and determining the state of the manhole cover (a metal body) by monitoring its influence on the electromagnetic field, thus forming a non-contact monitoring method.

[0032] However, when applied to underground anti-theft scenarios, existing eddy current detection solutions often employ large-sized wound coils and require large drive currents to achieve sufficient detection distance (typically from the bottom of the well to the top), resulting in high overall power consumption and difficulty in meeting the constraints of long-term battery power supply. If the coil is reduced in size to decrease power consumption, the detection distance and signal-to-noise ratio will drop sharply.

[0033] The electrical parameters of the wound coil (such as inductance and Q value) are greatly affected by the winding process and material batches, resulting in performance dispersion of different detection units. This requires complex on-site calibration for each unit, which severely restricts the efficiency of large-scale standardized production and deployment.

[0034] While eddy current detection solutions avoid the lifespan impact caused by direct contact between the downhole environment and electronic sensors (coils), the downhole environment presents complex electromagnetic interference (such as in cable wells or communication wells). Traditional solutions have simple signal conditioning circuits, insufficient anti-interference capabilities, and are prone to false alarms due to temperature drift, humidity changes, or interference from nearby metals, making reliability difficult to guarantee.

[0035] Furthermore, most solutions rely solely on a simple threshold to detect the amplitude of the received signal to determine the presence or absence of a manhole cover. This method is highly susceptible to baseline drift caused by slow environmental changes (such as aging of coil packaging materials or water accumulation in the manhole), and may also produce false alarms due to momentary strong interference (such as the passage of large metal vehicles). It lacks in-depth analysis and utilization of the essential characteristics of the signal.

[0036] Therefore, this solution provides a device and method for detecting the status of external manhole covers. It uses a pair of miniature standardized eddy current coils to emit electromagnetic waves with extremely low duty cycle pulses. By analyzing the characteristic frequencies and their intensities generated by the eddy currents of the manhole cover in the received signal spectrum and comparing them with a pre-calibrated "distance-amplitude" curve, it can accurately determine whether the manhole cover is in place, thereby achieving theft prevention.

[0037] like Figure 1 As shown, an external manhole cover status detection device includes an MCU microprocessor, an electromagnetic signal transmitting coil, an electromagnetic signal receiving coil, and a signal conditioning circuit.

[0038] The MCU microprocessor is an STM32L431V, which is connected to the electromagnetic signal transmitting coil. The electromagnetic signal transmitting coil is driven by PWM to generate a natural signal of a set frequency, which is used to detect whether there is a metal object in the surrounding area (assuming that the manhole cover is metal or contains a metal frame).

[0039] The MCU microprocessor is also connected to an electromagnetic signal receiving coil. The electromagnetic signal receiving coil can convert the analog signal of the ADC into a digital signal through the signal conditioning circuit and peripheral circuit. The digital signal is then transmitted to the MCU microprocessor to obtain the intensity and frequency of the electromagnetic field.

[0040] The MCU microprocessor generates a PWM signal through an electromagnetic wave transmitting circuit, which in turn drives an electromagnetic signal transmitting coil to produce electromagnetic waves at a frequency of 5kHz. The structure of the electromagnetic wave transmitting circuit is as follows: Figure 2As shown, the VCC33 terminal is connected to the electromagnetic wave emitting coil. The other end of the electromagnetic wave emitting coil is connected in series with a current sampling resistor and then connected to the third terminal (drain or collector C) of the switching transistor (such as MOSFET). The second terminal (source or emitter E) of the switching transistor is grounded. The first terminal (gate or base B) of the switching transistor is connected to the gate drive resistor. The other end of the gate drive resistor is connected to a pull-up resistor and a capacitor respectively. The other end of the pull-up resistor is connected to the L-Ctrl pin of the MCU, and the other end of the capacitor is grounded.

[0041] The conduction process of the above circuit is as follows: the L-Ctrl pin of the MCU outputs a low level (0V). At this time, the pull-up resistor pulls the current to the L-Ctrl terminal, making the MOSFET gate voltage low, the MOSFET cut off, and the coil is not energized.

[0042] The transmission process is as follows: the MCU's L-Ctrl pin outputs a high level (3.3V). This high level charges the gate capacitance of the MOSFET through the gate resistor. When the gate voltage exceeds its threshold voltage Vgs(th), the MOSFET quickly turns on. The current path is formed as follows: VCC33 → transmitting coil → sampling / damping resistor → MOSFET drain to source → ground. Current flows through the coil, establishing a magnetic field.

[0043] The PWM control process involves rapidly switching the high and low levels of L-Ctrl (PWM wave) to control the on-off time ratio (duty cycle) of the current in the coil, thereby controlling the average transmit power and achieving the goal of "1:100 duty cycle to reduce power consumption".

[0044] The electromagnetic signal receiving coil receives electromagnetic waves, which are then converted into a 0-3V voltage by the signal conditioning circuit and input into the ADC analog-to-digital converter. The MCU microprocessor obtains the intensity and frequency of the electromagnetic field by reading the ADC value.

[0045] To adapt to the downhole environment, the coil size is kept as small as possible. Furthermore, to improve production and assembly consistency, this embodiment uses a SUMIDA antenna coil designed for the 0-125kHz low-frequency range as the transmitter. It features a small 9mm x 9mm size, ultra-high sensitivity, and an external structure as shown below. Figures 4-6 As shown.

[0046] Battery powered, a 1:100 PWM waveform drives a transistor amplifier circuit to control the coil's transmit power to reduce power consumption. To ensure consistency between transmitted and received signals, and to improve consistency in production and assembly, the same SUMIDA antenna coil used in the 0-125kHz low-frequency range is employed as the receiver. It features a compact 9mm x 9mm size, ultra-high sensitivity, and a shape as shown in the image. Figures 4-6 As shown.

[0047] Signal conditioning: The signal received by the SUMIDA coil is a millivolt-level signal (the amplitude is not exactly the same depending on the distance). The received signal is amplified by about 100 times by the instrumentation amplifier in the electromagnetic wave receiving circuit, so that the signal amplitude reaches the range of 0-3V.

[0048] like Figure 3 As shown, the electromagnetic wave receiving circuit includes an electromagnetic wave receiving coil connected to the -IN and +IN pins of the instrumentation amplifier. The electromagnetic wave receiving coil is connected in parallel with a first resistor and a first capacitor. A second resistor is connected between the RG1 and RG2 pins of the instrumentation amplifier to set the amplification factor.

[0049] The +Vs pin of the instrumentation amplifier is connected to the VCC terminal and simultaneously connected to the GND terminal via a third capacitor to power the chip and filter out power supply noise. The -Vs pin of the instrumentation amplifier is directly connected to the GND terminal.

[0050] The REF pin of the instrumentation amplifier is connected to the VCC terminal through a fourth resistor and to the GND terminal through a fifth resistor. The resistance values ​​of the fourth and fifth resistors are equal, thereby stabilizing the REF pin potential at VCC / 2 and providing a DC bias midpoint for the output signal.

[0051] The Vout pin of the instrumentation amplifier outputs a signal that is connected to the output terminal VOUT via a second capacitor in series to isolate the DC component and output an AC detection signal. It is also connected to the GND terminal via a sixth resistor to provide a DC bias circuit for the output. The VOUT terminal is also connected to GND via a third resistor, serving as both the output load and a pull-down resistor.

[0052] Under normal circumstances, when there is no metal body around the coil, the magnetic field strength and frequency received by the electromagnetic signal receiving coil are basically the same as the magnetic field strength and frequency emitted by the electromagnetic transmitting coil.

[0053] In abnormal situations, when a metallic object appears around the coil, the magnetic field strength and frequency received by the electromagnetic signal receiving coil should differ from those emitted by the electromagnetic transmitting coil. By analyzing the abnormal frequency of the periodic signal using Fourier analysis, the presence and composition of the metallic object can be determined. The distance to the metallic object can be analyzed by the intensity of the abnormal frequency.

[0054] Under normal circumstances, when an electromagnetic wave emits an electromagnetic wave of a certain frequency, the periodic analog signal received by the electromagnetic receiving circuit can be separated into the same frequency signal after Fourier transform. At this time, the emitted electromagnetic wave is not interfered with, which means that there is no metal body nearby. When a metal object approaches an electromagnetic coil, the metal object generates eddy currents, which in turn generate a reverse electromagnetic field. At the same time, the frequency of the eddy current magnetic field generated by the metal varies depending on the material of the metal. There is a unique correspondence between the material of the metal and the abnormal frequency.

[0055] If the electromagnetic receiving circuit detects the presence of an abnormal frequency, it can be determined that a metal object is approaching.

[0056] The electromagnetic receiving circuit analyzes that the amplitude (i.e. intensity) of the abnormal frequency has a linear relationship with the distance to the metal body, and distance estimation is achieved through calibration.

[0057] Before the device is installed, a curve (or table) showing the relationship between "signal strength (abnormal frequency amplitude) - distance to manhole cover" is established through experiments using manhole cover samples of the same material. This curve will be used for distance estimation.

[0058] After the device is installed and the manhole cover is confirmed to be properly closed, a reference learning is performed to collect and analyze the received signal spectrum at this time and store it as a "reference spectrum".

[0059] This solution can adopt the following detection strategy, including the following steps: Step 1: Signal Acquisition and Preprocessing; Step 2: Spectral Feature Extraction; Step 3: Status Judgment Logic (Theft Prevention and Loosening); Step 4: Anti-interference and confirmation mechanism.

[0060] The following are the specific steps.

[0061] Step 1: Signal acquisition and preprocessing.

[0062] The electromagnetic transmitting coil emits an electromagnetic signal at a set frequency (set to f_tx), and the voltage signal V_adc[n] output by the electromagnetic receiving coil is obtained through the electromagnetic receiving circuit.

[0063] Digital bandpass filtering (e.g., center frequency 5kHz, bandwidth 1kHz) is applied to V_adc[n] to remove power frequency interference and environmental noise, resulting in a clean AC signal V_filtered[n].

[0064] Step 2: Spectral feature extraction.

[0065] Perform a Fast Fourier Transform (FFT) on the filtered signal V_filtered[n] to obtain its spectral characteristics Spectrum[f].

[0066] Determine the amplitude A_main at the transmission frequency f_tx; Analyze the received signal spectrum Spectrum[f], and calculate the background noise floor amplitude A_noise_floor in the frequency band far from the transmission frequency f_tx; Search for abnormal peaks that meet the following conditions within a preset detection frequency band (e.g., f_tx±Δf): the frequency point is a local maximum value and its amplitude is higher than A_noise_floor (i.e., Spectrum[f]>K×A_noise_floor, where K is a preset threshold). The peak frequency is denoted as the anomalous frequency f_anomaly, and its net anomalous amplitude A_anomaly = Spectrum[f_anomaly] - A_noise_floor is calculated.

[0067] Among them, the frequency f_anomaly can be used to assist in material judgment, and the net anomaly amplitude A_anomaly reflects the intensity of the eddy current field.

[0068] Step 3: State judgment logic.

[0069] If the amplitude A_anomaly value remains stable within the set reference distance range, the manhole cover is considered to be in normal position, and the signal exhibits normal environmental drift and vibration. The "reference distance range" is determined through the normal installation distance in the experiment.

[0070] If the value of amplitude A_anomaly deviates from the reference distance range but does not reach the "loss" threshold, and the duration exceeds the set value, according to the calibration curve, an increase in amplitude A_anomaly will decrease the distance (manhole cover sinks), and a decrease in amplitude A_anomaly will increase the distance (manhole cover tilts), issuing a "loose / tilted" warning signal.

[0071] If the value of amplitude A_anomaly drops below the "missing" threshold, the manhole cover is missing. In this state, the manhole cover may have been moved or completely damaged, making it undetectable and resulting in an extremely weak eddy current effect. At this time, the amplitude A_anomaly will approach the ambient noise amplitude A_noise, triggering a "missing / stolen" alarm signal.

[0072] Step 4: Anti-interference and confirmation mechanism.

[0073] It adopts a "N sampling, M confirmation" mechanism. For example, if the "loss" condition is met 8 out of 10 consecutive detections, the "theft" alarm will be triggered, effectively avoiding false alarms caused by brief interference such as a vehicle passing by momentarily.

[0074] By performing a moving average or simple trend calculation on the amplitude A_anomaly, the signal changes can be visualized. Theft typically causes a step drop in the signal, while environmental interference (such as temperature changes) results in a slow drift. Distinguishing between different rates of change can further improve the accuracy of the judgment.

[0075] This solution, employing a non-contact, environmentally resistant, and easily scalable approach, addresses the challenges of sensor and manhole cover damage and high deployment costs. The device is independently installed within the manhole wall or shaft, without direct physical connection to the cover, thus avoiding the sensor's exposure to crushing, impact, and corrosion. This overcomes the lifespan and reliability issues caused by sensor exposure to extreme environments. Utilizing a miniature antenna coil and an extremely low duty cycle pulse drive, the device is small in size, consumes very little power, supports long-term battery power, and eliminates the need for cables to be laid for each manhole cover, enabling large-scale, low-cost "plug-and-play" deployment.

[0076] This system achieves precise sensing of manhole cover status, moving beyond simply detecting "presence / absence" to assessing "distance changes," thus addressing the insufficient early identification capability for progressive risks such as loosening and subsidence. Utilizing the eddy current effect, the device extracts characteristic signals reflecting the distance to the metal manhole cover through spectral analysis and makes judgments based on their calibration relationship with distance. This allows detection to move beyond simple binary "present / missing" judgments, enabling sensitive detection of millimeter-level distance changes caused by loosening, tilting, or subsidence. This provides early warnings before the manhole cover completely fails, achieving a leap from "post-event alarm" to "pre-event warning."

[0077] In signal processing, environmental noise is removed by calculating the "net amplitude," and a continuous confirmation mechanism of "N samplings, M confirmations" combined with signal trend analysis is employed. This effectively distinguishes between transient interference caused by vehicles passing by momentarily and persistent signal characteristic changes resulting from actual theft or loosening of manhole covers, thus greatly reducing the false alarm rate. Anti-interference judgment logic ensures high reliability of alarm information and reduces unnecessary maintenance verification.

[0078] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A device for detecting the condition of an external manhole cover, characterized in that, Includes an MCU microprocessor, an electromagnetic signal transmitting coil, an electromagnetic signal receiving coil, and a signal conditioning circuit; The MCU microprocessor is connected to the electromagnetic signal transmitting coil to generate a PWM drive signal, which controls the electromagnetic signal transmitting coil to emit electromagnetic waves at a set frequency. The electromagnetic signal receiving coil is connected to the signal conditioning circuit to receive the electromagnetic signal modulated by the metal body of the manhole cover; The signal conditioning circuit is connected to the MCU microprocessor and is used to condition the received signal before sending it to the MCU microprocessor for analog-to-digital conversion and analysis. The MCU microprocessor is configured to: analyze the abnormal frequency and its net amplitude caused by the eddy current generated by the metal body of the manhole cover based on the spectral characteristics of the received signal, and determine the status of the manhole cover being in place, loose, or missing according to the pre-calibrated correspondence between the net amplitude of the abnormal frequency and the distance to the metal body.

2. The external manhole cover status detection device as described in claim 1, characterized in that, Both the electromagnetic signal transmitting coil and the electromagnetic signal receiving coil adopt standardized miniature patch antenna coils.

3. The external manhole cover status detection device as described in claim 1, characterized in that, The MCU microprocessor drives the electromagnetic signal transmitting coil through an electromagnetic wave transmitting circuit. The electromagnetic wave transmitting circuit includes a switching transistor, a current sampling resistor, a gate drive resistor, a pull-up resistor, and a decoupling capacitor; One end of the electromagnetic signal transmitting coil is connected to the power supply, and the other end is connected to the main current input terminal of the switching transistor via a current sampling resistor; The main current output terminal of the switching transistor is grounded, and its control terminal is connected to the PWM output pin of the MCU microprocessor via the gate drive resistor; The pull-up resistor is connected between the control terminal of the switching transistor and ground, and the decoupling capacitor is connected between the control terminal of the switching transistor and ground.

4. The external manhole cover status detection device as described in claim 1, characterized in that, The signal conditioning circuit is an instrumentation amplifier; The electromagnetic signal receiving coil is connected to the differential input terminal of the instrumentation amplifier, and a gain resistor is connected between the gain setting pins of the instrumentation amplifier to set the amplification factor. The output of the instrumentation amplifier is connected to the output node via a coupling capacitor. The output node is grounded via a pull-down resistor and connected to the ADC input pin of the MCU microprocessor.

5. The external manhole cover status detection device as described in claim 4, characterized in that, The positive power supply pin of the instrumentation amplifier is connected to the power supply and grounded via the first decoupling capacitor, while the negative power supply pin is directly grounded. The reference voltage pin of the instrumentation amplifier is connected between the power supply and ground through a symmetrical resistor divider network to set the reference voltage to half of the power supply voltage.

6. The external manhole cover status detection device as described in claim 1, characterized in that, The duty cycle of the PWM drive signal output by the MCU microprocessor is not greater than the set value.

7. The external manhole cover status detection device as described in claim 1, characterized in that, The MCU microprocessor is also configured to perform the following steps to obtain the net amplitude of the abnormal frequency: S1: Perform analog-to-digital conversion and digital bandpass filtering on the conditioned received signal to obtain the target frequency band signal; S2: Perform a fast Fourier transform on the target frequency band signal to obtain its spectrum; S3: Estimated background noise floor amplitude in the spectrum; S4: Within the preset anomaly detection frequency band, search for the peak frequency of the spectrum whose amplitude is significantly higher than the noise floor amplitude, and record its frequency as the anomaly frequency; S5: Calculate the difference between the original amplitude at the peak of the spectrum and the amplitude of the noise floor to obtain the net amplitude of the abnormal frequency.

8. The external manhole cover status detection device as described in claim 1, characterized in that, The MCU microprocessor is also configured to perform the following status determination steps: The calculated net amplitude of the abnormal frequency is compared with the preset threshold range; If the net amplitude is within the first preset range, the manhole cover is determined to be in a normal position. If the net amplitude is within the second preset range, the manhole cover is determined to be in a loose or tilted warning state based on the direction and duration of its deviation from the first preset range. If the net amplitude is lower than the third preset threshold, the manhole cover is determined to be in a lost or stolen alarm state.

9. The external manhole cover status detection device as described in claim 1, characterized in that, The MCU microprocessor uses a continuous confirmation mechanism of "N samplings, M confirmations" to determine the final state, where N and M are preset positive integers and M≤N.

10. A method for detecting the status of external manhole covers based on the device described in claim 1, characterized in that, Includes the following steps: Drive the transmitting coil to emit electromagnetic waves of a specific frequency; Receive and modulate electromagnetic signals modulated by the manhole cover; Spectral analysis of the received signal was performed to extract the abnormal frequencies and their net amplitudes generated by the eddy current effect of the manhole cover's metal body. Based on the pre-defined correlation between the net amplitude of abnormal frequencies and the distance to the manhole cover, it can be determined whether the manhole cover is currently in a normal position, loose and tilted, or lost or stolen.

Images