Bolt looseness sensor and monitoring system based on interdigital capacitor patch antenna

Abstract

The invention provides a bolt loosening sensor based on an interdigital capacitor patch antenna and a monitoring system, the bolt loosening sensor comprises two mutually staggered components, the first component is T-shaped on the whole and comprises a T-shaped grounding plane, a T-shaped substrate and a T-shaped radiation patch; the second component is U-shaped on the whole and comprises a U-shaped grounding plane, a U-shaped substrate and a U-shaped radiation patch; the first assembly extends into the U-shaped space of the second assembly, and the first assembly and the second assembly are coupled to form an interdigital capacitor patch antenna which is integrally installed in a bolt to be monitored; the end part of the second assembly is fixed with the screw rod; the end of the first assembly is fixed to the top of the nut, and the first assembly and the nut generate relative displacement in the loosening process of the bolt. The sensor and the bolt are integrated, accurate monitoring of the bolt loosening initial stage is achieved, timely intervention is facilitated, and therefore the structural damage risk is greatly reduced; the method has the advantages of easy conformality, high sensitivity, strong anti-interference capability and the like, and has good economic benefits and social benefits.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of instrument industry, and particularly relates to a bolt loosening sensor based on an interdigital capacitive patch antenna and a monitoring system. BACKGROUND

[0002] Bolt connection is an important way to transfer load between structural members, and is widely used in bridges, buildings, wind turbines and other industrial facilities. Bolt loosening may lead to serious structural safety problems, such as reduced connection stiffness, weakened load transfer capacity, and even catastrophic consequences. Bolt loosening is divided into two stages. The first stage is characterized by plastic deformation at the root of the screw rod, reduction in screw rod length, and slow decline in pretightening force. When the pretightening force drops to a certain level, it enters the second stage, which is characterized by the rotational movement of the nut, resulting in a sharp decline in pretightening force and a significant reduction in connection interface stiffness. At this time, the overall reliability of the bolt connection is greatly reduced. In actual engineering applications, if the bolt loosening develops to the second stage, it is often accompanied by connection failure and even structural damage and other serious consequences. To accurately assess the performance of the bolt structure and monitor the degree of bolt loosening, bolt loosening monitoring methods are needed.

[0003] Traditional bolt loosening monitoring methods include torque wrench method, resistance strain gauge method, piezoelectric impedance method, acoustic method, and image recognition method. However, these methods have problems such as inability to measure in real time, large environmental influence, need for additional installation of equipment, high cost, and inability to monitor at the initial stage of bolt loosening.

[0004] Patch antenna sensors have attracted widespread attention due to their passive, wireless, high sensitivity, and small size. Developing antenna sensors based on wireless sensing technology for bolt loosening monitoring has gradually become a research hotspot in this field. CN112697183A provides a bolt loosening sensor and loosening monitoring system based on a double-arc patch antenna, which uses changes in the resonant frequency of the antenna to monitor the rotation angle of the bolt nut. If the rotation angle changes, it indicates that the bolt is loosening, thereby achieving wireless monitoring of bolt loosening. However, this solution is aimed at the second stage of bolt loosening, and when the rotation angle changes, the pretightening force has already sharply declined, making it difficult to intervene in time to effectively control connection failure. CN113125132A provides a bolt loosening sensing device system and method based on a double-layer substrate patch antenna, which is aimed at the first stage of bolt loosening. The change in screw rod length is transmitted to the antenna through an assembly unit, and then the bolt loosening condition is determined by changes in the resonant frequency of the antenna. However, this solution requires the installation of a patch antenna on the surface of the bolt, making it difficult to ensure that the shape of the antenna conforms to the deformation of the bolt, i.e., conformal. SUMMARY

[0005] In view of the above problems, the application provides a bolt loosening sensor and a monitoring system based on an interdigital capacitive patch antenna, which can perform high-sensitivity and passive monitoring on the bolt loosening process in the first stage of bolt loosening, i.e., the bolt pre-tightening force reduction stage, so as to intervene in time, thereby greatly reducing the structural damage risk and the bolt loosening monitoring cost.

[0006] The technical scheme of the application is as follows: A bolt loosening sensor based on an interdigital capacitive patch antenna comprises two interlaced components, wherein: The first component 1 is in the shape of a whole "T", and comprises a T-shaped ground plane 11, a T-shaped substrate 12 and a T-shaped radiating patch 13. The second component 2 is in the shape of a whole "U", and comprises a U-shaped ground plane 21, a U-shaped substrate 22 and a U-shaped radiating patch 23. The U-shaped space of the second component 2 matches the first component 1, the first component 1 extends into the U-shaped space of the second component 2 from the opening end of the second component 2, the two components are coupled and jointly form an interdigital capacitive patch antenna, and the whole is installed in the bolt to be monitored. The screw rod 31 of the bolt is provided with a mounting hole 311 along the axial direction, the mounting hole 311 has a larger diameter than the width of the second component 2, the second component 2 is installed in the mounting hole, and the curved end of the second component 2 is fixed to the root of the screw rod 31; the end of the first component 1 is fixed to the top of the nut 32 of the bolt; the first component 1 and the second component 2 are close to each other but not connected, and the two components generate relative displacement during the loosening process of the bolt.

[0007] A bolt loosening monitoring system comprises the bolt loosening sensor based on the interdigital capacitive patch antenna and further comprises a signal transmitting device 4, a data acquisition device 5 and a monitoring module. The signal transmitting device 4 comprises a transmitter and a transmitting antenna, and is used for transmitting a modulated electromagnetic wave signal to the first component 1, wherein the signal is a linear frequency modulation continuous wave. The data acquisition device 5 comprises a receiver and a receiving antenna, and is used for acquiring the resonant frequency of the bolt loosening sensor and transmitting data to the monitoring module 6. The monitoring module 6 runs on a computer, and is used for monitoring the resonant frequency change of the bolt loosening sensor to monitor the deformation of the screw rod 31 in the bolt member, so as to infer the loosening state of the bolt.

[0008] Compared with the prior art, the application has the following advantages and beneficial effects: The sensor is integrated with the bolt to realize accurate monitoring of the initial stage of bolt loosening, so as to intervene in time, thereby greatly reducing the risk of structural damage; meanwhile, the bolt loosening sensor has the advantages of easy conformability, high sensitivity, strong anti-interference ability and passivity, and has good economic benefit and social benefit.

[0009] (1) The interdigital capacitive patch antenna is integrated on the bolt, the sensitive characteristics of the electrode spacing to strain are utilized, the axial strain of the bolt is converted into the drift of the resonant frequency, the small deformation of the bolt structure can be measured, and high precision is maintained; the sensor has high sensitivity, can monitor the screw rod deformation of the order of 0.01mm in the first stage of bolt loosening, can accurately detect the early signal of bolt loosening, realizes a technical breakthrough in the field of early warning and structure health maintenance in the early stage of bolt loosening, and further guarantees the safety and stability of the engineering structure.

[0010] (2) The bolt loosening sensor based on the interdigital capacitive patch antenna of the application takes the resonant frequency of the antenna as the monitoring parameter, and is installed in the bolt, so that the influence of external environmental noise and distance change is minimal, thereby ensuring the stability and accuracy of the measurement results; meanwhile, the sensor will not deform during monitoring the bolt loosening process, and has strong anti-interference ability.

[0011] (3) The deformation of the screw rod is associated with the change of the electrode spacing of the patch antenna, so that the shape of the antenna is consistent with the deformation of the bolt, and good conformability is achieved.

[0012] (4) The drift amount of the resonant frequency of the interdigital capacitive patch antenna can be monitored by using a radar monitoring device (including but not limited to a frequency-modulated continuous wave radar, a step frequency continuous wave radar or a USRP software radio platform), and the deformation of the screw rod of the bolt is calculated accordingly, so that passive monitoring of bolt loosening is realized.

[0013] (5) The bolt loosening sensor based on the interdigital capacitive patch antenna of the application is small in size and designed passively, and is convenient to arrange and apply in different engineering scenes; the bolt loosening sensor is small in size and integrated in the bolt; the operation energy is provided by receiving electromagnetic waves, so that the bolt loosening sensor does not need a battery or a power line to support, the installation process is simplified, the maintenance cost is reduced, and the service life of the system is significantly prolonged.

[0014] (6) The bolt loosening sensor based on the interdigital capacitive patch antenna of the application is simple to process and low in manufacturing cost, can be prefabricated and installed synchronously with the bolt structure, is helpful to realize a high-performance-price-ratio real-time monitoring system, and is widely applicable to engineering projects of different scales. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1The overall schematic diagram of the bolt loosening sensor integrated with the bolt according to the present application.

[0016] Figure 2 The schematic diagram of the installation of the bolt loosening sensor and the bolt according to the present application.

[0017] Figure 3 The schematic diagram of the first component structure of the bolt loosening sensor according to the present application.

[0018] Figure 4 The schematic diagram of the second component structure of the bolt loosening sensor according to the present application.

[0019] Figure 5 The schematic diagram of the bolt loosening sensor according to the present application.

[0020] Figure 6 The top view diagram of the bolt loosening sensor according to the present application.

[0021] Figure 7 The schematic diagram of the bolt loosening sensor of the embodiment.

[0022] Figure 8 The S11 curve under the partial frequency band of the embodiment test.

[0023] Figure 9 The schematic diagram of the relationship between the resonant frequency and the electrode spacing of the embodiment test.

[0024] Figure 10 The schematic diagram of the bolt loosening monitoring system based on the bolt loosening sensor.

[0025] Reference signs: 1, the first component 1, 11, the T-shaped ground plane, 12, the T-shaped substrate, 13, the T-shaped radiation patch; 2, the second component, 21, the U-shaped ground plane, 22, the U-shaped substrate, 23, the U-shaped radiation patch; 31, the screw rod, 32, the nut, 311, the mounting hole, 312, the cylindrical member; 4, the signal transmitting device, 5, the data acquisition device, 6, the monitoring module. DETAILED DESCRIPTION

[0026] The technical solutions provided by the present application will be further described below in combination with specific embodiments and their accompanying drawings. The advantages and features of the present application will be more apparent in combination with the following description.

[0027] Embodiment 1: As Figure 1 , Figure 2 , Figure 3 , Figure 4 , Figure 5 , Figure 6As shown, a bolt loosening sensor based on interdigital capacitive patch antenna comprises two interlaced components, wherein: The first component 1 is in the shape of "T" as a whole, comprising a T-shaped ground plane 11, a T-shaped substrate 12, and a T-shaped radiation patch 13, which is composed of a rectangular patch, a feeding line connected to the rectangular patch, and a feeding point, as shown in Figure 3 Further, the T-shaped ground plane 11 and the T-shaped radiation patch 13 are plated on the front and back of the T-shaped substrate 12 by gold plating process and closely adhere to the T-shaped substrate 12; the thickness of the T-shaped ground plane 11 and the T-shaped radiation patch 13 is approximately the same. The second component 2 is in the shape of "U" as a whole, comprising a U-shaped ground plane 21, a U-shaped substrate 22, and a U-shaped radiation patch 23, as shown in Figure 4 Further, the U-shaped ground plane 21 and the U-shaped radiation patch 23 are plated on the front and back of the U-shaped substrate 22 by gold plating process and closely adhere to the U-shaped substrate 22; the thickness and size of the U-shaped ground plane 21 and the U-shaped radiation patch 23 are approximately the same. The U-shaped space of the second component 2 matches the first component 1, the first component 1 extends into the U-shaped space of the second component 2 from the opening end of the second component 2, and the two components are coupled to form an interdigital capacitive patch antenna as a whole, which is installed in the bolt to be monitored. The screw rod 31 of the bolt is provided with a mounting hole 311 along the axial direction of the screw rod, the diameter of the mounting hole 311 is slightly larger than the width of the second component 2, the second component 2 is installed in the mounting hole, and the curved end of the second component 2 is fixed to the root of the screw rod 31; the end of the first component 1 is fixed to the top of the nut 32 of the bolt; the first component 1 and the second component 2 are closely adjacent but not connected, and the two components produce relative displacement during the loosening process of the bolt.

[0028] Further, the mounting hole of the screw rod 31 of the bolt is further provided with a cylindrical member 312, and the second component 2 is fixed to the screw rod 31 of the bolt through the cylindrical member 312: the curved end of the second component 2 is fixed to the top of the cylindrical member 312, and the bottom of the cylindrical member 312 is fixed to the screw rod 31 of the bolt. Specifically, the bottom of the mounting hole in the screw rod is provided with a clamping groove (not shown in the figure), the bottom of the cylindrical member is provided with a clamping seat (not shown in the figure) matched with the clamping groove, and the bottom of the cylindrical member is fixed to the bottom of the screw rod by clamping. Preferably, the material of the cylindrical member is the same as that of the bolt, and the cylindrical member can transmit the strain of the screw rod to the sensor.

[0029] Further, the T-shaped ground plane 11, the T-shaped radiation patch 13 of the first component 1, and the U-shaped ground plane 21, the U-shaped radiation patch 23 of the second component 2 are all made of copper; the T-shaped substrate 12 of the first component 1 and the U-shaped substrate 22 of the second component 2 are made of FR4 dielectric board.

[0030] The performance of the above bolt loosening sensor based on the interdigital capacitive patch antenna is related to the number and size of the interdigital. In practical applications, the size of the first component 1 and the second component 2 and the number of interdigital are optimized considering the limitation of the installation scene on the overall size, which can further improve the sensitivity and measurement accuracy of the bolt loosening sensor in monitoring the bolt loosening process, and can effectively detect the early bolt loosening signal to provide early warning for structure safety.

[0031] Further, as shown in Figure 7 , taking the sensor for M12 grade bolts as an example, the overall size of the first component 1 is designed to be 8mm*40mm, and the overall size of the second component 2 is designed to be 8mm*40mm; wherein the electrode spacing is 2mm, and the interdigital spacing is 0.25mm, which can be adjusted under actual working conditions.

[0032] With the first stage of bolt loosening, the screw rod deforms and the length changes, and the screw rod deformation can be measured by measuring the relative displacement between the two components (the first component 1 and the second component 2).

[0033] The present application applies the resonant characteristics of the patch antenna. The first component 1 and the second component 2 form a coupling capacitor to constitute an interdigital capacitive patch antenna. When the screw rod deforms, the two components (the first component 1 and the second component 2) fixed on the nut and the screw rod will relatively dislocate, and the size of the coupling capacitor formed between the components will change accordingly. Figure 6 In the middle, l is the total length of the antenna, l 0 is the length of the electrode, g is the width of the electrode, h is the electrode spacing, b is the interdigital spacing. The equivalent capacitance of the coupling capacitor formed between the components is C : (1) In the formula, is the relative dielectric constant of the dielectric plate, is the vacuum dielectric constant. According to the equivalent circuit model of the interdigital capacitor, the resonant frequency of the bolt loosening sensor is: (2) In the formula, is the inductance.

[0034] The inductance remains relatively constant. When the bolt loosening causes the electrode spacing to change , the change of the capacitance value is: (3) That is, the capacitance change rate is: (4) As shown in the formula above, when the bolt loosens, causing relative displacement between the two components, the change in electrode spacing leads to a change in capacitance, which in turn causes a change in the antenna's resonant frequency. Furthermore, the change in capacitance is linearly related to the change in electrode spacing. Therefore, by monitoring the resonant frequency and comparing it with the resonant frequency when the bolt is not loose, the change in the coupling capacitance between the first component 1 and the second component 2 can be determined. This allows us to obtain the change in the electrode spacing, i.e., the relative displacement of the two components, thus revealing the change in the bolt's length and further inferring the bolt's loosening state.

[0035] To verify the performance of the bolt loosening sensor based on an interdigital capacitive patch antenna, a wired test was conducted. The test targeted a bolt loosening sensor for M12 grade bolts; relevant dimensional parameters are as follows: Figure 7 As shown. The experiment utilized a vector network analyzer for signal transmission and data acquisition. A feed cable was used to connect the feed point of the interdigital capacitive patch antenna to the vector network analyzer to better test the sensor's performance. The experiment connected the screw tail of the bolt loosening sensor to a high-precision single-axis displacement platform. The change in the screw length when the bolt loosened was altered by changing the axial dimension of the high-precision single-axis displacement platform. Specifically, the high-precision single-axis displacement platform was driven by a closed-loop stepper motor, and the vector network analyzer had a wide bandwidth scanning capability from 10MHz to 20GHz, capable of simultaneously acquiring information such as resonant frequencies.

[0036] During the experiment, the reference electrode spacing of the bolt loosening sensor was first precisely calibrated to 2 mm. Then, a high-precision single-axis displacement platform was driven in 0.05 mm steps to adjust the electrode spacing from 2 mm to 1 mm. The S11 curve was obtained by frequency scanning in the 1.5-5.0 GHz band. Figure 8 This paper presents the S11 curves for a portion of the frequency bands obtained by a vector network analyzer. The resonant frequency and return loss are extracted from the S11 curves, and the frequency-electrode spacing variation curve is obtained by fitting the resonant frequency obtained from the vector network analyzer with the corresponding electrode spacing variation using the least squares method in MATLAB software. Figure 9 As shown.

[0037] The test results show that the second-order operating frequency changes from 2.80 GHz to 2.90 GHz when the electrode spacing changes in the range of 1 mm. The resonance frequency and the change of the electrode spacing show a very stable linear relationship, and the correlation coefficient is as high as 0.99, which proves that the sensor can measure the deformation of 0.01 mm level and has high sensitivity. In addition, the return loss in the entire frequency range is always lower than -15 dB, which indicates that the interdigital capacitive patch antenna has good matching and low reflection loss in this frequency range, proving its strong anti-interference ability, and the resonance mode of the interdigital capacitive patch antenna is effectively excited.

[0038] The above test shows that for the first stage of bolt loosening, the bolt loosening sensor of the application can still sense the deformation of the screw rod of 0.01 mm level, can accurately monitor the initial loosening of the bolt for timely intervention, which has great significance for ensuring the safety and stability of the engineering structure.

[0039] Embodiment 2: As shown in Figure 10 A bolt loosening monitoring system, comprising the bolt loosening sensor based on the interdigital capacitive patch antenna of the above embodiment 1, further comprising a signal transmitting device 4, a data acquisition device 5 and a monitoring module 6; The signal transmitting device 4 comprises a transmitter and a transmitting antenna, and is used for transmitting a modulated electromagnetic wave signal to the first component 1. The signal is a linear frequency modulation continuous wave. The data acquisition device 5 comprises a receiver and a receiving antenna, and is used for acquiring the resonance frequency of the bolt loosening sensor and transmitting the resonance frequency data to the monitoring module 6. The monitoring module 6 runs on a computer and is used for monitoring the change of the resonance frequency of the bolt loosening sensor to monitor the deformation of the screw rod 31 in the bolt member, so as to infer the loosening state of the bolt. The above signal transmitting device 4 and data acquisition device 5 belong to the commonly used technology in the art and are not the innovation points of the application. As an embodiment, the resonance frequency of the bolt loosening sensor can be monitored by using a frequency modulation continuous wave radar (FMCW radar). The signal transmitting device 4 is composed of a wideband antenna connected with the transmitter of the FMCW radar and the transmitter, and the data acquisition device 5 is composed of a wideband antenna connected with the receiver of the FMCW radar and the receiver.

[0040] In the monitoring process, the resonance frequency of the bolt loosening sensor is used as a measurement parameter to reflect the relative displacement of the first component 1 and the second component 2, so as to reflect the deformation of the screw rod 31 in the monitored bolt member, and further reflect the state of the monitored bolt loosening.

[0041] The process of reading the resonant frequency of the bolt loosening sensor: The signal transmitting device 4 transmits a sweeping wave to the feed point of the T-shaped radiating patch 13 of the bolt loosening sensor; the bolt loosening sensor receives the sweeping wave signal transmitted by the signal transmitting device 4 and reflects the backscattered electromagnetic wave signal at the resonant frequency, at which the antenna return loss is minimal; the data acquisition device 5 receives the backscattered electromagnetic wave signal reflected by the bolt loosening sensor and analyzes the backscattered signal to obtain the signal frequency, which is the resonant frequency of the bolt loosening sensor during measurement.

[0042] The bolt loosening sensor of the present invention is a passive sensor. Passive means that the bolt loosening sensor of the present invention does not rely on a pre-installed battery for power, but obtains the required energy by receiving electromagnetic waves emitted by an external signal transmitting device 4.

[0043] The monitoring module 6 monitors the change in the resonant frequency of the bolt loosening sensor and calculates the relative displacement between the first component 1 and the second component 2, i.e., the change in electrode spacing. This is to infer the looseness of the bolts.

[0044] Taking M12 grade bolts as an example, the relationship between resonant frequency and relative displacement is as follows: (5) in, The resonant frequency (GHz) is a linear function of the relative displacement. For sensitivity, This is the resonant frequency without displacement. It can be determined using methods such as simulation and experimental testing. , The value of the resonant frequency is used to determine the specific deformation of the screw and further infer the loosening state of the bolt.

[0045] In application, the signal transmitting device 4 transmits a modulated electromagnetic wave signal to the first component 1; in the initial stage of bolt loosening, the screw undergoes slight deformation, and the first component 1 and the second component 2 fixed at both ends of the bolt move relative to each other, causing the coupling capacitance between the two components to change, thereby causing the resonant frequency of the bolt loosening sensor to drift; the data acquisition device 5 acquires the resonant frequency of the bolt loosening sensor; the monitoring module 6 monitors the drift of the resonant frequency of the bolt loosening sensor, and can calculate the relative displacement of the two components through formula (5), thereby deduce the deformation of the screw, and then infer the degree of bolt loosening.

[0046] The above description is merely a description of preferred embodiments of this application and is not intended to limit the scope of this application in any way. Any changes or modifications made by those skilled in the art based on the above-disclosed technical content should be considered as equivalent and valid embodiments and fall within the scope of protection of the technical solution of this application.

Claims

1. A bolt loosening sensor based on an interdigital capacitive patch antenna, characterized in that, The interdigital capacitance patch antenna comprises two interlaced components, wherein: The first component (1) is in the shape of a "T" as a whole, comprising a T-shaped ground plane (11), a T-shaped substrate (12) and a T-shaped radiation patch (13), the T-shaped ground plane (11) and the T-shaped radiation patch (13) being arranged on the front and back surfaces of the T-shaped substrate (12) respectively; The second component (2) is in the shape of a "U" as a whole, comprising a U-shaped ground plane (21), a U-shaped substrate (22) and a U-shaped radiation patch (23), the U-shaped ground plane (21) and the U-shaped radiation patch (23) being arranged on the front and back surfaces of the U-shaped substrate (22) respectively; The U-shaped space of the second component (2) is matched with the first component (1), the first component (1) extends into the U-shaped space of the second component (2) from the opening end of the second component (2), and the two components are coupled to form an interdigital capacitance patch antenna, which is installed in a bolt to be monitored as a whole; The screw rod (31) of the bolt is provided with an installation hole (311) along the axial direction of the screw rod, the diameter of the installation hole (311) is greater than the width of the second component (2), the second component (2) is installed in the installation hole, and the curved end of the second component (2) is fixed to the root of the screw rod (31); the end of the first component (1) is fixed to the top of the nut (32) of the bolt; the first component (1) and the second component (2) are close to each other but not connected, and the two components are relatively displaced during the loosening of the bolt.

2. A bolt loosening sensor based on an interdigital capacitive patch antenna as claimed in claim 1, wherein, The installation hole of the screw rod (31) is further provided with a cylindrical member (312), the second component (2) is fixed to the screw rod (31) through the cylindrical member (312), specifically, the curved end of the second component (2) is fixed to the top of the cylindrical member (312), and the bottom of the cylindrical member (312) is fixed to the screw rod (31) of the bolt.

3. A bolt loosening sensor based on an interdigital capacitive patch antenna as claimed in claim 2, wherein, The bottom of the installation hole in the screw rod is provided with a clamping groove, the bottom of the cylindrical member is provided with a clamping seat matched with the clamping groove, and the bottom of the cylindrical member is fixed to the bottom of the screw rod in a clamping manner.

4. A bolt loosening sensor based on an interdigital capacitive patch antenna as claimed in claim 1, wherein, The T-shaped radiation patch (13) comprises a rectangular patch, a feeding line connected to the rectangular patch and a feeding point.

5. The bolt loosening sensor based on the interdigital capacitance patch antenna according to claim 1, wherein: A coupling capacitance is formed between the first component and the second component to form an interdigital capacitance patch antenna; when the screw rod is deformed, the first component and the second component fixed to the nut and the screw rod respectively are relatively displaced, and the coupling capacitance formed between the components changes in size; Equivalent capacitance of coupling capacitance formed between components C Is: （1） wherein is the relative permittivity of the dielectric plate, is the vacuum permittivity, l 0 is the electrode length, g is the electrode width, h is the electrode spacing; When the bolt loosening causes the relative displacement between the two components, the change in the electrode spacing causes the change in the capacitance value, so that the resonant frequency of the antenna changes.

6. A bolt loosening sensor based on an interdigital capacitive patch antenna as defined in claim 1, wherein, The T-shaped ground plane 11, the T-shaped radiation patch 13 of the first component 1 and the U-shaped ground plane 21, the U-shaped radiation patch 23 of the second component 2 are all made of copper; the T-shaped substrate 12 of the first component 1 and the U-shaped substrate 22 of the second component 2 are made of FR4 dielectric plates.

7. A bolt loosening monitoring system comprising a bolt loosening sensor based on an interdigital capacitive patch antenna as claimed in any one of claims 1-6, characterized in that, Further comprising a signal transmitting device (4), a data acquisition device (5) and a monitoring module (6). The signal transmitting device (4) comprises a transmitter and a transmitting antenna, and is used for transmitting a modulated electromagnetic wave signal to the first component (1), wherein the signal is a linear frequency modulation continuous wave; The data collecting device (5) comprises a receiver and a receiving antenna, and is used for acquiring the resonant frequency of the bolt loosening sensor and transmitting data to the monitoring module (6); The monitoring module (6) is operated in a computer, and is used for monitoring the change of the resonant frequency of the bolt loosening sensor to realize the monitoring of the deformation of the screw rod (31) in the bolt component, so as to infer the loosening state of the bolt.

8. The bolt loosening monitoring system of claim 7, wherein, The reading process of the resonant frequency of the bolt loosening sensor is as follows: The signal transmitting device (4) transmits a sweep wave to the feeding point of the T-shaped radiation patch (13) of the bolt loosening sensor; The bolt loosening sensor receives the sweep wave signal transmitted by the signal transmitting device (4), and reflects a backscattering electromagnetic wave signal at the resonant frequency, wherein the back wave loss of the antenna is minimum at the resonant frequency; The data collecting device (5) receives the backscattering electromagnetic wave signal reflected by the bolt loosening sensor, and analyzes the backscattering signal to obtain the signal frequency, wherein the frequency is the resonant frequency of the bolt loosening sensor during measurement.

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