Bolt loosening state monitoring system based on distributed strain switch

Abstract

The invention discloses a bolt loosening state monitoring system based on a distributed strain switch, and belongs to the technical field of state monitoring. The system comprises a plurality of strain switches, each strain switch monitors the loosening state of one bolt, the strain switches monitor that the bolts are loosened, and the change rate of resistance of the strain switches is larger than a threshold value; the strain switch is a flexible strain switch or a disposable strain switch; the flexible strain switch comprises a flexible substrate with a micro-channel structure, and the interior of the micro-channel structure is coated with a strain sensitive material used for forming a conductive path. The two ends of the micro-channel structure are connected with first metal signal lines, and the first metal signal lines are used for outputting resistance signals of the flexible strain switch; the disposable strain switch comprises a hard substrate printed with a metal sensitive grid, two ends of the metal sensitive grid are connected with second metal signal lines, and the second metal signal lines are used for outputting resistance signals of the disposable strain switch. The method can meet the engineering requirements of bolt connection health monitoring.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of mechanical connection state monitoring, and particularly relates to a bolt loosening state monitoring system based on a distributed strain switch, and is particularly suitable for bolt connection health monitoring in a high-frequency vibration scene such as a fracturing manifold in an oil and gas field. BACKGROUND

[0002] Threaded fasteners have become the most widely used mechanical connection method due to their low cost, good interchangeability, and convenient installation and removal. A typical threaded connection structure is composed of a bolt, a nut, and a clamped component. When the bolt or nut is tightened, the bolt shaft body is axially elongated and a pre-tightening force is formed. Proper pre-tightening force plays a key role in ensuring connection fastening performance and improving equipment reliability. However, threaded fasteners are prone to loosening failure under harsh working conditions, especially in a vibration environment. Threaded connection loosening will directly lead to pre-tightening force attenuation and cause mechanical failure, for example, leakage of oil, water, and gas media in special vehicles is often caused by such problems, and the detection of loosening failure is always an important challenge for industrial safety.

[0003] The core goal of threaded connection loosening detection is to evaluate the dynamic changes of the pre-tightening force in the bolt connection. According to the principle of the detection device, the existing technology is mainly divided into three categories: sensor-based detection method, acoustic detection method based on knocking excitation, and morphological analysis method based on machine vision. Among them, the sensor-based detection scheme is the most widely used, which realizes loosening monitoring by implanting or attaching sensors on the bolt connection structure and using the time-varying characteristics of the sensing parameters. In the sensor-based technology route, according to the correlation characteristics of the sensing parameters and the pre-tightening force, it can be further divided into explicit detection and implicit detection. The explicit detection method realizes the quantitative measurement of the pre-tightening force by establishing the explicit mapping relationship between the sensor measurement parameters (such as ultrasonic transit time, piezoelectric impedance) and the pre-tightening force through experimental calibration, and the typical representatives include strain sensors, ultrasonic sensors, and Bragg fiber grating sensors. The explicit detection scheme mostly depends on the integration of sensors in the standard bolt connection structure, such as attaching or embedding strain sensors in the bolt structure. However, this drilling or cutting operation will inevitably damage the bolt and reduce its strength and reliability. The implicit detection method usually uses a piezoelectric ceramic (PZT) sensor to monitor the bolt connection system, and its response signal shows a nonlinear implicit correlation with the pre-tightening force change, which needs to be evaluated qualitatively through feature extraction. At present, five typical methods have been formed, which are ultrasonic attenuation method, time reversal method, acoustic vibration modulation method, electromechanical impedance method, and excitation analysis method. The PZT sensor used in the implicit detection scheme does not need to be directly integrated in the bolt connection structure, but it is easily affected by environmental factors such as temperature and humidity, which changes the properties of the sensor's adhesive layer and causes the detection result to be inaccurate.

[0004] Acoustic testing methods based on percussion excitation use a vibrating hammer to strike the structure under test and collect the acoustic response signal through a microphone. Similar to methods based on implicit detection sensors, this method detects looseness in bolted connection systems by extracting sensitive features from the acoustic response signal. Morphological analysis methods based on machine vision use a camera or smartphone to capture images of bolted connection structures and use image processing techniques and machine learning algorithms to calculate the rotation angle or exposed thread length for bolt loosening detection. This type of technology can achieve non-contact detection, but the detection cost is high and is severely restricted by environmental optical conditions.

[0005] In view of the limitations of traditional detection technology, there is an urgent need for a real-time monitoring method for the loosening status of bolts without destroying the standard bolt connection structure, so as to solve the practical engineering problem that the loosening status detection of bolt connection structures is difficult and costly. Summary of the Invention

[0006] In order to solve the problems in the prior art, the present invention provides a bolt loosening status monitoring system based on distributed strain switches.

[0007] The technical solutions of the present invention are as follows:

[0008] In a first aspect, the present invention discloses a bolt loosening state monitoring system based on distributed strain switches, comprising a plurality of strain switches for monitoring the loosening state of the bolts, each strain switch monitoring the loosening state of a bolt, one end of the strain switch being fixedly connected to the upper surface of the bolt, and the other end of the strain switch being fixedly connected to the upper surface of the workpiece being fixed by the bolt, and the strain switch being in an extended state after fixation; when the bolt monitored by the strain switch becomes loose, the resistance of the strain switch changes, and when the rate of change of the resistance of the strain switch is greater than a threshold value, it is determined that the bolt monitored by the strain switch is loose;

[0009] The strain switch is a flexible strain switch or a disposable strain switch; the flexible strain switch includes a top protective layer and a flexible substrate having a microchannel structure, arranged from top to bottom. The microchannel structure of the flexible substrate is coated with a strain-sensitive material, and the strain-sensitive material forms a conductive path. Both ends of the microchannel structure are connected to a first metal signal line, and the first metal signal line is used to output the resistance signal of the flexible strain switch; the disposable strain switch includes an insulating protective layer and a hard substrate printed with a metal sensitive grid, arranged from top to bottom. Both ends of the metal sensitive grid are connected to a second metal signal line, and the second metal signal line is used to output the resistance signal of the disposable strain switch;

[0010] When the bolt is loose, the strain switch is driven to produce torsion and tensile strain, the strain sensitive position of the strain switch is stretched, so that the resistance of the strain switch will become larger; wherein the strain sensitive position of the flexible strain switch is that the strain sensitive material forms a conductive path, and the strain sensitive position of the disposable strain switch is the metal sensitive grid.

[0011] In a second aspect, the application further discloses a bolt loosening monitoring method using the bolt loosening state monitoring system, comprising the following steps:

[0012] S1: after the bolt to be monitored is installed in a standard pre-tightening manner, one end of the strain switch is fixedly connected to the upper surface of the bolt, the other end of the strain switch is fixedly connected to the upper surface of the workpiece fixed by the bolt, and the strain switch after being fixed is in a straightened state;

[0013] S2: according to the bolt loosening state monitoring requirement, all the strain switches in the state monitoring system are connected in a series connection or a parallel connection; if the connection mode between the strain switches is series connection, the resistance of a to-be-measured resistance module integrated with all the strain switches is detected by an external processing unit, and the initial total resistance of the state monitoring system and the threshold value are calibrated; if the connection mode between the strain switches is parallel connection, the resistance of each strain switch is detected by the external processing unit, each strain switch is assigned a unique physical address, a strain switch-bolt mapping relationship table is established, and the initial resistance of each strain switch and the threshold value are calibrated;

[0014] S3: if the connection mode between the strain switches is series connection, when the external processing unit detects that the resistance change rate of the resistance detection module is greater than the threshold value, there are some bolts that are loose in all the bolts monitored by the state monitoring system, and regional alarm is triggered; if the connection mode between the strain switches is parallel connection, when the external processing unit detects that the resistance change rate of a certain strain switch is greater than the threshold value, the bolt monitored by the strain switch is loose, the bolt that is loose is determined according to the physical address of the strain switch with the resistance change rate greater than the threshold value through the strain switch-bolt mapping relationship table, and precise positioning alarm is triggered.

[0015] Further, in S2, if the connection mode between the strain switches is series connection, if the strain switch is a flexible strain switch, the threshold value is 200-500% of the initial total resistance, if the strain switch is a disposable strain switch, the threshold value is 10-20% of the initial total resistance; if the connection mode between the strain switches is parallel connection, if the strain switch is a flexible strain switch, the threshold value is 400-600% of the initial resistance, if the strain switch is a disposable strain switch, the threshold value is 10-20% of the initial resistance.

[0016] Compared with the prior art, the application has the following beneficial effects:

[0017] 1. The preparation materials of the present invention are polydimethylsiloxane (PDMS), conductive nanosilver paste (or carbon-based composite materials such as conductive ink), and copper-nickel alloy wires. They are inexpensive and have stable properties. In addition, the preparation process of the present invention is simple, with clear steps and a simple structure. It supports modular production, has the potential for large-scale production, and has strong engineering application capabilities.

[0018] 2. The present invention uses conductive nanosilver paste or carbon-based composite materials, such as conductive ink, as the sensitive material, which exhibits excellent conductivity and strain sensitivity. When the strain switch undergoes strain changes, the sensitive material's resistance changes significantly, resulting in extremely high strain detection sensitivity. The present invention detects the strain caused by the relative displacement between the bolt and the workpiece clamped by the bolt when the bolt is loosened. The response signal exhibits switch response characteristics, enabling real-time monitoring of the bolt's loosening status.

[0019] 3. In the application of looseness monitoring of bolt connection structures, the present invention only needs to deploy distributed strain switches after the bolts have completed standard pre-tightening installation, without the need for drilling or cutting operations on the bolts, and does not damage the standard bolt connection structure, thereby ensuring the strength and reliability of the bolts. The monitoring signal can realize remote processing of abnormal conditions through remote data transmission, and analyze the fault type and evaluate the fault severity in real time, thereby achieving simple fault site positioning. It is simple to operate, highly real-time, and has good detection effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a three-dimensional diagram of the structure of the flexible strain switch of the present invention;

[0021] Figure 2 is a front view of the flexible strain switch of the present invention;

[0022] Figure 3 is a top view of the flexible strain switch of the present invention;

[0023] Figure 4 is a side view of the flexible strain switch of the present invention;

[0024] Figure 5 is an exploded view of the flexible strain switch of the present invention;

[0025] Figure 6 is a front view of the disposable strain gauge switch of the present invention;

[0026] Figure 7 Schematic diagram of a serial connection mode of all strain gauge switches of the condition monitoring system of the present invention;

[0027] Figure 8 It is a schematic diagram of a parallel connection mode of all strain switches of the state monitoring system of the present invention. DETAILED DESCRIPTION

[0028] The present invention will be further described and illustrated below in conjunction with specific embodiments. The embodiments are merely illustrative of the present disclosure and do not limit its scope. The technical features of the various embodiments of the present invention may be combined accordingly, provided that there is no conflict between them.

[0029] To address the challenges of existing technologies, this paper investigates the effects of bolt loosening and designs a low-cost strain gauge switch that can be directly attached to the bolt surface to detect the relative displacement between the bolt and the workpiece being secured. This paper proposes a bolt loosening status monitoring system based on distributed strain gauge switches. This system enables real-time monitoring of bolt loosening without disrupting the standard bolt connection structure, addressing the current practical engineering challenges of difficult and costly detection of loosening in bolted connections.

[0030] The bolt loosening monitoring system based on distributed strain switches of the present invention includes multiple strain switches for monitoring the looseness of bolts. Each strain switch monitors the looseness of a single bolt. One end of the strain switch is fixedly connected to the upper surface of the bolt, and the other end is fixedly connected to the upper surface of the workpiece being secured by the bolt. After securing, the strain switch is in an extended state. When the bolt being monitored by the strain switch becomes loose, the resistance of the strain switch changes. If the rate of change in the resistance of the strain switch exceeds a threshold, the bolt being monitored by the strain switch is determined to be loose.

[0031] The strain switch is a flexible strain switch or a disposable strain switch. Figure 1-Figure 5 As shown, the flexible strain switch includes a flexible substrate 1 with a microfluidic structure 6 and a top protective layer 5 arranged from bottom to top. The microfluidic structure 6 of the flexible substrate 1 is coated with a sensitive material 2 to form a conductive path; both ends of the microfluidic structure 6 are connected to a metal signal line 3, which is used to output the resistance signal of the flexible strain switch.

[0032] The flexible strain switch's flexible substrate 1 and top protective layer 5 are both made of a flexible material, such as polydimethylsiloxane (PDMS). The flexible substrate 1 accommodates the sensitive material 2 and has specific dimensions. The top protective layer 5 prevents leakage and isolates the sensitive material from the external environment. Using the same flexible material ensures compatibility between the flexible substrate 1 and the top protective layer 5, ensuring the overall compliance of the flexible strain switch and enabling it to better follow the strain caused by the relative displacement of the bolt and the workpiece being secured.

[0033] like Figure 3As shown, the shape of the microfluidic structure 6 is serpentine, and the microfluidic structure 6 can be processed on the flexible substrate 1 by mold casting, wherein the size of the flexible substrate 1 is determined by the size of the bolt to be monitored. The serpentine microfluidic structure 6 can enhance the resistance change caused by strain, thereby improving the sensitivity of the flexible strain switch to strain.

[0034] Sensitive material 2 is a strain-sensitive material composed of a conductive nanosilver paste or a carbon-based composite material, such as conductive ink. This material is applied to the microfluidic structure 6 of the flexible substrate 1 through a coating process, forming a continuous conductive path. Metal signal lines 3, made of a copper-nickel alloy, connect the two ends of the microfluidic structure 6 and extend outward from both sides of the flexible substrate 1.

[0035] The present invention also provides a method for preparing a flexible strain switch, comprising the following steps:

[0036] 1) Molding of the flexible substrate 1: For PDMS (silane prepolymer to crosslinker mass ratio of 10:1), a casting mold with a serpentine microfluidic structure was designed. The PDMS material was poured into the mold. After removing bubbles under a vacuum environment, it was heated to 80°C and cured for 2 hours. After peeling, the flexible substrate 1 with the microfluidic structure 6 was obtained.

[0037] 2) Filling the sensitive material 2: Injecting conductive nanosilver paste or conductive ink into the microchannel structure 6 of the flexible substrate 1, and removing the sensitive material 2 that overflows the microchannel structure 6.

[0038] 3) Signal line integration: Copper-nickel alloy wires are attached to both ends of the microfluidic structure 6. The exposed metal is directly inserted into the sensitive material 2 to ensure good contact. Then, a top protective layer made of PDMS material is attached to complete the preparation of the flexible strain switch.

[0039] like Figure 6 As shown, the disposable strain switch includes a hard substrate with a metal sensitive grid 9 and an insulating protective layer arranged in sequence from bottom to top. Both ends of the metal sensitive grid 9 are connected to metal signal lines, which are used to output the resistance signal of the disposable strain switch (the metal signal line is not in the Figure 6 One end of the disposable strain switch is connected to the bolt, which is the bolt fixing end 7; the other end of the disposable strain switch is connected to the workpiece fixed by the bolt, which is the workpiece fixing end 8.

[0040] The rigid substrate is made of fiberglass, and the metal sensitive gate 9 is printed onto the rigid substrate using a printed circuit process. The metal sensitive gate 9 is made of a metal strain gauge resistor material, such as copper and gold. The insulating protective layer is an epoxy resin coating formed on the rigid substrate. This insulating protective layer protects the metal sensitive gate 9 from mechanical damage and isolates it from external environmental influences on its sensing performance. The second metal signal line is a copper-nickel alloy conductor.

[0041] When the bolt loosens, it causes the strain switch to generate torsional and tensile strain, and the strain-sensitive position of the strain switch is stretched, thereby increasing the resistance of the strain switch. Among them, the strain-sensitive position of the flexible strain switch is the conductive path formed by the strain-sensitive material, and the strain-sensitive position of the disposable strain switch is the metal sensitive grid 9.

[0042] For multiple distributed strain gauge switches, you can choose between series connection mode and parallel connection mode based on the bolt loosening status monitoring requirements. Figure 7 As shown, the series connection mode interconnects the metal signal lines of two adjacent strain gauge switches. All the strain gauge switches are integrated into a single resistance module for resistance detection. If the rate of change of the resistance of the resistance module exceeds a threshold, several of the bolts monitored by the condition monitoring system are loose.

[0043] like Figure 8 As shown, the parallel connection mode connects the metal signal lines of all strain gauge switches in parallel. Each strain gauge switch acts as a resistance module to be measured, and each strain gauge switch performs resistance detection as a resistance module to be measured. If the rate of change of the resistance of a strain gauge switch exceeds a threshold, the bolt being monitored by that strain gauge switch is loose.

[0044] Among them, the requirement for bolt loosening status monitoring is whether it is necessary to accurately locate the loose bolts; if it is necessary to accurately locate the loose bolts, all strain switches in the status monitoring system are connected in parallel; if it is not necessary to accurately locate the loose bolts, all strain switches in the status monitoring system are connected in series.

[0045] The present invention further provides a method for monitoring bolt loosening using the bolt loosening state monitoring system, comprising the following steps:

[0046] S1: System deployment phase: After the bolt to be monitored is pre-tightened and installed in a standard manner, one end of the strain switch is fixedly connected to the upper surface of the bolt, and the other end of the strain switch is fixedly connected to the upper surface of the workpiece fixed by the bolt. After the fixation is completed, the strain switch is in a straightened state.

[0047] In a specific embodiment of the present application, the flexible strain switch can be arranged across the bolt to be monitored with minimum strain, and the two ends of the flexible strain switch are attached to the upper surface of the workpiece, and the middle section of the flexible strain switch is attached to the upper surface of the bolt to be monitored.

[0048] S2: System initialization stage: according to the bolt loosening state monitoring requirement, all strain switches in the state monitoring system are connected in series or parallel connection mode; if the connection mode between the strain switches is series, the external processing unit detects the resistance of the integrated resistance module of all strain switches, and calibrates the initial total resistance of the state monitoring system and sets the threshold value; if the connection mode between the strain switches is parallel, the external processing unit detects the resistance of each strain switch, assigns a unique physical address to each strain switch, establishes a strain switch-bolt mapping relationship table, and calibrates the initial resistance of each strain switch and sets the threshold value.

[0049] In series connection, multiple strain switches are connected in series through metal signal lines to form a chain network, and the overall resistance change of the system reflects the cooperative state of the bolt group. It is a low-cost and fast deployment scenario, and is suitable for scenarios that do not require accurate positioning of loose bolts. In parallel connection, each strain switch is connected to the external processing unit (external signal acquisition and transmission module) through an independent channel, supporting independent data acquisition and address coding of single nodes, and accurately positioning the position of loose bolts.

[0050] For series connection mode, the initial total resistance R total of the system is calibrated, and the threshold value ΔR is set. For flexible strain switches, ΔR is 200-500% of R total ; for one-time strain switches, ΔR is 10-20% of R total . For parallel connection mode, a unique physical address is assigned to each strain switch, a strain switch-bolt mapping relationship table is established, and the reference resistance value R0 of each strain switch is calibrated under the condition of standard pre-tightening installation, and the threshold value ΔR th is set. For flexible strain switches, ΔR th is 400-600% of R0; for one-time strain switches, ΔR th is 10-20% of R0.

[0051] S3: Looseness monitoring and alarm triggering: When the strain switches are connected in series, if the external processing unit detects that the resistance change rate of the resistance detection module is greater than the threshold, then some bolts among all the bolts monitored by the condition monitoring system are loose, and a regional alarm is triggered; when the strain switches are connected in parallel, if the external processing unit detects that the resistance change rate of a certain strain switch is greater than the threshold, then the bolt monitored by the strain switch is loose, and the loose bolt is determined through the strain switch-bolt mapping relationship table according to the physical address of the strain switch with a resistance change rate greater than the threshold, and a precise positioning alarm is triggered.

[0052] Under normal conditions, the bolts have sufficient tightening force, and there is no relative displacement between the bolted workpiece and the bolted bolts. At this time, the strain switch is at the initial strain level, and the resistance signal output is equivalent to the calibration value. When the bolted connection structure under test becomes loose due to vibration or load changes, a relative displacement occurs between it and the surface of the bolted workpiece. The strain switch is driven by the bolts to produce higher torsional and tensile strains, and the resistance signal output differs greatly from the calibration value, which is manifested as a switching characteristic. In the series connection mode, the overall resistance offset exceeds the threshold value ΔR, triggering a regional alarm. In the parallel connection mode, the resistance value of each strain switch is monitored in real time. When the resistance change rate is greater than the threshold value ΔR th When the alarm is triggered, the precise positioning alarm is triggered.

[0053] Example 1

[0054] This embodiment adopts Figure 1 The flexible strain switch shown in the figure has a flexible substrate 1 with dimensions of 8 mm × 40 mm and a thickness H1 of 1.5 mm. The flexible substrate 1 has a microfluidic structure 6 inside. The microfluidic structure 6 is a serpentine structure with a channel width L1 of 0.6 μm and a channel spacing L2 of 0.6 μm. The top protective layer 5 has dimensions of 8 mm × 40 mm and a thickness H2 of 1 mm. The diameter of the metal signal line 3 is 200 μm. After filling with nano-silver paste (silver particles account for 75 wt%) as the sensitive material 2, the initial resistance value of the flexible strain switch is 100-1000 Ω.

[0055] When multiple flexible strain gauge switches are connected in series to monitor bolt loosening, the monitoring method includes the following steps:

[0056] S1. System deployment phase: After the bolt to be monitored is pre-tightened and installed according to standard procedures, one end of the flexible strain switch is fixedly connected to the upper surface of the bolt, and the other end of the flexible strain switch is fixedly connected to the upper surface of the workpiece being fixed by the bolt. After the fixing is completed, the flexible strain switch is in an extended state.

[0057] S2. System initialization phase: All flexible strain gauge switches in the condition monitoring system are connected in series. An external processing unit is used to detect the resistance of a resistance module to be measured integrated into all flexible strain gauge switches, and the initial total resistance of the condition monitoring system is calibrated and a threshold value is set, which is 200-500% of the initial total resistance.

[0058] S3. Looseness monitoring and alarm triggering: Under normal conditions, the bolts have sufficient tightening force and there is no relative displacement between them and the workpiece being bolted. At this time, the flexible strain switch is at the initial strain level, and the resistance signal output is equivalent to the calibration value. When the bolt connection structure under test loosens due to vibration or load changes, a relative displacement occurs between it and the surface of the workpiece being bolted. The flexible strain switch will form a higher strain level under the action of the bolt inertia. Due to the design of the microfluidic structure, the serpentine microfluidic structure increases the tensile length of the sensitive material and enhances the resistance change caused by the strain. The resistance signal output is significantly different from the calibration value, which manifests as a switching characteristic. In the series connection mode, the overall resistance offset exceeds the threshold, indicating that the bolt connection structure has loosened.

[0059] Example 2

[0060] This embodiment uses a dual microchannel structure flexible strain switch, which can be regarded as two Figure 1 The flexible strain switches are connected in series to further improve the strain detection sensitivity. The flexible substrate 1 of the dual-microchannel structure flexible strain switch has a size of 8mm×80mm and a thickness of 1.5mm; it has a dual microchannel structure inside, and the microchannel structure 6 is a serpentine structure with a channel width of 0.6μm, and the channel spacing of the serpentine structure is 0.6μm; the top protective layer 5 has a size of 8mm×80mm and a thickness of 1mm; the diameter of the metal signal line 3 is 200μm; after filling with nano-silver paste (silver particles account for 75wt%) as the sensitive material 2, the initial resistance value of the dual microchannel structure is 400-2000Ω.

[0061] Multiple dual-microchannel flexible strain switches are connected in series to monitor bolt loosening. The monitoring method includes the following steps:

[0062] S1. System Deployment Phase: After the bolt to be monitored has been pre-tightened to a standard standard, the dual-microchannel flexible strain switch is placed across the bolt to be monitored with minimal strain. The two ends of the dual-microchannel flexible strain switch are attached to the upper surface of the workpiece being bolted, and the middle section of the dual-microchannel flexible strain switch is attached to the upper surface of the bolt to be monitored. S2. System Initialization Phase: An external processing unit detects the resistance of each dual-microchannel strain switch, assigns a unique physical address to each dual-microchannel strain switch, establishes a strain switch-bolt mapping table, and calibrates the initial resistance of each dual-microchannel strain switch, setting a threshold value between 400% and 600% of the initial resistance.

[0063] S3. Looseness monitoring and alarm triggering: Under normal conditions, the bolt has sufficient tightening force, and there is no relative displacement between the bolted workpiece and the bolted workpiece. At this time, the dual-microchannel structure flexible strain switch is at the initial strain level, and the resistance signal output is equivalent to the calibration value; when the bolt connection structure is loose, the bolt and the bolted workpiece produce relative displacement, causing the dual-microchannel structure flexible strain switch to produce corresponding strain, causing the flexible substrate to stretch and cause the resistance of the sensitive material therein to change. Due to the design of the dual-microchannel structure, the resistance change caused by the strain is further enhanced, showing the switch response characteristics, and at the same time, it can detect smaller loosening phenomena; in the parallel connection mode, the resistance value of each dual-microchannel structure flexible strain switch is monitored in real time. If the resistance change rate of the dual-microchannel structure flexible strain switch is greater than the threshold, the bolt monitored by the dual-microchannel structure strain switch is loose. According to the physical address of the dual-microchannel structure strain switch with a resistance change rate greater than the threshold, the loose bolt is determined through the strain switch-bolt mapping relationship table, and a precise positioning alarm is triggered. Example 3

[0064] This embodiment utilizes a disposable strain switch, which is hourglass-shaped, with two fixed ends measuring 15mm wide, a strain-sensitive location measuring 5mm wide, a PCB thickness of 2mm, and a 0.4mm conductor width. The printed circuit on the PCB passes through the strain-sensitive location and returns to the fixed end of the clamped workpiece. Two terminals are drawn from the fixed end of the clamped workpiece, and the switch response is detected via a metal signal line. After the bolt is preloaded and the strain switch is deployed, there is no relative displacement between the bolt and the clamped workpiece, and the connection between the two terminals is equivalent to a short circuit. When the bolt connection structure loosens, the relative displacement between the bolt and the clamped workpiece can cause the strain-sensitive location to break, creating an open circuit between the two terminals. Therefore, by detecting the connection between the two terminals, the loosening of the bolt can be monitored.

[0065] In summary, the strain switch designed in the present invention for real-time monitoring of the loosening state of a bolted connection structure has a simple structure, a mature preparation process, and a good monitoring effect. It can realize the loosening state detection of a specific bolted connection structure and meet the engineering needs of mechanical structure operation and maintenance.

[0066] The above-described embodiments merely illustrate several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. Persons skilled in the art will readily appreciate that variations and modifications may be made without departing from the scope of the present invention, all of which fall within the scope of protection of the present invention.

Claims

1. A bolt loosening status monitoring system based on distributed strain switches, characterized in that: The invention comprises a plurality of strain switches for monitoring the loosening state of bolts, each strain switch monitoring the loosening state of a bolt, one end of the strain switch being fixedly connected to the upper surface of the bolt, and the other end of the strain switch being fixedly connected to the upper surface of the workpiece being fixed by the bolt. After the fixing is completed, the strain switch is in a straightened state; when the bolt monitored by the strain switch is loose, the resistance of the strain switch will change. When the rate of change of the resistance of the strain switch is greater than a threshold value, it is determined that the bolt monitored by the strain switch is loose; The strain switch is a flexible strain switch or a disposable strain switch; the flexible strain switch includes a top protective layer and a flexible substrate having a microchannel structure, arranged from top to bottom. The microchannel structure of the flexible substrate is coated with a strain-sensitive material, and the strain-sensitive material forms a conductive path. Both ends of the microchannel structure are connected to a first metal signal line, and the first metal signal line is used to output the resistance signal of the flexible strain switch; the disposable strain switch includes an insulating protective layer and a hard substrate printed with a metal sensitive grid, arranged from top to bottom. Both ends of the metal sensitive grid are connected to a second metal signal line, and the second metal signal line is used to output the resistance signal of the disposable strain switch; When the bolt loosens, the strain switch generates torsional and tensile strain, and the strain-sensitive position of the strain switch is stretched, thereby increasing the resistance of the strain switch. Among them, the strain-sensitive position of the flexible strain switch is the conductive path formed by the strain-sensitive material, and the strain-sensitive position of the disposable strain switch is the metal sensitive grid.

2. The bolt loosening status monitoring system according to claim 1, characterized in that: The shape of the microchannel structure is serpentine; the flexible substrate and the top protective layer are both made of flexible materials, and the flexible materials used to make the flexible substrate and the top protective layer are the same; the flexible material is polydimethylsiloxane; the top protective layer is used to prevent leakage of sensitive materials and isolate the influence of the external environment on the sensing performance of the sensitive materials.

3. The bolt loosening status monitoring system according to claim 1, characterized in that: The strain-sensitive material is a conductive nano-silver paste or a carbon-based composite material, and the carbon-based composite material is a conductive ink; the first metal signal line is a copper-nickel alloy wire.

4. The bolt loosening status monitoring system according to claim 1, characterized in that: The hard substrate is made of glass fiber material, and the metal sensitive grid is printed on the hard substrate using a printed circuit process. The material for making the metal sensitive grid is a metal strain resistor material, and the metal strain resistor material includes copper and gold; the material for making the insulating protective layer is epoxy resin, and the insulating protective layer is used to protect the metal sensitive grid from mechanical damage and isolate the influence of the external environment on the sensing performance of the metal sensitive grid; the second metal signal line is a copper-nickel alloy wire.

5. The bolt loosening status monitoring system according to claim 1, characterized in that: All strain gauge switches are connected in series or in parallel; When all the strain switches are connected in series, the metal signal lines of two adjacent strain switches are connected to each other, and all the strain switches are integrated into a resistance module to be measured. All the strain switches detect resistance in the form of a resistance module to be measured; If the rate of change of the resistance of the resistance module to be measured is greater than the threshold, then several bolts among all the bolts monitored by the condition monitoring system are loose; When all strain switches are connected in parallel, each strain switch acts as a resistance module to be measured, and each strain switch detects resistance in the form of a resistance module to be measured; if the rate of change of the resistance of the strain switch is greater than the threshold, the bolt monitored by the strain switch is loose.

6. A method for monitoring bolt loosening using the bolt loosening status monitoring system according to claim 1, characterized in that: The following steps are involved: S1: After the bolt to be monitored is pre-tightened and installed in a standard manner, one end of the strain gauge is fixedly connected to the upper surface of the bolt, and the other end of the strain gauge is fixedly connected to the upper surface of the workpiece to be fixed by the bolt. After the fixing is completed, the strain gauge is in a straight state. S2: Based on the bolt loosening status monitoring requirements, all strain switches in the status monitoring system are connected in series or in parallel. If the strain switches are connected in series, the resistance of a resistance module to be measured integrated in all the strain switches is detected by an external processing unit, and the initial total resistance of the status monitoring system is calibrated and the threshold value is set. If the strain switches are connected in parallel, the resistance of each strain switch is detected by an external processing unit, and a unique physical address is assigned to each strain switch. A strain switch-bolt mapping relationship table is established, and the initial resistance of each strain switch is calibrated and the threshold value is set. S3: When the strain switches are connected in series, if the external processing unit detects that the resistance change rate of the resistance detection module is greater than the threshold, then some bolts among all the bolts monitored by the condition monitoring system are loose, and a regional alarm is triggered; when the strain switches are connected in parallel, if the external processing unit detects that the resistance change rate of a certain strain switch is greater than the threshold, then the bolt monitored by the strain switch is loose, and the loose bolt is determined through the strain switch-bolt mapping relationship table based on the physical address of the strain switch with a resistance change rate greater than the threshold, and a precise positioning alarm is triggered.

7. The method according to claim 6, characterized in that In S2, the bolt loosening status monitoring requirement is whether the loose bolts need to be accurately positioned; if the loose bolts need to be accurately positioned, all strain switches in the status monitoring system are connected in parallel; if the loose bolts do not need to be accurately positioned, all strain switches in the status monitoring system are connected in series.

8. The method according to claim 6, characterized in that In S2, when the strain switches are connected in series, if the strain switch is a flexible strain switch, the threshold value is 200-500% of the initial total resistance; if the strain switch is a one-time strain switch, the threshold value is 10-20% of the initial total resistance; When the strain switches are connected in parallel, if the strain switch is a flexible strain switch, the threshold value is 400-600% of the initial resistance; if the strain switch is a one-time strain switch, the threshold value is 10-20% of the initial resistance.

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