Passive wireless stress monitoring device based on piezoelectric material

Abstract

The invention discloses a passive wireless stress monitoring device based on a piezoelectric material, and the device comprises the piezoelectric material which is configured to achieve the mutual conversion between an electric field and mechanical vibration; the upper electrode and the lower electrode are configured to be conductive thin films formed based on a thin film deposition process and are respectively arranged on two end surfaces of the piezoelectric material to jointly form a piezoelectric resonator; the metal workpiece is configured to be tightly connected with the piezoelectric resonator to form a piezoelectric resonance workpiece, the resonance frequency of the piezoelectric resonance workpiece changes along with the change of the external stress, and the frequency change and the magnitude of the external stress are in a single mapping relationship; and the alternating current direct drive frequency conversion sensing circuit is configured to modulate a resonant frequency signal of the piezoelectric resonant workpiece to a microwave input signal to form a modulation signal, and quantify the external stress by detecting the frequency deviation of the modulation signal. According to the invention, passive wireless real-time monitoring of the stress / pretightening force of the metal workpiece can be realized by using the piezoelectric resonance workpiece and the alternating current direct drive frequency conversion sensing technology.

Description

Technical Field

[0001] The present invention relates to the technical field of passive wireless stress sensing, and in particular to a passive wireless stress monitoring device based on piezoelectric material. Background Art

[0002] Bolts, anchor rods, anchor cables, tension bars, washers, nuts and other metal workpieces are common engineering connection fasteners. They are widely used in the field of engineering structures because of their advantages of easy disassembly and reliable fastening. In many engineering applications, metal workpieces play an important role in maintaining the connection structure of engineering facilities and mechanical equipment, ensuring construction operation, and protecting the safety of property and personnel. However, engineering failures and safety accidents caused by loose connections and structural fractures of metal workpieces often bring great losses. Therefore, it is also particularly important to regularly monitor the stress / preload force applied to metal workpieces.

[0003] For a long time, the monitoring method of the stress / preload force applied to metal workpieces has been highly valued in engineering. At present, the main methods for monitoring the stress / preload force applied to metal workpieces include manual monitoring, ultrasonic monitoring, laser monitoring and other methods.

[0004] Manual monitoring involves workers observing the rotation angle of metal workpieces, tapping and listening to sounds, twisting with torque wrenches, etc. to determine whether the connection of metal workpieces is loose and whether the force-bearing structure is reliable. However, due to the large number of metal workpieces in engineering, the complex distribution environment, and the high frequency of monitoring required, and the manual monitoring process is highly dependent on the subjective judgment of workers, it is time-consuming, labor-intensive, and labor-intensive, and it is difficult to obtain accurate monitoring results.

[0005] The ultrasonic monitoring method uses ultrasound to capture the sound waves generated by tiny cracks or deformations at the joints of metal workpieces to determine whether the connection is loose. The laser monitoring method uses a laser measurement system to accurately detect tiny changes in the length of metal workpieces to determine whether they are loose. These two intelligent monitoring methods have high monitoring accuracy and can monitor in real time, but the sensor devices are expensive and must be installed in a wired manner, which will introduce a large number of wires, making installation difficult, costly, and requiring a high operating environment, and have significant limitations. Summary of the invention

[0006] In order to solve the above problems, the present invention proposes a passive wireless stress monitoring device based on piezoelectric materials, which uses piezoelectric resonant workpieces and AC direct-drive variable frequency sensing technology to achieve passive wireless real-time monitoring of stress / preload of metal workpieces. The passive wireless stress monitoring device of the present invention has the characteristics of passive wireless, simple structure, low cost, convenient installation and layout, easy maintenance, simple operation, accurate and reliable measurement results, low labor cost, and long working life.

[0007] The technical solution adopted by the present invention is as follows:

[0008] A passive wireless stress monitoring device based on piezoelectric material, comprising:

[0009] Piezoelectric materials are configured to achieve mutual conversion between electric fields and mechanical vibrations;

[0010] The upper electrode and the lower electrode are configured as a conductive film formed based on a thin film deposition process and are respectively arranged on both end surfaces of the piezoelectric material to form a piezoelectric resonator together;

[0011] The metal workpiece is configured to be closely connected with the piezoelectric resonator to form a piezoelectric resonant workpiece, the resonant frequency of the piezoelectric resonant workpiece changes with the external stress, and the frequency change and the external stress magnitude present a single mapping relationship;

[0012] The AC direct-drive variable frequency sensing circuit is configured to generate a tuning signal based on the excitation of a microwave input signal, so that the piezoelectric resonant workpiece resonates and generates a resonant frequency signal according to the piezoelectric effect; and based on the nonlinear characteristics of the internal semiconductor device, the resonant frequency signal is modulated onto the microwave input signal to form a modulation signal, and the frequency offset of the modulation signal is detected to quantify the magnitude of the external stress.

[0013] Furthermore, the metal workpiece is tightly connected to the piezoelectric resonator to form a piezoelectric resonant workpiece, comprising:

[0014] The surface of the metal workpiece is directly and tightly connected to the piezoelectric resonator through a preset process, so that the lower electrode of the piezoelectric resonator is tightly fitted to the metal workpiece. The preset process includes conductive adhesive bonding, hot pressing synthesis, welding and inlaying, laser cutting, injection molding, thin film coating, 3D printing and piezoelectric nanomaterial technology.

[0015] Furthermore, before the surface of the metal workpiece is directly and tightly connected to the piezoelectric resonator through a preset process, the surface of the metal workpiece is first smoothed by means of grinding and polishing, or a groove is made on the surface of the metal workpiece to embed the piezoelectric resonator into the metal workpiece; when the groove process is used for connection, the lower electrode of the piezoelectric resonator is connected to the bottom of the groove on the surface of the metal workpiece through a preset process, and the side of the piezoelectric resonator is tightly fitted to the side wall of the groove on the surface of the metal workpiece through an insulating adhesive.

[0016] Furthermore, the types of the insulating adhesive include epoxy resin, silicone resin, polyimide resin, phenolic resin, polyurethane and acrylic resin; the conductive adhesive includes a matrix and a conductive filler, the types of the matrix include epoxy resin, silicone resin, polyimide resin, phenolic resin, polyurethane and acrylic resin, and the types of the conductive filler include powder of conductive metal material, graphite and conductive compounds.

[0017] Furthermore, the types of piezoelectric materials include piezoelectric single crystals, polycrystalline piezoelectric ceramics, polymer piezoelectric materials and polymer-piezoelectric ceramic composites; the types of piezoelectric resonators include piezoelectric resonators, surface acoustic wave resonators, bulk acoustic wave resonators and transducers; the metal workpiece includes force-bearing devices, and the types of the force-bearing devices include bolts, anchor rods, anchor cables, tension bars, washers and nuts.

[0018] Furthermore, the AC direct-drive variable frequency sensing circuit includes a first impedance matching network, a frequency sweeping circuit, a reflection matching network and a second impedance matching network. The frequency sweeping circuit is respectively connected to the first impedance matching network, the reflection matching network and the second impedance matching network, and is also connected to the piezoelectric resonant workpiece.

[0019] Furthermore, the first impedance matching network is configured to match the input impedance of the AC direct-drive frequency conversion sensing circuit with the input source impedance of the AC microwave signal, so that the return loss of the AC microwave signal with an input frequency of f1 at the input of the AC direct-drive frequency conversion sensing circuit is minimized.

[0020] Furthermore, the frequency sweeping circuit is configured to generate tuning signals of several frequency components based on the characteristic that the junction capacitance between the two electrodes of the transistor changes with the alternating voltage applied between the two electrodes; the frequency sweeping circuit includes an inductor and a transistor, and the inductor and the transistor are connected in series or in parallel.

[0021] Furthermore, after the tuning signal generated by the frequency sweeping circuit enters the piezoelectric resonant workpiece, the frequency components close to the inherent resonant frequency f2 of the piezoelectric resonant workpiece will cause mechanical resonance of the piezoelectric resonant workpiece, and based on the piezoelectric effect, the mechanical vibration in the piezoelectric resonant workpiece will be converted into a resonant frequency signal with a gain of frequency f2, and reflected to the transistor of the frequency sweeping circuit, while the remaining frequency components will be attenuated and filtered, thereby completing the frequency selection of the frequency within the range around the inherent resonant frequency f2 of the piezoelectric resonant workpiece; after the frequency components close to the inherent resonant frequency f2 of the piezoelectric resonant workpiece return to the transistor of the frequency sweeping circuit, they are amplified by the transistor gain and then enter the reflection matching network and the second impedance matching network.

[0022] Furthermore, the reflection matching network is configured to have high impedance to the inherent resonant frequency of the piezoelectric resonant workpiece. After the inherent resonant frequency f2 of the piezoelectric resonant workpiece amplified by the transistor gain of the sweeping frequency circuit enters the reflection matching network, if the second impedance matching network is configured as a low-impedance network of the resonant frequency f2, the signal of the frequency f2 will be output by the second impedance matching network; if the second impedance matching network is not configured, the frequency f2 will be reflected back to the transistor for a second time, and due to the nonlinear characteristics of the transistor, it will be mixed with the input frequency f1, and the modulated mixed signal will be output at the input port; the mixed signal will carry the inherent resonant frequency information of the piezoelectric resonant workpiece, and then carry the information of the stress magnitude loaded on the piezoelectric resonant workpiece.

[0023] The beneficial effects of the present invention are:

[0024] 1. Small size and high integration. The appearance of the piezoelectric resonant workpiece after combining with the piezoelectric resonator is generally the same as that of an ordinary metal workpiece. The piezoelectric resonant workpiece and the AC direct-drive variable frequency sensor circuit can be highly integrated in the packaging structure with a small size.

[0025] 2. Easy to install and arrange, and applicable to a wide range of scenarios. The entire device works based on an AC direct-drive variable frequency sensing circuit, does not require a DC source, and does not require the introduction of a large number of transmission lines, so it can meet the engineering applications of most scenarios.

[0026] 3. Easy to use and maintain. Because the stress / preload force loaded on the piezoelectric resonant workpiece can be directly read out by only using the receiving demodulation device to receive the mixed signal sent by the demodulation sensor device, the technology used in the present invention is very simple to operate compared with intelligent monitoring technologies such as ultrasonic and laser. At the same time, there is no need to replace the battery of the sensor device, which is convenient for maintenance.

[0027] 4. Low cost. Since the circuit structure of the sensor device is simple and the components used are common electronic devices, the production cost of the sensor is low. At the same time, since the sensor device is easy to use and maintain, it also greatly saves labor costs.

[0028] 5. Not dependent on changes in external mechanical force. The passive wireless stress monitoring device based on piezoelectric material designed by the present invention can enable the piezoelectric material to generate signals when there is no change in external mechanical force, thereby achieving real-time collection of stress / preload.

[0029] In summary, the present invention utilizes a piezoelectric resonant workpiece and an AC direct-drive variable frequency sensing circuit to realize stress / preload force monitoring, and has the advantages of being passive and wireless, simple in structure, low in cost, convenient in installation and layout, easy in maintenance, simple in operation, accurate and reliable in measurement results, low in labor cost, and long in service life, and has high practical value. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a schematic diagram of a piezoelectric resonant bolt structure for detecting bolt preload according to Example 2 of the present invention.

[0031] Figure 2 This is a schematic diagram of a piezoelectric resonant anchor structure for detecting anchor stress according to Example 3 of the present invention.

[0032] Figure 3 This is a schematic diagram of a piezoelectric resonant bolt structure for detecting bolt preload according to Example 4 of the present invention.

[0033] Figure 4This is a schematic diagram of the structure of a piezoelectric resonance bolt circuit for detecting bolt preload force according to Example 5 of the present invention.

[0034] Figure 5 This is a schematic diagram of an AC direct-drive variable frequency sensing circuit including a piezoelectric resonant workpiece according to Example 6 of the present invention.

[0035] Figure 6 This is a circuit diagram of a passive wireless bolt preload monitoring device based on piezoelectric material according to Example 7 of the present invention.

[0036] Figure 7 This is a spectrum diagram of a tuning signal generated in the AC direct-drive frequency conversion sensing circuit of Example 7 of the present invention.

[0037] Figure 8 This is a spectrum diagram of the modulation signal of the mixing of the natural resonant frequency of the 6.88 MHz piezoelectric resonant bolt provided in Example 7 of the present invention. DETAILED DESCRIPTION

[0038] In order to have a clearer understanding of the technical features, purposes and effects of the present invention, the specific implementation methods of the present invention are now described. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention, that is, the embodiments described are only part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without making creative work are within the scope of protection of the present invention.

[0039] Example 1

[0040] The present embodiment provides a passive wireless stress monitoring device based on piezoelectric material, including piezoelectric material, upper electrode and lower electrode, metal workpiece and AC direct-drive frequency conversion sensing circuit, wherein the piezoelectric material is configured to realize mutual conversion between electric field and mechanical vibration; the upper electrode and the lower electrode are configured as conductive films formed based on a thin film deposition process, and are respectively arranged on the two end surfaces of the piezoelectric material, and together constitute a piezoelectric resonator; the metal workpiece is configured to be closely connected with the piezoelectric resonator to constitute a piezoelectric resonant workpiece, the resonant frequency of the piezoelectric resonant workpiece changes with the external stress, and the frequency change and the external stress magnitude are in a single mapping relationship; the AC direct-drive frequency conversion sensing circuit is configured to generate a tuning signal based on the excitation of a microwave input signal, so that the piezoelectric resonant workpiece resonates and generates a resonant frequency signal according to the piezoelectric effect; and based on the nonlinear characteristics of the internal semiconductor device, the resonant frequency signal is modulated onto the microwave input signal to form a modulation signal, and the magnitude of the external stress is quantified by detecting the frequency offset of the modulation signal.

[0041] It should be noted that the resonant frequency of a piezoelectric resonant workpiece will change under the action of external stress or strain. Specifically, through the piezoelectric effect, the physical size and elastic modulus of the piezoelectric material will change slightly when subjected to force, resulting in changes in the internal charge distribution, thereby affecting the inherent resonant frequency of the piezoelectric resonant workpiece.

[0042] Preferably, the thin film deposition process for making the upper electrode and the lower electrode, ie, the conductive film, includes: a physical vapor deposition process (such as evaporation deposition and sputtering deposition) or an electrochemical deposition and spraying process.

[0043] Preferably, the metal workpiece and the piezoelectric resonator are tightly connected to form a piezoelectric resonant workpiece, including: the surface of the metal workpiece is directly tightly connected to the piezoelectric resonator through a preset process, so that the lower electrode of the piezoelectric resonator is tightly fitted to the metal workpiece, and the preset process includes conductive adhesive bonding, hot pressing synthesis, welding and inlaying, laser cutting, injection molding, thin film coating, 3D printing and piezoelectric nanomaterial technology, which are specifically described as follows.

[0044] Hot pressing synthesis process: The piezoelectric resonator and the metal workpiece substrate are placed in a mold together, and the piezoelectric resonator and the metal workpiece substrate are tightly combined by heating and applying pressure. The advantage of this connection process is that the piezoelectric resonator has a high bonding strength with the metal workpiece and can work stably under high temperature and high pressure environments.

[0045] Welding and inlaying process: The piezoelectric resonator is directly welded to the surface or inside of the metal workpiece using welding processes such as laser welding and spot welding. The advantage of this connection process is that the welding process can achieve high-strength bonding and is suitable for high-temperature environments.

[0046] Laser cutting process: Use laser cutting technology to form precise structures on the surface or inside of metal workpieces, and then integrate piezoelectric resonators into these structures. The advantage of this connection process is that laser cutting has high precision and can produce complex integrated structures, which is suitable for miniaturized design.

[0047] Injection molding process: The metal workpiece and the piezoelectric resonator are injected into the mold together using injection molding technology, and the piezoelectric resonator is embedded in the metal workpiece directly during the manufacturing process. The advantage of this connection process is that the processing cost is low and it is suitable for mass production.

[0048] Thin film coating process: The piezoelectric film is coated on the surface of the metal workpiece by sputtering, evaporation or spraying to form a piezoelectric material film on its surface. The advantage of this connection process is that the processed piezoelectric resonant workpiece is lightweight and miniaturized, and it can realize the integration of thin-layer piezoelectric resonators.

[0049] 3D printing process: Using 3D printing technology, the piezoelectric resonator and the metal workpiece material are printed together into a metal workpiece-piezoelectric resonator composite structure. The advantage of this connection process is that it can manufacture complex structures and meet highly customized requirements.

[0050] Piezoelectric nanomaterial technology: Use piezoelectric nanomaterials such as nanowires and nanofilms and integrate them into the surface or interior of metal workpieces. The advantages of this connection process are that the processed metal workpieces have high sensitivity in monitoring stress changes, are easy to miniaturize, and have excellent sensing accuracy and anti-interference performance.

[0051] It should be noted that after the piezoelectric resonator is tightly fitted or embedded in the metal workpiece, the deformation effect of the metal workpiece caused by the stress will also be transmitted to the piezoelectric resonator through solid extrusion. When the stress condition of the metal workpiece changes due to the change in the magnitude of the force, the stress condition of the piezoelectric material will also change, thereby causing the natural resonant frequency of the piezoelectric resonant workpiece to change accordingly. By combining AC direct-drive variable frequency sensing technology to load and read the natural resonant frequency of the piezoelectric resonant workpiece, real-time monitoring of the stress state of the metal workpiece can be achieved.

[0052] Preferably, before the surface of the metal workpiece is directly and tightly connected to the piezoelectric resonator through a preset process, the surface of the metal workpiece is first smoothed by means of grinding and polishing, or a groove is made on the surface of the metal workpiece to embed the piezoelectric resonator into the metal workpiece; when the groove process is used for connection, the lower electrode of the piezoelectric resonator is connected to the bottom of the groove on the surface of the metal workpiece through a preset process, and the side of the piezoelectric resonator is tightly fitted to the side wall of the groove on the surface of the metal workpiece through an insulating adhesive.

[0053] Preferably, the types of insulating adhesives include epoxy resins, silicone resins, polyimide resins, phenolic resins, polyurethanes and acrylic resins; the conductive adhesives include a matrix and a conductive filler, the types of the matrix include epoxy resins, silicone resins, polyimide resins, phenolic resins, polyurethanes and acrylic resins, and the types of the conductive fillers include powders of conductive metal materials, graphite and conductive compounds.

[0054] Preferably, the types of piezoelectric materials include piezoelectric single crystals, polycrystalline piezoelectric ceramics, polymer piezoelectric materials and polymer-piezoelectric ceramic composites; the types of piezoelectric resonators include piezoelectric resonators, surface acoustic wave resonators, bulk acoustic wave resonators and transducers; the metal workpiece includes force-bearing devices, and the types of force-bearing devices include bolts, anchor rods, anchor cables, tension bars, gaskets and nuts.

[0055] Preferably, the AC direct-drive frequency conversion sensing circuit includes a first impedance matching network, a frequency sweeping circuit, a reflection matching network and a second impedance matching network. The frequency sweeping circuit is respectively connected to the first impedance matching network, the reflection matching network and the second impedance matching network, and is also connected to the piezoelectric resonant workpiece, as described below.

[0056] The first impedance matching network is configured to match the input impedance of the AC direct-drive frequency conversion sensing circuit with the input source impedance of the AC microwave signal, so that the return loss of the AC microwave signal with an input frequency of f1 at the input of the AC direct-drive frequency conversion sensing circuit is minimized.

[0057] The frequency sweeping circuit is configured to generate tuning signals of several frequency components based on the characteristic that the size of the junction capacitance between the two electrodes of the transistor changes with the alternating voltage applied between the two electrodes; the frequency sweeping circuit includes an inductor and a transistor, and the inductor and the transistor are connected in series or in parallel.

[0058] After the tuning signal generated by the frequency sweeping circuit enters the piezoelectric resonant workpiece, the frequency components close to the inherent resonant frequency f2 of the piezoelectric resonant workpiece will cause mechanical resonance of the piezoelectric resonant workpiece, and based on the piezoelectric effect, the mechanical vibration in the piezoelectric resonant workpiece will be converted into a resonant frequency signal with a gain of frequency f2, and reflected to the transistor of the frequency sweeping circuit, while the remaining frequency components will be attenuated and filtered, thereby completing the frequency selection of the frequency within the range around the inherent resonant frequency f2 of the piezoelectric resonant workpiece; after the frequency components close to the inherent resonant frequency f2 of the piezoelectric resonant workpiece return to the transistor of the frequency sweeping circuit, they are amplified by the transistor gain and then enter the reflection matching network and the second impedance matching network.

[0059] The reflection matching network is configured to have high impedance to the inherent resonant frequency of the piezoelectric resonant workpiece. After the inherent resonant frequency f2 of the piezoelectric resonant workpiece amplified by the transistor gain of the sweeping frequency circuit enters the reflection matching network, if the second impedance matching network is configured as a low-impedance network of the resonant frequency f2, the signal of the frequency f2 will be output by the second impedance matching network; if the second impedance matching network is not configured, the frequency f2 will be reflected back to the transistor for a second time, and will be mixed with the input frequency f1 due to the nonlinear characteristics of the transistor, and a modulated mixed signal will be output at the input port; the mixed signal will carry the inherent resonant frequency information of the piezoelectric resonant workpiece, and then carry the information of the stress magnitude loaded on the piezoelectric resonant workpiece.

[0060] Example 2

[0061] This embodiment is based on embodiment 1:

[0062] like Figure 1As shown, this embodiment provides a piezoelectric resonant bolt for detecting bolt preload, wherein the upper surface of the bolt 4 nut made of metal material is grooved and embedded with piezoelectric material 2, and the lower electrode 3 of the piezoelectric material is tightly fitted with the bottom of the groove on the upper surface of the bolt 4 nut through a polyurethane matrix-copper powder filler conductive adhesive, and the upper and lower surfaces of the piezoelectric material 2 are the upper electrode 1 and the lower electrode 3 respectively, the upper electrode 1 is exposed to the air and connected to the upper surface of the piezoelectric material 2, and the lower surface of the lower electrode 3 is tightly connected to the bottom of the groove through a conductive adhesive, and then connected to the entire bolt 4, so that the entire bolt constitutes the lower electrode. The side surfaces of the piezoelectric material 2 and the upper electrode 1 are tightly fitted with the side walls of the groove on the upper surface of the bolt 4 nut through a polyurethane insulating adhesive.

[0063] Preferably, the conductive electrodes of the piezoelectric material 2 are formed into high-conductivity thin film upper and lower electrodes by physical vapor deposition (such as evaporation, sputtering) or electrochemical deposition, spraying process, and the upper and lower electrodes can be effectively connected to the AC direct-drive variable frequency sensing circuit.

[0064] Example 3

[0065] This embodiment is based on embodiment 1:

[0066] like Figure 2 As shown, this embodiment provides a piezoelectric resonant anchor for detecting anchor stress, and the lower electrode 3 of the piezoelectric material 2 is bonded to the side surface of the anchor 4 using a conductive adhesive. The upper and lower surfaces of the piezoelectric material 2 are the upper and lower electrodes, respectively, and the upper electrode 1 is on the upper surface of the piezoelectric material 2. The lower electrode 3 of the piezoelectric material 2 is tightly bonded to the side surface of the anchor 4 through a conductive bonding agent, and then connected to the entire anchor 4, so that the entire anchor 4 constitutes the lower electrode. The side surface of the piezoelectric material 2 is insulated by an acrylic resin insulating bonding agent. When the stress along the rod direction of the anchor 4 on the piezoelectric resonant anchor changes, the force condition of the piezoelectric material 2 changes, and the natural resonant frequency of the piezoelectric resonant anchor changes.

[0067] Preferably, the conductive electrodes of the piezoelectric material 2 are formed into high-conductivity thin film upper and lower electrodes by physical vapor deposition (such as evaporation, sputtering) or electrochemical deposition, spraying process, and the upper and lower electrodes can be effectively connected to the AC direct-drive variable frequency sensing circuit.

[0068] Example 4

[0069] This embodiment is based on embodiment 1:

[0070] like Figure 3 As shown, this embodiment provides a piezoelectric resonant bolt for detecting bolt preload, wherein an annular groove is opened on the side surface of the nut of the bolt 4 and a hexagonal annular piezoelectric material 2 is embedded. The inner and outer surfaces of the hexagonal annular piezoelectric material 2 are respectively a piezoelectric material lower electrode 3 and an upper electrode 1, and the lower electrode 3 is tightly attached to the inner wall of the hexagonal annular groove opened on the side surface of the nut of the bolt 4 through a conductive adhesive.

[0071] Example 5

[0072] This embodiment is based on embodiment 1:

[0073] like Figure 4 As shown, this embodiment provides a piezoelectric resonant bolt for detecting bolt preload, wherein the bottom of the bolt screw 4 is slotted and embedded with the piezoelectric material 2, and the upper electrode 1 of the piezoelectric material 2 is tightly fitted with the top of the inner wall of the slot on the lower surface of the bottom of the screw of the bolt 4 through a conductive adhesive. The upper surface of the piezoelectric material 2 is the upper electrode 1, and the lower surface is the lower electrode 3. The upper electrode 1 is connected to the top of the inner wall of the slot on the lower surface of the bottom of the screw of the bolt 4 through a silicone resin matrix-aluminum powder filler conductive adhesive, and then connected to the entire bolt 4, so that the entire bolt 4 constitutes the upper electrode. Furthermore, the side surface of the piezoelectric material 2 is tightly fitted with the side surface of the inner wall of the slot on the lower surface of the bottom of the screw of the bolt 4 through an insulating adhesive.

[0074] Example 6

[0075] This embodiment is based on embodiment 1:

[0076] like Figure 5 As shown, this embodiment provides an AC direct-drive variable frequency sensing circuit including a piezoelectric resonant workpiece, including an impedance matching network 1, an impedance matching network 2, a frequency sweeping circuit, a piezoelectric resonant workpiece, and a reflection matching network, as described in detail as follows.

[0077] The impedance matching network 1 matches the input impedance of the AC direct-drive frequency conversion sensor circuit with the input source impedance of the AC microwave signal, so that the return loss of the AC microwave signal with an input frequency of f1 at the input of the AC direct-drive frequency conversion sensor circuit is minimized.

[0078] The frequency sweeping circuit is composed of an inductor L1 connected in series or in parallel with a transistor. Due to the characteristic that the junction capacitance between the two electrodes of the transistor changes with the alternating voltage applied between the two electrodes, a tuning signal with rich frequency components is generated.

[0079] The piezoelectric resonant workpiece has a natural resonant frequency of f2. After the rich tuning signal generated by the frequency sweep circuit enters the piezoelectric resonant workpiece, the frequency components close to f2 will cause the piezoelectric resonant workpiece to resonate mechanically and be reflected into the transistor, while other frequency components will be attenuated and filtered, thereby completing the frequency selection within a certain range of the natural resonant frequency f2 of the piezoelectric resonant workpiece. After the frequency components close to the natural resonant frequency f2 of the piezoelectric resonant workpiece return to the transistor, they will be amplified by the transistor gain and then enter the reflection matching network and impedance matching network 2.

[0080] The reflection matching network is configured to have high impedance to the inherent resonant frequency of the piezoelectric resonant workpiece. After the inherent resonant frequency f2 of the piezoelectric resonant workpiece amplified by the transistor gain enters the reflection matching network, if the impedance matching network 2 is configured as a low-impedance network of the resonant frequency f2, the signal of the frequency f2 will be output by the impedance matching network 2; if the impedance matching network 2 is not configured, the frequency f2 will be reflected back to the transistor for a second time, and due to the nonlinear characteristics of the transistor, it will be mixed with the input frequency f1, and the modulated mixed signal will be output at the input port. The mixed signal will also carry the inherent resonant frequency information of the piezoelectric resonant workpiece, and then carry the information of the stress magnitude loaded on the piezoelectric resonant workpiece.

[0081] Example 7

[0082] This embodiment is based on embodiment 1:

[0083] like Figure 6 As shown, this embodiment provides a passive wireless bolt preload monitoring device based on piezoelectric materials, which realizes passive wireless bolt preload monitoring by using piezoelectric resonant bolts and AC direct-drive frequency conversion sensing circuits. The passive wireless bolt preload monitoring device includes a transistor Q, inductors L1, L2, L3, a capacitor C1, and a piezoelectric resonant bolt X1. An alternating signal with a frequency of f1 is input from the input end, and after impedance transformation through the impedance matching network formed by the inductors L1 and L2, it is fed into the transistor Q with minimum return loss. Due to the characteristic that the junction capacitance between the two poles of the transistor changes with the alternating voltage applied between the two poles, the inductor L1 in the circuit combined with the junction capacitance of the transistor Q will produce a tuning signal with rich frequency components. The spectrum diagram of the tuning signal is shown in FIG. Figure 7 shown.

[0084] The tuning signal is a time-varying frequency signal. After the signal enters the piezoelectric resonant bolt X1 connected to the gate of the transistor Q, the frequency components different from the inherent resonant frequency of the piezoelectric resonant bolt X1 will be attenuated and filtered, while the frequency components close to the inherent resonant frequency of the piezoelectric resonant bolt X1 will cause the piezoelectric resonant bolt X1 to mechanically resonate and return to the transistor Q. The frequency signal within a certain frequency band near the inherent resonant frequency of the piezoelectric resonant bolt X1 enters the LC parallel resonant circuit composed of L3 and C1 after being amplified by the gain of the transistor Q. The resonant circuit is designed to be a circuit that is high-impedance to signals within a certain frequency band near the frequency f2. Therefore, the signal with a frequency of f2 cannot flow back to the ground through the resonant circuit composed of L3 and C1, and the signal will be reflected back to the transistor Q for the second time. Due to the nonlinear characteristics of the transistor, the frequency-selective signal with a frequency of f2 will be mixed with the input AC signal with a frequency of f1. The mixed signal is based on the Menley-Lowe formula:

[0085]

[0086] Among them, P m，nis the frequency f m，n The signal power is f1, f2 is the resonant frequency, m and n are integers, and f m，n The following relationship is satisfied with f1 and f2:

[0087] f m，n =mf1+nf2

[0088] The mixing signal will generate the first and difference frequencies, second and difference frequencies, ..., nth and difference frequencies of the signal with frequency f1 and the signal with frequency f2. The impedance matching network is set to simultaneously meet the maximum efficiency input and output of the signal with frequency f1 and the first and difference frequencies of the signal with frequency f1 and the signal with frequency f2, so that the first and difference frequencies of the mixing signal can be output through the input port, and the effect is as follows: Figure 8 shown.

[0089] The above is only a preferred embodiment of the present invention. It should be understood that the present invention is not limited to the form disclosed herein, and should not be regarded as excluding other embodiments, but can be used in various other combinations, modifications and environments, and can be modified within the scope of the concept described herein through the above teachings or the technology or knowledge of the relevant field. The changes and modifications made by those skilled in the art shall not deviate from the spirit and scope of the present invention, and shall be within the scope of protection of the claims attached to the present invention.

Claims

1. A passive wireless stress monitoring device based on piezoelectric material, characterized in that: include: Piezoelectric materials are configured to achieve mutual conversion between electric fields and mechanical vibrations; The upper electrode and the lower electrode are configured as a conductive film formed based on a thin film deposition process and are respectively arranged on both end surfaces of the piezoelectric material to form a piezoelectric resonator together; The metal workpiece is configured to be closely connected with the piezoelectric resonator to form a piezoelectric resonant workpiece, the resonant frequency of the piezoelectric resonant workpiece changes with the external stress, and the frequency change and the external stress magnitude present a single mapping relationship; An AC direct-drive variable frequency sensing circuit is configured to generate a tuning signal based on the excitation of a microwave input signal, so as to make the piezoelectric resonant workpiece resonate and generate a resonant frequency signal according to the piezoelectric effect; Based on the nonlinear characteristics of internal semiconductor devices, the resonant frequency signal is modulated onto the microwave input signal to form a modulation signal, and the frequency shift of the modulation signal is detected to quantify the magnitude of the external stress.

2. A passive wireless stress monitoring device based on piezoelectric material according to claim 1, characterized in that: The metal workpiece is tightly connected with the piezoelectric resonator to form a piezoelectric resonant workpiece, including: The surface of the metal workpiece is directly and tightly connected to the piezoelectric resonator through a preset process, so that the lower electrode of the piezoelectric resonator is tightly fitted to the metal workpiece. The preset process includes conductive adhesive bonding, hot pressing synthesis, welding and inlaying, laser cutting, injection molding, thin film coating, 3D printing and piezoelectric nanomaterial technology.

3. A passive wireless stress monitoring device based on piezoelectric material according to claim 2, characterized in that: Before the surface of the metal workpiece is directly and tightly connected to the piezoelectric resonator through a preset process, the surface of the metal workpiece is first processed to be flat by means of grinding and polishing, or a groove is made on the surface of the metal workpiece to embed the piezoelectric resonator into the metal workpiece; When the slotting process is used for connection, the lower electrode of the piezoelectric resonator is connected to the bottom of the slot on the surface of the metal workpiece through a preset process, and the side of the piezoelectric resonator is tightly fitted to the side wall of the slot on the surface of the metal workpiece through an insulating adhesive.

4. The passive wireless stress monitoring device based on piezoelectric material according to claim 3, characterized in that: The types of the insulating adhesive include epoxy resin, silicone resin, polyimide resin, phenolic resin, polyurethane and acrylic resin; the conductive adhesive includes a matrix and a conductive filler, the types of the matrix include epoxy resin, silicone resin, polyimide resin, phenolic resin, polyurethane and acrylic resin, and the types of the conductive filler include powder of conductive metal material, graphite and conductive compounds.

5. The passive wireless stress monitoring device based on piezoelectric material according to claim 1, characterized in that: The types of piezoelectric materials include piezoelectric single crystals, polycrystalline piezoelectric ceramics, polymer piezoelectric materials and polymer-piezoelectric ceramic composites; the types of piezoelectric resonators include piezoelectric resonators, surface acoustic wave resonators, bulk acoustic wave resonators and transducers; the metal workpiece includes force-bearing devices, and the types of the force-bearing devices include bolts, anchor rods, anchor cables, tension bars, washers and nuts.

6. The passive wireless stress monitoring device based on piezoelectric material according to claim 1, characterized in that: The AC direct-drive variable frequency sensing circuit includes a first impedance matching network, a frequency sweeping circuit, a reflection matching network and a second impedance matching network. The frequency sweeping circuit is respectively connected to the first impedance matching network, the reflection matching network and the second impedance matching network, and is also connected to a piezoelectric resonant workpiece.

7. The passive wireless stress monitoring device based on piezoelectric material according to claim 6, characterized in that: The first impedance matching network is configured to match the input impedance of the AC direct-drive frequency conversion sensing circuit with the input source impedance of the AC microwave signal, so that the return loss of the AC microwave signal with an input frequency of f1 at the input of the AC direct-drive frequency conversion sensing circuit is minimized.

8. The passive wireless stress monitoring device based on piezoelectric material according to claim 6, characterized in that: The frequency sweeping circuit is configured to generate tuning signals of several frequency components based on the characteristic that the junction capacitance between the two electrodes of the transistor changes with the alternating voltage applied between the two electrodes; the frequency sweeping circuit includes an inductor and a transistor, and the inductor and the transistor are connected in series or in parallel.

9. The passive wireless stress monitoring device based on piezoelectric material according to claim 8, characterized in that: After the tuning signal generated by the frequency sweeping circuit enters the piezoelectric resonant workpiece, the frequency components close to the inherent resonant frequency f2 of the piezoelectric resonant workpiece will cause mechanical resonance of the piezoelectric resonant workpiece, and based on the piezoelectric effect, the mechanical vibration in the piezoelectric resonant workpiece will be converted into a resonant frequency signal with a gain of frequency f2, and reflected to the transistor of the frequency sweeping circuit, while the remaining frequency components will be attenuated and filtered, thereby completing the frequency selection of the frequency within the range around the inherent resonant frequency f2 of the piezoelectric resonant workpiece; after the frequency components close to the inherent resonant frequency f2 of the piezoelectric resonant workpiece return to the transistor of the frequency sweeping circuit, they are amplified by the transistor gain and then enter the reflection matching network and the second impedance matching network.

10. The passive wireless stress monitoring device based on piezoelectric material according to claim 9, characterized in that: The reflection matching network is configured to have high impedance to the inherent resonant frequency of the piezoelectric resonant workpiece. After the inherent resonant frequency f2 of the piezoelectric resonant workpiece amplified by the transistor gain of the sweeping frequency circuit enters the reflection matching network, if the second impedance matching network is configured as a low-impedance network of the resonant frequency f2, the signal of the frequency f2 will be output by the second impedance matching network; if the second impedance matching network is not configured, the frequency f2 will be reflected back to the transistor for a second time, and will be mixed with the input frequency f1 due to the nonlinear characteristics of the transistor, and a modulated mixed signal will be output at the input port; the mixed signal will carry the inherent resonant frequency information of the piezoelectric resonant workpiece, and then carry the information of the stress magnitude loaded on the piezoelectric resonant workpiece.

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