A piezoelectric passive wireless strain sensing system

Abstract

A piezoelectric passive wireless strain sensing system includes the following four parts: a piezoelectric strain sensing unit, a sensing coil, a transmitting coil, and a signal transmitting and collecting device. The sensing coil and the transmitting coil are connected by magnetic field coupling. The system applies the force on the surface of the material to be tested to the upper surface of the piezoelectric plate through a packaged three-point support structure, thereby displaying the deformation as a change in the resonant frequency. By combining a piezoelectric material with a high quality factor with a high-gain coil, the system of the present invention can detect structural deformation at the micro-strain level within a transmission distance of 1-10 cm. The sensing system can be attached to the surface of engineering structures such as large-scale high-altitude steel frame structures and buried pipelines to detect abnormal stress, and can also be embedded in the interior of non-metallic components for full life cycle stress monitoring. It has the advantages of simple structure, high sensitivity and a wide range of applications.

Description

Technical Field

[0001] The present invention belongs to the technical field of sensors, and in particular relates to a passive wireless strain sensing technology. Background Art

[0002] Strain sensors are a crucial component of structural health monitoring systems. Currently, passive wireless strain sensors, which lack internal batteries and chips, require no external cables, and rely on electromagnetic waves for power, can perform contactless strain detection. Therefore, they are particularly suitable for use in hard-to-reach areas such as high-altitude steel structures and buried pipelines. The current state of research on this type of sensor is as follows.

[0003] 1. "A Method for Fabricating a Wireless Passive Flexible Pressure Sensor Based on LC Resonance" (Application No.: 201911383720.X) describes an inductive-capacitive passive wireless strain sensing system. This system consists of a receiving coil, a sensing coil, and a capacitive sensor. It identifies strain by identifying the resonant frequency of the sensing coil connected to a variable capacitor. Due to the mismatch between the resonant frequencies of the receiving and sensing coils, the transmitter has high impedance at the sensor's resonant frequency, resulting in short signal transmission distances and a significant impact of the resonant frequency on distance. The transmission distance is typically 1-3 cm. The sensor, consisting of a μH-level coil and a capacitor of several to tens of pF, has a quality factor typically less than 1000, resulting in low resolution, typically greater than 100 με. Furthermore, during signal processing, it is difficult to distinguish microstrain-level resonant frequency shifts from noisy signals. Therefore, this type of sensor is more suitable for measuring large strains at short distances, such as in electronic skin for monitoring human physiological indicators.

[0004] 2. "A Long-Distance LC Passive Wireless Sensing System" (Application No. 201911033881.6). This sensing system utilizes the principle of parity-time symmetry. The transmitting coil is connected to an adjustable capacitor, and the sensor and transmitter have the same resonant frequency. A variable gain controller is set to compensate for sensor losses and improve the quality factor, thereby increasing the transmission distance to 5 cm. Compared with inductive and capacitive passive sensors, this improves the resolution and transmission distance of existing sensors. However, the parity-time symmetric sensing system requires the variable capacitor and variable gain controller in the transmitter to achieve symmetry between the transmitter and sensor signals, which requires cumbersome operation and complex hardware structure. Secondly, due to the principle of parity-time symmetry, this system can only improve the sensor's quality factor in the strong coupling domain (i.e., at close distances). When the coupled system enters the weak coupling domain, the sensor signal is masked by the transmitter due to the merging of eigenvalue bifurcations, and the quality factor drops to the level of a conventional inductive and capacitive sensor. Therefore, this system is suitable for sensing distances within 5 cm.

[0005] 3. "Monolithic Fabrication of Wireless Miniaturized Quartz Crystal Microbalance (QCM-R) Arrays and Their Application for Biochemical Sensors" describes a passive wireless sensing method for biochemical components using a quartz crystal microbalance combined with a coil antenna. This method is used to detect minute changes in the chemical composition of quartz surfaces. However, due to the limitations of near-field inductive coupling wireless transmission, the signal becomes difficult to detect when the separation distance exceeds 5 mm. Furthermore, this quartz crystal microbalance is not suitable for chemical composition measurement applications, as it is not suitable for engineering stress testing.

[0006] In summary, for stress measurement of actual engineering structures or components, existing passive wireless strain sensors have the problems of short transmission distance and low resolution. Summary of the Invention

[0007] To solve the above-mentioned problems of low resolution and short transmission distance of existing passive wireless strain sensing systems, the present invention proposes a piezoelectric passive wireless strain sensing system. The specific technical solution is as follows:

[0008] A piezoelectric passive wireless strain sensing system includes a sensor 1 and a transmitter 2. The sensor 1 is composed of a piezoelectric strain sensing unit 3 and a sensing coil 4 to form a closed loop. The transmitter 2 is composed of a transmitting coil 5, a matching capacitor 6, and a network analyzer 7 to form a closed loop.

[0009] The piezoelectric strain sensing unit 3 is a packaged piezoelectric force sensitive element with an internal three-point support structure. The lower side of the housing 12 is fixed to the base 13. The housing 12 encapsulates the piezoelectric sheet 8, the first lower fulcrum 9, the second lower fulcrum 10, and the upper pressure point 11. The first lower fulcrum 9 and the second lower fulcrum 10 are in contact with the lower edge of the piezoelectric sheet 8, and the upper pressure point 11 is fixed to the inner center of the upper surface of the housing 12. The first lower fulcrum 9, the second lower fulcrum 10, and the upper pressure point 11 form a three-point support for the piezoelectric sheet 8.

[0010] When the substrate 13 is attached to the measured surface and is not subjected to force, the upper pressure point 11 applies a pre-tightening force to the piezoelectric sheet 8. At this time, the fundamental frequency of the sensing system is higher than the fundamental frequency of the free state; when the measured surface is bent upward due to tension, the shell 12 is deformed along with the substrate 13, and the angle between the first lower fulcrum 9, the second lower fulcrum 10 and the piezoelectric sheet 8 changes, squeezing the piezoelectric sheet 8 toward the upper pressure point 11, thereby increasing the resonant frequency of the sensing system.

[0011] Furthermore, the material of the upper pressure point 11 is an insulating, low-damping, high-hardness material, which is used to reduce the mechanical energy loss caused by the contact between the upper pressure point and the piezoelectric piece 8, and is preferably ceramic.

[0012] Furthermore, the housing 12 is filled with an inert gas, preferably nitrogen or argon, to prevent the piezoelectric piece 8 from being corroded by air.

[0013] Compared with the prior art, the sensor system of the present invention has the following advantages:

[0014] 1. The present invention utilizes a high-quality piezoelectric strain sensor unit, fundamentally overcoming the low quality factor and resolution issues of existing inductive and capacitive sensors. The unique three-point support structure of the piezoelectric strain sensor unit enables passive piezoelectric wireless strain sensors. Existing piezoelectric strain sensors are unable to receive sensor signals through passive wireless sensing because the pressure exerted by the housing on the piezoelectric material results in energy loss at the contact surface. The three-point support structure reduces the contact surface area and damping coefficient with the piezoelectric sheet, minimizing mechanical energy loss.

[0015] 2. The comprehensive performance of the sensor is better than that of existing inductive and capacitive sensors. The sensing distance exceeds 10 cm, which is about twice that of existing inductive and capacitive sensors. The resolution is better than 5με, which is dozens of times that of existing inductive and capacitive sensors.

[0016] 3. By selecting sensors and transmitters with the same resonant frequency but a quality factor that differs by three orders of magnitude, the characteristic signal of the sensor can still be clearly displayed even when the coupling coefficient is extremely low (less than 0.05). At the same time, the influence of the coupling coefficient on the resonant frequency of the sensor is reduced. Even when the distance between the transmitting coil and the sensing coil fluctuates significantly, the characteristic frequency of the sensor can be obtained with high accuracy.

[0017] 4. The sensor is suitable for a variety of extreme environments and, through calibration, can operate in high-temperature, high-humidity, and high-vibration environments. The sensor is a permanent device and requires no additional maintenance once installed. This non-contact sensing method can be used by robots or inspection workers to perform remote measurements, or as an embedded device.

[0018] The sensing system can be used on large steel structures, such as steel bridges, construction cranes, and amusement rides. Sensors are installed using a drone equipped with a manipulator, and the sensor signals are collected using an onboard portable network analyzer. The system can also be embedded within non-metallic materials with a wall thickness of 1-10 cm, such as the exterior walls of aircraft made of carbon fiber composite materials, for contactless strain monitoring throughout their lifecycle. It offers advantages such as lightweight construction, durability, and high resolution. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a structural block diagram of the passive wireless sensing system of the present invention;

[0020] Figure 2 Schematic diagram of the structure of the piezoelectric force-sensitive element of the present invention;

[0021] Figure 3 This is a schematic diagram of the strain sensor signal of the passive wireless sensing system of the present invention;

[0022] Figure 4 A comparison diagram of the resonant frequency of the passive wireless sensor according to the present invention and the signal change of the measured structural deformation;

[0023] Figure 5 The following is a comparison chart of strain sensor signals of three passive wireless sensing systems;

[0024] Figure 6 This is a schematic structural diagram of a piezoelectric force-sensitive element according to an embodiment of the present invention;

[0025] Figure 7 This is a signal comparison diagram of the passive wireless sensor of the present invention at transmission distances of 4 cm and 10 cm;

[0026] Markings in the figure:

[0027] 1-sensor, 2-transmitter, 3-piezoelectric strain sensing unit, 4-sensing coil, 5-transmitting coil, 6-matching capacitor, 7-network analyzer, 8-piezoelectric sheet, 9-first lower fulcrum, 10-second lower fulcrum, 11-upper pressure point, 12-housing, 13-base, 14-transmitter initial signal, 15-sensor sensing signal, 16-buffer pad. DETAILED DESCRIPTION

[0028] The technical solution of the present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0029] A piezoelectric passive wireless strain sensing system includes a sensor 1 and a transmitter 2. The sensor 1 is composed of a piezoelectric strain sensing unit 3 and a sensing coil 4 to form a closed loop. The transmitter 2 is composed of a transmitting coil 5, a matching capacitor 6, and a network analyzer 7 to form a closed loop. The structural diagram is shown in FIG. Figure 1 .

[0030] The method of use is as follows: 1. Before use, perform temperature calibration on the sensor 1 to determine the relationship between the sensor's resonant frequency and temperature. The reason is that thermal expansion will change the preload force applied by the shell to the piezoelectric piece. 2. During use, the base 13 and the surface to be measured are bonded by a curing agent, and the installation direction is that the line between the first lower support point 9 and the second lower support point 10 is parallel to the strain change gradient of the surface to be measured. 3. During measurement, the transmitting coil 5 is placed opposite the sensing coil 4, with the spacing maintained between 1-10 cm, and the network analyzer 7 is used to collect the reflection coefficient curve S of the transmitter. 11.

[0031] When the sensing system is working, the network analyzer 7 sends a sweep frequency signal to the transmitting coil 5 and measures the reflection coefficient S of the transmitter circuit. 11 At the resonant frequency of the transmitting coil 5 and the matching capacitor 6, the impedance of the frequency sweep signal reaches its minimum value and the electromagnetic field intensity reaches its maximum value. This electromagnetic field can be captured by the sensing coil 4 of the sensor 1, generating an induced electromotive force, and generating a current in the closed loop according to the loss of the piezoelectric strain sensor unit 3. This current generates an electromagnetic field in the sensing coil 4, and through near-field coupling, it affects the current in the transmitting coil 5 and the reflection coefficient S measured by the network analyzer 7. 11 .

[0032] The piezoelectric strain sensing unit 3 is a packaged piezoelectric force sensitive element with an internal three-point support structure. The lower side of the housing 12 is fixed to the base 13. The housing 12 encapsulates the piezoelectric sheet 8, the first lower fulcrum 9, the second lower fulcrum 10, and the upper pressure point 11. The first lower fulcrum 9 and the second lower fulcrum 10 are in contact with the lower edge of the piezoelectric sheet 8, and the upper pressure point 11 is fixed to the inner center of the upper surface of the housing 12. The first lower fulcrum 9, the second lower fulcrum 10, and the upper pressure point 11 form a three-point support for the piezoelectric sheet 8.

[0033] When the substrate 13 is attached to the measured surface and is not subjected to force, the upper pressure point 11 applies a pre-tightening force to the piezoelectric sheet 8. At this time, the fundamental frequency of the sensing system is higher than the fundamental frequency of the free state; when the measured surface is bent upward due to tension, the shell 12 is deformed along with the substrate 13, and the angle between the first lower fulcrum 9, the second lower fulcrum 10 and the piezoelectric sheet 8 changes, squeezing the piezoelectric sheet 8 toward the upper pressure point 11, thereby increasing the resonant frequency of the sensing system.

[0034] The piezoelectric sheet 8 is circular, and its fundamental frequency mode is shaped like a gong with flat edges and a protruding center. Therefore, the closer the force position is to its center, the more sensitive the piezoelectric effect is, and the fundamental frequency will move toward higher frequencies as the force increases. The piezoelectric strain sensing unit 3 uses a three-point support structure to apply the stress on the substrate 13 to the center of the piezoelectric sheet 8. The three-point support structure consists of a first lower support point 9, a second lower support point 10, and an upper pressure point 11, wherein the first lower support point 9 and the second lower support point 10 are in contact with the edge of the piezoelectric sheet, so that the upper and lower surfaces of the piezoelectric sheet 8 are suspended. Since the upper pressure point 11 is fixed to the inner center of the upper surface of the shell 12, the shell will deform and apply a reaction force to the piezoelectric sheet, causing its resonant frequency to increase. When the substrate 13 is not subjected to force, the upper pressure point 11 will also apply a preload force to the piezoelectric sheet 8, so that the fundamental frequency at this time is higher than the fundamental frequency of the free state. When the surface of the material being tested is subjected to tension (when bending upward, the base 13 will bend with the same curvature, changing the angle between the first lower support point 9 and the second lower support point 10, and squeezing the piezoelectric plate 8 toward the upper pressure point 11), the contact between the two during the squeezing process will cause mechanical energy loss in the piezoelectric plate, which is determined by the contact conditions, damping coefficient and other conditions.

[0035] It is precisely because the piezoelectric sheet 8 is composed of a packaged piezoelectric sensitive element with a three-point support structure, which generates a current that reacts to the sensor 1, that the signal of the sensing system has the following characteristics: Figure 3 As shown, the signal collected by the network analyzer 7 is the reflection coefficient S of the transmitter 2 at different frequencies. 11 . When there is only the transmitting coil 5, the transmitter original signal 14 is approximately a broadband parabola with a quality factor less than 100, and the peak position is the resonant frequency of the transmitter. When the sensing coil 4 is close to the transmitting coil 5, the sensor sensing signal 15 will be superimposed on the transmitter original signal 14, appearing as a narrowband parabola protruding near the peak of the original broadband parabola, and its quality factor is better than 10000. When the force on the piezoelectric piece 8 increases, the peak of the sensor sensing signal 15 will move to a high frequency. At the same time, the mechanical energy loss caused by contact will reduce the quality factor and amplitude of the signal; when the distance between the sensing coil 4 and the transmitting coil 5 increases, the resonant frequency and quality factor of the sensor sensing signal 15 do not change significantly, but the amplitude decreases. Based on this, the surface strain of the material being measured can be judged according to the resonant frequency of the sensor sensing signal 15.

[0036] To find the sensor's resonant frequency through signal processing, it's necessary to extract the peak value of the sensor's sensing signal 15 from the background noise of the swept frequency signal. Therefore, the higher the signal-to-noise ratio and the higher the quality factor, the higher the sensor's resolution. Compared to existing passive wireless strain sensors, the sensing system of the present invention has higher resolution for the following reasons:

[0037] The quality factor of an inductive and capacitive sensor is affected by the size of the inductor and capacitor. For example, the inductance of a coil with a diameter of less than 5 cm is less than 10 μH, while the capacitance of the same size is generally between a few and tens of pF. Therefore, its resonant frequency is around 10 MHz, and the quality factor is less than 100. The present invention overcomes the technical difficulties of high-frequency piezoelectric materials in being difficult to apply to pressure and strain measurement, and develops a piezoelectric strain sensing unit, whose quality factor is mainly determined by the selected piezoelectric material and is almost unconstrained by the sensor structure design and size, and is usually greater than 10,000. Furthermore, based on the use of a piezoelectric sensor, the present invention adopts a three-point support structure. When the measured surface is under tension, the three-point support structure further concentrates the force on the geometric center of the piezoelectric sheet 8, making the change of the fundamental frequency sensitive to the change of the force, further improving the detection resolution. To realize a piezoelectric wireless strain sensor, the following two points need to be achieved: the sensor housing applies pressure to the surface of the piezoelectric material. As the housing pressure changes, 1. the oscillation amplitude of the piezoelectric material will not be significantly attenuated, and 2. there is a significant shift in the frequency. The special structural design of the sensor of the present invention is precisely to meet the above requirements, while minimizing the contact area between the housing and the piezoelectric piece and reducing the damping coefficient of the contact surface.

[0038] To verify the structural design principle of the sensor, the following experimental data was used to prove that the resonant frequency of the piezoelectric strain sensor unit can move with the strain applied to the structure. During the test, the network analyzer used a smoothing process to restore the sensor signal characteristics in a noise-free environment by taking the average of ten measurements. The piezoelectric plate 8 is a quartz resonator with a resonant frequency of 10MHz and a coil spacing of 10cm. The signal amplitude of the transmitter 2 is approximately 0.02dB. The strain of the substrate and the signal of the sensing system under different loads, such as Figure 4 The graph shows that the peak frequency of the sensor signal varies with load, and that the sensor signal amplitude exhibits no significant attenuation when the piezoelectric plate is squeezed, confirming the rationality of the structural design of the piezoelectric strain sensing unit. Data analysis shows that the sensor has a sensitivity of 4.5 Hz / με and a raw signal-to-noise ratio of 18 dB, corresponding to a resolution better than 5 με.

[0039] To demonstrate the superior performance of the sensor system's resolution, this patent was tested and compared with two other existing sensor systems. The test conditions were: the distance between the sensor coil 4 and the transmitting coil 5 was 5 cm, which is the limit of existing capacitive and inductive sensors. At the same time, the resonant frequency of each sensor was set at around 10 MHz. The specific test data analysis is as follows: Figure 5 As shown in the figure, A is a piezoelectric plus matching capacitive sensor, B is a piezoelectric non-matching capacitive sensor, and C is an inductive capacitive sensor. The signals shown are the original signals that have not been smoothed. Figure 5In A, the sensor 1 uses a 10 MHz quartz resonator as the piezoelectric plate 8, and the transmitter 2 has a matching capacitor 6. The signal-to-noise ratio and quality factor are approximately 56 dB and 16,000, respectively; Figure 5 In B, the sensing system is similar to the existing piezoelectric passive wireless biochemical sensor (Background Art 3). Its sensor 1 is the same as the former, but the transmitter 2 does not contain the matching capacitor 6. The signal-to-noise ratio and quality factor are approximately 16 dB and 13,000, respectively. Figure 5 In C, the sensing system corresponds to the existing inductive-capacitive passive wireless strain sensor (Background Art 1), and its sensor and transmitter are the same as the existing inductive-capacitive sensor, with a signal-to-noise ratio and quality factor of approximately 18dB and 20, respectively. Figure 5 A, 5B comparison Figure 5 As can be seen from C, the piezoelectric piece 8 of the sensor 1 improves the quality factor of the signal by more than 600 times. The results prove that the sensor system of the present invention has obvious advantages in parameters such as quality factor and signal-to-noise ratio. In addition, Figure 5 A comparison Figure 5 Figures B and 5C demonstrate that matching capacitor 6 in transmitter 2 improves the signal-to-noise ratio by a factor of 100. This indicates that without matching capacitors, the signal-to-noise ratios of piezoelectric and inductive-capacitive sensors are on the same order of magnitude. In summary, the present invention combines a high-quality piezoelectric plate 8 with a high-gain transmitting coil 5, improving both the quality factor and signal-to-noise ratio of existing inductive-capacitive sensors by two orders of magnitude within the weak-coupling domain, significantly enhancing the overall performance of passive wireless sensors.

[0040] Furthermore, the upper pressure point 11 is made of an insulating, low-damping, high-hardness material, such as ceramic, to reduce mechanical energy loss caused by contact between the upper pressure point and the piezoelectric sheet 8. The housing 12 is filled with an inert gas, such as nitrogen or argon, to prevent the piezoelectric sheet 8 from being corroded or oxidized by air.

[0041] The technical difficulties solved by the present invention and the technical means adopted are as follows:

[0042] 1. Piezoelectric elements have not been successfully applied to existing passive sensors. The reason is that the packaging method of existing piezoelectric elements is end-face fixed, that is, the two ends of the piezoelectric piece are fixed to the shell wall of the packaging shell, which hinders the mechanical vibration caused by the external excitation electric field. As the test progresses, the mechanical energy loss of the piezoelectric piece rapidly increases, resulting in significant signal attenuation and inability to accurately measure. In order to overcome the difficulty of mechanical energy loss, the present invention adopts a "three-point support" structure inside the piezoelectric strain sensing unit 3. The left and right edges of the lower surface of the piezoelectric piece 8 are suspended by the first lower support 9 and the second lower support 10; the center of the upper surface of the piezoelectric piece 8 is in contact with the upper pressure point 11, and downward pressure is applied to the piezoelectric piece through the deformation of the shell 12; this structure does not use adhesive to fix the piezoelectric piece, otherwise it will cause mechanical energy loss of the piezoelectric piece, resulting in significant signal attenuation. While the piezoelectric piece 8 is suspended, the stress of the measured surface is concentrated and applied to the center of the surface of the suspended piezoelectric piece.

[0043] 2. The higher the resonant frequency of the piezoelectric sheet 8, the thinner it is and the easier it is to be crushed. Under actual engineering conditions, its service life is also a difficulty that restricts its use. In order to reduce the rigidity of the top of the housing 12 to protect the piezoelectric sheet 8, the present invention is Figure 2 The following three methods can be further adopted based on the structural diagram. First, a flexible buffer structure is added between the upper pressure point 11 and the top of the shell 12 to absorb the stress of the upper pressure point 11, and at the same time, the wall thickness of the top of the shell 12 is reduced to increase its deflection when subjected to force. Second, when using three-dimensional printing to manufacture the package of the piezoelectric strain sensor unit 3, the top of the shell 12 is designed to be a flexible thin-walled hollow structure, so that there is no need to add a buffer element between the upper pressure point and the shell, and it plays a buffering and protective role, greatly extending the service life of the component. Third, a buffer element can also be added between the upper pressure point and the shell, see Figure 6 ; In this specific embodiment, each component is Figure 2 A variation of the middle structure adds 16 rubber buffer pads between the shell and the upper pressure point, thereby reducing the force on the piezoelectric piece.

[0044] 3. Even so, in engineering applications, the contact between the piezoelectric sheet 8 and the upper pressure point 11 still inevitably results in mechanical energy loss. To further reduce this mechanical energy loss, the upper pressure point of the present invention utilizes a material with a low damping coefficient and reduces the area of ​​the contact surface between the upper pressure point and the piezoelectric sheet. Preferably, the upper pressure point utilizes a high-hardness, low-damping ceramic sheet. For thicker piezoelectric sheets with frequencies between 1 and 6 MHz or for more flexible buffer structures, the upper pressure point can utilize a circular contact surface to further reduce losses.

[0045] 4. When the coupling coefficient is extremely low, the near-field passive wireless sensing system is unable to identify the difficulty of the resonant frequency of the sensor. The present invention compensates the transmitting coil so that the center frequency of the operating frequency band of the transmitting coil is equal to the resonant frequency of the sensor, which maximizes the energy efficiency of the sensing system at this frequency. At the same time, due to the difference of three orders of magnitude in the quality factor between the transmitter and the sensor, the second magnetic field generated by the sensor will reduce the current of the transmitter in an extremely narrow frequency band, causing the reflection curve of the transmitter to be sharply distorted at the resonant frequency. This distortion can still be identified even when the coupling coefficient is extremely low (less than 0.001). However, the existing inductive and capacitive sensing method has the problem of mismatch between the resonant frequency of the transmitter and the sensor, which reduces the gain of the transmitter at the resonant frequency of the sensor. For the existing parity-time symmetric inductive and capacitive sensors, since the quality factor of the transmitter and the sensor is the same, the above-mentioned distortion of the reflection coefficient curve does not exist when the coupling coefficient is extremely low, and thus the state of the sensor cannot be identified.

[0046] The quality factor is determined by energy loss; the lower the energy loss, the higher the quality factor; the quality factor determines the resolution. To improve the quality factor, one method is to use piezoelectric materials; another method is to use a parity-time symmetric LC-type passive sensor (Background Art 2) to improve the quality factor. The principle is to compensate for the lost energy of the sensor. However, if the parity-time symmetry method is used, once the distance is increased, the quality factor returns to the quality factor of the LC-type sensor, and the energy loss cannot be compensated. The method described in the present invention utilizes the properties of piezoelectric materials and can improve the quality factor at all distances, thereby improving the resolution.

[0047] To illustrate the signal parameter characteristics of the sensor of the present invention, specific components and retrograde data are used for testing as follows. The test sensing coil 4 and transmitting coil 5 are both planar spiral coils with a diameter of 4.5 cm and an inductance of 6.5 μH. During the test, the network analyzer uses a smoothing process of taking the average of ten measurements to restore the sensor signal characteristics in a noise-free environment. The piezoelectric plate 8 is a quartz resonator with a resonant frequency of 10 MHz, and the peak reflection coefficient of the transmitter 2 is -13.5 dB. Figure 7 As shown, when the distances between the transmitting coil 5 and the sensing coil 4 are 4 cm and 10 cm, respectively, the coupling coefficients are approximately 0.08 and 0.006, and the amplitudes of the sensor sensing signal 15 are 0.037 dB and 14.5 dB, respectively. These results demonstrate that even with a 10 cm separation between the two coils, a characteristic signal with a high signal-to-noise ratio and high quality factor can still be obtained, allowing the parabolic peak of the sensor signal to be accurately identified.

[0048] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A piezoelectric passive wireless strain sensing system, comprising a sensor (1) and a transmitter (2), wherein the transmitter (2) comprises a transmitting coil (5), a matching capacitor (6), and a network analyzer (7) forming a closed loop, characterized in that: The sensor (1) is composed of a piezoelectric strain sensing unit (3) and a sensing coil (4) to form a closed loop. The piezoelectric strain sensing unit (3) is a packaged piezoelectric sensitive element with an internal three-point support structure. The lower portion of the housing (12) is fixed on a base (13). The housing (12) contains a piezoelectric sheet (8), a first lower fulcrum (9), a second lower fulcrum (10), and an upper pressure point (11). The first lower fulcrum (9) and the second lower fulcrum (10) are in contact with the lower edge of the piezoelectric sheet (8). The upper pressure point (11) is fixed to the inner center of the upper surface of the housing (12). The first lower fulcrum (9), the second lower fulcrum (10), and the upper pressure point (11) form a three-point support for the piezoelectric sheet (8). When the substrate (13) is attached to the surface to be measured and is not subjected to force, the upper pressure point (11) applies a pre-tightening force to the piezoelectric sheet (8), and at this time, the fundamental frequency of the sensing system is higher than the fundamental frequency of the free state; when the surface to be measured is bent upward due to tension, the housing (12) is deformed along with the substrate (13), and the angle between the first lower support point (9), the second lower support point (10) and the piezoelectric sheet (8) changes, squeezing the piezoelectric sheet (8) toward the upper pressure point (11), thereby increasing the resonant frequency of the sensing system.

2. The piezoelectric passive wireless strain sensing system according to claim 1, characterized in that: There is a difference of three orders of magnitude between the quality factors of the transmitter (2) and the sensor (1). The quality factor of the transmitter is below 20, while the quality factor of the sensor is above 10,000. As a result, the second magnetic field generated by the sensor (1) will reduce the current of the transmitter within an extremely narrow frequency band, causing the reflection curve of the transmitter (2) to be sharply distorted at the resonant frequency. This distortion can still be identified even when the coupling coefficient is extremely low.

3. The piezoelectric passive wireless strain sensing system according to claim 2, wherein: The material of the upper pressure point (11) is an insulating, low-damping, high-hardness material, and is used to reduce the mechanical energy loss caused by the contact between the upper pressure point and the piezoelectric sheet (8).

4. The piezoelectric passive wireless strain sensing system according to claim 3, characterized in that: The material of the upper pressure point (11) is ceramic.

5. The piezoelectric passive wireless strain sensing system according to claim 3, characterized in that: The interior of the housing (12) is filled with inert gas to prevent the piezoelectric piece (8) from being corroded by air.

6. The piezoelectric passive wireless strain sensing system according to claim 5, characterized in that: The inert gas filled into the shell (12) is argon.

7. The piezoelectric passive wireless strain sensing system according to claim 3, characterized in that: The interior of the housing (12) is filled with nitrogen to prevent the piezoelectric piece (8) from being corroded by air.

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