A flexible ultra-small RFID strain sensing tag and strain detection method

Abstract

The present invention discloses a flexible ultra-small RFID strain sensor tag and a strain detection method, which are applied to the field of structural health monitoring and non-destructive testing. The flexible ultra-small RFID strain sensor tag provided by the present invention is attached to the surface of a component. When the tag is deformed, its impedance changes to Z. sensed , the radar scattering cross section changes accordingly; while communicating with the tag, the reader can remotely monitor the antenna parameters according to the changes in the RCS0 and RCS1 intensities, thereby inferring the changes in the echo signal strength RSSI and indirectly obtaining the tag deformation; when the tag antenna impedance changes, the tag's resonant frequency changes accordingly. By analyzing the resonant frequency offset, the tag's deformation information can also be indirectly obtained.

Description

Technical Field

[0001] The present invention belongs to the field of structural health monitoring and nondestructive testing, and in particular relates to a strain sensing tag and a strain detection technology. Background Art

[0002] When damage and defects accumulate to a certain extent, the bearing capacity and fatigue resistance of the structure will be reduced, leading to extremely serious consequences. Strain is an important indicator to characterize the stress state of engineering structures. The safety status of the structure can be effectively judged by strain detection at the stress concentration point. Therefore, it is of great significance to use reliable and practical technical means to monitor the strain on the surface of the structure and then take corresponding preventive measures to reduce or avoid the occurrence of structural failure accidents.

[0003] After decades of development, traditional strain detection and health monitoring technology has formed a relatively complete theoretical system. Strain detection methods include strain gauge electrical measurement method, fiber Bragg grating test method, etc. However, due to the heavy equipment, slow detection speed, small detection range and low degree of automation, traditional non-destructive testing technology is often not feasible and expensive in detecting potential damage in large-scale facilities, especially in complex environments. In response to the problem of surface stress health monitoring of large-scale facilities, wireless sensor networks (WSNs) have emerged. WSN networks are composed of a large number of cheap micro sensor nodes deployed in the monitoring area, forming a self-organizing network through wireless communication. RFID technology can be used to build wireless sensor networks. RFID, or radio frequency identification technology, refers to non-contact two-way data communication through wireless radio frequency, so as to achieve the purpose of identifying targets and exchanging data. The classic RFID system consists of a reader and multiple RFID tags. RFID tags have the advantages of low cost, array formation, and long-distance communication. After developing sensing functions, they can be used as sensor nodes in wireless sensor networks. Currently, RFID wireless sensing technology is divided into active and passive types. Active sensing requires batteries to provide energy to the antenna and sensor. Battery-based RFID wireless sensing technology has not become a mainstream technical solution for wireless detection technology due to its relatively large size, expensive replacement costs, low sensor life, and battery pollution to the environment. Passive RFID strain sensing technology does not require an additional integrated power supply, but only requires external RF signal power to work. It can be implanted inside the device without replacing parts. It has better robustness and stability, and can work unattended all year round, making it the current mainstream technical solution.

[0004] The relevant prior art is as follows:

[0005] An embedded wireless strain sensor based on printed RFID tag is proposed in this paper. The design uses the threshold power P of the RFID tag to wake up the reader. t To invert strain. Size is 8mm*97mm.

[0006] A Novel RFID-Based Strain Sensor for Wireless Structural Health Monitoring proposes an RFID strain sensor tag based on an inverted F antenna, which uses the resonant frequency of the antenna strain sensor as the characteristic value to characterize the strain. The design fixes the reader's transmission power P, sweeps the frequency in the UHF (ultra-high frequency) band, and calculates the frequency f i The maximum interrogation distance R under i , then the frequency corresponding to the maximum interrogation distance is the resonant frequency. When the antenna is deformed, the resonant frequency of the antenna will also change. As long as the offset △f of the resonant frequency is known, the strain of the antenna can be quantitatively inverted. The size is 72mm*92mm.

[0007] Reusable Passive Wireless RFID Sensor for Strain Measurement on Metals: A reusable passive radio frequency identification (RFID) sensor for wireless strain measurement of metallic materials is reported. The characteristics of the proposed sensor are evaluated on aluminum tensile specimens. The specimen strain is measured using the received signal strength intensity (RSSI) of the RFID sensor and the frequency corresponding to the maximum RSSI. In addition to strain, the yield, ultimate tensile strength, and failure point of the specimen can also be determined from the RSSI measurements. The size is 80.8mm*19.8mm.

[0008] Longest-Range UHF RFID Sensor Tag Antenna for IoT Applied for Metal and Non-Metal Objects: A passive cavity-type ultra-high frequency radio frequency identification (UHF) tag antenna with the longest reading distance is proposed. The size is 140mm*60mm.

[0009] Passive RFID Strain-Sensor Based on Meander-Line Antennas: Design a fully passive UHF RFID sensor for strain monitoring from a flexible meander-line dipole form factor and feed segment design to achieve the required sensing resolution and dynamic range. The size is 36mm*36mm.

[0010] At present, strain sensing tags generally use antenna structures such as microstrip and inverted F antennas, which means that the size of the tag antenna must be greater than or equal to half or one-quarter of the wavelength corresponding to the operating frequency (one-quarter is the current path folding using short-circuit via technology). In addition, the current strain sensing tags all use hard substrates as antenna substrates, which are difficult to install on curved components. Summary of the invention

[0011] In order to solve the above technical problems, the present invention proposes a flexible ultra-small RFID strain sensor and a strain detection method, which can obtain the structural changes of the tag antenna or the surrounding environment information of the tag by reading the physical signals related to the tag communication and performing analog processing.

[0012] One of the technical solutions adopted by the present invention is: a flexible ultra-small RFID strain sensor tag, comprising: a radio frequency chip and an antenna structure; the antenna structure specifically comprises: a metal patch, a dielectric substrate, and a metal floor; the metal patch is arranged on the top surface of the dielectric substrate, and the metal floor is arranged on the bottom surface of the dielectric substrate.

[0013] The metal patch includes: a square ring microstrip line, a first circular ring, a second circular ring, and a circular resonant plate, wherein the radius of the first circular ring is greater than the radius of the second circular ring, and the radius of the second circular ring is greater than the radius of the circular resonant plate;

[0014] The square ring-shaped microstrip line includes two notches of the same size located on the symmetry axis;

[0015] The first circular ring includes a symmetrical first opening and a second opening; the second circular ring includes an opening; the first opening of the first circular ring is connected to the opening of the second circular ring, and the second opening of the first circular ring is connected to a notch of the square annular microstrip line;

[0016] The circular resonance piece connects the opening center point of the second circular ring to the symmetrical point on the second circular ring;

[0017] Another notch of the square ring-shaped microstrip line is connected to the radio frequency chip.

[0018] The dielectric substrate is a non-metallic thin sheet structure that is bendable and stretchable.

[0019] The input impedance of the radio frequency chip is conjugate matched with the antenna impedance.

[0020] The second technical solution adopted by the present invention is: a flexible ultra-small RFID strain sensor and a strain detection method, comprising:

[0021] S1, attaching the RFID strain sensor tag to the surface of the component;

[0022] S2. Generate radio frequency signal: Place the radio frequency receiving and transmitting module device at a distance of h meters above the measured component, and the radio frequency receiving and transmitting module device generates a swept frequency radio frequency electromagnetic wave signal U1;

[0023] S3, generating a response signal: after the tag antenna attached to the surface of the component receives the RF signal U1, the tag chip rectifies U1 and modulates the signal in a backscattering manner to generate a RF response signal U2;

[0024] S4, receiving the RF response signal: the RF receiving and transmitting module receives the RF response signal U2, and inputs U2 into the data acquisition and processing module; the data acquisition and processing module analyzes the signal U2 to obtain the frequency-amplitude curve fA of the signal U2;

[0025] S5, extract characteristic frequency: extract the resonant frequency f0 of the frequency-amplitude curve fA detected in S4;

[0026] S6. Invert the magnitude of the component surface strain through the frequency shift of the resonant frequency.

[0027] Step S6 includes the following sub-steps:

[0028] S61. Establish the relationship between frequency shift and strain: set n experiments, where the i-th experiment applies strain ε on the specimen i , i=1,2,…,n, the corresponding resonant frequency shift Δω is measured i ; The measured resonant frequency shift is used as the horizontal coordinate, and the corresponding strain applied in time is used as the vertical coordinate, and the frequency shift-strain equation is obtained by fitting;

[0029] S62, measuring the current resonant frequency offset of the RFID strain sensor tag according to the resonant frequency extracted in step S5, and calculating the corresponding strain in combination with the frequency shift-strain equation obtained in step S61.

[0030] Beneficial effects of the present invention: The present invention adopts four resonant structure composites, and the four resonant structures specifically include: an outer square line, an outer open ring, an inner open ring, and a circular resonant plate. The advantage of the label structure of the present invention is that while ensuring that the thickness of the label itself is extremely thin, the surface area is further reduced; that is, the label of the present invention is smaller in size than the existing labels; the flexible ultra-small chip RFID strain sensor label provided by the present invention is more suitable for complex mechanical mechanisms, can detect the size of the surface strain of gold components, and can realize the monitoring of the surface strain of the structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a schematic diagram of the top patch;

[0032] Figure 2 It is the principle of RFID tag antenna sensor system;

[0033] Figure 3 This is a schematic diagram of strain loading simulation;

[0034] Figure 4 is the frequency domain response of strain;

[0035] Figure 5 is the strain-resonance frequency linear fit;

[0036] Figure 6 A COMSOL model of two labels placed orthogonally. DETAILED DESCRIPTION

[0037] To facilitate those skilled in the art to understand the technical content of the present invention, the present invention is further explained below with reference to the accompanying drawings.

[0038] With the development of intelligent demand and process technology, the size of RFID tags is constantly developing in the direction of miniaturization. The miniaturization of tag antennas reduces the cost of a single tag, and the smaller size makes it suitable for more application scenarios. For example, the reduction in the size of RFID sensor tags with sensing functions makes them suitable for complex mechanical mechanisms. In addition, with the maturity of the processing technology of flexible circuit boards, the planar flexible RFID antennas produced in batches using this as a substrate are more convenient for large-scale deployment, and the flexible substrate allows RFID tags to be arranged on the surface of objects with complex shapes. Therefore, the present invention designs a miniaturized flexible RFID sensor tag and proposes a strain detection method.

[0039] The present invention provides a flexible ultra-small patch antenna strain sensor tag based on RFID technology, and its surface patch structure is as follows Figure 1 shown.

[0040] The tag surface patch is mainly composed of a square microstrip line, an open ring structure and a circular resonant plate, wherein the dielectric substrate is a bendable and extensible non-metallic thin sheet structure with a thickness of h; the metal floor is arranged on the entire area of ​​the bottom surface of the dielectric substrate; the top metal patch is arranged on the top surface of the dielectric substrate;

[0041] Square microstrip line width w line, containing two gaps with a width of l1. One end of the gap is connected to the RF chip, and the other end is extended and connected to the outer open ring. The square microstrip line, the outer open ring, the inner open ring and the circular resonant plate are nested. The outer diameter of the outer open ring is r5 and the inner diameter is r4; the outer diameter of the inner open ring is r3 and the inner diameter is r2; the radius of the circular resonant plate is r1. The width of the microstrip line connecting the inner open ring and the circular resonant plate is l2. The label is a square with a width of w0.

[0042] The notch and opening on the label surface patch are located on the central axis of the surface patch, so that the label surface patch is symmetrical on the left and right, so that the current path is equal on the left and right sides.

[0043] The present invention adopts a composite of four resonant structures, which specifically include: an outer square line, an outer open ring, an inner open ring, and a circular resonant plate. The label structure of the present invention does not need to have a size that is one-quarter of the wavelength. The advantage of the label structure of the present invention is that while ensuring that the thickness of the label itself is extremely thin, the surface area is further reduced; that is, the label of the present invention is smaller in size than existing labels.

[0044] Detection principle

[0045] When RFID sensors detect structural changes (such as strain and cracks), they can regard the tag antenna structure as a sensor module, and the physical quantity to be measured is coupled with the controlled changes in the geometric and electrical characteristics of the tag antenna. By reading the physical signals related to tag communication and performing analog processing, the structural changes of the tag antenna or the information about the tag's surrounding environment can be obtained. The challenge in designing RFID strain sensors lies in the balance of tag sensing and communication performance. The working principle of the strain sensor system based on the passive chip RFID tag antenna is as follows: Figure 2 As shown. The reader transmits a continuous wave (CW) to activate nearby tags and requires the tags to respond with their ID. The RFID tag antenna obtains energy from the reader's RF field. In order to maximize the power obtained by the tag, the tag antenna is required to be designed to be conjugate matched with the chip's input impedance. The tag chip impedance (Z c =R c +jX c ) and the tag antenna impedance (Z a =R a +jX a The impedance matching degree between the two is characterized by the reflection coefficient Г:

[0046]

[0047] Since the tag chip has both resistance and reactance, Rc in formula (1) is the real part of the tag chip impedance, Xc is the imaginary part of the tag chip impedance, Zc is the total impedance of the tag chip, and R a is the real part of the tag antenna impedance, Xa is the imaginary part of the tag antenna impedance, Z a is the total impedance of the tag antenna, Indicates the overall impedance of the antenna after the change.

[0048] When the tag chip obtains energy exceeding the threshold, the tag chip changes its internal impedance and emits an amplitude or phase modulated backscattered signal controlled by the antenna loading. In essence, the tag changes its own radar reflection cross section to RCS0 and RCS1, which represent binary signals of "0" and "1" to the reader.

[0049] When the tag is deformed, its impedance changes to Z sensed , the radar cross section (RCS) changes accordingly. While communicating with the tag, the reader can remotely monitor the antenna parameters based on the changes in the RCS0 and RCS1 strengths, thereby inferring the changes in the echo signal strength RSSI and indirectly obtaining the tag deformation. When the tag antenna impedance changes, the tag's resonant frequency changes accordingly. By analyzing the resonant frequency shift (RFS), the tag's deformation information can also be indirectly obtained.

[0050] A quantitative nondestructive testing method for component surface strain based on RFID strain sensing tags comprises the following steps:

[0051] S1, attaching the RFID strain sensor tag to the surface of the component;

[0052] S2. Generate radio frequency signal: A radio frequency receiving and transmitting module device placed at a distance of h meters above the measured component generates a swept frequency radio frequency electromagnetic wave signal U1;

[0053] S3, generating a response signal: after the tag antenna attached to the surface of the component receives the RF signal U1, the tag chip rectifies and modulates the signal in a backscattering manner to generate a RF response signal U2;

[0054] S4. Receive RF response signal: The RF receiving and transmitting module receives the RF response signal U2, and inputs U2 into the data acquisition and processing module. The data acquisition and processing module analyzes the signal U2 to obtain the frequency-amplitude curve fA of the signal U2. Analyzing the signal U2 is an existing known technology. Specifically: the data acquisition module is an impinj reader. The reader scans the frequency within the working frequency band. Each frequency point will read the echo signal strength RSSI corresponding to the tag on the chip. The signal strength is the amplitude of U2, and the frequency point is the horizontal axis of the curve; the processing module is a host computer. The host computer can obtain the frequency-amplitude curve fA of the signal U2 based on the amplitude of U2 and the corresponding frequency point.

[0055] S5, extract characteristic frequency: extract the resonant frequency f0 of the frequency-amplitude curve fA detected in S4. Since the tag chip always absorbs electromagnetic field energy at the resonant frequency to reach the maximum power, the strongest echo signal strength RSSI exists at this time, so the frequency corresponding to the maximum amplitude point of the curve fA is the resonant frequency f0.

[0056] S6. Inversion of strain magnitude through frequency shift of resonant frequency

[0057] S61. Establish the relationship between frequency shift and strain: Set n experiments, where the i-th experiment (i = 1, 2, ..., n) is to apply strain ε on the specimen i , the corresponding resonant frequency shift Δω is measured i The frequency shift-strain equation ε=a*Δω+b is obtained by linear fitting with Δω as the horizontal coordinate and ε as the vertical coordinate, where a and b are constant coefficients determined by the label structure parameters. The strain is recorded as Figure 3 shown.

[0058] S62. When the specimen produces a strain of unknown magnitude, measure the resonant frequency shift Δω0 of the tag at this time, substitute Δω0 into the frequency shift-strain equation, and calculate the corresponding strain ε0.

[0059] Example:

[0060] The RF chip used is IMPINJ MONZA R6-P. The size of the tag chip is 464.1μm*442μm. At the operating frequency of 923MHz, the resistance of the chip is 11.831Ω and the reactance is -118.564Ω. According to the conjugate matching principle, the antenna impedance should be designed to be 11.831+118.564iΩ.

[0061] The specific geometric parameters of the top patch are shown in Table 1. The substrate material is polyester film, with a thickness h of 0.1 mm, a relative dielectric constant of 3.2, and a relative dielectric loss of 0.003. The size of the UHF band RFID tag is 9.6 mm*9.6 mm.

[0062] Table 1 Parameters

[0063]

[0064] Set the strain to 0-20% with a step size of 5%. Label the frequency domain response as Figure 4 shown.

[0065] As the strain increases, the resonant frequency of the tag shifts to a low frequency. The resonant frequency is extracted and linearly fitted. The fitting results are shown in Figure 5As shown, its R-Square is 0.9908, which proves that the tag has a linear response to strain. For every 1% deformation, the resonant frequency of the tag will shift 3.96MHz to a low frequency.

[0066] A simulation model is established in COMSOL to explore the strain detection capability of flexible RFID tags, and a strain detection method based on flexible RFID tags is proposed. The strain sensitivity of the flexible ultra-small RFID strain sensing tag is -0.396kHz / με, and its COMSOL simulation model is as follows: Figure 6 As shown. The size of the bottom iron substrate is 40mm*40mm*2mm.

[0067] Considering that the yield limit of steel in engineering is considered to be 0.2% (2000με), the strain is set to 300-1800με with a step size of 300με to ensure that the strain of the component is within the elastic range, and the applied specified displacement is 12-72μm with a step size of 12μm.

[0068] The tag's resonant frequency shift RFS is used to invert the strain, as shown in Table 2.

[0069] Table 2 Inversion strain and relative error

[0070]

[0071] The tag's detection error for strains between 300-1800με is less than 2.5%. The proposed flexible ultra-small RFID sensor tag has good strain detection capability.

[0072] Those skilled in the art will appreciate that the embodiments described herein are intended to help readers understand the principles of the present invention, and should be understood that the scope of protection of the present invention is not limited to such specific statements and embodiments. For those skilled in the art, the present invention may have various changes and variations. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of the claims of the present invention.

Claims

1. A flexible ultra-small RFID strain sensing tag, characterized in that: include: RF chip and antenna structure; The antenna structure specifically comprises: a metal patch, a dielectric substrate, and a metal floor; the metal patch is arranged on the top surface of the dielectric substrate, and the metal floor is arranged on the bottom surface of the dielectric substrate; The metal patch includes: a square ring-shaped microstrip line at the periphery, a first circular ring, a second circular ring, and a circular resonant plate nested in the center of the square ring-shaped microstrip line, the radius of the first circular ring is greater than the radius of the second circular ring, and the radius of the second circular ring is greater than the radius of the circular resonant plate; The square ring-shaped microstrip line includes two notches of the same size located on the central symmetry axis of the metal patch; The first circular ring includes a first opening and a second opening located on the central symmetry axis of the metal patch; the second circular ring includes an opening located on the central symmetry axis of the metal patch; the first opening of the first circular ring is connected to the opening of the second circular ring, and the second opening of the first circular ring is connected to a notch of the square annular microstrip line; the notch and the opening on the label surface patch are located on the central axis of the surface patch, so that the label surface patch is bilaterally symmetrical; The circular resonant plate is connected to the symmetrical point on the second circular ring where the center point of the second circular ring opening is located; Another notch of the square ring-shaped microstrip line is connected to the radio frequency chip.

2. The flexible ultra-small RFID strain sensor tag according to claim 1, characterized in that: The dielectric substrate is a non-metallic thin sheet structure that is bendable and stretchable.

3. The flexible ultra-small RFID strain sensor tag according to claim 2, characterized in that: The dielectric substrate is made of polyester film.

4. The flexible ultra-small RFID strain sensor tag according to claim 3, characterized in that: The input impedance of the radio frequency chip is conjugate matched with the antenna impedance.

5. A flexible ultra-small RFID strain sensor and strain detection method, characterized in that: The strain detection is performed by attaching the RFID strain sensing tag according to any one of claims 1 to 4 to a construction surface. The specific detection method comprises the following steps: S1, attaching the RFID strain sensor tag to the surface of the component; S2. Generate radio frequency signal: Place the radio frequency receiving and transmitting module device at a distance of h meters above the measured component, and the radio frequency receiving and transmitting module device generates a swept frequency radio frequency electromagnetic wave signal U1; S3, generating a response signal: after the tag antenna attached to the surface of the component receives the RF signal U1, the tag chip rectifies U1 and modulates the signal in a backscattering manner to generate a RF response signal U2; S4, receiving the RF response signal: the RF receiving and transmitting module receives the RF response signal U2, and inputs U2 into the data acquisition and processing module; the data acquisition and processing module analyzes the signal U2 to obtain the frequency-amplitude curve fA of the signal U2; S5, extract characteristic frequency: extract the resonant frequency f0 of the frequency-amplitude curve fA detected in S4; S6. Invert the magnitude of the component surface strain through the frequency shift of the resonant frequency.

6. A flexible ultra-small RFID strain sensor and strain detection method according to claim 5, characterized in that: Step S6 includes the following sub-steps: S61. Establish the relationship between frequency shift and strain: set n experiments, where the i-th experiment applies strain ε on the specimen i , i=1,2,…,n, the corresponding resonant frequency shift Δω is measured i ; The measured resonant frequency shift is used as the horizontal coordinate, and the corresponding strain applied in time is used as the vertical coordinate, and the frequency shift-strain equation is obtained by fitting; S62, measuring the current resonant frequency offset of the RFID strain sensor tag according to the resonant frequency extracted in step S5, and calculating the corresponding strain in combination with the frequency shift-strain equation obtained in step S61.

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