Method for crack detection of ultrahigh frequency RFID sensing system with reference tagTechnical Field
The application relates to the technical field of detection, in particular to a method for crack detection by an ultrahigh frequency RFID sensing system with a reference tag.
Background
Compared with other metals and composite materials, the aluminum alloy has the advantages of mature manufacturing process, good corrosion resistance, light weight and low cost, and is used as a structural material in the aerospace industry (fuselage skin, wings, supporting structures and the like). Under a complex working environment, the aluminum alloy structural member is easy to crack due to the action of frequent stress, the service life of the aluminum alloy structural member is seriously endangered, and huge economic loss and potential safety hazard are caused. The bearing part of the aluminum alloy structural member or the position where the crack is generated is detected, so that the safe and reliable operation of the aluminum alloy structural member is ensured. The existing crack detection and characterization methods applied to the metal materials have nondestructive detection technologies such as ultrasonic, eddy current, infrared thermal imaging, acoustic emission and the like. These nondestructive testing techniques have the disadvantages of complex system, expensive equipment, inconvenient installation, difficult networking, and the like, so that the application is limited. Passive wireless sensors, which can be attached to aluminum alloy structures for health monitoring, are the best options. Existing wireless sensing systems are widely powered using batteries, the primary challenge of which is power supply. Due to the limited life of the battery, periodic replacement is required, which is expensive and time consuming. Furthermore, the presence of the battery creates a long-term environmental risk and limits the layout of the sensor. Therefore, a method with high reliability, low cost and simple networking is needed, and the service life evaluation of the aluminum alloy structural member is realized through continuous monitoring. The small-sized, passive and universal wireless sensor can be operated in a complex environment and is applied to ground testing of aluminum alloy structural parts and defect monitoring in operation. Ultra-high frequency Radio Frequency Identification (RFID) sensing is attracting attention as an emerging technology. The working process is that the reader sends continuous electromagnetic waves to activate the chip of the tag, and the surface of the aluminum alloy material can generate induced vortex. When cracks are generated in the antenna coverage area or the surrounding environment changes, parameters such as electromagnetic field distribution, input impedance, gain and the like of the antenna also change. The reader receives the parameters in the form of analog quantity, and then extracts characteristic information through the signals to realize crack identification and characterization.
The passive wireless RFID sensor has the advantages of simple configuration, passive operation, long reading distance, low cost, unique identification and the like. Thus, some researchers have studied their application to aluminum alloy structural members in recent years. GEETHA CHAKARAVARTHI a reusable passive RFID sensor was designed for aluminum alloy specimens. By measuring the frequency corresponding to the maximum Received Signal Strength (RSSI), strain, yield strength, ultimate tensile strength and failure point of the test specimen are identified. The RFID sensor is proved to be applicable to wireless strain measurement and structural health monitoring of aluminum alloy structural members. However, the influence of the change of the surrounding environment on the RSSI is not considered in the experimental process. By changing the base material, the flexibility of the RFID tag can be achieved. When the aluminum alloy sample has cracks, the flexible label can be broken, so that the performance parameters of the label are changed, and the crack width can be sensed. The hard tag will not deform and the crack will interfere with the field distribution between the metal surface and the radiating patch, thereby changing the performance parameters of the sensor. Zhangjun et al realized crack depth detection based on ultra-high frequency RFID antenna sensors. The influence of crack position and the size of the aluminum alloy sample in the antenna coverage area on the sensitivity and reliability of crack detection is considered. The results demonstrate that the sensing performance of the RFID tag antenna based sensor is degraded when the crack is not in the optimal position.
The sensing can be realized by establishing correlation between the performance parameters of the RFID tag antenna sensor and the change of cracks, and the ultra-high frequency band (UHF) antenna sensor is easy to be interfered due to the fact that a wireless channel changes along with the environment. Tags are used in sensors to minimize interference due to measurement setup (e.g., changes in distance, direction, and other scatterers present in the read range between the reader and the tag antenna, etc.). RFID sensors that compromise tag transmission performance and crack sensing effectiveness are becoming more important.
At present, the prior art adopts RFID sensors without reference labels, adopts a single label sensor, and the crack position in the antenna coverage area and the size of an aluminum alloy sample have influence on the sensitivity and reliability of crack detection. And when the crack is not in the optimal position, the sensing performance of the RFID tag antenna-based sensor may be degraded.
Aiming at key parts of an aluminum alloy structural member, which are easy to crack, the crack depth is characterized by adopting a sensing system with a reference label. One of the tags is used for sensing and the other is used as a reference tag. The impedance characteristics and gain changes of the linearly placed and orthogonally placed dual tag systems were analyzed by modality. The reliability of crack detection by the tag sensing system is studied by the preset change of the experimental environment.
Disclosure of Invention
The invention aims to solve the technical problem of providing a method for detecting cracks by an ultrahigh frequency RFID sensing system with a reference tag, so as to solve the problem that the sensing performance of a sensor based on a single RFID tag antenna is reduced when the cracks are not in the optimal position.
In order to achieve the above purpose, the present invention provides the following technical solutions: the method for crack detection of the ultrahigh frequency RFID sensing system with the reference tag is characterized in that two RFID sensing systems with the reference tag are used for crack detection of an aluminum alloy structural member, the system consists of a reader-writer and a tag antenna, the tag antenna comprises the reference tag and a sensor tag, the reference tag and the sensor tag are placed on a sample, the middle of the reference tag and the sensor tag is kept at a certain distance, and the reader-writer is located right above the tag.
2. The method of claim 1, wherein a midline of the reader and a midline of the sensor tag are aligned.
3. The method of claim 1, wherein the substrate material of the reference tag and sensor tag antenna is FR4, the relative dielectric constant is 4.4, and a ground plate without a rectangular patch at the bottom of the tag can be used for crack sensing.
4. A method according to claim 3, wherein the reference tag and sensor tag sensor are comprised of a ground plate, a dielectric substrate, a radiating patch and a chip, wherein the radiating patch and ground plate are etched on the RF4 substrate, the ground plate and radiating patch are on either side of the substrate, respectively, and are connected by shorting pins; the radiation patch is smaller than the medium substrate, a horizontal T-shaped structure is arranged in the middle of the short side of the radiation patch, a tag chip is arranged on the transverse edge of the T-shaped structure, the right end of the tag chip is connected with the radiation patch, and the left end of the tag chip is connected with the transmission line.
5. The method of claim 1, wherein the crack characterization method based on the sensor tag relative to the reference tag is calculated as follows:
P＝Psns-Pref
where Psns is the back-scattered power of the sensing tag, Pref is the back-scattered power of the reference tag, and the back-scattered power Pb is expressed by the following equation:
Where r is the distance between the reader and the tag; lambda0 is the wavelength; gR is the gain of the reader antenna; χρ is the polarization mismatch between the reader and the tag antenna; pin is the transmitting power of the reader, and the calculation formula is:
Pth is the minimum power to activate the tag chip, Gn is the gain of the tag antenna:
RCS is the radar cross section of the tag, and its calculation formula is:
τ is a power transfer coefficient, and the calculation formula is:
GS is the gain of a single tag, and Zs and Zc are the impedance of the antenna and IC chip. Rs and RC are real parts of the antenna and IC chip impedance. ZM is the transimpedance between the two tags.
Drawings
FIG. 1 is a configuration of a tag antenna structure and a dual tag sensor
(A) The structure of the tag antenna; (b) a linear configuration; (c) Orthogonal arrangement
FIG. 2 is an electric field distribution of a reference tag when placed linearly
FIG. 3 shows the impedance change of two labels when placed linearly
(A) Sensing tag self-impedance Z11; (b) a transimpedance Z12; (c) reference tag self-impedance Z22;
FIG. 4 shows gain variation of two labels when placed linearly
(A) A sensor tag; (b) a reference tag;
FIG. 5 shows the electric field distribution of a tag in orthogonal placement
FIG. 6 shows the impedance change of two tags in quadrature placement
(A) Sensing tag self-impedance Z11; (b) a transimpedance Z12; (c) reference tag self-impedance Z22;
FIG. 7 shows gain variation for two tags in quadrature placement
(A) A sensor tag; (b) Reference label
FIG. 8 is a physical diagram of a tag arrangement and an experimental setup scheme
FIG. 9 shows the variation of Pb measured value with frequency in linear placement
(A) A reference tag; (b) Sensing label
FIG. 10 shows the variation of Pb measured value with frequency in orthogonal arrangement
(A) A reference tag; (b) Sensing label
FIG. 11 shows the variation of Pb measured value with frequency in linear placement
(A) A reference tag; (b) Sensing label
FIG. 12 is a graph showing the change rule of Pb measured value with frequency in orthogonal arrangement
(A) A reference tag; (b) Sensing label
FIG. 13 Power P vs. crack depth variation
(A) Reading distance 30cm (b) reading distance 40cm
Detailed Description
The technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention, and it is apparent that the described embodiments are only some embodiments of the present invention, but not all embodiments, and all other embodiments obtained by those skilled in the art without making any inventive effort based on the embodiments of the present invention are within the scope of protection of the present invention.
The substrate material of the antenna was FR4 and the relative dielectric constant was 4.4. A ground plate without a rectangular patch at the bottom of the tag can be used for crack sensing. ALIEN HIGGS-3 microchip was used in the design, and the input impedance at 915MHz was Zchip = (27+j201) Ω. The specific dimensions of the label are: l=85 mm, L1＝78mm,W＝28mm,Linset＝10mm,Winset＝12mm,Ls =11 mm, m=4 mm, n=7mm, h=3 mm. The sensor system is herein composed of two identical tags, one for sensing and the other as reference tag. When placed linearly, the distance d=20 mm between the two labels. When placed orthogonally, the distance d=5 mm between the two labels, as shown in fig. 1 (b) (c).
The crack depth can be characterized by establishing a relationship with the backscattering power Pb. When the distance r between the reader antenna and the tag antenna is fixed, the back-scattered power Pb is expressed by the following formula:
Where r is the distance between the reader and the tag; lambda0 is the wavelength; gR is the gain of the reader antenna; χρ is the polarization mismatch between the reader and the tag antenna; pin is the transmitting power of the reader, and the calculation formula is:
Pth is the minimum power to activate the tag chip, Gn is the gain of the tag antenna:
RCS is the radar cross section of the tag, and its calculation formula is:
τ is a power transfer coefficient, and the calculation formula is:
GS is the gain of a single tag, and ZS and ZC are the impedance of the antenna and IC chip. RS and RC are real parts of the antenna and IC chip impedance. ZM is the transimpedance between the two tags.
The back-scattered power Pb of the tag is related to the magnitude of the impedance and gain. In this section, cststudio suite 2018 was used to simulate dual labels in two placement modes. An RFID sensing system is placed on top of the aluminum alloy coupon. In the CST setup, a discrete edge port with an impedance of 50Ω is first defined between the two strips. The frequency range is set to 800-1000MHz. The minimum unit is set to 0.5 in Hexahedral mesh division. The electric field distribution of the label and the change of impedance and gain under different crack depths are obtained through simulation, and theoretical basis is provided for later experimental analysis.
For practical applications, the dimensions of the aluminum alloy structural member can also affect the sensing system. Fig. 2 is an electric field profile of a reference tag when placed linearly, and it can be seen that the reference tag has less coupling effect on the sensor tag. It is also seen that the bottom of the reference label has a stronger electric field built up along the sides of the ground plate. Due to the interference effect of the edges of the aluminum alloy sample, the electric field re-radiation affects the parameters of the tag. Thus, the reference tag is mainly affected by the bottom electric field. Fig. 3 shows the variation of the impedance of two labels with crack depth for linear placement. It is seen from fig. 3 (a) that as the crack depth increases in the sensing tag coverage area, the impedance Z11 of the sensing tag shifts in the low frequency direction. At the same frequency, the impedance Z11 shows a tendency to increase. This is because the increase in crack depth increases the equivalent inductance L, while the resistance value is unchanged, resulting in a change in the resistance value. In fig. 3 (b), we can see that the transimpedance Z12 also tends to increase at the same frequency. At a frequency of 930MHz, the transimpedance Z12 changes from 17.2 Ω to 20.6 Ω as the crack depth increases. As seen in fig. 3 (c), the impedance Z22 of the reference tag does not substantially change as the crack depth increases. Fig. 4 shows the gain Gn of two labels as a function of crack depth for linear placement. From fig. 4 (a), we can see that as the crack depth increases, the gain of the sensor tag tends to decrease, mainly because the increase in crack depth increases the self-impedance and transimpedance of the sensor tag, resulting in a decrease in gain. As can be seen from equation (3), an increase in the transimpedance results in a slight decrease in the gain of the reference tag. Under the condition of linear placement, a certain coupling effect exists between the two labels, and the change of crack depth at the coverage area of the sensing label has a certain influence on the gain of the reference label.
The electric field distribution of the reference tag when placed in quadrature can be seen that the reference tag has substantially no coupling effect on the sensor tag. After orthogonal placement, the electric field at the bottom of the reference label is not interfered by the edge of the aluminum alloy sample. Fig. 6 shows the variation of the impedance of two labels with crack depth for orthogonal placement. As seen in fig. 6 (a), the impedance Z11 of the sensor tag shifts in the low frequency direction as the crack depth increases. Fig. 6 (b) shows the variation of the transimpedance Z12, with a smaller variation of Z12 at the same frequency. At a frequency of 930MHz, the transimpedance Z12 is 0.6Ω. We can derive that the effect of coupling is negligible when the two tags are placed in quadrature. Fig. 6 (c) shows that the impedance Z22 of the reference tag does not change at all as the crack depth increases. Fig. 7 shows gain of two labels as a function of crack depth for orthogonal placement. From fig. 7 (a), we can see that the amplitude of the sensor gain variation is larger in quadrature compared to the gain variation in linear placement. Indicating that it is more sensitive to variations in the depth of the crack. Fig. 7 (b) shows that the gain of the reference tag is unchanged, which corresponds to the result expressed in fig. 6 (b) (c).
All measurements were made using Tagformance tag performance test system. The method can obtain the receiving power and the transmitting power of the tag response at each frequency point by setting the frequency range (860 MHz-960 MHz) of the sweep frequency, can calculate the performance parameters such as the starting power, the backscattering power, the theoretical reading distance and the like of the tag, and intuitively display the test result in a curve form. When in test, the communication command is consistent with the ISO18000-6C protocol specification and is consistent with a universal reader-writer.
4 Cracks are prefabricated on an aluminum alloy sample with the dimensions of 200mm multiplied by 100mm multiplied by 5mm by adopting an electric spark machining method, the crack depth is increased from 0mm to 4mm, each step is 1mm, the crack length is 50mm, and the opening width is 1mm. And taking a crack-free aluminum alloy flat plate as an initial state of the metal structure to be detected, wherein the corresponding crack depth is 0mm. The label system was mounted on top of the aluminum alloy coupon by double sided tape. To verify the sensing capability of the tag sensor, the communication distance between the reader/writer and the tag antenna is set to h=30 cm and h=40 cm. The reader antenna is manually moved during the test and the line is aligned with the tag. The experimental setup scheme and sensor layout is shown in fig. 8. The read distance remains unchanged and the backscatter power Pb of the sensing tag and the reference tag is measured by the RFID reader.
Fig. 9 and 10 show the frequency dependence of the back-scattered power Pb of the two sensor systems at different crack depths at a reading distance h=30 cm. As can be seen from fig. 9, the Pb value of the reference label remains substantially unchanged as the crack depth increases, while the Pb value of the sensor label exhibits a decreasing trend. It is seen from equation (3) that as the crack depth increases, the self-impedance and the transimpedance of the sensor tag increase, the gain decreases, and the real part of the antenna impedance RS and the real part of the IC chip impedance value RC are unchanged, so that the Pb value decreases. The experimental result accords with the change rule of the gain of the two labels simulated in linear placement, as shown in fig. 4 (a) (b). We can also see that the Pb value of the reference tag is less than the Pb value of the sensor tag without a crack when the frequency ranges from 920MHz to 940 MHz. This is mainly due to the fact that the sensing tag is close to the right of the metal sample, and the difference in edge effect affects Pb. Fig. 10 shows the variation of the back scattering power Pb with frequency when two tags are placed in quadrature. As can be seen from fig. 10 (a), the Pb value of the reference tag remains unchanged as the crack depth increases, which is the same as the variation law of the gain of the reference tag shown in fig. 7 (b). The crack depth increases and the Pb value decreases, and the sensor tag can characterize the crack, as shown in fig. 10 (b).
The distance between the reader and the tag antenna was changed to h=40 cm and other aluminum alloy specimens were placed around the test environment. Fig. 11 and 12 show the variation of the back scattering power Pb of the tag sensor with frequency at different crack depths when the reading distance h=40 cm. As can be seen from fig. 11, the increase in the reading distance h and the effect of the aluminum alloy sample, reduced the initial back-scattered power of the reference and sensor tags, indicating that a greater on-power Pin is required to activate the chip for operation than when h=30 cm. When the frequency range is 920MHz-940MHz, the backscatter power values of the reference tags have good consistency. The backscattering power of the sensor tag shows a monotonically decreasing trend as the crack depth increases. Fig. 12 shows the frequency variation of the measured value of Pb in the orthogonal arrangement. The reference tag has a large fluctuation in the Pb value, which is mainly caused by errors generated by manually moving the reader antenna and transforming the aluminum alloy sample during experimental operation. The backscattering power Pb of the sensor tag has a decreasing trend and can detect cracks well.
In practical application environments, aluminum alloy structural members generally work under complex working conditions. After a period of service, the environment surrounding it changes. The detection of cracks requires that the reference tag be able to compensate for changes from the environment. We have discussed a crack characterization method based on a sensor tag relative to a reference tag, the calculation formula is as follows:
P＝Psns-Pref (3)
Where Psns is the back-scattered power of the sensing tag and Pref is the back-scattered power of the reference tag.
We can achieve crack characterization by establishing a relationship between the difference in the backscattering power of the sensor tag and the reference tag and the crack depth. The relationship between the difference in the backscatter power of two tags and the crack at a frequency of 930MHz is shown in fig. 13, where the backscatter power of the reference tag is averaged over 5 measurements. It can be seen that in both configurations, the Δp value decreases with increasing crack depth, and the response exhibits a linear characteristic. Thus, a relation between the difference Δp in back-scattered power and the depth is established, and a crack can be sensed. Under the condition that two labels are placed in an orthogonal mode, the sensor has good detection sensitivity to crack depth, but occupies a larger area. It can be seen from fig. 13 (b) that the sensor also characterizes the crack depth well by adding a reference tag to compensate for changes from the environment. The sensitivity differences that occur with different distances are mainly due to errors during operation. To address the case where Δp is less than 0, we can design a special reference tag according to the actual working scenario in future research. Interference influence in a test environment is eliminated by means of the difference between the backscatter power values of the sensing tag and the reference tag, but the sensitivity of the sensing tag is reduced, so that the proposed sensing system can be suitable for crack detection of an aluminum alloy structural member in a complex environment.