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Review existing and proposed sensor designs
The strain gauge sensor developed is based on the electric LC (ELC) resonator used in various metamaterial-related publications, which is considered to be a base resonator by which strain sensing can be realized.
Because it has a small size and has a large center capacitance, which has been used in successful sensor designs for other work.
From the existing chipless RFID strain sensor design literature, it can be seen that the developed resonator uses a variety of different deformation mechanisms to change its resonant response.
These include elastic deformation, bending, and to some extent rigid body motion, which occurs when the substrate expands between conductive regions to cause a change in capacitance or a change in capacitance/inductive coupling.
Another observation from the current literature in the field is the use of elastic deformation or bending as the primary sensing method, with a great emphasis on the mechanical properties of deposited conductors.
Min et al. created a successful design in which AgNP/MWCNT-based deposition was utilized, supporting strain levels in excess of 20%.
Other work, such as that in Teng et al., relies heavily on the bending of the MLA antenna and supports strain levels of up to 50%.
The latter work utilizes the liquid metal Garlingstein to support these strain levels, and other designs focus on detecting strain levels below 10,000 με (1%), such as the cited strain level.
Notable works include the work of Thai et al. in developing a design for a highly sensitive strain sensor using a cantilever mechanism.
Although these projects achieved very impressive strain sensitivity, they required the manufacture of suspended cantilevers, which introduced significant manufacturing complexity to the use of direct write technology to implement these designs.
Chuang· Thomson and Bridges' work and other works have led to the development of strain-sensitive resonator designs that can be interrogated using RF frequencies.
However, the design and analysis presented in this work focuses on flat design so that it can be easily deposited in situ using techniques such as inkjet or aerosol deposition.
It is important for this discussion that many of the above sensor designs use different resonator types and different substrate materials and operate at different frequencies.
Some work has attempted to compare these different strain gauge designs using metrics such as strain coefficient, maximum range, and many other metrics to compare various chipless RFID strain gauge designs.
However, such a comparison does not seem to reveal the best chipless RFID strain sensor design.
This point is made because it seems impossible to compare strain sensors operating in a variety of different strain ranges, as not necessarily every resonator design (SRR, ELC, MLA) can achieve arbitrary strain sensitivity and range.
In general, the choice of substrate material and its height appear to determine the overall performance of the strain sensor, while the relative mechanical properties of the conductor and substrate will determine the main deformation mechanism within the resonator, an observation of the existing RFID sensor literature.
Sensors that exhibit multiple deformation mechanisms appear to exhibit more impressive performance than other sensors and may be more likely to support large changes in the range of stimuli, which can be customized by changing the substrate material.
This observation is based on the fact that large strain levels (>20%) require highly customized substrates to convert this strain to a level suitable for strain resonators operating purely by elastic deformation (<0,5%).
Use of dedicated substrate materials
The next point in this section is to highlight the advantages of using a dedicated substrate material located between the MUT and the resonator.
To a large extent, many dielectric strain sensing applications can avoid the need for this addition, but there are advantages to including a known dielectric material where a resonator is applied, and detecting the strain of a metal or generally conductive material will require the use of an intermediate material between the MUT and the resonator.
Having a consistent resonant response position in the RCS response would be advantageous, and the use of a dedicated substrate would help achieve this, as dielectric MUTs may have significantly different dielectric constants.
Some dielectric materials have significant loss tangents, and the use of intermediate dielectrics helps mitigate their adverse effects on the sensor's resonant response.
The strain performance (sensitivity and range) of the sensor can be adjusted by using a specific substrate material and height.
The high level of surface roughness and curvature of the MUT can make it difficult to successfully / accurately deposit the resonator into place, and the substrate material can help provide a smooth, flat surface for conductor deposition.
Examples of how this can occur can negatively affect the function of strain sensors include the effect of substrate expansion if the coefficient of expansion of such material is different from that of the MUT.
This will be a particularly interesting question, which is likely to happen that certain materials may easily absorb the strain caused by the MUT within their underside.
And this deformation cannot be successfully transmitted to the resonator on its top surface, and when the MUT is at a low strain level, it is most likely just a matter of flexible substrates such as soft rubber.
Although the substrate height can be changed, the thickness resolution of films that are prone to deposition is limited.
The choice of substrate material may not be within the freedom of choice of the sensor designer, as the environment in which the sensor will be used may determine the use of unfavorable materials.
Overall, a dedicated substrate is required to detect the metal, and since the proven strain of the metal is about 0.2%, it is likely that the substrate will need enough stiffness to transfer these strains to the resonator.
Similarly, other applications will require a larger strain range and require a more flexible substrate.
Therefore, the best way to support all of these possible scenarios is to develop sensor designs that can operate under all these conditions.
Sensor modeling
FEM software based on the Ansys campus was used to perform the relevant sensor simulations in this work, and Ansys HFSS was used to simulate the electromagnetic behavior of the device.
AnsysMechanical is used to perform relevant steady-state structural and thermal/humidity analyses, previous simulation environments have included all relevant material properties for electromagnetic simulations, and other parameters required for mechanical modeling are taken from relevant published literature.
The HFSS environment utilizes a built-in meshing system that iteratively increases mesh resolution so that results at specific (meshing) frequencies converge within a certain deviation between successive meshing iterations.
Plane wave excitation was used at a distance of 10 cm from the sensor and bistatic RCS results were used to explore the direction dependence of the zero position, and the Perfect Electrical Conductor (PEC) boundary condition was used to simulate the influence of the upper layer of the metal.
This sensor design is designed to support different substrate types so that the sensitivity and range of the sensor can be customized.
For this purpose, AnsysMechanicalFEA modeling is used to assess the extent to which different deformation mechanisms (expansion, bending, rigid body motion) occur during different types of loading.
Physical test results clearly demonstrate the strain sensing capability of this resonator when used with soft substrate materials.
Since polyimide is much harder than rubber, the degree of rigid body motion during sensor operation is undoubtedly reduced, so it is important to evaluate the contribution of each deformation mechanism to sensor operation.
Harder substrates may benefit from additional new substrate modifications, such as slots, so that device sensitivity can be tailored more specifically.
The performance of polyimides in aerospace environments has been well characterized, and their cross-sensitivity has been extensively discussed in the literature.
Electromagnetic simulation results
The outlined sensor design can be seen with a polyimide substrate, and the response includes the response of the design with and without a metal upper layer.
These simulations and physical tests show that the resonator appears to exhibit a separate resonance pattern on the metal super, which is thought to be caused by the coupling of monopoles, which occurs between the resonator sides.
The results mentioned here show two resonant positions, one from a finite-sized metal superlayer and the other due to the presence of the sensor.
Although promising strain-sensitive results have been collected in both simulation and testing, the "on-metal" resonance response appears to exhibit some strong dependencies related to substrate loss tangents, as well as other dependencies related to superlayer size.
Further research is needed into the "metallic" properties of the device, similar to those found in , but preliminary results suggest that it can operate as a viable sensor for these materials.
Overall conclusion
This work sets out to develop a new type of chipless RFID strain transducer and has been successfully realized, the strain transducer developed in this work exhibits an impressive strain coefficient that exceeds the unit, and there is enough evidence that it should operate successfully with a variety of different conductor-substrate material combinations.
More generally, this paper attempts to dismantle the current strategy used to develop chipless RFID strain sensors, which largely leads to a comparison of substrate materials.
Exploration of humidity and heat-induced expansion showed that these effects were sufficient to hinder strain sensing with a resolution of 10 με, furthermore, given that swelling was the only effect considered in this analysis.
The general issue of cross-sensitivity seems inevitably a larger problem than described in this small working body.
The future of work
This work proposes an improved ELC resonator, mainly because it is suitable for supporting large deformations and high sensitivity, and other designs with other advantages such as polarization insensitivity and/or strong operation on conduction supervalue.
The main reason for advancing a single design is that it will allow subsequent focused exploration of other challenges around strain sensing, such as various cross-sensitivity, orientation limitations, and complete in-situ manufacturing, with the following future goals for this overall work:
(1) Proof-of-concept strain sensing below 0.2% with an enhanced resonator design on a rigid substrate that should take advantage of other deformation mechanisms largely avoided by this work.
(2) sensor manufacturing uses proven in-situ manufacturing methods that will support consistent electrical, thermal, and mechanical sensor characteristics, and then, reliable physical testing should be performed under different environmental conditions such as humidity and temperature,
(3) Comprehensively characterize the performance of the sensor under dielectric and conductive overspeed below 0.2% strain.
(4) Explore design methods to mitigate/compensate for the possible transverse strain sensitivity of the current sensor design.