This is the accepted version of the journal article: Vélez, Paris; Martín, Ferran; Fernández-García, Raúl; [et al.]. «Embroidered textile frequency-splitting sensor based on stepped-impedance resonators». IEEE sensors journal, Vol. 22, Issue 9 (May 2022), p. 8596-8603. DOI 10.1109/JSEN.2022.3163165 This version is available at https://ddd.uab.cat/record/258463 under the terms of the license IEEE SENSORS JOURNAL, VOL. XX, NO. XX, MONTH X, XXXX 1 Embroidered Textile Frequency-Splitting Sensor based on Stepped-Impedance Resonators Paris Vélez, Senior Member, IEEE, Ferran Martín, Fellow, IEEE, Raúl Fernández-García, and Ignacio Gil Abstract— This paper presents an embroidered textile frequency-splitting microwave sensor based on a pair of identical stepped-impedance resonators (SIRs) loading a microstrip transmission line. The sensor is implemented by means of conductive threads. The sensing region of the proposed structure is the capacitive square patch of one of the SIRs. If such region is kept unaltered, the structure is symmetric, and the frequency response (transmission coefficient) exhibits a single transmission zero. However, if symmetry is broken (e.g., through liquid absorption in the sensing region), the frequency response of the proposed sensor exhibits two transmission zeros (frequency splitting). The difference (in frequency and magnitude) between such zeros (or notches) is intimately related to the dielectric properties of the absorbed liquids to be sensed/detected. The proposed sensing structure is applied to the detection of deionized (DI) water absorption, and to the quantification of the number of DI water drops. The maximum measured sensitivity is found to be 2.70 MHz/µl and 0.03 dB/µl for the incremental frequency and incremental magnitude of the notches. Index Terms— Absorption, conductive yarn, dielectric constant, drops, embroidered sensor, frequency splitting, microstrip, microwave sensors. I. INTRODUCTION the properties they provide, especially in the field of sensors, such as wash ability, and full direct implementation on the W EARABLE electronics and electronic textiles (e- textiles) areas are attracting the attention of researchers clothes [15-16]. On the other hand, planar microwave sensors can potentially satisfy many demanding requirements and in the last years. One of the most important aspects that offer provide technical solutions to several challenging aspects due these two technological areas resides in the possibility to to their low cost, small size and low profile. Also, microwave integrate the electronic components and complete systems planar sensors can be implemented in many different types of directly into the textile substrate. This fact has allowed the substrates, including flexible and textile substrates. printing of passive electronic components [1-3] and In the recent literature, various strategies have been embroidering directly on textile substrates by means of proposed for the optimization of the sensitivity (one of the conductive threads [4-6]. This fact has opened a new door in most important performance parameters in sensors) in planar the design of microwave circuits, more specifically in the microwave sensors implemented on rigid substrates. Most of design of wearable textile RF/microwave components (e.g. these works are based on the use of non-resonant [17, 18] and filters and antennas) and sensors [7-14]. Today, wearable e- resonant [19-22] structures. In the latter case, the most textile technologies are experiencing a massive growth due to extended working principle exploits the resonance frequency variation due to the dielectric properties of the materials This work was supported by MICIIN-Spain (projects PID2019- (solids and liquids) under test (MUT). However, there are also 103904RB-I00, TEC2016-79465-R, and PDC2021-121085-I00), Generalitat de Catalunya (project 2017SGR-1159), Institució Catalana resonant sensors where frequency splitting is the sensing de Recerca i Estudis Avançats (who awarded Ferran Martín), and by mechanism [23]. Planar microwave sensors, either non- FEDER funds. resonant or resonant, can be implemented by considering P. Vélez, and F. Martín are with GEMMA/CIMITEC, Departament d’Enginyeria Electrònica, Universitat Autònoma de Barcelona, 08193, reflective-mode [24, 25] or transmission-mode [18, 22, 23] Bellaterra, Spain (email: paris.velez@uab.cat). structures, and using single-ended [19-22] or differential R. Fernández-García, and I. Gil are with Departament of Electronic measurement techniques [26-28]. Many reported sensors to Engineering, Universitat Politècnica de Catalunya (UPC), Terrassa, Spain. date exhibit very good performance (high sensitivity, high resolution, low cost and easy fabrication process), and have XXXX-XXXX © XXXX IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. 8 IEEE SENSORS JOURNAL, VOL. XX, NO. XX, MONTH X, XXXX been applied to the dielectric characterization of solids [18], physical length, respectively. In reference to the SIRs, the liquids [23], liquid composition detection [29], or defect narrow strip and the rectangular patch are modeled by the detection in samples [27], among others. inductance Lsi and capacitance Csi, respectively, where i = 1, 2 Frequency-splitting sensors (the subject of interest in this denote the specific SIR. The dielectric losses are taken into paper) can be implemented following several strategies, but account by means of the resistance Rci, parallel connected to the simplest one utilizes two identical sensing resonant the SIR capacitance, and the magnetic coupling between elements, symmetrically loading a host transmission line. resonators is described by the mutual inductance M (with Sensing is based on the splitting in the resonance frequency negative sign due to the opposite directions of the current lines that result by symmetry perturbation. The most usual through the SIRs [see Fig.1(b)]. procedure for symmetry truncation in pairs of resonant Note that the equivalent circuit model is not symmetric. The elements is by placing asymmetric dielectric loads on top of reason is the disruption of symmetry caused by the presence of such resonators. Consequently, most reported frequency- a dielectric load (with complex permittivity ε* = ε’ jε’’) in splitting sensors have been applied to the dielectric the sensing region. The sensing region is the textile substrate characterization of solids and liquids. Such sensors are able to zone corresponding to the capacitive patch of the SIR detect differences between the so-called reference (REF) designated as 1 in Fig. 1. Thus, varying Cs1 and Rc1 can model sample, placed on top of one of the resonant elements of the any change in the complex dielectric constant (real and pair, and the sample (or material) under test (MUT), situated imaginary parts respectively) of the solid/liquid under test. If on top of the other resonator. the structure is symmetric, corresponding to an absence of This paper demonstrates, for the first time, the symmetry dielectric load in the sensing region, then Ls1 = Ls2; Cs1 = Cs2; properties of frequency-splitting microwave sensors Rc1 = Rc2, and the transmission coefficient exhibits a single implemented in textile substrates. The proposed topology is transmission zero at the resonance frequency given by [30] based on the work presented in [30], where frequency-splitting sensors based on stepped impedance resonators (SIRs) [31] and implemented in rigid substrates were proposed. The √( | |) frequency of operation of the textile sensor presented in this paper is set to 3 GHz, and the sensor is applied to the By contrast, if symmetry is disrupted (caused by a dielectric characterization of liquid absorption and drop detection. High material in contact with the sensing region or by changes in sensitivities and good repetitively in the measurements of the substrate in that region), a split in frequency appears, different prototypes are obtained, as it will be shown. causing the appearance of an additional resonance. The paper is organized as follows. Section II presents the Considering Ls1 = Ls2 = Ls (the inductances are not affected by sensor topology (including the equivalent circuit model), the the presence of absorbed liquid in the considered volume layout design of the proposed sensor, and the principle of range), the two resonance (angular) frequencies are given by operation, considering the electric properties of textile commercial materials (substrate and conductive threads [32, √[ ] 33]). In Section III, the fabrication process of the presented √ textile microwave sensor is detailed and validated trough simulation and measurement of the bare sensor. The results lc (a) (b) derived from this work concerning the characterization of the amount of deionized (DI) water absorbed by the textile wc substrate of the sensor, as well as the detection of drops of lTL wTL small volume, are presented in Section IV. A comparison with P1 ll P2 wl other textile microwave sensors for absorption characterization is carried out in Section V. Finally, Section VI summarizes the relevant conclusions of the presented work. (c) Sensing Rc1 II. SENSOR TOPOLOGY AND PRINCIPLE OF OPERATION region (d) Cs1 The proposed frequency-splitting sensing structure, first Ls1 reported in [30], is based on a microstrip transmission line P1 TL M TL P2 loaded with a pair of mirrored stepped-impedance resonators Z0, kl Z0, kl Ls2 (SIRs) magnetically coupled (such magnetic coupling is Cs2 consequence of the proximity between the narrow inductive Rc2 strips of the SIRs). The layout and the equivalent circuit model Fig.1. 2D Layout (a), electric field distribution at the resonance (including dielectric losses) are shown in Fig. 1. The host frequency (f0) of the coupled SIRs (b), 3D HFSS layout view (c), and microstrip line is described by the transmission line sections at equivalent circuit model (d) of the proposed differential microwave both sides of the SIRs, with characteristic impedance Z0 and sensor. The ground plane is depicted in soft grey color. The relevant electrical length kl, where k and l are the phase constant and dimensions are indicated. 8 IEEE SENSORS JOURNAL, VOL. XX, NO. XX, MONTH X, XXXX In Fig. 2, the resonance frequencies are designated as f1 and (3b) f2, and are related to the angular resonances of expression (2) where kij (with j = 1,2 and i = 1,2) are coefficients that can be by f1 = /2 and f2 = +/2. In such figure, circuit responses deduced from the simulated data points of Fig. 4. corresponding to the symmetric case and two asymmetric '' = 0.00585 '' = 0.00900 cases are depicted. Note that for the asymmetric responses, 0 '' = 0.00675 '' = 0.01012 only the capacitor Cs1 changes (the other electrical parameters -5 '' = 0.00787 '' = 0.01125 are kept unaltered). '' = 0.01237 To illustrate the electromagnetic behavior of the proposed -10 S21(dB) differential sensor, let us consider the topology shown in Fig. -15 1(c). Fig. 3 shows the full 3D electromagnetic simulation S21 ' = 1.3 -20 ' = 1.5 (using the Ansys HFSS simulator) of the transmission ' = 1.75 coefficient of the layout of Fig. 1(c), considering different -25 ' = 2 substrate dielectric conditions. The sensitive region (see Fig. -30 ' = 2.25 ' = 2.5 1c) has been defined with a hypothetical material that fz ' = 2.75 emulates the textile substrate conditions when liquid -35 1.0 1.5 2.0 2.5 3.0 3.5 4.0 absorption occurs. It is assumed that the liquid does not reach Frequency (GHz) the line region (a reasonable hypothesis for moderate Fig.3. Simulated frequency response of the transmission coefficient of absorption in the sensitive region). The real and the imaginary the sensor of Fig. 1(c) considering different complex dielectric part of the complex dielectric constant (ε’, ε’’) of this material constants. The geometrical parameters in reference to Fig. 1(a) are: has been linearly increased from the initial value of the textile wTL = 5.89 mm, lTL = 15 mm, lc = 12.5 mm, wc = 14.5 mm, ll = 3 mm, wl = 3 mm. substrate, i.e., ε’ = 1.30 and ε’’= 0.00585, up to ε’ = 2.75 and ε’ = 0.0123. As it can be expected, when the dielectric 1150 12.5 HFSS Simulation data constant of the sensitive region coincides with the nominal one 1100 12.0 Linear regression (symmetric configuration), the transmission coefficient 1050 2 11.5 (R = 0.9993) exhibits only a single notch in the vicinity 3 GHz. If the fz (MHz) S21 (dB) 1000 11.0 symmetry is truncated by changes in the conditions of the substrate in one of the SIR with respect to the other, frequency 950 10.5 splitting arises (Fig. 3). Under these conditions, the difference 900 10.0 in frequency (Δfz) and magnitude (ΔS21) between the two 850 9.5 generated transmission zeros can be related to the differential 800 9.0 complex dielectric constant of the MUT, with real and 750 HFSS Simulation data 8.5 imaginary parts designated by and . 2 Linear regression (R = 0.9897) 700 8.0 1.50 1.75 2.00 2.25 2.50 2.75 Symmetric case (Cs1=Cs2) 0 Non-symmetric case (Cs1>Cs2) Textile Substrate Dielectric Constant Non-symmetric case (Cs1