Elsevier

Sensors and Actuators A: Physical

Volume 283, 1 November 2018, Pages 386-395
Sensors and Actuators A: Physical

Reconfigurable microwave SIW sensor based on PBG structure for high accuracy permittivity characterization of industrial liquids

https://doi.org/10.1016/j.sna.2018.06.008Get rights and content

Highlights

  • The tunable microwave sensor for permittivity determination of industrial liquids have been considered.

  • The Substrate Integrate Waveguide (SIW) is used for making the cavity for improve sensing.

  • The Photonic Band Gap method is utilized for improve the electric field in the hot spots.

  • The cavity perturbation technique in order to calculate the permittivity is employed.

Abstract

In this paper, we present a novel tunable microwave sensor for permittivity determination of industrial liquids. The proposed sensor is cavity based which is developed on a Substrate Integrated Waveguide (SIW). To enhance the characterization accuracy, the reconfigurable sensor is equipped with a Photonic Band Gap method and variable capacitors. Moreover, we employ the cavity perturbation technique in order to calculate the permittivity. In the characterization process, we obtain the permittivity of an unknown material by considering a resonant frequency shift. In fact, a capacitance is the main parameter for controlling the sensor resonance. We herein change this capacitance via reconfigurable SIW cavity and applying different materials. The proposed tunable architecture lets us study the material characteristic in the wider frequency range. The structure is designed in 5–6 GHz in order to determine the electromagnetic behavior of a brand new and used transformer oil samples. The results present a highly accurate permittivity of these oil samples. Hence, the proposed method and setup is not only suitable for oil ageing programs, but also applicable for other industrial liquid applications.

Introduction

Microwave sensors are vastly utilized in industrial applications for the recognition of material attribution in solid or fluid situations. They are designed in various shapes based on the electric field in a limited area with different techniques such as metamaterial structure like Split Ring Resonator (SRR) for high-Q application [1] and Substrate Integrated Waveguide (SIW) [2]. Recently, SIW structures have been suggested because they have shown similar response like cavity structures. These structures will be useful for microwave sensing for various forms of materials such as liquid, based on the perturbation techniques to obtain the permittivity of an unknown material [3,4]. The SIW cavities are good candidates for material permittivity determination as they provide a high Q-factor, compact profile, low-cost platform and easy fabrication [5,6]. Moreover, different techniques have been reported for determination of the unknown material permittivity such as Transmission/Reflection (TR) technique, resonator, and resonant perturbation technique [[7], [8], [9]]. An electric field concentration is considered in all of these detection methods to improve the accuracy. For this aim, different schemes have been reported such as interdigital gap [10], metamaterial structure based on SRR in different shapes and arrangement [11,12] or parallel tapered line for bio-sensing and dielectric spectroscopy [13]. Metamaterials and periodic structures such as Frequency Selective Surface (FSS) [14,15], Electrical Band Gap (EBG) [16,17] and Photonic Band Gap (PBG) are attractive in a communication system for conducting the electromagnetic wave in special surface and waveguide [18,19] or concentrating the field and making a shield for reducing the mutual coupling and distribution of the electric field [20]. These conventional methods for material detection have a main shortcoming which is providing the narrowband characteristics of the materials [11]. For studying a parameter in wider bandwidth based on a narrowband resonance, we need to design a reconfigurable structure. For this purpose, the three different methods have been developed by using: 1) PIN diode [21], 2) varactor [22], 3) the ferrite load [23]. The PIN diode can be used as on/off switch and for making a frequency shift, we need to increase the effective length of the resonator [24]. Consequently, for making even a few number of frequency shift, we should make a complicated circuit. Ferrite and varactor can give continuous variation for frequency shift but the biasing system of the ferrite is complex and we need external inductance [25]. Hence, the varactor is the best choice due to its compact size, easy fabrication and biasing system. This varactor should be placed at the hot spot point where we have the maximum electric field for reaching the maximum shift of frequency [26].

In this paper, we propose a microwave sensor developed based on the cavity techniques. A SIW structure is utilized for sensing and concentrating the electric field. To add more frequency shift for material recognition with high resolution, a PBG structure is used to concentrate the energy in a limited area. Moreover, the Surface-Mount Device (SMD) type capacitors (which can be replaced by a varactor) are used to perform a reconfigurable structure for covering even a wider frequency range. To create such a reconfigurable structure, the capacitors must be placed close to the maximum electric field distribution to achieve the adequate sensitivity. Hence, a gap ring is created around the sensor center, where has the maximum electric field distribution, to put the capacitor there. By changing the capacitor value we get a resonant frequency shift from 5 to 6 GHz which is gives us a wide frequency range of material characterization by this sensor. Two industrial liquids: Clear as a brand new and Dark oil as used one, are employed in this study for material characterization purposes. Moreover, three reference materials: Butanol, Ethanol, and Gasoline are used for getting the perturbation factor and calibrating the resonator. The results reveal that the proposed setup is a good candidate for determination of transformer oil aging.

The rest of this paper is organized as follows. In the next section, we present the sensor design procedure and its architecture. Parametric studies via simulation modeling for the return loss and electric field are performed in Sections III and IV, respectively. We also provide a parameter study for the PBG design in the next section. Then, we present the measurement results and discussions in Section VI. Finally, Section VII concludes the paper.

Section snippets

Sensor design

The geometry of the proposed microwave sensor is depicted in Fig. 1-(a). The structure is designed on SIW cavity and fabricated on a sheet of Rogers RT/Duroid 5880 substrate with the dielectric constant of 2.2 and a thickness of 0.8 mm. In fact, we choose the SIW structure to make the resonant cavity due to its simplicity, low radiation loss, small size, easy fabrication and more importantly easy integration into microwave and millimeter wave circuits. The proposed sensor architecture is

Simulation results and parametric studies for return loss

The proposed model is simulated by HFSS as a full-wave software, and the return loss is extracted for various cases. First, we compare the simple SIW model in the absence of the PBG structure. Then, the centralized gap ring and the two capacitors are added to the sensor architecture with the PBG holes. We herein study the PBG structure effect for various capacitances from 0.2 to 1.4 pF. Moreover, the effect of different permittivity values on the return loss of the final structure is

Simulation results and parametric study for electric field

In this section, we compare the electric field distribution in the presence and absence of the PBG layer. As shown in Fig. 8-(a), the electric field is around 3.09E + 4 v/m for simple structures. While after implementation of PBG the electric field enhances up to 3.21E + 4 v/m (Fig. 8-(b)). Then, for placing, the capacitors in the structure, we etch a ring on the structure and place, the capacitors where we have the maximum value of the electric field. However, utilizing this ring reduces the

Parametric study of PBG

We now perform a parametric study over the PBG location and hole diameters. In this study, we focus on the radius of the PBG and the radius, angle between two PBG holes (see Fig. 9). The effect of each parameter on the maximum value of the electric field is presented in Table 1, Table 2.

In Table 1, we compare the result for r = 7–10 mm and the angle variation between 15–22.5 degrees. The parametric studies reveal that the best result is obtained for r = 8 mm and the angle of 20° where the

Summary of design procedure

In this section, a brief summary of proposed sensor design is presented in three steps. In this process, the final structure is designed based on the simulation and the parametric studies.

Measurement result and discussions

A network analyzer (HP8720c) based setup is used for the measurement campaign as shown in Fig. 10. The measurement procedure is performed in three steps. At the first step, the structure with empty sample holder is placed under the test. Then, we repeat the measurement for unused industrial oil, so called Clear oil. Finally, the used Dark oil is replaced with the Clear oil and the measurement is again performed. In the second step, we obtain the measurement results for Gasoline, which has a

Sensor sensitivity

In order to show the sensitivity of the proposed sensor, a sensitivity criteria is provided. According to the criteria given in [3], the sensitivity is =ΔfΔε',Δf=f0-fsfs,Δε'=εs'-ε0'. Thus, the sensitivity for the proposed PBG sensor is obtained 1%. Moreover, a sensitivity comparison obtained from different references is presented in Table 10.

The sensitivity of the sensor without capacitor and the etched ring is higher [28]. However, we aim to consider the behavior of materials in a wider

Conclusion

We propose a novel type of microwave sensor for detection of material constants. The sensor is equipped with a PBG structure, which improves the electric field in the hot spot. We optimize the proposed PBG structure to maximize the sensitivity and resolution of the sensor, via the simulations and experimental campaigns. Moreover, a configurable architecture using added capacitors are employed for more enhance the sensing performance. We also use the perturbation method to calculate the material

Fereshteh Sadat Jafari was born in Zahedan, iran in 1988. Received B.Sc./M.Sc degree in communication engineering from University of Sistan and Baluchestan, Zahedan, Iran, in 2010/2012. She is currently working toward the Ph.D. degree at university of Sistan and Baluchestan, Zahedan, Iran. Her Ph.D research concerns include, analysis and design of microwave sensor for determination electromagnetic properties of materials and permittivity measurement. Her other area of research interest are

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    Fereshteh Sadat Jafari was born in Zahedan, iran in 1988. Received B.Sc./M.Sc degree in communication engineering from University of Sistan and Baluchestan, Zahedan, Iran, in 2010/2012. She is currently working toward the Ph.D. degree at university of Sistan and Baluchestan, Zahedan, Iran. Her Ph.D research concerns include, analysis and design of microwave sensor for determination electromagnetic properties of materials and permittivity measurement. Her other area of research interest are design conformal antenna to place in curve structure, microstrip antenna design and working on metamaterial, frequency selective surface and periodic structures.

    Javad Ahmadi-Shokouh received the B.Sc. degree in electrical engineering from Ferdowsi University of Mashhad, Mashhad, Iran, in 1993, the M.Sc. degree in electrical engineering from the University of Tehran, Tehran, Iran, in 1995, and the Ph.D. degree in electrical engineering from the University of Waterloo, Waterloo, ON, Canada, in 2008. From 1998 to 2003, he was with the Department of Electrical Engineering, University of Sistan and Baluchestan, Zahedan, Iran, as a Lecturer, where he is currently an Associate Professor. He was also with the Department of Electrical Engineering University of Manitoba, Winnipeg, MB, Canada (2008–2009) as a Postdoctoral Fellow. His research interests are antenna and microwave systems for all applications, millimeter wave and ultrawideband systems, smart antennas, optimal and adaptive MIMO systems.

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