Authors: Vitalii Kirilov[1], Dmitry Kozlov1, Holger Claussen1, [2],[3] and Senad Bulja1,3
Venue: IEEE Access, July 2024. Read the full paper here.
What’s this paper all about?
Measurements of pertinent parameters of liquids such as temperature, density, viscosity or chemical composition within large, enclosed vessels, such as barrels, cisterns or tanks are important practical tasks used to control technological processes or storage conditions. This type of measurement requires the establishment of reliable communications links to convey relevant information among multiple sensors, preferably, but not necessarily uniformly distributed within the enclosed vessel. To obtain correct spatial information on the technological activity taking place inside an enclosed vessel (such as the extent of mixing of liquids and temperature measurements, as examples), the placement of communications equipped sensors inside the enclosed vessel is necessary. Such sensors will communicate vital information about the relevant parameters (exhibited at their own spatial locations inside the vessel) among themselves and, also, with the outside world. However, even though the measurement of pertinent characteristics inside an enclosed vessel, such as temperature, viscosity and density are relatively trivial tasks, effective communications among the sensors and the outside world is challenging. This is due to performance limitations of the existing traditional communication methods in the scenario of enclosed vessels filled with high-loss liquids.
As it is known, optical communication links are reliable under line-of-sight conditions, however, they are adversely affected by the opacity (not optically transparent) and turbidity (cloudiness) of liquids[4]. Acoustic communications[5] are, also, a well-established approach for such scenarios but it is hampered by environmental factors like temperature, pressure, and influence of external interference. Radio Frequency (RF) communications can be an attractive solution to overcome the above-mentioned limitations of optical and acoustic in-vessel communications. A standard RF link is established by the interaction of transmitting and receiving antennas, which are traditionally equal to half or a quarter of the wavelength[6]. This means that the operational frequency has to be relatively high (GHz-frequency range) to be able to use small antennas in the limited space of the vessel. However, in that frequency range, losses related to the propagation of Electro-Magnetic (EM) waves through liquids are too high to establish a reliable communication link. Thus, a new approach for an enclosed volume or in-vessel communications is required to overcome these challenges.
Fig. 1 Cylindrical resonant cavity resonator filled with liquid (a); electric field distribution for: first eigenfrequency (b); second eigenfrequency (c).
In this paper, we treat enclosed volumes as natural microwave resonators with communications sensors placed inside it. Any metal-wall enclosed volume resonates at its own natural (eigenmode) frequencies, which are determined by the size of the enclosure. It is well-known from filter theory that lowest losses are exhibited at resonant frequencies, since the losses in this case are purely dominated by the ohmic (joule) losses of the medium. By making the sensors operate at the natural frequencies of the enclosing vessel, the vessel will behave as a natural “amplifier” with minimum losses, with well-defined maxima and minima of the EM fields for the given frequency of operation, such as the one shown in Fig. 1. In this paper we present extensive simulations and experimental work carried out to evaluate the feasibility of proposed in-vessel communications with a special focus for the case when the enclosed volume is filled with lossy liquids.
What exactly have you discovered?
Our study investigated the influence of several factors on the performance (transmission losses and frequency of operation) of enclosed (in-vessel) communications, such as probe positions, their size and influence of the losses of the dielectric (insulating) medium. Dielectric loss is a measure on how good a dielectric (insulator) is – ideally, insulators should be fully non-conductive and hence without any loss, however, this is not achievable in practice as some power loss is inevitable. The setup, showing the positions of the antennas, is shown in Fig. 2. For this study, measurements were carried out using monopole probes with lengths of 2-22 cm in 4 cm increment in a metal barrel with a radius of 30 cm and a height of 80 cm, filled with tap water.
First, it was shown that the transmitting (TX) and receiving (RX) probes should be placed at the location corresponding to the maximum field intensity of the eigenmode to perform communication at the frequency of interest. Moreover, the positions of the field maxima for different eigenmodes can overlap with each other, and probes will excite multiple eigenmodes simultaneously.
Second, it was shown that for optimum transmission, the size of the excitation probes should be as long as practical. This is dictated by the size of the enclosed vessel, since longer probes in a space-constrained environment are impractical.
Third, the dielectric loss of the filling liquid affects the optimal transmission frequency. For example, our study has shown that for the case of tap water, the optimum transmission frequency changes from 14*f1 to 6*f1, for loss-free (tan(δ) = 0) and lossy (tan(δ) = 0.05) scenarios, respectively.
Fig. 2 Measurement setup (a); cylindrical cavity model with two probes (b) and schematic representation of the location of the probes (c)
In conclusion, our findings reveal that for in-vessel communication scenarios, the optimum operational frequency band is mainly determined by the size of the enclosed cavity, the dielectric characteristics of the liquids, probe size and their position inside an enclosed vessel. In case of static probes, their positions should be chosen by considering the field distribution at the frequency of interest. And vice versa, considering the system where probes can move inside the cavity, the operational frequency should be an adjustable parameter to obtain minimum transmission loss. This can be done by performing a quick scan in a predefined frequency range, from which the frequencies exhibiting the lowest losses are selected.
So what?
This paper presented the first ever comprehensive simulation and experimental study on the performance of enclosed volume (in-vessel) resonant communications. Communications inside such enclosed volumes have recently gained prominence due to the industrial need to measure a variety of physical parameters, such as temperature, density, or viscosity, to name but a few, at exact locations and communicate those values either among the sensors in the enclosed volume or outside. The insights gained through the course of this study are not only applicable to physical parameter measurements, but encompass a much broader application range, such as IoT communications where the need for communications using low RF powers is of paramount importance. Examples include home automation with applications related to devices to control and regulate electrical and electronic equipment such as windows, refrigerators, lights and fire alarms to name but a few.
Read the full paper here.
[1] Wireless Communications Laboratory, Tyndall National Institute, Dublin, Ireland.
[2] School of Computer Science and Information Technology, University College Cork, Cork 021, T12 CY82 Ireland.
[3] Trinity College Dublin, Dublin 2, D02 PN40 Ireland.
[4] G. Schirripa Spagnolo et al., “Underwater Optical Wireless Communications: Overview,” Sensors (Basel), vol. 20, no. 8, pp. 2261, 2020, DOI: 10.3390/s20082261.
[5] M.S. Martins, “Ultrasonic wireless broadband communication system for underwater applications”, Ph. D. thesis, Universidade de Minho, Escola de Engenharia, Portugal.
[6] J. Lloret et al., “Underwater Wireless Sensor Applications in the 2.4 GHz ISM Frequency band,” Sensors, vol. 12, pp. 4237-4264, 2012, DOI: 10.3390/s120404237.
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