Wireless RF sensing in ionic aqueous environment: modeling, design and validation

Abstract

In light of the growing crisis of climate change, there is an urgent call to preserve the available limited resources like water. One of the sources of water use in everyday life is for hygiene, which has not received much attention for technological advancements from electrical engineering point of view to date. Water use in bathrooms is not regulated or controlled and the amount of water used is completely dependent on the bathroom user. A wireless sensor designed to detect the amount and type of excreta in a toilet bowl would be a game changer facilitating ways to regulate or adjust the water consumption for automatic cleaning. The design and operation of such a sensor are, however, extremely challenging due to the complicated environment in a bathroom. A proper sensor for this task must satisfy many requirements to be finally worthy of being installed in bathrooms. While providing privacy for users, the sensor must operate wirelessly from the exterior and be sensitive only to the inside of the bowl. This requirement hides the sensor from users and cleaners and preserves it from direct contact with the waste water or excreta. Moreover, the sensor must be conformable to take the shape of most of the currently available toilet setups. The potentially low cost and low noise performance of microwave sensors make them possibly an excellent candidate for the task. Unfortunately, a wireless sensing technology that meets all these requirements does not currently exist. In this thesis, to meet this technological gap, a functionally novel near-field RF sensor operating at 680-700 MHz with a single port which requires rather cheap and easy-to-manufacture circuitry is proposed and demonstrated employing a unique combination of spectral response and time-transient behaviour together for the first time. The proposed sensor is designed to be specifically coupled in the near field to the porcelain wall of the toilet bowl and water inside it which makes it impervious to the surrounding environment. It is installed on the three-dimensional surface of the outer wall of the U-pipe at the bottom of the bowl and performs sensing according to the changes occurring inside the U-pipe. The developed sensor is capable of sensing the addition of ionic content to the water inside the toilet, the same as the case when urine is injected into the bowl. Using artificial urine, the sensor is shown to detect ionic concentrations (down to 31 mM) in water which perfectly suits the application. Moreover, the addition of solid objects into the aqueous medium is demonstrated to be sensed using the time modulation of the surface of the water. Such characteristic modulations created on the water surface (ripples) are detected by the sensor in terms of time-transient shifts in resonance frequency. Deriving the Fourier content of the derivative of the frequency shift read-out in time produces valuable information for specifically differentiating between solid insertion and liquid addition into the bowl. In fact, section-by-section Fourier transforms of the frequency shift derivative while recording the maximum magnitude helps constructing a decision-making graph. This contains the useful information in the Fourier domain to determine whether solids or liquids were inserted in the medium by simple thresholding. Such extracted information plays a key role in choosing suitable washing mechanism to save water. The findings of this thesis indicate that this developed technology of wireless RF sensing in an aqueous environment holds great promise for smart bathroom and green cleaning applications.