Diamagnetically Levitated Resonant Mass Sensor

Abstract

In the past few decades, there has been a growing demand for low-cost disposable sensors to be utilized in hazardous and toxic environments such as high temperature and chemically aggressive environments, which can lead to irreversible damage to the sensor. Moreover, by cost reduction, they become more attractive for utilization in developing countries, where the research of new biomedical technologies has been restricted due to a decentralized healthcare system, lack of infrastructure and shortage of investment. This research focuses on developing a low-cost, disposable and easy to use mass sensor utilizing diamagnetic levitation. Diamagnetic levitation is the only stable form of levitation at room temperature and does not require external sources of energy, feedback loops or cooling for stable levitation. It is a low cost, easy to use, disposable sensor that makes it suitable for potential point-of-care applications. The sensor consists of a pyrolytic graphite plate that levitates above a checkerboard arrangement of permanent magnets with alternating magnetization. The system is actuated using electromagnetic excitation and the material properties of the sample are extracted by conducting a multi-modal analysis of the resonance frequencies and mode shapes; measured using a laser Doppler vibrometer (LDV). An opto-electronic phase lock loop utilizing the LDV is used to bring the levitating pyrolytic graphite into sustained mechanical oscillation controlled by a PID controller. The experimental mass sensitivity is measured by analysing the frequency shift associated with the placement of glass beads. The frequency stability is measured by computing the Allan deviation and the experimental precision of mass sensor is then computed. The results show that the precision of a diamagnetically levitated resonant mass sensors decreases with the decrease of the size of the levitating oscillator. The precision of a levitating oscillator of side length 2mm and thickness 0.08mm is found to be 14.4pg at a gate time of 0.1s for an instantaneous change in frequency.