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In this paper, a complete wireless power transmission scenario is presented, including an external transmission antenna, an in-body embedded antenna, a rectifying circuit, and a powered sensor. This system operates at the Industrial, Scientific, and Medical bands (902.8–928 MHz). For the antenna design, important parameters including reflection coefficient, radiation pattern, and specific absorption rate are presented. As for the rectifying circuit, a precise model is created utilizing off-the-shelf components. Several circuit models and components are examined in order to obtain optimum results. Finally, this work is evaluated against various sensors' power needs found in literature.
Telemetry acquisition from rodents is important in biomedical research, where rodent behavior data is used to study disease models. Telemetry devices for such data acquisition require a long-term powering method. Wireless power transfer (WPT) via magnetic resonant coupling can provide continuous power to multiple small telemetric devices. Our loosely coupled WPT (LCWPT) system consists of a stationary primary coil and multiple freely moving secondary coils. Our previous LCWPT system was designed to transfer reasonable power to secondary coils at poor orientations but transfers excessively high amounts of power at favorable orientations. Reasonable power is needed for telemetry and radio electronics, but highly induced voltage on the secondary coil creates excess energy which must be dissipated by previous devices, and caused problems (localized heat damage and variations in component properties) leading to drift in operating frequency. To remedy these two problems, a novel scheme is proposed to automatically tune or detune the resonant frequency of the secondary circuit. Our closed-loop controlled tuning or detuning (CTD) approach can be used to prevent excessive power transfer by detuning, or to improve power transfer by tuning, depending on the need. Furthermore, this novel CTD scheme facilitates the use of multiple telemetric devices.
Radiofrequency surface coils used as receivers in magnetic resonance imaging (MRI) rely on cables for communication and power from the MRI system. Complex surface coil arrays are being designed for improving acquisition speed and signal-to-noise ratio. This, in-turn makes the cables bulky, expensive, and the currents induced on cables by time-varying magnetic fields of the MRI system may cause patient harm. Though wireless power transfer (WPT) can eliminate cables and make surface coils safer, MRI poses a challenging electromagnetic environment for WPT antennas because the antennas made using long conductors interact with the static and dynamic fields of the MRI system. This paper analyses the electromagnetic compatibility of WPT antennas and reveals that commercially available antennas are not compatible with MRI systems, presenting a safety risk for patients. Even when the risk is minimized, the antennas couple with surface coils leading to misdiagnosis. This paper presents an approach to eliminate safety risks and minimize coupling using a filter named “floating filter.” A WPT antenna without a filter has a distortion of 27%, and floating filters reduce the distortion to 2.3%. Secondly, the floating filter does not affect the power transfer efficiency, and the transfer efficiency of 60% is measured with and without filters.