Design of inductive power transmission system for low power application with movable receiver and large air gap

Inductive power transmission is very useful, not only for systems where energy transfer should take place in hazardous, humid and wet areas, but also for mobile and very small systems. It finds today a widespread use in several fields, such as industry, automotive, medicine and smart buildings. For a good efficiency and a high-power transmission, the sending and the receiving coils should be perfectly aligned and close to each other. A misalignment between the sender and the receiver becomes unavoidable especially for systems with movable parts. This thesis aims to improve the transmitted power, the mutual inductance, the power at the load, and consequently the power transmission efficiency in case of lateral misalignment between the sending and receiving coils and at large coil-to-coil distance. For this purpose, we adopt a multi input single output (MISO) coil system able to orientate the issued magnetic field to the receiving coil by powering the neighbouring sending coils of the active ones with a weak current in the opposite direction. Furthermore, an analytical model of the used coils and an accurate three-dimensional model of the system have been developed to calculate the induced voltage, the induced current, and the equivalent mutual inductance. Both simulation and experimental results prove that the proposed multi-coil inductive system having an hexagonal arrangement and the sending coils, which have the half diameter of the receiving coil, is able to improve significantly the transmitted power in case of lateral misalignment and big air gap. The novel MISO system reaches better efficiency beginning with an air gap of 50% of the sending coil diameter, and a misalignment of 28% of the sending coil diameter. It reaches the double of the transmitted power of the conventional two-coil inductive system at 50 mm air gap (corresponding to 166% of the sending coil diameter) and at 10 mm lateral misalignment (corresponding to 33% of the sending coil diameter). In order to improve the equivalent mutual inductance between the primary and secondary sides and to avoid energy losses, we propose a receiver detection method using the sending coils themselves as detectors. Thereby, only the sending coils, under the receiver, are activated and the others remain switched off. For that, the peak of the AC current of the sending coils, is measured and then compared to a detection threshold. The excitation strategy of the active sending coils is optimized corresponding to the receiving coil position. The novel excitation strategy increases the mutual inductance by 85% and the induced voltage by 13% at perfect alignment and by 30% and 10% respectively at 10 mm lateral misalignment, in comparison to the MISO system without a receiver detector and coil-excitation strategy. In order to increase the transmitted power by resonance, different system topologies have been investigated, such as series-series SS, series-parallel SP, parallel-series PS, and parallel-parallel PP topologies for different levels of load impedance. The results show that a multi-coil inductive system with parallel-parallel PP topology realizes a higher transmitted power than the other topologies for both high and low load impedance values. The proposed multi-coil inductive system is suitable for low-power systems, such as wireless sensors and biomedical implants, but can be also applied to higher range of power at a flexible position of the receiver.