Directional and omnidirectional inductively coupled wireless power transfer systems

Abstract

This thesis presents the analysis and design methods of directional and omnidirectional inductively coupled wireless power transfer systems. Such system utilizes multiple coils to generate high frequency alternating magnetic fields in space and allows receivers to pick up power wirelessly in any position. The objective of optimization is to achieve the maximum power efficiency. Such kind of system allows the mobile devices to be continuously charged without staying on a fixed place.

Self- and mutual- inductance values of the coils are critical parameters that affect the power efficiency of a wireless power transfer system. An improved numerical calculation method is proposed and is presented in this thesis. It combines the theory of partial equivalent element circuit (PEEC) and empirical equations for calculating certain types of straight conductors. The segmentation method to discretize the conductor is optimized and verified. The accuracy of the proposed method has been tested and compared to both theoretical equations and measurements of practical coils, and is proved to be accurate.

The time-varying magnetic fields induced by the coils in a wireless power transfer system is the \transporter" of the energy. A time-efficient visualization method is proposed and is presented in this thesis. While commercial finite element analysis (FEA) software such as ANSYS Maxwell can perform transient analysis, the execution time is extremely long as the accuracy greatly depends on the simulation time steps and the number of iterations. The proposed method calculates the time-instant currents in coils with the derived mathematic model and then directly plots the time-varying magnetic fields. The execution time is shortened to a few of minutes on a desktop computer while the FEA solver takes a couple of hours on the same model. Several types of multiple-coil systems are analyzed with their magnetic field patterns.

To transmit power to arbitrary directions, the magnetic field must be controlled with its directions. According to the principle of superposition, a three-orthogonal-coil transmitter structure is proposed. Several current control methods are discussed to achieve omnidirectional wireless power characteristics. One is a continuous scanning type current scheme. The receivers placed in the designated area can receive power with any locations and angles. Another one is a discrete scanning type. It can be used to detect the load position and therefore delivering power to the load with the maximum power efficiency of the overall system. Simulations and experiments have been carried out to verify the theory and they agreed well.

While the aforesaid omnidirectional system adopts the special three-orthogonal-coil structure, the theory is generalized to arbitrary numbers of transmitters. It is proved by mathematical derivation that in a system with multiple transmitters and single receiver, there exists an optimal efficiency point when currents in the transmitter coils fulfill the certain relationships, regardless of the compensation scheme of the circuit, the use of magnetic materials and the shapes of the coils. An implementable method to calculate the optimal currents in a practical multiple-transmitter-single-receiver system is also provided. The generalized theory is verified by both simulations and experiments.