Modeling and design of wireless magnetic-resonant power transfer system for a deep brain stimulation device

Abstract

According to a news report, implanted devices are gammg more and more attentions among global medical professionals, because these devices can help to extend and improve the patients' quality of life. However, replacing a battery can be rather traumatic for patients. To address these issues, the wireless power transfer (WPT) technology will provide an effective platform to recharge rechargeable batteries without any surgery which can greatly alleviate the trauma of patients. The most important step in analyzing the WPT system is to obtain the resonant frequency of the system. For systems constructed with distributive inductance and distributive capacitance, the finite element method (FEM) is a powerful tool for the numerical simulation of such systems. It is worth noting that in general, the displacement current is always being ignored in time-domain in low frequency analysis. In this system, however, the displacement current plays a key role to realize resonating operation. Thus, the A-φ formulations which include eddy current and displacement current are deduced and implemented using C++ code in this thesis. Based on the above program, a novel and general method to obtain quickly the frequency-domain solutions based on one or a few time-domain solutions is presented. No frequency sweeping, which is required in conventional studies, is necessary in the proposed algorithm. The time-domain FEM is firstly used to obtain a solution for step function excitations by taking into account all the necessary initial conditions. Then, based on time-domain convolution theorem and the principle of integration by parts, a numerical algorithm to find the solutions of any excitations, without additional FEM computation, is deduced. The numerical experiments show that the computing time of the proposed time-domain FEM is about 10.13% of that required when using conventional frequency-domain 3-D FEM. Furthermore, it is shown in this study that the proposed method can reduce the computing time effectively and readily as it is easy to adjust the time step size in time-domain FEM. The FEM can be used readily and conveniently in analyzing the WPT system which is constructed with distributive inductance and distributive capacitance. For the WPT system which is constructed with lumped inductance and lumped capacitance, the equivalent circuit method is however more suitable in analyzing and designing of the system. In order to reduce the AC resistance, Litz wires are used to design the external transmitter part in the WPT system. Based on the equivalent circuit method, the optimization method is used to find the optimal design parameters which are governed by the initial constraints. In the optimization process, the optimal results should guarantee that the current and induced voltage in the load loop will be large enough to recharge the implanted rechargeable battery. The experiment results of the proposed design show that the diameter of the implanted receiver is less than 10 mm, and the output current and voltage are large enough to recharge a 3.6 V rechargeable battery with a separation distance of 30 mm. From the specific absorption rate (SAR) simulation results, it can be seen the designed system is safe for the patients when it is implanted into the skull of the patients.