New Coupling and Power Regulation Technologies for Wireless Inductive Links


Student thesis: Doctoral Thesis

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Award date18 Apr 2017


Wireless inductive links have been successfully implemented in many applications, for instances, biomedical implants, and battery chargers. The technique is limited by a fundamental “bottleneck” – variation of power transfer efficiency and transmitted power due to coil misalignment. Since the transmitter and receiver in a wireless inductive link system have no physical contact, the receiver has a spatial freedom to be displaced freely from the transmitter. In practice, inevitable spatial misalignment between the coils leads to the variation of the strength of the magnetic coupling. The power transfer efficiency and transmitted power are significantly affected by the magnetic coupling between the two coils. This thesis presents the findings of research on new coupling and power regulation technologies for wireless inductive links. The variation on the power transfer efficiency and transmitted power due to coil misalignment are two major aspects being investigated.

Coil structures comprising two orthogonally-placed windings used in either receiver side or transmitter side with circuit implementations were presented. Such coil structure can effectively lessen the variation of the magnetic coupling due to the coil misalignment. An output current summing technique that keeps the windings in receiving coil concurrently energized and combines the output currents of the windings was studied. A canonical model was also derived to describe the interactions between the coils. A driving mechanism for the windings in transmitting coil to maximize the power transfer efficiency was discussed. Experimental prototypes have been built and evaluated on a test bed, which allows different degrees of lateral and angular misalignments. Results reveal that the proposed structure can effectively increase the minimum efficiency zone, expanding the allowable lateral and angular misalignments.

Estimation algorithms were proposed to regulate the load power of the inductive link under coil misalignemnts. Variation of transmitted power due to coil misalignment is practically unavoidable. Typically, it is tackled by conducting on-the-spot measurements of the electrical quantities together with sophisticated communication links and protocols to provide the transmitter with the operating condition of the receiver. To reduce system complexity, a new perspective by using the transmitter-side electrical information and component values in the inductive link to estimate the mutual inductance and regulate the receiver side power is presented. The nonlinear input voltage-current characteristics of the diode-bridge rectifier, which causes distortions in the input and output current of the inductive link, has been taken into account in time-domain mathematical models. The proposed techniques are successfully implemented on a wireless-powered LED driver prototype. Experimental results reveal that the load power can be regulated within ±25% spatial misalignment with respective to the coil dimension.

In addition, an evolutionary computation technique that processes the transmitterside electrical information to estimation several system parameters, including coil inductances and quality factors, resonant frequencies of the transmitting and receiving networks, and coupling coefficient, for the transmitter to manage power transfer will also be discussed. Due to manufacturing tolerance, temperature effect, and aging, electronic components used in inductive link are subject to parameter variations. The proposed technique has been applied to the wireless-powered LED driver prototype for regulating the load power under parametric variations.