Simultaneous Power and Signal Transfer via Capacitive Coupling

Abstract

Capacitive power transfer (CPT) technology has recently emerged as an alternative to Inductive Power Transfer (IPT) for driving electronic devices with great convenience. CPT has advantages in terms of flexible and lightweight coupling structures, less EMI (Electromagnetic Interference), and the ability to transfer power through metal barriers, but it has shortcomings of low coupling capacitance and power density. This Ph.D. research is to study the electric field coupling characteristics, robustness improvement on CPT system power transfer, and further develop inband communication channels to achieve simultaneous wireless power and data transfer (SWPDT). The dissertation first addresses capacitive couplers studies, focusing on metal plate configurations and coupler loss analyses. Based on mutual capacitance matrix modelling, the expression of maximum output power among possible configurations is derived. Coupler loss is categorised into innate loss and region loss, which are linked to coupler materials and leakage currents, respectively. It is found the innate loss is normally negligible. However, the leakage current caused by the interactions between the couplers and the surrounding environment is a major contributor of practical CPT systems with loose electric field couplings. The loss breakdown analysis is verified by finite element analysis (FEA) simulations and practical tests. For improving robustness, a H∞ output feedback controller is proposed based on the linear matrix inequality (LMI) approach for a Class-E inverter based rotary CPT system. The H∞ controller automatically adjusts the operating frequency, and can keep the output voltage constant against up to 30% of coupling capacitance and load variations. Two inband communication methods for CPT systems are developed using load shift keying (LSK) and frequency injection techniques. The LSK method is applied on top of an autonomous push-pull resonant converter (PPRC). As the loading conditions change, the operating frequency of the PPRC drifts. This unique feature makes the communication possible even through lossy dielectric materials. A prototype is built and data transfer from a sensor buried 12cm beneath the soil is achieved. The frequency injection method is realised by two additional current sensing transformers, and applied on a double-sided Capacitor-Inductor (CL) compensation network. An additional high-frequency channel is built without compromising the power transfer performance. A full CPT system with wide bandwidth communication is finally developed. The high-pass feature of the capacitive coupling is advantageously utilized to establish a flat frequency response for data transfer at high carrier frequencies. An integrated compensation technique which combines ladder filter and resonant circuit designs is presented to enable the system to transfer power and signal at the same time within a very wide bandwidth, thus SWPDT is achieved. A prototype CPT system rated at 10W is built, which demonstrates a DC-AC efficiency of 81.2% and a data transfer rate of 10kbps using the frequency injection method.