Induced Voltage Estimation From Class EF Switching Harmonics in HF-IPT Systems

One of the advantages of high-frequency inductive power transfer systems is the high tolerance to misalignment and large air-gaps. However, the inherently large magnetic field volumes can lead to coupling of additional foreign objects with the primary, causing possible detuning of the system and heating of the objects. These foreign objects and the conditions of the local environment can load the transmitter, which changes the induced voltage on the primary side. Unfortunately, the induced voltage is not directly measurable in an operating transmitter and the most straightforward way of calculating this variable, through a measurement of primary coil current and voltage, can cause a significant decrease in quality factor which reduces system performance. An integrated solution capable of estimating the induced voltage through other less invasive measurements in the primary is needed to ensure safety of operation through foreign object detection. Knowledge of the induced voltage can also be used to correct tuning mismatches where both sides of the link are active (i.e., in synchronous rectification and bidirectional systems). In this article, multiple candidate variables for estimating the induced voltage are assessed based on factors such as measurement practicality and estimation accuracy. It is demonstrated for the first time that a solution which is based on the measurement of only two variables, the amplitude of the fundamental frequency of the switching waveform and input current, can achieve state-of-the-art induced voltage estimation accuracy. These two variables, which can be obtained using simple cost-effective analogue circuitry, are used in a Gaussian process to generate a regression model. This is used to estimate induced voltages at any angle in an approximate magnitude range of 0–20 V with a normalized root-mean-square error of 1% for the real part and 1.5% for the imaginary part. This corresponds to detecting a plastic container with 1 kg of saline solution (0.4% concentration, to emulate the electromagnetic profile of muscle) at a distance of 15 cm (1.5 coil radii). The results presented in this article were obtained with a bidirectional Class EF transceiver operating at 13.56 MHz delivering up to 32 W with coupling factors ranging from 1.1% to 4%. This article is accompanied by a video file demonstrating the experiments.