Development of a fast-charging platform for buried sensors using high frequency IPT for agricultural applications

This paper describes the methodology and experimental results for wireless power delivery to a soil-sensors power and data distribution unit from an unmanned aerial vehicle (UAV), using a high frequency inductive power transfer (HF-IPT) link. The configuration features, at the transmit side, a lightweight single-turn air-core coil driven by a 13.56 MHz Class EF inverter mounted on a Matrice 100 drone by DJI, and at the receive side, a two-turn PCB coil with a voltage-tipler Class D rectifier, an off-the-shelf 42 V battery charger and a supercapacitors module for energy storage. The experiments were conducted with a coil-to-coil gap of 250 mm, which corresponds to a coupling factor lower than 5%. In the experiments, a 10 F, 42 V supercapacitors module was charged in eleven minutes with an energy efficiency of 34% from the 80 V DC source that feeds the inverter on the drone to the supercapacitor-based energy storage unit in the sensor module. At higher power (50 W) the HF-IPT system was able to achieve a 68% DC-DC efficiency with a coupling factor of 3.5%. The work reported in this paper is part of a multiple-discipline project which looks to enable the optimal usage of water in agriculture with broader sensing techniques and more frequent sensing cycles

Design and development of a test rig for 13.56 MHz IPT systems with synchronous rectification and bidirectional capability

This paper presents the development of a test rig for bidirectional 13.56 MHz wireless power using identical back-to-back Class EF converters. Theoretical principles of bi-directional wireless power are described and an operating chart representing the range of admissible complex voltages induced on the active transmit side is introduced. The implementation is achieved by driving the gate signals of two Class EF coil-drivers from a signal generator, allowing the relative phase of the currents in each coil to be controlled. The rig sets a constant input voltage for each of the two coil-drivers by implementing a source-sink configuration, emulating a bidirectional DC-DC conversion stage at each side. This setup can also be used to test for tuning mismatches and different loading conditions in the back-to-back Class EF configuration. Experimental results include bidirectional wireless power transmission of 20 W across a 13.56 MHz link with 6.56% coupling. The combination of low coupling factors and moderate power levels enables new classes of applications that require large air gaps and tolerance to misalignment such as in micro e-mobility. High efficiency can be maintained despite changes in coupling factors and load since active rectification ensures operation at the resonant point in both tanks

A reflected impedance estimation technique for inductive power transfer

This paper proposes a technique to estimate the equivalent reflected impedance on the transmit side of an inductive power transfer (IPT) system, produced by an inductively coupled load. This technique consists of analysing changes in the drain voltage waveform of the switching devices to estimate the reflected impedance, and hence not require feedback information from the receive side or the coupling between the coils. The correlation between the drain voltage waveform and the reflected impedance is done by training a model with machine learning techniques. The proposed impedance estimation method is demonstrated using circuit simulations and is verified experimentally, with an IPT transmitter driven by a load independent Class EF inverter operating at 13.56 MHz. The IPT receiver consists of an ac-load which allows changes in the residual reactance. The model trained from experimental data is capable of estimating the equivalent reflected impedance with an accuracy (coefficient of determination) of 0.9899 for the real part and 0.9743 for the imaginary part

A multi-MHz wireless power transfer system with mains power factor correction circuitry on the receiver

This paper proposes the implementation of a new system topology for multi-MHz inductive power transfer (IPT) systems, which achieves unity power factor when fed from a mains power supply without traditional active circuitry in the front-end as a mains interface. Experiments were performed using an IPT-link which consists of two 20 cm two-turn air-core printed-circuit-board (pcb) coils separated by an air-gap of 13 cm. At the transmit side, a push-pull load-independent Class EF inverter fed from a rectified 60 Hz power supply with no bulk capacitor was designed to drive the transmit coil at 13.56 MHz. This inverter, which has two choke inductors between the voltage source and the two switches, similar to that of an interleaved boost converter, is suitable to be fed directly from a rectified mains source because it tolerates large changes on the input voltage. The IPT rectifier in the experiments was built using a dual current-driven Class D-based topology which allows for higher output voltage when the induced electromotive force (emf) on the receive coil is low. The final power conversion stage on the receive side is a power factor correction (PFC) boost converter that regulates the output voltage and shapes the current waveform at the input of the system. This stage is the only part of the system with closed-loop control. The end-to-end efficiency was measured at 73.3% with 99.2% power factor, when powering a load of 150 W

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 so..

A 13.56 MHz bidirectional IPT system with wirelessly synchronised transceivers for ultra-low coupling operation

This paper presents a high-frequency inductive power transfer (HF-IPT) system with bidirectional capability employing a new wireless synchronisation method. Synchronisation is achieved by transmitting a reference ultra high frequency tone (433.92 MHz) that is stepped down to 13.56 MHz in each transceiver. This allows the operating frequency to be locked across the two sides of the system. Afterwards, a phase search is performed looking for maximum power throughput, determining the phase at the point of resonance (i.e., no reflected reactances). The experimental implementation is achieved with two back-to-back Class EF coil-drivers driven by independent synchronisation circuits. In the experimental setup a constant input voltage is set for each of the two coil-drivers by implementing a source-sink configuration, emulating a bidirectional DC-DC conversion stage at each side. Experimental results show successful transceiver synchronisation, and 4 W were transferred from one end to the other and conversely at an ultra-low coupling of 1.6%. This proves that the combination of the load-independent Class EF transceivers and the synchronisation technique introduced herein is suitable for applications that require large tolerance to misalignment and air gaps larger than one coil diameter, such as in micro e-mobility

Probability-based optimisation for a multi-MHz IPT system with variable coupling

This paper presents the analysis and design of a dynamic inductive power transfer (IPT) system, in which coupling is treated as a stochastic variable and is therefore modelled as a probability distribution. The purpose of this formulation is to optimise the tuning of the inverter and the rectifier to the coupling value that achieves the highest charging energy-efficiency when operating at a broad range of coupling. The analysis is supported by a case study in which two rectifier designs, using the hybrid Class E topology, are tuned at different coupling values in order to verify which version achieves the highest charging efficiency. The load in the experiments is a wirelessly powered drone without a battery hovering randomly over the charging pad, and the range of motion is set by a nylon string tether. The experiments show lower energy consumption when the rectifier is tuned to present the optimal load of the link at the coupling value with the highest probability, as opposed to the first, which was designed to present the optimal load of the link at minimum coupling

Characterisation of high frequency inductive power transfer receivers using pattern recognition on the transmit side waveforms

This paper demonstrates the characterisation of inductively coupled receivers for high frequency inductive power transfer (HF-IPT) systems using pattern recognition on the inverter waveforms at the transmit side. The impedance reflected by the candidate receivers to the transmit coil was estimated using a model programmed to associate the experimental drain-voltage waveforms of the inverter when it drives a receiver under test to those when driving known loads. The necessity of employing this technique is due to the difficulty of accurately measuring current and voltage across the coil given the parasitic effects of probing and the precise skewing required to measure an impedance, especially at high Q-factor. The proposed technique is convenient for characterising and comparing the impedance reflected by candidate receivers for a particular application where there is a choice to be made with respect to the rectifier topologies and semiconductor technologies. Experimental results, using a 13.56 MHz 100 W inductive power transfer system, were obtained for a full-wave Class D rectifier using silicon (Si) and silicon carbide (SiC) Schottky diodes, and two Class E rectifiers using SiC diodes

Generalised multistage modelling and tuning algorithm for class EF and class Φ inverters to eliminate iterative retuning

The additional complexity of Class EF and Class Φ inverters compared to their Class E counterparts, combined with parasitic effects becoming more prevalent as frequency and power levels increase, results in poor accuracy from traditional design methods, and usually additional iterations of manual retuning are required. In this work we propose an approach to simulating and tuning Class EF/Φ inverters, with various levels of accuracy depending on the level of knowledge of the system parasitics. Our method is comprised of a combination of analytic and numerical solving methods thus providing both insight on the progression of the algorithm and computational robustness. The aim of our algorithm formulation is to enable solutions to be found in an automated and fast way. The novelty in our work lies in the design method's concurrent capability to provide a generalised set of design inputs (e.g. DC to AC current gain, arbitrary drain voltage slope at turn on, Φ- branch resonance, etc.), inclusion of board and device non-linear parasitics, and the ability to design within the set of preferred component values. An example is shown for the design of a 50 W, 13.56 MHz inverter where the experimental setup approaches the theoretical efficiency of 97%. The algorithm changes the values of the components over 5% to 50% and improves the simulated waveform accuracy by 2 to 12 times compared to the design method based on first order approximations