This paper deals with a wireless power transfer system where a novel structure of transmitting/receiving double DD coils is applied. This system uses two identical double D (DD) transmitter coils stacked on each other to transfer power to two stacked receiver coils. The power is transmitted simultaneously and independently through both transmitter coils to the receiving coils. The magnetic field of the first coil does not interfere with the second coil. Both transmitter and receiver coils are placed on each other and occupy the same footprint, so there is no need for increased space. This can lead to an interesting wireless power transfer system—from single load to double the load and higher power transfer density.

This paper presents the evolution of an inductive wireless power transfer using a multicoil system. The double DD coil structure on the transmitter and the receiver side using two perpendicular bipolar DD coils is upgraded with an additional nonpolar quadrature coil. The proposed structure can be called the double DDQ coil structure. All three coils are not coupled, due to the nature of the directional double DD coil. If the transmitter and the receiver are not misaligned to one another, the system behaves as three separate, uncoupled IPT systems. The main advantage of the proposed coil topology is additionally increased power density and increased misalignment tolerance. Additionally, when the transmitter and the receiver coil are perfectly aligned, the proposed pad structure can transmit three different voltages and can be excited with different frequencies. In the case of this paper, the three coils on the transmitter side were excited by the same frequency. The proposed coil was evaluated experimentally and compared to the system using double DD coil structure.

This paper presents the design of the control of the system using a double DD coil structure. The double DD coil is a layered coil structure that consists of two single DD coils, rotated to each other by 90°. A large-signal and small-signal model of the proposed IPT system are designed for control synthesis. The small-signal model is derived from the large-signal using harmonic approximation and the extended describing functions (EDF). For the small-signal model, voltage and current control schemes were proposed for the purpose of wireless battery charging. The robustness of the control is tested on a small-scale IPT system using double DD coils and resistive load. The results are evaluated at different reference voltages, currents, loads and coupling coefficients.

This paper presents a platform developed for automated magnetic flux density measurement. The platform was designed to be used to measure the magnetic flux density of the transmitter/receiver coil of an inductive wireless power transfer system. The magnetic flux density of a transmitter was measured using a small, 3-axis search coil. The search coil was positioned in the 3D space above the transmitter coil using a 3D positioning mechanism and used to measure the magnetic flux density at a specific point. The data was then sent to a computer application to visualize the magnetic flux density. The measured magnetic field could be used in combination with electromagnetic field solvers to design and optimize transmitter coils for inductive wireless power transfer systems.

This paper presents a custom-made, computer-connected, and controlled 3D platform that enables the evaluation of the coupling coefficient between the transmitter and receiver coil parts of an inductive wireless power transfer (IPT) system. The platform includes a computer application, a 3D positioning mechanism, and an inductance measurement circuit. The positioning mechanism moves the coils to the point in 3D space, and the inductance circuit measures the mutual inductance between the transmitter and the receiver coil. The measured value can be used to calculate the coupling coefficient between the transmitter and the receiver coil. The data are sent to the computer for further visualisation. The transmitter and the receiver coil can be evaluated by measuring the coupling coefficient between them in multiple points in space. Measurements performed with the platform can be used in the design and evaluation phases of inductive wireless power transfer systems and to extrapolate the polynomial function of the coupling coefficient in relation to the distance between coils or their misalignment.

This paper proposes an on-line method for foreign object detection in a double DD coil system. The foreign object is detected by real-time measurement of the mutual inductance between the transfer pads. Measurement of the mutual inductance between coils can be performed at the start, during initialisation, or during the wireless power transfer. The coils in the double DD coil structure can be used separately; one coil can be used for power transfer and the other can be used for the mutual inductance measurement. The mutual inductance measurement is based on the voltage measurement across the open circuit receiver coil. The measured value of mutual inductance between the transmitter and the receiver pad can be used in a control algorithm and in a foreign object detection algorithm. Additionally, a 2DDq coil structure can be used as a replacement for the double DD coil structure, which increases the power transfer density. The DD coils in the double DD coil structure can also be driven using two phase-shifted voltages, which enables better location and detection of foreign objects. The method also helps to differentiate the mutual inductance change due to the distance change from the mutual inductance change due to the presence of a foreign object.

This paper describes the control of a wireless power transfer system using a double DD coil structure, when the transmitter and the receiver coil are rotated to one another. WPT systems using single DD coils are rotationally dependent, due to the directional magnetic field generated by the DD transmitter coil. This rotational dependance can be mitigated by using a different transmitter structure. One such possibility is the usage of the double DD coil on the transmitter and the receiver sides. A double DD coil includes two directional DD coils, which can be excited separately. The coils inside the coil structure are perpendicular to one another, which allows higher power density and additional rotation tolerance. The proposed system was tested on a low power laboratory experimental setup.