Inductive power transfer (IPT) remains one of the most common ways to achieve wireless power transfer (WPT), operating on the same electromagnetic principle as electrical transformers but with an air core. IPT has recently been implemented in wireless charging of consumer products such as smartphones and electric vehicles. However, one major challenge with using IPT remains ensuring precise alignment between the transmitting and receiving coils so that maximum power transfer can take place. In this paper, the use of additional sensing coils to detect and correct lateral misalignments in an IPT systems is modeled and tested. The sensing coils exploit magnetic-field symmetry to give a nonlinear measure of misalignment direction and magnitude. Experiments using such sensing coils give a misalignment-sensing resolution of less than 1 mm when applied to a common smartphone wireless charging system. Voltage readings from the sensing coils are used for feedback control of an experimental two-dimensional coil positioner. This system is able to reduce lateral misalignments to less than 2 mm in real time, allowing for efficient power transfer. The results of this experiment give confidence that similar sensing coils can be used to reduce lateral misalignments in scaled IPT systems, such as electric-vehicle wireless chargers.
