Foreign object detection in wireless power transfer by asymmetry detection

In one embodiment, a wireless power transfer system comprises a transmitter coil structure comprising a first transmitter coil, and a second transmitter coil coupled to the first transmitter coil in such a way that when a first current flows in the first transmitter coil in a first spatial direction the first current flows in the second transmitter coil in a second spatial direction different from the first spatial direction, a foreign object sensor coil structure comprising a first sensor coil having a central axis in common with the first transmitter coil, and a second sensor coil coupled in series to the first sensor coil, the second sensor coil having a central axis in common with the second transmitter coil, the first sensor coil coupled to the second sensor coil in such a way that when a first voltage induced in the first sensor coil has a first polarity a second voltage induced in the second sensor coil has a second polarity different from the first polarity, a voltage detector coupled to the foreign object sensor coil structure, the voltage detector configured to detect a net voltage across the foreign object sensor coil structure, and a controller configured to receive the net voltage and determine a presence or absence of a foreign object based on a magnitude of the net voltage.