Foreign object detection in wireless power transfer by asymmetry detection

A method for detecting foreign objects on a wireless power transfer device comprising:

• providing power to 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;
• detecting a net voltage across a foreign object sensor coil structure, the 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;
• comparing the net voltage to a predetermined threshold;
• and
• determining presence or absence of a foreign object based on a result of comparing the net voltage to the predetermined threshold.

A system for detecting foreign objects on a wireless power transmitter, comprising:

• a foreign object sensor coil structure comprising
• a first sensor coil, and
• a second sensor coil coupled in series to the first sensor 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.