Foreign object detection in wireless energy transfer systems

A method for detecting foreign object debris around a wireless power transfer system, the method comprising:

• measuring at least one of a voltage and a current in each one of a plurality of detectors, wherein the plurality of detectors are positioned so that each detector receives at least a portion of a common oscillating magnetic field generated by the wireless power transfer system;
• and
• determining, based on the measurements, whether foreign object debris is present around the wireless power transfer system,
• wherein each of the detectors comprises one or more loops of conducting material;
• and
• wherein at least some of the plurality of detectors comprise a first number of loops of the conducting material, and at least some of the plurality of detectors comprise a second number of loops of the conducting material larger than the first number.