Foreign object detection in a wireless power transfer system

A wireless power receiver comprising:

• a power receiving circuit arranged to extract power from a power transfer signal,
• wherein the power transfer signal occurs during at least one power transfer time intervals of a repeating time frame,
• wherein the repeating time frame comprises at least one power transfer time interval and at least one foreign object detection time interval;
• wherein a duration of the foreign object detection time interval is no more than 20% of the duration of the repeating time frame,
• wherein the power receiving circuit is arranged to receive a test drive signal during a foreign object detection adaptation time interval from a wireless power transmitter;
• and
• a message transmitter circuit, arranged to transmit a foreign object detection configuration message to the wireless power transmitter,
• wherein the foreign object detection configuration message comprises an indication of a time at which the wireless power transmitter is to determine a signal property of a varying test drive signal for foreign object detection, and
• wherein the wireless power receiver is arranged to receive, from the wireless power transmitter, during a foreign object detection time interval,
• wherein a foreign object detection signal is based on the indicated time in the foreign object detection configuration message.