Foreign object detection using hybrid inductive and capacitive sensing
In aspects, techniques for detecting and discriminating foreign objects near an inductive wireless power transfer system is disclosed. The method is based on hybrid inductive and capacitive sensing. The apparatus includes a sensing system, a measurement circuit, and a controller. The sensing system senses the object based on at least one of an inductive or a capacitive effect. The sensing system includes an inductive sense element, which has an electrical characteristic that changes in a presence of the object based on the inductive effect. The sensing system further includes a capacitive sense element, which has an electrical characteristic that changes in the presence of the object based on the capacitive effect. The measurement circuit measures the electrical characteristics in the inductive and capacitive sense circuits. The controller determines the presence of the object, and discriminates between metallic and non-metallic objects based on the change in the measured electrical characteristics.
1 . An apparatus for detecting foreign objects in an inductive wireless power transfer system, based on one or more of an inductive effect and a capacitive effect, the apparatus comprising:
a first sense circuit comprising a first electrical conductor, the first electrical conductor forming a loop of an inductive sense element and terminating in a first terminal and a second terminal;
a second sense circuit comprising a second electrical conductor, the second electrical conductor forming an electrode of a capacitive sense element and having a third terminal; and
a measurement circuit, the measurement circuit configured to:
measure a first electrical characteristic between the first terminal and the second terminal and a second electrical characteristic between the first terminal and the third terminal; and
a controller configured to:
jointly use the measured first and second electrical characteristics to determine a presence of the foreign object and to discriminate whether the foreign object is a metallic object or a non-metallic object based on a change in the measured first and second electrical characteristics.
2 . The apparatus of claim 1 , wherein the first electrical conductor forms a balanced loop of at least two turns forming a substantially symmetric structure with respect to a mirror axis, and wherein the first electrical conductor has at least one crossover located on the mirror axis.
3 . The apparatus of claim 1 , wherein the first sense circuit further comprises a capacitor coupled between two equal length sections of the first electrical conductor forming a series resonant circuit tuned to a first operating frequency.
4 . The apparatus of claim 1 , wherein the first sense circuit further comprises a first capacitor coupled between the first electrical conductor and the first terminal and a second capacitor coupled between the first electrical conductor and the second terminal forming a series resonant circuit tuned to a first operating frequency.
5 . The apparatus of claim 1 , wherein the second sense circuit further comprises an inductor coupled between the second electrical conductor and the third terminal forming a series resonant circuit tuned to a second operating frequency.
6 . The apparatus of claim 5 , wherein the second sense circuit further comprises a capacitor coupled between the second electrical conductor and the first terminal in parallel to the capacitive sense element forming a series resonant circuit tuned to the second operating frequency.
7 . The apparatus of claim 1 , wherein the second electrical conductor comprises a single-turn open loop.
8 . The apparatus of claim 1 , wherein at least one of the first electrical characteristic or the second electrical characteristic is a complex impedance.
9 . The apparatus of claim 1 , wherein:
the first sense circuit further comprises a first capacitor coupled between the first electrical conductor and the first terminal and a second capacitor coupled between the first electrical conductor and the second terminal forming a series resonant circuit tuned to a first operating frequency;
the second sense circuit further comprises an inductor coupled between the second electrical conductor and the third terminal forming a series resonant circuit tuned to a second operating frequency; and
the first operating frequency differs from the second operating frequency.
10 . The apparatus of claim 1 , wherein the first and second electrical characteristics are measured in different time intervals according to a time multiplexing scheme.
11 . A method of detecting foreign objects in an inductive wireless power transfer system, based on one or more of an inductive effect and a capacitive effect, the method comprising:
measuring a first electrical characteristic between a first terminal and a second terminal of a first sense circuit comprising a first electrical conductor, the first electrical conductor forming a loop of an inductive sense element and terminating in the first terminal and the second terminal;
measuring a second electrical characteristic between the first terminal and a third terminal of a second sense circuit comprising a second electrical conductor, the second electrical conductor forming an electrode of a capacitive sense element and terminating at the third terminal; and
jointly using the measured first and second electrical characteristics to determine a presence of a foreign object and to discriminate whether the foreign object is a metallic object or a non-metallic object based on a change in the measured first and second electrical characteristics.
12 . The method of claim 11 , wherein measuring at least one of the first and second electrical characteristics comprises measuring a complex impedance.
13 . The method of claim 11 , wherein the first and second electrical characteristics are measured in different time intervals according to a time multiplexing scheme.