Foreign object detection using combined inductive and thermal sensing
A method for detecting a foreign metal object near an inductive wireless power transfer system is based on combined inductive sensing and thermal sensing. A sensing system senses the object based on an inductive or thermal effect. The sensing system includes a sense element which has an electrical characteristic that changes in a presence of the object based on the inductive effect. A mediating heat-sensitive material has a property that varies as a function of temperature and changes the electrical characteristic of the sense element as the property changes. The mediating heat-sensitive material is heated by the foreign object when the foreign object is heated by a magnetic field generated by the wireless power transfer system. A controller measures the electrical characteristic and determines a presence of the object based on the change in the measured electrical characteristics.
1. An apparatus for detecting a presence of a foreign object in an inductive wireless power transfer system, the apparatus comprising:
a sensing system configured to sense the foreign object based on one or more of an inductive effect and a thermal effect, the sensing system including:
a sense element having an electrical characteristic, the sense element being configured to sense the foreign object based on the inductive effect related to a first alternating magnetic field; and
a mediating heat-sensitive material coupled to, but electrically separate from, the sense element, wherein the mediating heat-sensitive material has a property that varies as a function of temperature when heated by the foreign object exposed to a second alternating magnetic field generated by the inductive wireless power transfer system, wherein the mediating heat-sensitive material changes the electrical characteristic of the sense element as a function of the property; and
a controller coupled to the sensing system, the controller configured to:
measure the electrical characteristic of the sense element; and
determine the presence of the foreign object based on the change in the measured electrical characteristic caused by at least one of the inductive effect and the thermal effect through the mediating heat-sensitive material.
2. The apparatus of claim 1 , wherein the property of the mediating heat-sensitive material comprises one or more of a resistivity, a conductivity, an impedance, a capacitance, an inductance, a magnetic permeability, and an electric permittivity.
3. The apparatus of claim 1 , wherein the mediating heat-sensitive material is configured to substantially change the property as the temperature of the mediating heat-sensitive material passes a threshold that is higher than 373 Kelvin.
4. The apparatus of claim 3 , wherein the mediating heat-sensitive material includes one or more of a negative temperature coefficient (NTC) resistance material, a positive temperature coefficient (PTC) resistance material, a dielectric material with a low Curie point, and a ferrite material with a low Curie point.
5. The apparatus of claim 1 , wherein the sense element comprises one or more turns of an electrical conductor.
6. The apparatus of claim 5 , wherein the sense element is implemented on a printed circuit board.
7. The apparatus of claim 1 , wherein the sense element is embedded in a layer also comprising the mediating heat-sensitive material.
8. The apparatus of claim 1 , wherein the heat-sensitive material is in a first layer different from a second layer comprising the sense element.
9. The apparatus of claim 1 , wherein the sensing system comprises an array of sense elements, and the mediating heat-sensitive material is associated with at least one sense element of the array.
10. The apparatus of claim 1 , wherein the electrical characteristic measured by the controller comprises one or more of an impedance, a transimpedance, a direct current (DC) resistance, a voltage, a current, an induced voltage, an impulse response, and a response of another waveform.
11. The apparatus of claim 1 , wherein the controller is configured to determine the presence of the foreign object by correlating the measured electrical characteristic with a level of the second alternating magnetic field generated by the inductive wireless power transfer system.
12. The apparatus of claim 11 , wherein correlating the measured electrical characteristic with the level of the second alternating magnetic field comprises at least one of inductive thermal sensing (ITS), inductive ferromagnetic sensing (IFS), or inductive motion sensing (IMS).
13. The apparatus of claim 1 , further comprising a heat-resistant material configured to protect the sensing system against damage from heat from the foreign object.
14. The apparatus of claim 13 , wherein the heat-resistant material constitutes at least a portion of an enclosure of the wireless power transfer system.
15. The apparatus of claim 13 , wherein the heat-resistant material is in a first layer exposed to an exterior of the apparatus, and the heat-sensitive material is in a second layer covered by the first layer.
16. The apparatus of claim 13 , wherein the heat-resistant material is configured to have a melting point higher than 473 Kelvin and to withstand a temperature reached by the foreign object higher than 473 Kelvin without losing structural integrity.
17. A method of detecting a presence of a foreign object in an inductive wireless power transfer system based on one or more of an inductive effect and a thermal effect, the method comprising:
measuring an electrical characteristic of a sense element sensitive to the presence of the foreign object based on an inductive effect related to a first alternating magnetic field; and
while a second alternating magnetic field is being generated by the inductive wireless power transfer system:
measuring the electrical characteristic of the sense element;
determining that the electrical characteristic has changed due to the thermal effect when the electrical characteristic has changed in a manner that indicates that the change was due to a property of a mediating heat-sensitive material coupled to, but electrically separate from, the sense element, the property varying as a function of temperature when the mediating heat-sensitive material is heated by the foreign object exposed to the second alternating magnetic field; and
determining the presence of the foreign object based on the change in the measured electrical characteristic caused by at least one of the inductive effect or the thermal effect through the mediating heat-sensitive material.
18. The method of claim 17 , wherein measuring the electrical characteristic of the sense element includes measuring one or more of an impedance, a transimpedance, a direct current (DC) resistance, a voltage, a current, an induced voltage, an impulse response, and a response of another waveform.
19. The method of claim 17 , wherein determining the presence of the foreign object includes correlating the measured electrical characteristic with a level of the second alternating magnetic field generated by the inductive wireless power transfer system.
20. The method of claim 19 , wherein correlating the measured electrical characteristic with the level of the second alternating magnetic field comprises at least one of inductive thermal sensing (ITS), inductive ferromagnetic sensing (IFS), or inductive motion sensing (IMS).