Foreign object detection for ferromagnetic wire-like objects
Systems and methods are described for improving foreign object detection for ferromagnetic wire-like objects. In particular, aspects include increasing FOD sensitivity using at least one inductive sense loop in response to detecting a change in resistance that is substantially equal to the change in reactance of a sense coil. By increasing the FOD sensitivity, objects such as ferromagnetic wire-like objects can be detected that are potentially hazardous objects. Detecting these potentially hazardous objects that were previously undetectable can reduce the chances of damage to surrounding materials or harm to humans.
1. An apparatus for detecting an object, comprising:
a sense coil; and
a detection circuit configured to:
determine an amount of change in a reactance of the sense coil from a reference reactance value for the sense coil;
determine an amount of change in a resistance of the sense coil from a reference resistance value for the sense coil; and
selectively increase a sensitivity of the detection circuit as by reducing a detection threshold by an amount that is a function of a ratio of the amount of change in the resistance reactance and the amount of change in the reactance resistance of the sense coil approaches a value of one, the sensitivity increased by reducing a detection threshold by an amount that is a function of the ratio as the ratio approaches a value by one.
2. The apparatus as described in claim 1 , wherein the detection circuit is configured to determine a presence of the object based on a relationship between the amount of change in the reactance of the sense coil and the amount of change in the resistance of the sense coil.
3. The apparatus as described in claim 1 , wherein the object is a conductive ferromagnetic object.
4. The apparatus as described in claim 1 , wherein the sense coil and the detection circuit are implemented in a wireless electric vehicle charging system.
5. The apparatus as described in claim 1 , wherein the sense coil is implemented as an inductive power transfer coil of an inductive power transfer system.
6. The apparatus as described in claim 1 , wherein:
the reference reactance value for the sense coil includes a reactance of the sense coil when no object is present; and
the reference resistance value for the sense coil comprises a resistance of the sense coil when no object is present.
7. The apparatus as described in claim 1 , wherein the detection threshold is reduced as the ratio approaches the value of one from approximately 0.9 or from approximately 1.1.
8. The apparatus as described in claim 1 , wherein the detection circuit is further configured to determine an impedance change angle of the sense coil based on the amount of change in the resistance of the sense coil and the amount of change in the reactance of the sense coil, wherein the detection threshold is reduced as the impedance change angle approaches a value of 45 degrees from approximately 44 degrees or from approximately 46 degrees in a complex plane.
9. The apparatus as described in claim 1 , wherein the amount of change in the resistance of the sense coil is determined to be within five percent of the amount of change in the reactance of the sense coil.
10. The apparatus as described in claim 1 , wherein the detection circuit is further configured to:
perform an inverse tangent operation on a quotient of the amount of change in the reactance of the sense coil divided by the amount of change in the resistance of the sense coil; and
determine a type of the object based on a result of the inverse tangent operation coinciding with a predetermined range of values corresponding to the type of the object.
11. The apparatus as described in claim 10 , wherein if the result of the inverse tangent operation comprises an angle that is approximately 45 degrees, the sensitivity of the detection circuit is increased to detect that the object is a ferromagnetic object or a non-critical object.
12. A method for detecting an object, the method comprising:
detecting, by a detection circuit, an amount of change in a reactance of a sense coil from a reference reactance value for the sense coil;
detecting, by the detection circuit, an amount of change in a resistance of the sense coil from a reference resistance value for the sense coil;
selectively increasing a sensitivity of a detection circuit as by reducing a detection threshold by an amount that is a function of a ratio of the amount of change in the resistance reactance and the amount of change in the reactance resistance of the sense coil, the sensitivity of the detection circuit increased as the ratio approaches a value of one, the sensitivity increased by reducing a detection threshold by an amount that is a function of the ratio; and
detecting a presence of the object based on a relationship between the amount of change in the reactance of the sense coil and the amount of change in the resistance of the sense coil.
13. The method as described in claim 12 , wherein if the object is determined to have has a length that is substantially longer than a thickness of the object, the ratio of the amount of change in the reactance and the amount of change in the resistance of the sense coil approaches a value of one.
14. The method as described in claim 12 , wherein if the object is a conductive ferromagnetic object, the ratio of the amount of change in the resistance and the amount of change in the reactance of the sense coil is within a range of approximately −1 to approximately +1.
15. The method as described in claim 12 , wherein the detection circuit is configured to use at least two separate inductive sense loops.
16. The method as described in claim 12 , further comprising ceasing inductive power transfer responsive to detecting the presence of the object.
17. The method as described in claim 12 , wherein:
the reference reactance value for the sense coil includes a reactance of the sense coil when no object is present; and
the reference resistance value for the sense coil comprises a resistance of the sense coil when no object is present.
18. Non-transitory computer-readable media comprising code that, when executed, causes an apparatus for detecting an object to:
determine that a ratio of an amount of change in reactance of a sense coil from a reference reactance value for the sense coil is substantially equal to an amount of change in resistance of the sense coil from a reference resistance value for the sense coil;
adjust a sensitivity of a detection circuit based on the determination as a function of the determined ratio, the adjustment including a selective increase in the sensitivity of the detection circuit as a the ratio of the amount of change in the resistance of the sense coil and the amount of change in reactance of the sense coil approaches a value of one, the sensitivity increased based on a reduction to a detection threshold by an amount that is a function of the ratio; and
detect a presence of the object based on a relationship between the amount of change in the reactance of the sense coil and the amount of change in the resistance of the sense coil.
19. The computer-readable media as described in claim 18 , wherein if the object is determined to have has a length that is substantially longer than a thickness of the object, the ratio of the amount of change in the resistance and the amount of change in the reactance of the sense coil approaches a value of one.
20. The computer-readable media as described in claim 18 , wherein if the object is a conductive ferromagnetic object, the ratio of the amount of change in the resistance and the amount of change in the reactance of the sense coil is within the range of approximately −1 to approximately +1.
21. The computer-readable media as described in claim 18 , wherein the sense coil and the detection circuit are implemented in a wireless electronic vehicle charging system.
22. The computer-readable media as described in claim 18 , wherein the sense coil comprises an inductive sensing coil that generates a magnetic field.
23. The computer-readable media as described in claim 18 , wherein:
the reference reactance value for the sense coil includes a reactance of the sense coil when no object is present; and
the reference resistance value for the sense coil comprises a resistance of the sense coil when no object is present.
24. An apparatus for detecting an object, the apparatus comprising:
means for determining that a ratio of an amount of change in a resistance of a sense coil from a reference resistance value of the sense coil is substantially equal to an amount of change in a reactance of the sense coil from a reference reactance value of the sense coil;
means for selectively increasing a sensitivity of a detection circuit based on a determination that the amount of change in the resistance of the sense coil is substantially equal to the amount of change in the reactance of the sense coil, the sensitivity of the detection circuit increased as a ratio of the amount of change in the resistance of the sense coil and the amount of change in the reactance of the sense coil approaches a value of one, the sensitivity increased as the determined ratio approaches a value of one by reducing a detection threshold by an amount that is a function of the determined ratio; and
means for detecting a presence of the object based on a relationship between the amount of change in the reactance of the sense coil and the amount of change in the resistance of the sense coil.
25. The apparatus as described in claim 24 , further comprising means for determining that wherein if the object includes a length substantially longer than a thickness of the object, the ratio of the amount of change in the resistance and the amount of change in the reactance of the sense coil approaches a value of one.
26. The apparatus as described in claim 24 , wherein if the object is a conductive ferromagnetic object, the ratio of the amount of change in the resistance and the amount of change in the reactance of the sense coil is within the range of approximately −1 to approximately +1.
27. The apparatus as described in claim 24 , wherein the sense coil comprises an inductive sensing coil that generates a magnetic field.
28. The apparatus as described in claim 24 , wherein the sense coil and the detection circuit are implemented in a wireless electronic vehicle charging system.
29. The apparatus as described in claim 24 , wherein:
the reference reactance value for the sense coil includes a reactance of the sense coil when no object is present; and
the reference resistance value for the sense coil comprises a resistance of the sense coil when no object is present.
30. The method as described in claim 12, further comprising:
performing an inverse tangent operation on a quotient of the amount of change in the reactance of the sense coil divided by the amount of change in the resistance of the sense coil;
determining a type of the object based on a result of the inverse tangent operation coinciding with a predetermined range of values corresponding to the type of the object; and
based on the result of the inverse tangent operation comprising an angle that is approximately 45 degrees, increasing the sensitivity of the detection circuit to detect that the object is a ferromagnetic object or a non-critical object.
31. An apparatus for detecting an object, the apparatus comprising:
a wireless electric vehicle charging system including a sense coil and a detection circuit, the detection circuit configured to:
determine an amount of change in a reactance of the sense coil from a reference reactance value for the sense coil;
determine an amount of change in a resistance of the sense coil from a reference resistance value for the sense coil; and
selectively increase a sensitivity of the detection circuit by reducing a detection threshold by an amount that is a function of a ratio of the amount of change in the reactance and the amount of change in the resistance of the sense coil, the sensitivity increased as the ratio approaches a value of one.
32. The apparatus as described in claim 31, wherein if the object has a length that is substantially longer than a thickness of the object, the ratio of the amount of change in the resistance and the amount of change in the reactance of the sense coil approaches a value of one.
33. The apparatus as described in claim 31, wherein if the object is a conductive ferromagnetic object, the ratio of the amount of change in the resistance and the amount of change in the reactance of the sense coil is within the range of approximately −1 to approximately +1.
34. The apparatus as described in claim 31, wherein the sense coil comprises an inductive sensing coil that generates a magnetic field.
35. The apparatus as described in claim 31, wherein:
the reference reactance value for the sense coil includes a reactance of the sense coil when no object is present; and
the reference resistance value for the sense coil comprises a resistance of the sense coil when no object is present.
36. The apparatus as described in claim 31, wherein the detection circuit is further configured to:
perform an inverse tangent operation on a quotient of the amount of change in the reactance of the sense coil divided by the amount of change in the resistance of the sense coil; and
determine a type of the object based on a result of the inverse tangent operation coinciding with a predetermined range of values corresponding to the type of the object,
wherein if the result of the inverse tangent operation comprises an angle that is approximately degrees, the sensitivity of the detection circuit is increased to detect that the object is a ferromagnetic object or a non-critical object.