FIELD OF THE INVENTION

The present invention relates to wireless power transfer systems (either charging or realtime power systems) and apparatus for detecting foreign objects thereon.

BACKGROUND TO THE INVENTION

Figure 1 illustrates a prior art wireless power transfer charging system 10 for charging an electric vehicle 11. The charging system comprises a central charging unit 12 with a power supply 9 and controller 8 for controlling the supply of power to the electric vehicle 11, and also comprises a wireless charging unit 13 that inductively transfers power 15 to the electric vehicle 11. The central charging unit 12 is electrically coupled 16 to the wireless charging unit 13. The wireless charging unit 13 can take the form of a wireless charging pad 40 comprising one or more inductive coils 14 for transferring power across to the electric vehicle 11.

In operation, the central charging unit 12 induces an alternating current into the inductive coils 14, which generates a changing electromagnetic field/flux in and around the inductive coils 14. The electric vehicle has a receiver 17 that receives power inductively transferred from the wireless charging unit 13. The receiver 17 is electrically coupled to a rechargeable battery 18. If the receiver 17 of the electric vehicle 11 is in the vicinity of the wireless charging pad 13, the changing electromagnetic field/flux induces an alternating current in the inductive coils of the receiver 17, thereby creating a wireless transfer of power 15 to the electric vehicle 11.

If a foreign object is also located in the vicinity of the wireless charging pad 13, then the changing electromagnetic field will induce eddy currents within the foreign object. If the electrical resistance of the foreign object is low, then the foreign object will start to heat, and continuous exposure to the electromagnetic flux increases the risk of the foreign object becoming a heating and/or fire hazard.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide for sensing of objects in wireless power transfer.

The present embodiments provide a foreign object detection ("FOD") system for a wireless power transfer charging system 10, so that if a foreign object is on or close enough to the wireless charging pad 13, then remedial action can be taken. For example, the wireless charging pad 13 can power down and/or switch off to prevent the foreign object from becoming a safety hazard. In one aspect, the present invention is a charging unit for wireless power transfer, comprising : a least one coil for inductive power transfer, a sensing arm comprising : a ferrite bar, a sensor coil array disposed on the bar and having a sensing field for sensing the presence of at least one foreign object, and a controller configured to: move the sensor arm so that the sensing field can scan a sensing region, and detect the presence of the at least one foreign object in the sensing region based on the sensing array output.

Optionally the ferrite bar is between the sensor coil array and the inductive power transfer coil

Optionally each coil is up to one third the size of an object to be detected.

Optionally each coil has 10 turns and/or is 10mm x 10mm.

Optionally there are 49 sensor coils along the ferrite bar.

Optionally there is a gap of 2mm between each sensor coil.

Optionally the ferrite bar is 25mm x 590mm x 1 mm.

Optionally the ferrite bar has a cross section shape that is:

• Square

• U shaped

• Rectangular

Optionally the inductive power transfer coil is disposed on a magnetic plate.

Optionally the charging unit further comprises a metallic plate.

Optionally the charging unit further comprises an actuator to move the sensor arm.

Optionally the ferrite bar at least partially shields the sensor coils from electromagnetic fields from the inductive power transfer coils.

Optionally the ferrite bar reduces the induced voltages in the sensor coils. Optionally the ferrite bar improves ability to detect the foreign object. Optionally the ferrite bar improves ability to detect the foreign object by decreasing induced voltage in the sensor coils when a foreign object is present. Optionally the ferrite bar decreases induced voltage to by a greater margin than without the ferrite bar. Optionally the ferrite bar decreases the induced voltage to near zero value when the foreign object is present. Optionally the ferrite bar improves ability to detect the foreign object by increasing the difference in induced voltage in the sensor coils between when a foreign object is present and when a foreign object is not present. Optionally the ferrite bar improves ability to detect the foreign object by increasing the difference in an electrical parameter experienced by the sensor coils between when a foreign object is present and when a foreign object is not present, wherein optionally the electrical parameter is one or more of: • induced voltage • flux • inductance • frequency Optionally the gap between the sensor coils is between one tenth to one fifth of length of sensor coils. Optionally one or more sensor coils have a number of winding turn such that the presence of the foreign object enables a decrease in induced voltage in the sensing by a margin of at least 50mV, preferably greater than 100mV. Optionally in a subset of the one or more sensor coils number each sensor coil in the subset of the one or more sensor coils have the same number of winding turn. Optionally the induced voltage can be measured on an individual sensor coil or a group of sensor coils (e.g. 2 - 3 coils) in series or in parallel. Optionally the length of the ferrite bar is substantially the same as the width of the charging unit. Optionally the width of the ferrite bar is proportional to the diameter of the sensor coil.

Optionally the width of the ferrite bar is 1-3 times the diameter of the sensor coil.

Optionally the induced voltage can be measured on an individual sensor coil or a group of sensor coils (e.g. 2 - 3 coils) in series or in parallel.

Optionally the ferrite bar comprises one or more gaps to split the ferrite bar into two or more sections to reduce core loss in the ferrite bar.

Optionally each gap is 2mm thick.

Optionally each section is 10mm long.

Optionally each gap is 0.5mm thick.

Optionally each section is 11.5mm long.

Optionally the sensor coil array and/or sensing field is moveable such that the sensing field can be: a) positioned in the sensing region so that the sensing field does not overlap the at least one foreign object, and also b) positioned in the sensing region so that the sensing field does overlap the at least one foreign object.

In another aspect the present invention is a charging unit for wireless power transfer, comprising : at least one coil for inductive power transfer, a ferrite bara sensor coil array disposed on the ferrite bar and having a sensing field for sensing the presence of at least one foreign object, and a controller configured to: move the sensor coil array and/or sensing field so that the sensing field can scan a sensing region, and detect the presence of the at least one foreign object in the sensing region based on the sensing array output, wherein the sensor coil array and/or sensing field is moveable such that the sensing field can be: a) positioned in the sensing region so that the sensing field does not overlap the at least one foreign object, and also b) positioned in the sensing region so that the sensing field does overlap the at least one foreign object.

Optionally the charging unit is for wirelessly charging an electric vehicle through inductive power transfer. Optionally the ferrite bar and sensor coil array form a sensing arm and the controller can control the move the arm to position the sensing field in the sensing region.

Described herein is a charging unit for wireless power transfer, comprising : at least one coil for inductive power transfer, a sensor coil array with a sensing field for sensing the presence of at least one foreign object, and a controller configured to: move the sensor coil array and/or sensing field so that the sensing field can scan a sensing region, and detect the presence of the at least one foreign object in the sensing region based on the sensing array output, wherein the sensor coil array and/or sensing field is moveable such that the sensing field can be: a) positioned in the sensing region so that the sensing field does not overlap the at least one foreign object, and also b) positioned in the sensing region so that the sensing field does overlap the at least one foreign object.

Optionally, the charging unit is for wirelessly charging an electric vehicle through inductive power transfer.

Optionally, the charging unit further comprises an arm, wherein the sensor coil array is disposed on the arm, and the controller can control the move the arm to position the sensing field in the sensing region.

Optionally, the arm as a rotatable arm and the controller can rotate the arm to position the sensing field.

Optionally, the arm is a slidable arm and the controller can slide the arm to position the sensing field.

Optionally, the charging unit has an operational region and the sensing region overlaps the operational region.

Optionally, the sensor coil array and/or sensing field are smaller than the operational region.

Optionally, the charging unit further comprises one or more additional sensor arrays for sensing the presence the at least one foreign object.

Optionally, each additional sensor array is disposed on a respective additional arm.

Optionally, the charging unit comprises electromagnetic shielding material for shielding the sensor coil array from electromagnetic interference. Optionally, the charging unit is for wirelessly charging one or more of the following:

• electrical system

• battery

• scooter

• e-bike

• robot

• other electronic device

Also described herein is a charging unit for wirelessly charging an electric vehicle through inductive power transfer, comprising : at least one coil for inductive power transfer, a sensor coil array with a sensing field for sensing the presence of a foreign object, and a controller configured to: move the sensor coil array and/or sensing field so that the sensing field can scan a sensing region, and detect the presence of a foreign object in the sensing region based on the sensing array output, wherein the sensor coil array and/or sensing field is moveable such that the sensing field can be: a) positioned in the sensing region so that the sensing field does not overlap the foreign object, and also b) positioned in the sensing region so that the sensing field does overlap the foreign object.

Also described herein is a comprise a sensing unit for a charging unit for wireless power transfer comprising : a sensor coil array with a sensing field for sensing the presence of at least one foreign object, and a controller or in communication with a controller configured to: move the sensor coil array and/or sensing field so that the sensing field can scan a sensing region, and detect the presence of the at least one foreign object in the sensing region based on the sensing array output, wherein the sensor coil array and/or sensing field is moveable such that the sensing field can be: a) positioned in the sensing region so that the sensing field does not overlap the at least one foreign object, and also positioned in the sensing region so that the sensing field does overlap the at least one foreign object.

Optionally, the charging unit is for wirelessly charging an electric vehicle through inductive power transfer.

Optionally, the charging unit further comprises one or more sensor arrays for sensing the presence the at least one foreign object.

Optionally, the charging unit is for wirelessly charging one or more of the following:

• electrical system

• battery

• scooter

• e-bike · robot · other electronic device

Also described herein is a wireless power transfer system comprising a charging unit and/or a sensing unit according to any one of the preceding aspects of the present invention.

Also described herein is a power system for wireless power transfer for charging and/or real-time powering of a system or device, comprising: at least one coil for inductive power transfer, a sensor coil array with a sensing field for sensing the presence of at least one foreign object, and a controller configured to: move the sensor coil array and/or sensing field so that the sensing field can scan a sensing region, and detect the presence of the at least one foreign object in the sensing region based on the sensing array output, wherein the sensor coil array and/or sensing field is moveable such that the sensing field can be: a) positioned in the sensing region so that the sensing field does not overlap the at least one foreign object, and also b) positioned in the sensing region so that the sensing field does overlap the at least one foreign object.

The term "comprising" as used in this specification and claims means "consisting at least in part of". When interpreting each statement in this specification and claims that includes the term "comprising", features other than that or those prefaced by the term may also be present. Related terms such as "comprise" and "comprises" are to be interpreted in the same manner.

It is intended that reference to a range of numbers disclosed herein (for example, 1 to 10) also incorporates reference to all rational numbers within that range (for example, 1, 1.1, 2, 3, 3.9, 4, 5, 6, 6.5, 7, 8, 9 and 10) and also any range of rational numbers within that range (for example, 2 to 8, 1.5 to 5.5 and 3.1 to 4.7).

This invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more of said parts, elements or features, and where specific integers are mentioned herein which have known equivalents in the art to which this invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

BRIEF DESCRIPTION OF THE FIGURES

Preferred embodiments of the invention will be described by way of example only and with reference to the drawings, in which :

Figure 1 is an overview of a prior art wireless power transfer charging system. Figure 2 is a general overview of a wireless power transfer charging system according to the described embodiments.

Figure 3 is a block diagram overview of the wireless charging unit according to the described embodiments.

Figure 4 is a block diagram overview of the sensing unit according to the described embodiments.

Figure 5 is a plan view of one embodiment of the sensing unit.

Figure 6 is a line diagram showing an example of how one embodiment of the sensing unit senses and detects a foreign object.

Figure 7 is a plan view of another embodiment of the sensing unit.

Figure 8 is a line diagram showing an example of how another embodiment of the sensing unit senses and detects a foreign object.

Figure 9 is a plan view of another embodiment of the sensing unit.

Figure 10 is a plan view of another embodiment of the sensing unit.

Figure 11 is a plan view of a prior art foreign object detection system.

Figure 12 is a line diagram demonstrating why it can be difficult to detect a foreign object using a prior art foreign object detection system.

Figures 13A to 13D show a charging pad with a ferrite bar sensor arm according to one embodiment.

Figure 14 shows a flux density simulation of a charging pad without a ferrite bar.

Figures 15 and 16 show a flux density simulation of a charging pad with a foreign object, one a ferrite bar and one without a ferrite bar respectively.

Figure 17 shows charging pads with and without ferrite bars used for simulation.

Figure 18 shows a cross section of the charging pad used for simulation with a ferrite bar.

Figures 19A to 24B show the results of simulations.

Figures 25A,-D show comparisons of the simulations for charging pads with and without ferrite bars.

Figures 26A to 26B show relative magnetic flux diagrams of ferrite bars with gaps and without gaps.

Figures 27 and 28 show a ferrite bar with a U shaped cross section.

DETAILED DESCRIPTION

Overview of embodiments described

Figure 2 shows a wireless power transfer charging system 10 for charging an electric vehicle 11 configured to enable detection of foreign objects (foreign object detection "FOD") in an operational area of wireless charging unit 13. The wireless power transfer charging system 10 comprises a central charging unit 12 for controlling the supply of power to the electric vehicle 11, and also comprises a wireless charging unit 13 that inductively transfers power 15 to the electric vehicle 11. The electric charging unit 13 is electrically coupled 16 to the central charging unit 12. The wireless charging unit 13 can take the form of a wireless charging pad comprising one or more inductive coils 14 for transferring power 15 across to the electric vehicle 17, 18.

In operation, the central charging unit 12 induces an alternating current into the inductive coils 14, which generates a changing electromagnetic field/flux in and around the inductive coils 14. The electric vehicle 11 has a receiver 17 that receives power inductively transferred 15 from the wireless charging unit 13. The receiver 17 is electrically coupled to a rechargeable battery 18. If the receiver 17 of the electric vehicle 11 is in the vicinity of the wireless charging pad 13, the changing electromagnetic field/ flux induces an alternating current in the inductive coils of the receiver 17, thereby creating a wireless transfer of power 15 to the electric vehicle 13. A foreign object sensing unit 20 is provided to detect any foreign objects in an operational area of the charging unit 13.

In general terms, the embodiments described provide a sensing unit 20, wireless power transfer charging unit 13 with a sensing unit 20 and/or a wireless power transfer charging system 10 with a charging unit 13, whereby the sensing unit 20 can sense and detect foreign objects (on or near the charging unit) that might present a hazard as previously set out.

Embodiments disclosed herein are described in terms of sensing and detecting a singular foreign object as opposed to two or more foreign objects. This is for the purpose of providing a simplified explanation as to how foreign objects may be sensed and detected by the embodiments described herein. Those skilled in the art would understand that the described embodiments are capable of sensing and detecting a single foreign object, and are also capable of sensing and detecting two or more foreign objects. This is because the techniques used to sense and detect a single foreign object can also be used to detect two or more foreign objects.

Embodiments of the wireless power transfer charging system 10, sensing unit 20, and wireless power charging unit 13 described herein are all described in the context of wirelessly charging an electric vehicle 11. However, a skilled person would appreciate that the described embodiments may also be suitable for systems that power an electrical system in real time or suitable for charging a battery. Further, the described embodiments may be suitable for charging scooters, e-bikes, robots, and other similar electronic devices. Various embodiments of a wireless charging unit 13 that can sense foreign objects are described. In overview, referring to the diagrammatic Figure 3, the embodiments provide a sensing coil array 34 that can scan a sensing region 31 and a controller 32 that can detect a foreign object 33 in the sensing region 31. Upon detection, suitable action can be taken to prevent the hazard, such as shutting off the wireless charging unit 13. Typically, the sensing coil array will be in the housing of the charging unit 13.

The controller 32 can be used to detect a foreign object 33. In addition, the controller 32 can be used to operate any actuators for moving sensing array 34, and the controller 32 can be used to power the sensing coil array 34 (including the sensing coils themselves). The controller 32 can refer to one or more controllers that can be placed anywhere in wireless charging unit 13.

Referring to Figure 4, various general features of the wireless charging unit 13 and sensing unit 20 are shown in general diagrammatic form. The various geometric regions are shown for illustrative purposes only, and may not correspond to actual geometric regions of a charging unit according to the embodiments described.

The charging unit 13 takes the form of a charging pad 40 with a housing covering a region. In this case the pad 40 and housing is round, but they could take other shapes, such as square, rectangular or other shape. The charging pad 40 comprises wireless power transfer charging coils ("power coils") 14 - in this case one, although the actual number will depend on the charging unit 13. The coil 14 has an operational region 41, being the region over which the electromagnetic field of the power coils 14 can operate to provide power transfer 15 to an electric vehicle 11. Foreign objects 42 falling within the operational region 41 are at risk of creating an overheating hazard, as they receive the power intended for the vehicle 11. Therefore, the embodiments herein are intended to detect such foreign objects 42 within the operational region 41. The geometry of the operational region 41 may or may not coincide exactly with the geometry of the charging pad region 40. The operational area 41 may coincide with the charging pad area 40, partially coincide, be bigger or be smaller, depending on the range of the wireless power transfer 15. Typically, the operation region 41 and charging pad region 40 would be closely aligned. Reference to the term "overlap" in relation to the operational region 41 and the charging pad 40 will mean that the operational region 41 at least partially coincides (irrespective of whether it is bigger or smaller) with the charging pad 40, and may fully coincide (that is, the same size and substantially fully aligned) with the charging pad 40, or it may even be bigger than the charging pad 40. Reference to the term "covers" means that the region at least aligns with or is even bigger than the other region. The charging unit 13 has sensing unit (generally depicted as 43). The sensing unit 43 might be integrated with the charging unit 13, or a separate apparatus, and may be retrofittable or incorporated at the time of manufacture. The sensing unit 43 comprises a sensor arm 44, and a sensor coil array 45 disposed on the sensor arm 44. The sensor coil array 45 is shown as a single array of a plurality of sensing coils e.g. 45a, but this is exemplary only and other geometries and numbers of sensing coils are possible. The sensor coil array 45 has an electromagnetic sensing field ("sensing field") 46, which is the geometric region over which the sensor coil array 45 can operate/sense foreign objects 42 to a suitable level of measurement.

The sensor coil array 45 (and thus the sensing field 46) is smaller than the operational region 41, for advantageous reasons that will be explained later. As a result, the sensing field 46 does not cover/overlap and cannot sense foreign objects 42 over the entire operational region (when stationary). Therefore, the sensing unit 46 has a controller 32 and an actuator 47 that under control of a controller 32 can operate the sensor coil array 45 so that the sensing field 46 can scan a larger area. This might be by way of moving the sensor arm 44 and/or sensor coil array 45 so that the sensing field 46 scans a larger area. The area that the sensing field 46 scans is the sensing region 48. The sensing region 48 is the region over which the foreign object sensing unit 20 can sense and detect a foreign object 42. Preferably, the sensing region 48 is at least the same size of, and fully covers, the operational region 41 of the charging unit 13. The sensing region area 48 might be bigger than the operational region area 41. However, it might not be absolutely essential that the sensing region 48 is the same size or bigger than the operational region 41, and a sensing region 48 that covers only part of the operational region 41 might still be useful. Some benefits can still be gained even if the sensing area 48 doesn't coincide completely with the operational area 41, for example it overlaps but does not completely cover it. However, any operational region 41 not scanned is at risk of harbouring foreign objects 42 that won't be detected and may pose a heating hazard. Reference to the term "overlap" in relation to the operational region 41 and the sensing region 48 will mean that the sensing region 48 at least partially coincides (irrespective of whether it is bigger or smaller) with the operational region 41, and may fully coincide (that is, the same size and substantially fully aligned) with the operational region 41, or it may even be bigger than the operational region 41. Reference to the term "covers" means that the region at least aligns with or is even bigger than the other region. As shown, in Figure 4, the sensing region 48 is slightly bigger than the operation region 41. The sensing field 46 may not coincide with the geometry of the sensor arm 44. Typically, it might be bigger than the sensor arm 44, although that is not essential and it might have other relationships to the sensor arm 44 such as coinciding with the sensor arm 44, partially or fully overlapping the sensor arm 44 and/or being smaller than the sensor arm 44.

As shown in Figure 4, the arm 44 can be moved by an actuator 47 so that the sensing coil array 45 rotates to scan (sweep out) the sensing region 48. In this case, the arm 44 is rotated to sweep out a circular sensing region 48. It can be seen that the actual area that the arm 44 sweeps (arm movement region 49) is less than the sensing region 48, because the sensing field 46 is slightly longer than the arm 44 so it sweeps a larger radius. Having a rotating sensor arm 44 is only one option for moving the sensing field 46, and other options are possible creating a different shape sensing region 48 - some of which will be described below.

The sensing field 46 and sensing region 48 geometry are arranged in relation to the operational region 41 so that there is at least one position of the sensing field 46 such that the sensing field 46 does not overlap (that means it does not partially or fully coincides with) a foreign object 42 in the operational area 41, and at least one position of the sensing field such that it does overlap (that means at least partially or fully coincides with) a foreign object 42 in the operational area 41. In practice, there will usually be multiple positions of both, but as long as there is at least one position for each, the sensing field 46, sensing region 48 could and movement could take various different geometries and motions.

Shielding is helpful for attenuating the electromagnetic field emitted from the power coils 14, such that electromagnetic interference with the signals induced in the sensing coils 45a of the sensing array 45 is mitigated. The arm of this embodiment can incorporate more shielding material and/or can be made partially or fully from shielding material, because the sensing bar 44 occupies a smaller area relative to the sensing region 48.

It should be appreciated that the boundaries of the regions shown in Figure 4 that relate to electromagnetic fields may not be the actual shape of those regions. The shapes have been shown for exemplary purposes only. It should also be appreciated that the boundaries of the regions shown will not actually be "hard" boundaries - that is, the electromagnetic fields depicted will not necessarily become zero immediately outside the boundary. Rather, electromagnetic fields attenuate as a function of the distance from the source of the field. The boundaries of the regions depicted are indicative only, and show where it may be deemed that the effective field has ended. Those skilled in the art will appreciate this, and the boundaries should not be considered limiting in such a manner that they limit the scope of the invention in a manner that goes against the concepts described herein. The embodiments described herein have advantages over prior art sensing arrays as follows.

• The sensor coil array 45 does not need to be the same size as the operational area 41 to effectively detect foreign objects 42 in the operational region. Rather a smaller sensor coil array 45 can be used. A small footprint of sensor coil array 45 can be used. This saves on cost of coils, and the attendant control and sensing electronics.

• The coils of a prior art sensing array preferably are shielded so that the field of the wireless power transfer coils 14 do not interfere with the sensing coils 45. But, if the prior art coil array is of a substantial size, then there is a large area of shielding, which will prevent or attenuate power being transferred to the vehicle 11. In contrast, the smaller sensor coil array 45 of the embodiments described herein require a smaller area of shielding, so they do not shield all or a substantial portion of the power coils 14, and so do not attenuate power transfer to as large a degree. As the sensor coil array 45 requires a smaller area of shielding, a greater level (e.g. thickness) of shielding around the sensor coil array 45 can be provided without impeding on the operation of the wireless power transfer coils 14. The extra shielding reduces electromagnetic interference from the wireless power transfer coils 14, which improves the quality of the signal sensed by the sensor coil array 45. This improves the accuracy and/or precision of the sensor coil array 15 measurements. Further, the extra shielding thickness allows the controller 32 to be placed closer to the sensor coil array 45. Placing the controller 32 physically closer to the sensor coil array 45 also improves the accuracy and/or precision of the sensor coil array 45 measurements.

• When detecting foreign objects 42 using a sensor coil array, having a reference helps distinguish between whether a foreign object 42 is present or not. With a prior art sensor coil array 15 that covers a substantial portion or all of the operational region, as shown in Figure 11 for example, it is likely that a foreign object 42 will always be sensed to some degree. It is therefore hard to determine the difference from when an object 42 is there, to when it isn't, see Figure 12 for example. In contrast, the present embodiments have a situation where the size of the sensing field 46 is such that only it sometimes senses the object 42, and other times does not, so the two readings can distinguish between an object 42 being there or not.

Particular exemplary embodiments will now be described. These are not limiting, and are by way of example only. Various other embodiments could be envisaged by those skilled art that still meet the aspects of the invention. Specific embodiments according to the present invention will now be discussed.

1. First embodiment

A first embodiment will now be discussed with reference to Figures 5 and 6.

The charging unit 13 forming part of the system of Figure 2 could be as shown in Figures 5 and 6. The various features of this embodiment will be renumbered with respect to previous figures, but they still relate to the same aspects where context dictates. The charging unit 13 of this embodiment comprises a circular pad 50 comprising a housing and at least one wireless power transfer coil 57. The charging unit 13 has a rotatable sensor arm ("rotatable arm") 54 disposed above the pad 50 on a rotatable actuator, such as an electric motor. Preferably, the rotatable arm is in the charging unit housing, although this is not essential. The rotatable actuator is controlled to rotate by way of a controller (e.g. 32 in Figure 3), disposed in the charging unit 13 or disposed elsewhere in the system. An array of sensor coils 55 is disposed on the rotatable arm 54. The arm is sized appropriately to carry the sensor coil array 55. The sensor coils 55 can be any suitable used in the art. The rotatable arm 54 and/or sensor coils 55 can be shielded to reduce interference from the charging coil 14.

The following regions are also shown:

• A charging pad housing 50 that is circular.

• A circular operational region 51, being the region over which the wireless power transfer coil 14 can transfer power. This typically would be of similar region to the charging pad 50, that is that they overlap.

• A rotatable arm 54 with sensor coils 55 that covers an elongated rectangular region.

• A sensing field 53, which is the region that the sensor coils 55 can sense an object 52 when the rotatable arm 54/sensor coil array 55 is in a particular position.

• A circular sensing region 58, being the region over which the sensing field 53 scans or sweeps through rotatable movement of the arm 54.

The area covered by the sensor coil array 55 and the rotatable arm 54 and sensing field 53 is a small proportion of the overall operational area 51, which is advantageous for the reasons described above. In particular, the sensing field 53 and sensing region geometry 58 are arranged in relation to the operational region 51 so that there is at least one position of the sensing field 53 such that it does not overlap the foreign object 52 in the operational area (see e.g. position A, position C and position D), and at least one position of the sensing field 53 such that it does overlap the foreign object 52 in the operational area (see position B). In practice, there will be many such positions of each. There is vertical clearance between the rotatable arm 54 and the charging unit pad 50 so that as the arm 54 rotates it will pass above any foreign object 52 on the pad 50 or otherwise in the operational area 51.

In use, the rotatable arm 54 is rotated by the actuator under control of the controller 32 so that the sensor coil array 55 scans/sweeps out the circular arm movement region 59 and the sensing field 53 scans/sweeps out a sensing region 58, allowing the sensor coil array 55 to sense any foreign object 52 located in the sensing region 58/operational region 51. Four positions of the rotatable arm 54 are shown (positions A, B, C and D), although it will be appreciated that the arm 54 will rotate through a continuous range of different positions. As can be seen, as the arm 54 rotates, for most of the positions, the sensing field 53 does not cover the foreign object 52, but for a small part of the scan (for a smaller range of positions) the arm 54 covers (and therefore the sensing field 53 covers) the foreign object 52.

The sensor coil array 55 will not sense the object 52 when the rotating arm 54/sensor coil array 55 is positioned so the sensing field 53 does not overlap (such as not cover or is too far away to sense) any of the foreign object 52. See for example positions A and C. The controller 32 receives output from the sensor coil array that is essentially a null signal 60 (see Figure 6) or a signal indicative of no object. Once the sensor coil array 55 is in one of the positions such that the sensing field 53 overlaps (such as covers or is near enough to sense) a foreign object 52 (e.g. position B), the controller 32 receives output from the sensor coil array 55 that is a positive signal 61a, b (see Figure 6) from which the controller detects the foreign object 52. The controller 32 can use the relative outputs from when the sensing field 53 does not sense or overlap the object 52 to when it does overlap/sense the object 52 to assist in distinguishing the two events.

For example, at position A, the sensing bar 54 is oriented in a "north-south" direction, and the foreign objects 52 are not overlapped by (and is too far away from) the sensing field 53. The foreign object 52 is too far from the sensing field 53 to induce an electrical signal into any of the sensing coils 55 located along the arm 54. As no electrical signal is induced in any of the sensing coils 55, the controller 32 receives a ("low") zero reading 60 from the sensor coil array 55. The rotatable arm 54 begins to rotate clockwise towards position B. At position B, the arm 54 is oriented in a "north east - south west" direction. At this position, the sensor coil array 55 overlaps the foreign object 52. Since an electrical signal is induced in at least one of the sensing coils 55, the controller 32 receives a ("high") non-zero reading 61. The arm 54 continues to rotate clockwise towards position C. At position C, the sensor coil array 55 is oriented in a "west - east" direction. At this position, the foreign object 52 is too far from the sensor coil array 55 to induce an electrical signal into any of the sensing coils 55 located along the rotatable arm 54. As no electrical signal is induced in any of the sensing coils 55, the controller 32 receives a ("low") zero reading 60 from the sensor coil array 55. The arm 54 continues to rotate clockwise towards position D. At position D, the sensing bar 54 is oriented in a "north west - south east" direction. At this position, the foreign object 52 is too far from the sensor coil array 55/sensing field 53 to induce an electrical signal into any of the sensing coils 55 located along the rotatable bar 54. As no electrical signal is induced in any of the sensing coils 55, the controller receives a ("low") zero reading 60 from the sensor coil array. The arm 54 continues to rotate clockwise towards position A, and can continue to rotate clockwise.

By rotating the sensing bar 54 from position A to position D and position A again, the controller 32 receives a ("low") zero reading 60 and a ("high") non-zero reading 61. The controller is able reference the a ("high") non-zero reading 61 against the ("low") zero 60 reading to correctly determine that there is a foreign object 52 within the sensing region 58. The controller 32 then powers down the inductive coil 57 of the charging pad to prevent induction heating of the foreign object 52. The foreign object 52 is then no longer an induction heating hazard.

Once the detection takes place the controller 32 can communicate with the central charging unit 12 to take appropriate action, such as shutting down power transfer.

Those skilled in the art would understand that as alternative to continuously moving the bar 54 in a continuous clockwise direction, the bar 54 can rotate in other ways. For example, the rotatable bar 54 can move with an alternating "stop" and "start" movement. In another example, the rotatable bar 54 can rotate in an anti-clockwise direction. In another example, the rotatable bar 54 can alternate between moving in a clockwise direction and in an anti-clockwise direction.

The disclosed embodiment suggests sensing and detecting different voltage signals (i.e. a non-zero reading when a foreign object is present, and a zero reading when a foreign object is absent) to determine whether a foreign object 52 is present in the sensing region 53. Those skilled in the art would understand that the controller 32 can be configured to instead measure a different parameter to ascertain whether a foreign object 52 is present in the sensing region 53. That is, instead of using relative voltage levels, the controller can instead measure frequency, phase shift, or some other parameter, for example.

2. Second embodiment

A second embodiment will now be discussed with reference to Figures 7 and 8. The charging unit 13 forming part of the system of Figure 2 could be as shown in Figures 7 and 8. The various features of this embodiment will be renumbered with respect to previous figures, but they still relate to the same aspects where context dictates. The charging unit 13 of this embodiment comprises a rectangular pad 70 comprising a housing and at least one power coil 77 (in this case three coils). The charging unit 13 has a slidable sensor arm ("slidable arm") 74 disposed above the pad 70 on a sliding actuator, such as a linear motor. Preferably, the slidable arm is in the charging unit housing, although this is not essential. The actuator is controlled to slide the arm 74 by way of a controller 32, disposed in the charging unit 13 or disposed elsewhere in the system. An array of sensor coils 75 is disposed on the slidable arm 74. The arm 74 is rectangular and spans a width slightly less than the width of the sensing region 78. The sensing coils 75 are coupled to control electronics. The arm 74 is sized appropriately to carry the sensor coil array 75. The sensor coils 75 can be any suitable used in the art. The slidable arm 74 and/or sensor coils 75 can be shielded to reduce interference from the charging coil 77.

The following regions are also shown:

• A charging pad 70 housing that is rectangular.

• An operational region 71, being the region over which the wireless power transfer coil can transfer power. This typically would be of similar region to the charging pad 70. This may actually be quasi-oblong, or some other shape that is that reflects the total electromagnetic field of the power coils 77. Irrespective, the operational area 71 will most likely be of a similar magnitude of size and cover or extend beyond the rectangular pad area 70. That is, preferably the sensing region 78 coincides with the operational region 71. However, some useful gains could be obtained even if there is not full coincidence.

• A slidable arm 74 with sensor coils 75 that covers an elongated rectangular region.

• A sensor field 73, which is the region that the sensor coils 77 can sense an object 72 when the rotatable arm 74/sensor coil array 75 as in a particular position.

• An approximately rectangular sensing region 78, being the region over which the sensing field 73 scans or sweeps through sliding movement of the arm 74.

The area covered by the sensor coil array 75 and the slidable arm 74 and the sensing field 73 is a small proportion of the overall operational area 71, which is advantageous for the reasons described above. In particular, the sensing field 73 and sensing region geometry 78 are arranged in relation to the operational region 71 so that there is at least one position of the sensing field such 73 that it does not overlap the foreign object 72 in the operational area 71 (see positions A and C), and at least one position of the sensing field 73 such that it does overlap the foreign object 72 in the operational area 71 (see position B). In practice, there will be many such positions of each. There is clearance between the slidable arm 74 and the charging unit pad 70 so that as the arm 74 moves it will pass above any foreign object 72 on the pad 70 or otherwise in the operational area 71.

In use, arm 74 is manipulated by an actuator that can move the bar across the sensing region, e.g. position A through position B to position C. The actuator can be any suitable mechanism such an electromagnetic rail, for example. The actuator is controlled by a controller 32 in the charging unit, in the controller unit 32, or elsewhere in the system. The slidable bar 74 is moved laterally by the actuator under control of the controller 32 so that the sensor coil array 75 scans/sweeps across a rectangular movement region 79 and the sensing field 73 scans/sweeps out a quasi-rectangular sensing region 78, allowing the sensor coil array 55 to sense any foreign objects 72 located in the sensing region 78/operational region 71. Three positions of the slidable arm 74 are shown A, B, C, although it will be appreciated that the arm 74 will move through a continuous range of different positions. As can be seen, as the arm 74 moves, for most of the positions, the sensing field 73 does not cover the foreign object 72, but for a small part of the scan (for a smaller range of positions) the arm 74 covers (and therefore the sensing field 73 covers) the foreign object 72.

The arm 74 moves transversely across from one end of the sensing region 78 to the opposite end, allowing the area and sensing field 73 scan a sensing region 78 to sense for any foreign objects 72 located in the operational region 71. The sensor coil array 75 will not sense the object 72 when the arm 74/sensor coil array 75 is positioned so the sensor field 73 does not overlap (such as not cover or is too far away to sense) the foreign object 72. See for example positions A and C. The controller 32 receives output from the sensor coil array 75 that is essentially a null signal 80 (see Figure 8) or a signal indicative of no object. Once the sensor coil array 75 is in one of the positions such that the sensing field 73 overlaps (such as covers or is near enough to sense) the foreign object 72(e.g. position B), the controller 32 receives output from the sensor coil array 75 that is a positive signal 81 from which the controller 32 detects the foreign object 72. The controller 32 can use the relative outputs from when the sensing field 73 does not sense or overlap the object 72 to when it does overlap/sense the object 72 to assist in distinguishing the two events.

For example, referring to Figures 7 and 8, at position A, the arm 74 is at the edge of the sensing region 78, and the foreign object 72 is too far from the arm 74 to induce an electrical signal into any of the sensing coils 75 located along the arm 74. As no electrical signal is induced in any of the sensing coils 75, the controller 32 receives a ("low") zero reading 80 from the arm. The arm 74 begins to move further to the right towards position B. At position B, the sensing bar 74 is positioned in the middle portion of the sensing region 78. At this position, the sensing bar 74 overlaps with the foreign object 72, and the foreign object 72 is close enough to induce an electrical signal into at least one sensing coil 75 located along the arm 74. Since an electrical signal is induced in at least one of the sensing coils 75, the controller 32 receives a ("high") 81 non-zero reading. The arm 74 continues to move further to the right towards position C. At position C, the arm 74 is at the edge of the sensing region 78, and similar to position A, the foreign object 72 is too far from the arm 74/sensing field 73 and the controller 32 therefore receives a ("low") 80 zero reading from the arm 74. From position C, the travelling direction of the sensing bar 74 can reverse, so that the sensing bar 74 starts to move back towards position A.

By moving the sensing bar 74 from position A to position C, the controller 32 receives a ("low") 80 zero reading and a ("high") 81 non-zero reading. The controller 32 is able reference the a ("high") non-zero reading 81 against the ("low") zero reading 80 to correctly determine that there is a foreign object 72 within the sensing region 78. The controller 32 then powers down the inductive coils 77 of the charging pad 70 to prevent induction heating of the foreign object 72. The foreign object 72 is then no longer an induction heating hazard.

Once the detection takes place the controller 32 can communicate with the system controller to take appropriate action, such as shutting down power transfer. Those skilled in the art would understand that as alternative to continuously moving the bar 74 in a continuous motion, the bar 74 can rotate in other ways. For example, the slidable bar 74 can move with an alternating "stop" and "start" movement.

The disclosed embodiment suggests sensing and detecting different voltage signals (i.e. a non-zero reading when a foreign object is present, and a zero reading when a foreign object is absent) to determine whether a foreign object 72 is present in the sensing region 73. Those skilled in the art would understand that the controller 32 can be configured to instead measure a different parameter to ascertain whether a foreign object 72 is present in the sensing region 73. That is, instead of using relative voltage levels, the controller can instead measure frequency, phase shift, or some other parameter, for example.

3. Variations

It will be appreciated that for the embodiments described above are there could be a number of variations. For example, there does not need to be a single array of coils for the sensor coil array - there could be two, three or even more array of coils, each array comprising one or more columns of coils. Furthermore, each array of coils may each be disposed on its own arm - that is, some embodiments may have multiple sensing arms, with each sensing arm having its own array of coils. Each sensing arm could move independently, or together. Furthermore, the number of coils can be any number suitable. The consideration for the number of coils will of course be the sensitivity, but also the compromise between that and optimising the smallest size of the sensor coil array (and therefore the arm) so that the cost can be kept to a desirable level, the shielding can be kept to a desirable level and also the array can always be put in a position where it does not detect a foreign object and another position where it does detect foreign object to enable the referencing between the two readings to improve detection.

Referring to Figure 9, in slidable arm 94 arrangements where the operational area 91 and sensing region 98 are rectangular, typically the largest size of the sensor coil array 95 will generate a sensing field 93 with an area slightly less than half the area of the sensing region 98, so that at least a non-sensing and a sensing position of a foreign object can exist. In practice, it would be desirable to have a sensor coil array 95 that generates a sensing field 93 much smaller than this. For example, the sensing field 93 area could be less than 50% of the area of the sensing region 98, or less than 40% of the area, or less than 30% of the area, or less than 25% of the area, or less than 10% of the area of the sensing region 98.

Referring to Figure 10, in a rotating arm arrangement 104, where the operational area 101 and the sensing region are circular, the sensor coil array 104 could in theory be a significant proportion of a full circle, to produce a sensing field 103 that is a significant proportion of a full circle with a small area that the sensing field 103 does not occur. This means that when the rotation of the sensor coil array 105 occurs, there is still a small area of the sensing region 108 where no sensing occurs. However, in practice it would be more desirable to have a sensor coil array 105 that generates a sensing field 103 much smaller than this, more akin to what was described in the embodiment above.

The embodiments above relate to a charging unit. Wireless power transfer can also be used for real-time powering of systems and/or devices without or in addition to charging. The embodiments above could be used with such systems also.

4. Shielding

As discussed earlier, shielding is helpful for attenuating the electromagnetic flux emitted from the inductive coils 14, 57, 77, such that electromagnetic interference with the signals induced in the sensing coils of the sensing array 45, 55, 75 is mitigated. In comparison to the sensing arrays, the sensing bars 44, 54, 74 according to the disclosed embodiments can incorporate more shielding material, because the sensing bars 44, 54, 74 occupy a smaller area relative to the scanning region 48, 58, 78.

4.1 Implementation of shielding

A possible implementation of the first embodiment will now be described that implements shielding ("shielded embodiment"). This implementation could be applied to the other embodiments too, but for exemplary purposes will be described with reference to the first embodiment. The embodiment will be compared to embodiment without shielding to demonstrate the improvement. It should be noted that the embodiment without shielding is still useful, but by providing shielding the performances improved.

Referring to Figure 4, the embodiment described with respect to that Figure is used, wherein the ferrite shielding is provided. In one example this is done by making the sensor arm as a bar 44 (it could also interchangeably be called a "beam" or "rod") made from ferrite. Other magnetic shielding materials, such as ferromagnetic material, ferrimagnetic material, and nanocrystalline material, or a combination of these three materials may be used instead of ferrite, or in combination with ferrite. Figures 13A to 13D show a construction of the charging pad 40 arrangement in perspective and side elevation (Figure 13A), a zoomed in partial elevation cross-section view (Figure 13B), an end on elevation (Figure 13C) and a plan view (Figure 13D). The charging pad has a primary coil 14 is provided on a primary magnet plate 130, and a ferrite rod is provided as sensor arm 44. the ferrite bar is a ferrite shield. A primary metallic base 131 is also provided as part of the charging pad 40. It may solely form the sense round, or form part of the sensor arm. Sensor coils 45a are arranged as a sensor coil array 45 along the sensor arm 44/ferrite bar. All other aspects are as described with respect to e.g. Figure 4, or any of the other embodiments this implementation might be applied to.

Figure 13D shows various dimensions of a charging pad 40. Possible dimension values are as follows. These are exemplary only and should not be considered limiting.

In one embodiment the ferrite bar 44 can be an elongated bar, with a rectangular cross- section as shown in the end elevation of Figure 13C. In this example, the dimension of the ferrite bar 44 is 25mm(w) x 590mm(L) x lmm(H). Sensor coils 45a are 10 turns windings with the dimension of 10mm(W) x 10mm(L) x lmm(H). In this version, there are 49 sensor coils 45a in total with 2mm gap between each other. A diameter of 30mm metallic metal (copper) is used for a foreign object. It will be appreciated that this is just one example, and other dimensions and numbers and positioning of sensor coils 45a are possible. In general terms, and referring to Figure 13D, the length C of ferrite bar 44 is normally chosen the same size as the width B of the primary pad to cover the whole primary width range. In practice, C can be slightly smaller than B for assembling requirements. For example, the width of the primary pad can be 600mm, and the length of the magnetic beam 590mm.

The width F of the ferrite bar is in proportional to the diameter I of the sensor coils 45a to achieve good shielding performance. The range is normally between 1 times to 3 times to the sensor coil 45a. For example, the sensor coil 45a diameter I is 10mm, and the width range of the ferrite bar between 10mm to 30mm. These are exemplary dimension only and should not be considered limiting.

In another example, referring to Figure 27, 28 the ferrite bar 44 can be square U shape in cross-section. The U shape bar can provide stronger shielding effect than the square/rectangular cross section shape. The bar has side rails 44a, 44b along the length C of the bar 44. In this case, the dimensions of the bar can be as follows.

In more general terms, the sensor coil 45a dimension I and sensor coil 45a gaps L are determined by anticipated foreign object size M. In general, the sensor coil 45a length I is preferred no more than one third of the dimension M of the anticipated detected foreign object 33. For example, if the anticipated detected object 33 diameter is up to 30mm, then the preferred sensor coil 45a length I is less than 10mm to ensure detection.

The sensor coil 45a gap L is preferred in the range between 1/10 to 1/5 of the sensor coil 45a diameter. For example, sensor coil 45a gaps L are preferred between 1mm to 2mm when the sensor coil 45a diameter is 10mm. If the anticipated foreign object diameter is up to 20mm. The preferred diameter of sensor coils 45a is chosen between 6mm to 7mm. The sensor coil 45a gap L is changed to 0.6mm to 1.4mm. Generally, the more winding turns for a sensing coil 45a, the higher voltage can be induced on the sensing coil. The winding turns can be decided by a substantial voltage (for example a voltage margin of at least 50mV, but preferably greater than 100mV, and even more preferable greater than 150mV) can be induced when foreign objects are not presented.

The above is possible dimension design guidance but the dimensions and design guidance is not restricted to that. The cross section shape of the ferrite bar can be square, rectangular, U-shaped or any other shape.

The positioning, dimensions and shape of the ferrite bar 44 improve shielding of the coils from the electromagnetic field of the transmission coil, thus improving detection of foreign objects 33.

The improvements provide will be described with reference to Figures 13A to 25B. The description relates to the rectangular cross section beam of Figure 13C unless said otherwise. First, referring to Figure 14 in the flux density caused by the charging pad 40 ("transmitter) electromagnetic field is shown for the embodiment without shielding ("unshielded embodiment"). The sensor coils 45a in the sensing array 45 of this embodiment coils are weakly coupled to the charging pad 40 inductive charging coils 14 ("transmitter coils")and receiver 17 inductive charging coils ("receiver coils"). Therefore, in this unshielded embodiment, voltages are induced across the sensor coils 45a when currents flowing through the transmitter and receiver coils. The magnitudes of these induced voltages are determined by the coupling (flux density B) between sensor coils 45a and the transmitter/ receiver coils, as shown in 12, which shows an elevation view and a zoomed-in partial cross-section view of the transmitter 13 , without a foreign object present 33. The magnitude of the induced voltage on a sensing coil when there is no foreign object should be substantially different (by more than 100mV for example) than the induced voltage when a foreign object is present.

Referring to Figure 15, when a foreign metallic object 33 is located on top of the transmitter 13, both the distributions and magnitudes of flux density B around the sensor coils 45a are affected. Therefore, induced voltages across sensor coils 45a are changed, as shown in Figure 15. It can be seen that the flux is shielded by the foreign object (in this case, a coin), and the flux density around L_FOD0 coil, L_FOD1 coil and L_FOD25 coil is changed.

By detecting changes of induced voltages across sensor coils 45a, foreign metallic objects can be discovered once they are located above the detection coils, as previously discussed.

The dimensions of sensor coils 45a in the embodiments described, are designed to be up to one third of the dimension of the anticipated detected foreign object 33. In some embodiments the dimensions of the sensor coils 45a can be designed to be up to one half of the dimension of the anticipated foreign object 33. This ensures that at any one time, at least one sensor coil 45a is 100% covered by the foreign object, and at least one sensor coil 45a is up to 80% covered by the foreign object, regardless of the position of foreign object. For example, if the intention is to detect objects up to a size of a circle of 30mm diameter, the dimension of sensor coils 45a are designed to be up to 10mm x 10mm.

In the shielded embodiment, where the sensor arm 44 is a ferrite bar, the ferrite bar helps to concentrate magnetic flux going through the bar. In other words, the ferrite bar regulates a large amount of flux between the sensor coils 45a and the transmitter coils 14 of the charging pad 40. It at least partially shields the coils from electromagnetic fields from the inductive power transfer coil. When the foreign object (in this case a coin) is in place, both the foreign object and the ferrite bar shield the electromagnetic field from the charging pad 40 to significantly reduce the flux between sensor coils 45a and the transmitter/ receiver coils in comparison to the flux between the sensor coils 45a and the transmitter/ receiver coils when there is no ferrite bar (see Figure 16). The reduced flux results in much lower induced voltages (close to 0) in the sensor coils 45a of the shielded embodiment. This makes the sensitivity of sensor coils 45a higher when detecting foreign objections. This is because voltage change detection easier because: the voltage difference detected by sensor coils 45a between when a foreign object is present (compared to when it is not present) for the shielded embodiment is larger in comparison to the voltage difference detected by sensor coils 45a between when a foreign object is present (compared to when it is not present) for the unshielded embodiment.

More specifically, with the ferrite bar 44:

1) the voltage changing ratio increases, and

2) the induced voltage drops to almost to 0V when the metallic object is located right above a sensing coil.

So comparing some voltages with 0v is much easier than comparing some voltages with another smaller voltages. As an example, the induced voltage is amplified, e.g. by a 3.3V op-amp, with a certain amplifier ratio (e.g. 10 times). For the induced voltage on a sensing coil (as long as bigger than 330mV) when no metallic object around it, the output of the op-amp will be 3.3V because of 10 times amplification or being saturated, but because the induced voltage on the sensing coil drops to close to 0 (<20mV) when a metallic object is right above the sensing coil, the output of the operational amplifier will be below 200mV which is still very low, so the difference is crossing a whole output range of a op-amp which increases the detection sensitivity and reliability.

This can be demonstrated with reference to the following simulation results.

In general terms, the shielding effect by the ferrite bar under the detection coils minimises the interference to improve the sensitivity, accuracy and reliability of the electronic detection circuits and much fewer number of coils are required to cover the same detection area. The ferrite bar under the detection coils also increases the induce voltage ratio difference caused by a foreign object, and the increased differences improve the resolution, accuracy, and reliability for detection circuits, induced voltage on the detection coils is used as the detective electric parameter, hence improving induced voltage characteristics is desirable. Optionally, embodiments use (a) narrow ferrite beam(s) and small detection coils located above on the ferrite beam(s) to detect small metallic objects. This improves the resolution. That is, a smaller object can be detected. The smaller an object is, the smaller the sensing coils that are required so one of the sensing coils can be fully covered by the metallic object. Meanwhile, a narrower ferrite bar can be used for the smaller coils to reduce ferrite core loss and less affect for the magnetic flux for power transferring.

4.2 Simulation results

The reason for the reduced induce voltage will now be described, with reference to exemplary simulations. The simulations demonstrate how a ferrite bar under the sensor detection coils improves the accuracy and resolution for a foreign object detection. Note, any reference to "solution", "solution 1", "solution 2" in the Figures refers to "embodiment", "embodiment 1", "embodiment 2" respectively.

As shown in Figures 14 to 16, the flux density B (blue arrows) comprise three components along the X, Y and Z axis. The main contribution to magnetic coupling between the transmitter coils of the charging pad 40 and sensor coils 45a is the components along the Z-axis of the B vector (B_z), which is perpendicular to the axis of the sensor coils 45a and transmitter/receiver coils.

B_z is the predominant magnetic flux generating the induced voltage in a sensor coil 45a. Therefore, to better understand the principle behind the foreign object detection, only B_z is observed to quantify the magnetic flux changes caused by a foreign object (the coin).

4.2.1 Simulation set up

In the simulation, a charging pad as per the unshielded embodiment (Figure 17) is specified, and a charging pad as per the shielded embodiment (Figure 13A-D, 17) is specified. In the simulation, a foreign object (in this case, a coin) is placed in a range of different positions on top of the charging pad 40 of the shielded and unshielded embodiments to compare the two embodiments. At each position, two simulations are run, one for each of the shielded and unshielded embodiments. Embodiment 1 refers to the shielded embodiment ferrite beam and Embodiment 2 refers to the unshielded embodiment. The results of the two embodiments are compared to demonstrate the sensitivity improvement provided by the shielded embodiment. As can be seen from Figure 18 , the sensor coil 45a in the centre (X=0 Y=0) is named as L_FOD0, the most left one is L_FOD24 and the most right one is L_FOD48. There are 49 sensor coils in total in this example.

The dimensions of the embodiments in the simulations are as follows:

4.2.2 Simulation results

4.2.2.1 B_z Without coin (reference)

The simulation is first done without a coin. The results are shown in Figures 19A and 19B. Note, any reference to "solution", "solution 1", "solution 2" in the Figures refers to "embodiment", "embodiment 1", "embodiment 2" respectively.

Figure 19A, 19B show B_z (Flux density along Z axis) across the sensor coils 45a for embodiment 1 and embodiment 2 respectively. The X axis indicates the positions also of the sensor coils 45a - from the left most coil and to the right most coil. Therefore 0.00mm means the left edge of sensor coil-24, 300.00mm means the middle of sensor coil-0, and 600.00mm means the right edge of sensor coil-48. 600mm is also the width of the metallic sheet in the primary coil which normally defines the width of the primary coil's outline size.

The graph above sensor coils to show the B_z relative to the sensor coils 45a, as shown in Figure 18. The length of this line is 600mm, which is the same as the length of the primary metallic base. B_z for embodiment 1 is plotted in Figure 16A and B_z for embodiment 2 is plotted in Figure 16B. From Figures 19A, 19B, the following can be seen.

• The magnetic flux density is not evenly distributed, it varies at the different positions.

• The B_z in embodiment 2 is stronger than in embodiment 1. This is because the ferrite bar in embodiment 1 shields the vertical magnetic field coming from bottom side of the sensor coils.

• In embodiment 1, the ferrite bar shields the vertical magnetic field coming from bottom side, but there is still some vertical magnetic field coming from top side of the sensor coils. So there are still some voltages can be induced on the sensor coils in embodiment 1.

• There are 2 special positions where the B_z is 0. In this set up, the positions of 0 mTesla are 120mm and 480mm.

Table 1 lists the induced voltages of sensor coils 45a with embodiment 1 and embodiment 2

Generally, higher B_z means higher coupling, which causes higher induced voltages. For example, L_FOD0 is at 300mm position and occupies the range between 295mm to 305mm. From the table and figures, B_z at this range in embodiment 1 is lower than that in embodiment 2. The induced voltage in embodiment 1, therefore, is lower than that in embodiment 2.

The induced voltage at sensor coil 45a whose centre is at 120mm where B_z is 0 mTesla is shown also in Table 1 under L_FOD15. We can see the induced voltage is very low at sensor coil 45a in the both embodiments due to extremely low B_z crossing sensor coil 45a. 4.2.2.2 B_z with coin

The simulation is now done with a coin placed in various positions. The results are shown in Figures 20A to 25B. Note, any reference to "solution", "solution 1", "solution 2" in the Figures refers to "embodiment", "embodiment 1", "embodiment 2" respectively.

The selected positions that the coin is placed are: 300mm, 280mm, 150mm, 120mm, 38mm, and 27mm. The changes of the induced voltage on the affected sensor coils 45a before and after the foreign object is placed at each selected position with both embodiments are obtained by simulations, and then comparing the change ratios under both embodiments to verify the big increase for detection sensitivity by the FOD ferrite beam.

The change ratio of the induced voltage on a sensor coil is calculated by change ratio =(induced voltage without coin - induced voltage with coin) / induced voltage without coin

Figures 20A to 25B clearly show that with embodiment 1, B_z crossing the sensor coil right under the coin drops to extremely close to 0, whereas with embodiment 2, B_z crossing the sensor coil right under the coin drops somewhat but not close to 0 except for at position 120mm where B_z is already close to 0 even the coil is not placed.

The B_z charts demonstrates the principle behind the reason why detection sensitivity can be significantly increased by using the ferrite bar the shielded embodiment.

Some particular non exhaustive simulation to demonstrate this are now described.

4.2.2.2.1 Coin at position 300mm (Coin cover range 285mm - 315mm)

Referring to Figures 20A, 20B, when a coin (30mm diameter) is placed at 300mm position, it occupies the range of 285mm to 315mm. For comparison, B_z are plotted under cases: a. embodiment 1 and b. embodiment 2.

Right under the coin, B_z crossing the sensor coil is very close to 0mT in embodiment1, whereas B_z is 0.4mT in embodiment 2.

4.2.2.2.2 Coin at position 280mm (Coin cover range 265mm - 285mm)

Referring to Figures 21A, 21B, when a coin (30mm diameter) is placed at 280mm position, it occupies the range of 265mm to 285mm. For comparison, B_z are plotted under cases: a. Embodiment 1 and b. Embodiment 2. B_z in L_FOD2 range (271mm-281mm range) is very close to 0mT in embodiment1, whereas B_z in L_FOD2 is 0.4mT in embodiment 2.

4.2.2.2.3 Coin at position 150mm (Coin cover range 135mm - 165mm) Referring to Figures 22A, 22B, when a coin (30mm diameter) is placed at 150mm position, it occupies the range of 135mm to 165mm. For comparison, B_z is plotted under 2 cases: a. Embodiment 1 and b. Embodiment 2.

Right under the coin, B_z crossing the sensor coil is very close to 0mT in embodimentl, whereas B_z is 0.2mT in embodiment 2.

4.2.2.2.4 Coin at position 120mm (Coin cover range 105mm - 135mm)

Referring to Figures 23A, 23B, when a coin (30mm diameter) is placed at 120mm position, it occupies the range of 105mm to 135mm. For comparison, B_z are plotted under cases: a. Embodiment 1 and b. Embodiment 2.

As can be seen from the figures and table, L_FOD15 (115mm-125mm range) is 100% covered by the coin. L_FOD14 (127mm-137mm range) and L_FOD16 (103mm-113mm range) are 80% covered by the coin. 4.2.2.2.5 Coin at position 38mm (Coin cover range 23mm - 53mm)

Referring to Figures 23C, 23D, when a coin (30mm diameter) is placed at 38mm position, it occupies the range of 23mm to 53mm. For comparison, B_z are plotted under cases: a. Embodiment 1 and b. Embodiment 2.

In comparison, Right under the coin, B_z crossing the FOD coil is very close to 0mT in embodiment1, whereas B_z is 0.2mT in embodiment 2. 4.2.2.2.6 Coin at position 27mm (Coin cover range 12mm - 42mm)

Referring to Figures 24A, 24B, when a coin (30mm diameter) is placed at 27mm position, it occupies the range of 12mm to 42mm. For comparison, B_z are plotted under cases: a. Embodiment 1 and b. Embodiment 2.

In comparison, Right under the coin, B_z crossing the FOD coil is very close to 0mT in embodiment1, whereas B_z is 0.2mT in embodiment 2.

4.2.3 Summary of simulation and comparison with no ferrite bar

The simulation data shown in 19A-24B can be summarised in the graphs shown in Figure 25A-D. This will now be discussed.

Since the magnetic flux density B_z generated from the primary pad and secondary pad are not evenly distributed, 6 positions where the magnetic flux density is different are chosen from the simulation results to demonstrate that the detection method with the ferrite bar can work well on the all areas.

Figures 25A-D illustrate the effects of having ferrite underneath sensor coils 45a by comparing the B_z and the open-circuit induced voltages of sensor coils 45a with ferrite (embodiment 1) to those without ferrite (embodiment 2) at six different positions along the ferrite bar. The six different positions that the coin is placed are: 300mm (position 6 in Figures 25A-D), 280mm (position 5 in Figures 25A-D), 150mm (position 4 in Figures 25A-D), 120mm (position 3 in Figures 25A-D), 38mm (position 2 in Figures 25A-D), and 27mm (position 1 in Figures 25A-D). The changes of the induced voltage on the affected sensor coils 45a before and after the foreign object is placed at each selected position with both embodiments are obtained by simulations, and then comparing the change ratios under both embodiments to verify the big increase for detection sensitivity by the FOD ferrite beam.

Two sets of the simulation data are presented, one set is for embodiment 1 (with ferrite bar) is shown in Figure 25A and Figure 25B, and the other set is for embodiment 2 (without ferrite bar) is shown in Figure 25C and Figure 25D:

• Figure 25A shows a comparison of the magnetic flux density at each of the six positions when a foreign object is present (orange circle) or absent (green triangle).

• Figure 25B shows a comparison of the induced voltage (as a result of the magnetic flux density) in the coil at each of the six positions when a foreign object is present (red circle) or absent (green triangle).

• Figure 25C shows a comparison of the magnetic flux density at each of the six positions when a foreign object is present (orange circle) or absent (green triangle).

• Figure 25D shows a comparison of the induced voltage (as a result of the magnetic flux density) in the coil at each of the six positions when a foreign object is present (red circle) or absent (green triangle).

Data for induced voltage on detection coils located at six different positions with different B_z are presented here with red dots representing the induced voltages on the detection coil at the each location without foreign objects and green triangles representing induced voltages on the sensor coil 45a at each location with a foreign object located above . Green triangles are always lower than red dots because foreign objects block part of voltage-generating flux.

From Figure 25A it can be seen that with embodiment 1 when a foreign object is presented above a sensing coil, due to double shielding from the ferrite bar and the foreign object, B_z drops to close to 0 (few ~ tens of uT), the extreme low B_z causes the induced voltage on the sensing coil to drop to near 0V (<50mV) which is shown in Figure 25B. In contrast, Figure 25C shows that with embodiment 2 (without the magnetic shielding provided by the ferrite bar) when a foreign object is presented above a sensing coil, B_z remains relative high (about 50 times stronger) comparing to B_z in embodimenltl, therefore the induced voltage on the sensing coil also remains relatively high which is shown in Figure 25D. It can be seen from Figure 25B, due to the shielding effects of ferrite, when a foreign object, such as a coin, is placed on top of a sensor coil 45a, the induced voltage reduces significantly. In other words, the voltage is reduced by a large ratio (93% ~99%).

The change ratio of the induced voltage on a sensor coil is calculated by: change ratio =(induced voltage without coin - induced voltage with coin) / induced voltage without coin

For example, the induced voltage of sensing coil at position 3 reduces from 159.4mV to only 0.57mV after a coin is placed on top of sensing coil at position 3. The large drop ratio simplifies the detection of foreign objects. In contrast, the results in Figure 25D show a smaller drop ratio (70%~80%) of the induced coil voltages due to foreign objects. This makes the detection more difficult as smaller drop ratio are generally harder to detect.

In addition to the larger change ratio in coil voltage, the shielding effects of ferrite also make the value of induced coil voltage extremely low when a foreign object is present. In this example, the voltages in Figure 25B are nearly zero (with the max value at only 35.5mV). Larger scale simulations have also shown that the shielding ferrites can always reduce the coil voltage to near zero for a detection coil regardless of its position on the IPT pad when a foreign object is present and cover the detection coil. Normally the induced voltages are amplified by operational amplifiers to increase the detection sensitive and reliability. The near zero induced voltage with a foreign object presented can easily remain very low (<200mV) after being amplified, whereas the high induced voltage without a foreign object presented can reach to the rail voltage due to being amplified or saturated. This creates a large separation between coil voltages with and without foreign objects, which dramatically simplifies the amplifier circuits design and allows a universal detection circuit to be used for all sensor coils 45a. In contrast, without the shielding ferrite, the induced voltage with foreign objects can still be very large when a foreign object is on top. This is shown in Figure 25D where the sensing coil at position 5 still has 232mV across it with a foreign object. To make matters worse, still referring Figure 25D voltage of the sensing coil at position 3 without a foreign object (141.5mV) is surprisingly lower than the voltage of the sensing coil at position 5 with a foreign object (232mV). This means the detection circuits for sensor coils at positions 3 and 5 must be designed and tuned differently, which adds complexity to a detection system. What's more, as the sensor coil moves relative to the IPT coil, the sensor coil will exhibit different induced voltages with foreign objects on top, which further complicates the design and tuning of sensor detection circuits. Based on these facts, one can see that placing the ferrite under sensor coils 45a can simplify the design and tuning of detection circuits by amplifying the change in induced voltage, therefore leading to a more robust and simpler FOD system.

4.3 Variations

Variations on the shielding embodiment will now be described.

4.3.1 Alternative methods for detecting foreign object

The simulations in section 4.2 above have been set up such that addition of the ferrite bar reduces the induced voltage to a value closer to zero than would be achieved otherwise if a ferrite bar is not used. However a skilled person would understand there are other ways to observe the benefits of adding a ferrite bar. For example the system can be simulated or implemented such that the presence of the foreign object increases induced voltage. In such a situation, the addition of the ferrite bar still improves the ability to detect foreign object since the presence of the foreign object increases induced voltage in the sensing coil by a larger margin than what could be achieved without the ferrite bar.

More generally, the simulations above compare induced voltages in the sensing coil between embodiments 1 and 2 to show how having a ferrite bar improves detection of foreign objects. However, change in any other electrical parameter can be observed (as an alternative to induced voltage in sensing coil) to detect the presence of a foreign object, either in simulation or in implementation. The change in any other electrical parameter can be observed as a result of the change in magnetic flux density when the foreign object is present or absent. In those situations, the addition of a ferrite bar improves the ability to detect foreign objects, because the ferrite bar enables a larger change in the electrical parameter being observed when the foreign object is present versus when the foreign object is absent. As an example, but not limiting, the change in electrical parameter could be change in flux, inductance, frequency, phase shift, or some other parameter, for example.

4.3 Ferrite bar design

Another variation on the shielded embodiment will now be described. This can be applied to any embodiment of the ferrite bar 44.

In the shielded embodiment, the core loss of the ferrite bar used in the simulation is 61.63W. This decreases the wireless power transfer efficiency and generates lots of heat. This is because of the high flux density going through the ferrite bar. The ferrite bar causes core losses in magnetic fields. It is simulated and shown as Figure 26A. The max magnitude of flux density B is about 800mT.

To reduce losses, in a variation, the ferrite rod is separated into multiple sections. By inserting a gap between each section, the core loss can be greatly reduced. The principle is that flux path on the ferrite bar is reduced, and therefore the core loss is decreased.

The ferrite rod can be separated in any suitable way. One option is to create gaps to create sections. For example, a number of small size ferrite tiles are assembled on a non-conducting material such as plastic, PCB substrate with a small gap between.

4.3.1 Gap 2mm

Referring to Figure 25B, a 2mm gap (see 250) is formed to separate the bar into sections of 10 mm. If the gap between each 2 sections is 2mm, the flux density in each section is decreased to between lOmTT - 160mT, this is shown in Figure 23A. The decreased flux density in each section reduces the total loss of the ferrite beam.

The total core loss on the ferrite bar is 0.74W, which is much lower than that of the whole ferrite bar.

4.3.2 Cutting gap 0.5mm

Referring to Figure 25B, a 0.5mm gap (see 251) is formed to separate the bar into sections of 11.5 mm. By decreasing the gap between 2 sections of the ferrite bar to 0.5mm, the core loss increases from 0.74W to 7.18W due to the increase of flux density B. The flux density magnitude is shown in Figure 25C. The flux density can be as high as around 400mT. That is because more flux can go through the FOD ferrite beam with smaller gaps.

4.4 Summary

In summary, use of a ferrite rod for the sensing arm provides some of the follow

• causes the induced voltage across the sensor coil under a metallic object to drop to extremely close to 0V (around few milli-volt) .

• The extremely low induced voltage (close to 0V) on a sensor coil when it is fully covered by a metallic object makes a 93% to 99% change of the induced voltage when the sensor coil enters and exits the coverage area of the metallic object.

• The close to 100% change of the induced voltage greatly increases the sensitivity and reliability of the detection circuit. To increase the detection resolution, or in the other words to detect a small metallic object, sensor coils with smaller dimensions than the metallic object can be used.

To maintain a substantial induced voltage on a detection coil when metallic objects do not exist, more turns can be added to the detection coil.

The sensor coils can have a different number of turns. If there is some relatively low magnetic flux density at Z-axis exit at the moving path of a sensor coil, the sensor coil can have more turns.

The X, Y dimensions of sensor coils are recommended to be 1/3 of the X, Y dimensions of the smallest foreign object required to be detected. The enables that at any particular time that at least one sensor coil is 100% covered, and one sensor coil is covered at least 80% by foreign objects, regardless of the position of foreign object.

The induced voltage can be measured on an individual sensor coil or a group of sensor coils (e.g. 2 - 3 coils) in series or in parallel.

The ferrite bar can be about 5 - 8mm wider than the sensor coils to provide good shielding.

The core loss on the FOD ferrite beam can be reduced by combing multiple sections with a gap in a range of about 0.5mm to about 2mm between adjacent sections.

5. Advantages

The disclosed embodiments may have one or more of the following advantages:

• The sensing bars 44, 54, 74 are small relative to the size of the sensing region 48, 58, 78 that they need to scan. The sensing bars 44, 54, 74 are moveable to scan the entire sensing region 48, 58, 78. The combination of the size of the bar 44, 54, 74 (relative to size of sensing region48, 58, 78), and the means of moving the sensing bar 44, 54, 74 ensures that the wireless charging pad 40, 50, 70 can detect the presence of a foreign object 42, 52, 72, irrespective of the size of the foreign object 42, 52, 72, where it is located within the sensing region 48, 58, 78, or whether the foreign object 42, 52, 72 is stationary.

• The small area of the sensing bar 44, 54, 74 restricts the amount of coil material and/or restricts the number of sensing coils 45, 55, 75 that can be placed. Consequently, fewer supporting electronic components are required producing a simpler electronic circuitry to manufacture and repair. The cost to manufacture and maintain is therefore cheaper. • The sensing bars 44, 54, 74 can have more shielding than is possible with a sensing array that covers most of the sensing region 48, 58, 78 of a wireless charging pad 40, 50, 70. The extra shielding that the sensing bars 44, 54, 74 can have means the sensing coils 45, 55, 75 pick-up less interference from the electromagnetic flux generated by the inductive coils 14, 57, 77, while still ensuring there is a sufficient rate of wireless power transfer from the wireless charging pad 40, 50, 70 to the electric vehicle 11. As the sensor coil arrays 45, 55, 75 require a smaller area of shielding, a greater level (e.g. thickness) of shielding around the sensor coil arrays 45, 55, 75 can be provided without impeding on the operation of the wireless power transfer coils 14, 57, 77. The extra shielding reduces electromagnetic interference from the wireless power transfer coils 14, 57, 77 which improves the quality of the signal sensed by the sensor coil arrays 45, 55, 75. This improves the accuracy and/or precision of the measurements of the sensor coil arrays 45, 55, 75. Further, the extra shielding thickness allows the controller 32 to be placed closer to the sensor coil arrays 45, 55, 75. Placing the controller 32 physically closer to the sensor coil arrays 45, 55, 75 also improves the accuracy and/or precision of the sensor coil arrays 45, 55, 75 measurements.

As illustrated in Figure 11, 12, prior art arrangements normally comprise of an array of sensing coils (referred to as "sensing array" herein) located underneath the charging surface (referred to as "sensing region" herein) of the wireless charging pad. The array of sensing coils normally spans across the entirety (or at least a large portion) of the charging pad area. The large size of the sensing array relative to the size of the wireless charging pad is problematic for several reasons:

• First, prior art arrangements that use sensor coils to sense foreign objects can only detect foreign objects if an induced measurement ("high" and/or "non-zero") representing the presence of the foreign object can be referenced against a weaker or a substantially non-existent ("low" and/or "zero") measurement that represents the absence of the foreign object. Prior to operation, the prior art is calibrated so that the system can distinguish between a "high" and a "low" measurement and/or distinguish between a "non-zero" and "zero" measurement. If a stationary foreign object sits on the pad during start up, the prior art arrangement would not be able to detect the foreign object. This is because the sensing array is too big, such that it will always sense the presence of the foreign object; and because a "low" and/or "zero" measurement will always be absent, the "high" and/or "non-zero" measurement is "misinterpreted" as an indeterminate measurement. In the example shown in Figures 9 and 10, the foreign object is located, such that a portion of the sensing array always "overlaps" with the foreign object,

• Second, a wireless charging pad covers a relatively large surface area that needs to be scanned for foreign objects. The prior art arrangements would need a large number of sensing coils such that the entire charging pad can be scanned for foreign objects. This means a lot of coil material is required. Further, to detect small foreign objects, (which could be at least as small as a hair pin or a paper clip for example), the sensing coils need to be placed very close together to enable high-resolution sensing. Such placement of the sensing coils exacerbates the issue of requiring a lot of coil material needed to manufacture a wireless charging pad.

• Third, the sensing array has shielding material to attenuate the electromagnetic flux emitted from the inductive coils, such that electromagnetic interference with the signals induced in the sensing coils of the sensing array is mitigated. The shielding material should still be magnetically "porous" enough such that the electromagnetic flux emitted from the inductive coils can still pass through such that there is still a sufficient wireless transfer of power. Referring back to the example of Figure 11, the size of the sensing array is large relative to the top surface of the sensing array, which restricts the upper limit of how much shielding material can be applied to the sensing array. An excess amount of shielding would restrict the rate of wireless power transfer from the wireless charging pad to the electric vehicle.