Foreign object detection in a wireless power transfer system

Abstract

A power transmitter includes a transmitter coil (103) that generates an electromagnetic field. A set of balanced detection coils (207, 209) includes detection coils that are in series and mutually compensating. A foreign object detector (205) performs foreign object detection by potentially detecting a foreign object in response to a characteristic of an output signal from the set of balanced detection coils (207, 209) and in response to an electromagnetic test satisfying a foreign object detection criterion. A communicator (211) is coupled with a communication antenna (213) and communicates with a power receiver (105) via the communication antenna (213). The communication antenna (213) includes multiple communication coils (215, 217) in parallel. A first segment of a first communication coil (215) has a first coupling with a first detection coil, and a second segment of a second coil (217) has a second coupling with a second detection coil. The couplings are capacitive and / or inductive, and the first and second couplings mutually compensate in the output signal.

Description

TECHNICAL FIELD

[0001] The present application relates to foreign object detection in wireless power transfer systems, and in particular, but not exclusively, to foreign object detection for power transmitters providing inductive power transfer to higher power devices, such as kitchen appliances. BACKGROUND

[0002] Most electrical products today require a dedicated electrical contact in order to be powered from an external power source. However, this tends to be impractical and requires the user to physically plug in a connector or otherwise establish a physical electrical contact. Typically, power requirements also differ significantly and currently most devices are provided with their own dedicated power supply, resulting in a user typically having a large number of different power supplies, each dedicated to a particular device. Although the use of internal batteries can avoid the need for a wired connection to a power source during use, this only provides a partial solution as the batteries will need recharging (or replacing). The use of batteries can also significantly increase the weight and potential cost and size of the device.

[0003] In order to provide a significantly improved user experience, it has been proposed to use wireless power sources, in which power is inductively transferred from a transmitter inductor in a power transmitter device to a receiver coil in individual devices.

[0004] Power transfer by magnetic induction is a well-known concept, primarily applied in transformers with a tight coupling between a primary transmitter inductor / coil and a secondary receiver coil. By separating the primary transmitter coil and the secondary receiver coil between two devices, wireless power transfer between these becomes possible based on the principle of loosely coupled transformers.

[0005] Such an arrangement allows wireless power transfer to a device without the need to make any wires or physical electrical connections. Indeed, it can simply allow a device to be placed in the vicinity or on top of the transmitter coil in order to be recharged or powered from externally. For example, a power transmitter device can be arranged with a horizontal surface on which a device can simply be placed in order to be powered.

[0006] Furthermore, such wireless power transfer devices can advantageously be designed such that the power transmitter device can be used with a range of power receiver devices. In particular, a wireless power transfer scheme known as the Qi specification has been defined and is currently being further developed. This approach allows a power transmitter device conforming to the Qi specification to be used with power receiver devices also conforming to the Qi specification, without having to come from the same manufacturer or having to be dedicated to each other. The Qi standard also includes provisions for allowing some functionality to be adapted to a particular power receiver device (e.g. depending on the particular power draw).

[0007] The Qi specification is developed by the Wireless Power Consortium and more information can be found on their website, where in particular the defined specification documents can be found.

[0008] A potential problem with wireless power transfer is that power can be unintentionally transferred to e.g. metal objects that happen to be located in the vicinity of the power transmitter. For example, if a foreign object such as a coin, a key, a ring etc. is placed on the power transmitter platform arranged to receive the power receiver, the magnetic flux generated by the transmitter coil will induce eddy currents within the metal object, which will cause the object to warm up. The heat increase can be very significant and can be very detrimental.

[0009] To reduce the risk of such a situation occurring, it has been proposed to introduce foreign object detection, where the power transmitter can detect the presence of a foreign object and reduce the transmit power and / or generate a user warning when a positive detection occurs. For example, the Qi system includes functionality for detecting foreign objects, as well as functionality for reducing the power when a foreign object is detected. In particular, Qi Specification version 1.2.1, section 11 describes various methods of detecting foreign objects.

[0010] A method of detecting such foreign objects is disclosed in WO 2015018868A1. Another example is provided in WO 2012127335, which discloses a method based on determining unknown power loss. In the method, both the power receiver and the power transmitter measure their power, and the receiver communicates its measured received power to the power transmitter. When the power transmitter detects a significant difference between the power transmitted by the transmitter and the power received by the receiver, there can potentially be an unwanted foreign object, and the power transfer can be reduced or aborted for safety reasons. This power loss method requires accurate power measurements performed by the power transmitter and the power receiver in synchronisation.

[0011] For example, in the Qi power transfer standard, the power receiver estimates its received power, e.g. by measuring the rectified voltage and current, multiplying them and adding an estimate of the internal power losses in the power receiver (e.g. losses of the rectifier, receiver coil, metal parts etc. that are part of the receiver). The power receiver reports the determined received power to the power transmitter at a minimum rate of e.g. every four seconds.

[0012] The power transmitter estimates its transmitted power, e.g. by measuring the DC input voltage and current of the inverter, multiplying them and correcting the result by subtracting an estimate of the internal power losses in the transmitter (e.g. estimated power losses in the inverter, primary coil and metal parts that are part of the power transmitter).

[0013] The power transmitter can estimate the power loss by subtracting the reported received power from the transmitted power. If the difference exceeds a threshold, the transmitter will assume that too much power is consumed in the foreign object, and then it can proceed to terminate the power transfer.

[0014] Alternatively, it has been proposed to measure the quality factor or Q-factor of the resonant circuit formed by the primary and secondary coils and the corresponding capacitance and resistance. A decrease in the measured Q-factor can indicate the presence of a foreign object. This method is typically used before the power transfer.

[0015] In practice, using the method described in the Qi specification tends to be difficult to achieve sufficient detection accuracy. Many uncertainties about the specific current operating conditions exacerbate this difficulty.

[0016] For example, a specific problem is the potential presence of friendly metals, i.e. metal parts of the device comprising the power receiver or the power transmitter, since their magnetic and electrical properties can be unknown (and vary between different devices) and thus can be difficult to compensate for.

[0017] Moreover, even a relatively small amount of power dissipated in a metallic foreign object can lead to unwanted heating. It is therefore necessary to detect even smaller power differences between the transmitted and received power, and this can be particularly difficult when the power level of the power transfer is increased.

[0018] In many situations, the Q-factor degradation method has better sensitivity for detecting the presence of metallic objects. However, it can still not provide sufficient accuracy and can also be affected by friendly metals, for example.

[0019] The performance of foreign object detection depends on the specific operating conditions present when the test is actually performed. For example, if the foreign object detection measurement is performed in the selection phase during the power transfer initialization process, as described in the Qi specification, the signal provided by the power transmitter for the measurement must be small enough to prevent it from waking up the power receiver. However, for such small signals, the signal-to-noise ratio is typically poor, resulting in reduced measurement accuracy.

[0020] Another problem is that foreign object detection is typically a very sensitive test in which relatively small changes caused by the presence of a foreign object are expected to be detected in an environment whose operating conditions and situations can vary greatly while the test is being performed.

[0021] For higher power levels, these problems tend to be exacerbated, and current trends in wireless power tend to be towards higher power levels of transmission. For example, the Wireless Power Consortium is developing a cordless kitchen specification that is intended to support high power levels of up to 2.5 kW or potentially even higher. For higher power levels, the foreign object detection algorithm needs to be more accurate to prevent heating of the foreign object above a safe temperature. In fact, the temperature rise is given by the absolute power level, and thus for higher power levels, the relative power loss that needs to be detected can be greatly reduced.

[0022] Another challenge for foreign object detection is that metal components of the power transmitter and power receiver interfere with the detection and make it more difficult to detect the presence of other metal objects.

[0023] This fact that some other functions can use metal components can exacerbate such problems. In particular, power transfer is generally implemented via power transfer coils, which are typically relatively large and thus can have a significant impact on foreign object detection. In case foreign object detection is performed using a dedicated foreign object detection antenna or coil, the design of the power transfer device can seek to minimize the impact of the power transfer coils by keeping them at a distance from each other or by implementing a magnetic shield between them, for example. However, this can be difficult to achieve as it often contradicts the desire to position the power transfer coils of the two devices in close proximity.

[0024] Similarly, in many power transfer systems, communication between the power transfer devices can be implemented via dedicated communication antennas. For example, NFC communication can be implemented using a dedicated NFC communication coil. These can be expected to be positioned close to each other, which can limit the design freedom in minimizing the impact on foreign object detection.

[0025] Current methods for foreign object detection and communication in power transmitters tend to be suboptimal and can provide suboptimal performance in certain situations and examples. Interactions between the foreign object detection and communication functions often result in reduced communication performance (e.g., due to suboptimal coupling between the communication antennas) and / or degraded foreign object detection performance (e.g., due to the impact of the communication antennas close to the foreign object detection antennas). In particular, current methods can result in the presence of a foreign object not being detected or in false foreign object detection in the absence of a foreign object. Furthermore, more accurate methods tend to be complex and expensive.

[0026] It would therefore be advantageous to have an improved foreign object detection and communication for use in a power transmitter, and in particular, a method that allows increased flexibility, reduced cost, reduced complexity, improved foreign object detection, fewer false detections and missed detections, backward compatibility, improved stability for higher power level transmissions, improved communication, reduced impact of communication antennas on foreign object detection, and / or improved performance would be advantageous. SUMMARY

[0027] The present invention therefore seeks to preferably mitigate, alleviate or eliminate one or more of the above mentioned disadvantages singly or in any combination.

[0028] According to an aspect of the present invention, there is provided a power transmitter for transmitting power to a power receiver via an inductive power transfer signal, the power transmitter comprising: a transmitter coil arranged to generate an electromagnetic test field for foreign object detection; a set of balanced detection coils comprising two detection coils, the two detection coils being connected in series and such that signals induced in the two detection coils by the electromagnetic test field cancel each other out; a foreign object detector coupled to the set of balanced detection coils and arranged to perform foreign object detection, the foreign object detector being arranged to detect a foreign object in response to a characteristic of an output signal from the set of balanced detection coils meeting foreign object detection criteria; a communication antenna; a communicator coupled to the communication antenna and arranged to communicate with the power receiver via the communication antenna; wherein the communication antenna comprises at least a first communication coil and a second communication coil connected in parallel, the communication antenna being arranged such that a first section of the first communication coil has a first coupling with the first detection coil, a second section of the second coil has a second coupling with the second detection coil, the first coupling and the second coupling being at least one of a capacitive coupling and an inductive coupling, and the first coupling and the second coupling cancel each other out in the output signal.

[0029] In many embodiments, the present invention can provide improved foreign object detection. In many scenarios and systems, more accurate foreign object detection can be achieved. In many embodiments, the method can reduce complexity. In particular, the method can be particularly suitable for improving foreign object detection in power transfer systems of higher power levels.

[0030] The method can allow for improved accuracy and / or reliability of foreign object detection tests during power transfer phases. In many embodiments, the method can reduce uncertainty of foreign object detection tests, thereby improving performance. When using balanced detection coils, the method can provide an especially effective method for improving detection accuracy.

[0031] In many embodiments, the method can allow for efficient power transfer and / or communication functionality, and can in particular allow for improved coupling between power transfer coils and / or communication antennas of power transmitters and power receivers.

[0032] In many embodiments, the method can allow for advantageous implementation and generally can allow for a compact implementation, e.g. using planar coils, e.g. the planar coils can be implemented in different layers of a multi-layer printed circuit board.

[0033] Balancing the detection coils is in that the detection coils are arranged such that the signals induced within the two detection coils by the electromagnetic field generated by the transmitter coil are mutually compensating. The compensation can be such that the combined voltage over the two balanced detection coils is lower than the maximum of the voltages over each of the two balanced detection coils. The compensation can be at least a partial cancellation of the two signals.

[0034] The foreign object detector can be arranged to determine that a foreign object is detected when the phase and / or amplitude signal from the detection coil exceeds a threshold value.

[0035] The electromagnetic test signal can also be referred to as test electromagnetic field and these terms can be considered to be interchangeable. The detection coils / windings in series means that the current through the detection coils / windings is the same.

[0036] The combined resistance of the coupling between the set of balanced detection coils can be less than 100 Ohm.

[0037] The mutually compensating signals induced in the two detection coils by the electromagnetic test field can reflect that the signals at least partially cancel each other. The compensation can reduce the (combined) signal amplitude of the output signal relative to the maximum amplitude of the individual signals induced in the two coils. The mutually compensating couplings can reflect that the couplings at least partially cancel each other in the output signal. The compensation can reduce the (combined) signal amplitude of the coupling signal components in the output signal relative to the maximum amplitude of the coupling signal components of the individual couplings.

[0038] According to an optional feature of the application, the first coupling comprises both a capacitive coupling and an inductive coupling and the second coupling comprises both a capacitive coupling and an inductive coupling.

[0039] By compensating both the inductive and capacitive coupling between the foreign object detection coils and the communication antenna, the method can allow for advantageous operation and generally improved foreign object detection, power transfer and / or communication.

[0040] According to an optional feature of the application, the communication is arranged such that a first signal component induced in the first detection coil by the current in the first communication coil is compensated in the output signal by a second signal component induced in the second detection coil by the current in the second communication coil.

[0041] The method can allow for advantageous operation and generally improved foreign object detection.

[0042] According to an optional feature of the application, the direction of current in the first segment relative to the direction of current in the first detection coil is opposite to the direction of current in the second segment relative to the direction of current in the second detection coil.

[0043] The method can allow advantageous operation and generally improved foreign object detection. This can be particularly so during foreign object detection.

[0044] According to an optional feature of the application, the first communication coil and the second communication coil are arranged such that a spatial relationship between the first communication coil and the first detection coil corresponds to a spatial relationship between the second communication coil and the second detection coil.

[0045] The method can allow advantageous operation and generally improved foreign object detection. The spatial relationships can correspond to each other by being identical / similar. The spatial relationships can be transformable into each other using only translation, rotation and mirroring.

[0046] According to an optional feature of the application, the voltage potential of the first segment matches the voltage potential of the second segment.

[0047] This can be particularly so during foreign object detection. The method can allow advantageous operation and generally improved foreign object detection, and in many embodiments can particularly result in a close match of the capacitive coupling. The voltage potential can be an average voltage potential over the segments. The voltage potential can be matched by being substantially identical (e.g. within 10% or 5% of each other).

[0048] In some embodiments, the voltage distribution of the first segment matches the voltage distribution of the second segment. This can be particularly so during foreign object detection.

[0049] In some embodiments, the voltage distribution in the first communication coil relative to the first detection coil is symmetrical to the voltage distribution in the second communication coil relative to the second detection coil. This can be particularly so during foreign object detection.

[0050] The method can allow advantageous operation and generally improved foreign object detection.

[0051] In some embodiments, the first communication coil and the second communication coil are arranged such that a spatial relationship between the first segment and the first detection coil corresponds to a spatial relationship between the second segment and the second detection coil.

[0052] The method can allow advantageous operation and generally improved foreign object detection. The spatial relationships can correspond to each other by being identical / similar. The spatial relationships can be transformable into each other using only translation, rotation and mirroring.

[0053] In some embodiments, the orientation of the first segment relative to the first detection coil matches the orientation of the second segment relative to the second detection coil.

[0054] In some embodiments, the direction of current in the first segment relative to the first detection coil matches the direction of current in the second segment relative to the second detection coil.

[0055] According to an optional feature of the application, the first and second communication coils have substantially the same spatial configuration, and the first and second segments are corresponding segments of the first and second detection coils.

[0056] This can allow advantageous operation.

[0057] In some embodiments, the first and second communication coils have substantially the same spatial configuration, and the first and second segments are the same segments of the first and second detection coils.

[0058] According to an optional feature of the application, the set of balanced detection coils are formed in a first plane, and the communication coils are planar coils formed in a second plane substantially parallel to the first plane.

[0059] In many embodiments, this can provide particularly advantageous operation and / or implementation. The planes can be substantially parallel, with a relative angle between them of no more than 10°, 5° or 3°.

[0060] According to an optional feature of the application, the first detection coil spans a first region in the first plane, the second detection coil spans a second region in the first plane, and the spatial relationship of the orthogonal projection of the first segment onto the first plane relative to the first region is the same as the spatial relationship of the orthogonal projection of the second segment onto the first plane relative to the second region.

[0061] In many embodiments, this can provide particularly advantageous operation and / or implementation.

[0062] According to an optional feature of the application, the first detection coil spans a first region in the first plane, the second detection coil spans a second region in the first plane, and the orthogonal projection of the first communication coil onto the first region matches the orthogonal projection of the second communication coil onto the second region.

[0063] In many embodiments, this can provide particularly advantageous operation and / or implementation.

[0064] In some embodiments, the first and second detection coils are rotationally symmetric about a point of rotation, and the first and second communication coils are also rotationally symmetric about the point of rotation.

[0065] This can provide particularly advantageous operation and / or implementation in many embodiments.

[0066] According to an optional feature of the application, the first and second detection coils are rotationally symmetric about a point of rotation, and the first and second segments are also rotationally symmetric about the point of rotation.

[0067] This can provide particularly advantageous operation and / or implementation in many embodiments.

[0068] According to an optional feature of the application, the power transmitter comprises a plurality of sets of balanced detection coils, each set of balanced detection coils comprising at least two detection coils, the detection coils being rotationally symmetric about a point of rotation, each detection coil spanning an angular interval; and each of the first and second communication coils comprises segments distributed along a concentric closed curve about the point of rotation and rotationally symmetric about the point of rotation between the angular intervals of a set of balanced detection coils; and a segment located within an angular interval spanned by one detection coil of a set of balanced detection coils is rotationally symmetric to a segment located within an angular interval spanned by another detection coil of the set of balanced detection coils.

[0069] This can provide particularly advantageous operation and / or implementation in many embodiments.

[0070] According to an optional feature of the application, each angular interval comprises a plurality of segments distributed along different closed curves for each of the first and second communication coils.

[0071] This can provide particularly advantageous operation and / or implementation in many embodiments.

[0072] In some embodiments, adjacent segments of a detection coil distributed along different closed curves are coupled via a substantially radial segment of the detection coil with respect to the point of rotation.

[0073] This can provide particularly advantageous operation and / or implementation in many embodiments.

[0074] In some embodiments, the concentric closed curve is a circle.

[0075] This can provide particularly advantageous operation and / or implementation in many embodiments.

[0076] According to an optional feature of the application, different segments of adjacent angular intervals are distributed along different curves of the concentric closed curve.

[0077] This can provide particularly advantageous operation and / or implementation in many embodiments.

[0078] According to another aspect of the present application, there is provided a method of a power transmitter for transmitting power from the power transmitter to a power receiver via an inductive power transfer signal, the method comprising: providing a transmitter coil for generating an electromagnetic test field for foreign object detection; providing a set of balanced detection coils comprising two detection coils, the two detection coils being connected in series and arranged such that signals induced in the two detection coils by the electromagnetic test field cancel each other out; providing a foreign object detector coupled to the set of balanced detection coils and arranged to perform foreign object detection, the foreign object detector being arranged to detect a foreign object in response to a characteristic of an output signal from the set of balanced detection coils meeting foreign object detection criteria; providing a communication antenna; and providing a communicator coupled to the communication antenna and arranged to communicate with the power receiver via the communication antenna; wherein the communication antenna comprises at least a first communication coil and a second communication coil, the second communication coil being connected in parallel to the first communication coil, the communication antenna being arranged such that a first section of the first communication coil has a first capacitive coupling to the first detection coil, a second section of the second coil has a second capacitive coupling to the second detection coil, the first and second couplings being at least one of capacitive and inductive, and the first and second couplings cancel each other out in the output signal.

[0079] These and other aspects, features, and advantages of the present application will become apparent to those persons skilled in the art upon reading the details of the (one or more) embodiments described below. BRIEF DESCRIPTION OF DRAWINGS

[0080] Embodiments of the present application will be described, by way of example only, with reference to the drawings in which:

[0081] Figure 1 An example of elements of a power transfer system according to some embodiments of the present application is shown;

[0082] Figure 2 An example of elements of a power transmitter according to some embodiments of the present application is shown;

[0083] Figure 3 An example of a half bridge inverter for a power transmitter is shown;

[0084] Figure 4 An example of a full bridge inverter for a power transmitter is shown;

[0085] Figure 5 An example of a time frame for a wireless power transfer system for Figure 1

[0086] Figure 6 An example of a detection coil for a power transmitter according to some embodiments of the present application is shown;

[0087] Figure 7 ​An example of a segment design for a communication coil according to some embodiments of the application is shown;

[0088] Figure 8 An example of an arrangement of coils in a power transfer system according to some embodiments of the application is shown;

[0089] Figure 9 An example of a detection coil for a power transmitter according to some embodiments of the application is shown;

[0090] Figures 10 to 15 An example of a segment design for a communication coil according to some embodiments of the application is shown;

[0091] Figure 16 An example of a practical implementation of a communication antenna and detection antenna according to some embodiments of the application is shown;

[0092] Figure 17 An example of a practical implementation of a communication antenna and detection antenna adjacent to a power transmitter coil according to some embodiments of the application is shown; and

[0093] Figures 18 to 19 An example of a segment design for a communication coil according to some embodiments of the application is shown. DETAILED DESCRIPTION

[0094] The following description focuses on embodiments of the application that are suitable for use in a wireless power transfer system that utilizes a power transfer method such as known from the Qi specification or the Cordless Kitchen Specification. However, it should be understood that the application is not limited to this application, but can be applied to many other wireless power transfer systems.

[0095] Figure 1 An example of a power transfer system according to some embodiments of the application is shown. The power transfer system comprises a power transmitter 101 comprising (or coupled to) a transmitter coil / inductor 103. The system also comprises a power receiver 105 comprising (or coupled to) a receiver coil / inductor 107.

[0096] The system provides an electromagnetic power transfer signal that inductively transfers power from the power transmitter 101 to the power receiver 105. In particular, the power transmitter 101 generates an electromagnetic signal that propagates as magnetic flux through the transmitter coil or inductor 103. The power transfer signal can typically have a frequency between about 20 kHz to about 500 kHz, and typically for Qi compliant systems typically in the range of 95 kHz to 205 kHz (or for example for high power kitchen applications the frequency can for example typically be in the range of 20 kHz to 80 kHz). The transmitter coil 103 is loosely coupled with the power receive coil 107, and thus the power receive coil 107 picks up (at least part of) the power transfer signal from the power transmitter 101. Thus, via the wireless inductive coupling from the transmitter coil 103 to the power receive coil 107, power is transferred from the power transmitter 101 to the power receiver 105. The term power transfer signal is mainly used to refer to the inductive signal / magnetic field (magnetic flux signal) between the transmitter coil 103 and the power receive coil 107, but it should be understood that by equivalence it can also be considered and used as a reference to the electrical signal provided to the transmitter coil 103 or picked up by the power receive coil 107.

[0097] In examples, the power receiver 105 is specifically a power receiver that receives power via the receiver coil 107. However, in other embodiments, the power receiver 105 can comprise a metallic element, for example a metallic heating element, in which case the power transfer signal directly induces eddy currents, resulting in direct heating of the element.

[0098] The system is arranged to transfer significant power levels, and in particular in many embodiments the power transmitter can support power levels in excess of 500 mW, 1 W, 5 W, 50 W, 100 W or 500 W. For example, for Qi corresponding applications, for low power applications (basic power curve) the power transfer can typically be in the power range of 1-5 W, for Qi specification version 1.2 the power transfer can be up to 15 W, for higher power applications (e.g. power tools, laptops, drones, robots, etc.) the power transfer can be in the range up to 100 W, and for ultra-high power applications (e.g. kitchen applications) the power transfer can be in excess of 100 W and up to 1000 W and beyond.

[0099] In the following, the operation of the power transmitter 101 and the power receiver 105 will be described with specific reference to embodiments generally in accordance with the Qi specification (except for the (or corresponding) modifications and enhancements described herein) or embodiments suitable for the higher power kitchen specification developed by the Wireless Power Consortium. In particular, the power transmitter 101 and the power receiver 105 can follow or be substantially compatible with the Qi specification version 1.0, 1.1 or 1.2 (except for the (or corresponding) modifications and enhancements described herein).

[0100] In a wireless power transfer system, the presence of an object (typically a conductive element that extracts power from the power transfer signal and is not part of the power transmitter 101 or the power receiver 105, i.e. an unintended, undesired and / or interfering element for the power transfer) during power transfer can be very disadvantageous. Such undesired objects are referred to as foreign objects in the art.

[0101] Foreign objects can not only reduce efficiency by adding power loss to the operation, but can also reduce the power transfer operation itself (e.g. by interfering with the power transfer efficiency or extracting power that is not directly controlled by the power transfer loop, for example). In addition, current induction in the foreign object (in particular eddy currents in metallic parts of the foreign object) can lead to a typically very undesired heating of the foreign object.

[0102] To address such situations, wireless power transfer systems such as the Qi or the wireless kitchen specification include functionality for foreign object detection. In particular, the power transmitter includes functionality that seeks to detect whether a foreign object is present. If so, the power transmitter can for example terminate the power transfer or reduce the maximum amount of power that can be transferred.

[0103] Foreign object detection can be performed before the power receiver enters the power transfer phase (e.g. during initialization of the power transfer) or during the power transfer phase. Detection during the power transfer phase is typically based on a comparison of measured transmit power and received power, while detection that occurs before the power transfer phase is typically based on a measurement of the reflected impedance, for example by using a small measurement signal to measure the quality factor of the transmitter coil.

[0104] The current approach proposed by the Qi specification is based on detecting power loss (by comparing transmitted and reported received power) or detecting a degradation in the quality Q of the output resonant circuit. However, in current use, it has been found that these approaches provide suboptimal performance in many situations and they can in particular lead to inaccurate detection, resulting in missed detections and / or false positives (detecting a foreign object although no such object is present).

[0105] Conventional foreign object detection tends to be suboptimal, partly due to variations and uncertainties in the specific operating conditions and circumstances under which it is performed, including variations and uncertainties in power transmitter characteristics, power receiver characteristics, and application testing conditions.

[0106] An example of the challenges in foreign object detection testing is the need to perform sufficiently accurate measurements to achieve reliable foreign object detection. This can lead to the desire to generate the strongest possible signal to improve detection accuracy. However, this can increase power consumption in the receiver and in any foreign object present. Detection performance can be sensitive to the specific signal level applied and often involves conflicting requirements.

[0107] Figure 1 The system employs a method for foreign object detection that seeks to provide an improved compromise for foreign object detection. This method provides improved foreign object detection in many embodiments, and specifically, in many embodiments, more accurate and / or more reliable foreign object detection. The method also allows for low complexity and low resource requirements.

[0108] As will be described in more detail below, the method uses a time-division approach during the power transfer phase, wherein foreign object detection and power transfer can be performed, for example, in separate time intervals, thereby allowing interference between them (specifically the impact of power transfer on foreign object detection) to be significantly reduced.

[0109] The following text will describe this in more detail. Figure 1 The system is described above. In this example, the electromagnetic power transfer signal and the electromagnetic test signal for foreign object detection are generated by the same coil. Furthermore, the signal / field will be referred to using different terms: the electromagnetic signal / field generated during the power transfer time interval will be called the power transfer signal, while the electromagnetic signal / field generated during the foreign object detection time interval will be called the electromagnetic test signal, or simply the test signal. In some cases where a time division between power transfer and foreign object detection is not employed, the power transfer signal itself can also be used as the electromagnetic test signal.

[0110] Figure 2 Showing more details Figure 1 The power transmitter 101 is a component.

[0111] The power transmitter 101 includes a driver 201 that generates a drive signal fed to a transmitter coil 103, which in turn generates an electromagnetic power transfer signal, thereby providing power transfer to the power receiver 105. The power transfer signal is provided during power transfer time intervals in the power transfer phase.

[0112] The driver 201 generates the current and voltage that is fed to the transmitter inductor 103. The driver 201 is typically a drive circuit in the form of an inverter that generates an alternating signal from a DC voltage. The output of the driver 201 is typically a switching bridge that generates the drive signal by suitably switching the switches of the switching bridge. Figure 3 A half-bridge switching bridge / inverter is shown. The switches S1 and S2 are controlled so that they are never closed at the same time. Alternatively, S1 is closed while S2 is open, and S2 is closed while S1 is open. The switches are opened and closed at a desired frequency, thereby generating an alternating signal at the output. Typically, the output of the inverter is connected to the transmitter inductor via a resonant capacitor. Figure 4 A full-bridge switching bridge / inverter is shown. The switches S1 and S2 are controlled so that they are never closed at the same time. The switches S3 and S4 are controlled so that they are never closed at the same time. Alternatively, the switches S1 and S4 are closed, while S2 and S3 are open, and then S2 and S3 are closed, while S1 and S4 are open, thereby generating a square wave signal at the output. The switches are opened and closed at a desired frequency.

[0113] The power transmitter 101 further comprises a power transmitter controller 203 that is arranged to control the operation of the power transmitter 101 in accordance with the desired operating principle. In particular, the power transmitter 101 can comprise many of the functions that are required to perform power control in accordance with the Qi specification or the cordless kitchen specification.

[0114] The power transmitter controller 203 is in particular arranged to control the generation of the drive signal by the driver 201, and it can in particular control the power level of the drive signal, and thus the level of the generated power transfer signal. The power transmitter controller 203 comprises a power loop controller that controls the power level of the power transfer signal in response to power control messages received from the power receiver 105 during the power control phase.

[0115] Figure 1 The system of the application uses a method for foreign object detection that seeks to adjust the operation in order to provide an improved compromise for foreign object detection. The method can provide improved foreign object detection in many embodiments, and in particular can provide more accurate and / or more reliable foreign object detection in many embodiments. The method can also allow for low complexity and low resource requirements.

[0116] In this example, the driver 201 and the transmitter coil 103 are arranged to generate both an electromagnetic power transfer signal for the purpose of transferring power to the power receiver, and an electromagnetic test signal for foreign object detection. In a system that does not employ time division, the power transfer signal can also be used as the electromagnetic test signal or field.

[0117] However, in this example, the power transmitter can employ a repeating time frame of the drive signal during the power transfer phase, where the time frame comprises at least one power transfer time interval and one foreign object detection time interval. An example of such a repeating time frame is shown in Figure 5 where the power transfer time intervals are indicated by PT and the foreign object detection time intervals are indicated by D. In this example, each time frame FRM comprises only one foreign object detection time interval and one power transfer time interval, and these (and the time frame itself) have the same duration in each frame. However, it will be appreciated that in other embodiments, other time intervals can also be included in the time frame (e.g. communication intervals), or multiple foreign object detection time intervals and / or power transfer time intervals can be included in each time frame. Furthermore, in some embodiments, the duration of the different time intervals (and indeed the time frame itself) can vary dynamically. In some embodiments, the system can not employ a repeating time frame, and can not apply foreign object detection time intervals or power transfer intervals. In some such embodiments, foreign object detection can be performed simultaneously with power transfer and / or communication.

[0118] However, in this method described below, foreign object detection and power transfer are separated in the time domain, resulting in reduced cross-interference from power transfer to foreign object detection. Thus, variability and uncertainty due to variations in operating conditions for power transfer can be isolated from foreign object detection, resulting in more reliable and accurate foreign object detection.

[0119] In the power transfer phase, the power transmitter is thus arranged to perform power transfer during the power transfer time intervals of the time frame. In particular, during these time intervals, the power transmitter 101 and the power receiver 105 can operate a power control loop (which can be based on communication within the power transfer time intervals or can be based on communication outside of the power transfer time intervals, e.g. within a dedicated communication time interval, which can overlap or be the same as the foreign object detection time intervals). Thus, the level of power being transferred can vary dynamically. In the foreign object detection time intervals of the time frame of the power transfer phase, at least one parameter of the drive signal and at least one parameter of the electromagnetic test signal are typically set to a predetermined value, or a value determined during an adjustment operation performed prior to the foreign object detection time interval, for example. Thus, in the foreign object detection time intervals, the parameters can be set to a predetermined value (i.e. determined prior to the foreign object detection time interval, and typically determined prior to the power transfer phase). In contrast, during the power transfer time intervals, the parameters can not be limited to this predetermined value.

[0120] For example, during the power transfer time interval, the system operates a power control loop which allows the power level of the power transfer signal to be changed in response to power control messages from the power receiver. The power control loop can control / change at least one of the current, voltage and frequency of the drive signal / power transfer signal. In contrast, during the foreign object detection time interval, the parameters which are changed by the power control loop during the power transfer time interval can be set to predetermined values of current, voltage and / or frequency determined prior to the power transfer phase.

[0121] In many embodiments, a constant (typically lower) amplitude (typically voltage) of the drive signal is set during the foreign object detection time interval. Additionally or alternatively, a predetermined frequency can be set for the drive signal during the foreign object detection time interval, and this can typically be significantly higher than the drive signal during the power transfer time interval.

[0122] As a result, the electromagnetic signal generated during the power transfer time interval (the power transfer signal) in the time division approach typically has significantly different characteristics to the electromagnetic signal generated during the foreign object detection time interval (the electromagnetic test signal). The electromagnetic signal or field generated during the power transfer time interval will be referred to as the power transfer signal, while the electromagnetic signal or field generated during the foreign object detection time interval will be referred to as the electromagnetic test signal, or just the test signal. However, it will be appreciated that in Figure 2 In the system of Figure 1, the electromagnetic signal is generated from the same coil in both the power transfer time interval and the foreign object detection time interval, and indeed the same driver etc. is used for both the power transfer time interval and the foreign object detection time interval. Indeed, in many embodiments, reference to the test signal can be considered to be equivalent to the power transfer signal during the foreign object detection time interval. In other embodiments, the power transfer signal and the electromagnetic test signal can be generated by different circuits and / or different coils.

[0123] The power transmitter 101 comprises a foreign object detector 205 which is arranged to perform a foreign object detection test, i.e. to detect in particular whether any unwanted electrically conductive elements can be present in the generated electromagnetic field.

[0124] During the interval in which foreign object detection is performed, i.e. during the foreign object detection time interval, the foreign object detector 205 therefore evaluates the conditions to determine whether a foreign object is considered to be present. During the foreign object detection time interval, the power transmitter 101 generates an electromagnetic test signal and the foreign object detection is based on evaluating characteristics and properties of this signal.

[0125] In this system, foreign object detection is based on detecting a signal induced by the electromagnetic test signal in a set of balanced detection coils, the set of balanced detection coils comprising at least two detection coils 207, 209, which are arranged such that they are negatively offset to each other in the presence of a uniform magnetic field and / or the electromagnetic field generated by the transmitter coil 103 (e.g. specifically the electromagnetic test signal). Specifically, the power transmitter comprises a first detection coil 207 and a second detection coil 209, which are coupled such that the electromagnetic field generated by the transmitter coil is (at least partially) mutually compensated.

[0126] Thus, the electromagnetic field generated by the transmitter coil 103 will induce a signal in the first detection coil 207 and will induce a signal in the second detection coil 209. However, the induced voltages will have opposite polarity, such that the voltage (amplitude) of the series of the detection coils 207, 209 due to the electromagnetic field generated by the transmitter coil 103 is lower than the voltage (amplitude) of at least the largest and typically any of the individual detection coils 207, 209 due to the electromagnetic field generated by the transmitter coil 103. Thus, the first detection coil 207 and the second detection coil 209 are coupled such that the induced voltages from the electromagnetic field generated by the transmitter coil 103 at least partially cancel each other out. Thus, the compensation of the two signals can result in a reduction of the signal amplitude of the combined signal, and specifically at least a partial compensation.

[0127] The detection coils can be arranged such that in the presence of a uniform electromagnetic field (or the electromagnetic field generated by the transmitter coil 103), the amplitude of the combined signal (typically a voltage signal) from the two detection coils is smaller than the amplitude of each of the individual signals of the detection coils. In many embodiments, the voltage over the series of the two detection coils is smaller than the voltage amplitude over the detection coil with the largest voltage amplitude.

[0128] The detection coils are specifically arranged to correspond to at least two windings / coils in which opposite signals are generated by the electromagnetic test signal when no foreign object is present. Thus, the opposite signals can at least partially cancel each other out, and thus the level of the induced signal measured over the series of the detection coils 207, 209 will be reduced, and potentially substantially cancelled. This can allow for a much increased magnetic field strength to be used for foreign object detection. In practice, in many embodiments and scenarios, the resulting induced voltage can (ideally) only arise due to the difference in magnetic flux between the windings. This difference or asymmetry between the windings can be caused by a foreign object, and thus in many scenarios the effect of a foreign object on the magnetic field (and the induced signal) can be more accurately measured.

[0129] Figure 6An example of a detection coil arrangement is shown. In this example, a first detection coil 207 is formed as a first winding L1 and a second detection coil 209 is formed as a second winding L2 connected in anti-)series, such that the combined voltages of the two windings are offset from each other for a uniform electromagnetic field. In this example, the detection coils 207, 209 / windings L1, L2 are positioned relative to each other and symmetrically around a center point. They are further formed in a plane, and the transmitter coil 103 is also formed in the same plane (or at least one substantially parallel plane). In this example, the detection coils are formed inside the transmitting coil. Furthermore, the detection coils are formed to have substantially the same outline and cover substantially the same area.

[0130] As a result, the electromagnetic flux through the two detection coils is substantially the same but in opposite directions relative to the current direction in the detection coils. Consequently, the induced voltages in the two detection coils 207 and 209 are substantially the same but have opposite phases / polarities, and the combined voltage on the two series-connected detection coils 213 is canceled out to substantially zero.

[0131] The detection coils 207 and 209 can be arranged such that, in the presence of a uniform field and / or in the presence of an electromagnetic test field, for example generated by the transmitter coil 103, and in the absence of other objects, the induced signals / voltages at least partially cancel each other out / compensate, ideally resulting in a combined voltage of zero.

[0132] Figure 2 and Figure 6 The arrangement of the two detection coils results in the induced signal of the first coil having a voltage opposite to that of the second coil. For a uniform field, the induced signals of the two detection coils have opposite phases. The two detection coils are connected in series and have opposite phases, resulting in induced signals with opposite polarities. These characteristics exist for uniform fields and the undistorted field generated by transmitter coil 103. For non-uniform fields, cancellation may be only partial.

[0133] However, in the presence of metallic foreign objects, the magnetic field is usually distorted, leading to an asymmetry between the fields of the two detection coils 207 and 209. Typically, with metallic foreign objects, the generated electromagnetic test signal will induce eddy currents, causing the foreign object to generate an electromagnetic field, resulting in distortion of the combined electromagnetic field relative to the generated electromagnetic test signal. For example... Figure 7As shown, the resulting asymmetric field will result in different signals being induced in the first and second detection coils 207, 209. Thus, in contrast to a situation where there is no foreign object and the flux passing through both detection coils 207, 209 is symmetric resulting in a combined voltage of substantially zero, the presence of a foreign object results in an asymmetry and thus a resulting voltage. This difference in the induced signals of the two detection coils 207, 209 can be used to detect the presence of a foreign object.

[0134] In an ideal theoretical case, the electromagnetic field generated by the transmitter coil 103 can be perfectly uniform and identical in the balanced detection coils 207, 209, and similarly, the balanced detection coils 207, 209 are also perfectly identical / symmetric. In this case, the signals induced in the two balanced detection coils 207, 209 are exactly the same and will exactly cancel each other out, resulting in a combined signal from a set of balanced detection coils of exactly zero in the absence of any foreign object or any other asymmetry in the environment.

[0135] However, it will be appreciated that in practice, this scenario is very unlikely. Typically, even if the system is implemented to seek to provide as uniform a field as possible in the detection coils, various asymmetries caused by variations in the generated field, the electromagnetic environment and / or the physical characteristics of the detection coils (or transmitter coil 103) can result in the signals generated in the individual detection coils varying somewhat, and result in the signals induced in the detection coils not cancelling each other out completely. In this case, the combined output from a set of balanced detection coils can not be exactly zero. However, while this can reduce the sensitivity of foreign object detection, and even in some cases can prevent small foreign objects from being accurately detected, this approach can still provide a substantially improved performance, for example compared to using a single detection coil. The partial cancellation between the respective detection coils in a set of detection coils can provide a more accurate signal that can be used for foreign object detection. Indeed, typically, any cancellation between the detection coils will tend to provide improved foreign object detection.

[0136] Thus, improved foreign object detection can be achieved as long as the detection coils are arranged such that the signals induced in the two detection coils by the electromagnetic test field (generated by the transmitter coil 103) cancel each other out.

[0137] In practice, in some embodiments even a strong non-uniform electromagnetic test field can be used, for example resulting in compensation between the balanced detection coils being only partial and generating a relatively high combined (while still compensated) signal. This can be due to, for example, asymmetric characteristics in the setup (e.g. differences in the detection coils, asymmetry in the transmitter coil 103, asymmetry in the electromagnetic environment, for example caused by metallic elements or components of the system), which cannot be eliminated or which are needed for other purposes. In such a case, compensation can be removed, but still provide improved performance.

[0138] In practice, in some embodiments even a time-varying field can be generated, and in case possible even a non-uniform field. In such a case, the time-varying non-uniform field will result in a time-varying combined signal which is only partially compensated. However, depending on whether a foreign object is present or not, the combined signal resulting from the partial compensation will still be different, which can be used by the foreign object detector 205 for foreign object detection.

[0139] For example, for a given time-varying and non-uniform field generated by the transmitter coil 103, a combined (partially) compensated output signal in the absence of a foreign object can be determined. In some embodiments, such a determination can be made during manufacturing, for example based on analysis or simulation. In other embodiments, it can be determined for example in a calibration procedure performed during power transfer initialization and for example in response to a user confirming that no foreign object is present. Subsequently, when a foreign object detection test is performed and a time-varying and non-uniform test electromagnetic field is generated, the resulting combined signal from the balanced detection coils can be compared to the stored / expected result. If the difference exceeds a given amount (in relation to any suitable comparison and difference metric), it can be determined that a foreign object is present, otherwise it can be determined that it is not. For example, the measured combined signal can be correlated to the expected combined signal, and if the correlation is below a threshold, it can be detected that a foreign object is present.

[0140] For non-time-varying signals, the combined signal (e.g. voltage) from a set of balanced detection coils can be simply compared to an expected signal level, typically after some averaging or low-pass filtering, and if the difference exceeds a threshold, it can be considered that a foreign object is detected. In many embodiments, a foreign object can be considered to be detected if the measured combined signal exceeds or is below the expected level by a given amount.

[0141] In some embodiments, the decision criteria for detecting a foreign object can thus be adjusted based on specific preferences and requirements, and can for example be adjusted to reflect variations in the generated electromagnetic field, environment, detection coils and / or transmitter coil. Thus, the operation can be adapted to the specific conditions, requirements and preferences of the respective embodiments.

[0142] In some embodiments, the power transmitter can include a calibrator arranged to calibrate the system for asymmetries in the induced signal. For example, as part of power transfer initialization, or for example periodically during power transfer, the system can perform a calibration. For example, the power transmitter can require a user input, allowing the user to confirm that no foreign object is present. It can then proceed to measure the current combined signal level, and it can generate a calibration value equal to the opposite value. Then, by adding the calibration value to the combined signal (i.e. subtracting the value measured during the foreign object detection test from the value measured during the calibration), a revised detection value can be generated, and detection can be based on the revised value. The calibration value can be time-varying for time-varying electromagnetic fields. In such examples, a simple static foreign object detection assessment based on the revised value can be used, for example detecting a foreign object if the absolute value of the revised signal exceeds a threshold.

[0143] Thus, while the approach can benefit from generating a uniform field, and in many embodiments the system can seek to make the uniform field as uniform as possible, and as constant as possible where possible, this is by no means a necessary or essential feature. Indeed, in many embodiments, the desired performance can be achieved by intentionally creating a non-uniform field and / or a time-varying field.

[0144] In Figure 2 In the system of FIG. 2, the combined voltage of the pair of detection coils 207, 209 is not directly measured and is used to perform foreign object detection. However, in other embodiments, the detection coils can for example be in series with a measurement transformer, such that the current through the detection coils 207, 209 also flows through the primary winding of the measurement transformer 211. Thus, the detection coils 207, 209 and the primary winding can be part of a series circuit through which the current induced in the detection coils 207, 209 flows. The foreign object detector 205 can be coupled to the secondary of the measurement transformer, and detect a foreign object for example when the secondary voltage of the measurement transformer exceeds a threshold.

[0145] It is noted that the primary winding of a transformer is the winding from which power / energy is extracted, and the secondary winding is the winding to which energy is delivered, i.e. energy is transferred from the primary winding to the secondary winding.

[0146] The circuit can include other components and elements, but in the specific example the coupling between the detection coils 207, 209 is low-ohmic. In most embodiments, the combined resistance of the coupling between the detection coils 207, 209 is less than 100 Ohms, and in many embodiments less than 50 Ohms, 10 Ohms, 5 Ohms, or in many embodiments even less than 1 Ohm. In many embodiments, the primary winding of the measurement transformer can be directly coupled to the detection coils 207, 209.

[0147] In such examples, the measurement transformer can be implemented as a current transformer rather than a voltage transformer. In particular, the measurement transformer can be arranged to have a winding ratio with the number of turns of the secondary winding being significantly higher than the number of turns of the primary winding. In many embodiments, the number of turns of the secondary winding is no less than 10, 20, 50 or 100 times the number of turns of the primary winding.

[0148] In the described system, the foreign object detector 205 is arranged to perform foreign object detection during a foreign object detection time interval based on a characteristic of the signal from the balance detection coil (received via the measurement transformer where possible). If the signal meets suitable foreign object detection criteria, it is determined that a foreign object is present, and if not, it is determined that a foreign object is not present. The specific foreign object detection criteria will depend on the specific preferences and requirements of individual embodiments. In many embodiments, it can be required that the signal amplitude from the balance detection coil is above a threshold, for example it can be required that the voltage and / or current amplitude of the output of the balance detection coil is above a threshold. The output of the balance detection coil can in particular be the voltage across the series connection formed by the individual detection coils / windings.

[0149] In many power transfer systems, in order to achieve efficient and reliable operation, a large amount of communication is employed between the power receiver and the power transmitter. In Figure 2 In the described system, some or all of the communication between the power transmitter and the power receiver is performed using a dedicated communication system / method. Thus, in this system, rather than modulating the power transfer signal, at least some of the communication is performed using a separate communication carrier, and in this example, the communication carrier is transmitted using a dedicated communication antenna.

[0150] In the described example, the power transmitter 101 comprises a communicator 211 coupled to the power transmitter controller 203 and a communication antenna 213. Thus, the communicator 211 is arranged to transmit and / or receive data to / from the power receiver 105. The communicator 211 can use a dedicated communication system / standard for communication, in particular a NFC communication system. The communicator 211 can be arranged to generate or receive the communication carrier and modulate / demodulate it, which will be known to the skilled person.

[0151] A challenge faced by many wireless power transfer systems is how to implement different functions in the same device without introducing negative cross effects and interference. A particular challenge is the presence of different antennas and coils, as the influence of the antennas on the electromagnetic field will affect all operations. In particular, foreign object detection tends to be very sensitive and based on very fine measurements. In particular, while the use of balance detection coils can allow for improved detection, it also makes the operation dependent on accurate and efficient balancing of the detection coils and the electromagnetic field through these coils.

[0152] These problems are often further exacerbated by the desire to implement the antenna in a small and compact structure. In particular, due to practical implementation reasons, it is often preferred to implement the foreign object detection coil and the communication antenna close together, e.g. as different layers of the same printed circuit board.

[0153] Figure 8 An example of a practical arrangement of the antennas / coils for the power transmitter and the power receiver is shown. The figure shows a cross section of the transmitter coil 103 and the receiver coil 107, respectively, of the power transmitter 101 and the power receiver 105, where the devices are positioned in the (optimal) configuration for power transfer. In this example, the power receiver 105 is positioned on top of the power transmitter 101.

[0154] The power transmitter 101 comprises a transmitter power transfer coil 103 generating a power transfer signal. In this example, the transmitter power transfer coil 103 is shown in cross section through two regions, reflecting the cross section of the regions comprising windings and reflecting a central region without windings. Figure 8

[0155] In this example, a foreign object detection antenna in the form of balanced detection coils 207, 209 is positioned on top of the transmitter power transfer coil 103, towards the power receiver 105. The transmitter power transfer coil 103 and the foreign object detection antenna 207 are distributed around a central axis 801.

[0156] In this example, a communication antenna 213 is further distributed in substantially the same plane as the foreign object detection coils. For example, the foreign object detection coils 207, 209 and the communication antenna 213 can be provided on different layers of the same printed circuit board (PCB).

[0157] Similarly, the power receiver 105 comprises a receiver power transfer coil 107 for receiving the power transfer signal. In this example, the receiver power transfer coil 107 is shown in cross section through two regions, reflecting the cross section of the regions comprising windings and reflecting a central region without windings. Figure 8

[0158] In a specific example, the receiver power transfer coil 107 is coaxial and symmetrically distributed around the same central axis 801 as the power transmitter 101 (reflecting the optimal positioning of the power receiver 105 on the power transmitter 101).

[0159] The power receiver 105 further comprises a power receiver communication antenna 803 arranged to support communication between the power transmitter 101 and the power receiver 105, and in particular coupled to the communication antenna 213 of the power transmitter 101. ​​

[0160] although Figure 8 An example is shown in which the power receiver is optimally positioned relative to the power transmitter such that the central axis 801 is common to the coil arrangement of the power transmitter and the power receiver. However, it should be understood that the power receiver is usually placed somewhat off-center and the central axis 801 is not perfectly aligned with the power receiver and the power transmitter.

[0161] Figure 8 This arrangement can be specifically used in cordless kitchen appliances, where two large power transmission coils 103 and 107 are used to transmit power from the transmitter to the receiver. A foreign object detection antenna 207 is placed between the power transmission coils 103 and 107.

[0162] This arrangement provides excellent coupling and good foreign object detection between the power transmission coils 103 and 107 because the foreign object detection antenna 207 is positioned close to the area between the power receiver and the power transmitter. It also provides effective communication performance because the communication antennas are positioned very close together and have strong coupling.

[0163] The problem with this system is that the antennas may interact / affect / interfere. Figure 8 In the example, power transmission coils 103, 107 can use magnetic shielding elements 805, 807 (which are configured such that the magnetic shielding is not critical to the power transmission signal (e.g., due to differences in electromagnetic field strength or frequency)) to electromagnetically shield communication antennas 203, 803 and foreign object detection coils 207, 209. However, the very close proximity of detection coils 207, 209 to communication antenna 213 leads to potential interference between them, especially since the proximity of communication antenna 213 may affect highly sensitive foreign object detection.

[0164] Specifically, during the foreign object detection time interval, the transmitter coil 103 generates a defined test signal (with a known coil current and the generated magnetic field), which is picked up by the detection coils 207 and 209. The detection coils detect changes in the magnetic field of the transmitter coil 103 caused by the foreign object. The positioning and design of the detection coils, as well as the overall magnetic environment, are crucial because the system is highly sensitive.

[0165] exist Figure 8 In this method, the communication antenna 213 is positioned in the same plane as the foreign object detection coil (essentially), and therefore they are very close to each other and are generally similar in size.

[0166] Although it has been proposed to use a balanced detection coil (such as...) Figure 6The inventors have recognised that the presence of a relatively close communication antenna tends to reduce performance and make foreign object detection more difficult, even for methods that use a plurality of detection coils (e.g. a plurality of balanced detection coils) and / or a plurality of communication coils (e.g. a plurality of communication coils in a loop). The inventors have recognised that this problem is exacerbated in many practical implementations because it is often desirable to be able to position the communication antenna and the foreign object detection coils very close to each other. The inventors have recognised that this effect can be reduced by configuring the communication antenna so that the presence of the communication antenna is at least partially balanced across the different balanced detection coils.

[0167] Improved performance can generally be achieved by using a communication antenna that comprises two communication coils in parallel, wherein the two coils are arranged so that they couple to two different detection coils so that the coupling from one communication coil to one detection coil is at least partially compensated by the coupling from the other communication coil to the other detection coil in the output of the balanced detection coil. In particular, the communication antenna is arranged so that a first section of the first communication coil has a first coupling to a first detection coil of the balanced detection coil and a second section of the second communication coil has a second coupling to a second detection coil of the balanced detection coil. Furthermore, the arrangement is such that the first coupling and the second coupling cancel each other out in the output signal.

[0168] The couplings can be inductive and / or capacitive couplings.

[0169] In some embodiments, the effect of the capacitive coupling between the first detection coil and the first section of the first communication coil will be compensated / reduced in the output signal of the balanced detection coil by the effect of the capacitive coupling between the second detection coil and the second section of the second communication coil.

[0170] In some embodiments, the effect of the inductive coupling between the first detection coil and the first section of the first communication coil will be compensated / reduced in the output signal of the balanced detection coil by the effect of the inductive coupling between the second detection coil and the second section of the second communication coil.

[0171] In many embodiments, the couplings can be both inductive and capacitive couplings.

[0172] In some embodiments, the effect of the inductive and capacitive coupling between the first detection coil and the first section of the first communication coil will be compensated / reduced / at least partially cancelled in the output signal of the balanced detection coil by the effect of the inductive and capacitive coupling between the second detection coil and the second section of the second communication coil.

[0173] Compensating the first coupling by the second coupling can cause a signal component in the output signal (from the set of balanced coils) due to the first coupling to be reduced by a signal component in the output signal due to the second coupling. Thus, a signal component induced in the first detection coil by the first coupling can be reduced by a signal component induced in the second detection coil by the second coupling. In case of two detection coils in series, e.g. in the example of Figure 6

[0174] Thus, the method can use two parallel communication coils symmetrically coupled with two detection coils such that the coupling effects / signal components from the different couplings cancel each other (partially or completely) in the combined output signal.

[0175] In many embodiments, the spatial arrangement of the first communication coil relative to the first detection coil corresponds / symmetrizes with the spatial arrangement of the second communication coil relative to the second detection coil.

[0176] In many embodiments, the spatial arrangement of the first communication coil can be the same as the spatial arrangement of the second communication coil. For example, the first communication coil can be transformed into the second communication coil using only translations, rotations and mirror transformations. Similarly, the spatial arrangement of the first detection coil is the same as the spatial arrangement of the second detection coil. For example, the first detection coil can be transformed into the second detection coil using only translations, rotations and mirror transformations. Furthermore, the position / orientation of the first communication coil relative to the first detection coil corresponds to the position / orientation of the first communication coil relative to the second detection coil.

[0177] In many embodiments, the spatial relationship between the first communication coil and the first detection coil thus corresponds to the spatial relationship between the second communication coil and the second detection coil. The spatial relationship can be the same, and in many embodiments, the spatial relationship can be that the combined spatial configuration of the first detection coil and the first communication coil can be transformed into the combined spatial configuration of the second detection coil and the second communication coil by a (geometric / spatial) transformation comprising only translations, rotations and mirror transformations.

[0178] In many embodiments, the inductive coupling between the first communication coil and the first detection coil is balanced (at least partially) with the inductive coupling between the second communication coil and the second detection coil.

[0179] ​In many embodiments, the communication antenna is arranged such that a first signal component induced in the first detection coil by the current in the first communication coil is compensated in the output signal by a second signal component induced in the second detection coil by the current in the second communication coil.

[0180] Thus, the arrangement of the two pairs of detection and communication coils can be such that the opposite current directions result in a reduction of the combined induced signal in the two detection coils, and the induced currents can be out of phase with each other, and in particular can have opposite phase. This can be achieved by the current direction in the first communication coil being opposite to the current direction in the first detection coil relative to the current direction in the second communication coil relative to the current direction in the second detection coil. The current directions do not necessarily reflect the actual currents, but reflect the nominal direction of the conductors such that a current flowing in this direction is considered a positive current, while a current flowing in the opposite direction is considered a negative current.

[0181] In many embodiments, the relative spatial arrangement can be substantially the same, but the current directions are opposite directions.

[0182] In many embodiments, the capacitive coupling between the first communication coil and the first detection coil is balanced (at least partially) with the capacitive coupling between the second communication coil and the second detection coil. In addition to the spatial arrangement, the communication antenna 213 can be arranged such that the voltage potentials and voltage distributions on the communication coils are respective and symmetrical with respect to the detection coils.

[0183] In some embodiments, the voltage potential of a section of the first communication coil matches and is substantially the same as the voltage potential of a second section of the second communication coil, where the two sections have the same spatial relationship with respect to the first and second detection coils, respectively.

[0184] In many embodiments, the voltage distribution in the first communication coil relative to the first detection coil is symmetrical with the voltage distribution in the second communication coil relative to the second detection coil. In this case, the capacitive coupling between the first detection coil and the pair of communication coils will be substantially the same as the capacitive coupling between the second detection coil and the pair of communication coils, if the spatial relationship is substantially the same.

[0185] In many embodiments, the first communication coil and the second communication coil are substantially identical and directly connected in parallel, resulting in substantially the same voltage distribution on the communication coils. Thus, sections of the two communication coils having the same distance (along the coil conductors) from the common connection point can have the same voltage potential (or the average voltage potential of the potential distribution / extended section).

[0186] Furthermore, in case the first and second detection coils are substantially identical, and the communication coil is positioned identically / symmetrically with respect to the respective detection coils, the detection coils can experience the same effective capacitive coupling as the parallel communication coil, resulting in the effects and signal components from the capacitive coupling substantially cancelling out in the combined output of the balanced detection coils.

[0187] It should be appreciated that while in most cases, full cancellation of the effects of the coupling is preferred, partial cancellation / compensation / reduction of the effects is often also beneficial.

[0188] In many embodiments, the communication antenna, in particular the communication coils, can be arranged such that the inductive and capacitive coupling is mitigated and compensated. In particular, in many embodiments, the communication coils can be arranged to include more, some or even all of the above considerations relating to spatial arrangement, current in the coils and voltage distribution in the coils.

[0189] In some embodiments, the above described spatial limitations and characteristics can extend to the communication coil as a whole, i.e. to the entire length / extent of the communication coil. In other embodiments, the relationship can be limited only to one or more segments of the communication coil, e.g. only to segments of the communication coil that overlap with the respective detection coil. For example, the described relationship between the first communication coil and the first detection coil can apply (in particular / only) to segments of the first communication coil that overlap (e.g. in a direction perpendicular to the plane of the first detection coil) with the first detection coil. Similarly, the described relationship between the second communication coil and the second detection coil can apply (in particular / only) to segments of the second communication coil that overlap (e.g. in a direction perpendicular to the plane of the second detection coil) with the second detection coil.

[0190] Thus, the method can use balanced detection coils and a communication antenna comprising a plurality of parallel communication coils to reduce the effects and effects of coupling between the foreign object detection coils and the communication antenna. In many embodiments, the method can allow for a significant improvement in foreign object detection and can provide for an improved implementation. For example, the limitations on the physical location of the coils can often be reduced, and the foreign object detection coils and the communication antenna can for example be allowed to be positioned in close proximity.

[0191] It should be noted that the coupling and resulting interference is typically only due to the physical presence of the communication antenna in proximity to the foreign object detection coils, and thus cannot be eliminated by operating the foreign object detection and the communication in different time intervals, i.e. by using a time division approach. However, the described method can mitigate the coupling even without employing time division.

[0192] In the described examples, the foreign object detection antenna uses two (or more) balanced detection coils, and these coils are capacitively / inductively coupled with the communication antenna. For a conventional communication antenna using a single coil, inductive coupling can introduce asymmetry. Furthermore, even if the inductive asymmetry is reduced or even completely eliminated, the varying voltage distribution over the communication antenna will mean that the balanced detection coils will be capacitively coupled with antenna segments having different voltages, resulting in an asymmetric capacitive coupling. Typically, the foreign object detection coils are carefully designed to achieve very accurate and sensitive detection, and so they are subject to significant design constraints. However, by using multiple parallel communication coils, these coils can be designed to exhibit not only symmetric inductive coupling, but also to provide a more symmetric voltage distribution with respect to the detection coils, and so exhibit a more symmetric capacitive coupling. In many embodiments, this can allow for a significant improvement in foreign object detection.

[0193] In many embodiments, the foreign object detection coils and the communication antenna / coils can be planar antennas / coils.

[0194] In particular, the set of balanced detection coils are typically formed within a first plane, and the communication coils are planar coils formed within a second plane, with the first and second planes being substantially parallel. In most embodiments, the angle between the planes can be no more than 10°, 5°, 3° or 1°.

[0195] In many embodiments, the antennas / coils can be implemented as layers on a printed circuit board (PCB), and indeed, in many embodiments, the foreign object detection coils and the communication antenna can be implemented within different layers of the same PCB.

[0196] Figure 6 An example is shown of a set of balanced detection coils implemented as planar coils within a single plane. These coils can be implemented within a layer of a PCB.

[0197] In other embodiments, multiple sets of balanced detection coils can be used, and the foreign object detection can be arranged to perform foreign object detection based on output signals from one or more of the sets of balanced detection coils. Figure 9 An example is shown of a planar arrangement of three sets of balanced detection coils, with each set of balanced detection coils comprising two detection coils arranged opposite each other.

[0198] In Figure 9In an example, the power transmitter can be generated to include three sets of balanced detection coils, each set including two detection coils. In this example, foreign object detection can measure the output signals from each of the three balanced detection coil pairs and use these to perform foreign object detection. The exact criteria used will depend on the preferences and requirements of individual embodiments. As an example of low complexity, a foreign object can be determined to be detected if at least one of these sets of balanced detection coils generates a signal that exceeds a given threshold. In some embodiments, different signals can be evaluated and compared, e.g. to generate a position estimate for the detected foreign object.

[0199] Each of the spatial detection coils spans or encompasses a region of the plane in which the coil is formed. In this example, each detection coil has a substantially circular sector profile, and thus spans a circular sector region. It will be appreciated that in other embodiments the profile and shape can be different. For example, the outer sector can be linear rather than curved / circular (e.g. the shape can be triangular). As another example, in some embodiments the profile of the detection coils can be substantially rectangular.

[0200] In this example, the detection coils in a set of balanced detection coils are rotationally symmetric about the point of rotation. Indeed, in this example the detection coils in different sets of balanced detection coils are rotationally symmetric. In many embodiments, as in those of Figure 6 and Figure 9 one detection coil thus spans a certain angular interval, and different detection coils 207, 209 span different, non-overlapping angular intervals from the point of rotation. In this example, additionally, the detection coils are mirror symmetric about a central radial line extending from the point of rotation at an angle that is the midpoint of the angular interval covered by the detection coils.

[0201] In a specific example, for a set of balanced detection coils there is thus a high symmetry between the detection coils, and indeed there is a high symmetry between the detection coils from different sets of balanced detection coils.

[0202] The two communication coils in parallel can be designed to have a corresponding symmetry with respect to the detection coils, such that the effect of the coupling between one of the communication coils and one of the detection coils is at least partially compensated by the corresponding coupling between the other communication coil and the other one of the balanced detection coils.

[0203] In many embodiments, the communication coils can be arranged to have a symmetry like the detection coils. In particular, the arrangement of the first communication coil and the first detection coil can be transformed to match the second communication coil and the second detection coil. In this example, the communication coils can be rotationally symmetric around the same point of rotation around which the detection coils are rotationally symmetric. The rotation in which the symmetry occurs can also be the same, i.e. if a rotation of angle X transforms the first detection coil into the second detection coil, then a rotation of the same angle X also transforms the first communication coil into the second communication coil. In some embodiments, the transformation can specifically be limited to only include rotation, translation and mirror transformations or symmetries.

[0204] In embodiments in which the communication antennas and the foreign object detection coils are formed in two parallel planes (which can be very close to each other), the symmetry can generally be considered as two-dimensional symmetry. The coils can specifically overlap each other.

[0205] The spatial considerations of the arrangement can be considered by considering the projection of the communication coils on the (plane of) the foreign object detection coils, and in particular the projection is a vertical or orthogonal projection along a direction perpendicular to the plane in which the foreign object detection coils and / or the communication coils are formed. It should be noted that in practice, the distance between the planes is typically very small compared to the dimensions of the coils, and the projection tends to be insignificant, and in practice the coils can essentially be considered to be formed in the same plane.

[0206] The detection coils span (cover / draw / encircle) areas in the plane in which the detection coils are formed. In many cases, the communication coils will be such that the communication coils overlap with these areas, and thus the vertical projection falls within the spanned area. The coupling considerations described earlier can specifically apply between a segment of a communication coil within a given spanned area and the detection coil spanning that area. In many areas, the communication coils can be arranged such that the projected segment of a first communication coil within an area spanned by a first detection coil of a set of balancing detection coils is the same as the projected segment of a second communication coil within an area spanned by a second detection coil of the set of balancing detection coils. This will generally result in matching coupling effects that will be cancelled in the combined signal from the set of balancing detection coils for a planar coil arrangement.

[0207] Thus, in many embodiments, the first detection coil spans the first region in a first plane, the second detection coil spans the second region in the same plane, and the communication coil is arranged such that the orthogonal / vertical projection of the segment of the first communication coil on the first plane has the same spatial relationship to the first region as the orthogonal / vertical projection of the segment of the second communication coil on the first plane has to the second region. In particular, the orthogonal / vertical projection of the first communication coil on the first region can match the orthogonal / vertical projection of the second communication coil on the second region.

[0208] In the following, an example of a specific implementation of a pair of communication coils that meets most or all of the considerations described above will be described. The example will be described with reference to Figures 10 to 15 The advantage of this specific design is that it can be implemented in very few layers, and in particular, in many embodiments, it can be implemented in only two layers, e.g. different layers of a printed circuit board.

[0209] The design is based on distributing segments of the communication coil along concentric curves around a point of rotation. Figure 10 A set of concentric circles is shown, which can be used as a lower layer template or grid for the configuration of the communication coil. While this example shows a set of concentric circles that are equidistant, it should be understood that other closed curves and other distances between these curves can be used in other embodiments. However, in many designs, the closed curves can be rotationally symmetric around a point of rotation so as to rotate at an angle that corresponds to the angle between the two detection coils. In many embodiments, the closed curves can have the same rotational symmetry as the detection coils. Thus, for a rotation angle that results in the same configuration of the detection coils, the rotation of the closed curves will also result in the same configuration.

[0210] A design of a communication coil that is suitable for Figure 9 a detection coil of the type described above will be described, Figure 10 The outline of the angular portion of each of the six detection coils is shown by a radial line from the central point of rotation.

[0211] The formation of the first communication coil can be illustrated by considering a series of steps. First, as Figure 11 shown, a set of segments can be distributed along the curves. In this example, the communication coil follows one of the closed curves in each angular interval that corresponds to a detection coil. Thus, in this example, the first communication coil comprises a segment that corresponds to each angular interval, each segment following one of the concentric circular curves. For the concentric circles, each segment is a segment of a circle.

[0212] Further, in this example, the segments within adjacent angular intervals are along different concentric curves, so when transitioning from one angular interval to another (and thus from one detection coil to another), the coil also transitions from one concentric curve to another. The transition from one concentric curve to another is in many examples formed by a segment that is radial to the point of rotation. In this example, a first segment, distributed along an outer closed curve, is connected to a second segment, distributed along an adjacent closed curve, via a transition segment that is radial to the point of rotation and along the boundary of the two detection coils. Similarly, the second segment is connected to a third segment along a next inner closed curve via another radially interconnected transition segment.

[0213] Figure 11 Four partial circular segments are shown, corresponding to four detection coil regions distributed along four concentric circles. Then, as shown, the arrangement can be repeated with a rotation corresponding to the four detection coils. The end of the inner segment (the last segment of the first repetition) can be coupled to the outer segment of the second repetition (the first segment of the second repetition). This process can then be repeated to produce a pattern of segments of a third repetition, resulting in the coil arrangement shown in Figure 12 Figure 13 Figure 14 As shown, the end points of the coils can be fed to the center of the coils by radial connections, so as to provide a central connection point for the coils.

[0214] A second communication coil can be generated with the same pattern / shape / space configuration as the first coil, but rotated 180° around the point of rotation. Thus, the first and second communication coils are rotationally symmetric around the point of rotation. Further, due to this rotation, the parts of the concentric closed curves that are not occupied by the first communication coil are occupied by the second communication coil, and no part of the curves is occupied by both communication coils, that is, there is no overlap between them.

[0215] Figure 15 The resulting communication antenna 213 is shown, formed from two communication coils in parallel (the second communication coil is shown by dashed lines).

[0216] It can be seen that this method results in each angular interval comprising a plurality of segments, distributed along different closed curves of each of the first and second communication coils. The segments are connected via substantially radial connections.

[0217] ​​Furthermore, the coils are rotationally symmetric with respect to the detection coils, such that for two of the set of balanced detection coils, the detection coils are rotationally symmetric, and indeed, if the coils are switched, the same is true for the spatial relationship with respect to the communication coils. Thus, the spatial configuration of the first communication coil with respect to the first detection coil, in particular within the angular interval of the first detection coil, is the same as the spatial configuration of the second communication coil with respect to the second detection coil, in particular within the angular interval of the second detection coil. Similarly and additionally, the spatial configuration of the second communication coil with respect to the first detection coil, in particular within the angular interval of the first detection coil, is the same as the spatial configuration of the first communication coil with respect to the second detection coil, in particular within the angular interval of the second detection coil.

[0218] Thus, full symmetry is achieved between the first and second communication coils and the first and second detection coils, respectively. Furthermore, the communication coils are in parallel, and apart from possible actual deviations affecting the balance, there will be the same current and voltage distribution for the first and second communication coils. Thus, the coupling effect between the first communication coil and the first detection coil will be the same as the coupling effect between the second communication coil and the second detection coil. Similarly, the coupling effect between the first communication coil and the second detection coil will be the same as the coupling effect between the second communication coil and the first detection coil. Since the currents in the two communication coils are identical, and since the detection coils have a current direction that is different from the current direction in the communication coils, as shown in Figure 6 and Figure 9 the induced electromotive forces in the detection coils by the currents in the detection coils will cancel each other out. Similarly, for the two detection coils, not only the spatial configuration, but also the voltage potential along the segment is the same, so the capacitive coupling will be the same, and the effect of the capacitive coupling will be cancelled out. Thus, a substantially more accurate foreign object detection can be made.

[0219] Another substantial advantage is that the method allows for a very efficient implementation of the antenna of the wireless power transfer system. The communication antenna can be positioned very close to the foreign object detection coil, and in many embodiments can be implemented on different layers of a single PCB. Indeed, the described arrangement allows for both the foreign object detection coil and the communication coil to be implemented by a single two-layer PCB. Thus, a very compact and practical implementation can be achieved. This can further allow for positioning the coils between the transmitter coil and the receiver coil, for example in the example of Figure 8 while still allowing for a very good coupling for the power transfer.

[0220] Figure 16 a practical design of foreign object detection coils and communication coils of different layers of a single PCB is shown,Figure 17 A photograph showing an implementation of such a PCB adjacent to a high power transmitter coil.

[0221] It will be appreciated that the above described methods can result in many other possible implementations and layouts of the communication coil, and of the communication coil in particular. For example, Figure 18 Examples are shown corresponding to Figure 15 a system implementing eight foreign object detection coils, and Figure 19 Examples are shown corresponding to another possible layout of the communication coil, which can be used for both six and eight foreign object detection coils where possible.

[0222] It should be appreciated that the above description maybe described with reference to different functional circuitry, units and processors for clarity. However, it is apparent that any suitable distribution of functionality between different functional circuitry, units or processors can be used without departing from the application. For example, functionality illustrated to be performed by distinct processors or controllers can be performed by the same processor. Hence, references to specific functional units or circuitry are only to be seen as references to suitable means for providing the described functionality, and do not imply a strict logical or physical structure or organization.

[0223] The present application can be implemented in any suitable form including hardware, software, firmware or any combination of these. The present application can optionally be implemented at least partly as computer software running on one or more data processors and / or digital signal processors. The elements and components of an embodiment of the application can be physically, functionally and logically implemented in any suitable way. Indeed the functionality can be implemented in a single unit, in a plurality of units or as part of other functional units. As such, the application can be implemented in a single unit or can be physically and functionally distributed between different units, circuitry and processors.

[0224] Although the present application has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present application is limited only by the claims. Additionally, although various features can have been described in conjunction with a specific embodiment, one of ordinary skill in the art will recognize that variations and / or modifications of the described features can be implemented in accordance with the application. In the claims, the term comprising does not exclude the presence of other elements or steps than those claimed.

[0225] Furthermore, although items, elements or circuits can be described or claimed in the singular, the plural is contemplated to be covered by this disclosure unless otherwise stated. Furthermore, to the extent that the terms "including", "includes", "having", "has", "with", or variants thereof are used in either the detailed description and / or the claims, such terms are used expansively and each means for the sake of brevity, that at least one member of the named group is either present or is to be considered present. To the extent that any term in the claims is dependent on a preceding element, such term independent of the preceding element is meant to cover its meaning in the context of the claims. To the extent that any term in the claims is dependent on a preceding element, such term independent of the preceding element is meant to cover its meaning in the context of the claims. To the extent that the term "in response to" is used in the description or claims, such term is used to indicate a response to a previous action, event, or the like. To the extent that the term "substantially" is used in the description or claims, such term is used to indicate that a situation, action, event, or the like, is intended to be understood as being close to, but not necessarily exactly, the situation, action, event, or the like, that is described. To the extent that the term "substantially" is used in the description or claims, such term is used to indicate that a situation, action, event, or the like, is intended to be understood as being close to, but not necessarily exactly, the situation, action, event, or the like, that is described. To the extent that the term "substantially" is used in the description or claims, such term is used to indicate that a situation, action, event, or the like, is intended to be understood as being close to, but not necessarily exactly, the situation, action, event, or the like, that is described. To the extent that the term "substantially" is used in the description or claims, such term is used to indicate that a situation, action, event, or the like, is intended to be understood as being close to, but not necessarily exactly, the situation, action, event, or the like, that is described.

Claims

1. A power transmitter for transmitting power to a power receiver (105) via an inductive power transfer signal, the power transmitter (101) comprising: a transmitter coil (103) arranged to generate an electromagnetic test field for foreign object detection; a set of balanced detection coils (207, 209) comprising a first detection coil and a second detection coil, the first and second detection coils being connected in series and such that signals induced in the first and second detection coils by the electromagnetic test field cancel each other out; a foreign object detector (205) coupled to the set of balanced detection coils and arranged to perform foreign object detection, the foreign object detector (205) being arranged to detect a foreign object in response to a characteristic of an output signal from the set of balanced detection coils meeting foreign object detection criteria; a communication antenna; a communicator coupled to the communication antenna and arranged to communicate with the power receiver via the communication antenna; wherein the communication antenna comprises at least a first communication coil and a second communication coil connected in parallel, the communication antenna being arranged such that a first section of the first communication coil has a first coupling with the first detection coil and a second section of the second communication coil has a second coupling with the second detection coil, the first and second couplings being at least one of a capacitive coupling and an inductive coupling, and the first and second couplings cancel each other out in the output signal.

2. The power transmitter of claim 1, wherein, the first coupling comprises both a capacitive coupling and an inductive coupling, and the second coupling comprises both a capacitive coupling and an inductive coupling.

3. The power transmitter of claim 1 or 2, wherein, the communication antenna is arranged such that a first signal component induced in the first detection coil by a current in the first communication coil is cancelled in the output signal by a second signal component induced in the second detection coil by a current in the second communication coil.

4. The power transmitter of claim 1 or 2, wherein, a direction of current in the first section relative to a direction of current in the first detection coil is opposite to a direction of current in the second section relative to a direction of current in the second detection coil.

5. The power transmitter of claim 1 or 2, wherein, the first and second communication coils are arranged such that a spatial relationship between the first communication coil and the first detection coil corresponds to a spatial relationship between the second communication coil and the second detection coil.

6. The power transmitter of claim 1 or 2, wherein, a voltage potential of the first section matches a voltage potential of the second section.

7. The power transmitter of claim 1 or 2, wherein, the first and second communication coils have the same spatial configuration, and the first and second sections are corresponding sections of the first and second detection coils.

8. The power transmitter of claim 1 or 2, wherein, the set of balanced detection coils are formed in a first plane, and the communication coils are planar coils formed in a second plane parallel to the first plane.

9. The power transmitter of claim 8, wherein, The first detection coil spans a first area in the first plane, the second detection coil spans a second area in the first plane, and the orthogonal projection of the first segment on the first plane has the same spatial relationship to the first area as the orthogonal projection of the second segment on the first plane has to the second area.

10. The power transmitter of claim 8, wherein, The first detection coil spans a first area in the first plane, the second detection coil spans a second area in the first plane, and the orthogonal projection of the first segment on the first plane has the same spatial relationship to the first area as the orthogonal projection of the second segment on the first plane has to the second area.

11. The power transmitter of claim 8, wherein, The first detection coil and the second detection coil are rotationally symmetric around a point of rotation, and the first segment and the second segment are also rotationally symmetric around the point of rotation.

12. The power transmitter of claim 8, wherein, The power transmitter comprises a plurality of sets of balanced detection coils, each set of balanced detection coils comprising at least two detection coils, the detection coils being rotationally symmetric around a point of rotation, and each detection coil spanning an angular interval; and each of the first communication coil and the second communication coil comprises segments that are distributed along a concentric closed curve around the point of rotation and that are rotationally symmetric around the point of rotation between the angular intervals of the detection coils of one set of balanced detection coils; and segments located within an angular interval spanned by one detection coil of one set of balanced detection coils are rotationally symmetric to segments located within an angular interval spanned by another detection coil of another set of balanced detection coils.

13. The power transmitter of claim 12, wherein, Each angular interval comprises a plurality of segments that are distributed along different closed curves for each of the first communication coil and the second communication coil.

14. The power transmitter of any one of claims 12 to 13, wherein, Different segments of adjacent angular intervals are distributed along different curves of the concentric closed curve.

15. A method of transmitting power from a power transmitter to a power receiver (105) via an inductive power transfer signal, the method comprising: providing a transmitter coil (103) for generating an electromagnetic test field for foreign object detection; providing a set of balanced detection coils (207, 209) comprising a first detection coil and a second detection coil, the first detection coil and the second detection coil being connected in series and arranged such that signals induced in the first detection coil and the second detection coil by the electromagnetic test field cancel each other out; providing a foreign object detector (205) coupled to the set of balanced detection coils and arranged to perform foreign object detection, the foreign object detector (205) being arranged to detect a foreign object in response to a characteristic of an output signal from the set of balanced detection coils meeting foreign object detection criteria; providing a communication antenna; and providing a communicator coupled to the communication antenna and arranged to communicate with the power receiver via the communication antenna; wherein the communication antenna comprises at least a first communication coil and a second communication coil, the second communication coil being in parallel with the first communication coil, the communication antenna being arranged such that a first section of the first communication coil has a first coupling with the first detection coil, a second section of the second communication coil has a second coupling with the second detection coil, the first coupling and the second coupling being at least one of a capacitive coupling and an inductive coupling, and the first coupling and the second coupling are mutually compensating in the output signal.

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