Wireless power transfer system

Abstract

Disclosed is a wireless power transmitter (5) comprising a transmitter coil configured to transmit a wireless power transfer signal, the wireless power transfer signal being transmitted using a magnetic charging field configured for transmission of power, and a transmitter control unit configured to perform a Foreign Object Detection comprising obtaining one or more measurements on power harvested from the wireless power transmitter, wherein the wireless power transmitter is configured to transmit a signal at predetermined time intervals, the signal indicating a synchronisation time, wherein the transmitter control unit is configured to perform the one or more measurements on harvested power during one or more Foreign Object Detection time periods occurring at predetermined time intervals relative to the synchronisation time, and wherein the transmitter control unit is configured to determine as part of its Foreign Object Detection that associated power receivers do not harvest power from it during the Foreign Object Detection time periods and / or that associated power receivers substantially reduce power harvesting from it during the FOD periods.

Description

[0001] WIRELESS POWER TRANSFER SYSTEM

[0002] Field of the invention

[0003] The invention relates to a wireless power transfer system and Foreign Object Detection in a wireless power transfer system, in particularly for use in a toy system.

[0004] Background

[0005] Power transmission via magnetic induction is by now well-known, and is utilised widely to charge power receiver devices having one or more receiver coils. When charging of a power receiver device is desired, the device is placed such that at least one of its receiver coils can couple to a transmitter coil in a wireless power transmitter device. Such an arrangement allows for wireless power transfer between the power transmitter device and the power receiver device without requiring any wires or physical electrical connections between the transmitter and receiver devices to be made.

[0006] Wireless power transfer arrangements may be designed such that a power transmitter device can be used with a variety of power receiver devices, for example by configuring the devices according to standards such as e.g. the Qi Specifications developed by the Wireless Power Consortium.

[0007] A known concern in wireless power transfer systems is the unintentional transfer of power to objects near or on the power transmitter, such as conductive objects. For example, if a metal object, such as e.g. a coin, key, ring etc., is placed near the power transmitter, magnetic flux generated by the transmitter coil may introduce eddy currents in the metal objects causing the object to heat up, potentially to significant temperatures. For this reason, Foreign Object Detection has been introduced whereby the power transmitter attempts to detect the presence of a Foreign Object. In response to such a detection the power transmitter may act, for example by reducing charging power or turning off the charging field.

[0008] A number of methods for detecting Foreign Objects both before initiating power transfer and while power transfer is in progress are known, see e.g. the Qi power transfer standard. An example of the latter is basic power loss accounting, whereby the power transmitter estimates the amount of power dissipated in Foreign Objects by obtaining data from power receivers on their received power and comparing the obtained data with its own estimate of the power transmitted. For this, each power receiver must estimate 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 such as losses of the rectifier, the receiver coil, metal parts being part of the receiver etc. The power receiver may then report the determined received power as an average received power level over a time window at regular time intervals, e.g. every four seconds, to the power transmitter. 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 estimation of the internal power losses in the transmitter, such as e.g. the estimated power loss in the inverter, the primary coil, and metal parts that are part of the power transmitter. From this information, 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 may conclude or determine that too much power is dissipated in a Foreign Object, and it can then proceed to e.g. reduce charging power or abort power transfer.

[0009] Another example of Foreign Object Detection is provided in US2023 / 0017317 A1 which discloses a charging system wherein the power receiver sends a synchronisation message to the wireless power transmitter. The synchronisation message indicates repeating time intervals of time periods of Foreign Object Detection and time periods of wireless power transfer, and the Foreign Object Detection time periods correspond to periods of reduced power being transmitted by the power transmitter to improve detection of Foreign Objects. The power receiver communicates power that it received from the wireless power transmitter during the Foreign Object Detection time periods.

[0010] A factor complicating Foreign Object Detection (FOD) is the presence of Friendly Metals in the magnetic field. A Friendly Metal is similar to a Foreign Object in that it can dissipate power from the magnetic field, but unlike a Foreign Object, it is an integral part of the power receiver or power transmitter. For this reason, a power transmitter can advantageously use multiple FOD methods to maximize the probability of detecting Foreign Objects, while minimizing the probability of false alarms.

[0011] Hence, it remains desirable to implement Foreign Object Detection (FOD) in power transfer systems, and preferable multiple methods for Foreign Object Detection.

[0012] Generally, improved operation of a power transfer system would be advantageous, such as an approach allowing increased flexibility, reduced cost, reduced complexity, improved Foreign Object Detection and / or improved performance would be advantageous.

[0013] In systems where a high degree of safety is wanted, such as toy systems, it is particularly desirable to have a reliable Foreign Object Detection (FOD) procedure in place.

[0014] Additionally, a power transfer system suitable for use in toy systems may advantageously be user-friendly, even for smaller children, and either require no user involvement at all, or if any, only a user involvement that can be accomplished by smaller children, for example taking into consideration their available cognitive skills and hand control.

[0015] Various aspects of embodiments of a wireless power transfer system and Foreign Object Detection in a wireless power transfer system disclosed herein address one or more of the above needs and / or other needs that exist in the field of wireless power transfer systems.

[0016] Summary

[0017] Disclosed herein are aspects related to wireless power transmitters, wireless power receivers, wireless power transfer systems, methods of wirelessly transmitting power, a method of harvesting wirelessly transmitted power, a method of communicating wirelessly received power, and methods of wirelessly transmitting power from a wireless power transmitter to one or more wireless power receivers. In the aspects, the terms and features relate to the terms and features having the same name in the other aspects and therefore the descriptions and explanations of terms and features given in one aspect apply to the other aspects. Parts of the disclosure relates to utilising a charging field signature which is a characteristic of the magnetic charging field that is a measurable characteristic by a wireless power receiver. A charging field signature may be used for coordination between a power transmitter and a power receiver, and may be particularly useful in systems where it is desirable that the wireless power transmitter and receiver(s) do not communicate directly with each other about the coordination. Alternatively, or additionally, a charging field signature may be used as an identifier of a wireless power transmitter, possibly as a temporary identifier.

[0018] In the following is described embodiments, wherein FOD in a wireless power transfer system relies on status messages from wireless power receivers and a charging field signature being used for identification of a wireless power transmitter.

[0019] According to an aspect, a wireless power transmitter disclosed herein comprises: a transmitter coil configured to transmit a wireless power transfer signal, the wireless power transfer signal being transmitted using a magnetic charging field configured for transmission of power, a transmitter communication unit configured for obtaining one or more wirelessly transmitted status messages, each status message comprising data on a received power level, and a transmitter control unit configured to:

[0020] - determine a transmitted power level, where the transmitted power level is the level of wireless power transmitted by the wireless power transmitter,

[0021] - determine a total received power, and

[0022] - determine a Foreign Object Loss based on the total received power and the transmitted power level.

[0023] The wireless power transfer signal comprises a transmitter charging field signature, and each status message further comprises data on an associated charging field signature. The determination of the total received power is based on the obtained data on received power level(s) for which the associated charging field signature corresponds to the transmitter charging field signature. A wireless power transmitter may also be referred to as a charger or a charging device. It is configured for wirelessly transmitting power to a wireless power receiver.

[0024] The wireless power transmitter may be configured to transmit a wireless power transfer signal using radio frequencies that are designated for ISM applications, i.e. to use an ISM band. The wireless power transmitter may be configured to transmit a wireless power transfer signal having a centre frequency in the frequency range 100 kHz - 50 MHz. For example, the wireless power transmitter may be configured to transmit a wireless power transfer signal having a centre frequency in the frequency range 15 - 50 MHz, such as 18 - 45 MHz, such as 21 - 40 MHz, such as 25 - 35 MHz. In some embodiments, the wireless power transmitter is configured to transmit a wireless power transfer signal having a centre frequency in the frequency range 100 kHz - 200 kHz. In some embodiments, the wireless power transmitter is configured to transmit a wireless power transfer signal having a centre frequency in the frequency range 5 MHz - 8.5 MHz. In some embodiments, the wireless power transmitter is configured to transmit a wireless power transfer signal having a centre frequency in the frequency range 12 MHz - 15 MHz. The wireless power transmitter may be configured for transmission of wireless power transfer signals at more than one frequency. In some embodiments, the wireless power transmitter is configured to transmit a wireless power transfer signal having a centre frequency in an ISM band comprising 140 kHz, and / or an ISM band comprising 6.78 MHz, and / or an ISM band comprising 13.56 MHz, and / or an ISM band comprising 27.12 MHz, where, for a given centre frequency, the wireless power transfer signal may be configured to comply with a regulated bandwidth of an ISM band comprising the given centre frequency. For example, the wireless power transmitter may be configured to transmit a wireless power transfer signal having a centre frequency centred at or near 140 kHz, and / or centred at or near 6.78 MHz, and / or centred at or near 13.56 MHz, and / or centred at or near 27.12 MHz, where, for a given centre frequency, the wireless power transfer signal may be configured to comply with a regulated bandwidth of an ISM band comprising the given centre frequency.

[0025] In some embodiments, the transmitter charging field signature is a measurable characteristic of the wireless power transfer signal. The transmitter charging field signature may comprise a periodic component. For example, the wireless power transmitter may be configured to regulate the power of the wireless power transfer signal at regular time intervals, and the transmitter charging field signature is defined at least in part by the regular time intervals of the power of the wireless power transfer signal. The regulation of the power of the wireless power transfer signal may be effectuated by a regulation of the magnetic charging field, which again is done by regulating the amplitude of the alternating current in the transmitter coil. The transmitter control unit may control the regulation of the magnetic charging field by regulating the amplitude of the alternating current in the transmitter coil. The regulation of the power of the wireless power transfer signal may be measured e.g. by a wireless power receiver receiving power from the wireless power transmitter. In some embodiments, the transmitter charging field signature is defined by one or more of: a length of time, where the power of the wireless power transfer signal is reduced compared to a previous amplitude, such as an average of a plurality of previous amplitudes, a length of time, where the power of the wireless power transfer signal is increased compared to a previous amplitude, such as an average of a plurality of previous amplitudes, a length of time, where the wireless power transfer signal is turned off, and / or a length of time, where the wireless power transfer signal is turned on.

[0026] In some embodiments, the transmitter communication unit is configured for obtaining the one or more status messages as a broadcasted data stream, where a broadcasted data stream is a data stream that is provided as a wireless signal by a broadcast device. The data stream is provided using a broadcast standard, which means that it does not require pairing between broadcast devices and receiving device. In some embodiments, the communication range of the broadcast protocol used is more than 50 cm, such as more than 75 cm, such as more than 1 m, such as more than 2 m, such as more than 4 m, such as more than 6 m, such as more than 8 m, such as more than 10 m (meters). The communication range may be a standard communication range of the communication protocol used, as the actual communication range in a given situation may be affected by interference as is well known. In some embodiments, the transmitter communication unit is configured for obtaining the one or more status messages via a short-range protocol, such as NFC. In some embodiments, the wireless power transmitter is configured to select the transmitter charging field signature upon start-up, and, optionally, the transmitter charging field signature is selected from a predetermined set of possible charging field signatures and / or the transmitter charging field signature is selected after a period of time, wherein the wireless power transmitter has listened for status messages reporting charging field signatures already in use. For example, by the wireless power transmitter listening to status messages that are broadcasted by wireless power receivers. As the wireless power transmitter is able to obtain wirelessly transmitted status messages from wireless power receivers within a communication range, the wireless power transmitter may obtain status messages prior to turning on its own charging field and from the obtained status messages determine transmitter charging field signatures already being used by other wireless power transmitters within the communication range. The wireless power transmitter may be further configured to select a transmitter charging field signature that provides the most efficient charging, such as a transmitter charging field signature out of those available that provides for the more time on average, wherein a wireless power receiver may receive power.

[0027] In some embodiments, the wireless power transmitter is configured to reduce power output or shut down power transmission, if the determined Foreign Object Loss exceeds a predetermined threshold indicating that significantly more power is being transmitted by the wireless power transmitter than is being reported in obtained status messages.

[0028] In some embodiments, the wireless power transmitter is configured to reset the transmitter charging field signature, such as select a new transmitter charging field, if the determined Foreign Object Loss exceeds a predetermined threshold which indicates that more power is being received than is being transmitted by the wireless power transmitter. Such a situation may occur, if more than one wireless power transmitter within communication range of the status messages of each other are using the same transmitter charging field signature. In this case, one or more of the wireless power transmitters may arrive at a Foreign Object Loss that indicates that more power is being received than they are each themselves transmitting. Thus, they can reason out the presence of one or more other wireless power transmitters without communicating directly with them. This allows a power transmitter to act accordingly, for example with respect to selection of a transmitter charging field signature.

[0029] In some embodiments, the wireless power transmitter is part of a toy set, such as a toy construction set.

[0030] According to an aspect, a wireless power receiver disclosed herein comprises: a receiver coil configured to receive a wireless power transfer signal, a load coupled to the receiver coil, the load being configured to receive power from the receiver coil, a receiver control unit configured to determine a received power level, where the received power level is the level of wireless power received, and a receiver communication unit configured to wirelessly transmit a status message, the status message comprising the received power level.

[0031] The receiver control unit is further configured to determine a transmitter charging field signature associated with the received power, and the status message further comprises data on the associated charging field signature.

[0032] The wireless power receiver may be configured to receive power from a wireless power transfer signal using radio frequencies that are designated for ISM applications, i.e. using an ISM band. The wireless power receiver may be configured to receive, and charge from, a wireless power transfer signal having a centre frequency in the frequency range 100 kHz - 50 MHz. For example, the wireless power receiver may be configured to receive, and charge from, a wireless power transfer signal having a centre frequency in the frequency range 15 - 50 MHz, such as 18 - 45 MHz, such as 21 - 40 MHz, such as 25 - 35 MHz. In some embodiments, the wireless power receiver is configured to receive power from a wireless power transfer signal having a centre frequency in the frequency range 100 kHz - 200 kHz. In some embodiments, the wireless power receiver is configured to receive power from a wireless power transfer signal having a centre frequency in the frequency range 5 MHz - 8.5 MHz. In some embodiments, the wireless power receiver is configured to receive power from a wireless power transfer signal having a centre frequency in the frequency range 12 MHz - 15 MHz. The wireless power receiver may be configured to charge from more than one centre frequency of a wireless power transfer signal, each at a different time. In some embodiments, the wireless power receiver is configured to receive a wireless power transfer signal having a centre frequency in an ISM band comprising 140 kHz, and / or an ISM band comprising 6.78 MHz, and / or an ISM band comprising 13.56 MHz, and / or an ISM band comprising 27.12 MHz, where, for a given centre frequency, the wireless power transfer signal may be configured to comply with a regulated bandwidth of an ISM band comprising the given centre frequency. For example, the wireless power receiver may be configured to receive power from a wireless power transfer signal having a centre frequency centred at or near 140 kHz, and / or centred at or near 6.78 MHz, and / or centred at or near 13.56 MHz, and / or centred at or near 27.12 MHz, where, for a given centre frequency, the wireless power transfer signal may be configured to comply with a regulated bandwidth of an ISM band comprising the given centre frequency. The wireless power receiver may be configured to be able to receive power from a wireless power transfer signal comprising radio frequencies that are higher than those used in the 13.56 MHz RFID band, and the wireless power receiver may further comprise an NFC / RFID tag reader.

[0033] In some embodiments, the transmitter charging field signature is a measurable characteristic of the wireless power transfer signal, i.e. on that can be measured by a wireless power receiver. The transmitter charging field signature may comprise a periodic component. In some embodiments, the transmitter charging field signature is defined by one or more of: a length of time, where the power of the wireless power transfer signal is reduced compared to a previous amplitude, such as an average of a plurality of previous amplitudes, a length of time, where the power of the wireless power transfer signal is increased compared to a previous amplitude, such as an average of a plurality of previous amplitudes, a length of time, where the wireless power transfer signal is turned off, and / or a length of time, where the wireless power transfer signal is turned on.

[0034] In some embodiments, the receiver communication unit is configured for transmitting the status message as a broadcasted data stream. A broadcasted data stream is a data stream that is provided as a wireless signal by a broadcast device. The data stream is provided using a broadcast standard, which means that it does not require pairing between broadcast devices and receiving device. Thus, the wireless power receiver may simply broadcast the status message, which means that the wireless power receiver may not know whether one or more wireless power transmitters, or indeed any wireless power transmitter, is receiving the status message.

[0035] In some embodiments, the wireless power receiver is part of a toy set, such as a toy construction set.

[0036] According to an aspect, a wireless power transfer system disclosed herein comprises a wireless power transmitter as disclosed herein, and a wireless power receiver as disclosed herein.

[0037] In some embodiments of a wireless power transfer system, the communication range of the receiver communication unit is longer than the effective charging range of the wireless power transfer signal. The receiver communication unit is configured to send status messages and may additionally be configured to communicate with other wireless power receivers. The communication protocol used for transmission of the status messages by the receiver communication unit, and for transmission of the status messages by transmitter communication units, may be a protocol supporting broadcasting and the status messages may be transmitted as a broadcasted data stream. In some embodiments, the communication range of the receiver communication unit is more than 50 cm, such as more than 75 cm, such as more than 1 m, such as more than 2 m, such as more than 4 m, such as more than 6 m, such as more than 8 m, such as more than 10 m (meters). The communication range may be a standard communication range of the communication protocol used, as the actual communication range in a given situation may be affected by interference as is well known.

[0038] In some embodiments, the wireless power transfer system is part of a toy set, such as a toy construction set.

[0039] According to an aspect, a method of wirelessly transmitting power by a wireless power transmitter disclosed herein comprises the steps of: Transmitting a wireless power transfer signal, the wireless power transfer signal being transmitted using a magnetic charging field configured for transmission of power, Determining a transmitted power level,

[0040] Obtaining one or more wirelessly communicated status messages, each status message comprising data on a received power level, Determining a total received power, and

[0041] Determining a Foreign Object Loss based on the determined total received power and the determined transmitted power level.

[0042] The wireless power transfer signal comprises a transmitter charging field signature, and each status message further comprises data on an associated charging field signature. The determination of the total received power is based on the obtained data on received power level(s) for which the associated charging field signature corresponds to the transmitter charging field signature. The wireless power transmitter may be a wireless power transmitter as disclosed herein.

[0043] According to an aspect, disclosed herein is a non-transitory computer readable medium having stored thereon instructions that when executed by processing circuitry of a wireless power transmitter cause the processing circuitry to perform the steps of the method of wirelessly transmitting power.

[0044] According to an aspect, a method of communicating wirelessly received power by a wireless power receiver is disclosed herein, the method comprising the steps of:

[0045] Receiving wirelessly transmitted power,

[0046] Determining a received power level, and determining a charging field signature associated with the received power,

[0047] Communicating wirelessly a status message comprising the determined received power level and the determined associated charging field signature.

[0048] The wireless power receiver may be a wireless power receiver as disclosed herein.

[0049] According to an aspect, disclosed herein is a non-transitory computer readable medium having stored thereon instructions that when executed by processing circuitry of a wireless power receiver cause the processing circuitry to perform the steps of the method of communicating wirelessly received power.

[0050] According to an aspect, a method of wirelessly transmitting power from a wireless power transmitter to one or more wireless power receivers is disclosed herein, the method comprising the steps of:

[0051] Producing, by the wireless power transmitter, a magnetic charging field configured for transmission of power, the magnetic charging field having a transmitter charging field signature,

[0052] Determining, by the wireless power transmitter, a transmitted power level, Determining, by each of one or more power receivers, a received power level, and determining a charging field signature associated with the received power,

[0053] Communicating wirelessly, by each of the one or more power receivers, a status message comprising the received power level and associated charging field signature determined by the respective power receiver, Obtaining, by the wireless power transmitter, the status message communicated by each of the one or more power receivers,

[0054] Determining, by the wireless power transmitter, a total received power based on the obtained data on received power level(s) for which the associated charging field signature(s) correspond to the transmitter charging field signature,

[0055] Determining, by the wireless power transmitter, a Foreign Object Loss based on the determined total received power and the determined transmitted power level.

[0056] The wireless power transmitter may be a wireless power transmitter as disclosed herein. The wireless power receiver may be a wireless power receiver as disclosed herein.

[0057] In the following is described embodiments, wherein FOD relies on a time synchronisation between a wireless power transmitter and one or more power receivers. A synchronisation signal indicating a synchronisation time is transmitted by the wireless power transmitter and received by wireless power receivers which coordinate their power harvesting based on the detected synchronisation time. The synchronisation signal may be comprised in the charging field signature of the wireless power transmitter. Configurations of the charging field signature may be as described elsewhere herein, such as in aspects described above or below, or in the detailed description.

[0058] According to an aspect, disclosed herein is a wireless power transmitter comprising: A transmitter coil configured to transmit a wireless power transfer signal, the wireless power transfer signal being transmitted using a magnetic charging field configured for transmission of power, and a transmitter control unit configured to perform a Foreign Object Detection comprising obtaining one or more measurements on power harvested from the wireless power transmitter, wherein the wireless power transmitter is configured to transmit a signal at predetermined time intervals, the signal indicating a synchronisation time, wherein the transmitter control unit is configured to perform the one or more measurements on harvested power during one or more FOD time periods occurring at predetermined time intervals relative to the synchronisation time, and wherein the transmitter control unit is configured to assume, i.e. determine, as part of its Foreign Object Detection that associated power receivers do not harvest power from it during the FOD time periods and / or that associated power receivers substantially reduce power harvesting from it during the FOD periods.

[0059] An advantage of performing the measurements on harvested power during periods where power receivers which are associated with the wireless power transmitter do not harvest power or substantially, i.e. essentially, nullify power harvesting is a reduction of uncertainty in the estimation of harvested power as any power harvest during the FOD time periods must be due to one or more objects which do not conform to the predetermined FOD time periods. An associated power receiver is one that is configured to work together with the wireless power transmitter. Thus, any power receiver which might otherwise be able to charge from the wireless power transmitter is considered a Foreign Object by the wireless power transmitter if it is not configured to cease power harvesting during FOD time periods. The wireless power transmitter and associated wireless power receiver will thus be configured to conform to the same FOD time periods relative to the synchronisation time.

[0060] In some embodiments, the wireless power transfer signal comprises a transmitter charging field signature which is a characteristic that is measurable by a wireless power receiver, the transmitter charging field signature indicating a synchronisation time at predetermined time intervals. An advantage of using a charging field signature to indicate the synchronisation time may be that the communication between a charger and power receiver is, at least partly, radio-free. In some embodiments, the charging field signature comprises a repeating blanking period, where the blanking period is a time period of reduced power transfer, and wherein the repeating blanking period indicates the synchronisation time. In some embodiments, the synchronisation time is indicated by the beginning or end of a blanking period. The signal indicating a synchronisation time may alternatively, or additionally, be a radio signal. One or more time intervals related to a charging field signature, synchronisation time, and / or time intervals may be pseudo-random. In some embodiments, the predetermined time intervals between synchronisation times are pseudo-random. In some embodiments, the predetermined time intervals between FOD time periods are pseudo-random.

[0061] In some embodiments, the repetition period of the synchronisation time is between 0.5 seconds and 15 minutes, such as between 1 second and 10 minutes, such as between 5 seconds and 5 minutes, such as between 15 seconds and 1 minute. A shorter repetition period allows for a faster and possibly more robust synchronisation, but also means that a higher proportion of time is spent not charging due to blanking periods. Thus, there is a trade-off in the selection of a suitable synchronisation time.

[0062] A time period is a length of time during which an activity occurs or a condition remains. When an action is said to occur “during” a time period, the action may be executed at a time within the time period and / or the action may be executed for the duration of the time period. An FOD time period is a length of time during which a wireless power transmitter may assume, i.e. determine, as part of its FOD determinations that no power receivers with which it is associated are harvesting power. An FOD time period is a length of time during which a power receiver may reduce or substantially nullify its power harvesting from a wireless power transmitter. A power receiver may be configured to reduce or substantially nullify its power harvesting for a longer time period so as to encompass the FOD time period. This will reduce the amount of time a power receiver will harvest power from a charger, but the FOD time periods may be brief such that the overall reduction in power harvesting is modest. An FOD time period may have a length of 10 milliseconds to 500 milliseconds, such as 20 milliseconds to 400 milliseconds, such as 30 milliseconds to 300 milliseconds, such as 40 milliseconds to 200 milliseconds.

[0063] A time interval is the amount of time between two given times. For example, a signal being transmitted at predetermined time intervals means that there is a temporal gap between successive instances of signal transmission. A time period occurring at predetermined time intervals means that signifies that a specific time duration elapses between two occurrences of the time period.

[0064] In some embodiments, the transmitter control unit is further configured to perform one or more comparative measurements on power harvested from it at times outside of the FOD time periods.

[0065] The FOD is a determination of whether a Foreign Object is receiving power from the wireless power transmitter. If a clear determination that a Foreign Object is receiving power is receiving power from the power transmitter is made, a power transmitter will usually be configured to respond to such a determination. In some embodiments, the wireless power transmitter is configured to reduce power output, shut down, or shut down power transmission, if it determines that a Foreign Object is receiving power from it.

[0066] The wireless power transmitter may be configured to transmit a wireless power transfer signal using radio frequencies that are designated for ISM applications, i.e. to use an ISM band. The wireless power transmitter may be configured to transmit a wireless power transfer signal having a centre frequency in the frequency range 100 kHz - 50 MHz. For example, the wireless power transmitter may be configured to transmit a wireless power transfer signal having a centre frequency in the frequency range 15 - 50 MHz, such as 18 - 45 MHz, such as 21 - 40 MHz, such as 25 - 35 MHz. In some embodiments, the wireless power transmitter is configured to transmit a wireless power transfer signal having a centre frequency in the frequency range 100 kHz - 200 kHz. In some embodiments, the wireless power transmitter is configured to transmit a wireless power transfer signal having a centre frequency in the frequency range 5 MHz - 8.5 MHz. In some embodiments, the wireless power transmitter is configured to transmit a wireless power transfer signal having a centre frequency in the frequency range 12 MHz - 15 MHz. The wireless power transmitter may be configured for transmission of wireless power transfer signals at more than one frequency. In some embodiments, the wireless power transmitter is configured to transmit a wireless power transfer signal having a centre frequency in an ISM band comprising 140 kHz, and / or an ISM band comprising 6.78 MHz, and / or an ISM band comprising 13.56 MHz, and / or an ISM band comprising 27.12 MHz, where, for a given centre frequency, the wireless power transfer signal may be configured to comply with a regulated bandwidth of an ISM band comprising the given centre frequency. In some embodiments, the wireless power transmitter is configured to transmit a wireless power transfer signal having a centre frequency in an ISM band comprising 140 kHz, and / or an ISM band comprising 6.78 MHz, and / or an ISM band comprising 13.56 MHz, and / or an ISM band comprising 27.12 MHz, where, for a given centre frequency, the wireless power transfer signal may be configured to comply with a regulated bandwidth of an ISM band comprising the given centre frequency. For example, the wireless power transmitter may be configured to transmit a wireless power transfer signal having a centre frequency centred at or near 140 kHz, and / or centred at or near 6.78 MHz, and / or centred at or near 13.56 MHz, and / or centred at or near 27.12 MHz, where, for a given centre frequency, the wireless power transfer signal may be configured to comply with a regulated bandwidth of an ISM band comprising the given centre frequency.

[0067] According to an aspect, a wireless power receiver disclosed herein comprises: a receiver coil (207) configured to receive a wireless power transfer signal, a load coupled to the receiver coil, the load being configured to receive power from the receiver coil, a receiver control unit (209) configured to adjust power harvesting via the receiver coil, wherein the receiver control unit is configured to determine a synchronisation time from a signal received from a wireless power transmitter, and wherein the receiver control unit is configured to reduce or substantially, i.e. essentially, nullify power harvesting during one or more FOD time periods occurring at predetermined time intervals relative to the synchronisation time.

[0068] Wireless power harvesting may be adjusted in a number of known ways. For example, power harvesting may be adjusted by adjusting the load and / or by changing the resonance of the receiver coil.

[0069] In some embodiments, the receiver control unit is configured to determine a charging field signature of a power transmitter from which the power receiver is receiving power, the transmitter charging field signature indicating the synchronisation time, or wherein the signal indicating a synchronisation time is a received radio signal.

[0070] In some embodiments, the synchronisation time is indicated by a blanking period in the magnetic charging field. For example, the synchronisation time may be indicated by the beginning or end of a blanking period.

[0071] The wireless power receiver may be configured to receive power from a wireless power transfer signal using radio frequencies that are designated for ISM applications, i.e. using an ISM band. The wireless power receiver may be configured to receive, and charge from, a wireless power transfer signal having a centre frequency in the frequency range 100 kHz - 50 MHz. For example, the wireless power receiver may be configured to receive, and charge from, a wireless power transfer signal having a centre frequency in the frequency range 15 - 50 MHz, such as 18 - 45 MHz, such as 21 - 40 MHz, such as 25 - 35 MHz. In some embodiments, the wireless power receiver is configured to receive power from a wireless power transfer signal having a centre frequency in the frequency range 100 kHz - 200 kHz. In some embodiments, the wireless power receiver is configured to receive power from a wireless power transfer signal having a centre frequency in the frequency range 5 MHz - 8.5 MHz. In some embodiments, the wireless power receiver is configured to receive power from a wireless power transfer signal having a centre frequency in the frequency range 12 MHz - 15 MHz. The wireless power receiver may be configured to charge from more than one centre frequency of a wireless power transfer signal, each at a different time. In some embodiments, the wireless power receiver is configured to receive a wireless power transfer signal having a centre frequency in an ISM band comprising 140 kHz, and / or an ISM band comprising 6.78 MHz, and / or an ISM band comprising 13.56 MHz, and / or an ISM band comprising 27.12 MHz, where, for a given centre frequency, the wireless power transfer signal may be configured to comply with a regulated bandwidth of an ISM band comprising the given centre frequency. For example, the wireless power receiver may be configured to receive power from a wireless power transfer signal having a centre frequency centred at or near 140 kHz, and / or centred at or near 6.78 MHz, and / or centred at or near 13.56 MHz, and / or centred at or near 27.12 MHz, where, for a given centre frequency, the wireless power transfer signal may be configured to comply with a regulated bandwidth of an ISM band comprising the given centre frequency. The wireless power receiver may be configured to be able to receive power from a wireless power transfer signal comprising radio frequencies that are higher than those used in the 13.56 MHz RFID band, and the wireless power receiver may further comprise an NFC / RFID tag reader.

[0072] According to an aspect, disclosed herein is a method of wirelessly transmitting power by a wireless power transmitter comprising:

[0073] Transmitting a wireless power transfer signal, the wireless power transfer signal being transmitted using a magnetic charging field configured for transmission of power,

[0074] Performing a Foreign Object Detection comprising obtaining one or more measurements on harvested power, wherein the wireless power transmitter is configured to transmit a signal at predetermined time intervals, the signal indicating a synchronisation time, wherein the one or more measurements on harvested power is performed during one or more FOD time periods occurring at predetermined time intervals relative to the synchronisation time, and wherein performing the Foreign Object Detection comprises presuming, i.e. determining, that associated power receivers do not harvest power from the wireless power transmitter during the FOD time periods and / or presuming, i.e. determining, that associated power receivers substantially reduce power harvesting from the wireless power transmitter during the FOD periods. The wireless power transmitter may be a wireless power transmitter as disclosed herein.

[0075] According to an aspect, disclosed herein is a non-transitory computer readable medium having stored thereon instructions that when executed by processing circuitry of a wireless power transmitter cause the processing circuitry to perform the steps of the method of wirelessly transmitting power by a wireless power transmitter.

[0076] According to an aspect, disclosed herein is a method of harvesting wirelessly transmitted power by a wireless power receiver comprising:

[0077] Receiving wirelessly transmitted power,

[0078] Detecting a synchronisation signal,

[0079] Determining a synchronisation time from the synchronisation signal, Adjusting the power harvesting during one or more FOD time periods occurring at predetermined time intervals relative to the synchronisation time.

[0080] Adjusting the power harvesting may comprise reducing or substantially nullifying the power harvesting.

[0081] The wireless power receiver may be a wireless power receiver as described herein.

[0082] According to an aspect, disclosed herein is a non-transitory computer readable medium having stored thereon instructions that when executed by processing circuitry of a wireless power receiver cause the processing circuitry to perform the steps of the method of harvesting wirelessly transmitted power.

[0083] According to an aspect, disclosed herein is a method of wirelessly transmitting power from a wireless power transmitter to one or more wireless power receivers, the method comprising:

[0084] Producing, by the wireless power transmitter, a magnetic charging field configured for transmission of power,

[0085] Transmitting, from the wireless power transmitter to the one or more wireless power receivers, a signal at predetermined time intervals, the signal indicating a synchronisation time,

[0086] Receiving wirelessly transmitted power by the one or more wireless power receivers, Adjusting, e.g. reducing or substantially nullifying, the power harvesting, by the one or more wireless power receivers, during one or more FOD time periods occurring at predetermined time intervals relative to the synchronisation time,

[0087] Performing, by the wireless power transmitter, a Foreign Object Detection comprising obtaining one or more measurements on harvested power, the one or more measurements on harvested power being performed during one or more of the one or more FOD time periods.

[0088] The wireless power transmitter may be a wireless power transmitter as described herein and / or the one or more wireless power receivers may be wireless power receivers as described herein.

[0089] Brief description of the drawings

[0090] FIG. 1 shows a schematic block diagram of a wireless power transmitter according to some embodiments;

[0091] FIG. 2 shows an example of a wireless power receiver in the form of a wireless interactive toy construction element;

[0092] FIG. 3 shows a schematic block diagram of a wireless power receiver in the form of a toy construction element according to some embodiments;

[0093] FIG. 4A and 4B shows schematic block diagrams of embodiments of wireless power transmitters and wireless power receivers according to some embodiments;

[0094] FIG. 5 shows a schematic block diagram of a wireless power transmitter, a wireless power receiver, and a passive repeater according to some embodiments;

[0095] FIG. 6 shows a schematic block diagram of a passive repeater according to some embodiments;

[0096] FIG. 7 shows a schematic block diagram of a wireless power transfer system according to some embodiments; FIG. 8 illustrate examples of charging field signatures according to some embodiments.

[0097] FIG. 9 illustrates features of a method of wirelessly transmitting power from a wireless power transmitter to one or more wireless power receivers according to some embodiments;

[0098] FIG. 10 illustrates features of a method of wirelessly transmitting power from a wireless power transmitter to one or more wireless power receivers according to some embodiments;

[0099] FIG. 11 shows a flow chart of a method of wirelessly transmitting power by a wireless power transmitter and a method of communicating wireless received power according to some embodiments;

[0100] FIG. 12 shows a flow chart of a method of wirelessly transmitting power by a wireless power transmitter and a method of harvesting wirelessly transmitted power according to some embodiments; and

[0101] FIGS. 13-14 each shows a flow chart of a method of wirelessly transmitting power by a wireless power transmitter according to some embodiments.

[0102] Detailed description

[0103] Various aspects and embodiments of wireless power transmitters, wireless power receivers, wireless power transfer systems, methods of wirelessly transmitting power, a method of harvesting wirelessly transmitted power, a method of communicating wirelessly received power, and methods of wirelessly transmitting power from a wireless power transmitter to one or more wireless power receivers as disclosed herein will now be described with reference the drawings. In some embodiments, the aspects and embodiments will refer to toy construction elements in the form of bricks. However, the invention may be applied to other forms of toy elements, and / or other forms of toy construction elements, for use in toy sets. FIG. 1 shows a schematic block diagram of a wireless power transmitter 5 according to some embodiments;

[0104] The wireless power transmitter comprises a transmitter coil 10 which is connected electrically to inductive charging circuitry 25, a power adapter 30 for connecting the wireless power transmitter to a power grid or other power supply, a transmitter control unit 20, a transmitter communication unit 15, and a user interface 35. The user interface may comprise buttons with which a user can interact with the wireless power transmitter, and / or it may comprise visible and / or tactile elements by which the wireless power transmitter can relay information to a user. The wireless power transmitter is configured to transmit a wireless power transfer signal using a magnetic charging field configured for transmission of power. The wireless power transfer signal can be utilised by a suitably configured wireless power receiver for wireless charging.

[0105] The wireless power transmitter 5 may comprise an encasing 7, i.e. a housing, configured for facilitating charging of one or more wireless power receivers by placement of the one or more wireless power receivers on or near a charging surface 8 of the wireless power transmitter. The charging surface 8 may comprise one or more substantially horizontal surfaces to allow one or more objects, such as wireless power receivers, placed on it to be supported at rest by the wireless power transmitter, when the transmitter itself is positioned substantially horizontally. The charging surface may be a surface of the transmitter encasing 7 which is closest to the transmitter coil. The charging surface 8 may comprise a visible and / or tactile marking. The visible and / or tactile marking may be configured to aid a user in placement of wireless power receivers on or near the charging surface. The wireless power transmitter may be configured to produce a charging field that is strong enough and which extends far enough away from the charging surface 8 that a wireless power receiver is able to harvest energy, i.e. receive power, even while at a distance from the charging surface 8.

[0106] In some cases, the wireless power transmitter 5 is configured for charging of a single wireless power receiver, while in other embodiments, the wireless power transmitter 5 is configured to allow charging of multiple wireless power receivers simultaneously. For example, the wireless power transmitter 5 may be configured for charging one or more wireless power receivers similar to the wireless power receiver 200 shown in fig. 2. The wireless power transmitter may be configured to support charging of power receivers having one or more of a plurality of supported receiver coil sizes. The wireless power receiver may be configured to support charging of a wireless power receiver that comprises more than one receiver coil, where each receiver coil may be a different size and / or shape, and / or to support charging of various wireless power receivers that each comprise a coil that may be a different size and / or shape to that of the other wireless power receivers.

[0107] The wireless power transmitter may be configured to create a charging field which transmits a wireless power transfer signal that comprises a transmitter charging field signature. The charging field signature is a signature of the charging field produced by the wireless power transmitter and may be a measurable characteristic of the wireless power transmitter signal. As an example, the wireless power transmitter may be configured to regulate the power of the wireless power transfer signal at regular time intervals so as to produce a charging field signature which comprises a periodic component. Examples of charging field signatures are described in connection with figs. 8, 9, and 10.

[0108] The transmitter control unit 20 may comprise e.g. a microcontroller, a microprocessor, or other suitable processing unit. The transmitter control unit 20 may be configured to handle numerous control functions related to the wireless power transmitter. For example, the transmitter control unit 20 may control the functioning of the transmitter coil, the transmitter communications unit, etc. At least some functions of the transmitter control unit 20 may be implemented in hardware.

[0109] The wireless power transmitter 5 may be configured for Foreign Object Detection based on status messages received from one or more wireless power receivers as described in the following:

[0110] The charging field signature makes it possible for a wireless power receiver to determine the charging field signature associated with the charging power it receives as the charging field signature is part of the wireless power transfer signal received by the receiver. Thus, in a way, the charging field signature is a “fingerprint” of the wireless power transmitter, although the analogy is not entirely accurate, as the wireless power transmitter may be configured to change its charging field signature as discussed further herein.

[0111] The transmitter communication unit 15 is configured to receive status messages comprising data on a received power level and data on an associated charging field signature. The status messages may be sent by one or more wireless power receivers and may be sent via e.g. radio, Bluetooth, or other suitable wireless communication protocols. The status messages may be sent as a broadcasted data stream.

[0112] The communication range of the wireless communication protocol used to send and receive the status messages may be longer than the effective charging range of the wireless power transfer signal. In such embodiments, this means that a wireless power transmitter may receive one or more status message from wireless power receivers that are too far away from it to receive charge from the wireless power transmitter itself, but where the receivers are receiving power from one or more other wireless power transmitters. The associated charging field signature being reported in each status message thus allows the wireless power transmitter to ignore the reports on received power levels from wireless power receivers which are being charged by a wireless power transmitter with a different charging field signature, when determining the total received power.

[0113] The wireless power transmitter 5 uses the received status messages to determine a total received power based on the obtained data on received power levels, i.e. to determine a measure of how much power has been received, for example in a given time interval. In addition, the wireless power transmitter determines a transmitted power level, where the transmitted power level is the level of wireless power transmitted by the wireless power transmitter. The determined transmitted power level may be compared to the determined total received power to arrive at a determination of whether a Foreign Object is receiving power from the wireless power transmitter. The skilled person will know of various procedures for making Foreign Object determinations as such are well-known in the field of Foreign Object Detection. For example, the determination of the Foreign Object Loss may be made by a simple subtraction of the total received power from the transmitted power level. However, the wireless power transmitter is determining the total received power based only on the received power levels for which the associated charging field signature corresponds to the transmitter charging field signature, i.e. only the received power levels that are reported to have been received from a charger with the same charging field signature as the wireless power transmitter are used in the determination; Received power levels that are reported to have been received from a charger with a different charging field signature to that of the wireless power transmitter are ignored in the determination of the total received power.

[0114] The wireless power transmitter determines a Foreign Object Loss by comparing the total received power with the transmitted power level, for example such that a positive Foreign Object Loss means that the transmitted power level is greater than the total received power. The determined Foreign Object Loss may then be evaluated, for example with respect to predetermined thresholds.

[0115] For example, the wireless power transmitter may conclude or determine based on the Foreign Object Loss that no Foreign Objects are absorbing power transmitted by the wireless power transmitter and continue the present operation.

[0116] However, if the Foreign Object Loss indicates that more power is being output than is being reported received, the wireless power transmitter may be configured to conclude that one or more Foreign Objects are harvesting power transmitted by the wireless power transmitter. In response, the wireless power transmitter may take steps to prevent heating up of any Foreign Objects. For example, the wireless power transmitter may in response reduce its output of power.

[0117] If the Foreign Object Loss indicates that more power has been received than transmitted by the wireless power transmitter, the wireless power transmitter may be configured to reset the transmitter charging field signature. This may for example occur when the communication range of the wireless communication protocol used to send the status messages is longer than the effective charging range of the wireless power transfer signal and multiple wireless power transmitters are using the same charging field signature, as described further below in connection with fig. 6.

[0118] Alternatively, or additionally, to making use of status messages for FOD, the wireless power transmitter 5 may be configured for Foreign Object Detection based on time-synchronisation of FOD time periods with power receivers as described in the following.

[0119] The transmitter control unit 20 is configured to obtaining one or more measurements on power harvested from the wireless power transmitter as part of Foreign Object Detection and to do so during one or more FOD time periods. By presuming, i.e. determining, that associated wireless power receivers do not harvest power or have substantially reduced power harvesting, e.g.to only harvest a small trickle charge, during the FOD time periods, the transmitter control unit can expect that power harvested during the FOD time periods is due to a Foreign Object. Thus, the charger may for example measure total power and subtract known losses from within the charger itself as well as subtract estimated transmitter losses to arrive at an estimate of power consumed by Foreign Objects. In order to not trigger a Foreign Object response to only a small power excess, i.e. power not accounted for which may be due to e.g. measurement uncertainties and / or trickle charging, the transmitter control unit may compare the estimated power excess with a predetermined FOD threshold. The wireless power transmitter may be configured to only react to an estimated power excess if it exceeds the predetermined FOD threshold. Alternatively, or additionally, the transmitter control unit may calculate rolling averages, and may compare these against one or more thresholds.

[0120] The wireless power transmitter may be configured to additionally make one or more measurements on power harvested from it at times outside of the FOD time periods, i.e. to make measurements on power harvested at times where associated power receivers may harvest power. By doing so the wireless power transmitter can compare the power harvest measurements made during and outside of the FOD time periods and make determinations as to the presence of objects charging from it. If the estimated power excess increases during FOD time periods and decreases outside of FOD time periods, the wireless power transmitter may determine that one or more associated power receivers are charging from it. If the power excess estimated from measurements done during and outside of FOD time periods are similar, the wireless power transmitter may determine that only Foreign Objects, or nothing, is harvesting power from it. To time-synchronise with associated power receivers, the wireless power transmitter transmits a signal indicating a synchronisation time. The synchronisation signal may be a radio signal, for example Bluetooth or other suitable wireless communication protocol. A synchronisation radio signal may be sent by the transmitter communication unit 15 and may be sent as a broadcasted data stream. The time of transmission of the radio signal may be the synchronisation time. Alternatively, or additionally, the wireless power transfer signal may comprise a transmitter charging field signature which indicates the synchronisation time to a wireless power receiver that receives the wireless power transfer signal. Thus, the time-synchronisation may be based in the charging field signature. For example, the time-synchronisation may be based on periodic blanking periods as described in connection with fig. 10.

[0121] The transmitter communication unit 15 may be configured to receive messages from wireless power receivers, such as broadcasted messages. A message from a wireless power receiver that a transmitter communication receives may comprise information indicating a charging field signature as a way to identify a wireless power transmitter that the power receiver interacts with magnetically.

[0122] The following generally applies to the wireless power transmitter whichever method was used for FOD unless specifically noted, for example by reference to features of a particular FOD method.

[0123] In response to a determination by the wireless power transmitter that a Foreign Object is receiving power from it, the wireless power transmitter may take steps to reduce the power. For example, the wireless power transmitter may in response reduce the output of power to reduce the amount of power a Foreign Object can receive, or the wireless power transmitter may stop outputting power entirely. To reduce the output of power, the wireless power transmitter may enter a trickle charge mode. The wireless power transmitter may be configured to output a relatively low amount of power as long as the amount of output power exceeds a predetermined minimum which may be determined by operating parameters and design of the power transmitter. The wireless power transmitter may be further configured to allow for a small amount of power, such as a trickle charge, to be drawn from it without this harvesting of power causing a further reduction or shutdown of power. This, together with a suitably arranged predetermined threshold, will allow for charging of a wireless power receiver that is unable to send a status message and / or that is unable to adjust its power harvesting during FOD time periods due to it not having enough power to do so, e.g. due to a “dead” battery. Thus, the wireless power transmitter can provide a small charging power to a wireless power receiver that is in a state of having very little or substantially zero battery power.

[0124] The wireless power transmitter may be configured to transmit a wireless power transfer signal using radio frequencies that are designated for ISM applications, i.e. to use an ISM band. The wireless power transmitter may be configured to transmit a wireless power transfer signal having a centre frequency in the frequency range 100 kHz - 50 MHz. For example, the wireless power transmitter may be configured to transmit a wireless power transfer signal having a centre frequency in the frequency range 15 - 50 MHz, such as 18 - 45 MHz, such as 21 - 40 MHz, such as 25 - 35 MHz. In some embodiments, the wireless power transmitter is configured to transmit a wireless power transfer signal having a centre frequency in the frequency range 100 kHz - 200 kHz. In some embodiments, the wireless power transmitter is configured to transmit a wireless power transfer signal having a centre frequency in the frequency range 5 MHz - 8.5 MHz. In some embodiments, the wireless power transmitter is configured to transmit a wireless power transfer signal having a centre frequency in the frequency range 12 MHz - 15 MHz. In some embodiments, the wireless power transmitter is configured to transmit a wireless power transfer signal having a centre frequency in an ISM band comprising 140 kHz, and / or an ISM band comprising 6.78 MHz, and / or an ISM band comprising 13.56 MHz, and / or an ISM band comprising 27.12 MHz, where, for a given centre frequency, the wireless power transfer signal may be configured to comply with a regulated bandwidth of an ISM band comprising the given centre frequency. For example, the wireless power transmitter may be configured to transmit a wireless power transfer signal having a centre frequency centred at or near 140 kHz, and / or centred at or near 6.78 MHz, and / or centred at or near 13.56 MHz, and / or centred at or near 27.12 MHz, where, for a given centre frequency, the wireless power transfer signal may be configured to comply with a regulated bandwidth of an ISM band comprising the given centre frequency. By using frequencies for charging that are higher than those used in the 13.56 MHz RFID band, a wireless power receiver configured for use with the wireless power transmitter may be designed to use its receiver coil(s) for both charging and RFID tag communication.

[0125] When configured for receiving status messages from power receivers, the wireless power transmitter 5 may be configured to exclusively receive, i.e. not to transmit, messages using the communications protocol on which the status messages are transmitted. For example, by being configured to receive the status messages as broadcasted data streams. The wireless power transmitter 5 may be configured to receive status messages via a broadcasting system, i.e. a system allowing for reception of data without pairing between communicating devices. By listening to, i.e. obtaining, status messages from wireless power receivers, a wireless power transmitter may reason out the presence other wireless power transmitters without communicating directly with them, thereby allowing the power transmitter to act accordingly for example with respect to selection of a transmitter charging field signature.

[0126] The wireless power transmitter 5 may be configured to provide charging of wireless power receivers without requiring a handshake or pairing between the wireless power transmitter and any wireless power receiver, for example by the use of a broadcast system for the status messages or for synchronisation messages for coordination of time periods. Using a charging field signature as a way to indicate the synchronisation time to a power receiver is another way of providing charging without a handshake or any type of pairing. In computing, a handshake is a well- known type of coordination between two devices or programs. Via a handshake the two devices exchange information which may establish protocols such as communication protocol and / or charging parameters. Even if handshaking or a type of pairing is not used for FOD, it may still be implemented in the wireless power transmitter and wireless power receivers, for example as part of security measures.

[0127] In addition to being used for FOD, the use of status messages and / or timesynchronisation necessitates that a power receiver has to be configured to work with the wireless power transmitter. Thus, this provides an additional hurdle for persons producing unauthorized copies of the wireless power transmitters and / or wireless power receiver described herein. For example, a wireless power transmitter configured to obtain status messages will determine that more power is being transmitted than is being reported received if an unauthorized wireless receiver is charging from it without providing a status message. This will likely cause the wireless power transmitter to conclude that a Foreign Object is receiving power from it and in response the wireless power transmitter may reduce power output or shut down power output as also described above.

[0128] The wireless power transmitter may be configured to select a transmitter charging field signature at start-up, when there is a conflict with the charging field signature of another wireless power transmitter, and / or intermittently, e.g. at random times and / or at predetermined time intervals and / or at one or more fixed times. The wireless power transmitter may be configured such that whenever a transmitter charging signature is to be selected, the wireless power transmitter selects a transmitter charging field signature of the available transmitter charging field signatures that provides a more efficient charging. For example, if the charging field signature is a blanking period as described in connection with figs. 8 or 10, the wireless power transmitter may be configured to select a transmitter charging field signature with as short a blanking period as possible as this will provide more time wherein the wireless power transmitter provides charging power. Thus, the wireless power transmitter may be configured to select at transmitter charging signature which allows the wireless power transmitter to provide charging for as much time as possible on average.

[0129] The transmitter charging field signature may be selected from a predetermined set of possible charging field signatures and / or the transmitter charging field signature may be selected after a period of time, wherein the wireless power transmitter has listened for status messages reporting charging field signatures already in use by other wireless power transmitters. Thus, the wireless power transmitter may be configured to have a predetermined set of charging field signatures from which it may choose a charging field signature. In addition, or alternatively, the wireless power transmitter may wait for a predetermined period of time and if the wireless power transmitter receives any messages comprising information on a charging field signature, such as status messages, in that time, it may decide to not use a charging field signature reported in those messages. In this way, the wireless power transmitter may try to avoid choosing a charging field signature that is already in use by another wireless power transmitter within range of the wireless communication used by the wireless power receiver(s) to transmit messages comprising information on charging field signature.

[0130] A wireless power transmitter may be configured to enter a power save mode, wherein the power output is reduced (while still maintaining the charging field signature), and may do so after a time period of not detecting any power receivers actively harvesting power from it. In order to provide trickle charging to associated power receivers which are low on battery power, the wireless power transmitter may be configured to override the power save mode, for example an automated periodic override or in response to a user interaction. While in the power save mode, a wireless power transmitter may listen for messages comprising information on charging field signature, such as broadcasted messages, and to exit the power save mode if it receives a message comprising its own charging field signature.

[0131] FIG. 2 shows an example of a wireless power receiver in the form of a wireless interactive toy construction element, generally designated 200. On the left-hand side of FIG. 2 the wireless interactive toy construction element is shown with its top surface visible and, on the right-hand side, the wireless interactive toy construction element is shown with its bottom side visible. In particular, the wireless interactive toy construction element comprises a generally box-shaped housing 201 with coupling pegs 204 extending from its top surface and with a cavity extending into the element from the bottom. The cavity is defined by side walls 202 and by a central, downwardly extending tube 203. The coupling pegs of another toy construction element can be received in the cavity in a frictional engagement, e.g. as disclosed in US 3 005282. The construction elements shown in the remaining figures have this known type of coupling members in the form of cooperating pegs and cavities. However, other types of coupling members may also be used in addition to, or instead of, the pegs and cavities. The coupling pegs are arranged across the top surface in a square planar grid, i.e. defining orthogonal directions along which sequences of coupling pegs are arranged. The distance between neighbouring coupling pegs is uniform and equal in both directions. This or similar arrangements of coupling members at coupling locations defining a regular planar grid allow the toy construction elements to be interconnected in a discrete number of positions and orientations relative two each other, in particular at right angles with respect to each other. In an assembled toy construction model, the coupling members of multiple toy construction elements may thus be located on grid points of a three-dimensional grid defined relative to the toy construction model.

[0132] In some embodiments, the housing 201 of the wireless interactive toy construction element is made from plastics material, e.g. thermoplastic polymers, or from another suitable material. The housing may e.g. be made by an injection moulding process or by another suitable manufacturing process.

[0133] The wireless interactive toy construction element 200 may include a function device 205 accommodated within the housing 201 of the wireless interactive toy construction element. Generally, a function device may be any suitable device for performing a function, such as a function that provides a user-perceptible effect, such as e.g. a visible, tactile, and / or audible effect. Examples of function devices may include any suitable mechanical and / or electrical device, arrangement, and / or circuitry adapted to perform one or more mechanical and / or electrical functions.

[0134] Examples of a mechanical function include driving a rotatable output shaft, winding- up a string or a chain which enables pulling an object closer to a toy module, moving a hinged part of the wireless interactive toy construction element, etc. The mechanical function may thus enable opening or closing a door, ejecting an object, rotating a turntable, moving a linear actuator, etc. Such mechanical motions can be driven by an electric motor.

[0135] Examples of an electrical function include emitting constant or blinking light, activating several lamps in a predetermined sequence, emitting audible sound such as beep, alarm, bell, siren, voice message, music, synthetic sound, natural or imitated sound simulating and / or stimulating play activities, playback of a sound, and / or other audio content, etc.

[0136] Accordingly, the function device may be selected from a motor, a light source (e.g. one or more LEDs), a sound source (e.g. a loudspeaker). The function wireless toy construction element may include more than one function device. In some embodiments, the system includes different types of wireless interactive toy construction elements comprising respective, different types of function devices. The wireless interactive toy construction element 200 includes a sensor system 208 comprising one or more sensors accommodated within the housing 201 of the wireless interactive toy construction element. The sensor system 208 may include one or more sensors, e.g. including the position and / or orientation sensor described herein and / or one or more other sensors, e.g. a linear or rotary encoder, a light detector, and a sound detector (e.g. a microphone), an accelerometer, and / or the like.

[0137] FIG. 3 shows a schematic block diagram of a wireless power receiver in the form of a toy construction element, generally designated 200 e.g. of the wireless interactive toy construction element shown in FIG. 2, according to some embodiments.

[0138] The toy construction element 200 comprises a housing 201 defining a top face which is provided with coupling members 204, all as described above with reference to fig. 2. A toy set, for example a toy construction set, may comprise one or more wireless power receivers 200, as disclosed herein. For example, a toy construction set may comprise one or more wireless power receivers 200, as disclosed herein, in the form of construction elements within the toy construction set.

[0139] The toy construction element 200 further comprises a receiver control unit 209 which may comprise a processing unit and one or more receiver coils 207, all accommodated within the housing 201. The wireless interactive toy construction element may further comprise, also accommodated within the housing 201 , one or more additional sensors 208 and / or one or more function devices 205, e.g. as described in connection with the sensor system and function device of the embodiment of fig. 2 above. The wireless interactive toy construction element 200 further comprises an energy storage device 210, such as a rechargeable battery or a capacitor, e.g. a large capacitor, and a wireless communications interface 211, also accommodated within the housing 201.

[0140] The wireless communications interface 211 , also referred to as the receiver communication unit, may comprise a radio-frequency transceiver and an associated antenna. In some embodiments, the wireless communications interface may comprise a Bluetooth chip or circuit or another form of radio-frequency transceiver adapted for communication via a wireless communications network, e.g. using known low-power, short-range wireless networking technology, such as Bluetooth / Bluetooth Low Energy, ZigBee, Z-Wave, or a similar wireless technology for low-power personal area data networking in compliance with a standardized protocol. The wireless communications interface 211 may be operable for two-way communication with other wireless interactive toy construction elements of the system. Accordingly, the wireless interactive toy construction element may be operable to communicate its identity and / or operational characteristics, scheduling information, position information and / or the like.

[0141] The receiver control unit 209 may comprise one or more microcontrollers, one or more microprocessors, one or more ASICs, and / or one or more other suitable processing units, or combinations thereof. The processing unit 209 may be configured to control the functional behaviour of the wireless interactive toy construction element. The wireless interactive toy construction element may be provided, e.g. pre-programmed, with a default behaviour, e.g. with default executable instructions stored in a memory of the wireless interactive toy construction element and executable by the processing unit of the wireless interactive toy construction element. The default executable instructions may define a set of predetermined rules for reacting to external stimuli as sensed by the sensor system, for example for reacting to detected positions and / or orientations relative to other wireless interactive toy construction elements or to detected changes in such positions and / or orientations. In some embodiments, the behaviour of the wireless interactive toy construction element may be programmed or configured by the user, e.g. by receiving program data and / or configuration parameters. To this end, the wireless interactive toy construction element may receive program and / or control data and / or configuration parameters from a computer or from another external electronic device, e.g. directly or via another toy module of the system. An external electronic device may be e.g. be or comprise a desktop computer, a tablet computer, a smartphone, a laptop computer, or another programmable computing device. Alternatively, or additionally, the wireless interactive toy construction element may capture program and / or control data and / or configuration parameters from a wireless tag, e.g. from an RFID tag, or from other data storage devices. For example, the wireless interactive toy construction element may be operable to read out such wireless tag or other data storage device in a contactless manner as described herein.

[0142] Each receiver coil 207 defines a coil axis around which the receiver coil extends. In this example, the receiver coils 207 are arranged such that one coil is arranged with its coil axis extending out of the top face of the housing while another other coil is arranged with its coil axis extending out of one of the side faces of the housing. A third receiver coil (not explicitly shown in fig. 2) may be arranged with its coil axis extending out of another side face of the housing, e.g. such that the axes of all three coils are oriented orthogonal to each other or at least linearly independent from each other. In this manner, the wireless interactive toy construction element may use the receiver coils to detect position and / or orientation coordinates of another wireless interactive toy construction element having similar arrangements of receiver coils as described herein. It will be appreciated, however, that other arrangements of coils are possible. In some embodiments, the wireless interactive toy may use one or more of the receiver coils to perform additional functions, e.g. to detect, identify and / or capture data from a wireless tag. The control unit 209 of the wireless power receiver may be configured to control a function device responsive to configuration data read from a wireless tag. Additionally, or alternatively, the control unit 209 may be further configured to control a function device responsive to an input from a sensor comprised in the wireless power receiver. A tag reader comprised in the wireless power receiver may be an RFID / NFC reader or any other circuitry for reading data from a tag when it is positioned in sufficient proximity of one or more receiver coils.

[0143] The wireless power receiver may be configured to receive power from a wireless power transfer signal using radio frequencies that are designated for ISM applications, i.e. using an ISM band. The wireless power receiver may be configured to receive, and charge from, a wireless power transfer signal having a centre frequency in the frequency range 100 kHz - 50 MHz. For example, the wireless power receiver may be configured to receive, and charge from, a wireless power transfer signal having a centre frequency in the frequency range 15 - 50 MHz, such as 18 - 45 MHz, such as 21 - 40 MHz, such as 25 - 35 MHz. In some embodiments, the wireless power receiver is configured to receive power from a wireless power transfer signal having a centre frequency in the frequency range 100 kHz - 200 kHz. In some embodiments, the wireless power receiver is configured to receive power from a wireless power transfer signal having a centre frequency in the frequency range 5 MHz - 8.5 MHz. In some embodiments, the wireless power receiver is configured to receive power from a wireless power transfer signal having a centre frequency in the frequency range 12 MHz - 15 MHz. The wireless power receiver may be configured to charge from more than one centre frequency of a wireless power transfer signal, each at a different time. In some embodiments, the wireless power receiver is configured to receive a wireless power transfer signal having a centre frequency in an ISM band comprising 140 kHz, and / or an ISM band comprising 6.78 MHz, and / or an ISM band comprising 13.56 MHz, and / or an ISM band comprising 27.12 MHz, where, for a given centre frequency, the wireless power transfer signal may be configured to comply with a regulated bandwidth of an ISM band comprising the given centre frequency. For example, the wireless power receiver may be configured to receive power from a wireless power transfer signal having a centre frequency centred at or near 140 kHz, and / or centred at or near 6.78 MHz, and / or centred at or near 13.56 MHz, and / or centred at or near 27.12 MHz, where, for a given centre frequency, the wireless power transfer signal may be configured to comply with a regulated bandwidth of an ISM band comprising the given centre frequency. The wireless power receiver may be configured to be able to receive power from a wireless power transfer signal comprising radio frequencies that are higher than those used in the 13.56 MHz RFID band, and the wireless power receiver may further comprise an NFC / RFID tag reader. By using frequencies for charging that are higher than those used in the 13.56 MHz RFID band, the wireless power receiver may be configured to use its receiver coil(s) for both charging and RFID tag communication.

[0144] Additionally, the wireless power receiver may be configured such that the default state of its coil control system (which may be part of the receiver control unit 209) is charging so as to allow charging of the wireless power receiver from a low or zero power state, e.g. from a substantially “dead” battery, where any active control requiring processing power is not possible. At least one receiver coil 207 is configured to receive a wireless power transfer signal from a wireless power transmitter, i.e. from a wireless charger, for charging the battery 210 which in turn powers other components of the wireless power receiver, such as the receiver control unit 209, the function device 205 and the wireless communications interface 211. One or more components receiving power at a given time via inductive charging circuitry in a wireless power receiver are known as the load. The inductive charging circuitry receives the electrical signal from the receiver coil(s) and processes it before supplying it to the load, e.g. rectifies, filters and regulates it.

[0145] The receiver control unit 209 may be configured to handle numerous control functions related to the receipt of wireless power. For example, the receiver control unit 209 may control the functioning of receiver coil(s). At least some functions of the receiver control unit 209 may be implemented in hardware. Implementing some functions of the wireless power receiver in hardware has the advantage that at least some of those functions can be designed to work even when there is too little power, such as too little power in the battery 210, to operate a processing core. In some embodiments, the wireless power receiver is configured to receive power even when the power received is relatively low power, such as e.g. a “trickle” charge. This may allow the wireless power receiver to be able to receive power even at a distance from a charging surface of a wireless power transmitter.

[0146] In the following is described embodiments of a wireless power receiver configured for a wireless power transfer system, where FOD in the system relies on status messages from wireless power receivers and a charging field signature being used for identification of a wireless power transmitter.

[0147] The control unit is configured to determine a received power level, where the received power level is the level of wireless power received by the wireless power receiver 200. The control unit is further configured to determine a transmitter charging field signature associated with the received power. For example, the transmitter charging field signature may be a measurable characteristic of the wireless power transfer signal, and may comprise a periodic component, i.e. a component in the wireless power transfer signal that appears or occurs at regular intervals. Examples of charging field signatures are described in connection with figs. 8, 9, and 10.

[0148] The receiver communication unit 211 is configured to send a status message comprising the received power level and the charging field signature associated with the received power. Thus, the wireless power receiver determines how much power it receives and the charging field signature associated with the received power. It then sends a status message containing this information. A wireless power transmitter configured to rely on status messages from power receivers in its FOD as described herein may then receive the status message, compare the associated charging signature to its own, and in case of a match use it in its determination of the total received power. That is, if the charging field signature associated with a received power matches the charging field signature of the wireless power transmitter, it can add the reported received power level to the total received power being determined by it. On the other hand, if the charging field signature associated with a received power does not match the charging field signature of the wireless power transmitter, it can ignore the reported received power level reported in the status message with respect to its determination of the total received power. The wireless power transmitter may, however, save the information on the associated charging field signature for later, e.g. in case the wireless power transmitter wishes to change its own charging field signature.

[0149] A wireless power transmitter 5 disclosed herein may be configured to exclusively receive, i.e. to not transmit, messages using the communications protocol on which the status messages are transmitted by the wireless power receiver 200. For example, by being configured to receive the status messages as broadcasted data streams. The wireless power receiver 200 may be configured to send status messages via a broadcasting system, i.e. a system allowing for transmitting and receiving data without pairing between communicating devices. The wireless power receiver may be configured to use the same communication protocol for communication with other wireless power receivers and for sending of status messages.

[0150] A wireless power receiver, whether configured for being used with a wireless power transfer system relying on status messages or one relying on FOD time periods for FOD, may be configured to bar, i.e. not permit, charging from a charging field that does not have a charging field signature. For example, the receiver control unit 209 may be configured to put the wireless power receiver in a state in which wireless power received is not stored or used, if the control unit cannot determine a charging field signature associated with the received wireless power.

[0151] In the following is described embodiments of a wireless power receiver configured for a wireless power transfer system, where FOD in the system relies on a time synchronisation between a wireless power transmitter and one or more power receivers. A synchronisation signal indicating a synchronisation time is transmitted by the wireless power transmitter and received by wireless power receivers which coordinate their power harvesting based on the detected synchronisation time. The receiver control unit 209 is configured to determine a synchronisation time based on a signal received from a wireless power transmitter. The signal may be a radio signal received by the receiver communication unit 211 or it may be indicated by a charging field signature of the wireless power transfer signal from a wireless power transmitter. Thus, the receiver control unit may be configured to determine a transmitter charging field signature. For example, the transmitter charging field signature may be a measurable characteristic of the wireless power transfer signal, and may comprise a periodic component, i.e. a component in the wireless power transfer signal that appears or occurs at regular intervals. Examples of charging field signatures are described in connection with figs. 8, 9, and 10. Thus, the synchronisation time may be indicated by the occurrence of a periodic component in the wireless power transfer signal. The synchronisation signal from a wireless power transmitter allows for time-synchronisation between the power transmitter and the power receiver.

[0152] The receiver control unit 209 is further configured to reduce or substantially nullify, i.e. cancel, power harvesting during one or more FOD time periods occurring at predetermined time intervals relative to the synchronisation time. Once the wireless power receiver has determined the synchronisation time, the FOD time periods are predetermined, i.e. defined beforehand. Thus, the wireless power receiver time- aligns its power harvesting to a timing that is dependent on a timing provided by the wireless power transmitter. The power receiver may be configured to charge whenever able, while adhering to the scheduled FOD time periods such that the wireless power transmitter can perform a FOD as described herein.

[0153] The receiver communication unit 211 may be configured to send a message comprising information on the charging field signature. The message may be sent as a broadcasted message. A message comprising information on the charging field signature associated with the wireless power transmitter the power receiver is receiving power from may have a number of uses. For example, it may be used to “wake up” a power transmitter that is in a power save mode. Alternatively, or additionally, it may be used as a way to let other wireless receivers know whether they are on the same wireless power transmitter as the power receiver which sent the message. The receiver control unit 209 may be configured to control the behaviour of the wireless power receiver based on whether other power receivers are within the magnetic charging field of the same wireless power transmitter as it. For example, the receiver control unit 209 may be configured to control the behaviour of a function device such that a user-perceptible effect may be generated. This allows for the wireless power receivers to act in a coordinated manner.

[0154] FIGS. 4A and 4B show schematic block diagrams of embodiments of wireless power transmitters and wireless power receivers according to some embodiments.

[0155] In fig. 4A is shown a first wireless power transmitter 5, i.e. a first charger, and a second wireless power transmitter 5”, i.e. a second charger, seen in a perspective view from a side and slightly above the charging surfaces 8, 8” of the chargers 5, 5”. On the charging surface 8 of the first charger 5 is placed a first wireless power receiver 200 and a second wireless power receiver 200”. On the charging surface 8” of the second charger 5” is placed a third wireless power receiver 200”’. The wireless power transmitters 5, 5” and the wireless power receivers 200, 200”, 200”’ may be wireless power transmitters and wireless power receivers as disclosed herein.

[0156] The chargers 5, 5” and the wireless power receivers 200, 200”, 200’” may be part of one or more toy sets. The first and second charger may have different sizes and / or different functionality. For example, the second charger 5” may be smaller than the first charger 5, as shown, and may only be large enough to accommodate a single one of the wireless power receivers on its charging surface 8”. The second charger 5” may be part of a toy set comprising the second charger 5” and a single one of the wireless power receivers 200, 200”, 200”’. The first charger 5 may then, for example, be part of a toy set comprising the first charger 5 and the two wireless power receivers 200, 200”on it. Each of the wireless power transmitters 5, 5” may be configured such that the transmitter does not need to communicate directly, e.g. to pair or handshake, with any of the wireless power receivers 200, 200”, 200”’ in order to allow charging. Thus, a wireless power receivers 200, 200”, 200’” may charge using any of the wireless power transmitters 5, 5”, regardless of whether they are part of the same toy set or not, provided that the wireless power receiver is configured to function within the requirements of the wireless power transmitters FOD.

[0157] Fig. 4B shows the same wireless power transmitters 5, 5” and wireless power receivers 200, 200”, 200’” as are shown in fig. 4A, this time seen from directly above the charging surfaces 8, 8” of the wireless power transmitters 5, 5”. Further, fig. 4B is a partially transparent view as a communication unit 15, 15”, 211, 211”, 21 T” comprised within a housing of each of the wireless power transmitters 5, 5” and each of the wireless power receivers 200, 200”, 200’” is shown.

[0158] The transmitter communication unit 15 comprised in the first wireless power transmitter 5 has dashed lines extending between it and the receiver communication unit 211, 211”, 21 T” comprised in each of the first 200, second 200”, and third wireless power receiver 200’”, respectively. Similarly, dotted lines extending between the second wireless power transmitter 5” and the receiver communication unit 211 , 211 ”, 21 T” comprised in each of the first 200, second 200”, and third wireless power receiver 200’”, respectively.

[0159] The dashed and dotted lines signify status messages being transmitted by the wireless power receivers 200, 200”, 200’” and being received by both of the wireless power transmitters 5, 5” as each of the status messages is being broadcast using a communication protocol having a range that is longer than the effective charging range of the wireless power transfer signal. Each of the status messages comprise data on the amount of power being received by the respective wireless power receiver, i.e. of a received power level. The two wireless power transmitters 5, 5” each use obtained data on received power level to determine the amount of power being received from it by wireless power receivers. However, the wireless power transmitter needs to be able to distinguish which status message(s) is / are providing information on power that is actually received from itself as it may receive status messages from wireless power receivers that are harvesting energy from another wireless power transmitter as is the case in the example illustrated in figs. 4A and 4B.

[0160] Each wireless power transmitter resolves this by also obtaining data on a charging field signature associated with the reported received power level. Thus, each status message sent by a wireless power receiver also comprises data on an associated charging field signature, and each wireless power transmitter can restrict its determination of the total power received from it to be based on each report on a received power level for which its own charging field signature matches the one reported together with the received power level.

[0161] FIG. 5 shows a schematic block diagram of a wireless power transmitter, a wireless power receiver, and a passive repeater according to some embodiments. The wireless power transmitter 5 and wireless power receiver 200 are part of a wireless power transfer system.

[0162] In fig. 5 is shown a wireless power receiver 200 that is also a construction element which may be used in a construction toy set together with other construction elements. Also shown are passive construction elements 530 which are not wireless power receivers. All of the construction elements shown in fig. 5 may be shaped as toy construction elements as described above in connection with fig. 2. For example, the wireless power receiver 200 may be shaped as a construction element with a 2- by-4 planar grid of coupling members on what may be referred to as the top of the element. Some of the coupling pegs 204 are visible as they have not been used for coupling to other elements in the construction of the toy model 500 shown in fig. 5. The toy model 540 has been placed on a wireless power transmitter 5.

[0163] Also shown in fig. 5 is a passive repeater 550. The passive repeater 550 may also be a construction element with coupling members suitable for coupling it to the passive construction elements 530 and / or the wireless power receiver 200. The passive repeater 550 has been positioned in the toy model 540 to be in-between the wireless power receiver 200 and the wireless power transmitter 5. The passive repeater 550 is configured to extend the charging range of the wireless power transmitter 5 via near field magnetic resonant coupling. The charging field of the wireless power transmitter 5 energizes the passive repeater 550 via near field magnetic resonant coupling and the passive repeater 550 “repeats” the charging field of the wireless power transmitter. More than one passive repeater 550 may be used and any additional passive repeaters may also receive energy from the wireless power transmitter 5 or optionally from another passive repeater 550 via near field magnetic resonant coupling. The use of one or more passive repeaters may provide an extension of the charging field and thereby allow a wireless power receiver 200 that is relatively far away from a wireless power transmitter 5 to harvest energy from the wireless power transmitter, or to harvest more energy from the wireless power transmitter.

[0164] FIG. 6 shows a schematic block diagram of a passive repeater according to some embodiments.

[0165] As an example, a passive repeater 550 shaped as a square ring is shown in fig. 6. The passive repeater 550 has eighteen coupling pegs 204 on one side and cooperating cavities on the opposing side (not shown). The passive repeater shown is ring shaped, but other shapes may be suitable as well.

[0166] FIG. 7 shows a schematic block diagram of a wireless power transfer system according to some embodiments. The wireless power transmitter 5 and wireless power receiver 200 are part of a wireless power transfer system as disclosed herein.

[0167] FIG. 8 illustrates examples of charging field signatures according to some embodiments. Shown is an illustrative graph of the charging field level, i.e. a measure of output power, from a wireless power transmitter as disclosed herein, the charging field level being shown as a function of time. The charging field level is shown as having a substantially constant level in the course of a normal operation except during a blanking period 920 in which the charging field level is reduced to a lower level or to substantially zero. The blanking period 920 may be a predetermined time period. The wireless power transmitter may select the blanking period 920, for example at, or prior to, start-up or in response to an event occurring during operation of the wireless power transmitter. The wireless power transmitter may be configured to select from a list of available blanking periods. In the example shown in fig. 8, a maximum blanking period 910 available to the wireless power transmitter is shown with divisions into a number of possible time lengths that are each a multiple of a minimum blanking period 915, the divisions being shown as dotted lines. In fig. 8 shown as five divisions, but a different number of divisions is also possible. The wireless power transmitter used for the example in fig. 8 has a charging field signature identified by a blanking period of twice the minimum blanking period as shown by the full line showing the charging field level versus the dotted line showing the division of the maximum blanking period 910. The wireless power transmitter may be configured such that the blanking period is at least 0.1 milliseconds, such as at least 1 millisecond, such as at least 10 milliseconds, such as at least 100 milliseconds. In some embodiments, the blanking period is less than 800 milliseconds, such as less than 500 millisecond, such as less than 300 milliseconds, such as less than 200 milliseconds, such as less than 100 milliseconds.

[0168] A repetition period 905 is defined in fig. 8 from the beginning of a blanking period and until the beginning of the next blanking period, but may be defined in other ways. Within the repetition period then, the charging field level is reduced to a lower level or to substantially zero for a period of time, the blanking period, and for the rest of the repetition period the charging field level is shown at a substantially constant level for ease of illustration. The wireless power transmitter may be configured such that the repetition period is at least 0.1 seconds, such as at least 1 second, such as at least 2 seconds, such as at least 3 seconds, such as at least 4 seconds, such as at least 10 seconds, such as at least 20 seconds, such as at least 30 seconds. The wireless power transmitter may be configured such that the repetition period is around 30 minutes or less, such as 15 minutes or less, such as 10 minutes or less such as 5 minutes or less, such as 1 minute or less. The wireless power transmitter may be configured such that the repetition period is between 0.5 seconds and 15 minutes, such as between 1 second and 10 minutes, such as between 5 seconds and 5 minutes, such as between 15 seconds and 1 minute The power transmission period, where the wireless power transmitter provides power, is in the example of fig. 8 the period between an end of a blanking period and until the beginning of the next blanking period. If the wireless power transmitter is designed for a system wherein FOD is based on status messages, i.e. reports on received power, from wireless power receivers, a wireless power receiver can harvest power for the entirety of the power transmission period. If designed for a system wherein FOD is based on the wireless power receivers not harvesting during predetermined time periods, the FOD time periods, a wireless power receiver will only be charging during some of the power transmission period.

[0169] As illustrative examples, the repetition period may be 5 seconds and the minimum blanking period may be 150 ms (milliseconds). To continue with this example as shown in fig. 8, a wireless power transmitter may then have e.g. five possible blanking periods to choose from: 150 ms, 300 ms, 450 ms, 600 ms, and 750 ms (milliseconds). Alternative, or additionally, the length of the repetition period may be selected by a wireless power transmitter as part of its charging field signature. A charging field signature may be any modified property of the charging field which a wireless power receiver can detect. A charging field signature may be a combination, for example a combination of a blanking period and repetition period such that two wireless power transmitters with the same blanking period could be distinguished by e.g. having different repetition periods. As described above, e.g. in connection with fig. 1, a wireless power transmitter may be configured to select a charging field signature at specific events, such as e.g. start-up or in case the determined total received power indicates a conflict with another wireless power transmitter. In the latter case, the wireless power transmitter may be configured to select a different charging field signature than the one it had when the conflict occurred. A wireless power transmitter may be configured to select a charging field signature intermittently, e.g. at random times and / or at predetermined time intervals and / or at one or more fixed times. A wireless power transmitter may be configured to select a charging field signature based on associated charging field signatures in messages from power receivers, such as from status messages. Advantageously, the wireless power transmitter may then select a charging field signature that does not appear in any of the obtained messages. A wireless power transmitter may be configured such that whenever a transmitter charging signature is to be selected, the wireless power transmitter selects a charging field signature of the available charging field signatures that provides a more efficient charging. For example, if the charging field signature is a blanking period as described here, the wireless power transmitter may be configured to select a charging field signature with as short a blanking period as possible if this provides more time wherein the wireless power transmitter provides charging power. Thus, the wireless power transmitter may be configured to select at transmitter charging signature which allows the wireless power transmitter to provide charging for as much time as possible on average.

[0170] The charging field signature may be defined by e.g. length of a blanking period, repetition period, and / or time passed since last blanking period. Thus, a charging field signature may have a periodic component such as for example length of a blanking period, repetition period, and / or time passed since last blanking period. In some embodiments, the wireless power transmitter is configured to regulate the power of the wireless power transfer signal at regular time intervals, and the transmitter charging field signature is defined at least in part by the regular time intervals of the power of the wireless power transfer signal. A wireless power receiver may then detect one or more characteristics of the wireless power transfer signal while receiving power from a wireless power transmitter, and determine a charging field signature from this; The wireless power receiver may then transmit a message comprising data on the charging field signature associated with the received power and possibly on a received power level.

[0171] In some embodiments, the charging field signature may be configured to not provide communication at radio wavelengths which would fall within the regulations of radio communication.

[0172] Generally, the charging field signature is designed to act primarily as an identifiable signature of the charging field produced by a wireless power transmitter that may be part of a toy set.

[0173] FIG. 9 illustrates features of a method of wirelessly transmitting power from a wireless power transmitter to one or more wireless power receivers according to some embodiments. The method comprises status messages being sent by wireless power receivers and being used by a wireless power transmitter for FOD. Fig. 9 shows a graph of the charging field level of a wireless power transmitter similar to that described in detail in connection with fig. 10. The charging field signature comprises a blanking period 920, wherein the charging field is reduced significantly or turned off. The top part of fig. 9 shows a somewhat idealized graph, while a more representative graph of a charging field level as measured by a wireless power receiver is shown below it.

[0174] A wireless power receiver which is receiving power from the wireless power transmitter, is configured to determine an estimate of the power received, a received power level, and send a status message comprising the received power level. The received power level may be determined as an average power received, for example as a rolling average power received 930 which is an average of the power received by the wireless power receiver over a predetermined time period. In fig. 9 a rolling average power is symbolised by a black circle. For example, the wireless power receiver may be configured to determine a rolling average power received over a time period that is substantially the same as the repetition period 905. In other embodiments, the wireless power receiver may be configured to determine a rolling average power received over a time period that is shorter or longer than the repetition period. In some embodiments, the wireless power receiver may be configured to determine a rolling average power received over a time period that is longer than 3 seconds, such as longer than 4 seconds, such as longer than 5 seconds, such as longer than 6 seconds, such as longer than 7 seconds, such as longer than 8 seconds, such as longer than 10 seconds, such as longer than 20 seconds. In some embodiments, the wireless power receiver may be configured to determine a rolling average power received over a time period that is shorter than 30 seconds, such as shorter than 20 seconds, such as shorter than 10 seconds. The wireless power receiver may be configured to determine the received power level, such as the rolling average power received, at a predetermined regular time interval, and / or at a predetermined time, absolute or relative.

[0175] The wireless power receiver is further configured to determine a charging field signature related to the charging field providing the power, and to send a status message comprising information on both the determined charging field signature and the determined rolling average power received. The wireless power receiver may be configured to send a status message at a predetermined regular time interval, and / or at a predetermined time. In a preferred embodiment, the wireless power receiver is configured to send a status message at a predetermined regular time interval that is shorter than the time period over which the rolling average power received is determined. In fig. 9, the time between status messages is denoted by the reference number 925. In some embodiments, the wireless power receiver is configured to send a status message with the same frequency as a received power level, such as a rolling average power received, is determined. In fig. 9 the large arrows represent a status message being sent by a wireless power transmitter and the smaller arrows pointing to each large arrow represent the determined blanking period and rolling average power received being comprised in the respective status message.

[0176] A wireless power transmitter is configured to determine a transmitted power level, where the transmitted power level is the level of wireless power transmitted by it. Similarly to the configuration of the wireless power receiver described above, the transmitted power level may be determined as an average power transmitted, for example, as a rolling average power transmitted which is an average of the power transmitted by the wireless power transmitter over a predetermined time period. For example, the wireless power transmitter may be configured to determine a rolling average power transmitted over a time period that is substantially the same as the repetition period. In other embodiments, the wireless power transmitter may be configured to determine a rolling average power transmitted over a time period that is longer than the repetition period. In some embodiments, the wireless power transmitter may be configured to determine a rolling average power transmitted over a time period that is longer than 3 seconds, such as longer than 4 seconds, such as longer than 5 seconds, such as longer than 6 seconds, such as longer than 7 seconds, such as longer than 8 seconds. The wireless power transmitter may be configured to determine the transmitted power level, such as the rolling average power transmitted, at a predetermined regular time interval, and / or at a predetermined time. The wireless power transmitter and wireless power receiver may be configured to determine the transmitted power level, such as the rolling average power transmitted, and the received power level, such as the rolling average power received, at the same predetermined regular time interval, and / or at the same predetermined time, absolute or relative.

[0177] In the example illustrated in fig. 9, the wireless power transmitter may e.g. have a charging field with a repetition period of 5 seconds and a selected blanking period of 300 milliseconds; Further, the wireless power receiver may be configured to determine a rolling average power received every second and over a period of 5 seconds, and to send a status message every second with the latest determined rolling average power received together with data on a determined charging field signature, such as the blanking period of 300 milliseconds.

[0178] FIG. 10 shows an example of a method of wirelessly transmitting power from a wireless power transmitter to two wireless power receivers according to some embodiments. The method comprises a synchronisation signal being transmitted by a wireless power transmitter and received by wireless power receivers which in response time-synchronise their power harvesting.

[0179] In the top of fig. 10, at A), is shown an illustrative graph of the charging field level, i.e. a measure of output power, from a wireless power transmitter as disclosed herein, the charging field level being shown as a function of time. The charging field level is shown as having a substantially constant level in the course of a normal operation except during a blanking period 920 in which the charging field level is reduced to a lower level or to substantially zero. The blanking period 920 and repetition period 905 may be as described in connection with fig. 8.

[0180] Also shown in A) are FOD time periods 922 which are time periods during which the wireless power transmitter is configured to assume, i.e. determine, that wireless power receivers configured to charge wirelessly from the wireless power transmitter are not harvesting power from it. The wireless power transmitter is configured to obtain one or more measurements on power harvested from it during these time periods and to perform a Foreign Object Detection, wherein a determination on the presence of a Foreign Object is made.

[0181] The FOD time periods 922 occur at predetermined time intervals relative to a synchronisation time. In the embodiment shown in fig. 10, the synchronisation time 921 is the beginning of a blanking period 920. As an example, the first FOD time period may occur a time after the synchronisation time, an offset time interval 923. By having an offset time, it may be arranged such that the blanking period 920, where the charging field is reduced or substantially zero, does not overlap with the first FOD time period. Further, a wireless power receiver may have a delay in its detection of the synchronisation signal resulting in that wireless power receiver being “off’ in its synchronisation with the charger as will be further described below. Multiple FOD time periods may occur within a repetition period 905 and may, as an example, be separated by a FOD time interval 924 between the beginning of one FOD time period and the beginning of the next FOD time period.

[0182] In B) and C) of fig. 10 are shown an illustrative graph of power harvesting as a function of time by two different wireless power receivers, referred to as power receiver B and power receiver C, respectively.

[0183] The wireless power receiver B is harvesting power, when it detects the synchronisation signal as the beginning of a blanking period, the detection being illustrated by a thick line 921 B. The power receiver B is configured to not harvest power during the predetermined FOD time periods 922 and substantially, i.e. essentially, nullifies its harvesting, for example by reducing its load or by changing the resonance of the receiver coil circuit, during those periods. This causes the power harvesting graph in B), where the power harvesting can be seen to drop to substantially zero during the FOD time periods 922 after which time period power harvesting is resumed. At a later time, another synchronisation signal is detected 921 B’ and the power receiver re-synchronises to the wireless power transmitter schedule of FOD time periods 922 relative to the detected synchronisation time.

[0184] Likewise, the wireless power receiver C is harvesting power, when it detects the synchronisation signal as the beginning of a blanking period, the detection being illustrated by a thick line 921C. However, the power receiver C detects the synchronisation signal a little late, and later than power receiver B which detected it almost immediately. A wireless power receiver may, however, be configured to reduce its power harvesting at a time before the beginning of a FOD time period 922 and to start power harvesting at a time after the end of a FOD time periods 922. In this way, even if a wireless power receiver detects the synchronisation signal a little late, as is the case for power receiver C, its power harvesting is still reduced within the FOD time periods 922 as shown in fig. 10 C).

[0185] Fig. 10 illustrates how a wireless power transmitter may time-synchronise with one or more wireless power receivers using a charging field signature. In an alternative embodiment, the synchronisation signal may be a radio signal. In an embodiment, multiple synchronisation signals are transmitted, for example a synchronisation signal via the charging field signature and a synchronisation signal as a radio signal.

[0186] In some embodiments, the wireless power transmitter may additionally obtain one or more measurements on power harvested from it at times outside of the FOD time periods. By measuring power harvested from it during both FOD time periods and outside of these time periods, the wireless power transmitter may compare the two and make determinations as to the presence of objects charging from it. For example, if the power harvested from it increases and decreases according to the pattern of FOD time periods, the wireless power transmitter may determine that one or more associated power receivers are charging from it. If the power harvested from it is similar whether at a time during a FOD time period or outside of a FOD time period, the wireless power transmitter may determine that only Foreign Objects, or nothing, is harvesting power from it.

[0187] FIG. 11 shows a flow chart of a method of wirelessly transmitting power by a wireless power transmitter and a method of communicating wireless received power according to some embodiments. The method comprises status messages being sent by wireless power receivers and being used by a wireless power transmitter for FOD. The wireless power transmitter may be a power transmitter as described herein and / or the wireless power receiver may be a wireless power receiver as described herein.

[0188] In step S10, the wireless power transmitter transmits a wireless power transfer signal having a charging field signature.

[0189] In step S12, the wireless power receiver receives power from a wireless power transfer signal. In step S18, the wireless power transmitter determines a transmitted power level which is a measure of the power that the wireless power transmitter has transmitted. This may, for example, comprise measuring and storing transmitted power values. It may further comprise determining a windowed average of transmitted power, for example, based on the measured and stored transmitted power values.

[0190] In step S19, the wireless power receiver determines received power level which is a measure of the power that the wireless power receiver has received. This may, for example, comprise measuring and storing received power values. It may further comprise determining a windowed average of received power, for example, based on the measured and stored transmitted power values.

[0191] In step S25, the wireless power receiver determines the charging field signature associated with the received power.

[0192] In step S26, the wireless power receiver sends a status message comprising the determined received power level, such as a determined windowed average of received power, and the determined charging field signature associated with the received power.

[0193] In step S30, the wireless power transmitter obtains one or more status messages such as e.g. the status message sent by the wireless power receiver in step S26 as well as possibly status messages sent by other wireless power receivers. Each status message obtained comprises a received power level and an associated charging field signature

[0194] In step S35, the wireless power transmitter filters obtained status message(s) based on the associated charging field signature(s) reported in each status message.

[0195] In step S40, the wireless power transmitter determines a total received power, for example a windowed average of total received power, based on the received power level(s) reported in the obtained status message(s) for which the associated charging field signature(s) match the charging field signature of the wireless power transmitter, if any. In step S45, the wireless power transmitter determines a Foreign Object Loss based on the determined total received power and the determined transmitted power level.

[0196] FIG. 12 shows a flow chart of a method of wirelessly transmitting power by a wireless power transmitter and a method of harvesting wirelessly transmitted power according to some embodiments. The method comprises a synchronisation signal being transmitted by a wireless power transmitter and received by a wireless power receiver which in response time-synchronises its power harvesting. The wireless power transmitter may be a power transmitter as described herein and / or the wireless power receiver may be a wireless power receiver as described herein.

[0197] In step S10’, the wireless power transmitter transmits a wireless power transfer signal and a synchronisation signal indicating a synchronisation time. The synchronisation signal may be a radio signal or may be transmitted via the wireless power transfer signal, e.g. as part of a charging field signature.

[0198] In step S12, the wireless power receiver receives power from a wireless power transfer signal. The wireless power receiver also receives the synchronisation signal and determines a synchronisation time therefrom.

[0199] In step S21, the wireless power receiver time-synchronises its power harvesting based on the determined synchronisation time such that power is not harvested during predetermined FOD time periods.

[0200] In step S22, the wireless power transmitter performs one or more measurements on transmitted power during one or more FOD time periods as part of its Foreign Object Detection. This may, for example, comprise measuring and storing transmitted power values. It may further comprise determining a windowed average of transmitted power, for example, based on the measured and stored transmitted power values.

[0201] In optional step S25’, the wireless power receiver determines a charging field signature associated with the received power and sends a message comprising information on the determined charging field signature. In step S45’, the wireless power transmitter determines an estimate of power consumed by Foreign Objects based at least partly on the measurements on transmitted power during one or more FOD time periods.

[0202] FIG. 13 shows a flow chart of a method of wirelessly transmitting power by a wireless power transmitter according to some embodiments. The wireless power transmitter performs a FOD based on status messages received from wireless power receivers. The wireless power transmitter may be a wireless power transmitter as described herein.

[0203] In step S5, the wireless power transmitter selects a (new) charging field signature.

[0204] In step S10”, the wireless power transmitter begins transmission of a wireless power transfer signal with the selected charging field signature, or the wireless power transmitter continues transmission of a wireless power transfer signal with the selected charging field signature.

[0205] In step S15, the wireless power transmitter measures and stores transmitted power values.

[0206] In step S20, the wireless power transmitter determines a windowed average of transmitted power based on the measured and stored transmitted power values. In this way, the wireless power transmitter determines a transmitted power level which is a measure of the power that the wireless power transmitter has transmitted.

[0207] In step S30, the wireless power transmitter obtains one or more status messages, where each status message comprises a received power level and an associated charging field signature.

[0208] In step S35, the wireless power transmitter filters obtained status message(s) based on the associated charging field signature(s) reported in each status message.

[0209] In step S40, the wireless power transmitter determines a total received power, for example a windowed average of total received power, based on the received power level(s) reported in the obtained status message(s) for which the associated charging field signature(s) match the charging field signature of the wireless power transmitter, if any. In step S45, the wireless power transmitter determines a Foreign Object Loss based on the determined total received power and the determined transmitted power level.

[0210] In step S50, the wireless power transmitter determines whether it must take an action in response to the Foreign Object Loss determined in step S45. For example, the wireless power transmitted may evaluate the determined Foreign Object Loss against a predetermined threshold.

[0211] If the wireless power transmitter determines that no action will be taken in response to the determined Foreign Object Loss, it may continue the transmission of a wireless power transfer signal with the selected charging field signature by returning to step S10”.

[0212] If the wireless power transmitter determines that an action will be taken in response to the determined Foreign Object Loss, it must in step S55 determine which action to take. In fig. 13 the determination of which action to take is shown as a decision step, S55, separate from the determination of whether to take an action, S50. However, in some embodiments, these two determinations may be part of the same decision step.

[0213] During decision step S55, the wireless power transmitter may determine that there is a conflict with another wireless power transmitting a wireless power transfer signal comprising the same charging field signature as itself. In response, the wireless power transmitter may return to step S5 and select a new charging field signature.

[0214] During decision step S55, the wireless power transmitter may determine that a Foreign Object is receiving power from it, and the wireless power transmitter may reduce the power output in step S60 to limit potential damage, or it may shut down the charging field. If the power output is being reduced, S60, the wireless power transmitter may then return to step S10” and continue the transmission of a wireless power transfer signal with the same charging field signature, now at a reduced power.

[0215] During operation, the wireless power transmitter may go to step S5 intermittently, for example at random times and / or at predetermined time intervals and / or at one or more fixed times; The wireless power transmitter may then select a new charging field signature before initiating or continuing transmission of a wireless power transfer signal with the selected charging field signature.

[0216] FIG. 14 shows a flow chart of a method of wirelessly transmitting power by a wireless power transmitter according to some embodiments. The wireless power transmitter performs a FOD based on a synchronisation signal being transmitted by a wireless power transmitter and received by a wireless power receiver which in response time-synchronises its power harvesting. The wireless power transmitter may be a wireless power transmitter as described herein.

[0217] In optional step S5, the wireless power transmitter selects a (new) charging field signature.

[0218] In step S10’”, the wireless power transmitter transmits a wireless power transfer signal and a synchronisation signal indicating a synchronisation time. The synchronisation signal may be a radio signal or may be transmitted via the wireless power transfer signal, e.g. as part of a charging field signature. The charging field signature may be the one selected in step S5.

[0219] In step S22, the wireless power transmitter performs one or more measurements on transmitted power during one or more FOD time periods as part of its Foreign Object Detection. This may, for example, comprise measuring and storing transmitted power values. It may further comprise determining a windowed average of transmitted power, for example, based on the measured and stored transmitted power values.

[0220] In optional step S30’, the wireless power transmitter obtains one or more messages (from wireless power receivers) comprising information on charging field signatures.

[0221] In step S45’, the wireless power transmitter determines an estimate of power consumed by Foreign Objects based at least partly on the measurements on transmitted power during one or more FOD time periods.

[0222] In step S50, the wireless power transmitter determines whether it must take an action in response to the estimate of power consumed by Foreign Object determined in step S45’. For example, the wireless power transmitted may evaluate the determined estimate of power consumed by Foreign Objects against a predetermined threshold.

[0223] If the wireless power transmitter determines that no action will be taken in response to the determined Foreign Object Loss, it may continue the transmission of a wireless power transfer signal with the selected charging field signature by returning to step S10’”.

[0224] If the wireless power transmitter determines that an action will be taken in response to the determined Foreign Object Loss, it must in step S55 determine which action to take. In fig. 14 the determination of which action to take is shown as a decision step, S55, separate from the determination of whether to take an action, S50. However, in some embodiments, these two determinations may be part of the same decision step.

[0225] During decision step S55, the wireless power transmitter may determine that there is a conflict with another wireless power transmitting a wireless power transfer signal comprising the same charging field signature as itself. The determination may, for example, be based on the one or more messages obtained in optional step S30’. In response, the wireless power transmitter may return to optional step S5 and select a new charging field signature.

[0226] During decision step S55, the wireless power transmitter may determine that a Foreign Object is receiving power from it, and the wireless power transmitter may reduce the power output to limit potential damage, or it may shut down the charging field. If the power output is being reduced the wireless power transmitter may then return to step S10” and continue the transmission of a wireless power transfer signal now at a reduced power.

[0227] During operation, the wireless power transmitter may go to optional step S5 intermittently, for example at random times and / or at predetermined time intervals and / or at one or more fixed times; The wireless power transmitter may then select a new charging field signature before initiating or continuing transmission of a wireless power transfer signal with the selected charging field signature.

Claims

CLAIMS1. A wireless power transmitter (5) comprising:- A transmitter coil (10) configured to transmit a wireless power transfer signal, the wireless power transfer signal being transmitted using a magnetic charging field configured for transmission of power, and a transmitter control unit (20) configured to perform a Foreign Object Detection comprising obtaining one or more measurements on power harvested from the wireless power transmitter, wherein the wireless power transmitter is configured to transmit a signal at predetermined time intervals, the signal indicating a synchronisation time, wherein the transmitter control unit is configured to perform the one or more measurements on harvested power during one or more FOD time periods occurring at predetermined time intervals relative to the synchronisation time, and wherein the transmitter control unit is configured to determine as part of its Foreign Object Detection that associated power receivers do not harvest power from it during the FOD time periods and / or that associated power receivers substantially reduce power harvesting from it during the FOD periods.

2. The wireless power transmitter according to claim 1 , wherein the wireless power transfer signal comprises a transmitter charging field signature which is a characteristic that is measurable by a wireless power receiver, the transmitter charging field signature indicating a synchronisation time at predetermined time intervals.

3. The wireless power transmitter according to claim 2, wherein the charging field signature comprises a repeating blanking period which is a time period of reduced or substantially nullified power transfer, and wherein the repeating blanking period indicates the synchronisation time.

4. The wireless power transmitter according to claim 1 , wherein the signal indicating a synchronisation time is a radio signal.

5. The wireless power transmitter according to any of the preceding claims, wherein the predetermined time intervals between synchronisation times are pseudo-random and / or wherein the predetermined time intervals between FOD time periods are pseudo-random.

6. The wireless power transmitter according to any of the preceding claims, wherein the transmitter control unit is further configured to perform one or more comparative measurements on power harvested from it at times outside of the FOD time periods.

7. The wireless power transmitter according to any of the preceding claims, wherein the wireless power transmitter is configured to reduce power output, shut down, or shut down power transmission, if it determines that a Foreign Object is receiving power from it.

8. A wireless power receiver (200) comprising: a receiver coil (207) configured to receive a wireless power transfer signal, a load coupled to the receiver coil, the load being configured to receive power from the receiver coil, a receiver control unit (209) configured to adjust power harvesting via the receiver coil, wherein the receiver control unit is configured to determine a synchronisation time from a signal received from a wireless power transmitter, and wherein the receiver control unit is configured to reduce or substantially nullify power harvesting during one or more FOD time periods occurring at predetermined time intervals relative to the synchronisation time.

9. The wireless power receiver according to claim 8, wherein the receiver control unit is configured to determine a charging field signature of a power transmitter from which the power receiver is receiving power, the transmitter charging field signature indicating the synchronisation time, or wherein the signal indicating a synchronisation time is a received radio signal.

10. A wireless power transfer system comprising a wireless power transmitter (5) according to any of claims 1 - 7, and a wireless power receiver (200) according to any of claims 8 or 9.

11. A method of wirelessly transmitting power by a wireless power transmitter, the method comprising:Transmitting a wireless power transfer signal, the wireless power transfer signal being transmitted using a magnetic charging field configured for transmission of power,Performing a Foreign Object Detection comprising obtaining one or more measurements on harvested power, wherein the wireless power transmitter is configured to transmit a signal at predetermined time intervals, the signal indicating a synchronisation time, wherein the one or more measurements on harvested power is performed during one or more FOD time periods occurring at predetermined time intervals relative to the synchronisation time, and wherein performing the Foreign Object Detection comprises determining that associated power receivers do not harvest power from the wireless power transmitter during the FOD time periods and / or determining that associated power receivers substantially reduce power harvesting from the wireless power transmitter during the FOD periods.

12. The method according to claim 11 , wherein the wireless power transmitter is a wireless power transmitter according to any of claims 1-7.

13. A method of harvesting wirelessly transmitted power by a wireless power receiver, the method comprising:Receiving wirelessly transmitted power,Detecting a synchronisation signal,Determining a synchronisation time from the synchronisation signal, Reducing or substantially nullifying the power harvesting during one or more FOD time periods occurring at predetermined time intervals relative to the synchronisation time.

14. The method according to claim 13, wherein the wireless power receiver is a wireless power receiver according to any of claims 8 or 9.

15. A method of wirelessly transmitting power from a wireless power transmitter to one or more wireless power receivers, the method comprising:Producing, by the wireless power transmitter, a magnetic charging field configured for transmission of power,Transmitting, from the wireless power transmitter to the one or more wireless power receivers, a signal at predetermined time intervals, the signal indicating a synchronisation time,Receiving wirelessly transmitted power by the one or more wireless power receivers,Reducing or substantially nullifying the power harvesting, by the one or more wireless power receivers, during one or more FOD time periods occurring at predetermined time intervals relative to the synchronisation time, Performing, by the wireless power transmitter, a Foreign Object Detection comprising obtaining one or more measurements on harvested power, the one or more measurements on harvested power being performed during one or more of the one or more FOD time periods.

16. The method according to claim 15, wherein the wireless power transmitter is a wireless power transmitter according to any of claims 1-7 and / or the one or more wireless power receivers are wireless power receivers according to any of claims 8 or 9.

17. A non-transitory computer readable medium having stored thereon:- instructions that when executed by processing circuitry of a wireless power transmitter cause the processing circuitry to perform the steps of the method of claim 11, or - instructions that when executed by processing circuitry of a wireless power receiver cause the processing circuitry to perform the steps of the method of claim 13.

Images