WO2021234399A1 - Relay wireless charging system - Google Patents

Abstract

A system for wirelessly charging at least one device (2, 4, 6; 110; 136) is disclosed. The device has a photovoltaic cell (32; 70; 108; 140; 166) for converting incident light into electrical energy. The system has a hub unit (8; 104; 114; 124; 148) with a laser source (22; 146, 144) and at least one relay unit (38, 98; 122; 138; 154). The relay unit is arranged to direct a laser beam (30; 102a; 112b; 128; 168) to the photovoltaic cell of the device.

Description

Relay Wireless Charging System With the continued proliferation of electronic devices, particularly those where it is not convenient or possible to provide a permanent wired connection to a mains power supply, and growing expectations for the functionality and battery life that these provide, there remains an important focus on how such devices are charged. There have been a number of developments in charging technology in recent years, most notably the introduction of magnetic induction charging to avoid the need for a physical coupling between the charger and the device being charged. Whilst this technology may be well suited to personal portable devices such as smart phones, smart watches, tablets etc, the need for a close physical proximity between the device and the charging surface does not make this technology suitable in all circumstances.

There have also been proposals to use lasers to provide power to charge devices by using the laser to illuminate a suitable photocell on the device. This has the advantage of removing the need for the device to be held close to a charging surface. However it suffers from some significant drawbacks. One of these is the requirement to have in place a suitable feedback system to ensure alignment between the laser and the photocell. Another is that a line of sight is required between the charging unit and the device which may cause difficulties in some environments or mean that additional charging units are needed.

Moreover, the above-mentioned laser charging methods are only capable of providing low charging currents. On one hand, although there are steady improvements being made, the efficiency of photovoltaic cells is still in general a long way below the theoretical maximum. On the other hand, there are stringent safety restrictions on the power levels for lasers that can be used in ordinary public, workplace or domestic settings.

For these and other reasons, the aforementioned remote laser charging has yet to be widely adopted. The present invention seeks to address at least some of the above and when viewed from a first aspect provides a system for wirelessly charging at least one device, said device comprising a photovoltaic cell for converting incident light into electrical energy, the system further comprising a hub unit comprising a laser source and at least one relay unit arranged to direct a laser beam to the photovoltaic cell of said device.

Thus it will be seen by those skilled in the art that in accordance with the invention a device to be charged (DTC) may be charged by a laser transmitted or deflected from the relay unit. This advantageously enables the charging laser beam to be transmitted from the relay unit(s) located at a different place either, instead of, or as well as transmitting from the hub unit. This may allow the charging laser beam to reach and charge DTCs which would have been blocked by an occlusion of the line of sight of a laser beam from the hub.

The relay unit could be powered by a mains electrical connection or on board battery. In a set of embodiments the relay unit comprises a photovoltaic cell. This could be used to receive power simply from ambient light such as sunlight but in a set of embodiments the hub unit is arranged to transmit a laser beam from the laser source to the photovoltaic cell of the relay unit to power said relay unit. This means that the relay unit can be wirelessly charged by the hub unit.

The relay unit could have its own laser source for providing the charging beam but in a set of embodiments the system is arranged so that the relay unit deflects a laser beam from the hub unit to the DTC. In a preferred set of embodiments the laser beam that is deflected by the relay unit to the DTC is the same laser that is used to charge the relay units. In such embodiments the relay unit may comprise a reflective element suitable for deflecting the incident laser beam from the hub unit to the DTC. In this case, the relay unit may receive a wireless communication e.g. from the hub, e.g. via WiFi, Bluetooth™, LTE, Zigbee, instructing the relay unit to generate and/or direct a laser beam toward a DTC.

The Applicant has recognised that by virtue of utilising the relay unit(s) to deflect the laser beam to the DTC, the laser beam on average has to travel a further distance before being incident on the photovoltaic (PV) cell of the DTC than if the laser beam travelled directly from the hub unit. In a set of embodiments, the relay unit comprises a collecting lens to refocus or collimate the laser beam to mitigate this.

In a set of embodiments the relay unit comprises a transmitter and is arranged to transmit a signal which is received by the hub unit and used to determine a position of the relay unit. The signal transmitted by the relay unit could be for example an optical, ultrasound or radio frequency signal. Such a signal could be in the form of a dedicated beacon or could be a signal transmitted for other purposes - e.g. as part of a WiFi, Bluetooth™ or LTE connection. The hub unit may employ any suitable technique or combination thereof for locating the position of the relay units, e.g. beamforming, time of flight measurement, time difference of arrival, signal strength measuring etc. In such a set of embodiments the hub unit comprises a receiver arranged to receive said signal.

The hub unit may comprise a single receiver element. However, in a set of embodiments the hub unit comprises an array of receiver elements for receiving the above-mentioned positioning signal. The array of receiver elements could be used to determine the position of the relay units more accurately. For example, if the transmitted signal from the relay unit was an optical signal, the hub unit could comprise an optical sensor, for example a camera with low resolution which cannot form detailed images as such (to allay potential privacy concerns). In other embodiments the signal is acoustic - either audible or ultrasonic. In such embodiments echo-location could be used i.e. employing beam-forming. Similarly the array of receiver elements would allow beam-forming to be used for RF signals.

As outlined in the above, the relay unit could be located by transmitting a signal for the hub unit to receive and subsequently determine the location of the relay unit from the signal the hub receives from it. In another set of embodiments, the relay unit is located by the hub unit by passive means on the relay unit. For example, in a set of embodiments the relay unit comprises a visible marker, e.g. a binary ArUco code, which may allow the hub unit to estimate the position of the relay unit visually. Furthermore, the relay unit’s position and orientation in relation to the camera, i.e. the pose, may be estimated from the detection of such visible markers using the correspondence of the two-dimensional image pixels to the object points. In such a set of embodiments, the hub unit may comprise a camera to detect the visible marker, e.g. a camera with low resolution to protect the privacy of the users of the relay wireless charging system. Alternatively, the hub unit may transmit a signal, e.g. optical, for the relay unit to reflect, e.g. using a retroreflector which reflects a signal back to the source. The reflected signal received by the hub unit may allow the system to determine an approximate position of the relay unit.

In an alternative set of embodiments, the positions of the relay units are programmed or hardcoded onto the system - e.g. during a setup phase.

In a set of embodiments the DTC is located by the hub unit. Equally, the relay unit or a plurality of relay units may locate the DTC. Advantageously the hub and relay unit(s) may be used dynamically to locate the DTC depending on its location and any obstructions in the zone.

In a set of embodiments the relay unit and/or the hub unit, receives a signal from the DTC - for example, an optical, ultrasound or radio frequency signal - which may be used to locate the DTC. In such a set of embodiments the relay and/or hub unit comprises a receiver suitable for receiving the signal, for example an optical, ultrasound or radio frequency receiver. The receiver may comprise a single receiving element or equally an array of receiving elements.

In a set of embodiments the DTC transmits a signal, e.g. an RF signal, to be received by at least two relay units, or alternatively, the hub unit and one or more relay unit(s). In such embodiments, the transmitted signal may be received by the multiple receivers and the location of the DTC can be determined from a triangulation calculation. This arrangement of receivers, e.g. the relay unit and the hub unit placed at ceiling level spaced apart across a room, typically results in a large, known baseline which is defined by the distance between the receivers. This large baseline gives the advantage of allowing the system to more precisely calculate the location of the DTC, as this reduces the relative uncertainty in the calculation. In such a set of embodiments, the location of the DTC may be calculated using the known baseline and angles of arrival. The angles of arrival may be calculated from the time difference of arrival (TDOA) at each of the receiver elements of a phased array of receiver elements at the hub or relay unit. Alternatively the DTC may comprise a suitable receiver and the relay unit(s) and/or the hub unit may transmit signals e.g. RF, for the DTC to receive and determine through suitable techniques, e.g. trilateration, the position of the DTC. Equally, any other suitable technique may be used to estimate the position of the DTC by processing a signal received at a phased array of receiver elements, e.g.

Frequency Difference of Arrival, Time Difference of Arrival, triangulation, trilateration, multilateration, and beamforming.

In one set of embodiments the received signal is processed by the relay unit. Alternatively the received signal(s) could be wirelessly transmitted to the hub unit to be processed there. The selected site of processing - the relay unit or the hub unit - may employ any suitable technique or combination thereof for locating the DTC, e.g. beamforming, time of flight measurement, time difference of arrival, signal strength measuring, line intersection, plane intersection, triangulation etc.

In a set of embodiments the system can locate the DTC by scanning a laser beam over a charging zone. In one set of such embodiments, the search starts with scanning the laser beam over a scan volume during a first mode with a first divergence angle. When the laser beam impinges on the PV cell of the DTC, the system receives a notification, prompting the system to change to a second mode with a second narrower divergence angle.

The notification could comprise a signal transmitted by the DTC, e.g. over an RF communication channel such as Bluetooth™. Equally, the notification could comprise a reflected signal transmitted by the hub or relay unit which reflects back to the hub or relay unit after becoming incident on a reflecting element situated on the DTC, e.g. for an optical signal the reflecting element could be a retroreflector. Such a scan could be performed by the hub unit. In another set of embodiments, the scan is performed by the relay unit.

As will be appreciated, such an arrangement allows the system to determine at least an approximate location of the DTC by correlating receipt of the notification (or a time stamp in the notification) with a control algorithm for the laser beam scan which can establish a direction in which the laser beam was pointing when it impinged on the PV cell. Depending on the precision of the location information, the narrower beam of the second mode may just be used immediately to charge the DTC. However in a set of embodiments during the second mode the system scans the beam over a second, smaller scan volume based on said location information. This may allow more accurate location determination - e.g. by scanning the smaller beam more slowly. The beam may then be left pointing at the PV cell to commence charging. Equally one or more further iterations of beam reduction and scanning may be envisaged. The hub unit could perform the above-mentioned scan to locate the DTC. In an alternative set of embodiments, the scan could be performed by the relay unit.

In a set of embodiments, the relay unit is arranged to perform an optical search to locate a device to be charged (DTC) having a retroreflector. In a set of such embodiments, the relay unit comprises illumination means arranged to illuminate a zone and a low resolution camera arranged to image at least part of the zone illuminated. It will be appreciated of course that multiple cameras could be used to image an illuminated zone. The low resolution camera may detect where the retroreflectors are located (and thus where the DTCs are located). For example, areas where retroreflection occurs may be detected as brighter areas in an image obtained by the low resolution camera. To be located, the DTCs merely need to be detected as brighter areas (spots) on an image. The Applicant has appreciated that such bright spots can be reliably imaged with very limited resolution cameras as long as a central point can be estimated. Therefore a higher resolution camera would not be necessary. This may be beneficial from the point of view of alleviating privacy concerns as well as reducing power consumption.

In a set of embodiments the low-resolution camera has a resolution of less than 50,000 pixels, e.g. less than 10,000 pixels - e.g. less than 5,000 pixels. For example, the camera may comprise between 50 X 50 and 200 X 200 pixels - e.g. 100 X 100 pixels. Such cameras may only consume a few milliwatts of power.

According to the above-described approach, the relay units may therefore be used to find a DTC which cannot be ‘seen’ by the hub unit (or other relay units). Furthermore, having a retroreflector on each DTC provides a low-power passive component that can possibly replace an active uplink communication channel (e.g. WiFi, Bluetooth etc) between the DTC and the hub. This advantageously removes the need for active beacons on the DTC, which may be difficult to practically implement. Having a low resolution low-power camera on the relay unit helps to limit the power consumption of the relay unit. Furthermore, owing to privacy concerns, low resolution cameras may be preferred over higher resolution alternatives. The limited resolution employed in accordance with embodiments of the invention would typically mean that identification of any faces or personal information in the area would not be possible. However, further measures may optionally be taken to further enhance the privacy of users of the relay wireless charging system - e.g. extra steps may be taken to blur the image which would effectively prevent manual or automatic facial recognition but would still allow the identification of bright spots.

In a set of embodiments the low-resolution camera and illumination means are adjacent to each other. Typically, the low-resolution camera and illumination means are situated within the observation angle of a retroreflector on the device.

The illumination means may comprise an active light source at the relay- e.g. one or more LEDs, a flash lamp or a bulb. However, in a set of embodiments, the illumination means comprises a diffusive, scattering or reflecting element arranged to scatter and/or reflect light and thus illuminate the zone when a beam of light is incident thereon.

In a set of embodiments therefore the hub unit comprises a light source - e.g. a laser source - arranged to generate a beam of light to be incident upon said diffusive, scattering or reflecting element on the relay unit. The light source may generate a beam that is a different wavelength to the charging beam - e.g. within the Infrared (IR) part of the spectrum. Preferably, the wavelength of the beam generated by the light source is below 1100 nm - e.g. below 1000 nm. Such a wavelength allows the low resolution camera to be a silicon camera. Silicon cameras are advantageously inexpensive and have low power consumption. The beam from the light source may be transmitted along the same beam line as the charging laser beam (and optionally other lasers - e.g. a visible laser beam for communication). Alternatively, the light source may be physically separate to the charging laser beam source. The diffusive, scattering or reflecting element allows the relay unit to passively provide a ‘flash’, without needing an active light source on the or each relay unit. The passive flash from the relay unit can therefore illuminate the zone from a different angle than is possible from the hub unit, and thus may illuminate DTCs which would not be reached by a flash from the hub unit (or another relay unit) but without requiring power from the relay unit.

The diffusive, scattering or reflecting element may comprise a material having a relatively high scattering coefficient - e.g. a white plastic material, a structured surface causing diffuse reflection or a curved mirror. The primary function of the diffusive, scattering or reflecting element is to change the direction of the illumination beam or ‘flash’ and to widen the angular distribution to a suitable width - e.g. the angular distribution may be hemispheric or smaller, e.g. matched to the field of view of the low resolution camera.

The approach described above, directed to optically searching for a DTC having a retro reflector, may also be performed by the hub unit. In a set of embodiments therefore, the hub unit also comprises illumination means arranged to illuminate a zone (which may be the same as that illuminated by the relay unit or different) and a low resolution camera arranged to image at least part of the zone. The illumination means on the hub unit may be an active light source since typically the hub unitwill have a power source such as mains power.

When viewed from another aspect the invention provides a method of wirelessly charging at least one device, said device comprising a photovoltaic cell for converting incident light into electrical energy and a retroreflector, the method comprising: illuminating a zone using an illumination means; imaging at least part of said zone from a point substantially adjacent to said illumination means; determining a location of said device by detecting light from said illumination means reflected by said retroreflector; and directing a laser beam to the photovoltaic cell of said device. By having the imaging point substantially adjacent the illumination means, light reflected by the retro-reflector can be detected even if the reflection has only a narrow beam angle.

As outlined above the illumination means could comprise an active light source or could comprise a diffusive, scattering or reflecting element illuminated by a remote light source - e.g. on a hub unit. A lens could be provided as the imaging point.

This could be part of a camera but this is not essential. In a set of embodiments detecting light from said illumination means reflected by said retroreflector is carried out by a low resolution camera as defined herein.

In another set of embodiments, locating the DTC is performed by the system in two phases, a first, coarse search phase and a second, fine search phase. In the coarse search phase, an initial location of the DTC may be determined by the DTC transmitting a notification signal, e.g. a Bluetooth™ signal, for the hub unit to process. The hub unit may process the received signal and determine a scanning volume smaller than the initial charging zone where the DTC can be found. The hub unit may then select the relay unit which has the best line of sight to the DTC based on the information received in the coarse search phase. This may trigger the fine search phase, in which the selected relay unit can more precisely locate the PV cell of the DTC by scanning the laser beam over the scanning volume determined in the coarse search phase. In a set of embodiments the fine search phase comprises a first mode in which the charging laser has a first divergence angle. When the laser beam impinges on the PV cell of the DTC, the system may receive a notification, prompting the system to change to a second mode in which the charging laser has a second, narrower divergence angle. This allows for finer scanning over the immediate vicinity of the DTC. Such an approach may allow the DTC to be located more quickly than using a single, narrow divergence angle for all the scanning.

The above-described approaches to searching for the DTC by scanning the laser beam, transmitted from the relay unit or hub unit, could equally be used to locate the relay unit. In such a set of embodiments, the laser beam scanning may be performed by the hub unit. The scan for locating relay units may be performed at a scheduled time to avoid interrupting the relay wireless charging system. For example, the relay units may be located by scanning at night or equally when the system is idle as the relay units are typically permanently fixed in position. This may mean that the scan time is less critical and so such a scan could be performed with a single divergence angle.

In a set of embodiments, the hub unit may charge the DTC directly without relaying the charging laser beam to the relay units. This may be advantageous for when the hub unit has a clearer line of sight to the DTC than the relay unit does. In such embodiments, the hub unit may comprise a moveable reflector, for example a steerable micro-mirror, such that the charging beam can be directed to the DTC directly from the hub unit at a range of angles. In such an embodiment, the moveable reflector may also direct the laser beam from the hub unit to at least one of the relay units in the relay wireless charging system. In one embodiment, the hub unit may decide whether to transmit the laser beam directly to the DTC, or alternatively, to deflect the laser beam from the relay units based on information relating to the position of the DTC.

In an alternative embodiment, the hub unit may comprise a servo-mounted laser, to transmit the laser beam to the one or more relay units. A servo-mounted laser can rotate the laser beam 360° in one plane such that, when the hub unit and the relay unit are arranged to be situated in a common plane, for example on a ceiling, the laser beam can be transmitted to the relay unit. In this arrangement, the hub unit may wirelessly charge the relay unit and the relay unit may deflect the laser beam to the DTC. In such an embodiment the relay unit increases the range of the wireless charging system to incorporate the volume of the room because the relay unit can deflect the charging beam at a wider range of angles e.g., by using a steerable micro-mirror, even if the hub unit can only direct the laser beam in one plane e.g., one at ceiling level. This arrangement also supports the advantage achievable in accordance with the invention of providing multiple angles for reaching a DTC, thereby mitigating the line of sight occlusion problem previously mentioned.

The Applicant appreciates that the charging laser beam of a relay wireless charging system, according to the present invention, is likely to be invisible to the naked eye. Therefore, it would be advantageous to show a visible indication to the user of information relating to the operation of the relay wireless charging system. In a set of embodiments, the system comprises a visible light beam which provides a projection of an image for the user to see. The projection may provide user feedback relevant to charging levels, e.g. a projection of the charging status of the device. The relay unit may comprise a visible laser source for projecting a visible laser beam. Equally the hub unit may comprise said visible laser source. If the source of the visible beam is the hub unit, the hub unit may direct the beam to the relay unit to deflect the visible beam to a position near the DTC for projecting the image.

In one set of embodiments, there may be separate mirrors for the visible laser beam and charging laser beam. In another set of embodiments, the visible laser beam and charging laser beam share a common mirror. The visible beam may be pulsed. If the visible beam is pulsed, the charging laser beam may operate between pulses i.e. when the visible beam is switched off. Alternatively, the visible beam may be operated continuously.

Features of any aspect or embodiment described herein may, wherever appropriate, be applied to any other aspect or embodiment described herein.

Where reference is made to different embodiments, it should be understood that these are not necessarily distinct but may overlap. It will be appreciated that all of the preferred features of the relay wireless charging system according to the first aspect described above may also apply to the other aspects of the invention.

One or more non-limiting examples will now be described, by way of example only, and with reference to the accompanying Figures in which:

Fig. 1 illustrates a system for wirelessly charging a plurality of different devices in a room;

Fig. 2 is a schematic diagram of certain parts of the system of Fig 1 ;

Fig. 3a is a schematic illustration of a relay wireless charging system in accordance with the invention;

Fig. 3b demonstrates how the system of Fig. 3a overcomes the problem of occlusions to the line of sight;

Fig. 4a is a schematic representation of a relay unit, in detail, which forms part of the relay wireless charging system embodied in Figs. 3a and 3b; Fig. 4b is a detailed view of a micro-mirror which can be used in accordance with the invention;

Fig. 5 is a schematic illustration of the relay unit transmitting a signal to the hub;

Fig. 6 is a flow chart explaining how the system locates the relay units;

Fig. 7 is a flow chart explaining how the system locates the devices;

Fig. 8 is a schematic view of a further embodiment in which a servomotor integrated with the hub allows a plurality of relay units in one plane to be charged by the hub; Fig. 9 schematically shows a further embodiment of a relay wireless charging system comprising a projection of the charging status of the device;

Fig. 10 schematically shows another embodiment comprising a projection of the charging status of the device;

Fig. 11 schematically shows another embodiment comprising a low resolution imaging system for locating the devices and

Fig. 12 is a flow chart explaining how the system shown in Fig. 11 locates the devices.

Fig. 1 shows an example of a system for wirelessly charging a plurality of different electronic devices in a room e.g. wireless earphones 2, a wireless mouse 4, a mobile phone 6 etc. In this exemplary wireless charging system, a supply unit in the form of a central hub 8 is mounted to the ceiling of the room. When mounted to a surface in this way, the hub 8 can be powered by the mains electrical supply.

The hub 8 has the capability to wirelessly charge devices anywhere within a charging zone. The charging zone in Fig. 1 is defined by the boundaries of the room i.e. the walls 10, ceiling 12, and floor 14. The hub unit 8 charges the devices 2, 4, 6 by means of a laser charging beam 16 represented by a dotted line. The devices to be charged (DTCs) 2, 4, 6 all comprise a suitable photovoltaic device to convert power from the beam 16 into electrical power for charging an on-board battery or otherwise being stored (e.g. in a super capacitor). The beam 16 could also be used directly to power some functioning of the respective device to be charged (DTC). It is important for the wavelength and power of the beam to be chosen with consideration of eye-safety regulations. For a charging laser, the beam must be safe, but must also transfer enough energy to charge the device in a reasonable time. Up to 0.5 W certain near-infrared wavelength lasers are considered to be safe. Alternatively, the source could generate a higher power charging laser (e.g. P > 0.5 W) with a safety interlock switch mechanism which turns off the power when the line of sight to the DTC is broken, but this is significantly more complex.

Fig. 2 shows schematically some more details of the hub 8 charging a DTC 4. The hub 8 comprises a laser source 22, a processor 24, beamshaping optics 36, and a steerable reflector herein referred to as the mirror 28 for directing the charging beam 16 toward the DTCs e.g. 2, 4, and 6. The DTC 4 comprises a photovoltaic device 32, e.g. the C30665GH available from Excelitas. Here, the charging laser beam 16, which is emitted from the hub 8, could be generated by an off-the-shelf laser diode 22, e.g. the T09-175 available from SemiNex. As can be seen in Fig. 2, after passing through beamshaping optics 26, the beam is incident on a steerable mirror 28. The mirror 28 then deflects the beam 16 so that it is output from the hub 8 and impinges on the photovoltaic cell 32 of the DTC 4.

The processor 24 within the hub 8 has the capability to process steering instructions to control the tilting angle of the mirror 28, based on information obtained during localisation of the DTC 4. The processor 24 also connects to a wireless communication module to communicate with the DTC 4 over a radio communication channel - e.g. BlueTooth™.

Figs. 3a and 3b each show a schematic of a relay wireless charging system, according to an embodiment of the present invention. The relay wireless charging system according to the present invention, is similar to the wireless charging system depicted in Figs. 1 and 2, with the addition of a plurality of relay units 38a-d. In particular the hub unit 8 has the features described above with reference to Fig.

2. The relay units 38a-d are positioned within the charging zone, e.g. around the ceiling. The relay units 38a-d can collaboratively locate the DTCs by scanning the charging zone and can transmit the charging beam to the DTCs, helpfully overcoming occlusions to the line of sight between the beam source, e.g. the hub 8, and the DTC. The relay units 38a-d, in this embodiment, are wirelessly charged by the hub 8.

Turning to Fig. 3a, there can be seen a schematic view of a room, with the relay wireless charging system installed and in operation, according to an embodiment of the present invention. The hub 8 and a plurality of relay units 38a, 38b, 38c, 38d are mounted to the ceiling. The hub 8 is mounted to the centre of the ceiling. The relay units are arranged around the hub 38a, 38b, 38c, 38d, preferably, such that the installation of the relay units spans the perimeter of the ceiling. In this example, the hub 8 is powered by the mains power supply and the relay units 38a, 38b, 38c, 38d are wirelessly charged by the hub 8.

In the embodiment exemplified in Fig. 3a, the hub 8 can independently charge a DTC 4b. This is illustrated by the hub 8 transmitting a beam incident upon the photovoltaic cell 32b of the DTC 4b laying on a table 50 in the charging zone. If it is preferable, e.g. because of an occlusion 46 blocking the line of sight to the DTC 4c, then one or more of the relay units 38c can deflect the charging beam that is initially output from the hub 8, to the DTC 4c. In Fig. 3a, each relay unit 38a-d comprises a moveable mirror 44a-d which facilitates deflection at each relay unit 38a-d.

In Fig. 3b, there can be seen the same relative arrangement of the hub and relay units as depicted in Fig. 3a. How the invention deals with the line of sight problem is highlighted in Fig. 3b, as the charging beam deflected from one of the relay units 38a cannot reach the DTC 4 due to the occlusion of the seated person 58. The relay wireless charging system overcomes this problem as two of the other relay units 38b, 38c, which have an uninterrupted line of sight to the DTC 4, each redirect a charging beam output from the hub 8 to the DTC 4.

A further advantage of the relay wireless charging system is depicted in Fig. 3b. In this embodiment, more than one charging beam may be output from the hub 8 at one time. This allows a first charging beam 30b and a second charging beam 30c to be incident upon the photovoltaic cell 32 of the DTC 4 such that they overlap. If the beams are from separate sources, e.g. more than one laser diode, then overlapping the beams upon the photovoltaic cell 32, will result in increased power delivery to the DTC 4 without breaking laser safety standards which require observance of exposure limits to prevent eye injuries. This limit is known as the ‘maximum permissible exposure’ (MPE) which is a calculated value dependent on inter alia the properties of the laser source that is used. The international standard for laser safety is I EC 60825-1:2014 and equivalently for the US is (ANSI) Z136, and both standards include methods for calculating the MPE. A relay unit 38, which forms part of the relay wireless charging system, is schematically shown in more detail in Fig. 4a. The relay unit 38 comprises a photodetector in the form of a photovoltaic cell 70 e.g. the C30665GH available from Excelitas. The photovoltaic cell 70 receives the charging laser beam 30 transmitted by the hub 8 so that relay units 38a-d can be wirelessly charged by the hub 8. The relay unit further comprises a micro-controller 72 comprising control logic for sending instructions to the moveable reflective element in the form of a MEMS mirror 44, which deflects the charging beam 30b, 30c from the hub 8 toward the DTC 4. The micro-controller 72 also connects to a wireless communication module to allow radio signals to be sent to the hub 8.

The relay unit 38 further comprises optics, e.g. a convex focusing lens 62, to re focus the beam such that the range of the charging beam can be extended.

The system is arranged such that in use, the charging beam 30a enters the relay unit 38 from the hub 8 through the optical window 68 and is refocused by the convex focusing lens 62 and incident upon the moveable mirror 44 which redirects the beam 30b to the DTC e.g. 4.

One example of a moveable micro-mirror that can be used in the relay unit is shown in Fig. 4b and described in more detail in US 9,250,418. Fig. 4b illustrates a MEMS micro-mirror 44 based on a ring shaped membrane 204 providing a coupling means between a rigid optical element 206 e.g. of silicon and a frame 208. An actuator is provided which is split around its circumference into four arcuate sections 210a, 210b, 210c, 210d. Corresponding inner actuator parts (not shown) are also provided. Piezo-resistors 212 are provided for position measuring. These piezo resistors 212 are positioned in the gaps between the actuator sections.

The actuator sections 210a, 210b, 310c, 210d are positioned on the membrane 204 defining a coupling area between the frame 208 and the rigid element 206. The actuator sections 210a, 210b, 210c, 210d deflect the rigid element disc 206. The optical element 206 is rigid so as to maintain essentially the same shape when moved by the actuator elements 210a, 210b, 210c, 210d. The actuator elements 210a, 210b, 210c, 210d are preferably positioned close to either the frame 208 or the rigid element 206, so that when the piezoelectric material contracts, the part of the actuator positioned on the membrane is bent upward thus pulling the membrane in that direction.

A number of laser and micro-mirror parings may be provided to give full 360° coverage around the room. The micro-mirror 44 may be controlled by the processor 72 or a separate controller unit might be provided. Although the MEMS mirror 44 described above is described as being part of the relay unit 38, the hub 8 may equally house such a MEMS micro-mirror 28 for directing the laser beam 30 to the relay unit 38 or a DTC 4.

In order for the relay units to be located by the hub for receiving the charging beam or to collaboratively locate the DTCs, a communication network is necessary. This can be made possible in a variety of ways. In a preferred embodiment, the relay units and hub communicate wirelessly. One possible example of a wireless communication channel is shown in Fig. 5.

Fig. 5 schematically shows the hub 8 communicating wirelessly with a relay unit 38 via Bluetooth™, according to an embodiment of the present invention. Both the hub 8 and the relay unit 38 have had most of their components omitted for simplicity of the drawings. The relay unit 38 comprises a photovoltaic cell 70 for receiving the charging beam, and a radio transducer, e.g. a transmitter 76 for transmitting a Bluetooth™ signal 84. The hub 8 comprises a radio transducer e.g. an antenna 94 for receiving the Bluetooth™ signal 84. The hub 8 further comprises a processor 24 which connects to a wireless communication module.

In order for the relay wireless charging system to operate effectively, the hub 8 must know the location of the relay unit 38 that it has selected to direct the laser beam 30 towards, e.g. either for deflecting the beam 30 to a DTC 4 or for directly charging the selected relay unit 38. Likewise, the hub 8 and the relay units 38a-d must know the location of the DTC 4. There are several ways that the relay units 38a-d can be located, the operation of which will now be described.

One method of locating the relay units 38a-d, is exemplified in the flow chart of Fig. 6. Starting at step 300, the relay unit 38 transmits a radio frequency signal, e.g. a Bluetooth™ signal 84 which is received by the hub 8 at step 302 and used to determine a position of the relay unit 38. The signal 84 transmitted by the relay unit 38 could be from a Bluetooth Low Energy™ (BLE) beacon, e.g. the RN4871 available from Microchip Technology Inc. The Bluetooth™ signal 84 is received at the antenna 94 of the hub 8, and at step 304 the processor 24 processes the received signal data and identifies the relay unit 38, then at step 306 the system determines a coarse estimate of the location of the relay unit 38 based on the direction of arrival of the signal and signal strength.

As mentioned above, the signal 84 transmitted to the hub unit 8 from the relay unit 38 could comprise identity data, which allows the hub unit 8 to recognise which of the relay units 38a-d is to be targeted, e.g. if a relay unit determines that it requires charging. The identity data of the relay unit 38 may be associated with the position of the relay unit 38 which may be stored in the memory of the hub unit 8 from a previous determination. Thus on identifying the relay unit 38 that has transmitted a signal 84, the hub 8 directs the laser beam 30 toward the correct relay unit 38. If the laser beam successfully hits the PV cell of the relay unit 38, then at step 310, charging can be confirmed. Following confirmation of charging, e.g. via a wireless confirmation signal transmitted by the relay unit 38, at step 314, the search can be stopped.

However, if the laser beam 30 is misaligned such that the relay unit 38 does not report that it is charging, a refinement step 312 of the laser beam position 30 is performed to locate the PV cell of the relay unit 38. This involves scanning the laser beam 30, through minor adjustments of the tilt of the MEMS mirror 28, over an area where the hub 8 has determined the relay unit 38 should be situated. If, through this adjustment, the relay unit 38 reports charging, then the mirror is fixed into position at step 318 and scanning is halted. If the relay unit is not found at all after a certain time then the search may be repeated from the beginning step 300.

In another example, the relay unit may be located by the hub unit by a passive means e.g. the relay unit comprises a visible marker, e.g. a binary ArUco code, which allows the hub unit to estimate the position and pose of the relay unit. Here, the hub unit 8 comprises a low resolution camera and an image recognition algorithm to identify and locate the visible marker. Alternatively, the hub unit 8 could transmit a signal for locating the relay units 38a-d, e.g. an optical signal, and the relay unit 38 could be arranged with a reflecting element to reflect the signal back to the hub 8. This can be achieved by using a retroreflector which reflects an optical signal back along its incident direction to return to the source, e.g. the PS974-B available from Thorlabs. The reflected signal received by the hub unit 8 would allow the system to determine an approximate position of the relay unit 38. For example, if the laser beam 30 is directed from the hub 8 such that it scans the plane where the relay units 38a-d are situated (see Fig. 3a and Fig. 3b). When the laser beam 30 illuminated the PV cell 70 of the relay unit 38, the retroreflector would return the light to the hub 8 which gives a notification that the position of the relay unit 38 has been found and the laser beam 30 is directed to the approximate position of the relay unit 38.

Furthermore, the DTC 4 must be located by the system, using the hub unit 8 and/or relay units 38a-d, in order for the relay wireless charging system to operate successfully. This can be achieved using similar methods as those described above for locating the relay units 38a-d and set out in more detail with reference to Fig. 7. In this method, the hub unit 8 and relay unit 38 work collaboratively to locate the DTC 4. Starting at step 320, the DTC 4 transmits a signal, e.g. an RF signal such as a Bluetooth signal, to be received by the hub unit 8 and the relay unit 38, at step 322. If the signal is not received at two units, they continue listening.

A phased array of receiver elements is provided on the hub unit, and another phased array of receiver elements is provided on the relay unit. These allow the time difference of arrival of the signal at each of the receiver elements to be used to determine the angle of arrival (AOA) at the hub 8 or relay unit 38. The received signal is processed and an AOA at each receiver, i.e. the relay unit 38 and hub unit 8, is determined at step 324. The location of the DTC 4 can then be determined from a triangulation calculation at step 326. It will be appreciated of course that more relay units, depending on how many are installed and whether any of them are occluded, may be involved to increase the accuracy of the calculation.

This arrangement of receivers, i.e. the relay unit and the hub unit being placed at ceiling level spaced across the room, results in a large known baseline which is defined by the distance between the receivers. The location of the DTC 4 is calculated using the known baseline and angles of arrival at the relay unit 38 and hub 8 at step 326. It is preferable for the relay unit 38 and hub unit 8 to be well- separated as a large baseline gives the advantage of allowing the system to more precisely calculate the location of the DTC 4, reducing the relative uncertainty in the calculation. When the position of the DTC 4 is known, the hub unit 8 transmits the laser beam 30 to a selected relay unit 38 which is suitable, i.e. with a clear line of sight to the DTC 4, for directing to the DTC 4. The relay unit 38 comprises a MEMS mirror 44 which tilts to direct the beam 30 according to instructions processed by the control logic 72 of the relay unit 38 and/or the hub unit 8, such that the laser is pointing towards the DTC 4 at step 328.

The above-mentioned positioning methods give the system an estimate of the position of the DTC 4 at various levels of precision. The direction that the laser beam 30 points can be refined such that the DTC 4 is charged more efficiently by the relay unit 38, e.g. by improving the alignment of the laser beam 30 upon the PV cell 32 of the DTC 4. The refinement of laser beam 30 alignment will now be explained with reference to steps 330-338 of the flow chart depicted in Fig. 7.

When the laser beam is directed toward the DTC 4 such that the beam 30 illuminates its PV cell 32, at step 330 the DTC 4 may send out a notification of charging for the hub unit 8 to receive, e.g. in the form of a wireless signal. This instructs the relay unit 38 to fix the position of the mirror 44 as the laser beam 30 has reached the correct position for charging the DTC 4, and the search ends at step 332.

However, if the laser beam 30 is misaligned such that the DTC 4 does not report that it is charging at step 330, a refinement step 334 of the laser beam position 30 is performed to locate the DTC 4. This involves scanning the laser beam 30, through minor adjustments of the tilt of the MEMS mirror 44, over an area where the system has determined the DTC should be situated 334. If, through this adjustment, the DTC 4 reports charging 336, then the mirror 44 is fixed into position and scanning is halted at step 338. If the DTC 4 is not found at all after a certain time then the search may be repeated from an earlier step 320. Fig. 8 represents a relay wireless charging system according to another embodiment of the present invention. There can be seen an alternative to the moveable MEMS mirror 28 depicted in Fig. 2 for directing the charging laser beam 102a, 102b from the hub 104 to the relay units 98a, 98b. In such a set of embodiments, a servomotor 106 facilitates 360° rotation of the direction of the charging beam in one plane e.g. the ceiling. With the relay units 98a, 98b at known positions on the same plane, the charging beam 102a, 102b can be directed towards the correct angle for charging the relay units 98a, 98b, i.e. such that the laser beam is incident upon the photovoltaic cell of the relay unit, by the servomotor driving the rotation of the hub 104. When the laser beam 102b is directed toward the relay unit 98b such that the beam 102b illuminates the PV cell of the relay unit 98b, the relay unit begins charging. A Bluetooth transmitter therein communicates a confirmation that the relay unit is charging by transmitting a notification signal for the hub unit 8 to receive. This instructs the hub 104 to halt the 360° scan as the laser beam 102b has reached the correct position for charging the relay unit 98b.

In the arrangement described above, it can be inferred that is not necessary for the hub 104 to have a moveable mirror e.g. with a MEMS architecture 44, as the servomotor facilitates charging of a plurality of relay units. The charged relay units 98a, 98b can then go on to redirect the charging beam 102a, 102b to other DTCs in the charging zone, using moveable MEMS mirrors 44 on the relay units as shown in Fig. 4a. Furthermore, the relay wireless charging system can work without the hub 40 being able to directly charge a DTC. The relay units 98a, 98b can be thought of as peripheral extensions of the more simplified hub 104, allowing the supply portion of the relay wireless charging system to span a larger area e.g. an entire ceiling.

The relay wireless charging system preferably uses an invisible laser for charging, meaning that the user may benefit from an informative visual display, e.g. being notified that a DTC is being charged by the system. The system may, therefore, comprise visual elements to enhance the user’s experience. For example, Fig. 9 shows a visible projection of the charging status 116 of the DTC 110.

Fig. 9 schematically shows the relay wireless charging system according to an embodiment of the present invention, further comprising a projection of the charging status 116 of the DTC 110, wherein the visible beam 118a and charging beam 118b are each directed to respective MEMS mirrors 120a, 120b.

The visible beam 118a is generated by a laser source within the hub 114, which is separate to the infrared charging laser source within the hub 114. Both the charging laser beam 118b and the visible laser beam 118a are output from separate laser sources within the hub 114 in the direction of one of the relay units 122 of the relay wireless charging system. The relay unit 122 comprises a first moveable MEMs mirror 120b to deflect the incident charging beam 118b so that the deflected charging beam is incident upon the photovoltaic cell 108 of the DTC 110. The relay unit 122 further comprises a second moveable MEMs mirror 120a to deflect the incident visible beam 118a so that the deflected visible beam 112a forms a visible projection 116 which allows the user to view information, here in the form of the battery charging status 116.

There are several ways, known in the art, to achieve a projected image using lasers. In a preferred embodiment, as shown in Fig. 9, the projection is be achieved by a visible laser 112a rapidly scanning a surface, to effectively ‘draw’ the image with light, by fast operation of the MEMs mirror 120a. The provision of a dedicated MEMS mirror 120a for the visible light and a dedicated MEMS mirror 120b for the charging beam 118b, 112b allow both lasers to be used simultaneously.

The operation of a MEMS mirror is energy expensive. A way to reduce the energy cost of the system would be to have the visible beam and charging beam sharing a common MEMS mirror, which requires the laser to be operated in a pulsed mode.

Fig. 10 schematically shows the relay wireless charging system according to an embodiment of the present invention further comprising a projection of the charging status of the DTC, wherein the visible beam and charging beam share a common mirror. In this arrangement, the charging laser 128a is pulsed. The visible projector laser 128b is pulsed such that it is “switched on” in between the pulses of the charging laser 128a. This is demonstrated in Fig. 8 where it can be seen that at a time t=0 the near-infrared charging beam 126a is first deflected from the shared MEMs mirror 130 of the relay unit 138 and subsequently directed towards the photovoltaic cell 140 of the DTC 136. Then at time t=1 the visible laser 126b is deflected from the shared MEMs mirror 130 of the relay unit 138 and the deflected visible beam 128b is subsequently directed to a surface near to the DTC 136 and projects information to the user while the charging laser 126a, 128a is switched off 136. The projected information is shown as a projection of the battery status 132 of the DTC 136.

Fig. 11 schematically shows a relay wireless charging system according to another embodiment of the present invention comprising a low resolution imaging system for locating the DTCs.

Similar to other embodiments shown in Figures 3a-b, 9 and 10 the system comprises a hub 148, a relay unit 154 and a DTC 172. Although only one DTC 172 is shown in Fig. 11, this embodiment is easily extended to a system with a plurality of DTCs. The scene of Figure 11 schematically shows an object (tree) 164 in the line of sight of the hub 148. This represents an occlusion to the line of sight between the hub 148 and the DTC 172 - presenting the same challenge as the objects 48, 46, 58, 60 in Figs 3a and 3b.

The hub 148 comprises a charging laser source 146; an infra-red (IR) flash laser source 144; a wavelength combiner 142; a moveable mirror 160; an IR flash lamp 152 and a low resolution camera 150.

The charging laser source 146 generates a charging beam for charging the relay unit 154 and the DTC 172. The solid line 168a-b represents the charging laser beam 168a-b directed to the PV cell 166 of the DTC 172.

The infra-red (IR) flash laser source 144 generates an IR beam which is used to help locate the DTCs (e.g. 172) as will be explained below.

The relay unit 154 comprises a PV cell 170; a moveable mirror 156 as well as a diffusive element 162 and a low resolution camera 158. The lens of the camera 158 is situated next to the diffusive element 162 (i.e. within the observation angle of a retroreflector) and the camera 158 is thus arranged to image from that point. The diffusive element 162, in this example, is comprised of a material having a high scattering coefficient - e.g. white plastic. The DTC 172 comprises a PV cell 166 for receiving a charging beam. In the same area on the DTC 172, the DTC 172 has a retroreflector 167. A wireless communication network (not shown) between the relay unit 154 and hub 148 is established. This communication network may also include the DTC 172. However, in this example, the method used to locate the DTC 172 requires no active uplink channel from the DTC 172.

The operation of the system shown in Fig. 11 will now be described with reference to the flowchart in Fig. 12.

The process begins at step 402, where a search for the DTCs (e.g. 172) begins.

The hub 148 begins a search for any DTCs within the line of sight of the hub 148.

At step 403, the IR flash lamp 152 illuminates the room and any DTCs in the field of view of the low resolution camera 150 are imaged by the point reflection of the illuminating light by their respective retroreflectors. Their approximate location can then be identified from that image. It will be appreciated that because of the camera’s low resolution, the reflected light from a given device is only likely to be captured by a small number of pixels and that only an approximate location is obtained. In the example shown in Fig. 11, there are no DTCs in the line of sight of the hub 148, so none are identified at this stage, but there is a DTC 172 hidden from the hub’s view. In order to find this DTC 172, the search continues via the relay unit 154. At step 404 the relay unit 154 is used to search for the DTC 172. During the localisation process, the hub may be wirelessly powering the relay unit 154. A first dashed line 178 represents the charging laser beam 178 directed to the PV cell 170 of the relay unit 154. At step 405 the IR flash laser source 144 at the hub 148 generates an IR flash beam for directing to the relay unit 154. A second dashed line shows the IR flash beam 174a reflected from the moveable mirror 160, impinging on the diffusive element 162.

When the beam 174a is incident on the diffusive element 162 the light is scattered and the zone is illuminated by the IR flash from the diffusive element on the relay unit 154. A ray 174b of the scattered light is shown between the diffusive element 162 and the retroreflector 167.

The narrow angle between the illumination direction and the viewing direction of the retroreflector 167 is called the observation angle 180. The light reflected by the retroreflector 167 is returned within this observation angle 180. The lens of the camera 158, being within this observation angle 180, receives a strong signal from the retroreflector 167 of the DTC 172 at step 407. It is important that the diffusive element 162 on the relay unit 154 is situated adjacent to the lens of the low resolution camera 158, within the observation angle 180 of the retroreflector 167. If it is mounted outside this angle 180, the retro- reflected signal 167 may not reach the low resolution camera 158.

At step 408 the camera 158 on the relay unit 154 acquires a low resolution image of the zone. As the DTC 172 is in the line of sight of the relay unit 154 (but not the hub 148), the DTC 172 is successfully identified and located within the zone.

Therefore, the answer to “are all DTCs found?”, at step 409, is “yes”. Step 409 is optional, however, as there may be no active communication link between the DTC and the wireless charging relay system, and therefore, no way to determine if all DTCs in the charging zone are found.

As the retroreflector 167 and PV cell 166 are very close (possibly overlapping) on the DTC 172, the camera may detect a strong reflected signal indicating not just the location of the DTC 172, but more particularly the location of the PV cell 166 on the DTC 172. The charging beam 168a, 168b may then be directed to the PV cell 166 of the DTC 172. The system may optionally proceed on to a fine search phase to better align the charging beam with the PV cell 166.

The process ends when all DTCs are located at step 410. If the answer at step 409 was “no”, however, and there were a plurality of relay units in the system, then the process may progress to step 411 and steps 404-409 may be repeated using a relay unit situated in a different area of the zone. The search may progress automatically to the next relay unit, and the process shown in steps 404-409 may repeat for a selection of the relay units or all the relay units in the system. Figs 11 and 12 therefore demonstrate a semi-passive optical search for DTCs- which locates devices to be charged by illuminating a retroreflector (e.g. 167) on each DTC (e.g. 172). Using an IR laser beam 174a generated by the hub 148 and directed by a moveable mirror (e.g. MEMS) 160 to illuminate the diffusive element 162 on a relay unit 154 provides a solution to the problem caused by interruptions to the line of sight between the hub 148 and a device (e.g. DTC 172). In simple terms, the relay units can be used to find a device (DTC) which cannot be seen by the main hub (or other relay units).

Incorporating a diffusive element 162 allows for each relay unit to have a ‘passive’ flash source for illuminating the zone, without having to have an (active) IR flash lamp (e.g. 152) on every relay unit. Furthermore, the retroreflector (e.g. 167) on each DTC (e.g. 172) provides another passive component that can possibly replace an active uplink communication channel (e.g. WiFi, Bluetooth etc) between the

DTCs and the rest of the system. This advantageously removes the need for active beacons on the DTC, which may be difficult to practically implement. Furthermore, the low resolution cameras 150, 158 use very little power (e.g. of the order of a few milliwatts) compared to typical medium or high resolution cameras. Choosing such passive and low-power components for the DTC search helps to reduce the power consumption of the entire system. Furthermore, low resolution cameras preserve the privacy of any users of the wireless charging system.

While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.

Claims

Claims

1. A system for wirelessly charging at least one device, said device comprising a photovoltaic cell for converting incident light into electrical energy, the system further comprising a hub unit comprising a laser source and at least one relay unit arranged to direct a laser beam to the photovoltaic cell of said device.

2. The system of claim 1 , wherein the relay unit comprises a photovoltaic cell.

3. The system of claim 1 or 2, wherein the hub unit is arranged to transmit a laser beam from the laser source to the photovoltaic cell of the relay unit to power said relay unit.

4. The system of any preceding claim, arranged so that the relay unit deflects a laser beam from the hub unit to the device.

5. The system of claim 4, wherein the laser beam that is deflected by the relay unit to the device is the same laser that is used to charge the relay units.

6. The system of any preceding claim, wherein the relay unit comprises a reflective element suitable for deflecting the incident laser beam from the hub unit to the device.

7. The system of any preceding claim, wherein the relay unit is arranged to receive a wireless communication instructing the relay unit to generate and/or direct a laser beam toward the device.

8. The system of any preceding claim, wherein the relay unit comprises a collecting lens to refocus or collimate the laser beam.

9. The system of any preceding claim, wherein the relay unit comprises a transmitter and is arranged to transmit a signal which is received by a receiver on the hub unit, said hub unit being arranged to determine a position of the relay unit using said signal.

10. The system of claim 9, wherein the hub unit comprises an array of receiver elements for receiving the signal.

11. The system of any preceding claim, wherein hub unit is arranged to locate the relay unit by passive means on the relay unit.

12. The system of claim 11 , wherein said passive means comprises a visible marker.

13. The system of claim 12, wherein the hub unit comprises a camera arranged to detect the visible marker.

14. The system of any preceding claim, wherein the hub unit is arranged to transmit a signal, said signal being reflected by the relay unit and received by the hub unit to allow the system to determine a position of the relay unit.

15. The system of any preceding claim, wherein the relay unit and/or the hub unit is arranged to receive a signal from the device and to use said signal to locate the device.

16. The system of claim 15, wherein the relay and/or hub unit comprises an array of receiving elements for receiving the signal.

17. The system of any preceding claim, arranged to scan the laser beam over a scan volume during a first mode with a first divergence angle and, upon receipt of a notification that the laser beam is impinging on the photovoltaic cell of the device, to change to a second mode with a second, narrower divergence angle.

18. The system of claim 17, wherein the notification comprises a signal transmitted by the device.

19. The system of claim 17, wherein the notification comprises a reflected signal transmitted by the hub or relay unit and reflected back to the hub or relay unit by a reflecting element situated on the device.

20. The system of any one of claims 17 to 19, arranged during the second mode to scan the beam over a second, smaller scan volume based on said location information.

21. The system of any one of claims 17 to 20 wherein the relay unit is arranged to perform the scanning.

22. The system of any preceding claim, wherein the relay unit is arranged to perform an optical search to locate a device having a retroreflector.

23. The system of claim 22, wherein the relay unit comprises illumination means arranged to illuminate a zone and a low resolution camera arranged to image at least part of the zone illuminated.

24. The system of claim 23, wherein the low-resolution camera has a resolution of less than 50,000 pixels.

25. The system of claim 23 or 24, wherein the low-resolution camera and illumination means are adjacent to each other.

26. The system of any of claims 23 to 25, wherein the illumination means comprises a diffusive, scattering or reflecting element arranged to scatter and/or reflect light and thus illuminate the zone when a beam of light is incident thereon.

27. The system of claim 26, wherein the hub unit comprises a light source arranged to generate the beam of light to be incident upon said diffusive, scattering or reflecting element on the relay unit.

28. The system of claim 27, wherein the wavelength of the beam generated by the light source is below 1100 nm.

29. The system of any of claims 23 to 28, wherein the hub unit also comprises illumination means arranged to illuminate a zone and a low resolution camera arranged to image at least part of the zone.

30. The system of any preceding claim, arranged to locate the device in a first, coarse search phase and a second, fine search phase.

31. The system of claim 30 wherein the hub unit is arranged during the coarse search phase to process a notification signal transmitted by the device and thereby to determine an initial location of the device.

32. The system of claim 30 or 31 , wherein the hub unit is arranged during the fine search phase to transmit in a first mode in which the charging laser has a first divergence angle and, upon receipt of a notification that the laser beam is impinging on the photovoltaic cell of the device, to change to a second mode in which the charging laser has a second, narrower divergence angle.

33. The system of any preceding claim, wherein the hub unit comprises a moveable reflector arranged to direct the laser beam to the device directly from the hub unit at a range of angles.

34. The system of claim 33, wherein the hub unit is arranged to determine whether to transmit the laser beam directly to the device, or alternatively, to deflect the laser beam from the relay units based on information relating to the position of the device.

35. The system of any preceding claim, the hub unit comprising a servo- mounted laser, to transmit the laser beam to the one or more relay units.

36. The system of any preceding claim, arranged to transmit a visible light beam providing a projection of an image.

37. The system of claim 36, wherein the hub unit or relay unit comprises a visible laser source for transmitting a visible laser beam, the system comprising separate mirrors for the visible laser beam and charging laser beam.

38. A method of wirelessly charging at least one device, said device comprising a photovoltaic cell for converting incident light into electrical energy and a retro reflector, the method comprising: illuminating a zone using an illumination means; imaging at least part of said zone from a point substantially adjacent to said illumination means; determining a location of said device by detecting light from said illumination means reflected by said retroreflector; and directing a laser beam to the photovoltaic cell of said device.

39. The method of claim 38, comprising detecting light from said illumination means reflected by said retroreflector using a low resolution camera.