Abstract:
A wireless energy transfer system comprising: a transmitter configured to beam scan RF radiation across a plurality of sectors at a first frequency, a receiver storing energy from the RF radiation, and sending acknowledgements at a second frequency, the first frequency being significantly different from the second frequency, and a controller configured to direct wireless energy transfer from the transmitter substantially at the receiver based on the acknowledgements.

Description:
FIELD 
       [0001]    The present invention relates to a wireless energy transfer system. 
       BACKGROUND 
       [0002]    With mobile electronic devices becoming more popular, ease and flexibility of charging the mobile device&#39;s battery is of increasing importance. Typically most prior art devices use a mains connected converter which is hard wire connected to the mobile device to provide a low voltage DC supply for charging. 
         [0003]    An alternative to wired charging is wireless charging. Prior art examples of wireless energy transfer include induction, resonant coupling, electromagnetic radiation and laser. Induction may only be useful where the device is very close, such as wireless dock charging for electric toothbrushes, or a transformer. At mid distances resonant coupling is used, such as in some RFID and smart cards. Because the efficiency reduces dramatically with distance, for larger distances a high degree of directionality is required. Longer distance options include EM radiation and laser. However such methods maybe sensitive to the device orientation. Thus the user may have to keep the device stationary and perpendicular to the flux to maintain the power transfer. 
         [0004]    For mobile electronic devices, it may be more convenient if the user did not have to dock the device for charging. For example it may be desirable if the device was able to charge when the user was simply in the same room as the charging station, (perhaps with the device in his or her pocket), similar to WiFi hotspots. In this scenario induction and laser are inappropriate, and EM radiation may be more desirable. 
         [0005]    Thus for EM radiation it is necessary to focus the radiation on the device, and therefore to track the device&#39;s location. One technical challenge may be how to locate a receiver accurately at very low power consumption at the receiver. Prior art solutions such as RFID may prove difficult because: 
         [0006]    (a). A generic RFID module at UHF band, if mounted in the transmitter and receiver, may not allow for beam scanning and the omni-directional radiation is very inefficient. 
         [0007]    (b). Because of the ultra low power level, it may be difficult to resolve between the signal from the TX, acknowledgement from the RX, any reflections and other interference, to allow for accurate 3D location estimation. 
         [0008]    Prior art attempts at wireless energy transfer include U.S. Pat. Nos. 6,856,291; 7,057,514; 7,383,064 and 7,639,994, and Japanese Patent Publication number 08-103039. However these do not provide suitable solutions to the problem mentioned. 
       SUMMARY OF THE INVENTION 
       [0009]    In general terms, the invention relates to a wireless energy transfer system that is capable of: 
         [0010]    1. Transmitting RF energy to a single or multiple specific directions rather than omni-directionally or a front-side, 
         [0011]    2. Wirelessly charging mobile electronic devices which consume less than a dozen millwatts, yet avoiding unnecessary radiation to humans, 
         [0012]    3. accurately detecting the 3D location of a mobile electronic device that needs energy transfer, and/or 
         [0013]    4. Tracking the mobile electronic device whilst in motion. 
         [0014]    The detecting and tracking may done by a transmitter (Tx) or base station, using beam scanning across the volume/area of coverage, which is divided into sectors. The beam scanning is done at 2.45 GHz. If a receiver (Rx) or mobile electronic device receives the beam scan it sends an acknowledgement at 860 MHz. The strongest acknowledgement indicates to the TX which sector the RX is in, after which energy transfer is focussed towards that sector. 
         [0015]    In a first specific aspect there is provided a wireless energy transfer system comprising: a transmitter configured to beam scan RF radiation across a plurality of sectors at a first frequency, a receiver storing energy from the RF radiation, and sending acknowledgements at a second frequency, the first frequency being significantly different from the second frequency, and a controller configured to direct wireless energy transfer from the transmitter substantially at the receiver based on the acknowledgements.
   The first frequency may be in an ISM band   The ISM band may be substantially located about 2.45 GHz or 5.80 GHz.   The second frequency may be in an RFID band.   The RFID band is substantially located about 866-869 MHz or 310 to 320 MHz.   The transmitter comprises a steerable phased array antenna.   The receiver may comprise a first omni-directional antenna to receive the first frequency and a second omni-directional antenna to send on the second frequency.   The receiver may further comprise a battery or super capacitor configured to store the energy from the first omni-directional antenna.   The receiver may further comprise a function generator configured to generate very low frequency pulses from the battery or super capacitor and a voltage controlled oscillator to generate the second frequency from the very low frequency pulses.   In a second specific aspect there is provided a method of locating a receiver relative to a transmitter comprising: scanning a beam of RF radiation over a plurality of sectors; receiving an acknowledgement from one or more sectors; and determining the location of the receiver based on which sector had the strongest acknowledgement.   In a third specific aspect there is provided a method of wireless energy transfer comprising: locating a receiver according to the preceding paragraph; and focussing RF radiation at the receiver&#39;s location   The method may further comprise tracking any change in the receiver&#39;s location.   The acknowledgement may be at a substantially lower frequency than the beam of RF radiation.   
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    One or more example embodiments of the invention will now be described, with reference to the following figures, in which: 
           [0029]      FIG. 1  is a block diagram of the overall RF based wireless energy transfer system with receiver searching and tracking functions, 
           [0030]      FIG. 2  is a block diagram of the proposed circuits for RX acknowledgement, 
           [0031]      FIG. 3  is a schematic diagram of the sensing circuit in the receiver, 
           [0032]      FIG. 4  is a schematic diagram of two possible constructions of small profile compact RX, 
           [0033]      FIG. 5  is a block diagram of the RFID detection circuits at the TX, 
           [0034]      FIG. 6  is operations of different blocks in  FIG. 5 , 
           [0035]      FIG. 7  is a calculated radiation pattern of proposed system with single radiation beam, and 
           [0036]      FIG. 8  is a calculated radiation pattern of proposed system with multiple radiation beams. 
       
    
    
     DETAILED DESCRIPTION 
       [0037]    The system  100  is shown in  FIG. 1  for wireless energy transfer between a base station  102  and a mobile electronic device  104 . The base station  102  includes a 2.4 GHz steerable antenna  106  for transmitting and a 860 MHz antenna  108  for receiving acknowledgements. A Field Programmable Gate Array (FPGA)  110  acts as a controller. The FPGA  110  controls the steerable antenna  106  to send focused burst of RF radiation scanning across a range of sectors  112  searching for any devices  104 . Based on any acknowledgements received, the FPGA  110  will make a determination on the location of any identified devices  104 . The steerable antenna  106  then focuses continuous RF radiation towards the location to transfer energy to the device  104 . The location is tracked and if the deice  104  moves to another sector, the location is updated. 
         [0038]    The steerable antenna  106  is a phased array with M×N elements. It transmits RF energy at 2.45 GHz and has a range of a couple of meters. The coverage area is divided into sectors which may be 1D or 2D. For example if the sectors are 1D, then each sector is defined by a horizontal angle from a reference. In  FIG. 1  the coverage area is over approximately a 180° angle and there are 7 sectors. The dimensions and configuration of sectors may be determined to suit the application. 
         [0039]    The mobile electronic device  104  may be a mobile phone, digital camera, portable media player, radio, LED lighting devices or the like. Typically the device  102  will be low power consumption, for example less than 1W. 
         [0040]    The device  104  is shown in more detail in  FIG. 2 . Generally the device  104  includes a 2.4 GHz receiving antenna  200 , a circuit or IC  202  and a 860 MHz transmitting antenna  204 . The circuit  202  operates when a pulse is received on antenna  200 , and sends an acknowledgement signal on the antenna  204 . Once the device  104  has been located, IC  202  stores the energy transferred to the antenna  200  for later use by the device  104  during normal operation. 
         [0041]    Both the receiving antenna  200  and transmitting antenna  204  are omni directional. For example  FIG. 4  shows two possible antenna configurations. Either a folded dipole or normal dipole are shown, although the particular antenna may depending on the actual layout of electronics it is attached to. 
         [0042]    The IC  202  may be an ASIC (application specific integrated circuits) design (such as a low cost CMOS process) which is ultra low power consumption. It may include an RF-DC rectifier  206 , a battery or super capacitor  208  and an acknowledgement circuit  210 . The RF-DC rectifier  206  converts the RF energy and rectifies it into DC, which is stored in the battery or a super capacitor  208 . 
         [0043]    The acknowledgement circuit  210  is shown in more detail in  FIG. 3 . A comparator  300  determines whether the battery  208  needs charging by comparing its voltage with an external voltage reference  302 . There is no acknowledgement sent to the base station  102  if the battery voltage is above the threshold voltage. 
         [0044]    If the battery voltage is below the threshold  302 , the comparator  300  enables a function generator  304 . The enabled function generator  304  generates pulses at very low frequency (˜kHz or lower). Normally data pulses have a duty cycle of 50%. To save energy as much as possible, its duty cycle may be reduced to 1% or even lower. However, its pulse width may be reasonably wide, and may be limited by the available bandwidth in RFID. If the antennas in  FIG. 5  have a 3 MHz available bandwidth, the on-period may be no smaller than 6.7 us. 
         [0045]    Each receiver has a unique ID  306  and this data is multiplied  308  with the low frequency clock output from the function generator  304 . An oscillator  310  will be powered on and tuned by the coded pulses from the multiplier  308 . The oscillator  310  is a gated voltage controlled oscillator with a 867.5 MHz central frequency. By using ultra-low duty cycle pulse trains, the overall power consumption of the oscillator  310  may be minimized and will be only a fraction of the received power. The oscillator  310  output is transmitted by the transmitting antenna  204 . 
         [0046]    The receiving antenna  108  is shown in more detail in  FIG. 5 . The receiving antenna  108  may an omni directional antenna tuned to 0.86-0.89 MHz, 310-320 MHz, or other RFID band. The antenna  108  output is amplified by a low noise amplifier  500  followed by an envelope detector  502 . This removes the carrier frequency (867.5 MHz for example) and leaves only a baseband waveform. The baseband waveform is demodulated  504  to determine the device ID, which is stored in the FPGA  110 . The baseband waveform is also integrated  506  and sampled by an ADC  508 . The digital signal is provided to the FPGA  110 . A switch  510  is closed to reset the voltage on the integrator after the scan moves to the next sector. 
         [0047]    Operation of the FPGA  110  is shown by the various waveforms in  FIG. 6 . When the steerable antenna  106  starts scanning  600 , the receiving antenna  108  is enabled awaiting for responses  602  from the device  104 . Since two separate frequencies are used, they are working independently and there is no talk-and-listen period required. The envelope  604  of the received acknowledgement  602  is demodulated to data  606 , so the FGPA  100  recognizes the device  104 . This envelope is also integrated  608  to measure the feedback signal strength. A reset signal  610  will be given at the end before measuring the feedback strength. After one sector, the steerable antenna  106  moves to the next sector and starts scanning again. 
         [0048]    The system  100  will operate in at least two modes: 
         [0049]    1. Searching for receivers 
         [0050]    The FGPA  110  scans and stores the sampled peak voltage of the feedback. It then compares all the sectors and the highest voltage peak is the estimate of the device  104  location. 
         [0051]    2. Charging and tracking of receivers 
         [0052]    In the course of charging, the device  104  keeps acknowledging at very low duty cycles. If the battery is fully charged, no acknowledgement will be sent. The device  104  stops charging. The FGPA  110  also stores the peak detected energy. If there is a big variation in peak detected energy, the steerable antenna  106  enters mode  1  and starts scanning again. 
         [0053]    In most applications, the steerable antenna  106  will focus an RF beam at a single direction. However, it is also possible to configure the steerable antenna  106  to send focus beams. With 8 antennas in a row, the radiation pattern of transmitting at +30 degrees  700  is plotted in  FIG. 7 . If the steerable antenna  106  was controlled to focus two beams instead of one, the feed is reconfigured with the 8 elements split into 2 sub-arrays, each consisting of 4 elements. Radiation pattern of two sub-arrays delivering power to +30  802  and −30 degrees  800  are plotted in  FIG. 8 . The penalty of doing this may be wider beam width, since less elements are used, and may be reduced power by a factor of 2. 
         [0054]    The advantages of using two widely separated frequencies transmit and receive frequencies rather than one single frequency may include: 
         [0055]    1. Less or no interference between RF transmit and receive frequency. 
         [0056]    2. The ability to conduct beam scanning allowing higher efficiency of energy transfer. 
         [0057]    3. Low power consumption at the device  104 . 
         [0058]    4. Smaller device  104  size. 
         [0059]    5. Because the acknowledgement signal is such low power, this system allows relatively accurate detection. 
         [0060]    6. Since no talk and listen period is required, the acquisition time is very fast and the system can dynamically track device movement with minimal delay. 
         [0061]    While example embodiments of the invention have been described in detail, many variations are possible within the scope of the invention as claimed as will be clear to a skilled reader.