Abstract:
A tracking system comprising: a transmitter configured to steer an RF beam across a detection range, a passive RFID tag configured to be enabled for locating substantially when located in the center of the RF beam of the transmitter, and an RFID reader configured to detect the tag  104  once enable.

Description:
RELATED APPLICATION DATA 
     The present application claims the benefit of priority to Singapore Patent Application No. SG 2012-08202-0 filed on Nov. 2, 2012 in the Singapore Patent Office, the entirety of which is incorporated by reference herein to the extent permitted by law. 
     BACKGROUND 
     The present invention relates to a system and method for tracking. 
     In modern wireless communication, power efficiency is an important design consideration. To achieve higher power efficiency, directional or focused RF beams, (instead of omni-directional beams), are increasingly being employed. However, transmission by directional beam forming either requires a fixed spatial relationship or real-time information on the relative position of the two communicating devices. 
     One method to detect relative position is to send an acknowledgement signal from a receiver to a transmitter either at the same frequency or at a second frequency. This acknowledgement signal may sometimes include position information. However, this method requires that the receiver to be active and generate the acknowledgement signal using a battery or a super capacitor that stores previously received power. Moreover, to build such a system, peripheral blocks like VCOs, power meters and control circuitries may be required at the receiver and transmitter. It should be noted that all of these blocks are active and will consume power. Also complexity and synchronization of the receiver and transmitter may be problematic. 
     SUMMARY OF THE INVENTION 
     In general terms the invention provides a passive receiver which does not need to generate an acknowledgement signal. A modified RFID tag and an RFID reader are used to detect the position of the receiver. 
     One or more embodiments may have the advantage(s) of: 
     1. Transmitting RF energy to a single or multiple directions rather than omni-directionally or the front-side; 
     2. Wirelessly charging low power electronics which consumes less than a dozen milliwatts, avoiding unnecessary user exposure; 
     3. Detecting the location of a rectifying antenna; and/or 
     4. Tracking the receiving antenna when the rectifying antenna is in motion. 
     In an embodiment, there is provided a tracking system comprising: a transmitter configured to steer an RF beam across a detection range, a passive RFID tag configured to be enabled when the tag and/or an RF antenna is located substantially in the RF beam, and an RFID reader configured to detect the tag once enabled. 
     In an embodiment, the tag is configured to connect to the RF antenna and to toggle a switch to enable the tag when the RF antenna is located substantially in the RF beam. 
     In an embodiment, the antenna is a rectenna. 
     In an embodiment, the RF beam is at a first frequency in an ISM band substantially located about 2.45 GHz or 5.80 GHz. 
     In an embodiment, the RFID reader is configured to detect the tag using a second frequency in an RFID band substantially located about 866-869 MHz, 928-950 MHz or 2.4-2.5 GHz. 
     In an embodiment, the transmitter comprises a steerable phased array antenna for the RF beam and/or for the RFID reader to detect the tag. 
     In an embodiment, the antenna is a rectenna and the rectenna is omni-directional or directive. 
     In an embodiment, the RF beam is sequentially steered at a series of spaced discrete angles, and if the RFID tag is detected, a tag location is determined as one of the discrete angles. 
     In an embodiment, the RF beam is continuously steered across the detection range, and if the RFID tag is detected, a tag location is determined as the centre of the range of RF beam steering angles where the RFID tag was detected, or as the angle at which a RSSI is the highest. 
     In an embodiment there is provided a method comprising: scanning a beam of RF radiation over a plurality of sectors; switching a modified RFID tag from invisible to visible when the tag receives the RF beam is in the currently scanned sector; detecting visible RFID tags; and determining the location of the tag of locating a tag relative to the RF beam origin based on the current sector in which an RFID tag can be detected. 
     In an embodiment the method further comprises focusing power RF radiation at the determined location. 
     In an embodiment, the method further comprises tracking any change in the determined location. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       One or more example embodiments of the invention will now be described, with reference to the following figures, in which: 
         FIG. 1  is a block diagram of the overall RF based wireless energy transfer system with receiver searching and tracking functions, 
         FIG. 2  is a schematic diagram of the modified RFID tag, 
         FIGS. 3 a  and 3 b    show the integration of an RF switch with the RFID tag, 
         FIG. 4  is radiation pattern of the transmitter in two different states, using a 2×4 array antenna, 
         FIG. 5  is radiation pattern of the transmitter transmitting at +30 degrees, using an 8 element linear array antenna, 
         FIG. 6  is radiation pattern of the transmitter split into 2 sub-arrays delivering power to +30 and −30 degrees, using an 8 element linear array antenna, and 
         FIG. 7  is a photo of a rectenna. 
     
    
    
     DETAILED DESCRIPTION 
     A tracking system uses transmitter with a steerable phased array antenna and an RFID reader, to locate a RFID tag. The Phased array antenna scans different angular sectors and sends power toward those specific sectors during the scanning time. The RFID tag includes a rectenna that receives RF power from phased array antenna and converts it to DC power. If the tag is in the active sector, the rectenna receives the RF power and produces a DC voltage. This voltage switches the RFID tag from being invisible to visible. The RFID reader is constantly scanning for visible tags and logs the current sector when a tag is detected as the tag location. 
     Sector here is used to mean a discrete angle range, or in terms of beam steering, a sequential series of spaced discrete beam steering angles. For example if detection range is ±80°, and there were 8 sectors, then the beam steering angles for each sector might be −70, −50, −30, −10, 10, 30, 50, 70. 
     A transmitter  100  is shown in  FIG. 1 . A phased array with M×N elements  102  transmits RF energy at a first frequency such as an ISM band for example: 2.45 GHz or 5.8 GHz. The signal for the array  102  is generated by signal generator/RF power generator  101  and coupled to each array element via feeding network  103 . 
     A RFID reader  106  transmits at a second frequency such as an RFID band for example: 866-869 MHz or 928-950-MHz or 2.4-2.5 GHz. A CPU  105  communicates with to the RFID reader  106  and signal generator  101 . The CPU  105  is also connected to a control board  107 , which in turn connects to the feeding network  103 . 
     The CPU  105  determines the power level to be transmitted by controlling the signal generator  101 . The signal sent during scanning may differ in power, frequency etc. compared to the power signal sent once the RFID tag is located. The CPU  105  determines the sector or direction of the array  102  by controlling the feeding network  103 . The RFID reader  106  is enabled by the CPU  105  and provides a status signal back to the CPU  105  when a RFID tag is located. The scanning process (and thus power consumption by the RFID reader  106 ) may be continuous, periodic or may be event activated. 
     The RFID reader  106  may incorporate a separate antenna or may share the phased array antenna  102  if the first and second frequencies are similar or identical. 
     The RFID reader  106  may be off the shelf and retrofitted into the system. The RFID tag  104  may include a dipole antenna  111  including an inductive loop  112 , and an RFID chip  110  coupled to antenna  111  via the inductive loop  112 . The RFID tag  104  may be specifically modified as shown in  FIGS. 2, 3   a  and  3   b , depending on the application. 
       FIG. 2  shows the operation of an unmodified RFID chip; which uses a semi-switching method (modulation) to communicate with RFID reader  106 .  FIG. 2  also shows a modified RFID tag  104 , with an added RF switch  114  in series with an RFID chip  110 . 
     A directional rectenna  109  designed for the second frequency converts any received RF energy from the array  102  and rectifies it into a DC bus  116 . The rectenna  109  is shown in more detail in  FIG. 7 . The rectenna includes a patch radiator  700  fed by an antenna feed  702  and a DC bus  704 . A voltage doubler  706  is connected between the antenna feed  702  and the DC bus  704  to rectify the signal from the antenna feed  702  to a DC voltage. Two capacitors  708  are provided to store energy received by the patch radiator  700  and to reduce the ripple on the DC bus  704 . An inductor  710  is connected between the voltage doubler  706  and the antenna feed  702  for impedance matching. 
     The DC bus  116  is connected to the control input of the RF switch  114 . As shown in  FIG. 2  when the RF switch  114  is closed the loop  112  is closed and enabled, and when is open the loop  112  is open and disabled. When the inductive loop  112  is open, the coupling from the RFID chip  110  to the dipole  111  is insignificant and thus the RFID tag  104  appears invisible to the RFID reader  106 . Alternatively if the RFID chip  110  may have an enable or disable input port to which DC bus  116  can be connected. In that case the RFID chip  110  itself is directly disabled rather than indirectly via the RF switch  114 . 
     The modified RFID tag  104  can only be read once the transmitter is transmitting the RF beam toward its corresponding sector. So, the system can detect that a rectenna is located in this sector and the system can either stop at this step or it can continue to find other rectennas in other sectors. After detecting the sector(s) of the available rectenna(s) the system can transmit power toward the detected sector(s). 
     The DC bus  116  is coupled to the mobile device(s)  118  as shown in  FIG. 1 . The mobile device(s)  118  or DC bus  116  may incorporate energy storage such a battery or super capacitor to store the received energy from the rectenna  109 . 
     Each RFID chip  110  has its unique ID and this data is stored as the ID of the attached modified RFID tag by CPU  105 . 
     The advantages of using different first and second frequencies are: 
     1. The TX is sending watt level power. If a single frequency or slightly shifted frequency is used, the TX power may jam detections. 
     2. Conventional RFID does not support beam steering. In this technology, one frequency is used for beam steering while another frequency at RFID band is used for detection. However, since the detection process and power transmission process are separate from each other, the same frequency for both of these processes can be used. 
     The system will operate in at least two modes: 
     1. Searching for Receivers 
     The TX scans and stores the ID of the read modified tags along with their corresponding sectors. It then can decide that to which of these sectors should transmit power. 
     2. Power Transmission and Tracking of Receivers 
     In the course of transmitting power to a rectenna, in some specified intervals the RFID reader tries to read the RFID tag of this rectenna again. If the RFID reader was able to read the modified tag again, TX will continue transmitting power to this rectenna. If not, it means that the rectenna has changed its sector and the system will start searching for the new sector of this rectenna. 
     The array  102  may deliver power to a single direction. However, it is also possible to configure the feeding network  103  to send multi-beams (&gt;1). With an 8 element linear array, the radiation pattern of transmitting at +30 degrees is plotted in  FIG. 5 . The feeding network  103  can alternatively split the 8 elements into 2 sub-arrays, each consisting of 4 elements. Radiation pattern of two sub-arrays delivering power to +30 and −30 degrees are plotted in  FIG. 6 . The penalty of doing this may be wider beam width, since fewer elements are used, and reduced power by a factor of 2. 
     Moreover, its also possible to steer the beam continuously. As the beam is steered across its complete arc, the RFID reader  106  will continue to monitor for responses by a RFID tag  104 . In this scenario the RFID tag  104  may become enabled or detected over a region of beam steering angles, rather than a discrete beam steering angle as described above. The CPU  105  can determine the RFID tag  104  location by either: 1) monitor the start and end angle of the detected region and designate the centre of this region or) determine the beam steering angle at which the RSSI (Received Signal Strength Indication) of the reflected RFID tag signal is highest. 
     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.