Abstract:
A method for identifying objects including fixing tags to respective objects, each such tag comprising at least one optical emitter. The at least one optical emitter on each of the tags is driven to emit optical radiation of a respective color, selected from among the first plurality of colors emittable by the tags, during a respective time slot, selected from among a second plurality of time slots during which the tags may emit the optical radiation. A camera captures sequence of electronic images of an area containing the objects to which the tags are fixed. The electronic images in the sequence are processed in order to identify, responsively to the colors of the optical radiation emitted by the tags and the time slots in which the optical radiation is emitted, the objects to which the tags are fixed.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to object identification and location systems, and specifically to optical systems for simultaneously identifying and tracking locations of multiple objects. 
     BACKGROUND OF THE INVENTION 
     Various methods are known in the art for remote identification and tracking the location of a movable object within a controlled area. For example, radio frequency (RF) identification tags may be fixed to objects in the area. Each tag typically comprises a RF transceiver, which transmits a unique identification code when queried by a signal from a central antenna. Such systems may be capable of identifying multiple objects, but they generally give only a rough indication of the location of each object. Optical markers can be tracked using video cameras to obtain more accurate position information. Optical systems of this sort, however, generally require the use of sophisticated and costly image processing equipment, and are limited in the number of objects that they can track simultaneously. 
     SUMMARY OF THE INVENTION 
     In embodiments of the present invention, an optical identification and tracking system uses both time and wavelength multiplexing in order to identify and accurately track the locations of a number of objects in a defined area. An optical tag is fixed to each object. Each tag is programmed to emit optical radiation of a certain color during a predetermined time slot, among a number of different colors and a plurality of synchronized time slots that are available. One or more cameras, typically video cameras, capture a sequence of electronic images of the radiation emitted by the tags in each time slot. A processing unit analyzes the images in order to determine location coordinates of each tag. In this manner, the system is able to identify and accurately track the location of multiple moving objects simultaneously—up to at least a number of objects equal to the product of the number of time slots times the number of different colors. 
     There is therefore provided, in accordance with an embodiment of the present invention, apparatus for identifying objects, including: 
     a multiplicity of tags, each such tag being adapted to be fixed to a respective one of the objects and including:
         at least one optical emitter, which is adapted to emit optical radiation of a respective color, selected from among a first plurality of colors emittable by the tags; and   a controller, which is coupled to drive the at least one optical emitter to emit the optical radiation during a respective time slot, selected from among a second plurality of time slots during which the tags may emit the optical radiation;       

     at least one camera, which is adapted to capture a sequence of electronic images of an area containing the objects to which the tags are fixed; and 
     an image processor, which is adapted to process the electronic images in the sequence in order to identify, responsively to the colors of the optical radiation emitted by the tags and the time slots in which the optical radiation is emitted, the objects to which the tags are fixed. 
     In some embodiments, for each of at least some of the tags, the at least one optical emitter includes at least first and second optical emitters of different, first and second colors, and the controller is configurable to select one of the first and second colors to be emitted by the tag. 
     Typically, the tags are configured so that no more than one of the tags emits any one of the colors during any of the time slots. 
     In an aspect of the invention, the controller is adapted to receive a synchronization input and to synchronize the respective time slot responsively to the synchronization input, so that all the tags are in mutual synchronization. In a disclosed embodiment, the apparatus includes a synchronization transmitter, which is adapted to transmit a synchronization signal over the air in the area containing the objects, wherein each of the tags includes a synchronization module, which is coupled to receive the synchronization signal and responsively thereto, to generate the synchronization input to the controller. Typically, the synchronization signal includes a radio frequency (RF) signal or, alternatively, an infrared (IR) signal. 
     In an embodiment of the invention, the at least one optical emitter includes at least one light-emitting diode (LED). 
     In some embodiments, the image processor is further adapted to process the electronic images in the sequence in order to determine, responsively to the colors of the optical radiation emitted by the tags and the time slots in which the optical radiation is emitted, location coordinates of the objects. The apparatus may include a memory, coupled to the image processor, which is adapted to create a location database in the memory, containing records of motion of the objects in the area, based on the location coordinates determined by the image processor. In one embodiment, the objects include animals. 
     There is also provided, in accordance with an embodiment of the present invention, a method for identifying objects, including: 
     fixing tags to respective objects, each such tag including at least one optical emitter; 
     driving the at least one optical emitter on each of the tags to emit optical radiation of a respective color, selected from among a first plurality of colors emittable by the tags, during a respective time slot, selected from among a second plurality of time slots during which the tags may emit the optical radiation; 
     capturing a sequence of electronic images of an area containing the objects to which the tags are fixed; and 
     processing the electronic images in the sequence in order to identify, responsively to the colors of the optical radiation emitted by the tags and the time slots in which the optical radiation is emitted, the objects to which the tags are fixed. 
     The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic, pictorial illustration of a system for tracking animals, in accordance with an embodiment of the present invention; 
         FIG. 2  is a block diagram that schematically illustrates elements of an optical tag, in accordance with an embodiment of the present invention; and 
         FIG. 3  is a block diagram that schematically illustrates a central processing and control unit used in an optical tracking system, in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG. 1  is a schematic, pictorial illustration of a system  20  for tracking animals  22 , in accordance with an embodiment of the present invention. In the present example, animals  22  comprise cows, and system  20  is used to identify and track the locations of the cows within a large, enclosed area, such as a dairy barn. It will be understood, however, that this application of the present invention is shown here solely by way of example, and the principles of system  20  may be applied in a wide range of other applications, for tracking people, animals or other movable objects. 
     An optical tag  24  is fixed externally to each animal  22 . All of tags  24  operate in mutual synchronization, in accordance with RF synchronization signals broadcast by an antenna  26 . Typically, antenna  26  transmits these signals at approximately 433 MHz in the ISM band, with 1.5 MHz bandwidth. Alternatively, antenna  26  may transmit in the 846 MHz band, or in any other suitable band permitted by regulatory authorities. Further alternatively, system  20  may use optical synchronization signals, such as infrared (IR) pulses transmitted by a suitable IR transmitter, in place of antenna  26 . 
     As yet another alternative, tags  24  may synchronize on an external signal, such as a beacon provided by a cellular communication network or a Global Positioning System (GPS). 
     In response to the synchronization signal from antenna  26 , each tag  24  transmits light of an assigned color in an assigned time slot. Each tag is programmed in advance with its time slot and color assignments. For example, each tag may be programmed to transmit in one of 480 successive time slots, each typically 1-4 sec long, during which the tag emits either red, green or blue light. Alternatively, there may be a larger or smaller number of available time slots, which may be longer or shorter in duration, and a larger or smaller number of colors may be used. Further alternatively, tags  24  may be configured to emit IR or ultraviolet (UV) radiation. The term “optical radiation,” as used in the present patent application and in the claims, should thus be understood to refer to any radiation in the visible, IR or UV range, while the term “color” refers to any distinguishable wavelength band in any of these ranges. 
     In the above example, it will be observed that there are 1440 possible combinations of different time slots with different radiation colors, so that system  20  is capable of distinguishing among at least 1440 different animals  22  with their individual tags  24 . More advanced techniques, such as transmission by certain tags  24  of multiple colors, transmission by certain tags in a predetermined sequence of time slots, or selective transmission, whereby only certain tags transmit after any given synchronization signal, may be used to increase the capacity of system  20  still further. For example, tags  24  may operate dynamically, requesting a time slot and then transmitting optical radiation only after a certain event occurs, such as movement of animal  22  to which the particular tag is fixed. Image processing techniques may further be used to distinguish among multiple tags transmitting in the same time slot. 
     Video cameras  28 , which are typically mounted above animals  22 , capture images that include the radiation emitted by tags  24 . Typically, cameras  28  comprise standard CCD- or CMOS-based solid state image sensors, spaced about 10-20 m apart, depending on the mounting height and the resolution required of system  20 . For example, cameras  28  may comprise model CV7017H CCD cameras, produced by Appro Technologies (Taiwan), which are ceiling-mounted, face down, within a protective plastic cover. The video signals are input to a central processing and control unit  30 , which analyzes the signals to determine the location of each tag  24 , based on the timing and color of the radiation emitted by each tag. Unit  30  is thus able to maintain a location log for each animal  22 , showing its movement over time within the area monitored by system  20 . 
       FIG. 2  is a block diagram that schematically shows details of tag  24 , in accordance with an embodiment of the present invention. A timing controller  32  determines the color and time slot in which tag  24  is to emit radiation, wherein the time slot is determined in relation to the synchronization signals transmitted by antenna  26 . The time slot and color assignments of tag  24 , and possibly other configurable operating parameters, as well, are input to controller  32  via a control interface  34 . Operating power for the components of tag  24  is typically supplied by an on-board battery (not shown), although power may alternatively be supplied externally, via solar cells or RF induction, for example, as is known in the art. The components of the tag may be integrated into a single microelectronic chip, contained within a package that is capable of withstanding the stresses and wear present in the operating environment of system  20 . Alternatively, tag  24  may comprise a circuit board or other substrate on which two or more chips are mounted. 
     A RF synchronization module  36  receives the synchronization signals from antenna  26  via an internal antenna  38  within tag  24 . Based on these signals, module  36  generates a synchronization input to controller  32 . Typically, the synchronization signal transmitted by antenna  26  comprises a pulse or a train of pulses in a predetermined pattern, indicating the beginning of a global synchronization period (GSP) for all of tags  24 . (Different pulse trains may also be used to encode data representing the current time slot number.) Module  36  filters, amplifies and discriminates the RF signals received by antenna  38  in order to detect the pulse or pattern of pulses transmitted by antenna  26 . When the synchronization signal comprises a pulse train (for synchronization purposes and possibly to represent the current time slot number), module  36  correlates the pattern of received pulses with a predetermined reference pattern in order to detect the exact synchronization, and accordingly signals the beginning of the GSP to controller  32 . It is generally desirable that synchronization modules  36  in all of tags  24  synchronize on the signals from antenna  26  with a maximum tag-to-tag deviation no greater than 1/10 of a time slot. 
     Controller  32  uses a clock provided by a local oscillator  40  in order to determine when its assigned time slot occurs within the GSP, relative to the synchronization input from module  36 . 
     When the assigned time slot arrives, controller  32  triggers a LED driver circuit  42  to actuate one of LEDs  44 ,  46  and  48 . Typically, each of the LEDs emits radiation of a different color. For example, LED  44  may emit red light, LED  46  green light, and LED  48  blue light. The choice of which LED to actuate is typically pre-programmed via interface  34 , so that no more than one tag  24  emits radiation of a given color during any given time slot. Alternatively or additionally, system  20  may comprise different groups of tags  24 , wherein each tag has a single LED, and a different color LED is used in the tags of each group. Further alternatively, other types of variable-wavelength or fixed-wavelength light sources may be used. 
     A time-slot configuration management (TCM) device  50  is used to program controller  32  via interface  34 . For this purpose, interface  34  may comprise a plug (not shown), which mates with TCM device  50 , or the TCM device may alternatively communicate with interface  34  over a wireless link, such as a RF or IR link. TCM device  50  is used to configure the timing parameters of each tag  24 , including:
         The GSP duration (typically between 1 and 8 min).   Time slot duration (typically between 1 and 4 sec).   Time slot selection (typically from time slot #1 to #480).
 
As noted above, TCM device  50  may also be used to set other operating parameters of tag  24 , such as color selection. Although certain ranges of GSP and time slot duration are listed above by way of example, larger or smaller durations may also be used.
       

       FIG. 3  is a block diagram that schematically shows details of processing and control unit  30 , in accordance with an embodiment of the present invention. Unit  30  is built around a server  60 , which typically comprises a personal computer running the Microsoft Windows® operating system. Server  60  controls a RF synchronization interface  62 , comprising a RF transmitter, which transmits the RF synchronization signals via antenna  26 , as described above. Video signals from cameras  28  are received and digitized by a video interface  64 , such as the PV  143  WDM video capture card, produced by Professional Video (Taiwan), which inputs the digitized video signals from the cameras to server  60  for analysis. A video storage repository, typically comprising a large-capacity hard disk or disk array, may be provided in order to store selected frames (or all frames, if desired) from the video streams that are received by interface  64 . 
     Server  60  processes the digitized video image output from each of cameras  28  in turn, in order to locate the bright, colored lights emitted by different tags  24  during successive time slots. Locations of cameras  28  are registered and calibrated, so that server  60  is able to associate the pixels in the images produce by each of the cameras with specific location coordinates in the area monitored by system  20 . Thus, when server  60  finds that radiation of a particular color was detected at a given pixel (or group of pixels) in the image received by a particular camera  28  during a particular time slot, the server is able to determine unequivocally the identity of the tag that emitted the radiation and the location coordinates of the tag. Server  60  records this information in a location database  68 , which is typically held in disk memory. 
     Users of system  20  may access the information in database  68 , as well as in repository  66 , via server  60 . The server may have a communication interface to a network  72 , allowing a client computer  70  to access the information remotely, via the network. The information in database  68  indicates to the user which animals  22  were located in the monitoring area of system  20  at any point in time, and also provides a record of the locations and movements of the animals within the area. The user may similarly access server  60  in order to find the current locations of particular animals in real time. 
     It will be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.