Patent Publication Number: US-2009219210-A1

Title: Wireless Tracking System And Method With Multipath Error Mitigation

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     The Present Application is a divisional application of U.S. patent application Ser. No. 11/672,047, filed on Feb. 7, 2007, which is a Continuation-In-Part Application of U.S. patent application Ser. No. 10/968,814, filed on Oct. 18, 2004, now U.S. Pat. No. 7,312,752, which claims priority to U.S. Provisional Application No. 60/572,690, filed on May 19, 2004, now abandoned, U.S. Provisional Application No. 60/528,052, filed on Dec. 9, 2003, now abandoned, and U.S. Provisional Application No. 60/513,784, filed on Oct. 22, 2003, now abandoned. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is related to wireless tracking systems and methods. More specifically, the present invention relates to a system and method for mitigating multipath errors associated with the wireless tracking of objects. 
     2. Description of Related Art 
     The ability to quickly determine the location of objects located within a facility is becoming a necessity of life. To the uninformed observer, the placement of transponders, also known as tags, on numerous non-stationary objects whether in an office or home would appear to be an unnecessary use of resources. However, the uninformed observer fails to appreciate the complexity of modern life and the desire for efficiency, whether at the office or home. 
     For example, in a typical hospital there are numerous shifts of employees utilizing the same equipment. When a new shift arrives the ability to quickly locate medical equipment not only results in a more efficient use of resources, but also can result in averting a medical emergency. Thus, the tracking of medical equipment in a hospital is becoming a standard practice. 
     The tracking of objects in other facilities is rapidly becoming a means of achieving greater efficiency. A typical radio frequency identification system includes at least multiple tagged objects, each of which transmits a signal, multiple receivers for receiving the transmissions from the tagged objects, and a processing means for analyzing the transmissions to determine the locations of the tagged objects within a predetermined environment. One exemplary method triangulates the strongest received signals to determine the location of a tagged object. This method is based on the assumption that the receivers with the strongest received signals are the ones located closest to the tagged object. However, such an assumption is sometimes erroneous due to common environmental obstacles. Multipath effects can result in a further located receiver having a stronger signal from a tagged object than a more proximate receiver to the tagged object, which result in a mistaken location determination. 
     Tekinay, U.S. Pat. No. 6,259,894 for a Method For Improved Line-Of-Sight Signal Detection Using RF Model Parameters, discloses a method for reducing time-shift due to multipathing for a RF signal in an RF environment. 
     Close, U.S. Pat. No. 3,869,673 for a Method And Apparatus For Measuring Multipath Distortion, discloses a method for indicating multipath distortion in a received signal. 
     Lennen, U.S. Pat. No. 5,402,450 for a Signal Timing Synchronizer, discloses a method and apparatus for reducing the effects of multipath induced distortions on the accuracy of detecting the time of arrival of a received signal. 
     Fortune et al., U.S. Pat. No. 5,450,615 for a Prediction Of Indoor Electromagnetic Wave Propagation For Wireless Indoor Systems, discloses techniques for predicting RF propagation within a structure. 
     The prior art has yet to resolve mistaken location calculations based on multipath effects. 
     BRIEF SUMMARY OF THE INVENTION 
     One aspect of the present invention is a method for determining a real-time location of an object within an indoor facility. The method begins with obtaining a plurality of sensor readings from a transponder attached to the object. Next, a reading set is generated from the plurality of sensor readings. The reading set is then sorted by a plurality of physical regions. Then, a first physical region is selected from the plurality of physical regions. The first physical region is composed of a first plurality of sensor readings that have the highest average signal strength. Next, the first plurality of sensor readings is sorted into a second plurality of sensor readings. Each of the second plurality of sensor readings corresponds to sensor located in a zone within the first physical region. A selected zone having the highest average reading is then selected. Next, a real-time location of the object is calculated using only the second plurality of sensor readings that correspond to the selected zone. 
     Each sensor reading preferably comprises a signal strength, link quality, time and identification of the transponder. The method may further comprise displaying the real-time location of the object on a graphical user interface. The method may also include comparing the calculated real-time location of the object to a previously calculated location for the object. The method may include monitoring the motion state of the object to confirm movement of the object from the previously calculated location to the real-time location. In a preferred embodiment, the indoor facility is a hospital, with each of the plurality of physical regions being a floor of the hospital, and the selected zone being a room on a floor of the hospital. The plurality of sensor readings of the reading set preferably comprises from eight to thirty sensor readings for the transponder, and each sensor reading originates from a single stationary sensor positioned within the indoor facility. Each sensor reading is preferably a radio frequency transmission from the transponder. The step of obtaining a plurality of sensor readings from the transponder attached to the object preferably comprises, transmitting a radio frequency transmission from the transponder, the radio frequency transmission comprising a signal strength, link quality, time of transmission and identification of the transponder, receiving the radio frequency transmission at a plurality of stationary sensors positioned within the indoor facility, and transmitting the signal strength, the link quality, the time of transmission and the identification of the transponder from each of the plurality of stationary sensors to a server for processing. 
     Another object of the present invention is a system for providing real-time location information for a plurality of non-stationary objects within an indoor facility. The system includes a plurality of sensors, a plurality of transponders and a processing means. Each of the stationary sensors is positioned within the indoor facility. Each of the transponders is attached to one of the non-stationary objects. Each of the transponders has means for wirelessly transmitting to each of the stationary sensors transponder-specific data. The processing means processes the transponder-specific data to obtain a real-time reading set for the transponder. The processing means also processes the real-time reading set to determine a first plurality of sensor readings. The first plurality of sensor readings corresponds to a physical region within the indoor facility having the highest average reading. The processor means then processes the first plurality of sensor readings, which are associated with the selected physical region, to select a zone within the physical region having the highest average reading. The processing means then calculates the position of the non-stationary object using the sensor readings from the stationary sensors positioned within the selected zone of the selected physical region. 
     The transponder-specific data preferably comprises a signal strength, link quality, time and identification of the transponder. In a preferred embodiment, the indoor facility is a hospital with the physical region preferably a floor of the hospital, and the selected zone is a room on a floor of the hospital. The processing means is preferably a server in communication with the plurality of stationary sensors through a network. Each transponder preferably transmits a radio frequency transmission of approximately 2.48 gigahertz, and each stationary sensor preferably communicates utilizing a 802.15.4 protocol. The system may further comprise means for eliminating those sensor readings not associated with (i.e., located within) the selected zone. 
     Another aspect of the present invention is a method for determining a location of an object within a predetermined environment. The method begins with transmitting a plurality of radio frequency signals for a wireless tracking device to a positioning engine. The wireless tracking device is attached to the object and each of the radio frequency signals corresponds to a fixed signal transmitter within the environment. Each radio frequency signal is processed to determine the location of the respective fixed signal transmitter. A probable region of the object is determined based on the location of a majority of the fixed signal transmitters for the plurality of radio frequency signals. The radio frequency signals that correspond to fixed signal transmitters located outside of the probable region of the object are eliminated from the location determination. The position of the object within the predetermined environment is calculated using only the radio frequency signals that correspond to fixed signal transmitters located within the probable region of the object. 
     The predetermined environment is preferably a hospital, and the probable region of the object is preferably a room in the hospital. The step of transmitting a plurality of radio frequency signals for a wireless tracking device to a positioning engine preferably comprises transmitting radio frequency signals from the wireless tracking device, each radio frequency signal comprising a signal strength, link quality, time of transmission and identification of the transponder, receiving the radio frequency signals at a plurality of stationary sensors positioned within the predetermined environment, and transmitting the signal strength, the link quality, the time of transmission and the identification of the wireless tracking device from each of the plurality of stationary sensors to a server for processing. 
     Yet another aspect of the present invention is a system for providing real-time location information for a plurality of non-stationary objects within an indoor facility. The system includes a mapped space and a processor. The mapped space is of a physical environment of the indoor facility. The processor includes means for updating the mapped space in response to received measurements of the physical environment from one or more stationary sensors located within the indoor facility, means for generating a plurality of location hypotheses for a non-stationary object within the physical environment, at least one of the location hypotheses computed in response to measurement received from the non-stationary object and the mapped space, and means for generating a location estimate based on one or more of the plurality of location hypotheses, wherein one or more of the plurality of location hypotheses are selected based on a probability associated respectively therewith. The probability is computed in association with known barriers in the physical space. 
     Having briefly described the present invention, the above and further objects, features and advantages thereof will be recognized by those skilled in the pertinent art from the following detailed description of the invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is schematic view of a system of the present invention. 
         FIG. 2  is a multi-floor view of a facility employing the system of the present invention. 
         FIG. 3  is a floor plan view of a single floor in a facility employing the system of the present invention. 
         FIG. 4  is a two-floor view of a facility including a tagged object and sensors of the system of the present invention. 
         FIG. 5  is a flow chart of a general method of the present invention. 
         FIG. 6  is a flow chart of a specific method of the present invention. 
         FIG. 7  is a flow chart of a specific method of the present invention. 
         FIG. 8  is a flow chart of a single sensor reading input. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in  FIGS. 1-4 , a system is generally designated  50 . The system  50  is capable of determining real-time location of an object  100  within an indoor facility  70 . The system  50  preferably includes a plurality of sensors  55 , a plurality of bridges  56 , a plurality of tags  60  and at least one server  65 . One example of the components of the system  50  is disclosed in U.S. Pat. No. 7,312,752 for a Wireless Position Location And Tracking System, which is hereby incorporated by reference in its entirety. A more specific example of the sensors  55  is disclosed in U.S. Pat. No. 7,324,824 for a Plug-In Network Appliance, which is hereby incorporated by reference in its entirety. Another example of a system  50  is set forth in U.S. Pat. No. 6,751,455 for a Power-And Bandwidth-Adaptive In-Home Wireless Communications System With Power-Grid-Powered Agents And Battery-Powered Clients, which is hereby incorporated by reference in its entirety. 
     The system  50  is preferably employed within an indoor facility  70  such as a business office, factory, home, hospital and/or government agency building. The system  50  is utilized to track and locate various objects positioned throughout the facility  70 . The tags  60  continuously transmit signals on a predetermined time cycle, and these signals are received by sensors  55  positioned throughout the facility  70 . The sensors  55  transmit the data to a bridge  56  for transmission to a server  65 . If a sensor  55  is unable to transmit to a bridge  56 , the sensor may transmit to another sensor  55  in a mesh network-like system for eventual transmission to a bridge  56 . In a preferred embodiment, a transmission may be sent from a transmission distance of six sensors  55  from a bridge  56 . The server  65  preferably continuously receives transmissions from the sensors  55  via the bridges  56  concerning the movement of objects  100  bearing a tag  60  within the facility  70 . The server  65  processes the transmissions from the sensors  55  and calculates a real-time position for each of the objects  100  bearing a tag  60  within the facility  70 . The real-time location information for each of the objects  100  bearing a tag  60  is preferably displayed on an image of a floor plan of the indoor facility  70 , or if the facility  70  has multiple floors, then on the floor plan images of the floors of the facility  70 . The floor plan image may be used with a graphical user interface so that an individual of the facility  70  is able to quickly locate objects  100  within the facility  70 . 
     As shown in  FIG. 1 , the system  50  utilizes sensors  55  to monitor and identify the real-time position of non-stationary objects bearing or integrated with tags  60 . The sensors  55   a - f  preferably wirelessly communicate with each other (shown as double arrow lines) and with a server  65  through a wired connection  66  via at least one bridge  56 , such as disclosed in the above-mentioned U.S. Pat. No. 7,324,824 for a Plug-In Network Appliance. The tags  60   a - c  transmit signals (shown as dashed lines) which are received by the sensors  55   a - e , which then transmit signals to bridges  56  for eventual transmission to a server  65 . The server  65  is preferably located on-site at the facility  70 . However, the system  50  may also include an off-site server  65 , not shown. 
     Each tag  60  preferably transmits a radio frequency signal of approximately 2.48 GigaHertz (“GHz”). The communication format is preferably IEEE Standard 802.15.4. Those skilled in the pertinent art will recognize that the tags  60  may operate at various frequencies without departing from the scope and spirit of the present invention. 
     As shown in  FIGS. 2-4 , the facility  70  is depicted as a hospital. The facility  70  has a multitude of floors  75   a - c . An elevator  80  provides access between the various floors  75   a ,  75   b  and  75   c . Each floor  75   a ,  75   b  and  75   c  has a multitude of rooms  90   a - i , with each room  90  accessible through a door  85 . Positioned throughout the facility  70  are sensors  55   a - o  for obtaining readings from tags  60   a - d  attached to or integrated into non-stationary objects  100   a ,  100   b  (see  FIGS. 2 and 4 ). A bridge  56  is also shown for receiving transmissions from the sensors  55  for processing by the server  65 . 
     As shown in  FIG. 4 , the tag  60   a  is attached to movable bed  100   a  positioned on an upper floor  75   c . The tag  60   a  transmits a signal which is received by sensors  55   a ,  55   b  and  55   c . If the signal to sensor  55   c  is the strongest, then an analysis of the readings from the sensors  55   a - c  may place the tag  60   a , and thus the movable bed  100   a , at position  60 ′ on the lower floor  75   b . This type of faulty reading would likely occur with triangulation. To prevent such a faulty positioning reading, the present invention processes the readings preferably according to one of the methods illustrated in  FIGS. 5-7 , which would eliminate the reading from sensor  55   c  from the location calculation for movable bed  100   a.    
     A general method  200  of the present invention is illustrated in  FIG. 5 . At block  202 , the sensors  55  of the system  50  generate readings from the tags  60 . These single sensor reading inputs  600  are illustrated in  FIG. 8 . As shown in  FIG. 8 , the inputs preferably include the tag identification  604 , the signal strength  606 , the link quality  608  and the time of the reading  610 , which are inputted as a single sensor reading  602 . At block  204 , a plurality of readings sets are generated from the sensor readings. In a preferred embodiment, each of the plurality of readings sets represents an area of a facility  70 . At block  206 , the readings are further sorted by a particular zone of the facility  70  thereby eliminating readings that may lead to an incorrect location. In a preferred embodiment, a zone is a subset of an area. At block  208 , the zone with the highest average reading is selected for calculation of the position of the object  100 , again eliminating readings that may lead to an incorrect reading. At block  210 , the location of the object  100  is calculated based on the readings from the selected zone. 
     A more specific method  300  of the present invention is set forth in  FIG. 6 . At block  302 , the sensors  55  of the system  50  generate readings from the tags  60 . As discussed above, the single sensor reading inputs  600  are illustrated in  FIG. 8 . At block  304 , a reading set is generated for readings from a single tag  60 . The generation of the reading set is typically in response to an inquiry from a user of the system  50  in search of an object  100  bearing tag  60 . At decision block  306 , the server  65  determines if there is sufficient data to proceed with the location analysis. If there is insufficient data, the method is restarted at block  302 . If there is sufficient data, then the method proceeds to block  308 . At block  308 , the reading sets are separated by floor  75  of the facility  70 . At block  310 , the floor  75  with the highest average reading set is selected for further processing. At block  312 , the readings for the selected floor are sorted by zones. Each zone may represent any physical boundary on the selected floor  75  of the facility  70 . Preferably, the zones represent a room  90 , station  95  or other easily determined physical location. At block  314 , the zone with the highest average reading is selected. At block  316 , the location of the object  100  is calculated based on the readings from the selected zone. At block  318 , the location is inputted to the location database for dissemination to users of the system to locate the object  100 . 
     An even more specific method  400  of the present invention is set forth in  FIG. 7 . At block  402 , the sensors  55  of the system  50  generate readings from the tags  60 . As discussed above, the single sensor reading inputs  600  are illustrated in  FIG. 8 . At block  404 , a reading set is generated for readings from a single tag  60 . The generation of the reading set is typically in response to an inquiry from a user of the system  50  in search of an object  100  bearing tag  60 . At decision block  406 , the server  65  determines if there is sufficient data to proceed with the location analysis. If there is insufficient data, the method is restarted at block  402 . If there is sufficient data, then the method proceeds to block  408 . At block  408 , the reading sets are separated by floor  75  of the facility  70 . At block  410 , the floor  75  with the highest average reading set is selected for further processing. At block  412 , the readings for the selected floor are sorted by zones. Each zone may represent any physical boundary on the selected floor  75  of the facility  70 . Preferably, the zones represent a room  90 , station  95  or other easily determined physical location. At block  414 , the zone with the highest average reading is selected. At block  416 , the location of the object  100  is calculated based on the readings from the selected zone. 
     At decision block  418 , the server  65  inquires if the new calculated location is consistent with available data for the object  100 . The available data includes the motion sensor state of the object  100  which is tracked at block  424 . If the motion sensor has not detected a motion threshold of the object  100 , then that is one indication that the new calculated location is in error. However, if the motion sensor has detected movement (a motion threshold) of the object  100 , then that is one indication that the new calculated location is correct. Additional data for the decision block  418  includes recently calculated locations for the object  100  which are available from database  426 . Yet further data available for decision block  418  is data from the possible hypotheses database  428 . The possible hypotheses database includes data such as the timing between the last calculated location and the new calculated location. If the object  100  has moved one end of the facility  70  to another end of the facility  70  within seconds, then the new calculated location may be in error. If the response to decision block  418  is yes, then at block  420  the location is inputted to the location database for dissemination to users of the system to locate the object  100 . If the response to decision block  418  is no, then the new calculated location is held as an unproven hypothesis at block  422 . 
     The following example illustrates the information that is utilized and eliminated in practicing the present invention. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE ONE 
               
               
                   
               
               
                 Sensor 
                 Signal Strength 
                 Link 
                   
                 Sensor Location 
               
               
                 # 
                 dB 
                 Quality 
                 Time 
                 (floor/region) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 1 
                 −95 
                 −95 
                 Sep. 14, 2006 
                 5/B 
               
               
                   
                   
                   
                 11:22:35 
               
               
                 2 
                 −10 
                 −10 
                 Sep. 14, 2006 
                 4/C 
               
               
                   
                   
                   
                 11:22:35 
               
               
                 3 
                 −20 
                 −20 
                 Sep. 14, 2006 
                 4/C 
               
               
                   
                   
                   
                 11:22:36 
               
               
                 4 
                 −25 
                 −25 
                 Sep. 14, 2006 
                 4/C 
               
               
                   
                   
                   
                 11:22:35 
               
               
                 5 
                 −40 
                 −40 
                 Sep. 14, 2006 
                 4/C 
               
               
                   
                   
                   
                 11:22:36 
               
               
                 6 
                 −50 
                 −50 
                 Sep. 14, 2006 
                 4/C 
               
               
                   
                   
                   
                 11:22:36 
               
               
                 7 
                 −70 
                 −70 
                 Sep. 14, 2006 
                 4/D 
               
               
                   
                   
                   
                 11:22:36 
               
               
                 8 
                 −80 
                 −80 
                 Sep. 14, 2006 
                 4/D 
               
               
                   
                   
                   
                 11:22:36 
               
               
                 9 
                 −90 
                 −90 
                 Sep. 14, 2006 
                 4/E 
               
               
                   
                   
                   
                 11:22:37 
               
               
                 10 
                 −95 
                 −95 
                 Sep. 14, 2006 
                 4/E 
               
               
                   
                   
                   
                 11:22:37 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
             
               
                   
                 TABLE TWO 
               
               
                   
                   
               
               
                   
                 Floor 
                 Average Reading per Floor 
               
               
                   
                   
               
             
            
               
                   
                 2 
                 N/A 
               
               
                   
                 3 
                 −120 
               
               
                   
                 4 
                 −30 
               
               
                   
                 5 
                 −85 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
             
               
                 TABLE THREE 
               
               
                   
               
               
                 Region 
                 Peaks 
                 Average Reading per Region 
               
               
                   
               
             
            
               
                 C 
                 −20 
                 −20 
               
               
                 D 
                 −10 
                 −70 
               
               
                 E 
                 −70 
                 −95 
               
               
                   
               
            
           
         
       
     
     As shown in Table One, the signal strength from each tag  60  is provided dBm with a full strength value of zero, which is a ratio of power relative to 1 milli-Watt. The Link Quality value is provided as a similar value as the signal strength. The time is a date stamp of the time and date that the signal is received by the sensor  55 . The sensor location is preferably a floor and region on the floor. In a preferred embodiment, the regions on the floors overlap each other. The regions are preferably determined based on the facility  70 . 
     In Table One, ten readings from sensors  55  positioned on various floors of the facility  70 . Each of the readings is transmitted from a single tag  60  to the sensors  60 . The sensors  60  transmit the data from the tag  60  to the server  65  via bridges  56 . The server  65  uses the data to calculate the location of the object  100  as discussed. The sensor location may also be provided in terms of a X-Y position which is based on a floor plan image of each floor of the facility  70 . The X-Y position may be based on the pixel location on the image of the floor plan. 
     The average reading from all of the sensors  55  on each floor is provided in Table Two. More specifically, if the fifth floor has ten sensors  55  that each received a signal from a specific tag  60 , then the readings from those ten sensors  55  are averaged to obtain the average reading per floor value provided in Table Two. The readings from the floor with the highest value are then further processed to determine the location of the object  100 . The readings from the sensors  55  on the other floors are eliminated from the calculation for the location of the object  100 . 
     The average reading from all of the sensors  55  in each region on the selected floor is provided in Table Three. As mentioned above, the regions preferably overlap so that a single sensor  55  may be in two or more regions, and used in the average reading for both regions. The peak reading for each region is also set forth in Table Three. In an alternative embodiment, if the peak reading exceeds a threshold, then that region is selected even if the average readings for that region are less than another region. In calculating the location of the object  100 , the highest readings within a selected region are used for the calculation. The number of readings used preferably ranges from 2 to 10, and is most preferably 3 to 5. The more readings used in the calculation, the longer the processing time for the calculation. Thus, using 10 readings may provide a more accurate location, however, the processing time will be longer than using 3 readings. In a preferred embodiment, a radial basis function is utilized in calculating the location of the object  100 . The location of the object  100  is preferably conveyed as an XY coordinate on an floor plan image of the facility  70 . 
     From the foregoing it is believed that those skilled in the pertinent art will recognize the meritorious advancement of this invention and will readily understand that while the present invention has been described in association with a preferred embodiment thereof, and other embodiments illustrated in the accompanying drawings, numerous changes modification and substitutions of equivalents may be made therein without departing from the spirit and scope of this invention which is intended to be unlimited by the foregoing except as may appear in the following appended claim. Therefore, the embodiments of the invention in which an exclusive property or privilege is claimed are defined in the following appended claims.