Abstract:
The invention relates to a method and a system for monitoring and position determination of objects and/or living beings within an area, such as, e.g. a room in a building or a road tunnel. The system comprises a plurality of identification tags equipped with an ultrasonic receiver and radio transmitter which is attached to the objects that have to be monitored. The identification tags receive ultrasonic signals whose arrival time they measure. This information together with the identification tags&#39;ID code are transmitted by radio waves to a central unit which calculates the position of each of the identification tags.

Description:
INTRODUCTION/TECHNICAL FIELD 
   The invention relates to a method and a system for monitoring and position determination of objects and/or living beings within an area such as, e.g. a room in a building or a road tunnel. The system comprises a plurality of electronic units called identification tags, which are attached to the objects to be monitored. Each identification tag has its own identification code (ID code) and is equipped with an ultrasonic receiver and radio transmitter. The ultrasonic signals it receives are transmitted synchronously from a plurality of transmitter units. The ultrasonic receiver in the identification tag is connected with a calculating unit which calculates the arrival time of received ultrasonic signals. This information together with the identification tag&#39;s ID code and any additional information are transmitted in the form of radio waves to a central processing unit which calculates the identification tag&#39;s position and presents it to a user of the system. 
   BACKGROUND TO THE INVENTION 
   There are a number of different positioning systems in existence today based on various principles including ultrasound and radio waves. These systems all have both advantages and disadvantages. The system according to the invention employs identification tags which are placed on the objects that have to be monitored. The identification tags contain both ultrasonic receivers and radio transmitters. 
   Systems based on radio waves, for example in one of the unlicensed ISM bands (Industrial, Scientific, Medical), for example 2.4 GHz for WLAN or Bluetooth™ plug-in wireless network card in PCs have the following advantages: they have a high data transfer capacity, they can tolerate a lot of movement (Doppler shift) and have a long range. The disadvantages of such a system are that the long range makes it necessary to use 3 or more base stations in order to obtain a position. 
   Location systems based on ultrasound have the following advantages: they have a short range, the waves do not pass through walls, i.e. they are easy to position at room level and the detectors are cheap. The disadvantages are that these systems have a low transfer capacity and cannot tolerate much movement. 
   The invention outlined herein combines the best features of radio and ultrasound-based systems. An identification tag according to the invention is equipped with both an ultrasonic receiver and a radio transmitter. The ultrasonic signals it receives are transmitted synchronously from a plurality of transmitter units. The ultrasonic receiver in the identification tag is connected with a calculating unit which calculates the arrival time of received ultrasonic signals. This information together with the identification tag&#39;s ID code etc. are transmitted in the form of radio waves to a central processing unit which calculates the identification tag&#39;s position and presents it to a user of the system. 
   By equipping an identification tag with a radio receiver in addition to a radio transmitter, the ultrasonic receiver in the identification tag can be activated only if ultrasonic signals are to be transmitted in the room or the area in which it is located. 
   The system may advantageously be employed in areas which are traditionally used for only radio-based systems or only ultrasound-based systems. By using ultrasound for calling up a single or several tags, the area in which tags respond will be restricted, while the use of radio waves for transferring the ID code and timing gives the identification tag a high data transfer capacity and makes it insensitive to Doppler shift. 
   PRIOR ART 
   Several different principles exist today for locating objects within a limited area. The systems are generally based on ultrasonic and/or radio communication. U.S. Pat. No. 6,317,386 describes a system which combines ultrasonic and radio waves. The system works in such a manner that the identification tags are called up by means of radio waves, while the actual communication between transmitter units and base stations is conducted by means of ultrasound. The object of this system, which is for indoor use, is to increase the capacity of an ultrasound-based system. This is accomplished by periodically calling up each tag, which has a unique address, by means of radio waves. The identification of each tag therefore does not need to be transmitted to the base station by means of ultrasound. The area of application of this system is restricted, the system requires accurate location of all the receivers in advance and complicated signal processing, and it functions best when there is an unobstructed view between the transmitter and the receivers. 
   The present invention is also a system which combines ultrasonic and radio waves. However, unlike U.S. Pat. No. 6,317,386 the identification tags receive ultrasonic pulses, and the actual communication from the identification tags to a central unit is implemented by means of radio waves. By using such a system a wider area of application is achieved than with the system described in U.S. Pat. No. 6,317,386. 
   U.S. Pat. No. 6,121,926 also describes a system for location of identification tags. In this case the tags are attached to objects in a logistics system. When an identification tag transmits a signal with its identification, it has to be received on 3 or more base stations, where an analysis is made of the arrival time and the position is determined. In practice, the accuracy of such a system will be a few metres, thus making it difficult to determine which room an object is located in without extensive calibration. Moreover, the cost per base station with antenna for the radio is substantially greater than the cost of the ultrasonic transmitters, even though one ultrasonic transducer often has to be placed in each room. U.S. Pat. No. 612,196 essentially describes an improved signal processing method for improving the location of identification tags by distinguishing between direct waves and reflected waves. This requires processing power which is also not required in the present invention. 
   BRIEF DESCRIPTION OF THE INVENTION 
   The present invention relates to a method and a system for monitoring and position determination of identification tags which can be attached to objects. The object of the invention is to provide a flexible system that combines the use of ultrasonic waves and radio waves. The system and the method according to the invention make it possible to achieve high data transfer capacity and to measure position regardless of movement. 
   The system comprises, more closely described, electronic transmitter units or identification tags for attaching to the objects that have to be monitored. Each identification tag is equipped with a radio transmitter and ultrasonic receiver. Furthermore, each room is equipped with one or more units which transmit different encoded ultrasonic pulses. The transmission of ultrasonic pulses from these stationary units is controlled from a central processing unit which may be a server with which the system is in network connection. 
   The stationary units in the system consist of master units and slave units. A master unit comprises an ultrasonic transmitter and means for transmitting and receiving synchronisation information. The master units may receive synchronisation signals from a central processing unit, e.g. a server. Moreover, they may transmit this synchronisation information to the slave units which are stationary units comprising ultrasonic transmitters and means for receiving the synchronisation information. The connection between master units and slave units is either wireless or wire-based. For a wireless connection the use of radio waves is preferred. The connection between master units and server may also be wireless or wire-based, where the use of radio waves is preferred. 
   In the simplest embodiment there may be only one stationary master unit in each room or area, where each master unit receives control signals from the central processing unit. The master unit will then transmit an ultrasonic pulse in the room in which it is located. In such a system it will only be possible to perform a rough positioning, i.e. whether or not there is an identification tag in a room. 
   In a preferred embodiment there is one stationary master unit in each room or area that is connected with a central processing unit, e.g. a server, and at least three slave units connected with the master unit. When a user of the system wishes an update of the position of the various identification tags, this starts by the user of the system implementing an initiation from a server or terminal connected to the server. The server successively transmits a message to the master units in each room with which it is connected. In the first room, e.g. room 1, the master unit will transmit a synchronisation message to slave units with which it is connected. The master unit and the slave units will simultaneously transmit an ultrasonic pulse, each encoded in its own way, which is received by the identification tags in room 1. Each identification tag will calculate arrival times for received ultrasonic pulses. This information together with the identification tag&#39;s ID code is transmitted in the form of radio signals to the server which in turn updates its database with the position of the various identification tags and thereby the object to which they are attached. After the positions of the identification tags in room 1 have been updated, room 2 will be called up, and the same procedure will be carried out until all the rooms included in the system have been examined. 
   In a power-saving embodiment the identification tags may be equipped with a radio receiver, thus enabling them to intercept the radio waves transmitted from the server to the master unit in the room in which they are located and thereby switch on the ultrasonic receiver for a certain period after receiving the radio waves. 
   The object of the invention is achieved with a system and a method as described in the set of claims, and which will be described in greater detail below. 

   
     LIST OF DRAWINGS 
     The invention will be further described with reference to the drawings, in which: 
       FIG. 1  illustrates the construction of an identification tag for this system, 
       FIG. 2  illustrates the construction of a master unit, and 
       FIG. 3  illustrates how the whole system is combined in a network. 
   

   DETAILED DESCRIPTION 
   The system according to the invention is constructed in such a manner as to obtain high data transfer capacity and insensitivity to Doppler shift. There are several technical features with regard to transmitter, receiver and central unit that contribute to this. As a whole it represents a system that is well suited to different environments. The advantages of the invention are achieved by combining the use of radio waves with ultrasonic waves in the manner described below. 
     FIG. 1  illustrates which units may typically be incorporated in each identification tag  100 . The identification tag  100 , which is intended for use in a location system  400  ( FIG. 3 ) for determining the identification tag&#39;s  100  location in a room in a building or areas that require to be monitored, comprises an ultrasonic transducer  190  adapted to receive ultrasonic signals, together with a radio transmitter  170  connected to an antenna  195  for transmitting signals with information containing the identity of the identification tag. The ultrasonic transducer  190  is connected to a receiver unit  180  for detecting ultrasonic pulses with different codes (e.g. frequencies, scattered spectrum, etc.) transmitted from one or more master units  200  and slave units  300  ( FIG. 3 ) which transmit on the various frequencies. The identification tag  100  further comprises a calculating unit  165  connected to the receiver unit  180 , arranged to calculate transit time differences for the received ultrasonic pulses. The calculating unit  165  is further connected to a control unit  160  which coordinates and controls all the signals received by and transmitted from the identification tag  100 . The control unit  160  is arranged to cause the radio transmitter  170  to transmit the radio signals in response to the identification chip&#39;s  100  calculating unit  165  having calculated transit time differences for all received ultrasonic pulses. 
   The identification tag  100  may further consist of a sabotage sensor  110 , which is connected to the control unit  160 , in order to detect any attempt to remove and/or open the identification tag  100 , and where, after such detection, the control unit  160  is adapted to add such additional information to the radio signal transmitted from the identification tag  100 . Furthermore, the identification tag  100  may comprise a timer unit  120 , motion detector  130 , identification unit  140  and battery monitor  150 , which monitors the status of the battery  155 . 
   The identification tag  100  may also be equipped with a radio receiver  175  connected to an antenna  195 . The radio receiver is then connected to the control unit  160  in order to receive the radio signals transmitted in the room or area in which it is located, with the result that the identification tag  100  does not need to listen continuously for ultrasonic signals. In this case the identification tag  100  will listen for ultrasonic pulses for a short period after receiving radio signals. Since the ultrasonic receiver requires more power than a radio receiver, this will represent a power saving. 
   The tag or identification tag  100  may contain all or only some of the units mentioned here. 
     FIG. 2  shows which units may typically be incorporated in a master unit  200 . The master unit  200  comprises an ultrasonic transducer  265  for transmitting ultrasonic signals in the form of ultrasonic pulses and a receiver unit depicted as a radio receiver  270  for receiving instructions from at least one central processing unit  410 . In the figure a radio receiver  270  is illustrated connected to an antenna  295 . This type of connection to the central processing unit  410  may be replaced by a typical cabled connection such as, e.g. Ethernet. The signals coming into the receiver unit  270  via the antenna  295  are fed to a data demodulator  230  which in turn is connected to a network interface  215  via a controller  210 . 
   To this network may be connected corresponding units, preferably slave units  300 . The master units use the network to transmit synchronisation information to other connected units in the same room or area. The network connection may be wireless (not shown) or wire-based. 
     FIG. 3  shows an overview of the whole system  400  according to the invention. The figure illustrates the interplay between master units  200 , slave units  300 , identification tags  100  and a central unit  410  in the form of a server that processes all received data. Several client terminals  420  may also be coupled to the system in order to gain access to information from different locations. 
   A system  400  for position determination of at least one identification tag  100  comprising:
         at least one stationary master unit  200 , with an ultrasonic transducer  265  for transmitting ultrasonic signals in the form of ultrasonic pulses and a receiver unit  270  for receiving instructions from at least one central processing unit  410 ,   at least one identification tag  100  as described above for transmitting the identification tag&#39;s  100  identification as well as measured transit time differences for received ultrasonic pulses together with any additional information,   a network  215  interconnecting master units  200  with the central processing unit  410  for transfer of instructions,   means in the central processing unit  410  for calling up identification tags  100  as well as detecting, collecting and interpreting received radio signals from the identification tags  100 , and   processing means in the central processing unit  410  for determining the position of the identification tags  100 .       

   The system further comprises at least one stationary slave unit  300  with an ultrasonic transducer for transmitting ultrasonic signals in the form of ultrasonic pulses. 
   In a preferred embodiment the system comprises one master unit  200  and at least three slave units  300 . 
   The network  215  further interconnects stationary master units  200  and slave units  300  for transfer of a synchronisation message from the master unit  200 . 
   The stationary master units  200  and the slave units  300  each transmit ultrasonic waves on their own frequencies and with their own coding. 
   The master units  200  comprise means  210 ,  230  for transmitting a synchronisation message to all stationary slave units  300  with which they are connected via a network. 
   The connection between master units  200  and the central processing unit  410  may be based on radio waves or be wire-based. 
   The stationary slave units  300  comprise means for receiving a synchronisation message from stationary master unit  200  with which they are connected via a network. The network connection connecting stationary master units  200  and slave units  300  may be radio-based or wire-based. 
   The method for using the system according to the invention will now be described. In order to determine the position of one or more identification tags  100  in a room in a building or areas that require to be monitored, the method comprises: 
   a) transmitting a radio message concerning updating from a central processing unit  410  to stationary master units  200 , 
   b) transmitting a synchronisation message from the stationary master units  200  to stationary slave units  300 , 
   c) transmitting ultrasonic pulses synchronously from the stationary master units  200  and the slave units  300 , 
   d) receiving the ultrasonic pulses on the identification tags  100  described above, 
   e) calculating transit time differences for received ultrasonic pulses in the identification tag  100 , 
   f) transmitting radio signals from the identification tag  100  containing transit time differences for received ultrasonic pulses together with identification of the identification tag  100  to a central processing unit  410 , 
   g) calculating the position of the specific identification tag  100  in the central processing unit  410  on the basis of received identification and transit time differences transmitted from the identification tag  100 , together with knowledge of the position of each individual stationary master unit  200  and slave unit  300  in each room or area. 
   The synchronisation message transmitted from the master units  200  includes information concerning on which frequencies transmission should be made or which coding should be employed. 
   The stationary master units  200  and slave units  300  each transmit on their own code 1-n. The radio message from the central processing unit  410  to the stationary master units  200  is initiated by a user requesting an update of positions via a user interface on the central processing unit  410 , or by an identification tag  100  transmitting a request via radio signals to the central processing unit  410 . 
   Identification tags  100  containing a radio receiver  175  in addition to an ultrasonic receiver  180  will be able to switch on the ultrasonic receiver  180  only when master units and slave units are to transmit ultrasonic pulses in the area where they are called up by listening for radio signals transmitted from the stationary calculating unit  410  to the master units. 
   The identification tag  100  will transmit a request concerning updating of position together with additional information to the central processing unit  410  when any attempt is made to open or move it.