Abstract:
A tag has first and second modes of operation, and uses substantially less battery power in the first mode. In the first mode, the tag is responsive to receipt of a first wireless signal from a remote location with a first transmission range for shifting to the second mode. In the second mode, the tag transmits a second wireless signal with a second transmission range. In one configuration, the second transmission range is greater than or equal to the first transmission range. In a different configuration, the tag periodically checks for the first wireless signal during the first mode at points in time spaced by a time interval. The second transmission range is less than the first transmission range by a difference that is greater than or equal to the time interval multiplied by a speed of movement of the tag toward the remote location.

Description:
[0001]     This application claims the priority under 35 U.S.C. §119 of U.S. provisional application No. 60/693,200 filed Jun. 23, 2005, the disclosure of which is hereby incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates in general to radio frequency identification tags and, more particularly, to techniques for conserving battery power in tags.  
       BACKGROUND  
       [0003]     A known technique for tracking items is to mount a radio frequency identification (RFID) tag on each item to be tracked. These RFID tags transmit wireless signals, and readers are provided to receive these signals. RFID tags almost always run on battery power. Consequently, it is always desirable to conserve a tag&#39;s battery power, in order to maximize the length of time from insertion of a newly-charged battery until the battery becomes too discharged to properly operate the tag. Existing techniques for conserving battery power have been generally adequate for their intended purposes, but have not been satisfactory in all respects.  
       SUMMARY OF THE INVENTION  
       [0004]     One of the broader forms of the invention relates to a tag having a battery and having first and second operational modes that are different, the tag using substantially less power from the battery in the first operational mode than in the second operational mode. This form of the invention includes: responding to receipt by the tag in the first operational mode of a first wireless signal originating from a remote location and having a first transmission range by shifting the tag from the first operational mode to the second operational mode; and transmitting from the tag in the second operational mode a second wireless signal that is different from the first wireless signal and that has a second transmission range greater than or equal to the first transmission range.  
         [0005]     Another of the broader forms of the invention relates to a tag having a battery and having first and second operational modes that are different, the tag using substantially less power from the battery in the first operational mode than in the second operational mode. This form of the invention includes: moving the tag relative to a remote location so that the tag has a speed of movement toward the remote location; periodically checking during the first operational mode at points in time spaced by a time interval for receipt by the tag of a first wireless signal originating from the remote location and having a first transmission range; responding to receipt by the tag in the first operational mode of the first wireless signal by shifting the tag from the first operational mode to the second operational mode; and transmitting from the tag in the second operational mode a second wireless signal that is different from the first wireless signal and that has a second transmission range less than the first transmission range, the difference between the first and second transmission ranges being greater than or equal to the time interval multiplied by the speed of movement.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     A better understanding of the present invention will be realized from the detailed description that follows, taken in conjunction with the accompanying drawings, in which:  
         [0007]      FIG. 1  is a diagrammatic top view of an apparatus that embodies aspects of the invention, and that includes a reader and a plurality of shipping containers with radio frequency identification tags thereon.  
         [0008]      FIG. 2  is a diagrammatic top view of a further apparatus that embodies aspects of the invention, and that is an alternative embodiment of the apparatus of  FIG. 1 .  
     
    
     DETAILED DESCRIPTION  
       [0009]      FIG. 1  is a diagrammatic top view of an apparatus that embodies aspects of the invention, and that includes a reader  16 , a plurality of members  21 - 26 , and a plurality of radio frequency identification (RFID) tags  31 - 36 . In  FIG. 1 , the members  21 - 26  are each a shipping container of a known type. The reader  16  is stationarily supported, and the shipping containers  21 - 26  are disposed at various locations around the reader  16 . Although the shipping containers  21 - 26  are capable of being transported to and from the locations shown in  FIG. 1 , for the sake of simplicity in the discussion that follows, the containers  21 - 26  are each assumed to be stationary. The environment shown in  FIG. 1  may, for example, be be a yard located adjacent to a factory. Shipping containers filled with component parts are delivered to the yard, and then sit and wait within the yard until the component parts therein are needed for use in making products in the factory.  
         [0010]     The RFID tags  31 - 36  are each a battery-operated device that is mounted on a respective one of the shipping containers  21 - 26 . Each tag has multiple operating modes, including a sleep mode and an active mode. In the sleep mode, most of the circuitry within the tag is disabled, so that the tag draws very little power from its battery. The sleep mode thus helps to maximize the operational life of the battery, or in other words the time interval from installation of a fully charged battery until that battery is too discharged to properly and reliably operate the tag. In the active mode, more of the circuitry within the tag receives operating power, and the tag is capable of transmitting a wireless tag signal that contains an identification code unique to that particular tag.  
         [0011]     The tags  31 - 36  can each transmit the wireless tag signal with a power level that provides a distance or range indicated diagrammatically in  FIG. 1  by the length of a broken-line arrow  46 . A broken-line circle  47  extends around that reader  16 , and has a radius equal to the length of the arrow  46 . Thus, when a tag is located inside the circle  47 , tag signals transmitted by the tag will be received by the reader  16 . On the other hand, when a tag is located outside the circle  47 , the reader  16  will not receive tag signals transmitted by that tag, because the reader  16  is beyond the range of those wireless signals. As evident from  FIG. 1 , two tags  31  and  36  are at locations where their wireless tag signals will not reach the reader  16 , and four tags  32 - 35  are at locations where their wireless tag signals will reach the reader  16 .  
         [0012]     The reader  16  is capable of transmitting a wireless wakeup signal to any tag  31 - 36  that is currently within the range of the wakeup signal. The International Organization for Standardization (ISO) has promulgated an international standard for active RFID that is known in the art as ISO 18000-7. According to this standard, a wakeup signal is a 30 KHz single tone waveform with a duration of at least 2.5 seconds. The tags  31 - 36  are each configured so that, when they are in the sleep mode, they periodically check for this  30  KHz wakeup signal. According to ISO 18000-7, the tags check for the wakeup signal at points in time spaced by time intervals of no more than 2.5 seconds. If a tag detects the presence of the wireless wakeup signal, the tag shifts from its sleep mode to its active mode, and then transmits its tag signal.  
         [0013]     If the wakeup signals from the reader  16  were transmitted with the same power level as the tag signals from the tags  31 - 36 , it would be easier for the tags to detect the wakeup signal than for the tags to reliably deliver a tag signal to the reader  16 . Stated differently, the wakeup signals and the tag signals have different effective ranges, even if their power levels are the same. Consequently, the discussion that follows will tend to focus more on effective ranges than on specific power levels.  
         [0014]     It would be possible for the reader  16  to transmit its wireless wakeup signals with a relatively high power level, so that each wakeup signal has a long range that is sufficient to reach all of the tags  31 - 36  shown in  FIG. 1 . However, as discussed above, the tag signals are transmitted by the tags with a power level that provides the effective range represented by the length of the arrow  46 . Thus, in  FIG. 1 , only the tag signals transmitted by the tags  32 - 35  located within the circle  47  will reach the reader  16 . If the reader  16  transmits its wakeup signal with a high power level that is sufficient to reach the tags  31  and  36 , then the tags  31  and  36  will shift from the sleep mode to the active mode, and will transmit their tag signals. However, this wastes power from the batteries in the tags  31  and  36 , because these tags are outside the circle  47 , and the reader  16  will therefore not receive any of the tag signals they transmit.  
         [0015]     Accordingly, to avoid unnecessary waste of battery power, the reader  16  transmits its wireless wakeup signal with a power level selected so that the effective range  56  of the wakeup signal is approximately equal to (but no greater than) the effective range  46  of the tag signals. Thus, the wakeup signal will wake up the tags  32 - 35  that are within the circle  47  and that are capable of delivering a wireless tag signal to the reader  16 . However, the wakeup signal will not wake up the tags  31  and  36  that are outside the circle  47  and that cannot transmit a tag signal far enough to reach the reader  16 .  
         [0016]     As an alternative, the reader  16  can be configured to transmit its wireless wakeup signal with a power level having an effective range  66  that is represented diagrammatically by the circle  67 , and that is slightly less then the range  46  of the tag signals. In that case, the wakeup signal will wake up the three tags  33 - 35  that are located within the circle  67 , but will not wake up the three tags  31 - 32  and  36  that are located outside the circle  67 . This ensures that each tag receiving the wakeup signal is located where it will be able to reliably deliver its tag signal to the reader  16 .  
         [0017]      FIG. 2  is a diagrammatic top view of an apparatus  110  that embodies aspects of the invention, and that is an alternative embodiment of the apparatus  10  of  FIG. 1 . The apparatus  110  of  FIG. 2  includes a reader  16 , and three shipping containers  21 - 23  that each support a respective RFID tag  31 - 33 . The reader  16 , containers  21 - 23  and tags  31 - 33  are equivalent to their counterparts in the embodiment of  FIG. 1 , and are therefore identified with the same reference numerals.  
         [0018]     A fence  139  has a gate  138 , and a road  137  extends through the gate  138 . The reader  16  is mounted on or adjacent the gate  138 . The containers  21 - 23  move along the road  137  in a direction generally toward the gate  138  and the reader  16 , as indicated diagrammatically by arrows  141 - 143 . For example, the containers  21 - 23  may each be supported on a respective vehicle that is not illustrated, such as a truck.  
         [0019]     The tag signals transmitted by the tags  31 - 33  have a transmission range that is indicated diagrammatically by the length of an arrow  146 . Stated differently, the reader  16  is capable of receiving a tag signal transmitted by a tag when the tag is located between the reader  16  and a line  147 . The reader  16  transmits its wireless wakeup signal with a power level that provides an effective range indicated diagrammatically by the length of a broken-line arrow  156 . Thus, a tag can receive the wakeup signal if the tag is located between the reader  16  and a line  157 .  
         [0020]     In the embodiment of  FIG. 1 , the transmission range  56  of the wireless wakeup signals was selected to be less than or equal to the transmission range  46  of the tag signals. In contrast, in the embodiment of  FIG. 2 , the transmission range  156  of the wakeup signals is selected to be greater then the transmission range  146  of the tag signals. The transmission range  156  exceeds the transmission range  146  by a distance that is indicated diagrammatically by the length of a double-headed arrow  162 . More specifically, the distance  162  is selected to be equal to the speed of movement of the containers  21 - 23 , multiplied by the time interval between the periodic checks by the tags  31 - 33  for the presence of the wakeup signal. Stated differently, the distance  162  is the distance that each of the tags  31 - 33  will travel during the time interval between two successive checks by that tag for the presence of the wakeup signal. Consequently, each of the moving tags  31 - 33  will detect one of the periodic wakeup signals at some point while the tag is traveling between the lines  147  and  157 . Each of the tags will thus be in the active mode and transmitting its tag signal by the time it reaches the line  147 .  
         [0021]     Since the tags  31 - 33  are all moving, and will eventually travel through the gate  138  and then move away from the reader  16 , there is a limited window of time available for the reader  16  to read the tag signals transmitted by each of the tags  31 - 33 . As discussed above, each tag will be active and transmitting its tag signal by the time that it reaches the line  147 . This helps to maximize the amount of time available for the reader  16  to read the tag signal transmitted by each tag. This approach also ensures that the tags do not receive the wakeup signal before they reach the line  157 . Thus, they do not leave the sleep mode too early and simply waste battery power.  
         [0022]     In the foregoing discussion of  FIG. 2 , it has been assumed that the containers  21 - 23  are all moving at approximately the same predetermined speed, for example because the road  137  has an established speed limit to which most vehicles adhere. In a variation of the embodiment of  FIG. 2 , a speed detector of a known type can be provided to detect the speed of each container before the container reaches the line  157 , as indicated diagrammatically with broken lines at  171  in  FIG. 2 . The output of the speed detector  171  is supplied to the reader  16 , so that the reader  16  knows the exact speed of each container as the container approaches the line  157 . The reader can then dynamically change the power level of its wireless wakeup signal to adjust the effective range  156  of the wake up signal (and the position of the line  157 ), in a manner that is customized for each container. Stated differently, the distance  162  is customized for each container, so that the distance  162  from the line  147  is equal to the actual speed of that particular container multiplied by the time interval between successive checks by each tag for the presence of the wakeup signal.  
         [0023]     The foregoing discussion of  FIG. 2  also assumes that the tags  31 - 33  each check for the wakeup signal at points in time spaced by time intervals of no more than 2.5 seconds, for example as specified by the industry standard set forth in ISO 18000-7. However, in situations where compliance with ISO 18000-7 is not essential, it would be possible to dynamically vary the time interval between checks by the tags for the wakeup signal. As one example, the tags  31 - 33  could be programmed to use one time interval (such as 1 second) during the day, and a different time interval (such as 2 seconds) at night. The reader would then use the current time of day to dynamically adjust the power level of its wakeup signal, and thus the effective range  156  of the wakeup signals, so that the distance  162  is consistent with the time interval currently being used by the tags  31 - 33 .  
         [0024]     As discussed above, the members  21 - 26  in the disclosed embodiments are shipping containers. However, the members  21 - 26  could alternatively be any of a variety of other types of members, such as shipping pallets, or individual items that are being tracked. In the embodiment of  FIG. 2 , the containers  21 - 23  are supported on not-illustrated vehicles that are moving along a road  137 . However, the approach described in association with  FIG. 2  can be applied to other scenarios. For example, a moving conveyor could be provided instead of the road  137 , and the containers  21 - 23  could be supported on the conveyor. The conveyor would move the containers past the stationary reader  16 .  
         [0025]     Although selected embodiments have been illustrated and described in detail, it should be understood that a variety of substitutions and alterations are possible without departing from the spirit and scope of the present invention, as defined by the following claims.