Patent Application
20060092040
Radio frequency identification (RFID) tags have become commonplace for various types of use, such as inventory control, toll road collection, controlled access badges, etc. In general, an RFID tag is an electronic device that uses radio frequency wireless communication to transfer information (typically, but not exclusively, a unique ID) to an interrogator. Typically (but not necessarily) the tag is powered from the received energy of a received radio signal, and it uses that received energy to power itself and transmit a sequence that identifies the tag. RFID readers are devices that transmit the energizing signal and receive the identification sequences from RFID tags within range. Further processing may be performed once the identification number(s) is identified in this manner, either by the reader or by another device in communication with the reader. Although the technology has improved in various ways, in many instances the radio exchange is still generally a simple binary operation: either an identification number is received by the reader or it is not. In a conventional system, this binary operation only provides information that the tagged item is within range of the RFID reader, but provides no information about possible movement of the tagged item within that range.
The invention may be understood by referring to the following description and accompanying drawings that are used to illustrate embodiments of the invention. In the drawings:
In the following description, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known circuits, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.
References to “one embodiment”, “an embodiment”, “example embodiment”, “various embodiments”, etc., indicate that the embodiment(s) of the invention so described may include a particular feature, structure, or characteristic, but not every embodiment necessarily includes the particular feature, structure, or characteristic. Further, the different embodiments described may have some, all, or none of the features described for other embodiments.
In the following description and claims, the terms “coupled” and “connected,” along with their derivatives, may be used. It should be understood that these terms are not intended as synonyms for each other. Rather, in particular embodiments, “connected” may be used to indicate that two or more elements are in direct physical or electrical contact with each other. “Coupled” may mean that two or more elements co-operate or interact with each other, but they may or may not be in direct physical or electrical contact.
The term “processor” may refer to any device or portion of a device that processes electronic data from registers and/or memory to transform that electronic data into other electronic data that may be stored in registers and/or memory. A “computing platform” may comprise one or more processors.
In the context of this document, the term “wireless” and its derivatives may be used to describe circuits, devices, systems, methods, techniques, communications channels, etc., that may communicate data through the use of modulated electromagnetic radiation through a non-solid medium. The term does not imply that the associated devices do not contain any wires, although in some embodiments they might not.
As used herein, unless otherwise specified the use of the ordinal adjectives “first”, “second”, “third”, etc., to describe a common object, merely indicate that different instances of like objects are being referred to, and are not intended to imply that the objects so described must be in a given sequence, either temporally, spatially, in ranking, or in any other manner.
The invention may be implemented in one or a combination of hardware, firmware, and software. The invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by a computing platform to perform the operations described herein. A machine-readable medium may include any mechanism for storing, transmitting, or receiving information in a form readable by a machine (e.g., a computer). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other form of propagated signals (e.g., carrier waves, infrared signals, digital signals, the interfaces that transmit and/or receive those signals, etc.), and others.
Various embodiments of the invention may use changes in the nature of the response received from an RFID-tagged object to derive parameters that indicate probable movement of the object. In some embodiments, multiple RFID tags and/or multiple RFID readers may be used in conjunction with one another to further refine the probability of movement and/or to indicate the probability of a particular type of movement.
For simplicity in the drawings, the illustrated signals in
In a related manner, rotating the RFID tag may change the orientation of its antenna, which may change the perceived strength of the signal received from the direction of the RFID reader. For example, if the antenna is initially oriented such that obtains maximum reception from the direction of the RFID reader, and then rotates 90 degrees so that it obtains much weaker reception, the energy received by the RFID tag may be significantly reduced. The strength of the signal transmitted by the RFID tag may be similarly directional, so that after rotation it no longer sends its maximum signal in the direction of the RFID reader.
Although either or both of the RFID tag and RFID reader may be moved, their relative distance and orientation from each other may be the primary factors in signal strength, and this orientation is described herein with respect to movement of the RFID tag only. Further, only a two dimensional orientation between the RFID reader and the RFID tag are described herein, although three dimensional motion may be obtained. It should be obvious to a person of ordinary skill in the art to extend the principles described herein to three dimensions and to movement by either or both of the tag and the reader.
In some embodiments the strength of the signal received by the RFID tag 120 is not directly measurable, although it may affect the strength of the signal transmitted from the RFID tag 120. In some RFID systems, an RFID tag responds to any receipt of the proper signal (e.g., a carrier wave of the correct frequency), provided the received energy is sufficient to power the circuitry of the RFID tag. The strength of that response may or may not be strong enough to be detected by the RFID reader. As a result, the reader may perceive only a binary result: either it receives a response identifying the RFID tag or it does not. Other than proximity and orientation, many external factors may affect whether a response is received. Such factors may include, but are not limited to: reflections of signals off nearby objects, signals passing through objects between the transmitter and receiver, interference caused by other signals, electrostatic disturbances, etc. Because of such factors, some signals transmitted from an RFID reader may not result in a response from a particular RFID tag, and some of the responses from an RFID tag may not be detected by the RFID reader, even in the absence of movement by the reader and the tag. To overcome this problem, the reader may transmit a signal for an extended period of time (or a series of transmitted signals over the period of time), monitor the number of responses received, compare that number to a reference number (such as a theoretical maximum number of responses that might be obtained) to obtain a value that is a statistical indicator of the relevant signal strengths. If this process is repeated over a sufficiently large period of time, so that a sufficiently large number of indicators are determined over that period of time, a change in this indicator may indicate that the RFID tag has moved relative to the RFID reader and/or that external influences that affect signal strength have changed.
The graph of
In another example, the four ranges A-D may represent different orientations of the antenna of the RFID tag, with an antenna substantially facing the RFID reader producing the range of response rates shown for A, while turning the antenna progressively away from the RFID reader would produce the ranges shown for B, C, and D, respectively. As can be seen, a response rate of 1.0 is a theoretical maximum and no further improvement in signal strength via closer distance or improved antenna orientation may be detectable through this technique. Similarly, a response rate of 0.0 is a theoretical minimum, and no further reduction in signal strength via greater distance or degraded antenna angle may be detectable through this technique.
Using the described techniques, all response rates may be expected to fall between 0.0 and 1.0, inclusive. The period of time that is used to determine a single value for response rate may represent a tradeoff between various factors—if the time period it too short, the number obtained may not be statistically accurate, but if the time period is too long, the system may not be able to detect movement quickly enough. Similarly, the time period used to determine the trend of the response rates may also be a tradeoff, for similar reasons. Different applications may require different periods of time to achieve the desired results.
For example, an increase in the response rate for a single RFID tag might indicate either that the attached object is moving closer, or that the object is rotating such that the antenna angle is improved, but it may be difficult to determine which. In the illustrated example of
In some embodiments, RFID readers 411, 412 may be at approximate right angles to one another with respect to RFID tag 420, although other embodiments may not be so limited. In the illustrated example, RFID reader 411 and RFID reader 412 may pass information to processor 430 for combined analysis, although the various embodiments of the invention are not limited in this manner. The position of processor 430 may take various forms. For example, processor 430 may be located with RFID reader 411, with RFID reader 412, or may be external to both RFID readers 411 and 412. The connection between each RFID reader and processor 430 may take any feasible form, such as direct connection, shared bus, wired and/or wireless telecommunications, a combination of techniques, etc. In some embodiments each RFID reader may derive its own response rates and pass those response rates to processor 430, but other embodiments may use other techniques (e.g., each RFID reader may pass the detected tag identifications to processor 430, which determines response rates and compares those ratios for both RFID readers.
In an example of the type of coordination that various embodiments might use, if RFID reader 411 detects an increasing response rate while RFID reader 412 detects no change in response rate, it may be inferred that RFID tag 420 is moving laterally towards RFID reader 411, but moving at right angles to RFID reader 412. Rotating RFID tag 420 might increase the response rate seen by one reader while decreasing the response rate seen by the other reader (as the antenna turns toward one reader but away from the other reader). However, various directions of lateral motion might give the same results. To resolve such ambiguities, additional RFID readers may be used. In one embodiment, three RFID readers may be used, located in orthogonal directions from RFID tag 420 such that the directional vectors between the RFID tag and the three RFID readers correspond approximately to x, y, and z axes at mutual right angles. Additional readers may also be used to further reduce ambiguities.
Because the signals from the various RFID readers might sometimes interfere with one another to produce confusing results, various techniques may be used to reduce such interference. Such techniques may comprise one or more of the following, but may not be limited to these:
1) The RFID readers may coordinate their transmissions so that only one reader is transmitting at any given time.
2) Each response from an RFID tag may be received and counted by more than one RFID reader, regardless of which RFID reader the RFID tag is responding to. As long as responses to one reader are not mingled with responses to another reader, the resulting response rates should remain meaningful. In some operations, this technique may be preferable. For example, if the location and/or antenna configuration of RFID tag 420 is such that its responses to RFID reader 411, as received by RFID reader 411, are saturated at 1.0, and its responses to RFID reader 412, as received by RFID reader 412, are at 0.0, these rates may change little or not at all when RFID tag 420 moves. However, if the response rates to RFID reader 411, as received by RFID reader 412, and the responses to RFID reader 412, as received by RFID reader 411, are both within the more useful range between 0.2 and 0.8, then changes in the response rates in either direction could be detected.
At 540 the multiple values for response rate may be compared to one another, and/or to some other reference value, to detect changes in those values, with a sufficient change in the values providing an indication that the RFID tag has moved. Statistical treatments may be used in this comparison process to improve the probability that the observed changes actually represent movement rather than other external influences such as random noise, interference, reflections, movement of other external objects, etc.
While 510 through 540 may represent a process involving responses received from a single RFID tag using a single RFID reader, the results may be improved by using multiple RFID tags and/or multiple RFID readers to get multiple sets of response rates, and processing those multiple sets at 550 to get improved results as compared with the results obtained from a single RFID tag and RFID reader. For example: 1) a single RFID reader may receive responses from multiple RFID tags at different places on the same object to derive multiple sets of response rates, 2) multiple RFID readers at different locations may receive responses from a single RFID tag to derive multiple sets or response rates, or 3) multiple RFID readers at different locations may receive responses from multiple RFID tags at different places on the same object to derive multiple sets of response rates. For each reader/tag combination, a separate indication of motion may be determined based on the differences in the associated response rates, and the separate indications of motion may then be processed to determine a combined indication of motion As before, statistical treatments may be used to improve the probability that the observed responses represent actual movement of the object rather than that lateral movement may be distinguished from rotation.
The foregoing description is intended to be illustrative and not limiting. Variations will occur to those of skill in the art. Those variations are intended to be included in the various embodiments of the invention, which are limited only by the spirit and scope of the appended claims.
