Radio frequency identification device for increasing tag activation distance and method thereof

ABSTRACT

A radio frequency identification (“RFID”) device for increasing tag activation distance comprises a first exciter electrode (), a second exciter electrode (), a dielectric substrate (), and a first exciter voltage source (). The second exciter electrode is positioned behind the first exciter electrode. The dielectric substrate is disposed between the first and second exciter electrodes. The dielectric substrate isolates the first exciter electrode from the second exciter electrode. The first exciter voltage source is coupled to at least one of the first exciter electrode and the second exciter electrode.

INFORMATION

DETAILED DESCRIPTION OF THE INVENTION

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Radio frequency identification (“RFID”) systems based on electric field coupling are affected by the ubiquitous parasitic capacitances inherent in the presence of conducting, semi-conducting and non-conducting materials present in the environment and required to build practical devices. The effects of these considerations are ameliorated by the present invention disclosed below. The following embodiments are described as being applied to monopole electric field RFID devices, although the concepts can be applied to dipole electric field RFID devices as well.

Referring now to the first preferred embodiment of the present invention, FIG. 6 illustrates the addition of the second exciter electrode to the electric field RFID device of FIG. . Second exciter electrode is separated from and positioned behind first exciter electrode . The separation between first exciter electrode and second exciter electrode is formed by dielectric substrate that is disposed between them, and can be a material such as but not limited to a glass-epoxy printed circuit board. First exciter electrode and second exciter electrode are connected together by a conductor so that they are each driven by exciter voltage source , and are therefore at the same alternating current (“AC”) potential and phase. The resulting charge distribution across the surface of second exciter electrode that is adjacent to displacement current control surface is represented in FIG. 6 by the + and −charge symbols on each surface respectively. The magnitude of charge density is represented by the number of + and −charge symbols on each surface, and is similar to that illustrated between exciter electrode and displacement current control surface in FIG. .

Although first exciter electrode and second exciter electrode in FIG. 6 are in two spatial locations, they may be driven by the same potential, so that the differential voltage between them, V, is zero. Because V is zero, the charge density between the adjacent surfaces of first exciter electrode and second exciter electrode is also zero. Other than electric fringe fields that exist at the narrow outer edges of exciter electrode , essentially all of the internal displacement current flows from the exciter electrode rather than exciter electrode . As a result, second exciter electrode buffers first exciter electrode from the loading effects of displacement current control surface . Because of this buffering action and that the first exciter electrode is located in front of the second exciter electrode (i.e., the first exciter electrode being positioned farther away from displacement current control surface than the second exciter electrode ), a higher charge density can be developed on the outer surface of first exciter electrode . This is represented by the increased number of +charge symbols on the outer surface of first exciter electrode in FIG. 6 than are present on the outer surface of exciter electrode in FIG. . The increase of available charge on the outer surface of first exciter electrode in FIG. 6 enables an increased level of induced charge on tag . The higher charge density is represented by a larger number of −charge symbols on tag electrode than are present in FIG. . The increased charge as a result of the present invention increases the electric field strength in front of the reader, thereby increasing tag activation distance.

The mechanical packaging of the first preferred embodiment of the invention in the RFID device is more clearly illustrated in FIG. . Enclosure contains all functional elements of electric field RFID device , and is entirely or partially composed of a dielectric material. The enclosure is attached to mounting surface , which may or may not be at external ground potential. The RFID device electronics printed circuit board (“PCB”) is comprised of substrate , electronic circuitry , conductors , and displacement current control surface . Electronic circuitry occupies the region within the dotted line on the back of RFID device electronics PCB , and includes exciter voltage source . Exciter voltage source return node is electrically coupled to displacement current control surface through the conductor , which may be implemented as a wire, a conductive via on RFID device electronics PCB , or any other suitable means. Exciter voltage source return node is usually electrically connected to electric field RFID device circuit common (not shown) and external system ground , all of which terminate on RFID device electronics PCB .

An antenna PCB comprises a substrate , a first exciter electrode , and a second exciter electrode . First exciter electrode and second exciter electrode are shown as two electrically conductive layers that are disposed on substrate . First exciter electrode and second exciter electrode are electrically coupled by conductor , which is illustrated as a PCB electrically conductive via. Conductor can be implemented as a wire or any other suitable means. Second exciter electrode is electrically coupled to exciter voltage source by the conductor , which can be a wire, connector, or any other suitable means. Antenna PCB is positioned in front of RFID device electronics PCB such that first exciter electrode is closest to the front of enclosure , to which RFID tags (not shown) are presented. The mechanical means for spacing antenna PCB from RFID device electronics PCB is not shown, but may be implemented through the use of a wide variety of materials and fasteners. It is implied in FIG. 7 that antenna PCB is not electrically coupled through mechanical spacers to RFID device electronics PCB , although this can be done if desired. For example, conductor may be implemented as a metal spacer or standoff between antenna PCB and RFID device electronics PCB . In general, however, minimizing electrical conductors in the vicinity of the exciter electrodes is preferred in order to maximize tag activation distance.

FIG. 7 illustrates, for example, that the present invention can be implemented on existing materials and assemblies of an electric field RFID device with minimal impact. The addition of second exciter electrode to antenna PCB as a conductor layer does not increase the overall assembly thickness. Adding the conductor layer that forms second exciter electrode is a commonly available technique in printed circuit manufacturing processes, and increases cost a negligible amount. Therefore, the use of the present invention can be implemented at essentially no increase in cost or size, and does not require exciter circuit redesign. Although it might be considered that the exciter electrode area has been doubled, this is not the case because the two exciter electrodes are not coplanar, but are stacked instead. Because second exciter electrode “hides” behind first exciter electrode , there is no increase in electric field RFID device package area.

The implementation of the present invention illustrated in FIG. and described above has produced improvements in tag activation distance ranging from 5% to 25%, depending upon the size of the exciter electrodes and and displacement current control surface , the spacing between first exciter electrode and second exciter electrode , and the distance between second exciter electrode and displacement current control surface . Computer simulations have shown that implementing the present invention in a manner that increases electric field RFID device package thickness can yield further improvements in tag activation distance. Although increasing product thickness may be undesirable, the incremental improvement in tag activation distance is several hundred percent greater than the increase in electric field RFID device package thickness.

A second embodiment of the present invention is illustrated in FIG. 8, which is similar to FIG. 7 except that second exciter electrode is not electrically coupled to exciter voltage source . Instead, the second exciter electrode is floating (i.e., the second exciter electrode is not electrically coupled through a conductor to any electrical potential within the reader). First exciter electrode is, however, still electrically coupled to exciter voltage source . While this second embodiment does not perform as well as the first embodiment of FIG. 7, the presence of second exciter electrode still produces an increase in tag activation distance. Second exciter electrode creates an electrically conductive surface that causes the potential across its entire surface to be constant, that potential being between that of exciter voltage source and that of displacement current control surface . The second exciter electrode acts to somewhat buffer or isolate first exciter electrode from the adverse loading effects of displacement current control surface . This embodiment can provide an improvement in tag activation distance if, for example, it is undesirable to electrically couple the exciter voltage source to both exciter electrodes due to packaging or manufacturing restrictions. In an alternative second embodiment, first exciter electrode may be left electrically floating, while second exciter electrode is electrically coupled to exciter voltage source . This can also yield an improvement in tag activation distance when compared to a single exciter electrode, but in general is not as productive as the first preferred embodiment in which both exciter electrodes are electrically coupled to exciter voltage source .

An advantage of the alternative second embodiment occurs if only one of the exciter electrodes is electrically terminated. It may be more convenient to terminate second exciter electrode than to terminate first exciter electrode . If desired, exciter electrode could be terminated through a voltage divider network (not shown) such that it develops a potential between that of exciter voltage source and displacement current control surface .

FIG. 9 illustrates a third embodiment of the invention, in which a third exciter electrode is added to the electric field RFID device of FIG. . First exciter electrode , second exciter electrode and third exciter electrode are all electrically coupled to exciter voltage source . This configuration can be used to further enhance the tag activation distance. An alternative third embodiment of the invention is to use any quantity greater than one of non-coplanar exciter electrodes that may be desired to further improve tag activation distance. As described in the second embodiment of the invention, one or more additional exciter electrodes may be electrically floated if desired.

An example of a fourth embodiment of the invention is illustrated in FIG. 10, in which multiple exciter electrodes described previously are replaced with the thick single exciter electrode , which has a thickness equal to the total distance spanned from the outermost surface of exciter electrode to the innermost surface of exciter electrode in FIG. . Thick single exciter electrode could be a continuously electrically conductive sheet, forming a conductive volume. FIG. 10-indicates the location of displacement current control surface , as defined previously. Other than displacement current control surface , the remainder of the electric field RFID device electronic circuitry is represented by the functional electronics , which among other elements, includes exciter voltage source . The front area of thick single exciter electrode is unchanged from its multilayer exciter electrode counterpart. The conductive edge surfaces of thick single exciter electrode are not directly exposed to tag as is its front surface. However, because thick single exciter electrode has continuous conducting surfaces along its edges, termination of the electric field to displacement current control surface is not as extensive as in the multilayer exciter electrode examples. More electric charge is available on the edge areas of thick single exciter electrode , and the electric field is able to fringe outward, providing more charge to become available to tag .

Variations of this fourth embodiment exist, such as using the five-sided conducting shell instead of a six-sided box, as shown in FIG. 10. Alternately, as shown in FIG. 10, a combination of thick exciter electrode and a thin (or thick) second exciter electrode could be used as described in the preferred first embodiment. An electrically conductive exciter electrode could also be formed on the front-inside part of enclosure using various processes and materials such as spray coating, sputtering deposition, flexible films, inks, etc., as illustrated in FIG. 10. The first exciter electrode is formed by the conductive layer disposed on the inside of enclosure , and is electrically coupled by the connection to second exciter electrode , and then electrically coupled to exciter voltage source (not shown). Alternatively, first exciter electrode can be capacitively coupled (not shown) to second exciter electrode to fulfill the intent of the present invention in a cost effective manner.

An approximation of the thick single exciter electrode could be implemented on a printed circuit board with two or more parallel exciter electrodes, as illustrated in FIG. . The thick exciter electrode is comprised of PCB substrate , first exciter electrode , second exciter electrode , and the plurality of the conductive vias . Conductive vias electrically couple first exciter electrode to second exciter electrode , and are evenly distributed along all edges of the PCB, to approximate a conductive surface along its edges. If desired, additional conductive vias may be distributed internally, away from the edges of thick exciter electrode . It can be appreciated to one skilled in the art that many variations can be employed to produce or simulate a thick exciter electrode.

A fifth embodiment of the invention is schematically represented in FIG. 12, and is characterized by non-identical geometries for first exciter electrode and second exciter electrode . FIG. 12 indicates the location of displacement current control surface , as defined previously. Other than exciter electrode , exciter electrode , connection and displacement current control surface , the remainder of the electronic circuitry contained in electric field RFID device is represented by functional electronics , which among other elements, includes exciter voltage source . While only two exciter electrodes are illustrated in FIG. 12, the essence of this embodiment is not limited to only two exciter electrodes. A variety of different sizes and shapes for multiple exciter electrodes can be implemented, in varying combinations of electrical terminations to exciter voltage source (not shown) and remain within the spirit of the present invention. The geometry and size of different exciter electrodes can be utilized, for example to accommodate special packaging features or restrictions. Or, non-identical geometries of exciter electrodes may be used for the purpose of controlling the shape of the tag activation volume. This may be desired, for example, if a narrow tag activation volume is required for focused reading of tags. The use of multiple exciter electrodes having differing geometries can be used to extend the tag activation distance only for localized areas, such as might be located about the central axis of an electric field RFID device. An example of this is illustrated in FIG. 13, in which first exciter electrode , second exciter electrode and third exciter electrode all differ in size. They are all electrically coupled to exciter voltage source by connections .

A sixth embodiment of the invention utilizes multiple exciter voltage sources to drive multiple exciter electrodes. The simplest form of this is illustrated in FIG. 14 in which first exciter electrode is driven by first exciter voltage source , and second exciter electrode is driven by second exciter voltage source . Both exciter voltage sources and produce AC voltages that are of the same frequency and phase. Multiple exciter voltage sources may be utilized to allow the physical distribution excitation voltage to optimize circuit design and packaging constraints. This configuration can be used to control the potential applied to first exciter electrode independently of second exciter electrode . For example, first exciter voltage source may be varied in response to the magnitude of the received tag signal or external loading, so the radiated power can be dynamically controlled. By keeping the drive on second exciter electrode constant, the loading effects of displacement current control surface may be managed independently. By combining different exciter electrode geometries with different exciter voltage sources, greater control over the tag activation volume can be achieved.

A seventh embodiment of the invention is similar to the sixth embodiment, except that the multiple exciter voltage sources can differ from one another in amplitude or phase or both in order to further alter the shape of the effective tag activation volume. This can be used to focus or spread the field, or to limit radiated emissions at distances greater than the read area. Also, the multiple exciter voltage sources can vary from one another in frequency if desired and remain within the spirit of the present invention.

An eighth embodiment of the invention is similar to the sixth and seventh embodiments, except that the magnitude and phase of multiple exciter voltage sources are dynamically varied to alter the effective tag activation volume, position, or both. This could be used, for example, to scan an area for tags that occupy different physical locations within the environment of the electric field RFID device. For example, the voltage applied to exciter electrodes used to buffer other exciter electrodes from the adverse effects of nearby electrically conductive surfaces could be dynamically reduced to cause tag activation distance in those specific location(s) to also be reduced.

While the invention has been described in conjunction with specific embodiments thereof, additional advantages and modifications will readily occur to those skilled in the art. The invention, in its broader aspects, is therefore not limited to the specific details, representative apparatus, and illustrative examples shown and described. Various alterations, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. For example, it can be appreciated by those of ordinary skill in the art that one or more embodiments of this invention can be combined with other exciter electrode structures, such as multiple co-planer electrodes to obtain an even greater degree of spatial tag activation volume, shape and position control. The present invention could be fully integrated into a single multilayer PCB, for example, combining multiple exciter electrodes, displacement current control surface and electronic circuitry into one assembly to realize low cost compact electric field RFID devices. Thus, it should be understood that the present invention is not limited by the foregoing description, but embraces all such alterations, modifications and variations in accordance with the spirit and scope of the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention are now described, by way of example only with reference to the accompanying drawings in which:

FIG. 1 illustrates a simplified side pictorial/schematic view of the excitation portion of a system containing an electric field RFID device and tag;

FIG. 2 illustrates the introduction of conductive materials near the system of FIG. 1;

FIG. 3 illustrates the net effect upon tag activation distance resulting from the presence of nearby conductive surfaces;

FIG. 4 illustrates an electric field RFID system having a compact electric field RFID device;

FIG. 5 illustrates the introduction of a displacement current control surface into the system of FIG. 4;

FIG. 6 illustrates the introduction of a second exciter electrode to the RFID device of FIG. 5 that is electrically coupled to an exciter voltage source in accordance with a first embodiment of the present invention;

FIG. 7 illustrates the mechanical packaging of FIG. 6 in accordance with the first embodiment of the present invention;

FIG. 8 illustrates the introduction of a second exciter electrode to the RFID device of FIG. 5 that is not electrically coupled to an exciter voltage source in accordance with a second embodiment of the present invention;

FIG. 9 illustrates the introduction of a third exciter electrode to the RFID device of FIG. 6 in accordance with a third embodiment of the present invention;

FIG. 10 illustrates the introduction of a thick single electrode to the RFID device in accordance with a fourth embodiment of the present invention;

FIG. 11 illustrates a printed circuit implementation that approximates the single thick exciter electrode of FIG. 10 in accordance with the fourth embodiment of the present invention;

FIG. 12 illustrates non-identical geometries for the first and second exciter electrodes in accordance with a fifth embodiment of the present invention;

FIG. 13 illustrates non-identical geometries for the first and second and third exciter electrodes in accordance with a fifth embodiment of the invention; and

FIG. 14 illustrates utilizing multiple exciter voltage sources to drive multiple exciter electrodes in accordance with a sixth embodiment of the present invention.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is related to U.S. Pat. No. 6.611,199, filed Apr. 16, 1998, by Geiszler et al., titled “Remotely Powered Electronic Tag with Plural Electrostatic Antennas and Associated Exciter/Reader and Related Method” which is commonly assigned to Motorola, Inc. the disclosure of which is hereby incorporated by reference, verbatim and with the same effect as though it were fully and completely set forth herein.

Additionally, this application is related to U.S. Pat. No. 6,411,213, filed Feb. 27, 1998, by Vega et al., titled Radio Frequency Identification Tag System Using Tags Arranged for Coupling to Ground” which is commonly assigned to Motorola, Inc. the disclosure of which is hereby incorporated by reference, verbatim and with the same effect as though it were fully and completely set forth herein.

Additionally, this application related to U.S. Pat. No. 6,229,442, filed Mar. 14, 2000, by Rolin et al., titled “Radio Frequency Identification Device Having Displacement Current Control and Method Thereof” which is commonly assigned to Motorola, Inc. the disclosure of which is hereby incorporated by reference, verbatim and with the same effect as though it were fully and completely set forth herein.

CLAIMS

1. A radio frequency identification (“RFID”) reader for increasing tag activation distance, the RFID reader comprising: at least a first exciter electrode and a second exciter electrode, the second exciter electrode positioned behind the first exciter electrode; a dielectric substrate disposed between the first and second exciter electrodes, and isolating the first exciter electrode from the second exciter electrode; and a first exciter voltage source coupled to at least one of the first exciter electrode and the second exciter electrode for generating an electric field that radiates in a front portion of the RFID reader.

2. The RFID reader of claim 1 wherein the first exciter voltage source is coupled to the first exciter electrode, and further comprising a second exciter voltage source coupled to the second exciter electrode.

3. The RFID reader of claim 2 wherein the first exciter voltage source produces alternating currents in phase with the second exciter voltage source.

4. The RFID reader of claim 2 wherein the first exciter voltage source produces alternating currents out of phase with the second exciter voltage source.

5. The RFID reader of claim 2 wherein the first exciter voltage source and the second exciter voltage sources produce alternating currents at the same frequencies.

6. The RFID reader of claim 2 wherein the first exciter voltage source and the second exciter voltage source produce alternating currents at different frequencies.

7. The RFID reader of claim 2 wherein the first exciter voltage source and the second exciter voltage source produce alternating currents at different voltages.

8. The RFID reader of claim 2 wherein the first exciter voltage source and the second exciter voltage source produce alternating currents at the same voltages.

9. The RFID reader of claim 1 further comprising a conductor connecting the first exciter electrode to the second exciter electrode.

10. The RFID reader of claim 1 wherein the dielectric substrate is a printed circuit board.

11. The RFID reader of claim 1 wherein the first exciter electrode and the second exciter electrode have non-identical geometries.

12. The RFID reader of claim 1 wherein the first exciter electrode and the second exciter electrode have identical geometries.

13. The RFID reader of claim 1 wherein the first exciter electrode and the second exciter electrode are not coplanar.

14. The RFID reader of claim 1 wherein the first exciter electrode is electrically coupled to the first exciter voltage source, and the second exciter electrode is not electrically coupled through a conductor to any electrical potential within the RFID device.

15. The RFID reader of claim 1 wherein the first exciter electrode is electrically coupled to the first exciter voltage source, and the second exciter electrode is electrically coupled to a voltage divider.

16. The RFID reader of claim 1 further comprising electronic circuitry coupled to the first exciter electrode and the second exciter electrode.

17. A radio frequency identification (“RFID”) reader for increasing tag activation distance, the RFID device comprising: at least a first exciter electrode and a second exciter electrode, the first and second exciter electrodes electrically coupled continuously along their edges to form a conductive volume; and a first exciter voltage source coupled to the first and second exciter electrodes for generating an electric field that radiates in a front portion of the RFID reader.

18. The RFID reader of claim 17 wherein the electrical connection and the first and second exciter electrodes are formed in a continuous electrically conductive sheet without any intervening dielectric material.

19. A method for increasing tag activation distance in a radio frequency identification (“RFID”) reader, the method comprising: providing at least a first exciter electrode and a second exciter electrode; positioning the second exciter electrode behind the first exciter electrode; disposing a dielectric substrate between the first and second exciter electrodes; and electrically coupling a first exciter voltage source to at least one of the first exciter electrode and the second exciter electrode for generating an electric field that radiates in a front portion of the RFID reader.

20. The method of claim 19 wherein the step of disposing comprises isolating the first exciter electrode from the second exciter electrode.

21. The method of claim 19 further comprising electrically coupling the first exciter electrode to the second exciter electrode.

22. The method of claim 19 further comprising dynamically varying at least one of a magnitude and phase of the first exciter voltage source.

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