The invention relates to a shelf antenna configuration for an RFID system for recognizing objects which are provided with an RFID label and which are located in a shelf system with one or more shelves, and to a shelf system provided with at least one shelf antenna configuration. RFID (Radio Frequency Identification) systems have been known for many years for diverse applications. Such systems usually comprise a transmitting/receiving device, which, with the aid of an antenna system having at least one loop antenna lying in a substantially flat plane, generates a magnetic interrogation field in an interrogation zone or detection zone. Further, an RFID system comprises a number of detection labels, also called labels, for short, or detection plates, transponders, responders or tags. These labels each comprise at least one loop antenna lying in a flat plane to allow communication with the interrogation field when a label is in the interrogation field. A good coupling between the antenna system and the label antenna is of great importance. The loop antenna of the label should comprise as many field lines of the interrogation field as possible. In practice, this means that the label antenna should have a minimum size of a few centimeters and that the label antenna should be in one plane which preferably extends parallel to the loop antenna of the transmitting/receiving device or at an angle thereto that is not too large. According as the angle between the label antenna and the transmitting/receiving antenna approaches 90°, the coupling between the two antennas becomes worse. If it is desired to identify books or similarly shaped objects, such as, for instance, CDs and DVDs, that are located on a shelf, with the aid of an RFID system, the books or the like can each be provided with a detection label. In practice, due to the minimum required dimensions of the label antenna, this label can only be arranged on or to the upper cover or the lower cover, or possibly flat between the pages of the book. The label, and hence the label antenna, then lies in a plane extending transversely to the plane of the bookshelf. As a consequence, use of a transmitting/receiving antenna lying in the plane of the bookshelf or product shelf is not straightforwardly possible. In the past, in libraries this problem has been solved in practice by using a number of bookends extending transversely to a bookshelf, with transmitting/receiving antennas arranged in the bookends. Such bookends have the drawback of hindering a free arrangement of the bookshelves. Also, relatively many bookends with the associated antennas are needed, which is costly and also requires much wiring in a bookcase. From the American patent publication US 6,844,859 B2 (Noda et al.) a product shelf for CDs and videos is known, which product shelf is provided with an antenna system consisting of one or two antenna loops. With the aid of the antenna system, an interrogation field can be generated in the space above the product shelf. The antenna loops according to this publication have a special form, which has as a purpose to create an interrogation field such that a good communication with RFID labels located in a plane extending transversely to the surface of the product shelf is possible.

To this end, according to the American patent publication 6,844,859, a product shelf provided with one or two antenna loops lying in the plane of the product shelf is used, which have a first long, straight, forward conductor extending lengthwise of the product shelf and linking up with a second, backward conductor completing the antenna loop, which has a sawtooth shape. In the use of two of such antenna loops, which are alternately energized, a rotary field is formed in a spatially highly limited area. Although with this configuration at a short distance from the sawtooth-shaped conductor an interrogation field with a horizontal component can indeed be obtained, the coupling between the transmitting/receiving antenna(s) and the label antenna remains weak, and there remain dead areas, in which no communication between the antenna system and a label antenna is possible. A faultless identification is therefore not ensured. In WO 2005/081808 (3M Innovative Properties Company) an RFID antenna system is described for detecting and/or identifying various kinds of objects, such as manufactured articles, books, files, or animals or persons wearing a detection label. This known antenna system comprises two or more antenna loops located one within the other in one plane, which reinforce each other's field, reduce dead areas and enlarge the effective range of the thus obtained interrogation field. This antenna system, however, does not seem suitable for detecting labels that extend transversely to an elongated antenna plane. The same holds for an RFID antenna configuration described in

WO 2006/008705 Al (Philips), consisting of one or more closed loops formed by an antenna wire and located in one plane and/or widened regions separated by narrowed sections. The closed loops and the widened areas form specific locations for identifying objects provided with an RFID label present in those regions.

However, these known antenna configurations also seem unsuited for detection and/or identification of labels extending transversely to the antenna plane and fixed in or to upstanding books or similar objects. From French patent application 2836581 an RFID antenna configuration for a shoplifting detection system is known, which comprises a separate transmitting antenna, consisting of a single loop, and two receiving antennas lying one on top of the other and working according to a multiplex method. One receiving antenna comprises two loops and the other receiving antenna three. The receiving antennas have cross points which are shifted with respect to each other in the direction in which a detection label normally passes the antennas. Furthermore, the receiving antennas are jointly shifted with respect to the transmitting antenna. With this known antenna configuration, situations are possible where no or insufficient coupling occurs between the transmitting antenna and a detection label, and also dead zones occur. The object of the invention is to make available a shelf antenna configuration with which the above-mentioned drawbacks are obviated or at least reduced and with which both labels extending transversely to an antenna system lying in the plane of a shelf and labels extending parallel to the antenna system, or labels with a different orientation, can be detected or identified in a reliable manner.

To this end, according to the invention, a shelf antenna configuration comprises a first and a second antenna loop, which antenna loops form transmitting/receiving antennas substantially identical in shape and each lie in a substantially flat plane, which flat planes extend parallel to each other at a short distance from each other, wherein the antenna loops have a length direction and a width direction, and wherein each antenna loop has a forward conductor which forms a number of first merlons and a backward conductor which forms a number of second merlons, wherein the first and the second merlons alternate with each other and the flanks of the first and the second merlons are at such a distance from each other that between these flanks no coupling or only a slight coupling occurs, while the branches of the first and second merlons of an antenna loop that extend in the length direction are shifted with respect to each other in the width direction and wherein the antenna loops overlap each other but are also positioned shifted with respect to each other in the width direction and the length direction, wherein only a slight coupling between the antenna loops occurs and wherein the flanks of the merlons of one antenna loop lie in a central area between flanks of the other antenna loop and vice versa. In the following, the invention will be further described with reference to the appended drawings.

Fig. 1 schematically shows in front view an example of a cupboard with shelves on which objects can be placed or laid; Fig. 2 schematicaDy shows in top plan view an example of a shelf provided with an exemplary embodiment of an antenna configuration according to the invention;

Fig. 3 schematically shows a rear view of the shelf of Fig. 2; Fig. 4 schematically shows a bottom view of the shelf of Fig. 2; and

Fig. 5 shows in perspective a bottom view of a shelf according to the invention.

Fig. 1 shows in outline in front view an example of a shelf system 1 in which the invention can be applied. It is noted that in this description and in the claims, the term "shelf system" is understood to mean an assembly of several bearing shelves, storage shelves, product shelves, etc., mounted between uprights and/or to a wall, as well as a single shelf, which may be attached to a wall with, for example, brackets or the like.

The shelf system shown as an example in Fig. 1 is designed as a bookcase with two side walls 2, a top wall 3, four bookshelves 4, of which the lowermost forms the bottom of the bookcase, and legs 5.

The bookcase may, at will, be further provided with a rear wall or not.

In the bookcase 1 shown in Fig. 1, on the top shelf and the shelf under it, a few books 6, 7 are placed. The books are provided with detection labels 8, which, for the sake of clarity, in the books indicated at 6, have been drawn in somewhat thicker lines and have been arranged in or on the back cover of the books. At the top shelf 4, there is further shown a separator 9 functioning as bookend. As already indicated in the foregoing, hitherto, in practice, such bookends 9 were sometimes provided with transmitting/receiving antennas to create an interrogation field for the labels 8. The fact is that the bookend 9 lies in a plane substantially parallel to the front and back cover of an upstanding book. An antenna loop in such a bookend is hence also substantially paraDel to the labels 8, thus allowing a good communication between an antenna loop of the bookend 9 and the labels 8.

A drawback of the use of antenna loops in bookends is that relatively many bookends are then required, which occupy often scarce shelf space and which hinder free placement of the books and a free arrangement of the bookshelves.

According to the invention, the RFID labels 8 are identified with the aid of special transmitting/receiving antennas provided in the bookshelves 4, described in more detail in the following. Bookends provided with transmitting/receiving antennas are then superfluous. Moreover, with the antenna configuration according to the invention, also horizontally oriented labels, such as, for instance, a label 8 of a horizontally oriented book, can be identified. This means that with an antenna configuration according to the invention not only books and similarly shaped objects provided with RFID labels, such as CDs, DVDs, video tapes, etc., can be identified, but also other types of articles, such as, for instance, store articles, for instance jeans lying on a shelf in a pile, or medicines in boxes, can be identified if they are provided with RFID labels.

Also, other objects than objects having a more or less flat shape that are provided with an RFID label can be identified. One application is, for instance, in a cupboard for medical or other instruments.

Figs. 2, 3 and 4 schematically show an example of a configuration of transmitting/receiving antennas provided in a shelf 4 according to the invention in top plan view, rear view and bottom view, respectively. The antenna configuration shown comprises two specially formed antenna loops 10, 11, which are provided in recesses in a top surface 12, and bottom surface 13, respectively, of a core layer 14 of a shelf 4.

The two antenna loops 10, 11 each form a transmitting/receiving antenna, i.e., with each antenna loop, both an interrogation field can be generated and a detection label can be detected. The antenna loops jointly extend substantially throughout the length and width of a shelf 4. The two antenna loops 10, 11 are furthermore substantially identical in shape and overlap for the greater part, but are shifted with respect to each other both in the length direction of the shelf 4 and in the width direction thereof. The core 14 in which the antenna loops are located can be manufactured from a suitable electrically insulating material, such as a suitable plastic, or MDF, or a foam material. The core 14 is covered with a top plate 15, not shown in Figs. 2 and 4, and a bottom plate 16, see Figs. 3 and 5. In this example, the core 14 has a recess 17 near one end of the shelf on the rear side, which provides room for a connector (not shown) for connection of the antenna configuration to an RFID transmitting/receiving device schematically indicated at 18 in Fig. 2. In this example, the recess 17 is also provided in the bottom plate 16.

The antenna loops 10, 11 are both of merlon-shaped design, with the merlons of one, for instance the forward, path of an antenna loop being positioned in shifted relation with respect to the merlons of the other, for instance the backward, path of the same antenna loop.

In Figs. 2 to 4, the right-hand terminal of the first antenna loop 10 is indicated with 19 and the left-hand terminal with 20. Likewise, the right-hand terminal of the second antenna loop 11 is indicated with 21 and the left-hand terminal with 22. From the right-hand terminal 19, the conductor of antenna loop 10 connected thereto has a first horizontal (in the drawing) branch or section A extending over a distance a in the length direction of the shelf 4 at a short distance from the nearby longitudinal edge 24 of the recess 17. Then, in this example, the conductor turns off at a right angle in the direction of the front longitudinal edge 25 of the shelf located opposite the recess 17 for forming a first vertical (in the drawing) branch B having a length b.

The length b is selected such that the branch B extends to a point well over half the width of the shelf 4, in the example shown to a point at approximately three-fourths of the width of the shelf 4. After this, the conductor has a horizontal branch C extending in the length direction of the shelf again over a distance c, which in this example is slightly longer than the distance a, then to proceed with a vertical branch D over a distance b to a point p located at a short distance from the longitudinal edge 24. The branches BCD form a first merlon form, with the branches B and D forming the flanks of the merlon extending in the width direction of the shelf, which interconnect the lengthwise extending branches A and C, and C and E ₁ respectively. In the example shown, after the point p and a horizontal branch E which is located near the longitudinal edge 24 of the recess and the rear longitudinal edge 26 of the shelf and in this example has a slightly greater length than the branch A, the antenna loop 10 forms a second merlon form FGH, followed by a last horizontal branch I. The first and second merlon forms in this example are substantially identical in shape. Linking up with the horizontal branch I is a first backward merlon form JKL, of which the horizontal branch K located near the front longitudinal edge 25 of the shelf 4 is closer to the front longitudinal edge 25 than are the horizontal branches G and C. Furthermore, the downward (in Fig. 2) branch L of the first backward merlon form extends substantially parallel to and spaced to the right from the downward branch H of the second forward merlon form FGH. The path L links up at some distance before the path I with a horizontal backward branch M, which in turn links up with a second backward merlon form NOP, which is followed again by a backward horizontal branch Q and a third merlon form RST. The branch T ends at the second terminal 20 of the first antenna loop 10. The first and second backward merlon forms JKL and NOP in this example are substantially identical in shape, while the last merlon form RST is slightly wider.

The vertical branches of the forward merlon forms, in operation, always carry currents flowing in the same direction as the nearby vertical branches of the backward merlon forms, but always lie at such a distance from the nearby vertical branches of the backward merlon forms that between the mutually adjacent conductors of the different merlon forms no coupling or only a slight coupling with a slight self-induction occurs.

The first antenna loop accordingly consists of a number of merlon forms or substantially right-angled tooth forms in both the forward and the backward path of the antenna loop, with the merlon forms of the forward path and the backward path being shifted with respect to each other both in horizontal direction and in vertical direction.

The merlons of one path, as it were, fill up the interspaces between the merlons of the other path. As a result, there are formed a number of juxtaposed composite loops ABST, QDCR, EFOP, MHGN and IJKL having in operation in each case an opposite current direction and corresponding opposite magnetic fields. In clarification, in Fig. 4 the opposite current directions in the juxtaposed composite loops of the antenna loop 11 are schematically indicated with curved arrows 30 to 34. The antenna loop 11 has the same shape as the antenna loop 10 described in detail hereinabove, but is shifted with respect to the antenna loop 10 over such a distance to the left (in the drawing) that the vertical branches in each case are situated at the central area of the composite loops of the antenna loop 10. Conversely, the vertical branches of the antenna loop 10 are situated at the central area of the composite loops of the antenna loop 11.

Furthermore, the antenna loop 11 as a whole is shifted with respect to the antenna loop 10 in the direction of the longitudinal edge 25 of the shelf 4, so that also the horizontal branches of the antenna loop 10 are spaced apart from the corresponding horizontal branches of the antenna loop 11. Owing to the geometry of the layout of the two antenna loops as described and shown by way of example, the coupling between the antenna loops is slight.

Due to the shift of the two antenna loops 10, 11 with respect to each other, the dead areas (neutral points) of one antenna loop are compensated by active areas of the other antenna loop and vice versa, so that effectively no dead zones occur. Furthermore, the antenna loops 10, 11 are energized by the device 18 according to a multiplex method or with a phase difference of 90°. This last may be effected, for example, with a delay line or a splitter or through a suitable choice of the coupling between the antenna loops. As a result of the juxtaposed composite antenna loops formed in each antenna loop 10, 11 and having opposite current directions, in operation a magnetic field with horizontal and vertical components is created, whilst by the use of two antenna loops shifted with respect to each other and controlled in the manner described above, locally in time alternately horizontal and vertical field components are active. As a result, both vertically and horizontally oriented labels can be identified, as well as labels having an intermediate orientation.

Fig. 5, for the sake of completeness, shows in perspective bottom view a shelf according to the invention, where the core 14 with the antenna configuration shown in Figs. 2 to 4 is covered by a top plate 15 and a bottom plate 16. The top plate 15 also covers the recess 17, while the bottom plate 16 leaves it clear.

Furthermore, the shelf 4 may be enclosed by a metal cover edge 36 (Fig. 3), which extends along the front edge, side edges and rear edge. Such a cover edge not only protects the core 14 but also forms a short-circuit conductor limiting the interrogation field. Of course, one or two short-circuit conductors may also, if desired, be arranged in the core itself around the antenna loops.

Also, if desired, one side of a shelf may be provided with a shielding layer if the interrogation field should be active exclusively on the other side, for example, only above or only under the shelf. It is noted that where in the description of the drawing reference is made to "horizontal" and "vertical" and "left" and "right", these terms relate solely to the positions and orientations in the figures themselves. In a practical situation, the orientation and location of the various elements of the antenna loops depends on the specific use of the shelf. Furthermore, it is noted that after the foregoing, various modifications of the embodiments described will be evident to those skilled in the art. Thus, the number of merlons can be greater or smaller than in the example described. Also, the merlons could have a different shape, for example, a trapezoid shape or a shape having slightly bent branches, without departing from the principle of merlons or teeth shifted with respect to each other in two directions, per antenna loop, while a sufficient distance between the various branches of the forward and backward paths is preserved, and from the principle of antenna loops shifted with respect to each other in two directions. In the case where the merlons have a different shape, the flanks do not need to extend exclusively in the width direction (vertical direction), but the flanks can also have a horizontal component. Likewise, the parts situated between the flanks could have a vertical component, whether or not in combination with flanks that have a horizontal component.