Electromagnetic coupler arrangement

Abstract

The present disclosure relates to a wireless electromagnetic coupler arrangement with a sequential array of coupling elements. The arrangement is self-adaptive in that it automatically selects a single one out of a plurality of coupling elements forming the array for being used as an active coupling element in coupling to planar metallic traces (in particular: RFID inlays) of arbitrary geometric shape without the need for a specific calibration or location procedure. This is achieved by means of feeding an unmodulated wave sequentially to each of the coupling elements, one after the other, until an element providing sufficient interaction suitable for performing signal coupling has been reached, in an initial sensing state. The existence of efficient interaction is detected by means of a threshold comparison of the DC output wave of a harvester associated with each coupling element and feeding back the result to a bypass switch. By restricting the activation to a single coupling element of the array only, the occurrence of destructive interference between plural coupling elements is avoided.

Description

[0001] ELECTROMAGNETIC COUPLER ARRANGEMENT

[0002] FIELD

[0003] The present disclosure broadly relates to electromagnetic coupling technology. More specifically, a non-limiting embodiment relates to an electromagnetic coupler arrangement suitable for use in a printer for coding RFID inlays or other near field encoding applications.

[0004] BACKGROUND

[0005] Radio frequency identification (RFID) is a technology that uses radio waves to transfer data from electronic tags (known as RFID tags or RFID inlays). The information is stored electronically in the tag. For reading out the information, an RFID reader transmits an encoded radio signal to interrogate the tag. The RFID tag therefore includes a planar metallic trace, more specifically, a current loop suitable for inductive coupling. The same planar metallic trace (current loop) can also be used for encoding the RFID tag (writing information into the tag) by means of electromagnetic coupling.

[0006] An RFID device that includes a planar metallic trace is usually referred to as an inlay. In particular, an inlay is an RFID device including a planar metallic trace film supported on a flexible substrate, which is connected to a transponder. The transponder, included in the current loop, is an integrated circuit for deciphering signals sent to the inlay and received by the planar metallic trace, and also for sending a signal to the planar metallic trace, which is then transmitted by the planar metallic trace (sometimes also called “antenna”). The inlay antenna may be tuned (i.e. sized) to communicate at a certain target frequency with a transceiver, which includes at least a coupling element for communication with the RFID inlay and an interrogator. Regarding the term “antenna”, for better understanding it is noted that within the present disclosure the term “antenna” is interpreted as meaning a radiating device. The present disclosure, however, relates to coupling in the reactive near field as opposed to the radiation far field. The usage of the term “antenna” has therefore been avoided in the present disclosure.

[0007] In recent years, printing devices have become known that enable dislocating an RFID inlay on a medium such as a sheet of paper, and at the same time, enable encoding the RFID inlay with the desired information during the printing process. The encoding is performed by means of electromagnetic coupling, preferably in the reactive near field. For this purpose, an RFID printer / encoder is provided with an electromagnetic coupler arrangement that fits in a cavity of the printer so as to couple electromagnetic power bearing the coding information into the RFID inlay which is located on the medium, while the medium is guided through the printer / encoder along a media path.

[0008] Coupler arrangements with only a single coupling element may be used, as well as those with a plurality of coupling elements forming a one- or two-dimensional array.

[0009] A problem arises due to the need to encode different inlays with various geometries (form factors) efficiently and without substantive changes to the coupler arrangement.

[0010] The coupling element, whether it is in a single configuration making up the whole coupler or in an array configuration, is generally a non-optimized design, since there is a vast variation of inlay geometries. In many cases, transmission line (TRL) technology is used for leakage field coupling from different TRL geometries. In other cases, a more antenna-like radiating structure is used, at a cost of extensive shielding for radio frequency (RF) isolation. Regardless of which of the mentioned configurations is used, non-optimized coupling is achieved. One solution for geometry adaptation may then be the array configuration, with controlled excitation of the individual elements for optimized RF coupling field, so that the imperfection of the individual element is overcome.

[0011] However, this has the drawback that calibrations are needed in order to adapt the coupling arrangement to a particular inlay to be encoded, before an actual encoding can be performed. Either the positioning of the inlay for optimum coupling must be known or the inlay profile must be obtained and stored.

[0012] In the former (“static”) case, each inlay type has its own unique required positioning, which cannot be changed for a fixed coupler. In the latter (“externally controlled adaptive”) case, there is a need for a scanning functionality in order to know the inlay profile.

[0013] In the particular case of an array of electromagnetic coupling elements, a calibration step is initially necessary to be performed, for each inlay geometry, in order to determine (select) one or a limited plurality of elements of the array to be used (activated) for coupling power into this particular inlay type. In addition or at the same time, there is a need to determine the (relative) position between the inlay to be communicated with (in particular: be encoded) and the coupler arrangement.

[0014] In order to address the drawbacks of the above approaches, document WO 2021 / 180319 A1 provides a self-adaptive array of wireless electromagnetic coupling elements. In a sensing state, during which a non-modulated electromagnetic wave is forwarded to each of the coupling elements of the array at the same time, it is determined, which one or which plural ones of the array engage in a highly efficient coupling with an RFID inlay, based on a sensed interaction strength between each coupling element and an inlay present in the vicinity of the coupler arrangement. As a result of the determination, a resistance of the feeding path of those coupling elements (a single one or a plurality), which establish a highly efficient interaction, is switched from a high to a low value. Thereby, out of the plurality of coupling elements only those which enable highly efficient coupling with the particular inlay are automatically activated, whereas the rest does not practically contribute to interaction and is thus inactive. Hence, in a subsequent step of coupling with a modulated wave for providing information, only the coupling elements enabling the most efficient coupling are active in order to concentrate the fed coupling energy on those coupling elements. In other words, spatial selectivity is automatically attained among coupling elements of an array interacting with the particular tag without the need for a dedicated calibration procedure.

[0015] For achieving this, harvester elements are connected to the feeding paths of each individual coupling element of the array. The harvester elements provide a feedback for switching the respective resistances based on a coupling strength between the coupling element and the inlay determined during the sensing phase.

[0016] A drawback of the approach applied in the above identified patent document resides in that, in view of the concurrent provision of the sensing wave to all couplers of the array in the sensing state and the resulting possible selection and activation of a plurality of coupling elements at once, destructive interferences between plural coupling elements may occur. This decreases the overall coupling efficiency and should therefore be avoided as far as possible.

[0017] The present invention aims to provide an improved self adaptive array of coupling elements that avoids destructive interferences between individual coupling elements of the array.

[0018] SUMMARY

[0019] According to a first aspect, a wireless electromagnetic coupler arrangement for providing spatial selectivity in interaction with an RF terminated planar metallic trace of arbitrary geometric shape by means of electromagnetic energy, is provided. The electromagnetic coupler arrangement comprises a sequential array of coupling elements for coupling the electromagnetic energy to the RF terminated planar metallic trace by means of reactive near field coupling. Each of the coupling elements is connected to a differential transmission line via a respective first switch, so as to alternatively either feed any electromagnetic energy provided by the differential transmission line to the respective coupling element or forward the electromagnetic energy provided by the differential transmission line to the next coupling element in the array, thereby bypassing the respective coupling element, by controlling the respective first switch. The arrangement further comprises a respective impedance element being included in the path between the differential transmission line and a respective one of the coupling elements forming the array. Each of the coupling elements is associated with a respective harvester element for collecting a part of energy that is fed from the differential transmission line to the respective coupling element and providing a DC output voltage and a respective feedback loop, for providing feedback from the harvester element to the respective first switch, for switching the respective first switch to a state bypassing the respective coupling element when the DC output voltage provided by the harvester element exceeds a first reference DC voltage.

[0020] According to a second aspect, a method of operating a wireless electromagnetic coupler arrangement according to the first aspect is provided. The method comprises the step of initially controlling the respective first switches via which each of the coupling elements is connected to the differential transmission line to be in a state in which the coupling element is coupled to the differential transmission line for feeding electromagnetic energy. The method further comprises the steps of entering a sensing state with feeding an unmodulated RF carrier wave of electromagnetic energy to the differential transmission line and of switching the first switch connected to a first coupling element in the array to the state for bypassing the first coupling element in the array, when the DC output voltage provided by the harvester element exceeds the first reference DC voltage. Further, the method comprises the step of repeating the switching step for subsequent coupling elements of the sequential array, until a coupling element has been reached, for which the DC output voltage provided by the harvester element remains below the first reference DC voltage. Finally, the method comprises the step of entering a communication state with feeding a modulated RF carrier electromagnetic wave as a communication signal to the differential transmission line, after a coupling element has been reached for which the respective first switch remains in the state connecting that coupling element, so as to encode an RF terminated planar metallic trace via that coupling element.

[0021] It is the particular approach of the present invention to feed, in a sensing state, unmodulated electromagnetic energy sequentially to each of the coupling elements of an array, starting with a first one. If an efficient coupling of a coupling element with an RFID inlay located in the vicinity is detected, the coupling element remains active and is used for subsequent signal communication for reading / writing the inlay. If no efficient coupling is detected, the coupling element is bypassed and thereby deactivated, and the sensing wave is forwarded to the next coupling element in the array. Thereby, only a single coupling element is activated at any time and any destructive interferences between plural coupling elements are avoided. In order to reach selectivity, by activating or bypassing each individual coupling element, a respective (first) switch is used to connect each coupling element with the feeding transmission line. The respective switch is triggered by a respective harvesting element associated with each coupling element. Lack of efficient interaction is indicated by the DC output voltage of the harvester element exceeding a reference, which triggers the switch to bypass.

[0022] According to a non-limiting embodiment, the electromagnetic coupler arrangement further comprises an interrogator for feeding electromagnetic energy to be coupled to the planar metallic trace to the differential transmission line. Further preferably, the arrangement comprises a balun that is electrically coupled between the interrogator and the differential transmission line. The balun forms a differential transmission line input by splitting an output of the interrogator into parts equal in amplitude and shifted by 180° in phase.

[0023] According to preferred non-limiting embodiments, the interrogator is configured to feed unmodulated electromagnetic energy to the differential transmission line in a sensing state. The unmodulated electromagnetic energy is an RF carrier wave guided by the differential transmission line. Thereby, in the sensing state, unmodulated electromagnetic energy is used in order to establish certain DC voltages at the harvester elements, depending on the strength of interaction between the respective coupling element and a planar metallic trace (inlay) that is present in the vicinity of the coupling element.

[0024] Further preferably, the interrogator is configured to subsequently modulate the RF carrier as a communication signal to the coupling elements. In accordance therewith, when a stable connection status has been established for one of the coupling elements, on the basis of the sensing result of the sensing step, a modulated communication signal can be sent for encoding the inlay.

[0025] In accordance with preferred non-limiting embodiments, an energy supply for the operation of the feedback loop including the generation of the DC reference voltage is derived from the electromagnetic energy fed by the interrogator. Hence, the power for operating the feedback loop is provided by the feeding energy, in particular, the energy of the unmodulated carrier wave during the sensing state. According to preferred non-limiting embodiments, the differential transmission line is terminated.

[0026] Further preferably, the termination includes a further coupling element for coupling the electromagnetic energy to the RF terminated planar metallic trace by means of reactive near field coupling, without being associated with a respective harvester element and a respective feedback loop.

[0027] According to preferred non-limiting embodiments, each of the coupling elements includes a transmission line loop for interacting with the RF terminated planar metallic trace by inductive coupling. The transmission line loop is a continuous transmission line of finite length, formed into a loop so that two terminals thereof approach each other and are differentially fed through the differential transmission line. Such a configuration is also known as a “differential transmission line loop” (DTLL).

[0028] According to a preferred non-limiting embodiment, the sequential array of coupling elements is of a one-dimensional geometric arrangement.

[0029] According to an alternative preferred non-limiting embodiment, the sequential array of coupling elements is of a two-dimensional geometric arrangement.

[0030] According to preferred non-limiting embodiments, the RF terminated planar metallic trace is an RFID inlay. Further preferably, the electromagnetic coupler arrangement is adapted to encode the RFID inlay by coupling electromagnetic power thereto.

[0031] According to preferred non-limiting embodiments, the impedance elements each comprise an inductor and / or a capacitor. More preferably, the impedance elements comprise a series connection of a resistor and an inductor. With such a construction, a particularly simple arrangement can be achieved, without having any internal feedback or active elements in the path between the first switch and the coupling element. In particular, having an inductive impedance element takes into account the basically capacitive character of the coupler and the harvester.

[0032] According to alternative non-limiting preferred embodiments, each of the impedance elements comprises a switchable resistor element, so as to alternatively switch the resistance of the electric connection between the differential transmission line and the respective coupling element to a first or second value. Here, the second value is smaller than the first value. The switching is achieved by controlling a respective second switch. More preferably, the resistance is initially switched to the first value for coupling elements of the array. Still more preferably, there is an additional (“internal”) feedback loop from the harvester to the second switch, and the resistance is switched from the first to the second value, when the DC output voltage provided by the harvester is below the second reference DC voltage. The second reference DC voltage is lower than the first reference DC voltage. This construction enables to reduce the resistance of the feeding path for an element, for which highly efficient coupling has been sensed so as to be used for the coupling (writing / encoding and reading).

[0033] Further preferably, the feedback from the harvester to the first and the second switch is provided by means of a window comparator circuit. Preferably, the parameters of the window comparator circuit are adjusted in such a way that the first and second reference voltages are relatively close to each other, so that most likely either the first switch switches to bypass state or the second switch switches to reduce the path resistance.

[0034] Still further, according to non-limiting preferred embodiments, the switchable resistor element comprises a first resistor with the first resistance value and a second resistor with the second resistance value arranged in parallel so as to be alternatively switched into the electric path between the differential transmission line and the respective coupling element by the respective second switch. More preferably, each of the harvester elements is permanently coupled to the differential transmission line, via an electric connection with the first resistance value. This enables a stable operation with particularly small low resistance values.

[0035] Also preferably, the coupler arrangement is employed in a printer, wherein RFID inlays to be encoded are arranged on a medium guided in the printer along a media path.

[0036] According to a non-limiting embodiment, an RFID printer / encoder, comprising a multi-layer electromagnetic coupler arrangement according to the first aspect or any of non-limiting embodiments thereof, is provided. Since the electromagnetic coupling for transferring coding information to an RFID inlay occurs in the reactive near field, the multi-layer electromagnetic coupler arrangement can be easily fitted into a printer cavity, in proximity to the media path.

[0037] The non-limiting embodiments disclosed broadly aim to provide an improved self-adaptive electromagnetic coupler arrangement that is applicable to couple electromagnetic power into inlays of arbitrary shape efficiently.

[0038] These and other aspects and features of non-limiting embodiments will now become apparent to those skilled in the art upon review of the following description of specific non-limiting embodiments in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Additional features and advantages of non-limiting embodiments will become apparent from the following description as illustrated in the accompanying drawings, wherein:

[0040] Fig. 1A is a block diagram of an electromagnetic coupler arrangement in accordance with a nonlimiting embodiment;

[0041] Fig. 1 B is a block diagram of an electromagnetic coupler arrangement in accordance with another non-limiting embodiment;

[0042] Fig. 2 is a simplified illustration of an arrangement of an impedance element as employed in one non-limiting embodiment;

[0043] Fig. 3 is a simplified illustration of an impedance element as employed in another non-limiting embodiment;

[0044] Fig. 4 is a simplified functional illustration of a differential transmission line loop as employed as a coupling element in a non-limiting embodiment; and

[0045] Fig. 5 is a flowchart illustrating an exemplary operation method for a wireless electromagnetic coupler arrangement according to an aspect of the present invention.

[0046] DETAILED DESCRIPTION

[0047] Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

[0048] The present disclosure relates to an electromagnetic coupler arrangement comprising a self- adaptive array of coupling elements for enabling a selective and highly efficient coupling of electromagnetic energy into planar metallic traces of arbitrary shape, in particular, for encoding RFID inlays. The non-limiting embodiments disclosed are particularly suitable for encoding inlays which are provided on a medium such as in an RFID printer / encoder.

[0049] In non-limiting embodiments of the electromagnetic coupler arrangement an interrogator (sometimes also called “RFID reader”) is connected via a balun to a transmission line, thereby transforming the transmission line into a differential transmission line having two portions being fed with an output of the interrogator with equal amplitudes and phases shifted by 180°. A sequential array of coupling elements is shunted onto the differential transmission line via controllable switches (“first switches”). Each of the switches either bypasses or connects the associated coupling element to the differential transmission line.

[0050] Between the differential transmission line and each coupling element, an impedance element is foreseen. Further, an output terminal of each of the impedance elements is connected with a respective harvester element that is further connectable to the respective coupling element. Thereby, the harvester element collects energy that is fed from the differential transmission line to the respective coupling element.

[0051] The harvester element collects energy when the device is powered and thereby converts RF power fed into the system into a DC voltage. The harvester element is coupled via a feedback loop to the respective controllable switch connecting the coupler element via the impedance element to the differential transmission line. In preferred embodiments, the feedback loop may include a window comparator.

[0052] A harvester element (“power harvester”) and the operation thereof is as such well-known in the art. A detailed description thereof is therefore herein omitted. For further explanations, reference is made, for instance, to the article “Self-Reconfigurable RFID Reader Antenna”, by Pavel Nikitin, IEEE RFID Conference, May 9 - 11, 2017 (cf. also patent publication US 10,096,898 B2).

[0053] The goal to be achieved with the above described structure is to selectively activate communication only between a particular one of the plurality of couplers forming the array and an inlay (transponder) present in the vicinity of the array. More specifically, only one of the coupling elements is to be activated, which allows for a highly efficient communication with the particular inlay (transponder) having a specific geometry.

[0054] For this purpose, the arrangement is initially set into a sensing state for sensing efficiency of the near-field electromagnetic interaction and identifying a coupler of the array that provides for an efficient coupling. Once such a coupling element has been identified, this coupling element remains in an active state and is used for signal communication (reading and / or writing) with the inlay (transponder). The remaining coupling elements are inactive. This is achieved by activating all switches connecting a coupling element of the array, that is electrically connected between the interrogator and the particular coupling element, to the bypass state. Since the switch connecting the particular coupling element is not in the bypass state, the (electrically) subsequent coupling elements are not fed and are, therefore, inactive as well.

[0055] In accordance with non-limiting embodiments, the sensing state is established as follows. At the beginning, when the interrogator is switched on, all (first) switches of the array are switched to the connecting, i.e. non-bypassing state. The interrogator then powers the system by feeding an (unmodulated electromagnetic) wave towards the differential transmission line. While being fed, the DC voltage, established at the harvester element coupled to the first coupling element in the electrical sequence of the array, gradually increases. However, as soon as efficient coupling with an RFID inlay (planar metallic trace) located in the vicinity of the first coupling element occurs, the DC voltage output by the harvester element saturates. In that case the respective first switch is not triggered and remains active for being used in coupling.

[0056] On the other hand, when there is no efficient interaction with an inlay, the DC harvester element output voltage increases and eventually exceeds a reference voltage (“first reference DC voltage”). It is noted that in practical embodiments, the necessary energy for operation of the arrangement is taken out of the feeding energy (unmodulated electromagnetic wave) as well, and there is no static power supply. As a consequence, the said reference voltage is not set in advance as a predetermined voltage, but increases together with the DC output voltage of the harvester element, until it reaches a saturated value. In other words, the whole coupler arrangement is powered by the feeding energy (“self-biased”). Initially, the DC voltages increase, therefore starting from zero. In addition, the self-biased power-up needs a time for power up, due to internal time constants of the circuitry. A typical power up time duration may be in the order of a few microseconds (ps), although yet still significantly shorter than the response time of the feedback loop, i.e. the time after which a change at the input of the feedback loop is reflected at the output. Without any limitation, in case of a window comparator, this time may be in an order of 200 ps. In general, the relative timing between saturated levels of the control signals, such as the reference levels and the element harvester DC output, entering the input of the feedback loop cannot be assumed to be fixed. The relative timing between the control signals and the DC bias can also not be assumed to be fixed. But the time constants of the coupler circuitry are assured to be short enough, by design, to enable fully saturated levels of the DC bias and control signals within the response time of the feedback loop. The control circuitry is therefore guaranteed to operate as desired.

[0057] It is further noted that the first switches may be arranged in such a way that the bias voltage for powering the elements is even fed to a coupling element in the bypass state. When the DC harvester voltage exceeds the first reference voltage, the feedback loop triggers the respective switch so as to bypass the first coupling element. The respective operation then repeats for the second coupling element in the electrical sequence of the arrangement, and so on, until a coupling element establishing efficient interaction with an RFID transponder is reached, which then remains active.

[0058] Further details of the operation will be described below, in accordance with individual embodiments.

[0059] Thereby, a single one out of the coupling elements of the array is automatically selected as an active one. Via this coupling element, the actual communication with the inlay is then performed by feeding the system with a modulated signal, in order to write (or read). As indicated, the other couplers are inactive, so that the communication concentrates on one single active coupler element.

[0060] Thus, a self adaptive array of coupling elements, avoiding destructive interference between individual couplers, is provided that ensures highly efficient communication with transponders (tags, inlays) of various shapes and geometry, to be flexibly enabled without the need to know or determine an exact location of an individual transponder.

[0061] In the following, further details of non-limiting embodiments will be described with reference to drawings.

[0062] Figs. 1A and 1 B are block diagrams providing a system overview of an electromagnetic coupler arrangement 1 , 1a according to alternative non-limiting embodiments.

[0063] Specifically, Fig. 1A shows an embodiment, wherein termination is realized by a generic impedance element. Fig. 1 B shows an embodiment, wherein termination is realized by means of a further coupling element that is not associated with a harvester and feedback loop.

[0064] In the following, the common features of the arrangements in Figs. 1A and 1 B will be described. For simplification of representation, reference numerals 3, 3a, and 5 are indicated only once, at a single element in these drawings but are equally applicable to the corresponding portions of other elements.

[0065] The electromagnetic coupler arrangement comprises an interrogator 10, a balun B, and a plurality of sequentially coupled elements (Element 1 ,... Element n,... Element N) of a differential transmission line TRL. “Sequentially coupled” means a sequential electric coupling of the individual elements to the differential transmission line, which must be distinguished from the geometric shape, which may, for instance, represent a one-dimensional or two-dimensional array. For simplification, further elements arranged between the elements explicitly shown in the drawing and arranged between the wavy lines have been omitted. Further, the drawing has been broken between the balun and Element 1. To each of the elements, a coupling element TLL is coupled via impedance elements Z, together with a respective harvester element H. Non-limiting embodiments of the impedance elements will be described in more detail below with reference to Figs. 2 and 3.

[0066] For each connection (“shunting”) of a coupling element TLL to the differential transmission line TRL via an impedance element Z, a respective switch (“first switch”) 5 is provided. The switch allows for either establishing or bypassing the connection between the TRL and the coupling element.

[0067] A feedback loop 3 from each of the harvester elements H to the respective switches 5 is indicated with dashed lines. As can be seen from the drawings, a feedback from the harvester is indicated not only to the switches 5, but also to the impedance elements Z. This second feedback loop (3a, dotted) is optional and only necessary in embodiments, wherein the impedance elements Z include further controllable switches. This kind of embodiments will be described in more detail below with reference to Fig. 2.

[0068] More specifically, the interrogator 10 is adapted to feed electromagnetic energy (in the form of an unmodulated wave or a modulated wave representing a signal) to power the system, which is indicated by a single line extending from the interrogator 10 to the balun B. The balun splits the electromagnetic wave (signal) into two waves (signals) of equal amplitude and with a phase shift of 180°. Thereby, the balun B converts the transmission line extending from the interrogator into a differential transmission line. This is indicated by the two parallel lines forming the differential transmission line, onto which the coupling elements TLL and the harvesters H are shunted via the switches 5 and impedance elements Z. It is further noted that in the preferably used copper transmission lines, high-frequency (radio frequency RF or ultra-high-frequency UHF) losses are quite small.

[0069] According to a non-limiting embodiment, a frequency range of operation is about 860 to 960 MHz (Megahertz).

[0070] In the drawing of Fig. 1A, the differential transmission line is terminated behind element N (on the right-hand side of the drawing) by means of a terminating impedance Zterm. The terminating impedance serves for compensating mismatches along the differential transmission line. The presence of the termination is, however, optional, and, depending on the system parameters, an un-terminated differential transmission line is within the scope of the present disclosure.

[0071] In the drawing of Fig. 1 B, the terminating impedance Zterm is replaced by a termination last element comprising a combination of an impedance ZTLL and a further (“last”) coupling element. In other words, there is a last coupling element terminating the arrangement that is neither associated with a harvester element, nor connected to the TRL via respective switches. In a case, where no sufficiently efficient interaction between any of the couplers in elements 1 to N has been detected in order to select any of these couplers for being active, a final coupling attempt is nevertheless possible via the terminating coupling element. This coupling element may be a TLL identical to the elements 1 - N, such as indicated in Fig. 1 B, but is not limited hereto. The coupling element may also be of a different geometry. In particular, it may be designed with the purpose of the coupler to overcome any deficiencies of the array 1 - N, when the metallic trace to be coupled to has a geometry leading to inefficient coupling and triggering, despite having a location above the coupler array.

[0072] The harvester-based control feedback loop 3 cannot be externally interfered with and, whether or not a switch 5 is triggered, is not monitored outside the coupling element.

[0073] Regarding the geometric dimensions, it is noted that the label size of an individual label is generally smaller than the geometric size of the self-adaptive array of coupling elements. In other words, the processing for activating only a single one of the coupling elements forming the array (subarray activation) is foreseen for short pitch applications.

[0074] The size of the array (number of coupling elements in the coupler arrangement) is generally not limited. Typical values may be 8, 16 or 32, but without being limited to these. Out of the coupling elements of the array, only a single one may be activated in the self-adaptive sensing procedure for a particular inlay shape and arrangement (position and orientation).

[0075] It is further noted that for the connection of the harvester and the coupling element to the differential transmission line, two identical arrangements Z, as shown in Fig. 1 and illustrated in detail in Figs. 2 and 3, are connected in each section (element) of the differential transmission line.

[0076] In the following, non-limiting exemplary arrangements of circuitry in the boxes labeled Z in Figs. 1A and 1 B, which are illustrated in Figs. 2 and 3, shall be explained. Fig. 2 shows an exemplary arrangement of circuitry in the box Z in accordance with a non-limiting embodiment.

[0077] In accordance therewith, the electric input path splits into two parallel paths between the TRL (first switch 5 of Fig. 1 , not illustrated in Fig. 2) and the respective coupling element TLL characterized by having different resistances, illustrated in the form of different resistors Ri and F?2. In accordance with the convention used in this specification, the resistance value of the first resistor, Ri, is larger than the resistance value of the second resistor, F?2. A switch 2 (“second switch”), arranged at an output side of the box, enables switching between the two parallel paths. The harvester element H remains permanently connected to the electric path with the higher resistance, Ri. This is achieved by arranging the switch 2 at a position behind the terminal to which the harvester element H is connected, in the current flow from the differential transmission line TRL towards the coupling element TLL. Thereby, only the connection between the differential transmission line TRL and the coupling element TLL is switched, whereas the connection to the harvester element H remains unaffected by the switching. As can be seen from the drawing, there is a short circuited connection between the harvester element H and the coupling element TLL when the switch 2 is on the side of resistor Ri. However, when switch 2 is switched towards resistor F?2, the galvanic connection between harvester H and coupling element TLL is interrupted.

[0078] Operation of the second switch 2 is controlled by a signal that is provided through the second feedback loop 3a from the harvester element H. This is indicated by the dashed line in the drawing.

[0079] In the following, the operation for automatically reaching a stable activation status of an individual coupling element in the presence of a particular inlay, will be described.

[0080] At the beginning, i.e. before feeding of energy from the interrogator 10 starts, the system is in a default unpowered state. Due to the fact the that no control signals will exist in this initial state in view of the power up time, the switch will be powered up with its DC bias well in time before any control can take place, explained by the response time of the feedback loop, generating the control signals. Consequently, when a stable DC bias exists, the switch will have its default low state, with the TLL connected to the harvester (H) and Ri.

[0081] When an inlay approaches, electromagnetic interaction between the inlay and the coupling element TLL leads to a saturation in the DC voltage at the harvester element H being lower than the second reference voltage. This is a result of mismatch and, thus, to an impedance change in the respective element of the array due to the interaction between the coupling element and the transponder. In particular, the higher the efficiency of interaction between a particular coupling element and the transponder is, the lower the DC voltage at which the harvester output saturates, becomes. The DC reference voltage (“first reference DC voltage”) for triggering the respective first switch 5 is, thus, never reached or exceeded. As a consequence, the respective switch 2 is triggered.

[0082] After the switch 2 is triggered, the resonance condition is interrupted by the triggered switch 2. The harvester element remains out of the resonance circuit. Consequently, the low DC voltage at the harvester element H remains stable, even in the case of a very low second resistance value R2 (one or even several orders of magnitude smaller than R-).

[0083] It is again emphasized that any triggering of switches is not caused by transient voltage relations, but only by the situation as established upon the end of the delay time due to self-biased power- up. In a preferred embodiment, the feedback loop 3, 3a comprises a window comparator circuit (“window detector circuit”). Such a circuit compares an input voltage (here: the DC output voltage of the harvester element) with both a lower and a higher threshold (reference voltage). An output is “high-level” when the input voltage is in the “window” between the lower and the higher reference voltage. A first output is high, when the input voltage is below the higher reference value. A second output is high, when the input voltage is above the lower reference value. Again it is noted that the comparison is made between saturated values of the respective voltages and both reference voltages saturate well in time before any reaction occurs at the outputs of the comparator.

[0084] Thus, the first output of the window comparator circuit will trigger the first switch 5 in case of a high harvester DC output voltage (no efficient inlay interaction present) and the second output of the window comparator will trigger the second switch 2 in case of low harvester DC output voltage (efficient inlay interaction present). Triggering of the second switch will further enhance efficiency of interaction for the subsequent coding process, by reducing the resistance in the feeding path. For most efficient operation, it is desirable to reduce the “gap” between both reference voltages as much as possible, although it cannot be reduced to 0 for principal constructional reasons of the window comparator. In practical implementations, size of the gap between the reference voltages may be in the order of a few percent such as 2% of the reference voltages itself.

[0085] Should the DC output of the harvester saturate in the (to be made small) gap between both reference voltages, none of the switches is triggered. Hence, the respective coupling element remains active and the resistance of the feeding path nevertheless remains high. This allows for a reduced, but in many cases nevertheless, still sufficient coupling strength. Fig. 3 shows an exemplary arrangement of an impedance element in the box “Z’ in accordance with another non-limiting embodiment.

[0086] In this embodiment, the impedance element simply comprises a series connection of a resistor R and an inductor L, without including any further switches. The output is split between the coupling element TLL and the harvester H.

[0087] This arrangement enables a much simpler and, thus, more cost-efficient construction. Selectivity is maintained by triggering the remaining first “(external”) switch 5 based on feedback 3 of the DC output voltage of the harvester. On the other hand, a resistor may be selected having a voltage in-between preferred resistance values of the two resistors in the example of Fig. 2, which nevertheless guarantees sufficient interaction in both the sensing and signal coupling stages. In other words, in that case the same circuitry can be used in the sensing and the coupling state.

[0088] It is noted that, more generally, any complex impedance can be used as the impedance Z in Fig. 3. A basically inductive embodiment, as illustrated, is, however, preferable in most cases in view of the overall capacitive character of the combination of the coupling element and the harvester. The respective compensation achieved ensures a stable operation. Stability can be improved by appropriately choosing the values of the complex impedance (in the particular example: R and L).

[0089] The differentially fed coupling element TLL may be one of various geometric arrangements, shapes etc. An exemplary and non-limiting embodiment will be described with reference to the drawing of Fig. 4.

[0090] In accordance with preferred non-limiting embodiments, the structures as explained below, serving as coupling elements TLL in an electromagnetic coupler arrangement according to the present disclosure, are generally embedded in a multi-layer structure (using one of well-known microstrip or strip line technologies) comprising the feeding layer, a metallic ground plane layer and the top surface layer to be arranged closest to the terminated planar metallic trace (inlay to be encoded). The top surface layer comprises the coupling elements, an example of which will be explained in more detail below. It is noted that although the reference label “TLL” (for: “transmission line loop”) is generally used for the coupling elements throughout the present disclosure, this does not imply any limitation of the structure of suitable coupling elements, in principle.

[0091] In typical arrangements, there is only a small distance between the top surface layer (coupling elements) and the planar metallic trace (inlay) to be coupled, in the order of about 1 mm, or between 0.5mm and 1.5mm, without being limited to that range. A typical size of an inlay may be in the order of 4” (inches), i.e. approximately 100mm (millimeters), conforming to 4” wide media used in bar code printers. More compact inlay designs also exist, being in the order of 30-50 mm, which are compatible with narrower media widths.

[0092] Fig. 4 is a simplified functional illustration of an example of a coupling element, which will be called “Differential Transmission Line Loop” (DTLL). The coupling element is constituted of a continuous transmission line (which may, for instance, be manufactured in the microstrip technology), which is shaped so as to form an (almost) closed loop 6. The open ends of the loop 6 constitute terminals 21 and 22 that are fed with signals 7 and 8 (illustrated in form of dashed lines), having a phase difference of 180° with respect to each other. As was explained above with reference to Fig. 1 , signals 7 and 8 are output by a balun, as a result of splitting an input signal into two signals of equal amplitude and inverted phase, and fed via a differential transmission line TRL and the shunted connection as described in more detail above with reference to Figs. 1 to 3.

[0093] As a consequence of the differential feeding, the direction of the current flowing in the loop 6, illustrated by an arrow in the loop, can be the same throughout the loop (at a given instance of time).

[0094] The design of the coupling element as a continuous transmission line loop, representing a distributed field theoretic component, can be performed without the consideration of matching. On the contrary, if discrete components were to be included at intermediate locations in the loop structure, for the purpose of input matching, unnecessary iteration would be needed between field theoretic coupling optimization and component value optimization. Thus, a continuous transmission line loop offers a considerable design advantage as compared to mixing distributed and discrete components inside the coupling structure. For appropriate functioning, the DTLL must be arranged so as to oppose a ground plane (not illustrated). In accordance with a non-limiting embodiment, all discrete components are arranged on the feeding side, or balun side of the arrangement, that is, the opposite side of the ground plane relative to the coupling element.

[0095] Hence, a non-limiting embodiment facilitates, firstly (and independently), optimizing the geometry of the transmission line loop and, secondly, after the geometry of the transmission line loop has been optimized, performing the impedance matching on the balun side, by choosing appropriate electric components. The balun has three functional properties. The first property consists of splitting the input signal into two parts equal in magnitude. The second property consists of shifting the two parts 180° apart in phase. The third property consists of an impedance transformation from a non-differential impedance of an external feeding system interface, such as, for example, 50Q, to a differential impedance level, as seen at the input of the loop. In other words, the balun according to a non-limiting embodiment may be regarded as including a transformer and designated as a “balun transformer” since it also fulfils the third function of impedance transformation. The impedance transformation is generally characterized by the impedance transmission ratio k. In the case relevant for a non-limiting embodiment, wherein a non-differential impedance is transformed into a differential impedance, k equals twice the ratio of the differential impedance value (at the input of the DTLL) and the non-differential impedance value (of the external feeding system interface). In case of a 50Q external feeding system, and assuming the differential impedance level at the input of the loop to be 500Q, the impedance transmission ratio would be k = 5.

[0096] Generally, the balun “sees”, at its output, a high reactive impedance (or high Q-value), which is due to the inductive character of the transmission line loop and the presence of the ground plane. By including the respective electric components, said impedance is matched with the impedance on the feeding side.

[0097] A high Q-value (high value of the Q-factor or quality factor) corresponds to a highly efficient inductive coupling but at a reduced bandwidth, since the Q-factor generally expresses the relation of the resonance frequency of a circuit to the bandwidth (half power bandwidth). This means that the range of frequencies where it is possible to deliver power with high efficiency, is limited. Therefore, the matching is preferably made in a way that reduces the Q-value to a certain acceptable extent. This can be done, for instance, by including an internal resistor, at the output of the balun. This is possible, taking into account the potentially very high coupling factors that can be achieved between the differentially fed transmission line loop and the inductive loop of an inlay at a single frequency, where some reduction in delivered power to the transponder chip is accepted, with a still overall high coupling factor exhibited by the DTLL over the desired bandwidth.

[0098] The size of the coupler element (TLL) is highly dependent upon the electrical length of the transmission line forming the TLL. The electrical length of the transmission line (TL) portion being confined to the top surface of the multi-layer arrangement may not exceed 180° (one half wavelength), to avoid zero crossings of the current. As reference, a straight TL with 1 mm width on an FR4 substrate having a thickness of 1 mm has a 180° electrical length corresponding to approximately 92 mm of physical length. Hence, any TLL has a surface size restriction corresponding to this TL length. It shall be noted that for an arbitrary geometrical shape of the TLL, sub sections of the loop being very close to each other, interact electromagnetically. Hence, the actual size of the TLL may be affected by the shape of it. For instance, a super elliptical shape may have a TL length being rather close to the reference straight TL. On the contrary, a spiral shape has several sub sections being close to each other. For any DTLL realization, the differential input impedance corresponding to a 180° electrical length has a real value.

[0099] Fig. 5 is a flowchart illustrating an exemplary operation method of an electromagnetic coupler arrangement according to an embodiment of the present invention.

[0100] At an initial stage (step S10), all first switches are put in the non-bypassing state (preferably: nonenergized), so as to connect the respective coupling elements to the differential transmission line.

[0101] At subsequent step S20, the sensing state is entered by feeding electromagnetic energy in the form of an unmodulated RF carrier wave. In particular, the sensing carrier wave is initially fed to the first coupling element (element 1), because the first switch of element 1 is (as initially all first switches) in the non — bypassing state and does, thus, not forward the sensing wave further. The initial feeding of element 1 is expressed in step S30.

[0102] At subsequent step S40, it is checked whether the output DC voltage of the harvester of element 1 (DCout ) exceeds an established (saturated) first reference DC voltage (DCref,i).

[0103] If this is the case (S40: Y), a state has been established, in which there is no RFID transponder (inlay) present, with which an efficient electromagnetic interaction is possible. Consequently, the first switch of the respective element number 1 is triggered and element 1 is bypassed (step S50).

[0104] The sensing wave is thus forwarded to the next (second) element (the element number is set to the previous element number (i = 1) plus one, i.e. i = 2 in this case). The processing of step S40 then repeats for the second element (element number 2). Should there again be no efficient interaction with an RFID transponder detected, the subsequent processing of step S50 repeats, until the last element (element number N) has been reached, as expressed by the loop including steps S60 and S70. If the last element has been processed (i.e. all first switches are now in bypass state without an interaction being detected) the processing of this flowchart ends. Depending on the hardware embodiment, this may mean that no interaction is possible (e.g. Fig. 1A) or that an interaction will finally be attempted through the terminating last loop (e.g. Fig. 1 B). Alternatively, an operation is possible, wherein after all first switches were switched to bypass, all first switches are then put into a state, wherein they forward the feeding energy in both directions, i.e. both to the associated coupler element and to the next coupling element. In this way, coupling can be attempted through all coupling elements at once.

[0105] On the other hand, if the output DC voltage of the harvester in element 1 (or any of the other elements with number i > 1 when the loop is repeated) does not exceed the first reference DC voltage, the first switch is not triggered and the respective coupling element is automatically selected to be the single one that remains active. As a consequence, a transition occurs from the sensing state to the signal communication state, wherein a modulated electromagnetic carrier wave is fed to the selected active coupling element (step S80). In subsequent step S90, the RFID transponder is encoded via a selected coupling element. Consequently, processing ends without the elements following after element number i having been processed in this case.

[0106] It is noted that the example of Fig. 5 illustrates the principal steps of an operation method for selecting and selectively operating a single one out of coupling elements of an array according to the present invention.

[0107] The invention is, however, by no means limited to this example. For instance, a more sophisticated operation scheme is possible, wherein the individual elements include internal (second) switches, the operation of which is triggered by the harvester output voltage so as to adjust an impedance element (resistance) in the feeding path, as described above with reference to Fig. 2.

[0108] It is noted that the foregoing has outlined some of the more pertinent non-limiting embodiments. It will be clear to those skilled in the art that modifications to the disclosed non-limiting embodiments can be effected without departing from the spirit and scope thereof. As such, the described non-limiting embodiments ought to be considered as merely illustrative of some of the more prominent features and applications. Other beneficial results can be realized by applying the non-limiting embodiments in a different manner or modifying them in ways known to those familiar with the art. This includes the mixing and matching of features, elements and / or functions between various non-limiting embodiments being expressly contemplated herein, so that a person of ordinary skill in the art would appreciate from this disclosure that features, elements and / or functions of one embodiment may be incorporated into another embodiment, unless described otherwise. Although the description is made for particular arrangements and methods, the intent and concept thereof may be suitable for and applicable to other arrangements and applications.

[0109] In summary, the present disclosure relates to a wireless electromagnetic coupler arrangement with a sequential array of coupling elements. The arrangement is self-adaptive in that it automatically selects a single one out of a plurality of coupling elements forming the array for being used as an active coupling element in coupling to planar metallic traces (in particular: RFID inlays) of arbitrary geometric shape without the need for a specific calibration or location procedure. This is achieved by means of feeding an unmodulated wave sequentially to each of the coupling elements, one after the other, until an element providing sufficient interaction suitable for performing signal coupling has been reached, in an initial sensing state. The existence of efficient interaction is detected by means of a threshold comparison of the DC output wave of a harvester associated with each coupling element and feeding back the result to a bypass switch. By restricting the activation to a single coupling element of the array only, the occurrence of destructive interference between plural coupling elements is avoided.

Claims

CLAIMS1. A wireless electromagnetic coupler arrangement for providing spatial selectivity in interaction with an RF terminated planar metallic trace of arbitrary geometric shape by means of electromagnetic energy, the electromagnetic coupler arrangement comprising: a sequential array of coupling elements (TLL) for coupling the electromagnetic energy to the RF terminated planar metallic trace by means of reactive near field coupling, wherein each of the coupling elements is connected to a differential transmission line (TRL) via a respective first switch (5), so as to alternatively either feed any electromagnetic energy provided by the differential transmission line (TRL) to the respective coupling element or forward the electromagnetic energy provided by the differential transmission line (TRL) to the next coupling element in the array, thereby bypassing the respective coupling element, by controlling the respective first switch (5); and a respective impedance element (Z) being included in the paths between the differential transmission line (TRL) and a respective one of the coupling elements (TLL) forming the array; wherein each of the coupling elements (TLL) is associated with: a respective harvester element (H) for collecting a part of energy that is fed from the differential transmission line (TRL) to the respective coupling element (TLL) and providing a DC output voltage; and a respective feedback loop (3) for providing feedback from the harvester element (H) to the respective first switch (5) for switching the respective first switch (5) to a state bypassing the respective coupling element (TLL) when the DC output voltage provided by the harvester element (H) exceeds a first reference DC voltage.

2. An electromagnetic coupler arrangement according to claim 1 , further comprising an interrogator (10) for feeding electromagnetic energy to be coupled to the planar metallic trace to the differential transmission line (TRL).

3. An electromagnetic coupler arrangement according to claim 2, further comprising a balun (B) that is electrically coupled between the interrogator (10) and the differential transmission line (TRL), the balun (B) forming a differential transmission line input by splitting an output of the interrogator (10) in two parts equal in amplitude and shifted by 180° in phase.

4. An electromagnetic coupler arrangement according to claim 2 or 3, wherein the interrogator (10) is configured to feed unmodulated electromagnetic energy to the differential transmission line (TRL) in a sensing state, the unmodulated electromagnetic energy being an RF carrier wave guided by the differential transmission line (TRL).

5. An electromagnetic coupler arrangement according to claim 4, wherein the interrogator (10) is configured to subsequently modulate the RF carrier as a communication signal to the differential transmission line (TRL).

6. An electromagnetic coupler arrangement according to any of claims 2 to 5, wherein an energy supply for the operation of the feedback loop (3) including the generation of the DC reference voltage is derived from said electromagnetic energy fed by the interrogator (10).

7. An electromagnetic coupler arrangement according to any of claims 1 to 6, wherein the differential transmission line (TRL) is terminated.

8. An electromagnetic coupler arrangement according to claim 7, wherein the termination (Zterm) includes a coupling element (TLL) for coupling the electromagnetic energy to the RF terminated planar metallic trace by means of reactive near field coupling, without being associated with a respective harvester element and a respective feedback loop.

9. An electromagnetic coupler arrangement according to any of claims 1 to 8, wherein each of said coupling elements (TLL) including a transmission line loop (6) for interacting with the RF terminated planar metallic trace by inductive coupling, the transmission line loop being a continuous transmission line of finite length formed into a loop (6) so that two terminals (21 , 22) thereof approach each other and are differentially fed through the differential transmission line (TRL).

10. An electromagnetic coupler arrangement according to any of claims 1 to 9, wherein the sequential array of coupling elements (TLL) being of a one-dimensional geometric arrangement.

11. An electromagnetic coupler arrangement according to any of claims 1 to 9, wherein the sequential array of coupling elements (TLL) being of a two-dimensional geometric arrangement.

12. An electromagnetic coupler arrangement according to any of claims 1 to 11 , wherein the RF terminated planar metallic trace is an RFID inlay.

13. An electromagnetic coupler arrangement according to claim 12, adapted to encode the RFID inlay by coupling electromagnetic power thereto.

14. An electromagnetic coupler arrangement according to any of claims 1 to 13, wherein said impedance elements (Z) comprise an inductor (L) and / or a capacitor.

15. An electromagnetic coupler arrangement according to claim 14, wherein said impedance elements (Z) comprise a series connection of a resistor (R) and an inductor (L).

16. An electromagnetic coupler arrangement according to any of claims 1 to 13, wherein saidimpedance elements (Z) each comprise a switchable resistor element, so as to alternatively switch the resistance of the electric connection between the differential transmission line (TRL) and the respective coupling element (TLL) to a first (Ri) or a second value (R2), the second value (R2) being smaller than the first value (R1), by controlling a respective second switch (2).

17. An electromagnetic coupler arrangement according to claim 16, wherein the resistance is initially switched to the first value (R1) , for all coupling elements (TLL) of the array.

18. An electromagnetic coupler arrangement according to claim 17, further comprising a feedback loop (3a) from the harvester to the second switch, wherein the resistance is switched from the first to the second value when the DC output voltage provided by the harvester is below a second reference DC voltage, the second reference DC voltage being lower than the first reference DC voltage.

19. An electromagnetic coupler arrangement according to any of claims 16 to 18, wherein the feedback from the harvester (H) to the first (5) and the second (2) switch is provided by means of a window comparator circuit.

20. An electromagnetic coupler arrangement according to any of claims 16 to 19, wherein the switchable resistor element comprises a first resistor (R1) with the first resistance value and a second resistor (R2) with the second resistance value that are arranged in parallel so as to be alternatively switched into the electric path between the differential transmission line (TRL) and the respective coupler element (TLL) by the respective second switch (2).

21. An electromagnetic coupler arrangement according to claim 20, wherein each of the harvester elements being permanently coupled to the differential transmission line (TRL) via an electric connection with the first resistance value (R1).

22. An RFID printer / encoder comprising an electromagnetic coupler arrangement (1 , 1a) according to any of claims 1 to 21.

23. A method of operating a wireless electromagnetic coupler arrangement (1 , 1a) according to any of claims 1 to 21 , comprising the steps of initially controlling (S10) the respective first switches (5) via which each of the coupling elements (TLL) is connected to the differential transmission line (TRL) to be in a state in which the coupling element (TLL) is coupled to the differential transmission line (TRL) for feeding electromagnetic energy, entering a sensing state with feeding (S20) an unmodulated RF carrier wave of electromagnetic energy to the differential transmission line (TRL), switching (S30, S50) the first switch (5) connected to a first coupling element in the array to the state for bypassing the first coupling element in the array, when the DC output voltage provided by the harvester element (H) exceeds the first reference DC voltage (S40: Y), repeating (S60, S70: Y) said switching step (S50) for subsequent coupling elements (TLL) of the sequential array, until a coupling element has been reached, for which the DC output voltage provided by the harvester element (H) remains below the first reference DC voltage (S40: N), and entering a communication state with feeding (S80) a modulated RF carrier electromagnetic wave as a communication signal to the differential transmission line (TRL), after a coupling element has been reached for which the respective first switch (5) remains in the state connecting said coupling element, so as to encode (S90) an RF terminated planar metallic trace via said coupling element.

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