A METHOD AND A SYSTEM FOR REMOTE DETECTION OF MARKERS

Abstract: A method and a system is provided for remote detection of markers (10), each marker comprising at least two magnetic elements (13) arranged in a predetermined relationship providing an identity of the marker, by either exciting a respective magnetic element (13) to resonate mechanically or by exciting an electrical resonant circuit (14), to which the respective magnetic element (13) is coupled, to oscillate electrically. A resonant frequency (f?res?) of the respective magnetic element or of the electrical resonant circuit depends on an applied magnetic field (H), which is given a varying orientation. A corresponding variation in the resonant frequency (f?res?) is monitored, and an extreme value (f?min local?) of the variation is detected. A momentary orientation ( alpha ?min local?) of the magnetic field is determined in response to the detection of the extreme value, and an orientation of the respective element (13) is determined from this momentary field orientation.

Description: A METHOD AND A SYSTEM FOR REMOTE DETECTION OF MARKERS Technical Field The present invention relates to a method and a system for remote detection of markers, each marker comprising at least two magnetic elements arranged in a predetermined relationship providing an identity of the marker, by either exciting a respective magnetic element to resonate mechanically or exciting an electrical resonant circuit, to which the respective magnetic element is coupled, to oscillate electrically, wherein a resonant frequency of the respective magnetic element or of the electrical resonant circuit depends on an applied magnetic field. Description of the Prior Art Many applications require a reliable and contactless detection of the presence, identity or position of objects. Common examples of such applications are for instance price labelling of commercial articles or identification of object type in production lines or at recycling plants. A marker or label is provided with a number of magnetic sensor elements arranged in predetermined angular relation- ships or at predetermined distances from each other. A method of detecting such markers is previously known from WO88/01427, wherein an article detection system is provided with excitation means for exciting the magnetic elements to resonate mechanically and detection means for detecting a resonant frequency of a magnetic signal generated by the respective resonating magnetic element. The magnetic elements are formed as magnetostrictive strips or ribbons made of an amorphous ferromagnetic material. The resonant frequency of each strip depends on the length and mass of the strip. Furthermore, due to a particular property of the amorphous material, known as the delta-E effect, the reso- nant frequency of each element is also dependent on the magnetic field strength or flux intensity along the main (longitudinal) direction of the element. This material property is used according to WO88/01427 to allow simul- taneous detection of several identical markers. A coil device is arranged to generate a heterogeneous magnetic bias field, wherein any two markers present at the same time in the detection zone will be exposed to different magnetic field strengths and will therefore also exhibit different resonant frequencies for the magnetic elements comprised in the markers. 093/14478 discloses a similar system, wherein, however, the markers are provided with one or more than one electrical resonant circuit, each of which is inductively coupled to a respective magnetic sensor element. The relative permeability μ r of the magnetic element is affected by the heterogeneous magnetic field, and due to the inductive coupling between the magnetic element and the resonant circuit, also the resonant frequency of the resonant cir- cuit is affected by the heterogeneous magnetic field. The markers of 093/14478 are excited and detected by electromagnetic or magnetic signals. The magnetic elements of each marker are arranged to represent a specific information entity, such as an identi- ty of each marker, an article number, etc. For markers where the magnetic elements are arranged in angular relationships with respect to each other, the information is represented by the respective angles between pairs of magnetic elements, and for markers where the magnetic elements are arranged in parallel to each other, with fixed distances between adjacent elements, the information is represented by the distances. Additional information may be represented by using elements of different types, e.g. of different lengths or with different masses . One way of ob- taining magnetic sensor elements with different masses, and consequently different resonant frequencies, is known from 095/29534. A method of simultaneously detecting several markers is known from W095/29467. The markers, and the magnetic elements comprised therein, are exposed to a sequence of different magnetic field situations. The resonant frequencies of all elements present in the detection zone are detected for every magnetic field situation, and the component of the magnetic field vector along the longitudinal direction of each element is determined from the corresponding detected resonant frequency. All possible combinations of angular relations between pairs of elements of a marker are determined, and for a combination, resulting magnetic vectors are computed from different pairs of the determined components of a particular magnetic field situation, using the angular relations of the particular combination. Any combination not having identical resulting magnetic vectors computed for different pairs of elements is eliminated, and the procedure is repeated until only those combinations remain, which correspond to actual element combinations of markers within the detection zone. The method of 095/29467 is advantageous in that it is capable of processing and identifying several identical markers present simultaneously in the detection zone. How- ever, if the total number of possible code values (the number of different angular element configurations) is large, the total number of computations that have to be carried out grows extremely large, and the identification process may have to be continued for a considerable time, until all elements present in the detection zone have been correctly identified. Therefore, there is a need for a faster detection method and system, which may identify the markers more quickly. Summary of the Invention It is a purpose of the present invention to provide more rapid detection of markers for remote detection of objects. The purpose is achieved by a method and a system according to the appended independent patent claims. In essence, the purpose has been achieved by the understanding that a marker may be identified very quickly by exposing the marker to a rotating magnetic field, the magnetic field vector of which rotates 360°, while continuously monitoring a variation in the resonant frequencies originating from the respective magnetic elements, so as to detect the moment at which the resonant frequency variation reaches an extreme value, such as a minimum value. The actual orientation of the magnetic field vector is registered at the moment when this extreme value is detected, and the orientation of the respective magnetic element is determined from the momentary magnetic field orientation. In summary, as the magnetic field vector is rotated 360°, the field vector will become more and more aligned with a first one of the elements of the marker and will subsequently become parallel to this element, wherein the extreme value is detected and the orientation of the first element is determined. As the field vector is rotated further, it will become less and less aligned with the first magnetic element but instead become more and more aligned with a second magnetic element. The orientation of the second magnetic element is determined when the magnetic field vector is in alignment with the second element, and the procedure is repeated for the remaining elements of the marker. Once the correct element orientations have been determined, the type of each element may be determined according to a preferred embodiment during a second step, wherein a magnetic field with varying field strength is applied along each determined element orientation so as to detect a global minimum/maximum resonant frequency of each element and determine the type thereof by comparing this resonant frequency to a set of prestored' data . Other purposes, features and advantages of the present invention appear from the following detailed dis- closure, from the drawings as well as from the dependent claims . Brief Description of the Drawings The present invention will now be described in more detail, reference being made to the accompanying drawings, in which: FIG 1 is a schematic view of a remote detection system according to the present invention, FIG 2 is a schematic illustration of the method according to the invention, FIG 3 is a frequency diagram illustrating the method of the present invention, FIG 4 illustrates a second step of the method according to a preferred embodiment of the invention and FIG 5 is a frequency diagram illustrating the second step of FIG 4. Detailed Disclosure of the Invention FIG 1 illustrates a detection system according to an illustrative embodiment of the present invention. The detection system comprises excitation means 15, detection means 16, both of which are operatively connected to a system controller 17, and a magnetic field generating means 18. A marker 10 is provided with four magnetic sensor ele- ments 13 arranged in an angular configuration. In the illustrated embodiment these magnetic elements are of any magnetoelastical (magnetostrictive) type known per se, e.g. formed as thin strips, ribbons or wires made from an amorphous metal alloy with certain magnetic properties. How- ever, the invention is equally applicable to markers com- prising electrical resonant circuits, to which a respective magnetic element 13 is coupled. Markers of these two types are described in e.g. O88/01427 and 093/14478, which are referred to above and are incorporated herein by reference. The individual magnetic elements 13a-d (FIG 2) are of different types and exhibit different resonant frequencies. The different types may be selected among elements of different lengths and/or elements of different masses, shapes, etc . The excitation means is arranged to generate magnetic or electromagnetic excitation signals, which force the magnetic elements 13 into a state of mechanical resonance. The magnetic signals generated by the resonating magnetic elements 13 are detected by the detection means 16 and supp- lied to the system controller 17. Furthermore, the magnetic field generating means 18 is arranged to produce a varying magnetic field H, as will be described in more detail below. In response to the variations of the magnetic field H, the material properties of the magnetic elements 13 will cause a corresponding variation in the resonant frequency f res of the respective elements 13. The system controller 17 is arranged to drive the magnetic field generating means 18 to produce a magnetic field with rotating field vector. Preferably, the field vector is rotated 360°. The rotation of the magnetic field vector is illustrated in FIG 2, and the resulting variation in resonant frequency f res of a respective magnetic element 13 is illustrated in FIG 3. As the field vector of the magnetic field H begins to rotate from 0° towards 360°, the field vector will first become more and more aligned with a first magnetic element 13a of the marker 10. As the field vector H becomes more and more aligned with this magnetic element, the resonant frequency f res thereof will decrease in value (illustrated by an arrow I in FIG 3) , since the projection of the magnetic field vector H along the longi- tudinal direction of the first magnetic element 13a becomes larger and larger. At a certain angle of rotation a min . local the projection of the field vector H will reach maximum (ideally, fully parallel with the first magnetic element 13a) , wherein the resonant frequency f res reaches a minimum value f min local - A further rotation of the field vector H will make it less perfectly aligned with the first element 13a, wherein the resonant frequency f res starts to increase, as illustrated by an arrow II in FIG 3. The moment the re- sonant frequency reaches its local minimum point f min local , the momentary angular rotation α min local of the field vector H is registered. Since the local minimum resonant frequency will occur when the field vector H and the first magnetic element 13a are parallel, the orientation of the element 13a is directly obtained from the momentary orientation of the field vector H, i.e. min local . As the field vector H is continuously rotated, it will become less and less aligned with the first element 13a but more and more aligned with the second element 13b. At a certain angular rotation, the field vector H will become parallel to the second magnetic element 13b, wherein the orientation thereof is determined in similarity to the above. Subsequently, a corresponding determination is made for the orientation of the third and fourth magnetic ele- ments 13c and 13d, respectively. In a preferred embodiment of the present invention the marker 10 is provided with magnetic elements 13a-d of different types, i.e. exhibiting resonant frequencies within different (but possibly overlapping) frequency ranges. For such a marker, once the orientations of the respective elements 13a-d has been determined as set out above, the type of each magnetic element 13a-d is determined as follows. For each element orientation determined above the system controller 17 operates the generating means 18 to produce a magnetic field H with varying strength along the respective orientation, as illustrated in FIGs 4 and 5. As the magnetic field strength along the longitudinal direction of a respective magnetic element 13a-d is varied, the resonant frequency thereof will de- crease and eventually reach a global minimum value f mingιobaι at a magnetic field strength H fmin global . The global minimum resonant frequency is directly related to a respective element type, and the mapping between a global minimum resonant frequency and a particular element type is represen- ted by a prestored set of reference data in a storage device operatively connected to the system controller 17. Once the global minimum resonant frequency f min global has been determined, the controller 17 will compare this value to the prestored set of reference data and select the parti- cular element type, for which the corresponding frequency value or range matches the detected global minimum resonant frequency. For some marker types it may be possible to detect a global maximum of the resonant frequency instead of its global minimum. Thanks to the present invention, markers representing a respective code value among a very large number of possible code values may be accurately and rapidly detected. For instance, for a marker like the one illustrated in the drawings, comprising four different magnetic elements, each of which is selected among four different types and is arranged in any of 15 possible angular positions, the total number of different code values is 327,600. If the four elements are selected among a total of 10 different types, each marker may represent one of 13,628,160 different code values for the same number of different angular positions. It is possible within the scope of the present invention to detect more than one marker simultaneously, even if the elements are identically oriented on the markers . For simultaneously detecting e.g. two markers, one of which is rotated at a certain angle relative to the other, it is possible to differentiate the two markers by considering the sequence of absolute angular position values obtained for all the elements of the two markers. Assuming that the respective first elements of the two markers have a 0° ori- entation with respect to a given reference orientation, while the second elements have a 10° orientation and the third elements have a 25° orientation, this exemplary sequence of absolute angular position values may be obtained during the rotation of the magnetic field: {0°, 7°, 10°, 17°, 25°, 32°, ...} It is concluded from the above sequence that every second value is displaced by 7° from the preceding value. This implies that one of the markers is displaced by 7° relative to the other, thereby allowing the two markers to be differentiated from each other. If, on the other hand, the two markers are fully parallel, they may nevertheless be individually identified by applying a field gradient to the rotating magnetic field. Hence, the rotating field vector is not uniformly strong but of increasing magnitude, wherein the two markers will exhibit different variations in resonant frequency, when exposed to the rotating magnetic field. The invention has been described above with reference to an illustrative embodiment for exemplifying but not limiting purposes. As will be readily realized by a man skilled in the art, other embodiments than the one disclosed herein are possible within the scope of the invention, as defined by the appended independent patent claims. For instance, in a situation where the plane in which the marker elements are oriented is substantially different from the plane of rotation of the magnetic field, it may be advisable to expose the marker to a sequence of two, or even more, different rotating magnetic fields, the rotational planes of which are non-parallel (preferably: or- thogonal) , so as to ascertain an accurate determination of the element orientations.

Claims: CLAIMS 1. A method for remote detection of markers (10), each marker comprising at least two magnetic elements (13) arranged in a predetermined relationship providing an identity of the marker, by either exciting a respective magnetic element (13) to resonate mechanically or exciting an electrical resonant circuit, to which the respective magnetic element (13) is coupled, to oscillate electrically, a resonant frequency (f res ) of the respective magnetic element or of the electrical resonant circuit being dependent on an applied magnetic field (H) , c h a r a c t e r i z e d in that the magnetic field (H) is given a varying orientation, a corresponding variation in the resonant frequency (f res ) is monitored, an extreme value ( f min local ) of the variation is detected, a momentary orientation ( min local ) of the magnetic field is determined in response to the detection of said extreme value and an orientation of the respective element (13) is determined from said momentary field orientation. 2. A method according to claim 1, wherein the momentary field orientation is essentially parallel to a longitudinal axis of the respective magnetic element (13) . 3. A method according to claim 1 or 2 , wherein the orientation of the respective element (13) is determined as being essentially parallel to the determined momentary field orientation. 4. A method according to any preceding claim, wherein said extreme value is a maximum or minimum value . 5. A method according to any preceding claim, further comprising the steps of determining, after the determination of the respective element orientation, a type of the respective element (13) by: generating a magnetic field (H) with varying field strength along a respective determined element orientation, detecting a minimum or maximum value of the resonant frequency of the respective element (13), comparing said detected resonant frequency minimum or maximum value with a prestored set of reference data relating a plurality of frequency values or ranges to respec- tive element types, and selecting a particular one of the prestored element types, for which the related prestored frequency value or range matches the detected resonant frequency minimum or maximum value . 6. A method according to any preceding claim, wherein the magnetic elements (13) comprise an amorphous metal alloy. 7. A method according to any preceding claim, wherein the magnetic elements (13) are formed as strips, ribbons or wires . 8. A method according to any preceding claim, wherein at least two of the magnetic elements (13) are arranged in a mutual angular relationship with respect to a longitudinal direction of said elements (13) . 9. A method according to any preceding claim, wherein at least two of the magnetic elements (13) are arranged with a mutual distance relationship. 10. A system for remote detection of markers (10), each marker comprising at least two magnetic elements (13) arranged in a predetermined relationship, the system comprising excitation means (15, 17) , detection means (16, 17) and magnetic field generating means (18) and being arranged to detect a resonant frequency (f res ) related to the respective magnetic element and dependent on a magnetic field (H) generated by the magnetic field generating means, c h a r a c t e r i z e d in that the magnetic field generating means (18) is arranged to generate a rotating magnetic field (H) , and in that the detection means (16, 17) is arranged to detect an extreme value (f min loca χ) of a variation in the resonant frequency (f res ) caused by the rotating magnetic field (H) and to determine, from a momentary orientation ( miπ Iocal ) of the rotating magnetic field (H) , an orientation of the respective magnetic element (13) . 11. A system according to claim 10, wherein the magnetic field generating means (18) is furthermore ar- ranged to generate a magnetic field with varying field strength for determining a respective type of the respective magnetic element (13) .

Inventors: TYREN CARL, TYREN, CARL
Applicants: RSO CORP, TYREN CARL, RSO CORPORATION N.V., TYREN, CARL
Application year: 1999
Main classification: G06K7
IPC codes: G01B7, G01D5, G01V3, G06K7, G06K19, G08B13