Description of the Prior Art Many applications require a reliable and contactless detection of the presence, identity or position of objects within a detection zone. Common examples are for instance price labeling of commercial articles, identification of components in production lines, identification of material type at recycling plants or electronic article surveillance in e. g. shops.

For some applications it is sufficient to detect the presence of the object or article. One example is a simple electronic article surveillance system, which is arranged to provide an alarm signal, once a protected article is carried into a detection zone. Such a simple application uses one single sensor element in the form of a thin metal

strip or wire with magnetic properties. The sensor element may be detected magnetically by means of arc-shaped magnetic generators/detectors, which expose the sensor element to a alternating magnetic field, which affects a physical property of the sensor element. Use is often made of the fact that the alternating magnetic field causes a periodical switch of the magnetic momentum of dipole of the sensor element, which is also known as Barkhausen jumps.

Sensors of this kind are for instance disclosed in US-A-5 496 611, EP-A-0 710 923 and EP-A-0 716 393.

A different single-element sensor technology is described in W097/29463 and W097/29464, wherein each sensor comprises a wire-shaped element of amorphous or nano- crystalline metal alloy. An important feature of the amorphous or nano-crystalline metal alloy is that the permeability thereof may be controlled by an alternating magnetic modulating field. Through a physical effect known as Giant Magnetoimpedance, the amplitude of an electromag- netic reply signal from the sensor is modulated by the magnetic modulating field, when the sensor is excited by an electromagnetic interrogation signal. The modulation in amplitude in the reply signal is detected and used for determining the presence of the sensor in the detection zone.

None of the electronic article surveillance applica- tions described above provides a remotely detectable iden- tity for each sensor. However, for advanced applications it is necessary to provide such identity information, representing e. g. an article number, serial number, material code etc for the respective object, to which each sensor is attached. Such sensors or markers are disclosed in W088/01427, wherein each sensor or marker is provided with a number of magnetostrictive strips or ribbons made of an amorphous ferromagnetic material and arranged in predetermined angular relationships or at predetermined

distances from each other. The identity of such a sensor is represented by the predetermined relationships as well as the respective type of individual sensor elements. The sensor elements are excitable to mechanical resonance by magnetic energy. The magnetic signals generated by the resonating sensor elements may be detected magnetically or inductively.

A similar system is described in W093/14478, wherein the sensors or markers are provided with one or more than one electrical resonant circuits, each of which is induc- tively coupled to a respective magnetic sensor element.

Each electrical resonant circuit is excited to oscillate electrically, and the resonant frequency thereof is con- trollable, through the permeability of the magnetic element, by an external magnetic field, wherein a simul- taneous detection of several identical sensors is possible.

In summary, prior art sensors for remote detection of objects are either of a single-element type, allowing only the presence of each sensor to be detected, or of a multi- element type, allowing also an identity of each sensor to be detected. Single-element sensors are easier to design and produce and therefore have a lower unit cost. On the other hand, multi-element sensors require a supporting carrier (particularly for mechanically resonating sensor elements) and/or capacitive and inductive components (for the electric resonant circuit versions). Naturally, this implies a higher cost per unit. Additionally, since the multi-element sensors described above mainly operate by a magnetic or inductive link, the operating distance of the detection system is quire narrow.

Summary of the Invention An objective of the present invention is to provide a sensor for remote detection of objects, which is capable of representing an identity of the sensor, or of the object to

which the sensor is attached, at a substantially lower cost than the prior art sensors. A subsidiary objective of the present invention is to provide a sensor, the operating distance of which is far better than that of the prior art multi-element sensors described above.

The objectives are achieved by a method for remote detection of objects, wherein each object is provided with a sensor comprising at least two magnetic elements arranged in a predetermined mutual relationship representing an identity of the sensor, wherein electromagnetic signals are generated for exciting the sensor elements to produce electromagnetic reply signals and wherein an amplitude of the electromagnetic reply signal from each sensor element is modulated by a first magnetic field having a magnitude- variant and a magnitude-invariant component.

The method further comprises the steps of generating a second magnetic field with rotating field vector; detec- ting a frequency shift in a component of said reply signal occurring when a magnitude-invariant component of said second magnetic field balances the magnitude-invariant component of said first magnetic field, wherein the respective sensor element is momentarily exposed to a resulting magnetic field with essentially no magnitude- invariant component; and determining an orientation of the respective sensor element from the orientation of the magnitude-invariant component of said second magnetic field, when said frequency shift occurs.

Furthermore, the objectives are achieved by a sensor for remote detection of objects, comprising at least two magnetic sensors elements, which are arranged in a mutual relationship selected from a set of predetermined relation- ships and representing an identity of the sensor, or of an object to which the sensor is attached, said sensor ele- ments being electromagnetically detectable and comprising a magnetic material, the permeability of which is control-

lable by a magnetic field and the high-frequency impedance of which depends on said permeability.

Other objectives, features and advantages of the present invention appear from the following detailed dis- closure, from the drawings as well from the appended patent claims.

Brief Description of the Drawings A preferred embodiment of the present invention will now be described with reference to the accompanying drawings, in which: FIG 1 illustrates a system for remote detection of objects, in which the method and sensor according to the present invention may be applied, FIG 2 is a diagram illustrating a frequency shift effect, which is used in the method according to the present invention, and FIGs 3 and 4 are vector diagrams illustrating the principle of detection according to the present invention.

Detailed Disclosure of the Invention FIG 1 illustrates an exemplary embodiment of a system for remote detection of objects, in which a sensor according to one embodiment of the present invention is used. A transmitter antenna 11 and a receiver antenna 12 are arranged in a detection zone 10. The transmitter antenna 11 is operatively connected to an output stage 13, which in turn is connected to a controller 14. The output stage comprises various commercially available driving and amplifying circuits and means for generating an alternating electric current of high frequency fHF, said current flowing back and forth through the transmitter antenna 11 when supplied thereto, wherein a high-frequency electromagnetic field is generated around the transmitter antenna. This electromagnetic field is used, as will be

described in more detail below, for exciting a sensor present in the detection zone 10, so that the sensor will transmit, at the reception of electromagnetic energy from the transmitter antenna 11, an electromagnetic reply signal, which is received by the receiver antenna 12.

The receiver antenna 12 is operatively connected to an input stage 15, which comprises conventional means with amplifying and signal processing functions, such as band- pass filtering and amplifying circuits. The input stage 15 also comprises means for demodulating the electromagnetic reply signal and supplying it to the controller 14.

The transmitter antenna 11 as well as the receiver antenna 12 thus have the purpose of converting, in a known way, between an electrical signal of high frequency and an electromagnetic signal. Preferably, the antennas are helically formed antennas with rotating polarization (for optimal coverage in all directions), or alternatively conventional end-fed or center-fed halfwave whip antennas, but other known antenna types are equally possible.

The detection zone 10 is additionally provided with means 16 for generating a first magnetic field Hmode The means 16 is connected to the controller 14 via a driving stage 17. The driving stage 17 comprises means for generating a modulating current, which is supplied to the means 16, wherein a magnetic modulating field Hmod is gene- rated in essential portions of the detection zone 10. The magnetic modulating field Hmod preferably has a frequency of about 500-800 Hz, and the electromagnetic excitation and reply signals preferably has a frequency within the GHz band, such as 1.3 GHz or 2.45 GHz.

An object 20, which has been schematically illustra- ted in FIG 1 in the form of a box-shaped package, is provi- ded with a sensor according to the invention, comprising at least two magnetic sensor elements, which are arranged in a mutual relationship and represent an identity of the sen-

sor, or of the object 20, to which the sensor is attached.

The sensor elements are electromagnetically detectable and comprise a magnetic material, the permeability of which is controllable by a magnetic field and the high-frequency impedance of which depends on said permeability.

According to a preferred embodiment, the material of the sensor elements are essentially identical to the ones described in the abovementioned W097/29463 and W097/29464, both of which are fully incorporated herein by reference.

In other words, in the preferred embodiment the sensor elements are made from a cobalt-rich amorphous metal alloy, such as (FeO. gSigB. The sensor elements of the preferred embodiment are formed as metal wires with a length of about 5-100 mm and a typical transversal diameter of between 7 and 55 Hm. Furthermore, the wires are provided with a thin coating of glass or another dielectric material, the thickness of which is preferably less than the thickness (diameter) of the metal wire core. Such a wire is commonly referred to as a microwire and is produced by rapidly pulling a molten metal alloy and a surrounding molten glass tube.

According to other embodiments, the material of the sensor elements may be nanocrystalline rather than amorphous. Furthermore, the glass coating may be dispensed with, and the thickness (transversal diameter) may be larger than for the preferred embodiment. Transversal diameters of between 100 and 200 pm have proven useful, particularly about 125 m. However, such wires are not referred to as microwires and are produced in other ways than the one mentioned above, as is well known per se in the technical field of magnetic sensor elements. In summary, the sensor of the present invention may comprise magnetic sensor elements of various kinds, as defined by the appended independent sensor claim.

According to the preferred embodiment the at least two sensor elements are arranged at a certain angle to each other. Preferably, one sensor element is arranged"on top" the other sensor element. The sensor elements may be mounted to a carrier, such as an adhesive label, or alter- natively attached directly to the related object, for in- stance by adhesion. A further alternative is to sew or weave the sensor elements into or onto e. g. an article of clothing or another article of merchandise. In such a case the identity of the sensor may represent an article class or type. Yet another alternative is to integrate the sensor elements into a packaging material, such as cardboard, paper or plastic film, or into an article of recycling (e. g. a plastic container, a glass bottle, a cardboard package, etc.). In such cases, the identity of the sensor may represent e. g. a type of material for each recycling article.

The identity of the sensor (or its related object) is provided by the value of the angular deviation between the sensor elements. When assembling the sensor, the sensor elements are arranged according to one specific predeter- mined orientation selected from a set of such predetermined orientations.

A second magnetic field generating means 18 is arranged in the detection zone 10 for generating a second magnetic field Hbias This second magnetic field is magnitude-invariant and is given a rotating magnetic field vector (i. e. the magnetic field has a field strength, which does not vary with time but which changes in direction).

As previously mentioned, the Giant Magnetoimpedance effect causes a modulation in amplitude of the electro- magnetic reply signal transmitted from the sensor and received by the receiver antenna 12. For high frequency signals the Skin Depth of the wire (sensor element) material controls the impedance of the wire. The Skin Depth

is controlled by the magnetic permeability of the wire, and the magnetic permeability is in turn controlled by the magnitude of the magnetic field surrounding the sensor in the detection zone 10. The magnetic modulating field Hmod generated by the means 16 has a magnitude-variant ("AC") as well as a magnitude-invariant ("DC") component. Basically, the frequency of the amplitude-modulation of the electro- magnetic reply signal is determined by the AC component, and the phase of the modulation is determined by positive or negative DC level.

In FIG 2 the impedance of the sensor element at high frequencies is illustrated as a function of the magnetic modulating field. The rightmost vertical sinewave illust- rates the normal situation, where the magnetic modulating field Hmoa has an AC as well as a DC component. The AC component of Hmod has a frequency fmOd. As illustrated by the uppermost horizontal sinewave, the resulting variations in the impedance of the sensor element have the same frequency fmOd. The amplitude of the electromagnetic reply signal transmitted from the sensor will follow these variations in impedance, and therefore the reply signal will be modulated <BR> <BR> <BR> in amplitude by the varying magnetic modulating field H.

The demodulated reply signal will thus contain a signal of frequenCY fmOd- However, as illustrated by the leftmost vertical sinewave in FIG 2, if the magnetic modulating field has no DC component (magnitude-invariant component), a frequency doubling effect will occur in the amplitude modulation of the electromagnetic reply signal from the sensor element, since the magnetoimpedance effect is insensitive to sign (i. e., direction of the magnetic modulating field). The resulting frequency doubling is illustrated by the lower- most horizontal halfwave, which has a frequency of 2-for.

Consequently, the demodulated signal will have a frequency,

which is twice as high as the frequency fmOd of the magnetic modulating field H.

The frequency doubling effect described above provides a distinct signal feature, which is used according to the present invention for detecting also the identity of the sensor. Normally, at least a portion of the magnitude- invariant component of the magnetic modulating field in the detection zone 10 originates from the magnetic field of the earth and/or other magnetic material sources present in the detection zone.

As illustrated in FIG 3, by adding the rotating magnetic field vector of the second magnetic field Hbias/ which is generated by the second magnetic field generating means 18, the direction or orientation of each sensor element may be determined. A prerequisite for a successful detection of the sensor element angle is that the magnitude <BR> <BR> <BR> <BR> of the rotating field vector of the magnetic field Hbias is equal to or greater than the magnitude-invariant component <BR> <BR> <BR> <BR> of the magnetic modulating field Hmod Initially, in absence of the second magnetic field, each sensor element is exposed to a projection of the DC component of the first magnetic field Hmod along a longitudinal direction of the sensor element. As the magnitude-invariant field vector of <BR> <BR> <BR> <BR> the second magnetic field Hbias is applied and rotated, each sensor element will also be exposed to a corresponding <BR> <BR> <BR> <BR> projection of the applied field vector of Hbias-Through the rotation of this field vector, the projection of the second <BR> <BR> <BR> <BR> field vector Hbias will sooner or later exactly balance the magnitude-invariant field vector of the first magnetic <BR> <BR> <BR> <BR> field Hmoa. thereby causing a frequency shift to 2-for<BR> <BR> <BR> <BR> <BR> <BR> <BR> (i. e., two times the modulating frequency of Hmod) as illustrated in FIG 2. Since the momentary orientation of the second magnetic field vector Hbias is known, the orientation of each sensor element may be determined, as will be further described below.

As illustrated in FIG 4, the resulting zero DC field condition will occur at symmetrical angles for a respective <BR> <BR> <BR> sensor element. If the rotating DC vector of Hbias is larger<BR> <BR> <BR> <BR> <BR> than the DC vector of Hmoa. the resulting magnetic DC field imposed on the sensor element will decrease to zero for a first time at an angle al and for a second time at an angle a2, where al = a2. Therefore, even if the orientation or <BR> <BR> <BR> magnitude of the DC vector of the first magnetic field Ho, osa is unknown, the orientation of the sensor element may be calculated by detecting the angles al and a2 and then, for reasons of symmetry, determining the orientation as being the center angle between al and a2. However, in the event <BR> <BR> <BR> that the DC vector of Hmod has exactly the same magnitude as<BR> <BR> <BR> <BR> <BR> the rotating DC vector of Hbias/the resulting zero DC field condition will occur only once, i. e. when the DC vector of <BR> <BR> <BR> Hbias is parallel to the sensor element, wherein the orien- tation of the sensor element is immediately obtained.

It is possible, within the scope of the invention, to perform the identification in several steps, wherein the <BR> <BR> <BR> magnitude of Hbias is changed between each full rotation of the field. In this way the identification of both sensor elements is facilitated.

The present invention has been described above by way of a few exemplary embodiments. However, other embodiments than the ones described above are possible within the scope of the invention, as defined by the appended independent patent claims.