Communications: electrical – Condition responsive indicating system – Specific condition
Patent
1996-12-16
1998-04-14
Swarthout, Brent A.
Communications: electrical
Condition responsive indicating system
Specific condition
340572, 324309, G08B 1324
Patent
active
057397520
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention refers to an improved method in detecting magnetic elements with a high magneto-mechanical coupling factor. In detecting many such elements, which exist in certain predetermined arrangements, a complicated detection method is carried out.
DESCRIPTION OF THE PRIOR ART
In previous patent specifications it has been suggested to use heterogenous bias fields to separate identical sets of elements located at different places within an interrogation zone. Regardless of how the elements are configured in order to provide each set with a certain code, a problem exists when it comes to rapidly linking together signals from individual elements into a group, corresponding to a label or the like.
WO-A-93/14478 discloses a method and a device for detecting objects in an interrogation zone. Each object is provided with a label, comprising a set of magnetic elements arranged in a predetermined code configuration so as to provide the label with an identity. The magnetic properties of the elements are determined by exciting the elements to oscillation and detecting the resonance frequency of each element. By exposing the interrogation zone with a plurality of different heterogeneous magnetic bias fields, it is possible to detect and separate all labels present in the interrogation zone. This is true also for labels with identical element code configuration, since the nominal values of the element resonance frequencies are offset to different extents thanks to the heterogeneous magnetic bias fields. If the number of possible element codes is large and/or if a large number of labels are present in the interrogation zone, many different bias fields have to be generated in order to completely and accurately detect all the labels.
BRIEF SUMMARY OF THE INVENTION
An object with the present invention is to render the detection of magnetic elements more effective by means of a number of preparatory measurements. This object is obtained by the method according to claim 1. Further objects and advantages are apparent from the following description and claims.
DESCRIPTION OF THE DRAWINGS
In the accompanying drawings,
FIG. 1 is a graph showing the frequency response variation in relation to the magnitude of the applied bias field for different angles between the element and the magnetic field,
FIG. 2 is a graph showing the maximum value of H.sub..alpha.min in relation to the frequency,
FIG. 3 is a schematic view of the positions for three elements,
FIG. 4 is a graph showing the frequency variation as a function of
FIG. 5 is a graph showing the theoretical frequency response from element number 2.
DETAILED DESCRIPTION OF THE INVENTION
In order to facilitate a following detection of magnetic elements a series of settings for the magnetic bias field is initially carried out and followed by the detection of signals generated by the elements in the interrogation zone. Two series of settings for the bias field, the first of which having a constant bias field in any direction and the second of which having a bias field oriented in a particular direction with a gradient in any direction, aim at reducing the infinite number of possible positions for the elements to a finite number. A third series of bias fields aims at finding the exact number of elements in the interrogation zone, either by elimination of such positions, where there are no elements, or by separating the frequency response from a hidden element.
An element may be hidden, if for each bias field it responds at the same frequency as another element does. Theoretically, this is a very rare situation, but practically it is all the more frequent, as the frequency resolution of the electronic circuitry is poor. It has been found, that when two resonance frequencies are approaching each other, one of them suddenly disappears, before the two frequencies are equal. One solution to avoid hidden elements is therefore to increase the frequency resolution.
The first two series of bias fields are absolutely necessary and use a set of very differ
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RSO Corporation N.V.
Swarthout Brent A.
Trieu Vam T.
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