Electricity: measuring and testing – Magnetic – With means to create magnetic field to test material
Patent
1996-12-16
1998-06-02
Snow, Walter E.
Electricity: measuring and testing
Magnetic
With means to create magnetic field to test material
324232, 340551, 340572, G08B 1324, G08B 1318, G01R 3312
Patent
active
057605807
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
Elements made of a material, the magnetic or magneto-mechanical properties of which depend on and are influenced by external magnetic field(s), may be detected by means of magnetic detection means. The magnetic and mechanical properties of such elements are changed, when the elements are exposed to a so-called bias field. A common way of obtaining a signal from a basically passive element is to excite the element to mechanical resonance. The self-oscillation of such a resonance element is magnetically detectable.
DESCRIPTION OF THE PRIOR ART
In e.g. WO 88/01427, a method and device for automatic and remote identification of objects are disclosed and described. The objects are provided with labels, comprising tapes of a material with a high magneto-mechanical coupling. The tapes are mechanically movable and are forced into mechanical self-oscillation by a magnetic signal from an excitation means.
When the magnetic signal emitted from the tapes or the resonance elements according to WO 88/01427 is going to be recorded, a problem arises. The signal from the resonance elements exists simultaneously with, or in immediate connection with, the exciting signal. Furthermore, the signals from the resonance elements have the same frequency. As a result, the normally considerably stronger excitation signal completely hides a simultaneously incoming resonance element signal, which is intended to be used for detection and which is considerably weaker.
Various systems have been suggested for the purpose of avoiding the above-mentioned problem. A common way is to excite the resonance elements at their fundamental frequency, but to detect them at a harmonic frequency. Alternatively, the excitation signal may be the total of two signals at completely different frequencies.
A drawback with the solutions described above is that harmonics are generally considerably weaker than the fundamental and that the frequency relationship between the fundamental and a harmonic is complicated, especially in the case of magneto-elastical elements.
BRIEF SUMMARY OF THE INVENTION
An object with the present invention is to substantially remove the above-mentioned problem. According to the invention, a signal from the resonance elements is detected at the same frequency as the excitation signal and in immediate connection with the emission of the excitation signal. The above is possible due to the fact that the detection signal is recorded symmetrically at both sides of a symmetry plane in an interrogation zone. Due to the fact that the excitation signal is generated by two excitation means located on different sides of the symmetry plane, the symmetrically recorded detection signal completely lacks influence from the excitation signal.
BRIEF DESCRIPTION OF THE DRAWING
The DRAWING schematically shows an arrangement for excitation and detection of magnetic resonance elements according to the invention.
DETAILED DESCRIPTION
In a preferred embodiment, two excitation means 12 are connected to an electronic controller 15 through two driver and amplifier units 16A and 16B. The excitation means 12A and 12B are realized as magnetic coils, which are parallelly and coaxially arranged with respect to each other. Consequently, a symmetry plane 14 is obtained between the excitation coils with regard to the excitation field generated by the excitation coils. If the signals from both the excitation coils are of equal strength, the symmetry plane is obtained at half the distance between the coils. Two detection means, preferably in the form of magnetic coils, are symmetrically arranged with one means at each side of the symmetry plane. The detection coils 13A and 13B are operatively connected to a central electronic controller 15. In the embodiment shown, the detection coils 13A and 13B are connected to the amplifier units 17A and 17B, respectively, which in turn are connected to the controller 15.
By the use of the arrangement according to the drawing, identical excitation signals are generated in the two excitation coils
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RSO Corporation N.V.
Snow Walter E.
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