Electricity: measuring and testing – Magnetic – With means to create magnetic field to test material
Reexamination Certificate
1998-06-18
2001-05-08
Snow, Walter E. (Department: 2862)
Electricity: measuring and testing
Magnetic
With means to create magnetic field to test material
C324S225000, C324S202000, C235S449000, C194S213000
Reexamination Certificate
active
06229306
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method and an apparatus for measuring magnetic properties of sheet material such as bank notes.
2. Description of Related Art
Such an apparatus is known from DE 40 22 739. The apparatus has at least one measuring head with at least two pole shoes disposed so as to form a measuring gap. The magnetic signals produced by the magnetic properties of the sheet material at the measuring gap are converted into electric signals by a detector on at least one of the pole shoes.
In order to permit in particular magnetic particles with low remanence to be detected with sufficient reliability, one pole shoe is made of permanent-magnetic material and the other pole shoe of soft magnetic material. The magnetic field produced by the material of the permanent-magnetic pole shoe is selected to be so great that the soft magnetic material of the other pole shoe has a suitable magnetic operating point. This measure permits even low magnetic signals at the measuring gap to produce sufficient electric signals on the detector.
In particular bank notes generally have low magnetic particle densities in the area of the printed image and high ones e.g. in the area of the security thread, so that electric signals with low or high amplitudes arise on the detector during measurement of bank notes.
SUMMARY OF THE INVENTION
The problem of the invention is to propose a method for measuring magnetic properties of sheet material which delivers reliable signals at both low and high magnetic particle density.
The basic idea of the invention is substantially to amplify the electric signals produced by the detector of a measuring head in a certain signal range such that the lower-amplitude electric signals produced by the areas with low magnetic particle density are amplified to a greater extent than the higher-amplitude electric signals produced by the areas with high magnetic particle density. For this purpose the signal range is divided into at least two signal ranges which are each amplified constantly. The amplification in the range for lower-amplitude signals is greater than in the adjoining signal range.
The signal range is preferably divided into at least three ranges which are each amplified constantly. The amplification in the two outer ranges is selected to be equal, and the amplification in the middle range is selected to be greater than the amplification in the outer ranges. The electric signal is preferably shifted by a constant value before amplification so that the zero point of the signal range is in the middle of the middle range.
One advantage of the method is that the low-amplitude electric signals produced by the areas with low particle density are located in the middle range of the signal range and amplified to a great extent, while the high-amplitude electric signals produced by the areas with high particle density fall within the two outer ranges of the signal range and are amplified to a lesser extent than the low-amplitude electric signals. The amplitudes of the electric signals at the amplifier output are therefore distributed more uniformly over the output range of the amplifier and can then be optimally processed by a following analog-to-digital converter for example. Suitable shift of the electric signal from the detector permits amplification of both positive and negative amplitudes.
In an apparatus for carrying out the method for measuring magnetic properties of sheet material one can use for example a measuring head known from DE 40 22 739 for converting the magnetic properties of the sheet material into electric signals.
A preferred embodiment of an apparatus for carrying out the method has at least one measuring head with at least two pole shoes which are decoupled magnetically, each having a detector and preferably soft magnetic material. Further, means are provided for producing a time-invariant magnetic field perpendicular to the detector plane, the strength of the magnetic field being different on each detector. The strength of the time-invariant magnetic field is preferably selected so that the soft magnetic material of one pole shoe has a suitable magnetic operating point.
One advantage of the preferred embodiment is that the asymmetrically constructed measuring head causes the signals produced by the detectors and amplified to have different symmetries depending on whether soft magnetic particles with low remanence or hard magnetic particles with high remanence are measured. This permits differentiation of whether the measurement involves particles with low or high remanence.
In a further embodiment, means are provided in the measuring gap of the measuring head for producing a defined, time-variant, magnetic signal. One preferably provides an electric conductor through which a defined, time-variant, electric signal is conducted.
Using the electric signal measured at the amplifier output one can for example vary the amplification of the amplifier such that the electric signals measured at the amplifier output are adapted to a certain value. Alternatively, one can obtain an adaptation of the electric signals measured at the amplifier output to a certain value by suitably normalizing the electric signals. This procedure permits elimination of undesirable variations in amplification which arise e.g. from temperature influence or from wear of the measuring head.
Disposing a plurality of measuring heads in an array permits a plurality of tracks of the sheet material to be measured simultaneously.
REFERENCES:
patent: 3920960 (1975-11-01), Ditman
Schütt Lothar
Stein Dieter
Bacon & Thomas PLLC
Giesecke & Devrient GmbH
Snow Walter E.
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