Electricity: measuring and testing – Magnetic – With means to create magnetic field to test material
Patent
1986-05-20
1989-01-17
Eisenzopf, Reinhard J.
Electricity: measuring and testing
Magnetic
With means to create magnetic field to test material
324237, 324240, G01N 2782, G01N 2790, G01R 3312
Patent
active
047990112
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the detection, by eddy currents, of typical defects on a product in relative movement with a detector, and more particularly to the detection of cracks on a slab or like product.
A slab as produced in a continuous casting installation may present on its surface, along its edges, defects in the form of cracks which it is important to locate before the subsequent treatment of the product.
2. Description of the Related Art
For detecting surface defects on a metal product in relative movement with a detector, it is known to use a process of detection employing eddy currents, process of the type whereby an alternating magnetic field is generated in order to provoke the circulation of eddy currents on the surface of the product, a signal representative of the variations of the eddy currents is collected by means of a differential sensor placed in the vicinity of the surface of the product, and the collected signal is demodulated by projection along an angle of adjustable phase in order to detect the disturbances introduced by the defects to be detected.
Detection of cracks on a slab by a process of this type is particularly difficult. In fact, in addition to the unfavourable operational conditions (particularly the temperature), there are numerous sources of noise, particularly the presence of surface irregularities (waviness or wrinkle formation), edge effects, random variations of the distance between sensor and product, etc . . .
SUMMARY OF THE INVENTION
Despite such unfavourable conditions, which result in a low signal
oise ratio, it is an object of the present invention to provide a reliable, automatic process of detection, particularly adapted to recognize typical defects such as cracks on a slab in relative movement with a detector.
This object is attained thanks to a process of the above-mentioned type whereby, according to the invention, for the whole duration of detection, the angle of phase for which the projection of the collected signal has a minimum mean value, is automatically and periodically determined and the projection of the collected signal is effected along said angle of phase in order to eliminate disturbances of the collected signal due to repetitive surface irregularities; the component of the collected signal obtained by said projection is converted into digital form; the digital signal thus obtained is filtered by means of a digital filter whose characteristics are predetermined as a function of a type of defect to be detected; and a defect detection signal is generated when the output signal of the digital filter exceeds a predetermined threshold.
The reliability of detection effected by this process is the consequence of a considerable improvement in the signal
oise ratio due to the combination of the following elements:
use of a differential sensor with which the parasitic signals in common mode are eliminated;
control of the angle of phase of projection of the collected signal in order to eliminate the influence of repetitive surface irregularities, such as waviness or wrinkle formation, which do not constitute defects proper and which are characterized by a particular angle of phase which can vary during the process of detection, and
selective digital filtering thanks to an adapted filter having characteristics predetermined as a function of a type of defect to be detected and of the differential sensor already mentioned.
The component of the collected signal obtained by projection along the determined angle of phase is converted into digital form with a sampling frequency controlled by the speed of advance of the product.
The digital signal obtained is thus independent of possible variations in the speed of advance of the product, this simplifying the design of the digital filter. Digital filtering is for example effected by correlation with a digital signal representing the pre-recorded signature of a particular type of defect to be detected.
It is a further object of the invention to pr
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Edmonds Warren S.
Eisenzopf Reinhard J.
Institut de Recherches de la Siderurgie Francaise--IRSID
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