GB2169080A

GB2169080A - Measurement system for the detection of defects in electrically conductive material

Info

Publication number: GB2169080A
Application number: GB08522491A
Authority: GB
Inventors: Federico Monti
Original assignee: Centro Sperimentale Metallurgico SpA
Current assignee: Centro Sperimentale Metallurgico SpA
Priority date: 1984-09-25
Filing date: 1985-09-11
Publication date: 1986-07-02
Also published as: IT1177991B; DE3533311A1; GB8522491D0; BE903300A; NL8502529A; IT8448901A0; FR2570828A1; IT8448901A1

Abstract

A measurement system for the non-destructive detection of defects in iron and steel products includes a single eddy current or resonance pick-up coil which is coupled to computing means for compensating for lift-off. The computing means is arranged to subtract from the coil output voltage a voltage value due to the distance of the coil from the surface of the product being tested and a voltage value which would be obtained with a defect-free product. The result of the subtraction operation is a voltage value ascribable to the presence of a defect in the product.

Description

SPECIFICATION Differential probes for the detection of defects in electrically conductive material This invention relates to a measurement system for the non-destructive inspection of steelworks products, for example. More precisely, it relates to a differential probe for detection of defects in an electrically conductive material, particularly for hot and cold detection of defects in slabs, blooms and billets.

The system described is an alternative to, and an improvement on, commonly used differential probe measurement systems based on the Foucault current or eddy-current principle, and can be applied to other measurement system of the electromagnetic type (electromagnetic resonance priciple).

To facilitate understanding of the method, the eddy-current measurement principle will now be explained. An eddy-current probe is used consisting of a coil having primary and secondary windings. A current is caused to flow in the primary winding by an oscillator.

This current generates an alternative magnetic field which produces eddy-currents in the material under inspection. In their turn these eddy-currents generate a magnetic field which induces a voltage in the secondary winding of the coil. The voltage varies depending on the distance of the inspected material from the probe and on the nature of the material concerned.

In known measurement systems, the above principle is generally applied by means of two indentical coils placed side-by-side an appropriate and adjustable distance apart and connected in series opposition.

In this way, since the two coils 1 and 2 are indentical and are connected in series opposition, it follows that the voltages in the coils cancel out when the portion of material in front of each coil is indentical in both cases.

In other words the following relation holds good: (AV1a+AV1c+AV1c)- (AV2a+AV2b+ AV2c)=O (1) where: AVa=voltage due to the distance from the material under inspection, AVb=voitage which would be obtained with defect-free material and which is due only to the surface roughness of the material (taken as being constant), and AVc=voltage due to defects in the material.

Since the two coils are arranged side-byside, it is assumed that the distance between them and the material under inspection is the same in both cases, so it follows that: AV,a=AV2a (2) Equally, since it is assumed that the surface roughness of the material is constant, then: AV,b=AV2b (3) When the coil 1 faces defect-free material, and coil 2 faces material having a defect, which is a condition obtained by appropriate adjustment of the distance between the centre lines of the windings, then it follows that: AVlc=O AV2c 0 (4) The value AV2c is therefore a voltage which indicates the presence and size of the defect.

According to the present invention there is provided a differential probe for detection of defects in electrically conductive material comprising a single coil and computing means adapted to subtract from a measured voltage value from the coil during detection of the material a calculated voltage value AVa due to the distance of the coil from the surface of the material and also a calculated voltage value AVb,,,, which would be obtained with defect-free material, so that the result of the subtraction operation is a voltage value AVc ascribable to the presence of a defect in the material.

In accordance with an exemplary embodiment of the invention which will now be described, the above-described measurement can be carried out using a differential probe having only one coil, the differential operation being performed by a computer in which the voltage data is stored in a predetermined program.

The measured voltage values for such a probe are in keeping with the relation: AV=AVa+AVb+AVc By statistical methods, starting from stored data corrsponding to previously made measurements, the means values of the voltages due to the distance of the winding from the surface Va,,) are computed, as are the mean values of the voltages Vb,,) which would be obtained with defect-free material.

For each point of measurement, the values AVa and AVb are subtracted from the measured total voltage value AV, so that it follows that: AV AVa-AVbrn=AVc The resultant AVc values are the voltage values due to any defects.

On the basis of this priciple, only one coil is utilised, as indicated, thus eliminating the difficulty of having to use two perfectly indentical coils and simplifying the mechanical task of adjusting the coil distances, which is obviously a delicate manoeuvre in a system operating at high temperature, for example, and critical when the mechanical regulation, however perfect this may be, is innately subject to errors which lead to a fall off in the sensitivity and precision of measurement.

Claims

1. A differential probe for detection of defects in electrically conductive material, comprising a single coil and computing means adapted to subtract from a measured voltage value output from the coil during detection of the material, a calculated voltage value AVam due to the distance of the coil from the surface of the material and also a calculated voltage value AVbm which would be obtained with defect-free material, so that the result of the subtraction operation is a voltage value AVc ascribable to the presence of a defect in the material.

2. A differential probe according to claim 1, wherein the computing means adapted to calculate the voltage values AVam and AVbm by means of a computer program which processes stored measurement data.

3. A differential probe according to claim 1 and substantially as hereinbefore described with reference to the exemplary embodiment.