GB2161602A

GB2161602A - Examination over a surface

Info

Publication number: GB2161602A
Application number: GB08417798A
Authority: GB
Inventors: Graham Bernard Hesketh; Dr Maurice Geoffrey Silk
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1984-07-12
Filing date: 1984-07-12
Publication date: 1986-01-15
Also published as: GB2161602B; GB8417798D0

Abstract

In a method for carrying out an examination over a surface of an object, particularly one having a complex shape, a measuring probe (136) is scanned over the surface. For each position of the probe, the co-ordinates of the probe (x,y,z) are measured with respect to predetermined axes, and new co- ordinates (X,Y,Z) are then calculated with respect to axes defined by three points (A,B,C) on the surface. The plane triangle having those points as vertices is subdivided into coplanar elements (1-25), and for each element the corresponding probe measurement is recorded. A visual display (106) may indicate those elements for which probe measurements have been recorded. Further triangles can then be generated and scanned over, each sharing at least one side with a previously scanned triangle. Ultrasonic B-scans may be displayed as grey-scale images on another visual display (108). Coupling between the probe (136) and the surface is taken to be satisfactory when the rear-wall echo is within preset limits. <IMAGE>

Description

SPECIFICATION Examination over a surface This invention relates to an apparatus and a method for carrying out an examination over a surface of an object, and particularly but not exciusively for performing an ultrasonic B-scan over an object whose surface is curved.

When carrying out for example an ultrasonic scan of an object, or an examination for surface contamination, it may be important to ensure that measurements are taken over the entire surface. Where the surface of the object is plane, cylindrical or spherical the position of any point on the surface at which a measurement is taken can simply and readily be defined by reference to predetermined coordinate axes. However where the object has a more complex shape (such as a pipe junction), or where the shape is not known, it may be impractical to choose suitable co-ordinate axes.

According to the present invention there is provided a method for carrying out an examination over a surface, comprising: (a) choosing three points on the surface so as to define the vertices of a plane triangle and determining the co-ordinates of the points with respect to a predetermined set of axes x, y and z; (b) subdividing the triangle into a plurality of coplanar elements; (c) scanning a measuring probe over the surface, and for each position of the probe: determining its x, y and z co-ordinates; calculating the transformed co-ordinates X, Y, Z with respect to an X-axis along one side of the triangle, a Y-axis along another side, and a Zaxis perpendicular to the plane of the triangle; determining, from the co-ordinates X and Y, the corresponding element of the triangle; and recording the measurements taken by the probe for the corresponding element of the triangle.

The above procedure enables the examination to be performed over the whole of the one triangle. To carry out an examination over the whole surface in a case where the surface is curved the procedure is repeated by defining (and scanning over) other triangles in succession, each triangle sharing one or two sides with previously examined triangles.

Desirably the elements of the triangle are of triangular shape, being generated by three sets of lines, each set being parallel to one side of the triangle and dividing the other two sides into an equal plurality of segments. If the number of segments is N, then the number of elements so generated is N2. One measurement is recorded for each element, so if the quantity being measured varies significantly over a short distance d the number of segments, N, is preferably chosen so that the length of each segment is less than the short distance d.

If the probe is an ultrasonic probe it is important to ensure good coupling between the probe and the surface. The degree of coupling may be assessed for example by the amplitude of the signal reflected from the opposite surface of the object. Desirably the signal received by the probe is analysed to assess the degree of coupling, and the signal is recorded only if the coupling is adequate.

In the case of an ultrasonic probe, the signal received represents an A-scan at the position of the corresponding element. In the preferred mode of operation, after recording the signal received for each element of the triangle, a B-scan along a straight line across the triangle is generated by selecting the recorded signals from those elements lying along that line, and displaying all the selected recorded signals as grey-scale lines adjacent to each other.

The invention also provides an apparatus for carrying out an examination over a surface, comprising a measuring probe; means for determining the position of the probe with respect to a predetermined set of axes x, y and z; means for transforming the x, y, z coordinates to co-ordinates X, Y, Z with respect to an X-axis defined by a straight line between two points on the surface, a Y-axis defined by a straight line between one of the points and a third point on the surface, and a Z-axis perpendicular to the plane of the triangle defined by the three points as vertices; means for determining from the values of co-ordinates X and Y, a corresponding element of the triangle; and means for recording a measurement taken by the probe for the corresponding element of the triangle.

Preferably the apparatus includes means for indicating those elements of the triangle for which no measurement has been recorded, and preferably includes means for displaying simultaneously the measurements recorded for a selected plurality of elements.

In the preferred embodiment the position determining means comprises a base and an arm for supporting the probe, the arm comprising a number of elements linked together and encoders for indicating the orientation of each element with respect to the elements to which is is linked.

The invention will now be described, by way of example only, and with reference to the accompanying drawings, in which: Figure 1 shows a diagrammatic perspective view of an apparatus for performing an ultrasonic scan over a surface; Figure 2 shows a network of triangles covering part of a surface; Figure 3 shows one of the triangles of Figure 2; Referring to Figure 1, an apparatus for performing an ultrasonic scan over the surface of an object comprises an articulated arm 100, a computer 102 incorporating a keyboard 104, and two television monitors 106 and 108 connected to the computer 102 by cables 110 and 112 respectively. The arm 100 comprises a base 120 from which a rotatable support member 122 extends upwards as shown. The support member 122 is connected at its upper end to one end of a first arm portion 124 by a hinge 126.Similarly the first arm portion 124 is connected at its other end by a hinge 128 to one end of a second arm portion 130, which is itself connected at its other end by an orthogonal pair of hinges 1 32 to one end of an end portion 1 34. At the other end of the end portion 1 34 is an ultrasonic pulse-echo. probe 1 36. The base 120 includes an electrical encoder (not shown) which produces an electrical signal indicative of the angle through which the support member 1 22 has turned about its longitudinal (vertical) axis, and each hinge 126, 1 28 and the pair 132 includes an electrical encoder (not shown) which produces an electrical signal indicative of the angle between the two components linked by the hinge 126, 1 28 or the pair 1 32 respectively.

The electrical signals from the probe 1 36 and from the encoders are carried by a cable 1 38 to the computer 102.

The computer 102 is programmed to determine, from the signals generated by the encoders, the co-ordinates of the probe 1 36 with respect to predetermined mutually orthogonal axes x, y, z whose origin is in the base 1 20 of the arm 100 and to perform this determination in response to an instruction entered by an operator on the keyboard 104.

In operation of the apparatus, the operator first places the probe 1 36 at three points on the surface of the object to be scanned, and for each point instructs the computer 102 to determine the x, y, z co-ordinates. Scanning of the triangular region of the surface with those points as vertices can then be performed as described below. After scanning this first triangular region, the operator can define a second triangular region by placing the probe 1 36 at a point outside the first triangular region, the second triangular region sharing a side with the first triangular region.

The computer 102 can be instructed to display on the monitor 106 the vertices and sides of the triangular areas which have been or are being scanned. In Figure 2, to which reference is now made, A, B and C are the vertices of the first triangular region to be defined; the second triangular region has vertices A, C and D; the third triangular region has vertices C, D and E; the fourth triangular region has vertices D, E and F; and so on. By defining and scanning triangular regions in this manner, a large area of the surface can be scanned. No gaps are left as each new triangular region shares at least one side with a previously scanned region, and no line is shared by more than two triangular regions.

It will be understood that if the surface is curved the triangular region of the surface over which the probe 1 36 is scanned will not be coplanar with the plane triangle defined by the three vertices, although the vertices do lie on the surface.

Referring to Figure 3, which shows the vertices A, B and C of the first triangular region to be defined, the vertices A, B and C lie on the surface of the object, while the straight lines AB, AC, and BC form a plane triangle. In scanning over the triangular region, the probe 1 36 is placed by the operator on the surface of the object, and the computer 102 instructed to transform its co-ordinates x, y, z into co-ordinates X, Y, and Z with respect to an X-axis along the straight line AB, a Yaxis along the straight line AC, and a Z-axis perpendicular to the plane of the triangle.The triangle is subdivided by straight lines parallel to each of its sides dividing each side into N equal segments (in Figure 3, N is 5), so dividing the triangle into N2 congruent triangular elements, each element being similar in shape to the triangle itself. Each element is allotted a unique reference number (in Figure 3, from 1 to 25). From the values of coordinates X and Y the computer 102 then determines the reference number of the corresponding element.

For every position of the probe 136, the computer 102 analyses the signal received from the probe 1 36 as to whether the reflection from the rear side of the object lies within acceptable limits (which indicates if the coupling between the probe 1 36 and the surface is satisfactory), and if so records the signal on a disk file as relating to the corresponding element. Only one signal is recorded for each element, and the monitor 106 indicates both the elements for which a signal has been recorded and the position of the probe 136.

When a signal has been recorded for each element, the operator can be sure that the entire triangular region has been scanned.

Ultrasonic B-scans can then be displayed on the monitor 108 along lines parallel to any side of the triangle, by selecting the recorded signals from the elements lying on the line in question, and displaying each signal as a grey-scale line. The grey-scale lines for the elements, when displayed adjacent and parallel to each other, create a B-scan display, i.e.

an ultrasonic sectional view in the plane perpendicular to the surface.

The length of each segment, and so the value of N, must be chosen so that the signal received from the probe 1 36 would not be expected to vary significantly across any one element. The value of N can be set by the operator, so as to provide adequate resolution of any defects within the object. The larger the value of N the greater the resolution, but the more information there is to record and the longer it will take to scan the whole triangle.

In order to ensure that there are no gaps in the coverage of the surface of the object, signals may be recorded not only for those elements lying within the triangle ABC (see Figure 3) but also for an additional line of identical elements just outside each side AB, AC and BC of the triangle.

Claims

1. A method for carrying out an examination over a surface, comprising: (a) choosing three points on the surface so as to define the vertices of a plane triangle and determining the co-ordinates of the points with respect to a predetermined set of axes x, y and z; (b) subdividing the triangle into a plurality of coplanar elements; (c) scanning a measuring probe over the surface, and for each position of the probe: determining its x, y and z co-ordinates; calculating the transformed co-ordinates X, Y, Z with respect to an X-axis along one side of the triangle, a Y-axis along another side and a Zaxis perpendicular to the plane of the triangle; determining, from the co-ordinates X and Y, the corresponding element of the triangle; and recording the measurements taken by the probe for the corresponding element of the triangle.

2. A method as claimed in Claim 1 wherein the elements are of triangular shape, being generated by three sets of lines, each set being parallel to one side of the triangle and dividing the other two sides into an equal plurality of segments.

3. A method as claimed in Claim 2 wherein the number of segments is chosen so that the length of any one segment is less than the distance over which the measured quantity is expected to vary significantly.

4. A method as claimed in any one of Claims 1 to 3 including assessing the measurement taken by the probe, and recording the measurement only if the measurement is assessed to be satisfactory.

5. A method as claimed in any one of Claims 1 to 4 wherein the measuring probe is an ultrasonic probe and the measurements taken by the probe represent an A-scan at the position of the probe, the method further comprising generating a B-scan along a line across the triangle by selecting the recorded signals from those elements lying along that line, and displaying all the selected recorded signals as grey-scale lines adjacent to each other.

6. A method for carrying out an examination over a surface comprising performing the method as claimed in any one of Claims 1 to 6, and repeating this method for a succession of triangles, each successive triangle sharing one or two sides with previously examined triangles.

7. A method for carrying out an examination over a surface substantially as hereinbefore described with reference to, and as shown in, Figures 1, 2 and 3 of the accompanying drawings.

8. An apparatus for carrying out an examination over a surface, comprising a measuring probe; means for determining the position of the probe with respect to a predetermined set of axes x, y and z; means for transforming the x, y, z co-ordinates to co-ordinates X, Y, Z with respect to an X-axis defined by a straight line between two points on the surface, a Yaxis defined by a straight line between one of the points and a third point on the surface, and a Z-axis perpendicular to the plane of the triangle defined by the three points as vertices; means for determining from the values of co-ordinates X and Y, a corresponding element of the triangle; and means for recording a measurement taken by the probe for the corresponding element of the triangle.

9. An apparatus as claimed in Claim 8 including means for indicating those elements of the triangle for which no measurement has been recorded.

10. An apparatus as claimed in Claim 8 or Claim 9 including means for displaying simultaneously the measurements recorded for a selected plurality of elements.

11. An apparatus as claimed in any one of Claims 8, 9 or 10 wherein the position determining means comprises a base and an arm for supporting the probe, the arm comprising a number of elements linked together and encoders for indicating the orientation of each element with respect to the elements to which it is linked.

1 2. An apparatus for carrying out an examination over a surface substantially as herein before described with reference to, and as shown in, Figure 1 of the accompanying drawings.