GB2073878A - A method of monitoring surface quality - Google Patents

Abstract

The quality of a surface, e.g. the coated or uncoated surface of a metal sheet, is monitored by directing a laser beam at the surface, preferably obliquely, and receiving all or most of the reflected divergent beam by means of a photosensitive detector. The reflected beam is passed through a moving shutter presenting transparent and opaque zones alternately, so that the detector provides a periodic signal having an alternating component which is measured and is a function of the irregularity of the surface.

Description

SPECIFICATION A method of monitoring surface quality The present invention relates to a method of monitoring surface quality, whether or not the surfaces are coated with one or a plurality of layers, e.g. any type of paint coating, and whatever their shape. The method may also be used when the articles whose surfaces are to be measured are being displaced.

The present description is based on the surface quality of sheet metal, but this is purely by way of non-limiting example as the method of the invention may be applied to any type of surface of any size.

The quality of sheet metal may be judged in two ways: on one hand, with respect to the material of the sheet itself, which should have well defined properties from a point of view of mechanical strength, structure, homogeneity, and isotropy, and, on the other hand, with respect to its surface appearance, whether this surface is coated or not with paint or any coating. The present invention relates to the second of these factors.

The surface quality is directly linked to the degree of smoothness or brilliance of the surface, as well as to its homogeneity and uniformity of appearance.

With respect to coated surfaces and in particular painted metal surfaces which are produced in large quantities for automobile bodywork, the various surfaces of an assembly must not only be as brilliant as possible but must also have the same degree of brilliance, for their quality to be judged satisfactory. It is therefore necessary to be able to judge the brilliance of a painted surface, irrespective of colour, in order to monitor such production.

Methods have already been used in this field which are based on the power of resolution of the painted surface used as a mirror.

However, the results obtained in this way depend on subjective evaluation by the operator, with all the known drawbacks of a method of this type.

With respect to non-coated surfaces and in particular the surfaces of sheet metal, which may be seen as a succession of peaks and valleys under the microscope, it is known that the morphology of these surfaces, which is of considerable importance, is characterised by various parameters such as arithmetic mean roughness, number of peaks per unit length, mean plateau and valley length, which may only be obtained from relatively difficult measurement operations.

We have already proposed a method in this field which enables the above-mentioned drawbacks to be remedied and in which a beam of light of the laser type having a low power is directed onto the surface with an incidence which is preferably oblique, the angle of aperture of the cone of reflected light is ascertained under suitable conditions, and the quality of the surface is deduced from this, taking into account that surface irregularity increases with angle of aperture.

As the above-mentioned method is based on the measurement of the intensity of a beam relected by the sheet whose brilliance is to be measured, there are, in practice, certain difficulties in measuring the reflected intensity in a low angle of aperture in the centre of the beam, or in determining, by means of a screen, the angle of aperture of the reflected beam. The latter case requires either a screen having large dimensions, which is therefore costly, or an accurate auxiliary device designed to cause the beam to approach the measuring screen.

The present invention provides a method in which a laser beam is projected onto the surface whose properties of reflectance and brilliance are to be measured, the beam reflected by the surface, as a beam which diverges a greater or lesser degree, has its major portion, and preferably its totality, received by a photo-sensitive detector, whilst passing through one or several transparent zones of a moving shutter comprising alternate transparent and opaque zones, and the alternating component of the signal produced by the photo-sensitive detector is measured, the said signal representing the brilliance of the sheet. The alternating component is a function of the irregularity of the surface.

The movement of the shutter may be such that the reflected beam comes successively into contact with at least one transparent zone and one opaque zone, and preferably comes successively into contact with several alternating transparent and opaque zones.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagram of a shutter having transparent and opaque zones of equal width, together with the signal produced by a lightsensitive receiver when struck by a reflected laser bear whose local diameter is equal to the width of each zone; Figure 2 is similar to Fig. 1, but with the local diameter of the reflected beam greater than the zone width; Figure 3 is similar to Fig. 1, but with the transparent zones narrower than the opaque zones and narrower than the reflected beam; Figure 4 is similar to Fig. 3, but with the beam wider than the opaque zones; Figure 5 shows one embodiment of the shutter shown in Figs. 1 and 2; Figure 6 shows one embodiment of the shutter shown in Figs. 3 and 4;; Figure 7 shows another embodiment of the shutter shown in Figs. 1 and 2; and Figure 8 shows another embodiment of the shutter shown in Figs. 3 and 4.

The features of the shutter and the quality of the surface to be measured may give rise to several different cases: (a) The shutter is constituted by a sequence of alternating transparent and opaque zones, whose width is substantially equal to the diameter, at the location of the shutter, of a beam reflected by a sheet whose surface is considered to be very brilliant and very uniform. In this case, the zone of the sensitive surface of the receiver which is struck by the beam (or a major part of the beam) may receive, with the passage of time, a degree of illumination as shown by the graph of Fig. 1, the passage of an opaque zone of the shutter being shown by an intensity of illumination which is almost zero. The alternating component of the signal is represented by A1 in Fig.

1.

If, using this type of shutter, a surface having distinctly inferior qualities of brilliance and/or uniformity is involved, the diameter of the reflected beam will be substantially greater at the location of the shutter, and the receiver will record, as a function of time, a graph of the type of that of the signal A2 af Fig. 2.

Comparison of such graphs enables the evaluation of whether one sheet is more brilliant than another, or whether it has acceptable brilliance characteristics with respect to a given scale.

(b) The shutter is constituted by a sequence of alternating opaque and transparent zones, in which the width of the transparent zones is much smaller than the diameter of a spot reflected by a surface considered to be brilliant and smooth.

In this case, the zone of the sensitive surface of the receiver which is struck by the beam of small diameter (corresponding to a smooth sheet) may receive, with the passage of time, a degree of illumination as shown by the graph of Fig. 3, the passage from an opaque zone to the following zone being shown by the recording of a signal which is very similar to a pulse (C3). If, on the other hand, using such a shutter, the diameter of the reflected beam is substantially greater than in the previous case, and even at times greater than the width of the opaque strip of the shutter, the zone of the sensitive surface of the receiver which is struck by the reflected beam during the displacement of the shutter leads to the recording of a signal of the type shown in Fig. 4 (whose alternating value C4) is indicated in the drawing).

The method of the present invention is advantageous in that it considerably facilitates the measurement of the degree of illumination caused by the reflected beam which may be readily produced within an annular aperture of 1 or, whereas in the case of known methods measurements had to be carried out at the centre of the reflected beam, for example using an aperture of the order of (1 /50)t.

In accordance with the invention there are various ways of designing a shutter which satisfies the conditions required to carry out the above-mentioned method. However, preference is given to a shutter either designed as a slotted disc (having a reception cell behind the disc-Figs. 5 and 6) or having a cylindrical shape and also provided with slots (with the reception cell inside the cylinder-Figs. 7 and 8).

Preferably, the total reflected intensity of the sheet at the point in question is also measured, which enables its brilliance to be ascertained (relationship or difference between the values measured using the above method and the total reflected intensity).Two procedures may advantageously be used: eitherthe measurement of the continuous component of the signals received through the shutters, -or deflecting a portion of the reflected beam to a second photometric cell by means of a semi-transparent reflector located upstream of the shutter (with respect to the path of reflected radiation).

According to a further advantageous variant, when surfaces whose relative position may vary within wide limits are involved, the variations in direction and position of the reflected beam may be decreased by causing the laser radiation to pass successively onto two plane mirrors before reaching the surface whose reflectance is to be measured, the first of these mirrors being fixed with respect to the laser and the second being parallel to the surface of the sheet at its point of measurement.

The forced parallelism between the radiation striking the second mirror and that reflected by the surface leads, in the case of displacement of the surface, to a simple displacement of the reflected beam, which remains substantially parallel to itself during its displacement, thus facilitating the measurement to be made.

In the case involving a particularly smooth and brilliant surface, the very slight degree of divergence of the laser beam may be artificially increased by passing this beam (before and/or after reflection) through a telescope, for which (in the case of a laser beam) there is the relationship: output divergence = input divergence x (input diameter/output diameter), which facilitates the accuracy of measurement, even in the above-mentioned case.

The scope of the invention includes displacing the beam which is incident on the surface in question, along any suitable path, for example a circular path, in order to obtain as a result a mean value of those values obtained with a sequence of point measurements.

Claims (14)

1. A method of monitoring the quality of a surface, comprising directing a laser beam at the surface, whereby a divergent reflected beam is produced; passing the reflected beam through a moving shutter presenting alternating transparent and opaque zones; receiving the reflected beam after it has passed through the shutter, by means of a photosensitive detector which would, in the absence of the shutter, receive at least the major part of the reflected beam, whereby a signal having an alternating component is produced; and measuring the alternating component of the signal.

2. A method as claimed in claim 1, in which the movement of the shutter is such that the reflected beam comes successively into contact with at least one transparent zone and one opaque zone.

3. A method as claimed in claim 1 or 2, in which the shutter has a sequence of alternating transparent and opaque zones, whose width is substantially equally to the local diameter of the reflected beam when reflected by a surface which is brilliant and uniform.

4. A method as claimed in claim 1 or 2, in which the shutter has a sequence of alternating transparent and opaque zones, in which the width of the transparent zones is smaller than the diameter of the reflected beam when reflected by a surface which is brilliant and smooth.

5. A method as claimed in any of claims 1 to 4, in which the shutter-is in the form of a slotted disc.

6. A method as claimed in any of claims 1 to 4, in which the shutter is in the form of a cylinder provided with slots.

7. A method as claimed in any of claims 1 to 6, further comprising measuring the total intensity of the reflected beam.

8. A method as claimed in claim 7, in which the measurement of the alternating component and the measurement of the total reflected beam are either divided by one another or subtracted from one another.

9. A method as claimed in claim 7 or 8, in which the total reflected intensity is measured by measuring the continuous component of the signal produced by the photosensitive detector.

10. A method as claimed in claim 7 or 8, in which the total reflected intensity is measured by deflecting a portion of the reflected beam to a second photosensitive detector by means of a semi-transparent reflector located in the path of the reflected beam to the shutter.

11. A method as claimed in any of claims 1 to 10, in which the position of the surface is variable, the laser beam being successively reflected by first and second plane reflectors before reaching the surface, the first reflector being fixed with respect to the laser, and the second reflector being parallel to the surface.

1 2. A method as claimed in any of claims 1 to 11, including passing the laser beam through a telescope before it reaches the photosensitive detector.

13. A method as claimed in claim 12, in which the laser beam passes through the telescope before reaching the surface.

14. A method as claimed in any of claims 1 to 13, in which the laser beam is oblique to the surface on incidence.

1 5. A method as claimed in claim 1, substantially as described with reference to Figs. 1 and 2 or Figs. 3 and 4 of the accompanying drawings.