GB2148488A - A non-contacting mensuration system for measuring profiles - Google Patents

Abstract

The system uses a laser scan profilometer wherein the laser beam 7 is split 9, 10, preferably into three beams, which are focussed onto the surface to be measured and the variation in spacing of the contact points of the focussed beams are used to calculate the profile of the surface. The spacings of the contact points are determined by a detector 8, such as a camera, which provides outputs to a computing means for calculating the profile. The system may be used to determine the profile of a helicopter rotor blade. <IMAGE>

Description

SPECIFICATION Improvements in or relating to non contacting mensuration The invention relates to non-contacting mensuration and more particularly, though not exclusively, to the mensuration of profiles and surfaces.

The art of profile and surface measurement is not new, for example profilometers are used for the measurement of surface roughness of mechanical items and touch probes are used for the measurement of profiles and surfaces on smaller objects. However, profilometers on their own are unsuitable for mensuration purposes on smaller objects and the use of touch probes is laborious if total surface measurement is required.

United States Patent No 4,325,640 teaches one method of non-contacting electrooptical measurement wherein it is adapted to measure the contour of a rotor blade by triangulation. This method though accurate is, by comparison with the method we are about to disclose, both expensive and complicated.

However, the background of the invention is basically the same and it is to a helicopter rotor blade that we direct our example of the use of the present invention. This is mainly because a helicopter rotor blade is a very good example of a three dimensional surface and one in which accurate measurement is required.

It is therefore an object of this invention to provide a relatively inexpensive and uncomplicated system of non-contacting mensuration for profiles and surfaces using a laser scan profilometer.

According to the invention we provide a non-contacting mensuration system comprising a driven scanning means with readout, a beam splitter and focussing means, a laser, and a detector means wherein the laser beam is split into two or more beams and focussed via the scanning means onto the surface to be measured, the detector means providing outputs proportional to the spacing of the contact points of the beams on the surface to be measured and said outputs are used as the inputs to a computing means for the purpose of calculating said profile.

The invention will now be described, by way of example only in conjunction with the accompanying drawings in which: Figure 1 shows a basic system Figure 2 shows the principle Figure 3 shows the errors in the principle Figure 4 shows the solution Figure 5 shows a true function of the solution and Figure 6 shows a basic profilometer unit according to the invention.

Referring now to Fig. 1 wherein the basic system is shown an object to be profiled is formed as an inclined plane (1) having a base (2) of 40cm. and a height (3) of 10cm. A laser scan profilometer comprising a driven scanning means with readout (4), such as a galvanometer scanner or the like, a beam splitter and focussing means (5), a laser (6) preferrably a low power He Ne laser capable of emitting a beam (7) in the order of 1 mm diameter and a detector means (8) such as a position sensitive device or the like.

In its simplest form the beam (7) from the laser (6) is split into two parallel beams (9) and (10) of approximately 10mm separation and focussed by the beam splitter and focussing means (5) through the driven scanning means (4) to a point coincident with the highest point of the inclined plane (1) where a spot formed by each beam overlaps the other completely. As the driven scanning means (4) is rotated counter-clockwise by approximately 30 the two beams rotate through approximately 60 and the spots formed by the two beams, now strike the inclined plane at its lowest point, have a diameter of approximately half a millimeter and are separated by approximately 3mm.

Viewing the inclined plane as from the detector means (8) the effect is similar to that shown at Fig. 2 where two dots of diameter d2, are merged at the highest point of the plane and would gradually separate into two spots of diameter d1 at the lowest point of the inclined plane as the scanning means (4) rotates counter-clockwise.

However, as can be seen from Fig. 3 this method is valid only for an inclined plane of precisely defined geometry and is shown here to illustrate that spot separation is directly proportional to the length of the two beams (9) and (10) between the driven scanning means (4) and the point of the spots on the inclined plane. The beams converge from the driven scanning means (4) with an angle 8 from a separation of 2a. At a distance L they encounter a plane, either plane 11 or plane 12, and depending on the inclination of the plane the spot projection will be separated by a distance of either b or c and spot separation is no longer a unique function of distance L.

To overcome this problem on complex surfaces, for example a helicopter rotor blade, it is necessary to use a minimum of three spots as shown in Figs. 4 and 5 wherein L is a unique function of b and c i.e.

L=f(b, c, 0) Referring now to Fig. 6 a basic profilometer unit for measuring complex surfaces and profiles using a minimum of three beams is constructed comprising a driven scanning means with readout (4); a laser (6), beam splitter and focussing means (5); and a detector means (8).

In this specific example the driven scanning means with readout (4) is a mechanical scanner of the type commonly used for document scanning and can be driven to a frequency in excess of 100 Hz carrying a mirror of an inch in diameter.

The laser is an He Ne laser operating at 0.633 microns wavelength with excellent beam properties. A standard lower port He Ne laser emits a beam with a diameter of around 1 mm and the intensity within the beam is distributed as a Gaussian. The Gaussian distribution is retained by utilising a good quality focussing means.

The detector means is a camera with a position sensitive detector having a sensitive surface of approximately 34mm and three outputs. The outputs, produced by the spot of light falling on the photo-sensitive surface, are processed to give a voltage proportional to the position of the spot of light on the surface. As the output is derived as a ratio, the intensity distribution is unimportant and as the spot of light is a laser spot, distributed as a Gaussian, the centre of this spot can be tracked to a very high accuracy.

The system is depicted working on an aerofoil section with dimensions as shown and the unit having a field of view of around 35cm.

with a camera magnification of X 10. The aerofoil section profile is given in polar coordinates of r and 9.

As the angle 8 is always known from the scanner drive readout the distance r is calculable from the separation of the three spots as registered by the camera. e.g. If the spots are separated by 1 Omm (2a of Fig. 5) then a height difference of 1 Ocm corresponds, approximately, to a change in separation of O to 5mm on the aerofoil - or O to 500 microns on the surface of the position sensitive detector. In the simplest method of measurement the positions of the three spots would be derived by sequentially switching the laser beams as the response times of the position sensitive detectors are adequate to allow sufficient channel separation. However, it is to be understood that several other configurations are possible.

Though the method as described shows profiling in one axis only it will be obvious to one skilled in the art that profiling may equally well be performed in an axis at 90 degrees and that a complete surface may be measured by serially scanning in both axes. It will also be obvious to one skilled in the art that a two dimensional scanner may be used.

Though the driven scanning means is generally described in conjunction with a mirror it will also be obvious to one skilled in the art that acusto optical, electro optical and holographic scanners may equally well be used without deviating from the invention as may the detector means be of the type known as a linear array.

Claims (9)

1. A non-contacting mensuration system comprising a driven scanning means with readout, a beam splitter and focussing means, a laser, and a detector means wherein the laser beam is split into two or more beams and focussed via the scanning means onto the surface to be measured, the detector means providing outputs proportional to the spacing of the contact points of the beams on the surface to be measured and said outputs are used as inputs to a computing means for the purpose of calculating said profile.

2. A non-contacting mensuration system as claimed in Claim 1 wherein the detector means is a camera with a position sensitive detector having a sensitive surface within the range of 10 to 100mm in length.

3. A non-contacting mensuration system as claimed in Claim 1 wherein the detector means is of the type known as a linear array.

4. A non-contacting mensuration system as claimed in Claims 1 or 2 wherein the driven scanning means is a mechanical scanner with reflector and readout.

5. A non-contacting mensuration system as claimed in Claims 1 or 2 wherein the driven scanning means is acousto-optical.

6. A non-contacting mensuration system as claimed in Claims 1 or 2 wherein the driven scanning means is electro-optical.

7. A non-contacting mensuration system as claimed in Claims 1 or 2 wherein the driven scanning means is a holographic scanner.

8. A non-contacting mensuration system as claimed in any previous claim wherein the scanning is in more than one dimension.

9. A non-contacting mensuration system substantially as described and claimed herein in conjunction with the accompanying draw ings.