GB2144290A - Measuring system - Google Patents

Abstract

A measuring system 1 for measuring the position and movement of a point within a predetermined plane perpendicular to the plane of the figure and intersecting it at 10 includes optical apparatus for projecting a divergent beam 8 onto the plane. A portion 14 of this beam is reflected by a cube-corner retroreflector 12 whose apex is at the point the position and movement of which is to be measured. The return beam 16 is focussed on a two-dimensional photodetector array 32. The optical system is such that movement of the retroreflector 12 corresponds on a reduced scale to movement of the focussed beam over the photodetector array. A scanning technique is then used to derive the position and movement of the focussed spot and hence of the retroreflector. Three different techniques for improving the speed of operation of the array scanning, compared with scanning the complete frame, are described; one of these (Fig. 2, not shown) involves splitting the reflected beam and projecting each sub-beam onto a linear array via a cylindrical lens. <IMAGE>

Description

SPECIFICATION Measuring Systems The present invention relates to measuring systems. More specifically the invention relates to systems for measuring the position within a plane of a predetermined movable point.

In various applications it is required to accurately measure the position of a point without affecting the motion of that point.

Examples of such applications include the measurement of torsional oscillations in tall towers, the measurement of the achieved dynamic performance of the tip of a robot arm under test, and the automatic tracking of airborne targets by airborne vehicles or ground stations. In such applications any system which requires contact with the moving point is generally unsuitable as it may cause the movement which is to be measured to change.

In certain types of application, linear or rotational displacement is required to be measured in the presence of directionally random motion, which may be of greater magnitude. This occurs when torsional oscillation of a tall tower about its major axis is measured by measuring the displacement of a point radially displaced from the tower axis. Such towers are also prone to swaying motion which causes an essentially planar motion of the specified point. This swaying motion may be larger than the substantially linear displacement of the point caused by the torsional oscillation. Since the swaying motion may be in any direction, the linear motion to be measured is superimposed upon larger random planar motion.

Errors that would arise as a result of this swaying motion may be eliminated by measuring the tangential motion of two points on opposite sides of the tower axis. The difference in these measured motions is solely due to the torsional motion and their sum represents the swaying motion alone.

Further problems are also encountered with present measuring systems, when the point whose planar displacement is to be measured is moving very rapidly.

The present invention accordingly provides a system for measuring the position within a plane of a predetermined point, said system comprising means for producing a radiation beam divergent about an incident optical axis which at said plane covers the area of movement of the predetermined point, reflecting means mounted at said point for reflecting incident rays back along substantially parallel paths producing a return beam divergent about said incident optical axis, and means for focussing said divergent return beam onto a photo-detector array, said system being arranged such that movement of the reflecting means within said plane is reproduced by linearly proportional movement of the focussed return beam on said array, and means for detecting the position of said focussed return beam on said array.

Preferably, the reflecting means comprises a cube-corner tetrahedral prism or an arrangement of three mirrors each respectively corresponding to one of the right-angled triangle sides of such a tetrahedron and with their reflective surfaces facing inwardly.

Such a prism or arrangement of mirrors, will hereinafter be referred to as a cube-corner retroreflector. The apex of the retroreflector as used herein refers to the apex at the junction of the three right-angles of the equal triangular sides.

Such a retroreflector enables the measuring system to be used when the measuring point is also subject to rotational movement as well as the planar displacement to be measured. This arises because a cube-corner retroreflector is insensitive to rotation in particular about axes perpendicular to the optical axis, provided always that the divergent beam is able to enter the retroreflector.

A measuring system embodying the present invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a diagram illustrating the measuring system; Figure 2 shows a modification of the system of Figure 1; and Figure 3 shows a plan view of one of the photodetector arrays of the system modified in accordance with Figure 2.

The apparatus 1 employed in the illustrated measuring system includes a radiation source 2 which, in the present example, is a laser light source. Other sources of monochromatic radiation may be employed. Non-monochromatic radiation may be employed but this would produce less accurate results due to chromatic aberration in the optical system of the apparatus.

The laser light source 2 is illustrated as producing a parallel beam 4 which is passed through a lens system 6 to produce a beam 8 diverging about an incident optical axis. The lens system 6 is illustrated as a single convergent lens L, which focusses the parallel beam 4 at a point S from which the divergent beam 8 spreads.

The illustrated measuring system 1, is intended for measuring the motion of a point within a plane which is perpendicular to the paper and has a line of intersection with the plane of the paper shown by the broken line 10. The divergent beam 8 projects a circle of diameter D onto this plane. A cube-corner retroreflector 1 2 which may be considered to have an optical aperture of diameter d is mounted with its apex at the point the movement of which is to be measured.

Wherever the retroreflector is within the circle of diameter D it will reflect a portion 14 of the divergent beam 8 back towards the light source as a beam 1 6 diverging about an optical axis joining the apex of the retroreflector to the effective point source S. The cube-corner retroreflector has the optical property that light received by it along a first optical path is reflected back along a parallel optical path thus the extreme rays 18 and 20 of the beam 14 which impinge on the retroreflector are reflected back along parallel ray paths 22 and 24 respectively which define the extreme rays of return beam 1 6.

The reflected energy in beam 1 6 is related to the energy of the divergent beam 8 by a factor of (d/D)2.

The beam 1 6 is redirected into a substantially perpendicular path by a semi-reflecting mirror (beam-splitter) 26. The return beam diameter is 2d at the point source S and at the corresponding virtual source produced by the beam-splitter 26.

In Figure 1 only one beam 28 is shown. The beam 28 is passed through a lens system 30 to produce a convergent beam 34 which is focussed on the photodetector array 32 which extends in a plane perpendicular to the paper. In the present example the lens system 30 comprises a convex lens L2 which focusses the beam 34 into a spot on the area photodetector array 32.

In the present example the distance between the point source S and the plane 10 in which movement is to be measured is represented by the distance y. The focal length of the lens L2 is the distance x. The centre of lens L2 is arranged to be the same distance from the centre of the beam splitter as is the point source S.

Therefore a movement m1 of the retroreflector 12 is reproduced as a movement m2 of the focussed beam 28 on the photodetector array 32.

The relationship between the magnitudes of these movements is: m2 x m1 y The distances x and y may therefore be selected so that the full range of movements of the retroreflector can be reproduced on a suitably sized photodetector array. It will be appreciated that movement of the retroreflector in a direction transverse to the paper will be reproduced as movement of the focussed beam on the photodetector array in a corresponding direction.

The photodetector array 32 consists, for example, of a square array of 500 by 500 individual photodetectors such as photodiodes or charge coupled devices. Each photodetector may be considered to represent a pixel. In order to detect and track the position of a focussed light spot moving over the photodetector array as produced by the optical system of Figure 1 in response to movement of the retroreflector, a scanning system such as used by electronic cameras may be employed, in which the output from each photodetector is scanned in turn. It is possible to scan the individual photodetector outputs at a rate of typically 10 Mhz.

Correspondingly a scan of the whole photodetector array can be carried out at a rate of 40 hz. In the described apparatus such a scanning method produces a large amount of redundant information as there will only be one focussed light spot on the whole photodetector array at any one instant in time. This system is also not ideal if the movement is taking place very rapidly where considerable movement of the light spot may occur in the 25 milliseconds between successive complete frame scans.

A method of improving the efficiency of detection which is suitable for use when the maximum slew rate of the point is known, can be estimated, or measured from acquired data, will now be described. In such cases the maximum sample to sample displacement can be predicted.

In this method only a portion of the photodetector array 32 adjacent the previously known position of the spot is scanned. The size of this portion is determined by the maximum possible displacement. Since only a fraction of the photodetectors are scanned during any one frame scan, the rate at which the position is sampled is increased. Further reduction in the scan area and hence increase in the sampling rate may be achieved by using directional track prediction techniques.

In such methods the scan size may either be limited by existing knowledge or may commence at the full scan size and be reduced automatically by an adapted scan length control system as knowledge of point to point displacements is acquired. In effect such a system may be designed to learn the optimum scan area.

Two further methods of detecting the position of the focussed beam or beams on their respective photodetector arrays will now be described. In both these methods the area scanned is reduced to two mutually orthogonal line scans and therefore the scanning time is reduced to the time for two line scans either sequentially 6r simultaneously performed. In the case of a 500 by 500 array this represents a time saving factor of 250 or 500 respectively.

The first method involves modifying the system of Figure 1 as shown in Figures 2 and 3. In this modified system a further beam splitter 40 is positioned in the path of the convergent beam 34.

The beam splitter 40 produces continuing beam 34 which passes through a cylindrical lens 38 and is focussed as an elongate strip extending in the plane of the paper onto photodetector array 32. A further beam 36 is also produced from the beam splitter 40 and this travels in a direction orthogonal to the beam 34 through a cylindrical lens 42 and is focussed onto a photodetector array 44. The array 44 is oriented orthogonally to the array 32. The beam 36 is focussed by the cylindrical lens 42 into an elongate strip on the array 44 extending orthogonally to the paper and also orthogonally to the focussed strip produced from beam 34 by cylindrical lens 38 on array 32. If the fields of view from beam splitter 40 are superimposed then the intersection point of the two focussed elongate strips would lie at what would have been the the position of the focussed spot produced by the convex lens L2.

Figure 3 illustrates the relative orientations of the focussed strip 36 and the photodetector array 44. In this case the photodetector array is a linear detector of, for example, 500 photodetectors arranged with a 500:1 aspect ratio. As illustrated the detector 44 is capable of detecting movement of the focussed strip in the direction of arrow 46, but is insensitive to movement of the focussed strip in the orthogonal direction indicated by arrow 48. In the case of the focussed strip 34 and photodetector 32 which is also a linear detector, the relative positions of the strip and detector are exchanged so that motion in the direction of arrow 48 can be detected but not motion of the strip in the direction of the arrow 46. In this way the position of the strip can be determined in two mutually orthogonal directions.

Whereas the foregoing method relies on elongation of the focussed light spot in two mutually orthogonal directions so as to enable conventional line scan detectors, which may consist of, say, 500 photodetectors each representing a pixel arranged with a 500:1 aspect ratio, to be employed; the next method to be described makes use of the elongation of the detector elements in two mutually orthogonal directions so as to enable the position of the spot to be determined in just two line scans of the elongate detector elements.

In the second method the outputs from each photodetector pixel in each row of an area photodetector are simultaneously summed into a corresponding row register. Each such register is then output scanned sequentially. In effect the area detector has been reduced to a linear detector having one column of transversely elongate elements. In the present example of a 500 by 500 area photodetector array, the linear detector would consist of a single column of 500 rectangular elements, each element having a geometrical aspect ratio of 500:1. This method enables a one-dimensional (single co-ordinate) measurement of the position of the light spot to be made.

In order to provide two-coordinate or area position sensing using this method the system is modified in a manner similar to that shown in Figure 2, except, in this case the cylindrical lenses 38, 42 are not present so that the beams 34, 36 are focussed as spots on their respective photodetector arrays 32, 44. Each of the arrays 32, 44 is a linear detector as previously described of one column of elongate elements. Each detector 32, 44 is arranged to measure the displacement of the two corresponding light spots in two mutually orthogonal directions in order to yield the two-dimensional geometrical coordinates in the time taken by one or two line scans of the 500 by 500 area detector array.

It will be appreciated that the above described measuring system readily allows the position of any remote point to be measured and its movement monitored, provided only that a retroreflector may be mounted with its apex at the point the position and movement of which are to be measured.

Claims (11)

1. A system for measuring the position within a plane of a predetermined point, said system comprising means for producing a radiation beam divergent about an incident optical axis which at said plane covers the area of movement of the predetermined point, reflecting means mounted at said point for reflecting incident rays back along substantially parallel paths producing a return beam divergent about said incident optical axis, and means for focussing said divergent return beam onto a photo-detector array, said system being arranged such that movement of the reflecting means within said plane is reproduced by linearly proportional movement of the focussed return beam on said array, and means for detecting the position of said focussed return beam on said array.

2. A system as claimed in claim 1, wherein the reflecting means comprises a cube-corner tetrahedral prism or an arrangement of three mirrors each respectively corresponding to one of the right-angled triangle sides of such a tetrahedron and with their reflective surfaces facing inwardly.

3. A system as claimed in claim 1 or 2, wherein the focussing means comprises means for focussing the return beam into a spot on said array.

4. A system as claimed in claim 1 or 2, wherein the focussing means comprises means for focussing the return beam into at least one elongate strip on said array.

5. A system as claimed in claim 4, wherein the focussing means produces two substantially perpendicular, intersecting, elongate strips on said array.

6. A system as claimed in any one of the preceding claims, wherein said detecting means comprises means for scanning only a portion of the photodetectors in the array in the region of the last detected position of the focussed return beam.

7. A system as claimed in any one of the preceding claims, wherein the photodetector array comprises a plurality of photodetectors arranged in a plurality of rows and columns, and the outputs of all the photodetectors in each row and/or column are summed into a respective row or column register, said detecting means scanning the outputs of at least some of said registers.

8. A system as claimed in any one of claims 1 to 6, wherein said photodetector array is a linear detector and comprises a single row or column of photodetectors.

9. A system for measuring the torsional motion and swaying motion of a tower, comprising two systems as claimed in any one of the preceding claims, the reflecting means of each system being mounted at either end of a diameter of a cross section through the tower so as to measure tangential motion of said reflecting means, and means for summing and differencing the position information derived from each system to derive the torsional motion and swaying motion respectively.

1 0. Apparatus for monitoring the movement of a point in a plane, comprising a laser light source arranged to direct a divergent beam onto said plane, a cube-corner retroreflector mounted with its apex at said point, a photodetector array.

means for focussing a return beam from said retroreflector onto said photodetector array, and scanning means for repeatedly scanning the output from at least some of the photodetectors to repeatedly sample the position of the focussed beam on the array which corresponds to the position of the point in the plane.

11. A system for measuring the position within a plane of a predetermined point substantially as herein described with reference to the accompanying drawings.

1 2. Apparatus for monitoring the movement of a point in a plane substantially as herein described with reference to the accompanying drawings.