GB2197151A - Rangefinder - Google Patents

Abstract

A rangefinder for robotic applications such as the determination of the position, orientation or profile of an object at a range of 1 to 5 metres comprises a projector 1 for projecting a beam or plane of light 12 onto the object 14, a receiver 2 whose axis is substantially coaxial with the projected beam or plane, and a light receptive surface 3 onto which an image of the object is directed. The distance between the optical elements of the receiver 2 and the light receptive surface 3 is fixed, while at no object distance is the light receptive means 3 conjugate to the object. The profile of the blur (caused by the image of object 14 being out of focus) is processed to determine the range of the object. The rangefinder requires no focusing and is both small and robust. <IMAGE>

Description

SPECIFICATION Rangefinders The present invention relates to rangefinders.

Rangefinders are used in robotic applications to determine the position of an object being operated on by a robot, its orientation and profile, the distance involved being typically between 1 and 5 metres. Hitherto known rangefinders are generally cumbersome and expensive, using complicated lens systems and electronics.

The present invention seeks to provide an improved rangefinder.

Accordingly, the present invention provides a rangefinder comprising a projector for projecting a beam of light into a space containing an object whose range is to be measured; an optical receiver whose optical axis is substantially coaxial with said beam for directing light reflected from the object onto a light receptive means operable to generate an electrical signal representative of the pattern of light incident on the light receptive means; storage means for storing said electrical signal and processing means for processing the stored signal so as to determine the range of the object from a datum position related to the position of the receiver; wherein the distance between the or each optical element of the receiver and the light receptive means is fixed, the aperture of the optical receiver is fixed during measurements over a range of object distances and at no object distance is the light receptive means conjugate to the object.

A rangefinder according to the present invention projects a bright beam or pattern of light from a projector unit into a scene containing an object or objects to be recognised and monitors the out of focus blur pattern produced at some strategic position behind an optical receiving unit arranged effectively coaxially with the projected beam of light. Information obtained from the profile of the blur enables range data to be predicted for any orientation setting of the rangefinder provided the aperture of the optical receiving unit is covered by the reflected light from the object.

The term "light" as used herein includes not only electromagnetic radiation in the visible spectrum but also in the infra red.

The present invention is further described hereinafter, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a preferred embodiment of rangefinder according to the present invention; Figure 2a and 2b illustrate measurement of blur pattern profile; Figures 3a, b and c are respectively a side elevation, plan view and front elevation of a preferred lens system for an optical receiving unit of the rangefinder of Fig. 1; Figure 4 is a light ray diagram illustrating the relationship between object position, lens/aperture position, observation position and image position of the lens system of the rangefinder of Fig. 1; Figures 5a and b diagrammatically illustrate the averaging of picture element intensities effected by the rangefinder in order to reduce speckle effect;; Figures 6a, b and c show respectively a side elevation, plan view and front elevation of a two-cylindrical lens form of the rangefinder of Fig. 1; Figure 6d & 6e show respectively a side elevation and front elevation of a single lens form of the rangefinder of Figure 1; Figure 7 is a diagrammatic view of a further form of rangefinder according to the present invention; and Figures 8a, b and c are respectively a side elevation, plan view and front elevation of a further form of rangefinder according to the present invention.

The general arrangement of one form of the rangefinder is shown in Fig. 1 which shows a projector unit 1, an optical receiving unit 2, a light receptive surface 3, temperal integration means 4 and timing means 5, conversion means 6, storage means 7 and processing means 8.

The projected beam and axis of the optical receiving unit and light receptive surface are rendered coaxial as far as the objects in the scene are concerned using the coaxial means 9, which can be a beam splitting prism, half silvered mirror or similar device, to produce a common optical axis which is orientated in object space by the orientation means 10. This orientation means could constitute a robot arm or a pan and tilt head or some other similar device.Based on information contained in the storage means (such as a computer memory) derived from the light pattern on the light receptive surface, range data concerning the illuminated part or parts of the object or objects in the scene is determined using the processing means, this information being the distance of the illuminated part of the object from a predetermined datum or reference usually associated with the position of the optical receiving unit and generally located on its optical axis.

In this invention no focussing of the optical receiving unit and minimal (if any) focussing of the projector is used throughout the operation of rangefinding within a scene. With this system the light pattern on the light receptive surface 3 due to reflections off the illuminated objects is not an in-focus image, consequently temporal integration of the signal using the integration means and timing means are usually necessary to improve the signal to noise ratio before storing in the storage means. If the light from the projector is coherent, as would be the case if a laser is used, then spatial averaging can be accomplished on the sotred information using the processing means.The invention is thus characterised by the fact that it uses a projected light beam of constant intensity and maintains the distances between all optical elements of the optical receiving unit and between these elements and the light receptive surface fixed throughout the procedure of measuring the distance and three dimensional profile of objects in the scene. In one embodiment, for example, where only a single lens is used as the optical receiving unit and a TV camera as a combination of units 3, 4 and 6 (and possibly 5), a preferred arrangement is one in which the active surface of the camera is placed at the back focal plane of the lens.

In order that a good distance resolution be obtained, it is advantageous that the optical receiving unit has a large aperture. However, in such wide aperture systems, spherical aberration can introduce complications and so it can sometimes be advantageous to obstruct the region of the aperture around the optical axis of the unit provided the blockage does not unduly restrict the amount of light. This restriction cannot only improve the quality of the light pattern on the light receptive surface but can also afford a means of achieving a compact coaxial arrangement between projector unit and the optical receiving unit. Alternatively, it can constitute a smaller lens within the aperture of the front lens of the optical receiving unit to form an integrated vision/rangefinder system.

The light pattern projected into the scene can be a simple spot or several spots arranged in a regular pattern, or can be a vertical line or a set of vertical lines of equal horizontal spacing. The type of pattern used will depend upon the arrangement for orientating the beam in space, the amount of scene coverage required and the type of lens or lenses used in the optical receiving unit. By suitable choice of pattern and lenses the number of orientation settings can be reduced for full scene coverage.

The very nature of the invention allows considerable latitude on the type and wavelength of projected beam used. For example, it can be monochromatic or white light and coherent or incoherent, thus the projector unit could be a gas laser, a semi conductor laser or a white light projector, the choice depending on such things as availability, cost and any size and weight restrictions. Similarly the type of lenses used in the optical receiving unit do not have to be centro symmetric. Effective rangefinding can be achieved using the following arrangements for the optical receiving unit: i) Centro symmetric lens (or lenses) with circular aperture situated symmetrically about the optical axis ii) Centro symmetrical lens (or lenses) with an aperture having at least one pair of parallel sides which would form the effective rangefinding aperture (these sides to be called the range pair).In practice these sides, which would be symmetric about the optical axis would probably be either horizontal or vertical but could in fact be at any angle ç relative to the horizontal axis-where 0 can vary from 0 to 90 Irrespective of the angle 0 it is necessary as will be shown below, that measurement of the blur pattern profile should be made across the width of the blur perpendicular to direction of the parallel edges resulting from the restriction of light rays due to the range pair (see Fig. 2).

iii) Cylindrical lens (or lenses) In this case the sides of the lens itself can be used to form the aperture although a more accurate solution would be to provide an aperture having at least one pair of parallel sides, with these sides parallel to the non magnifying axis of the lens. In the case when two lenses are used (Fig. 3), one would be used merely to act as a condenser lens 16 with its magnifying axis perpendicular to that of the other cylindrical lens. This condenser lens would be positioned between the object and the rangefinding lens 18 with care being taken to ensure that none of its sides restricted the light rays reaching the light receptive surface (see Fig. 3).

Although it Is possible to use circular apertures to produce blur patterns as is the case in (i) above, it is generally better to use apertures with straight edges as in (ii) and (iii) irrespective of whether the lenses used are centro symmetrical or cylindrical.

The basic principle can be explained by considering the two dimensional (2D) case of the imaging of a point source of light situated at Z=Z on the optical axis by a single lens at Z=Z2.

Fig. 4 shows such a situation with observation being carried out in the back focal plane of the lens at Z=Z4. It canbe shown that the principle applies to the 3D cases pertaining to the use of a cylindrical lens and to a spherical lens with parallel sided aperture and also, with modification to a spherical lens with circular aperture.

The relationship between the position X and the direction cosine of a ray in planes Z=Z, and Z=Z4 is given by the following matrix equation (1) below, where x, is the direction cosine of a ray in plane Z=Z2 with respect to the X, axis, similarly jlXq iS relative to the 4 axis, thus:-

assuming the lens is thin and paraxial conditions hold. Where f is the focal length of the lens, Z12 is the object distance from the point source to the lens and Z24 the observation distance from behind the lens (see Fig. 4).

From Matrix equation (1) it can be seen that:

which reduces to: X4 = Z24 #X1 = @ #X1 (3) if observation is carried out in the back focal plane when Z24=f.

This expression shows that there is a one to one relationship between rays in plane Z=Z, having direction y,l and position (X4, Z4), i.e. all rays in plane Z=Z1 having the direction described by Yxl will pass through X4 in the back focal plane.

If the single point source is considered to be situated on the optical axis of the lens, then the direction cosines of the extreme rays that can enter the lens aperture are given by:

where P is the half width of the aperture of the lens (considered symmetrically disposed about the optical axis).

The corresponding positions where these rays cross the Z=Z4=f plane are therefore:-

The blur width Bw (=X4T~X4H) is therefore given by:

Now is Z,22 P2 then this expression can be approximated to 2Pf Bw= where u=Z12 (7) u This leads to the following rangefinding relationship: 2Pf u= (8) Bw where u is the estimate of the range obtained by measuring the blur width Bw.

If diffraction effects are ignored this expression can be modified to accommodate sources of finite width as would be obtained by illumination of objects with a narrow beam of light, however, the aim of the method is to keep the source width small compared with the aperture and on or near the optical axis of the lens.

If diffraction effects are taken into account-an inevitable consequence of using coherent laser illumination-then it can be shown that the above relationships still remain valid (with reserva tions).

If the light received back from the object is of low intensity the signal to noise ratio can be improved, at the expense of acquisition time, by allowing time for the signal to build up using the integration and timing means (4 and 5). These features are built into all modern solid state cameras.

As can be seen from Fig. 4, range information is obtained by limiting the extent of the rays reaching the observation plane by means of an aperture in plane Z=Z2. If the divergence of rays coming from the object is small such that all are completely captured well within the aperture of the lens (optical receiving unit) then the blur width will not give range data. This state of affairs will only occur in practice if the surfaces of the objects being illuminated are flat and very smooth such that specular reflections occur. It can be shown that surface roughnesses of the order of one wavelength are sufficient to scatter light to an extent to cover apertures of around 20mm at ranges of 1 metre.

If a laser is used to illuminate objects having diffuse like surfaces then Lambertian reflectance characteristics will occur and this will produce an objective speckle effect in the blur pattern 20 which will to some extent tend to distort the pattern and hence invalidate equation (8) if the blur width is measured along the 4 axis only. This problem can be significantly reduced if information is taken over an area in the back focal plane rather than along a line. To do this information over an area in the back focal plane can be converted to electrical signals using a suitable sensor such as a TV camera and the information stored in the memory cells 24 of a computer memory 26. Averaging can then be carried out down columns 22 corresponding to different values of X4 in the back focal plane (see Fig. 5) by the use of a simple computer program.As information over the complete blur pattern can be acquired, by suitable electronics, in one field (or frame) scan of the camera it is possible to produce averaged blur pattern data at speeds that make if possible to detect the ranges of moving objects-say those moving along a conveyor.

Referring to Fig. 1, the conversion, storage and processing would be achieved by the conversion, storage and processing means (items 6, 7 and 8) already mentioned which, in the above case, would correspond to the TV camera, buffer memory and associated computer device.

One of the most important features of a rangefinder is its ability to acquire range data from a scene at the required vertical and horizontal resolution levels at a speed which enables on-line operation. If a vertical line (or horizontal line) is projected into space then range data along this line can be obtained for a given orientation of the rangefinder thus improving the speed at which scene range information can be obtained.

Considering first a spherical lens system with a rectangular aperture, the fact that observation is not in the conjugate plane of the object means that, for a particular point in the blur pattern, light will be received from a vertical region on the object (or horizontal region if a horizontal line is used) and the extent of this region will dictate the vertical resolution (or horizontal resolution) of the arrangement.

If a vertical line is considered the extent of this region is 2E where E is half the height of the aperture and it is centred on the position Y4Z,2 fR where Y4 is the ordinate of the blur arrangement in the plane Z=Z4 with Z24=fR the back focal length of the lens and Z12 the object distance. It can be shown that the region which contributes to the light being received at Y4 does not, in this case, alter with range.

The general expression for effective vertical range coverage at Z12 for a single ordinate measurement in plane Z4 (not necessarily in the back focal plane) is given by:

from which it can be seen VR is not generally equal to 2E except when Z24=f.

If a spherical lens is used and measurements are taken in the back focal then VR will automatically be 2E irrespective of object distance whereas if two cylindrical lenses are used with mutually perpendicular axes it is possible to get VR to vary with object distance Z12 and yet effect proper ranging by choosing the lenses to have different focal lengths. Thus by measuring the width of the blur profile in the X (horizontal) direction in the back focal plane Z24=fR of the ranger lens arranged to magnify in the X direction and setting the other cylindrical lens of focal length fc in front of the rangefinder lens with its magnifying axis perpendicular to it both rangefinding and dependence of VR on distance can be achieved.This dependency will depend upon whether fR is less than or greater than fc and the choice would depend upon the range of applications to be covered. In practice fR and fc would not differ very much from each other mainly because of the need to keep the intensity of illumination of the blur as high as possible.

If observation is not carried out in the back focal plane then the relationship of equation (8) will not hold as can be seen by referring to equation (2). However, if the distance A of the observation plane from the back focal plane is small and Zt2 P then it can be shown that rangefinding can still be achieved using the expression: 2P(f+#) 2#P Bw= - (10) u f provided A is known.

A physical embodiment of the invention is shown in Fig. 6 showing the two cylindrical lens (Figs. 6aj b and c) and one single lens (Figs. 6d and e) versions. The lens versions illustrated in Fig. 6 comprise a laser diode source 32 with suitable shaping lenses 34 to provide the vertical stripe and beam expansion, a beam splitting prism 36, two cylindrical lenses 16, 18 with their axes perpendicular to one another, a solid state area camera 38 and an aperture 40 of rectangular shape. The camera has a light receptive surface 42 in the back focal plane of the lens 18, with additional electronics 44 being provided for signal processing. In both examples the projected beam 30 is shown as a vertical stripe to aid speed of range data acquisition and to reduce the need for elevation control of the rangefinder.In order to achieve full scene coverage, the device shown would need to be orientated either using a pan and tilt head (or pan head alone) or a manipulating device of some sort such as a robot. A further arrangement for increasing the amount of data at one orientation setting is shown in Fig. 7 showing three lens units 46 (each having a condenser lens 16 and rangefinder lens 18 mounted on top of each other with a single beam splitting device 36 and projector unit 1).

Although the invention so far has been concerned with the technique of rangefinding, it is possible when using a spherical lens or two cylindrical lenses, both arrangements with a rectangular aperture, to incorporate a normal vision feature into the system. For example, if the optical receiving unit is fitted with a focussing unit then, based on range information on an object obtained previously using the optical receiving unit in the mode described in the invention so far, the position of the focussing unit could be adjusted to bring the object or part of object into a clear focus onto the light receptive surface.

Alternatively, if the presence of the adjustable focussing unit described above is unacceptable, then an integrated vision/rangefinding system could be used as shown in Fig. 8. In this figure the central portion of the optical receiving unit is shown to comprise of a spherical rangefinder lens 18 with another lens 50 having a shorter focal length than the effective focal length of the optical receiving unit. Images from this inner lens 50 could be arranged to be in focus over the likely range of object distances by means of the large depth of field afforded by such lens parameters. This in-focus image, albeit of rather limited quality, could be used to provide a two dimensional picture which could be used, together with the range data, to gain information about the content of a scene.

In this integrated system two light sources would generally be needed to illuminate the object.

When operating in the rangefinding mode the object would be illuminated by the narrow beam of light 30 (say a laser beam) to enable the profile of the object to be obtained whereas in the two dimensional vision mode ambient lighting or some other source of overall lighting would be needed.

The most likely mode of operation would be to obtain the two dimensional information first with an orientation setting suitable to cover the whole scene followed by the range data acquisition procedure where the scene is scanned by the beam and data amassed and processed at each orientation setting.

In order to keep the size of such an integrated system down, the image obtained from the inner (central) lens 50 would be transferred from the image plane 52 to some other optical detector or to the same light receptive surface by means of coherent fibre optic bundles 54-although other methods such as the use of mirrors could also be used. The position of the coherent bundle attached to the light receptive surface could be at a region which creates the least problems when measurement of the blur profile is needed-one such position is the central position symmetrically disposed about the optical axis of the light receptive surface.

Whereas in Fig. 8 the vision lens is shown as a plano convex lens fixed via its flat face to the flat face of a plano convex rangefinder lens, this shows only one possible configuration of the vision and rangefinding lenses. Throughout the descriptions of the possible rangefinder and integrated vision/rangefinder systems shown, plano convex lenses have been shown, whereas these shapes are generally suited for the application, the invention does not rely of these shapes, for example, biconvex lenses can be used when appropriate.

If the projected beam suffers from an excess of divergence (or convergence) with distance it is possible to offset unwanted effects due to this divergence (convergence) by offsetting the position of the light receptive surface from the effective back focal plane of the optical receiving unit.

In a further embodiment of the invention a concave mirror replaces the centro symmetrical spherical lens in the optical receiving unit. In a still further embodiment a concave cylindrical mirror replaces the cylindrical lens in the optical receiving unit.

Claims (39)

CLAiMS

1. A rangefinder comprising a projector for projecting a beam of light into a space containing an object whose range is to be measured; an optical receiver whose optical axis is substantially coaxial with said beam for directing light reflected from the object onto a light receptive means operable to generate an electrical signal representative of the pattern of light incident on the light receptive means; storage means for storing said electrical signal and processing means for processing the stored signal so as to determine the range of the object from a datum position related to the position of the receiver; wherein the distance between the or each optical element of the receiver and the light receptive means is fixed, the aperture of the optical receiver is fixed during measurements over a range of object distances and at no object distance is the light receptive means conjugate to the object.

2. A rangefinder as in claim 1 wherein the light receptive means is situated in the effective back focal plane of the optical receiver.

3. A rangefinder as in claim 1 or 2 wherein said projector is operable to project a beam whose cross-sectional area is small compared to the aperture of the optical receiver.

4. A rangefinder as in claim 1, 2 or 3 wherein the aperture of the optical receiver is symmetrically disposed and perpendicular to the optical axis of the optical receiver.

5. A rangefinder as in any preceding claim wherein the optical receiver has a single centro symmetrical lens.

6. A rangefinder as in any of claims 1 to 4 wherein the optical receiver comprises two cylindrical lenses whose cylinder axes are mutually perpendicular and both perpendicular to the optical axis.

7. A rangefinder as claimed in claim 6 wherein the focal lengths of said lenses are substantially the same.

8. A rangefinder as in any preceding claim wherein the aperture comprises one pair of parallel sides used for range determination.

9. A rangefinder as in claim 8 wherein said parallel sides are at an angle 0 to the horizontal where can have a value in the range 0 to 90 according to the configuration of the optical receiver.

10. A rangefinder as in claim 5 wherein the range information is obtained from signals derived from the intensity profile of the pattern across and extending through the diameter of the blur incident on the light receptive means including the optical axis.

11. A rangefinder as in claim 9 wherein the range information is obtained from signals derived from the intensity profile of the pattern across and extending through widths of the blur incident on the light receptive means perpendicular to the parallel sides of the blur which correspond to the parallel sides of the aperture of the optical receiver.

12. A rangefinder as in claim 9 wherein the range information is obtained from signals derived from the intensity profile of the pattern across and extending through the width of the blur incident on the light receptive means, perpendicular to the parallel sides of the blur which correspond to the parallel sides of the aperture of the optical receiver and passing through the optical axis.

13. A rangefinder as in any preceding claim wherein the projected light beam is a narrow beam of light of approximately circular cross section.

14. A rangefinder as in claim 5 and 9 wherein the projected light beam is a vertical narrow stripe of light.

15. A rangefinder as in any of claims 8, 11 and 12 wherein the projected light beam is a narrow stripe parallel to the parallel sides of the aperture of the optical receiver.

16. A rangefinder as in any of claims 1 to 15 wherein the light receptive means comprises an area sensor with random access facility.

17. A rangefinder as in any of claims 1 to 15 wherein the light receptive means comprises a TV camera.

18. A rangefinder as in any of claims 1 to 17 wherein said projector has a monochromatic light source.

19. A rangefinder as in claim 18 wherein said light source is a laser device.

20. A rangefinder as in any of claims 1 to 19 wherein the central portion of the aperture of the optical receiver is symmetrically disposed about the optical axis and has different optical properties to the remainder of the aperture.

21. A rangefinder as in any of claims 1 to 20 wherein the central portion of the aperture of the optical receiver provides an opening for the projected light beam such that the beam is coaxial with the optical receiver.

22. A rangefinder as in any of claims 1 to 19 wherein the central portion of the aperture of the optical receiver forms the aperture of a single lens of different effective focal length to that of the remainder of the optical receiver.

23. A rangefinder as in any of claims 1 to 22 wherein said directing means comprises a beam splitting prism.

24. A rangefinder as in any of claims 1 to 23 further comprising orientation means for directing the optical axis of the rangefinder formed by a wrist or end effector of a robot.

25. A rangefinder as in any of claims 1 to 24 wherein the optical receiver is optically coupled to the light receptive means by coherent fibre optics.

26. A rangefinder as in any of claims 1 to 25 wherein the projector is remote from said directing means and is optically coupled thereto by means of fibre optics.

27. A rangefinder as in any of claims 1 to 26 wherein the storage means is a buffer memory.

28. A rangefinder as in any of claims 1 to 27 wherein the processing means is a computer.

29. A rangefinder as in any of claims 1 to 28 wherein said light receptive means comprises temporal integration means and timing means for improving the signal to noise ratio of said signal.

30. A rangefinder as claimed in claim 29 wherein said timing means comprises a microprocessor.

3 1. A rangefinder as in any of claims 1 to 30 wherein the optical elements of the optical receiver comprise one or more Fresnel lenses.

32. A rangefinder as in any of claims 1 to 31 wherein the light receptive means is movable longitudinally along the optical axis for achieving an in-focus image of the object.

33. A rangefinder as in any of claims 1 and 3 to 32 wherein the light receptive means is not in the effective back focal plane of the optical receiver.

34. A rangefinder as claim 33 wherein the position of the light receptive means is offset from the effective back focal plane of the optical receiver to offset unwanted effects of the projected light beam.

35. A rangefinder as in claim 33 wherein the position of the light receptive means is offset from the effective back focal plane of the optical receiver to offset the effects of divergence or convergence of the projected beam of light.

36. A rangefinder as in any preceding claim wherein the optical receiver has a lens with spherical surfaces.

37. A rangefinder as in any of claims 1 to 4 wherein the optical receiver has a concave mirror.

38. A rangefinder as in claim 37 wherein said mirror is a cylindrical surface concave mirror.

39. A rangefinder system substantially as hereinbefore described with reference to the accompanying drawings.