GB2043387A

GB2043387A - Optical velocity measurement

Info

Publication number: GB2043387A
Application number: GB7850049A
Authority: GB
Original assignee: Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1978-12-27
Filing date: 1978-12-27
Publication date: 1980-10-01
Also published as: GB2043387B

Abstract

An optical velocity measuring apparatus for use in a hovercraft, navigation system, consists of two photo-detectors 8, 9, arranged to view a relatively moving surface 4 through a rotating disc bearing a spiral continuous grating line 6 which provides the effect of a grating like sensitivity pattern for each detector moving in orthogonal directions. The movement of each sensitivity pattern contributes a component of velocity which combines with the velocity of the surface relative to the apparatus so that over a given range of relative surface velocities of either sense about zero the velocity representative frequency component of the signal generated by each detector is always in respect of a velocity of one sense only. In another embodiment the grating and detectors are replaced by a pair of orthogonally disposed, scanned, charged coupled device arrays. <IMAGE>

Description

SPECIFICATION Improvements in or relating to optical velocity responsive apparatus This invention relates to optical velocity responsive apparatus and more particularly to such apparatus in which a velocity dependent frequency component is derived from a photo-sensitive device receiving light from a relatively moving surface.

A typical optical velocity reponsive apparatus for indicating the velocity of a moving surface comprises a photo-detector arranged to receive light from the moving surface via an optical grating which gives rise to an output signal from the photodetector which contains a component of frequency which depends upon the velocity of the surface passing the grating.

One problem which is commonly encountered in a simple system of the above kind is that the sense of the velocity measured (i.e. the direction of movement) is not normally presented and if the optical velocity responsive system comprises part of a navigation system for, for example, a hovercraft, information concerning the sense of the velocity is usually essential. Methods are known by which the sense of the velocity can be determined. One method is to provide two photo-detectors one of which receives light from one series of strips of the surface and the other of which receives light from a second interposed series of strips of the surface the relative phases of the outputs of the two detectors being representative of the sense of the velocity.

Such methods, however, are not easy to implement.

In addition, particularly when the system comprises a navigation system, velocities which pass through zero are required to be measured and this involves the difficulty that the dynamic range of frequencies presented for processing is infinite.

One object of the present invention is to provide an improved optical velocity responsive apparatus in which one or more of the above difficulties are at least reduced. A further object of the present invention is to provide an improved navigation system utilising an optical velocity responsive apparatus.

According to this invention an optical velocity responsive apparatus comprises means for generating a signal having a frequency component related to the velocity of a surface moving in operation across a field of view of said apparatus by creating a grating like sensitivity pattern across said field of view and means for causing said sensitivity pattern to move across said field of view whereby itself to contribute a component of velocity which combines with the velocity of said surface relative to said apparatus such that over a given range of surface velocities including velocities of either sense about zero the said velocity related frequency component of the signal generated is always in respect of a velocity of one sense only.

The means for creating said grating like sensitivity pattern may comprise an optical grating filter as such interposed between said surface and a photoelectric detector in which case said optical grating may be formed by lines upon a continuously rotating drum whereby to provide said required movement of said sensitivity pattern or by an helical grating formed like a screw thread with suitable optics for imaging said surface thereonto but preferably said means for creating said grating like sensitivity pattern comprises a disc rotatable about an axis which is substantially perpendicular to said surface, said disc bearing at least one continuous grating line having a spiral formation from an inner region of said disc towards the outer rim of said disc and a photodetector being arranged to view said surface through said disc whereby the effects of said spiral grating line as said disc is rotated creates an apparent movement of the grating provided by said spiral grating line. Whilst for simplicity a single continuous spiral grating line is prefered a plurality of interleaved grating lines may be provided each having a different start and a different finish.

Preferably where said means for creating a grating like sensitivity pattern is a disc as aforesaid preferably a further photo detector or diode is positioned to receive light from light source through said grating whereby to generate an electrical signal representative of the apparent velocity of said grating occasioned by the rotation of said disc.

Preferably again where said apparatus is utilised in a navigation system two photo-detectors are provided to view said surface via grating like sensitivity patterns extending in orthogonal directions whereby the signal developed by one detector represents the value of one velocity vector of a velocity parallelogram whilst the signal developed by the other represents the value of the other velocity vector of said parallelogram.

Preferably, in such a case, means, such as prisms, are provided in the path of light from said surface to said two detectors whereby the sensitivity patterns of each have a common centre.

Preferably also each detector or the detectors are arranged to view said surface via a servo controlled zoom objective system.

In one embodiment of the invention, a laser rangefinder is also provided which is arranged, preferably via a semi-silvered mirror in the path of light to said two detectors, to transmit light to and receive light from a patch of said surface which is centred on the common centre of said overlapping sensitivity patterns.

Where optical means are not provided to compensate for range or height variations between said apparatus and said surface, means may be provided for compensating for such variations as determined by the output of said laser rangefinder.

The true corrected value of a velocity vector of said velocity parallelogram may be calculated from two values as determined and the apparent velocity of movement of said sensitivity pattern and the two vectors resolved to provide an indication of the true course of a body such as a hovercraft carrying said apparatus and moving over said surface.

In another embodiment of the invention the generating means and the means for creating said sensitivity pattern or patterns comprises one or two arrays (as the case may be) of charged coupled devices each array comprising a plurality of grating lines formed by lines of charged coupled devices each line of which is arranged to be sequentially addressed at a rate determined by the required apparent velocity of movement of the sensitivity pattern thus created. Normally the rate at which each line of charge coupled devices is addressed is controlled by means of a clock signal generator which also provides an output indicative of the apparatus velocity of the sensitivity pattern or patterns. Said apparatus may be arranged to be responsive to infra-red but preferably said apparatus is responsive to visible light.

The invention is illustrated in and further described with reference to the accompanying drawings in which, Figure lisa semi-schematic diagram illustrating one form of optical velocity responsive apparatus utilised in a navigation system in accordance with the present invention, Figure 2 shows explanatory graphs, and Figure 3 illustrates a modification.

Referring to Figure 1, part of the hull of a hovercraft is represented at 1. The hull 1 has a window 2 by means of which an optical velocity measuring apparatus indicated generally by the reference 3 may receive light from a surface 4 over which the hovercraft is passing.

A grating effect is provided by means of a transparent discS which bears a single, continuous, spiral grating line 6 which starts towards the centre of the disc 5 and finishes towards the outer rim of the disc 5. The grating line is formed, as represented by the insert 7 which shows part of the section through the disc 5, by a groove cut in the underside of the disc.

What is normally referred to in the art as one "line pair" is shown between the arrows "1.p.".

Positioned at right-angles to one another are two condenser/photo - detectors 8 and 9 which receive light from the surface 4 through the discS via a servo-controlled zoom objective system 10.

If the discS is rotated about its axis 11 as represented by the arrow 12, each condenser/photo detector 8 or 9 will, because of the effect of the spiral grating line 6, view a different patch of the surface 4 via a grating which appears to be in constant motion.

Thus condenser/photo - detector8 will view the surface 4 through an effective grating providing a senstivity pattern as represented at 13 whilst condenser/photo - detector 9 will view the surface 4 through an effective grating providing a sensitivity pattern as represented at 14, with the sensitivity patterns 13 and 14 apparently in motion in orthogonal directions as represented by vectors 15 and 16 respectively in the velocity parallelogram represented at 17.

Introduced between the servo controlled zoom objective system 10 and the surface 4 and two prisms represented at 18 which act to cause the sensitivity patterns 13 and 14 to overlap as represented in dotted outline at 19 so that in fact both sensitivity patterns 13 and 14 are centred on the same point.

Positioned respectively above and below the grat ing discS are a light emitting diode 20 and a photo detector 21. Photo-detector 21 will provide an output signal which is representative at any instant of the apparent velocity of movement of the grating provided by the spiral grating line 6.

Also incorporated in the system is a laser rangefinder 22. Light from the laser rangefinder 22 is directed on to and received back from a region 23 centred on the common centre of the overlapped sensitivity patterns 13 and 14, by means of a semisilvered mirror 24 interposed in the path of light from the servo-controlled zoom objective system 10 to the grating disc 5.

As will be apparent, if the disc5 is rotated at a constant speed the grating provided in respect of each of the condenser/photo - detectors 8 and 9 will apparently have a given velocity, with which the surface velocity combines either subtractively or additively. In this present case it may be assumed that the required velocity range of the actual surface velocity is from -20 to +200 knots. If the scaled appar entvelocityofthe grating is 100 knots the dynamic range of the resultant will be from 80 to 300 apparent knots and for processing purposes measurements would have to be made only over this relatively small dynamic range rather than a dynamic range which was infinite because of the fact that the true velocity being measured could be expected to pass through zero.

Graphically the effect is shown in Figure 2 in which the frequency spectrum of a simple fixed grating system assuming a required velocity range of -20 to +200 knots is shown atA and the corresponding frequency spectrum of the apparatus shown in Figure 1 is shown at B.

For the computation of the final true velocity the scaled apparent velocity of the grating (as monitored by photo diode 21) is subtracted from the velocity computed from the electrical signal generated by condenser/photo-detector sensor 8 in the case of the vector 15 and condenser/photo-detector sensor 9 in the case of the vector 16. The computation involved may be expressed thus:- If grating spatial frequency = fs line pairs per unit length.

Velocity of surface image relative to grating = v Then received output frequency (fundamental) from surface sensor f=v,fs If grating movement generated velocity = v2 Then grating frequency f2 = v2fs Thus as the required velocity = v1-v2 Then (v1-v2) = 1 [f42] fs The determination of the velocities from the signals generated by the sensors 8 and 9 and the photo diode 21 are in practice carried out in a processing circuit represented at 25, as well known per se.In practice the computation of the true velocity values of the vectors 15 and 16 by simple subtraction or addition of velocities is also carried out in processing pattern across said field of view and means for causing said sensitivity pattern to move across said field of view whereby itself to contribute a component of velocity which combines with the velocity of said surface relative to said apparatus such that over a given range of surface velocities including velocities of either sense about zero the said velocity related frequency component of the signal generated is always in respect of a velocity of one sense only.

2. An apparatus as claimed in claim 1 and wherein the means for creating said grating like sensitivity pattern comprises an optical grating filter as such interposed between said surface and a photoelectric detector.

3. An apparatus as claimed in claim 2 and wherein said optical grating is formed by lines upon a continuously rotating drum whereby to provide said required movement of said sensitivity pattern.

4. An apparatus as claimed in claim 2 and wherein said optical grating is formed by an helical grating formed like a screw th read with suitable optics for imaging said surface thereonto.

5. An apparatus as claimed in claim 2 and wherein said means for creating said grating like sensitivity pattern comprises a disc rotatable about an axis which is substantially perpendicular to said surface, said disc bearing at least one continupus grating line having a spiral formation from an inner region of said disc towards the outer rim of said disc and a photo-detector being arrangedto view said surface through said disc whereby the effects of said spiral grating line as said disc is rotated creates an apparent movement of the grating provided by said spiral grating line.

6. An apparatus as claimed in claim 5 and wherein a plurality of interleaved grating lines is provided each having a different start and a different finish.

7. An apparatus as claimed in claim 5 and wherein a single continuous spiral grating line is provided.

8. An apparatus as claimed in any of claims 5 to 7 and wherein a further photo-detector or diode is positioned to receive light from light source through said grating whereby to generate an electrical signal representative of the apparent velocity of said grating occasioned by the rotation of said disc.

9. An apparatus as claimed in any of the above claims and wherein two photo-detectors are provided to view said surface via grating like sensitivity patterns extending in orthogonal directions whereby the signal developed by one detector represents the value of one velocity vector of a velocity parallelogram whilst the signal developed by the other represents the value of the other velocity vector of said parallelogram.

10. An apparatus as claimed in claim 9 and wherein means are provided in the path of light from said surface to said two detectors whereby the sensitivity patterns of each have a common centre.

11. An apparatus as claimed in claim 10 and wherein said last mentioned means comprises prisms.

12. An apparatus as claimed in claim 2 or in any of the above claims as dependent upon claim 2 and circuit 25 as is the calculation of the resultant vector 26 of the velocity parallelogram 17. It will be appreciated, however, that these last two functions may quite readily be carried out manually by an operator using simple mathematics.

It should be noted that the velocities v, and v2 are those appearing at the grating itself. The system is, of course, range or height conscious (the heighth of the apparatus above the ground 4 is shown fore shortened in Figure 1) and if the system is not range compensated by optical means the values of the vel ocities appearing at the grating require to be mod ified by the magnification factorm to give the actual surface velocity: Vsurface = m [f-f2] fs For this last mentioned purpose processing circuit 25 is shown as deriving an input from the laser rangefinder 22.

It will be noted that the direction of motion along a grating vector is determinable by determining if the frequency generated by a detector (8 or 9) is above or below the frequency generated by diode 21.

In addition the effect of "shifting-up" all of the frequency components in the measured band simplifies the filtering out of the fO component with its associated side bands which could otherwise erroneously be measured rather than the wanted signal. The reduction in the dynamic range also renders easier the rejection of harmonics. In the modification illustrated in Figure 3 the mechanically rotated disc 5 together with the condenser/photodetectors 8 and 9 and the light emitting diode 20 with its photo diode 21 are omitted from the apparatus of Figure 1 and in the place of detectors 8 and 9 are provided orthogonally extending arrays 8' and 9' of charged coupled devices (CCD's). As represented each array 8' and 9' consists of seven grating lines. In practice of course there would normally be many more. Each grating line of charge coupled devices in each array 8' or 9' is sequentially sampled by a sampling switch 26 or 27 at a rate which produces the required apparent grating velocity.

The output 28 of switch 26 corresponds to the output of detector 8 in Figure 1 and the output 29 of switch 27 corresponds to the output of detector 9 in Figure 1.

Switches 26 and 27 are driven by a synchronous drive unit 30 which in turn is controlled by a clock generator 31 which provides signal on output 32 which represents the apparent grating velocity and therefore corresponds to the output derived from photodiode 21 in Figure 1. In other respects the apparatus is similar to that already described with reference to Figure 1 including the function of processing circuit 25.

Claims

1. An optical velocity responsive apparatus comprising means for generating a signal having a frequency component related to the velocity of a surface moving in operation across a field of view of said apparatus by creating a grating like sensitivity wherein each detector or the detectors are arranged to view said surface via a servo controlled zoom objective system.

13. An apparatus as claimed in claim 9 or in any of claims 10 to 12 as dependent upon claim 9 and wherein a laser rangefinder is arranged to transmit light to and receive light from a patch of said surface which is centred on the common centre of said overlapping sensitivity patterns.

14. An apparatus as claimed in claim 13 and wherein said laser rangefinder is arranged to transmit light to and receive light from a patch of said surface which is centred on the common centre of said overlapping sensitivity patterns via a semisilvered mirror in the path of light to said two detectors.

15. An apparatus as claimed in claim 13 or 14 and wherein means are provided for compensating for range or height variations between said apparatus and said surface as determined by the output of said laser rangefinder.

16. A navigation system including an apparatus as claimed in any of the above claims 9 to 15 and wherein means are provided for calculating the true corrected value of a velocity vector of said velocity parallelogram from two values as determined and the apparent velocity of movement of said sensitivity pattern and the two vectors resolved to provide an indication of the true course of a body carrying said apparatus and moving over said surface.

17. A navigation system as claimed in claim 16 and wherein said body is a hovercraft.

18. An apparatus as claimed in claim 1 and wherein said generating means and the means for creating said sensitivity pattern or patterns comprises one or two arrays (as the case may be) of charged coupled devices each array comprising a plurality of grating lines formed by lines of charged coupled devices each line of which is arranged to be sequentially addressed at a rate determined by the required apparent velocity of movement of the sensitivity pattern thus created.

19. An apparatus as claimed in claim 18 and wherein the rate at which each line of charge coupled devices is addressed is controlled by means of a clock signal generator which also provides an output indicative of the apparent velocity of the sensitivity pattern or patterns.

20. A navigation system including an apparatus as claimed in claim 18 or 19.

21. A navigation system or an apparatus as claimed in any of the above claims and wherein said apparatus is responsive to infra-red light

22. A navigation system or an apparatus as claimed in any of the above claims and wherein said apparatus is responsive to visible light.

23. An optical velocity responsive apparatus substantially as herein described with reference to Figure 1 ofthe accompanying drawings.

24. An optical velocity responsive apparatus substantially as herein described with reference to Figure 2 of the accompany drawings.

New claims or amendments to claims filed on 21.3.80.

Superseded claims 1-24 New or amended claims 1-20 CLAIMS

1. Velocity responsive apparatus comprising a sensor arranged to receive radiation from a field of view and means for forming a pattern across the field of view so that a signal produced by the sensor has a frequency component related to the velocity of the pattern relative to features in the field of view or vice versa the means for forming the pattern having a spiral shape and having means for rotating the spiral shape about an axis thereof so that the said pattern apparently moves across the field of view when the apparatus has zero velocity relative to the field view. of view.

2. An apparatus as claimed in claim 1 and wherein the sensor is a photo-electric detector and the means for forming the pattern comprises a spiral optical grating interposed between a surface in the field of view and the photo-electric detector

3. An apparatus as claimed in claim 2 and wherein said optical grating is formed by lines upon a continuously rotating drum whereby to provide said apparent movement of said pattern.

4. An apparatus as claimed in claim 2 comprising an optical system for imaging said surface onto the grating.

5. An apparatus as claimed in claim 2 and wherein said means for forming the pattern comprises a disc rotatable about an axis which is substantially perpendicular to said surface, said disc bearing at least one continuous grating line having a spiral formation from an inner region of said disc towards the outer rim of said disc; sensor being arranged to view said surface through said disc.

8. An apparatus as claimed in any of claims 5 to 7 comprising a light source and wherein a further sensor is positioned to receive light from the light source through said grating whereby to generate an electrical signal representative of the apparent veloc ityofsaid grating occasioned by the rotation of said disc.

9. An apparatus as claimed in claim 5 and wherein two sensors are provided to view said surface via different parts of the disc, whereby the signal developed by one detector represents the value of one velocity vector of a velocity parallelogram whilstthe signal developed by the other represents the value of the other velocity vector of-said parallelogram.

10. An apparatus as claimed in claim 9 and wherein means are provided in the path of light from said surface to said two sensors whereby the sensitivity patterns of each have a common centre.

12. An apparatus as claimed in claim 2 or in any of the above claims as dependent upon claim 2 and wherein each sensor or the sensors is/are arranged to view said surface via a servo controlled zoom objective system.

13. An apparatus as claimed in claim 9 or in any of claims 10 to 12 as dependent upon claim 9 and wherein a laser rangefinder is arranged to transmit light to and receive light from a patch of said surface which is centred on the common centre of said different parts of the disc.

14. An apparatus as claimed in claim 13 and wherein said laser rangefinder is arranged to transmit light to and receive light from a patch of said surface which is centred on the common centre of said different parts of the disc via a semi-silvered mirror in the path of light to said two sensors.

18. A navigation system or an apparatus as claimed in any of the above claims and wherein said apparatus is responsive to infra-red light

19. A navigation system or an apparatus as claimed in any of the above claims and wherein said apparatus is responsive to visible light.

20. An optical velocity responsive apparatus substantially as herein decribed with reference to Figure 1 of the accompanying drawings.