GB2097215A - Range finder - Google Patents

Abstract

A range finder (Fig. 1) e.g. for a submersible 12 positioned near an object 10 comprises a pair of t.v. cameras 15, 16 spaced apart on the submersible with overlapping fields of view. Camera signals are fed to a control station on a surface vessel in which the camera signals are displayed on a video monitor Fig. 2 (not shown) in the form of interlaced scans. A variable time delay element can be operated to delay the signals from the right hand camera 16 until the images appear at the same place on the monitor. The delay required to achieve this is a measure of the angular displacement of the images and, knowing the separation of the cameras the range of the object can be calculated. Preferably the video signals applied to the monitor are also applied to a correlator which for a range of small fixed time delays correlates the video signals for a "best match" and the delay required to achieve this forms a correlation to the time delay obtained manually during the visual correlation. A change from maximum correlation coefficient may be used to drive the vessel to restore maximum correlation enabling it to be positioned and held. A similar correlation of a frame-to- frame basis of one of the cameras may control lateral positioning and movement. <IMAGE>

Description

SPECIFICATION Range finder This invention relates to range finders and in particular to range finders employing optical imaging to give an electrical output indicative of the distance of the surface of a target object.

Range finders employing purely optical imaging techniques such as coincidence or splitimage types receive images of the target object along separate paths which are diverted and brought together either coincidentally or adjacently for comparative viewing, the degree of diversion required between the paths being a measure of the range of the target object. Such range finders have disadvantages in requiring the presence of an operator to determine when range information is available, continuous attention when range monitoring is required and a degree of fragility imposed by the provision of optical elements and means to divert the optical paths.

Other forms of range finders employing electromagnetic radiation in radar and analogous systems can be utilised remotely without the constant attention of an operator and can be manufactured robustly with few moving parts but their complexity increases the expense and if used remotely there is no provision for ascertaining that the target image is correctly acquired.

Systems adapted for remote operation using a combination of optical ranging principles and electronic imaging techniques have been suggested where the prime interest is one of continuous monitoring automatically. One example is described in UK patent specification No. 1,458,274 in which a laser beam scattered from a target surface is caused to fall obliquly on a television camera tube, the position at which it falls being related to the range of the target. The system described essentially uses the television camera as a large area photodetector but still requires the use of a specific light source and carefully aligned optics making it suitable for use only in a relatively unharsh environment.

It is an object of the present invention to provide a method of range finding and a range finder employing optical imaging onto television apparatus which mitigates at least some of the disadvantages of known range finding systems.

According to one aspect of the present invention a range finder comprises a pair of television cameras mounted a predetermined distance apart from each other with their fields of view overlapping means operable to delay the signals from one camera until the signals from both cameras, forming part at least of the image of an object whose range is to be determined, occur simultaneously within each television frame period and means responsive to the angular displacement of the object between the fields of view corresponding to the time by which the signals of said one camera and to the separation and inclination of said field of view axes to determine the range of the object from the cameras.

The means for delaying the signal may include a time delay element controlled by an operator to delay the signal from one or other camera by a variable amount and image monitoring means, arragnged to display images received from camera delayed in accordance with the setting of the time delay element, from which the delay can be set to cause the images to occupy substantially the same position within the fields of the monitoring means.

Alternatively, or preferably additionally, the means for delaying the signal may include timing means operable to delay the signals of either camera relative to the other by different predetermined time intervals, correlation means operable to perform a correlation between a set of signals at sample points derived from the frame video signal of one camera and a set of signals from corresponding points derived from the frame video signal of the other camera for each of said predetermined time delays to determine which time delay produces a maximum correlation coefficient, the overall time delay required between the signals representing said angular displacement of the object between the fields of view.

According to another aspect of the present invention a method of determining the range of an object from a range finder having a pair of television cameras mounted side-by-side with overlapping fields of view comprises delaying the signals of one camera with respect to the other until such time as there is correlation between the parts of the signals representative of the object, and from the time delay, which represents the angular displacement of the object within the fields of view of the cameras, and the separation the axes of the fields of view of said cameras, determining the range of the object from the cameras.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which Figure 1 is a schematic representation of a submersible vessel carrying a range finder according to the preset invention stationed in the vicinity of a submarine structure and whose distance from a feature of the structure is required, and Figure 2 is a schematic representation of signal processing circuitry of the range finder of Fig. 1.

Referring to Fig. 1 a submarine structure 10, for example a platform leg, has a removable cover 11. A submersible 1 2 movable fore and aft by a drive system 1 3 and laterally by a drive system 1 4. The submersible also carries tools and/or measuring instruments (not shown) for application to the structure and for which the distance of the vessel from relevant part of the structure is required to be known and the correct distance held by manoeuvring of the vessel.

The range finder is in two parts, one part carried by the vessel comprises a pair of television cameras, 15, 1 6 mounted a distance D apart at either side of the fore-aft line 1 7 of the vessel aligned in their direction of scanning. The cameras are mounted substantially the same distance from the nose of the vessel and arranged to view forward of the vessel with their fields of view (indicated by boundary lines 18, 1 9 respectively) overlapping and centered on axes 20, 21 respectively substantially parallel to the fore-aft line 1 7 of the vessel.

Camera signals are amplified by video amplifiers 22, 23 and passed by way of lines 24, 25 to the second part of the range finder carried on a surface vessel, or the platform itself, and under the control of an operator.

The line 24 is passed by way of a variable time delay element 26 to a first video input terminal of a television display monitor 27.

The line 25 is passed by way of a similar time delay element 28 to a second video input terminal of the monitor 27. The time delay elements 26, 28 are operable independently of each other to delay a signal passing therethrough by an amount set by the operator.

Each delay element also has an output line 29 upon which a signal representing the magnitude of the delay applied to the video signal appears.

The video outputs of the delay elements 26, 28 are also fed to a correlator device 30.

The correlator takes a set of sample points from the video signals representing each TV frame and delays one of the sets of samples by increasing steps until a maximum correlation coefficient is achieved as represented at output terminal 31. The time delay required for maximum correlation coefficient is represented by a signal appearing at delay output 32.

The lines 29 and correlator output 32 are both connected to a delay time summation circuit 33 which provides an output signal on line 34 to a computation circuit 35.

Considering operation of the range finder thus far described the submersible vessile is manoeurved such that it is positioned with the portion of the structure which is to be workedon lying on or near the fore-aft line of the vessel so that the portion of structure appears towards the centre portion of the images displayed. In this case there are few distinguishing features about the structure other than the cover 11 and the images of this are used to give a range indication.

The images of the cover 11, are within the field of view of the cameras. The video signals are displayed on the monitor 27 in the form of interlaced rasters such that the two images appear, one 15' due to camera 1 5 in which the cover 11 appears nearer the centre of the screen and one (16') due to camera 1 6 in which the cover appears further to the left, that is earlier in the raster scan.

It will be appreciated that the scan length S and associated scan time T of the raster on the monitor represent the angle of the field of view a of each camera and the separation of the parts of the image of the cover 11, from the centre of the image, ssS1, dS2 and associated 3Tt, ssT2 as a proportion of scan length and time respectively, represent the angular separation as a proportion of the field of view S.

Thus if the submersible is positioned as shown in Fig. 1 such that the cover 11 makes an angle 8, to the axis 20 of camera 1 5 and an angle H2 to the axis 21 of camera 16 the displacement of the cover from each axis is given by h, = RB, and h2 = D + h, = Ras2.

Thus D = h2-h, = R(82-8.) and range R = D/(82-H,) From the above discussion of the image it will be seen that a = S,,2/S,,2 = AT, 2/T 2 so that # = .a/T and 82 = St.a/T (02-01) = a(STh-ST1)/T and giving R = D.T/a(ST,-ST,) (1) The picture elements of the image 16' appear earlier (to the left) in the scan and the video signal from camera 1 6 is delayed by operating the delay element 28 until the readily identifiable image parts of cover 11 are brought into registration as observed by the operator. The magnitude of the delay introduced by element 28 is fed to summation means 33.

The video inputs to the monitor 27, which by operation of delay element 28 now receive very similar signals are also connected to the correlator 30. This operates by selecting a set of signal samples from different, corresponding points in the frame for each video signal and obtaining a coefficient of the correlation between the sets of samples with the sets delayed relative to each other by differing predetermined small time intervals until the maximum correlation coefficient is found, the delay required to achieve it appearing as an output signal at terminal 32. This delay represents a fine adjustment to the manually set delay of element 28 and the delays are summed in summation means 33 to give an accurate value of the overall time delay (ST2- AT,) discussed previously.

The overall time delay is fed to the computation circuit 35 in which are stored the parameters of a, D and T, and the value of R computed in accordance with equation (1).

In the particular example given part of the object of interest to the submersible for access is not the same part used for range finding target but if the relationship between those parts is well understood reasonably precise location is possible. However when the target is able to be located or near to one of the camera axes 20, 21 the images thereof will occupy the centre position of the screen and the delay adjustment correlation can be performed on samples from only such a centre portion of the screen. The number of samples can be considerably less than when the whole field has to be considered, enabling a simpler and faster correlator to be employed, or the density of samples increased to improve the accuracy of correlation.

In general the object of interest will present a suitable target for range finding and so will be located on the axis of the field of view of one of the cameras. If for example, in Fig. 2 the cover 11 lay on the axis 20, then Oi and ssT, would be zero. While the operation of the time delay element 28 is the same, fineadjustment correlation may benefit as described.

It will be appreciated that by making use of the visual correlation performed by the operator the correlator has only to perform with a relatively small number of potential time delays. If desired the manual correlation can be omitted, the correlator 30 accepting both video signals from the cameras and establishing the time delay between them necessary to achieve correlation and provide an indication of the range.Clearly the range of time delays required is much greater than when a coarse manual correlation is performed and the initial correlation may take a considerable time (in computing terms) but subsequently operation with the small changes expected in correlation intervals operation will be as for a combined system, However as the monitor screen is used by the operator to direct and observe subsequent operations by the submersible either one of the camera outputs must not be displayed or measured delay signal used to impose a delay in the appropriate signal to restore registration between the images.

The display of images from the two cameras in the embodiment described are shown as interlaced rasters to enable the operator to make line-by-line comparisons to judge the registration between the images. Other techniques may be employed, such as side-by-side displays on the same or separate monitors, or combined in one, to give a stereoscopic view of the scene when viewed through a spectacle arrangement to ease subsequent submersible operations, the manual correlation being based on a 'sharpening' of the image of the target when the appropriate time delay is applied for the range.

Furthermore, in the example given in which the cameras are mounted side by side with aligned scanning then for a conventional left to right raster scan the image from the right hand camera will always appear further to the left of the field of view than that from the left hand camera and is the only image which can be moved to the right, by delay, to align the images. Consequently for this operation the delay element 28 only is required. However in a situation where a different scanning scheme is followed or where the cameras are not aligned in the direction of scan, for example, where this is some separation perpendicular to the line of scan, it may be desirable to delay the video signals from the left hand camera by less than a multiple of one complete scan period to produce visual alignment of images.

To permit such variations in operation it is desirable to have the variable time delay element 26 as well as the delay element 28.

The provision of the variable time delay elements in each video line is useful in respect of the range finding arrangement including range control means adapted to hold the submersible at a constant distance from the target whose range is found. The correlator 30 produces a signal at 31 as aforesaid related to the magnitude of the correlation coefficient for each applied time delay, the signal being a maximum for true correlation.

A deviation circuit 37 is arranged to receive the correlator coefficient signal and produce a signal related to any deviation of the coefficient from the maximum. The signal is passed by way of direction sensing arrangement 38 to the fore-aft drive 1 3 of the submersible.

Once the submersible is establised at a station and the range from the target has been determined and found satisfactory, the distance holding function may be brought into operation. Any deviation in range due to movement of the submersible causes a fall in the value of correlation coefficient between the signals, which results in motion of the vessel until the maximum correlation coefficient is restored at the desired range.

Thus by this basic addition the system is capable of maintaining a desired range without any action on the part of the operator who is free to perform other tasks. It will be appreciated that as an extension of this, if at any time one of the delay element 26 28, is disturbed to introduce an additional time delay to one of the video signals, the correlation will be upset and the submersible will move to take up a position at a new range for which the new range for which the new time delay holds. This then represents a means of making controlled movements of the submersible towards and away from the target to define, measure and maintain a particular range. Furthermore, by introducing a varying time delay the vehicle may be driven to or from the object at a velocity determined by the rate of change of time delay.

The above described range finder may also be employed to determine not only the range movement but also lateral movement of the submersible, that is orthogonally to the foreaft line 17.

The video output of one camera, such as on line 24, is applied to lateral control means comprising a lateral correlator 40.

The correlator 40 includes a television frame store in which the signals of one frame are stored for correlation with corresponding signals from a subsequent frame. The size of the correlator and store are determined by the number of signal samples employed in the correlation and as explained in relation to the correlator 30, may be minimised by ensuring that the target object always appears towards the centre of the screen and restricting correlation to the appropriate parts of the frame signal.

The correlator 40 has an output 41 representing the frame-to frame correlation coefficient for the one camera and this is connected to a deviation circuit 42 corresponding to that 37 and thence to a direction sensing circuit 43 corresponding to that 38. The direction sensing circuit 43 provides an output signal to lateral drive unit 14 of the submersible.

In operation, the video signal for one frame of the camera is applied to the correlator 40 where samples from predetermined portions of the frame are stored. When the video signal of the next frame is received the stored samples are read from the store and replaced with samples from the current frame. The samples from the current frame are also correlated with the retrieved samples of the previous frame and a correlation coefficient produced at 41. If this is less than the maximum the drive unit 14 is operated and the submersible moved laterally, in the appropriate direction. As the position is restored the deviation from maximum correlation coefficient is reduced for each frame of the signal and the drive signal lessened until the vessel rests at the desired location.Any tendency to movement of the submersible, whether by sea currents acting on it or as a reaction to unplanned contact with the structure 10 will be counteracted by the lateral control means.

Correlation need not take place for each frame of the video signal but only with such frequency as is justified by the movements of the vessel.

The lateral control means described above may be used to implement a controlled lateral movement of the submersible relative to the structure. A time delay element 44 is included between video line 24 and lateral correlator 40. The element is manually adjustable in the same manner as elements 26 and 28 and is operated to introduce a small delay in the video signal. This leads to a reduced correlation coefficient for each television frame generated and thus to a constant driving speed of the drive means 14 in an attempt to eliminate the 'error'. Where the stored frame is not replaced by each currently received frame any constant delay signal applied to it to effect lateral movement will result only in a change of lateral position. To effect a continual change the delay must be increased as a function of the number of times the stored frame is retrieved.It will be appreciated that any genuine positional errors superimposed on this manually introduced one will be eliminated by variations in the actual drive speed in accordance with fluctuations in the level of correlation coefficient at 41.

Similar means (not shown) relating to movement in the third orthogonal direction, that is, control of depth, may also be employed. Thus it is possible to cause the submersible to take up and hold position at any desired point relative to the structure, providing there is sufficient detail to be seen to effect correlation between video signals, and by means of the artificially introduced time delays the submersible may be caused to traverse the structure.

The range finder described above and positional control of a vehicle to which it is fitted is not limited to use with a submersible vessel and may be employed with any vehicle or machine where accurate positioning is required under the control of a remotely located operator or where the opertor does not have access to a direct optical system for construc- tional or safety reasons.

For this reason the mechanical layout of the range finder has been shown schematically only. The cameras 15, 1 6 may be located at any convenient separation, although preferably at the same distance from the nose of the vehicle. Also the axes 20, 21 of the fields of view of the cameras have been described as parallel to each other and to the fore-aft line of the vessel. The axes 20, 21 may be inclined to each other if necessary and appropriate corrections made to the signals in computing the range.

From the schematic arrangement shown it will be evident that the cameras may be rigidly mounted so that the range finder consists of no moving parts and apart from protective mounting of the television cameras all other apparatus can be mounted in a protected or remote location. The cameras may be other than rigidly mounted and in particular, where caneras already exist on a vehicle such as a submersible vessel such cameras may be used for the range finder.

Claims (16)

1. A range finder comprising a pair of television cameras mounted a predetermined distance apart from each other with their fields of view overlapping, means operable to delay the signals from one camera until the signals from both cameras, forming part at least of the image of an object whose range is to be determined, occur simultaneously within each television frame period and means responsive to the angular displacement of the object between the fields of view corresponding to the time by which the signals of said one camera are delayed and to the separation and inclination of said field of view axes to determine the range of the object from the cameras.

2. A range finder as claimed in claim 1 in which the means for delaying the signal includes a time delay element controlled by an operator to delay the signal from one by a variable amount and image monitoring means arranged to display images received from each camera delayed in accordance with the setting of the time delay element, from which the delay can be set to cause the images to occupy substantially the same position within the field of the monitoring means.

3. A range finder as claimed in claim 2 in which the image monitoring means comprises a single monitor arranged to display at least the partial images of the two cameras.

4. A range finder as claimed in claim 3 in which the monitor is arranged to display the images in raster form interlaced.

5. A range finder as claimed in any one of claims 1 to 4 in which the means for delaying the signals includes timing means operable to delay the signals of either camera relative to the other by different predetermined time intervals, correlation means operable to perform a correlation between a set of signals at sample points derived from the frame signal of one camera and a set of signals from corresponding points derived from the frame signal of the other camera for each of said predetermined time delays to determine which time delay produces a maximum correlation coefficient, the overall time delay required between the signals representing said angular displacement of the object between the fields of view.

6. A range finder as claimed in any of the preceding claims for mounting on a vehicle movable by drive means towards and away from an object observed by the cameras including range control means responsive to a change in a measured time delay between camera signals to apply a signal to the drive means to restore the measured time delay and thus demanded separation between the rangefinder and object.

7. A range finder as claimed in claim 6 when dependent from claim 5 in which a change in correlator output indicating less than a maximum correlation coefficient provides an error signal for the control means.

8. A range finder as claimed in claim 6 or claim 7 including means operable to set a predetermined time delay into one or other of the video signals to cause the vehicle to assume said predetermined range.

9. A range finder as claimed in any of claims 6 to 8 including means operable to set a predetermined rate of time delay change into one or other of the signals to induce a rate of change of range of the vehicle related to the magnitude of the delay rate.

10. A range finder as claimed in any one of claims preceding claims for mounting on a vehicle movable by lateral drive means perpendicular to said camera axes of field of view lateral control means comprising correlation means responsive to the signals representing the image received by one of the cameras to correlate at least part of the signal representing a television frame with corresponding parts of the signals of a stored preceding frame to produce a correlation coefficient and determine from a correlation coefficient less than maximum lateral movement between the corresponding image portions indictive of lateral movement between the camera and object, means responsive to a change in output of the lateral correlation means from a maximum correlation coefficient to apply a signal to the lateral drive means to cause the vehicle to move laterally in such a direction as to restore the maximum correlation between said successive image signals.

11. A range finder as claimed in claim 10 in which each frame signal applied to the lateral correlator is stored to replace that of the preceding frame.

1 2. A range finder as claimed in claim 10 of claim 11 including means operable to delay current frame signals used in the lateral correlator to induce a lateral motion at a velocity related to the delay.

1 3. A range finder substantially as herein described with reference to and as shown in the accompanying drawings.

1 4. A method of determining the range of an object from a range finder having a pair of television cameras mounted side-by-side with overlapping fields of view comprising delaying the signals of one camera with respect to the other until such time as there is correlation between the parts of the signals representative of the object, and from the time delay, which represents the angular displacement of the object within the fields of view of the cameras, and the separation and inclination of the fields of view of said cameras, determining the range of the object from the cameras.

1 5. A method as claimed in claim 1 4 including displaying the camera signals as images on monitoring means, delaying the signals of one until the object appears to match the positions on the screen by visual correlation determining the time delay necessary to achieve the match.

16. A method as claimed in claim 15 in which the camera signals are displayed on a single monitor screen in raster form interlaced.

1 7. A method as claimed in claim 1 5 or claim 1 6 in which the signals, one of which is delayed to achieve visual matching, are also applied to a correlator device to determine any adjustment of delay between the signals required to given an accurate correlation and thus a more accurate time delay from which to calculate the range.

1 8. A method of determining the range of an object from a range finder having a pair of television cameras mounted side-by-side with overlapping fields of view substantially as herein described with reference to, and as shown in the accompanying drawings.