CORRELATION FOLLOWER FOR TRACKING AN OBJECT

Abstract: A correlation follower comprising an image sensor (1) adjustable sideways and in height with a limited field of view, which is cyclically scanned by the sensor the output signal of which reflects the image content within the field of view and a video correlator (3) for controlling the alignment of the sensor in dependance of the output signal of the image sensor. The video correlator (3) has two addressable memories (12, 14) the one of which is a real time memory (12), i.e. in this memory a section of the field of view is stored for each cycle. The other memory is a reference image memory (14) and is updated with the content or the real time memory (12). During the correlation the contents of the memories are displaced in relation to each other and an error signal, corresponding to the position of displacement for which maximum correlation is achieved, is made to control, through said control circuits, the alignment of the sensor. To make the tracking process insensitive to disturbancies and image elements which appear momentarily in the field of view for each scanning cycle only a part of the positions of the reference image memory (14) are updated by selection the addresses to said positions randomly or according to a predetermined rule of selection. By that the positions which are updated during one cycle in positions are distributed over the entire area of the memory.

Description: Correlation follower for tracking an object Technical field The present invention relates to a correlation follower for track¬ ing an object, comprising an image sensor having a limited field of view and arranged to scan said field of view cyclically and to supply, in preselected form, a video signal representing the field of view, a video correlator with two addressable memories, one of which has the object of storing in digital form for each scanning cycle one section of the field of view, while the other memory is arranged to be updated with the content of the first memory, said video correlator being arranged to produce, once the image sensor has been so aligned that an object is encompassed within the sec¬ tion, an error signal controlling the alignment of the image sen- sor with the object, said error signal corresponding to a dis¬ placement of the section in the first memory relative to the sec- tion in the other memory, for which displacement a maximum corre¬ lation is achieved between the contents of the memories. Background art By correlation one can get, according to a predetermined rule of evaluation, a measure of how well the contents of the memories coincide at different relative displacements. Essential for the correlation and hence also for the tracking process is the way in which the other memory, subsequently referr- ed to as the reference image memory, is updated. At known corre¬ lation followers, see e.g. US Patent No. 3 828 122, updating is brought about in the course of one single scanning cycle, either periodically the updating process being repeated after a certain number of cycles, or when the maximum correlation drops below a preselected value. If an image element irrelevant to the track¬ ing process appears momentarily in the section the former updat¬ ing alternative entails the danger of the reference image memory being updated precisely when the image element appears, which may jeopardise the tracking process. With the other updating alternative the appearance of the image element may trigger up- « dating of the reference image memory. This leads in practice to tracking of the irrelevant image element. The object of the present invention is to bring about such updat¬ ing of the reference image memory that the above disadvantages are avoided and this is enabled in that the video correlator has means arranged to select at each scanning cycle addresses of posi¬ tions in the latter memory for the purpose of updating said memory randomly or according to a predetermined rule of selection so that said memory at each scanning cycle is updated only partly and in positions that are distributed over the area of the memory. Description of the drawing The invention is further explained below with reference to the attached drawing in which fig.l is a block diagram showing schema¬ tically the design of a correlation follower, fig.2 is a block diagram showing the design of a video correlator and fig. is a block diagram showing how updating is effected according to the invention. Description of a preferred embodiment In fig. 1 an image sensor of known type and consisting of a TV or IR camera with a field of view restricted in space is design- ated 1. The sensor is mounted on a platform 2 capable of being adjusted both sideways and in height and is so designed as to scan the field of view cyclically and to supply a video signal which reflects the image content within the field of view in electric form. The video signal is supplied, via a video corre- lator 3 connected with the sensor 1, to a monitor 4 on screen 5 of which the sensor's field of view is displayed. A cursor is superimposed over the video signal in the video correlator 3, the position of which on the monitor screen 5 is shown as a window 6 which an operator can move, with the aid of a control lever 7, to any position on the screen. Also the size of window 6 can be varied with the aid of control lever 7 and the said size OM so selected that the image 9 of an object 8 on screen 5 fits exactly within the window. In this way the effect of inter¬ ference from the surroundings of the object can be minimised, i.e. irrelevant background contrasts are screened off. Using control lever 7 the operator can also align the sensor 1 with object 8 and get the correlation follower to lock on the latter. In this process aligning signals from control lever 7 are trans¬ mitted via the video correlator 3 to an electronic platform system 10 in.which the aligning signals are converted into control signals for aligning the platform 2 and hence the sensor 1 both laterally and in height. After locking on, the sensor 1 tracks the movements of the object 8, whereby an error signal is extract¬ ed from the video signal of sensor 1 by means of a correlation process, which will be discussed in connection with fig. 2, the said error signal being converted in the electronic platform system 10 into control signals for aligning the sensor 1 as de¬ scribed above in connection with control lever 7. In fig. 2 twin-line arrows illustrate a flow of image information. This flow of information reaches an analog/digital-converter 11 from an image sensor of the above described type which is not shown in the figure. The A/D-converter 11 is designed to convert an analog video signal from the image sensor, the amplitude of which corresponds to the contrast at each point within the field of view of the sensor, to a digital signal containing in binary coded form the same data as the video signal. A part of the digital signal corresponding to the said section of the field of view of the sensor is read, during each scanning cycle, into a memory 12, designated henceforth as the real time memory. The reason for the designation "real time memory" consists in the fact that the information stored in the real time memory in real time corresponds to the image content in the scanned section. From the real time memory 12 the flow of image information passes, on the other hand, via an updating date 13 to a memory 14, which is the above-mentioned reference image memory, and on the other hand to a correlation computing circuit 15 which also receives OMPI image information from the reference image memory 14 forming an output from the latter. In the same way the designation "refer¬ ence image memory" points to the fact that this memory is to serve as a reference during the correlation process. The flow of image information is controlled by a control logic 15 in accordance with the result of the correlation circuit measure¬ ment as will be described below. As previously stated the image sensor can be made to lock on to the object, by means of- the control lever 7. When this is the case the content of the real time memory 12 is copied, during one scanning cycle, into the reference image memory 14. In the course of each scanning cycle, i.e. with each image read into the real time memory 12, the contents of the two memories are compared in the correlation computing circuit 15, the said con¬ tents being placed in different positions relative to one another. The comparison may be effected in accordance with any known method of correlation by means of which a factor of merit is calculated for each relative position of the image information in the two memories 12, 14. The relative position in which the highest factor of merit, i.e. the maximum correlation, occurs, is stored in a memory 17. Depending on the relative position in which maximum correlation is achieved the control logic 16 controls the flow of image information, i.e. the correlation computing process and the updating of the reference image memory 14. The control logic 16 has also the object to regulate, in response to signals from the control lever 7, the arrangement of window 6 and to supply error signals to the electronic platform system 10. Updating in accordance with the invention of the reference image memory 14 will now be explained with reference to fig. 3 in which as before twin-line arrows illustrate the flow of image informa¬ tion. In fig. 3 which shows especially how the real time memory 12 and the reference image memory 14 are addressed and the latter memory OM is updated, 18 designates an address counter by means of which partial elements in the memories 12, 14 are addressed sequenti¬ ally via the address correction circuits 19 och 20, respectively, so that these are passed through line for line until all partial elements have been covered. The address correction circuits 19, 20 are controlled by memory 17,as regards the relative displace¬ ment between the contents of the memories at which the highest factor of merit is achieved as described above. The amplitude values in the partial elements addressed during this process in the respective memory are read into an amplitude logic 21 or a balancing circuit in which, according to a special characteristic of the invention, the amplitude values in corresponding positions are combined with one another, it being stated according to a criterion applying to each combination of amplitude values with which amplitude value the addressed position in the reference image memory 14 is to be updated. Such a criterion may be, for instance, that with quick or large changes in contrast, i.e. with large amplitude differences between the contents in the addressed partial elements, a mean value should be formed by means of which the partial element in the reference image memory is updated. This is equivalent to a certain filtration which prevents track¬ ing of any image element which suddenly appears in the field of view. If the amplitude values in two corresponding positions are equal, updating will of course take place with this value, i.e. the content of the partial element in the reference image memory remains unchanged. According to the primary characteristics of the invention updating shall occur randomly or according to a predetermined rule of selection so that during each scanning cycle the memory is up¬ dated only partly and in positions that are distributed over the area of the memory. This is achieved by that the amplitude value stated in accordance with the above criterion being fed to the addressed partial element in the reference image memory 14 via a gate 22 controlled by a prime or random number generator 23. With prime number generation the control operates in such a way as to ensure that the gate 22 is opened for each p-th of the partial elements addressed by the address counter 18, p being a prime number the size of which is selected with a view of the required updating rate. Should p be set to 1 this would mean that the entire reference image memory 14 would be updated during one single scanning cycle as described above. Therefore the prime number is at least equal to 3. With random number genera¬ tion the gate 21 is opened once the address counter 17 has counted forward s partial elements, s being a random number, e.g. from a table of random numbers. Whenever the gate 21 is opened a new random number is supplied. In this case the updating rate varies owing to selection of different mean values for the table of random numbers. By that the updating is carried out according to the invention and thus is neither related to the result of the correlation nor carried out periodically, it is achieved that with great probabi¬ lity the reference image memory will not contains disturbing image elements and such that suddenly occur in the field of view of the sensor. This means that a correlation follower, the updating of the reference image memory of which is carried out as described above, is difficult to disturb and therefore the tracking of an object can be carried out with high accuracy. It is obvious that the invention can be modified in many ways within the scope of the inventive idea. It is possible, for instance, to utilize the video signals from a radar station for tracking an object. Further the updating may be carried out according to some other rule of selection than described above, e.g. according to a fixed pattern that is moved successively over the area of the memory. , . WI

Claims: Claims 1. A correlation follower for tracking an object, comprising an image sensor having a limited field of view and arranged to cyclically scan said field of view and to supply, in a preselected form, a video signal corresponding to the field of view, and a video correlator with two addressable memories one of which has the object of storing in 'digital form for each scanning cycle a section of the field of view while the other memory is arranged to be updated with the contents of the first memory, said video correlator being arranged to produce, once the image sensor has been so aligned that an object. is encompassed within the section, an error signal controlling the alignment of the image sensor with the object, said error signal corresponding to a displacement of the sec¬ tion in the first memory relative to the section in the other memory for which displacement a maximum correlation is achiev¬ ed between the contents of the memories, c h a r a c t e r-.. i s e d . in that the video correlator (3) has means (21, 22) arranged to select, for the purpose of updating the other memory (14), addresses of positions in the latter memory " randomly or according to a predetermined rule of selection so that the other memory (14) for each scanning cycle is up¬ dated only partly and in positions that are distributed over the area of the memory. 2. A correlation follower according to claim 1, c h a r a c t e r¬ i s e d in that, the video correlator (3) is arranged to select the addresses to the positions of the memory for the updating so that of addresses generated in a certain sequence each p-th is selected where p is a prime number. 3. A correlation follower according to claim 1, c h a r a c t e r- i s e d in that the video correlator (3) is arranged to read while updating each selected position in the other memory (14) , the contents stored in that memory and at the corresponding . IjlJREA t OMPI position of the first memory (12), to combine the contents of the said positions with one another and to state, in accordance with the criterion applying to each combination of contents, with which value the selected position in the other memory (14) is to be updated. BUR OA? ,< wip

Inventors: JONSSON R, LUDVIGSSON G, WARNSTAM L
Applicants: JONSSON R, LUDVIGSSON G, SAAB SCANIA AB, WARNSTAM L
Application year: 1978
Main classification: G01S3
IPC codes: G01S3, G01S13, G06T1, H04N5, H04N7