Position determination
This invention relates to methods of position determination. In particular the invention relates to methods of determining the relative positions of a first object and a second object. 
     Such methods are frequently necessary in robotics where, for example, it is required to place a manipulator adjacent to an object to be manipulated, the action taking place under the control of a computer. In order for the computer to receive information relating to the relative positions of the manipulator and the object, it is usual for two cameras to be provided, each camera viewing the scene including the manipulator and the object from a different angle, so as to provide stereoscopic information to the computer. The computer then uses the differences between the pictures provided by each of the cameras to determine the relative positions of the manipulator and the object. 
     It is an object of the present invention to provide a method of determining the relative positions of two objects wherein only one view, and thus only one camera need be provided, and also whereby the interpretation of the information provided by the camera to the computer may be simplified. 
     According to the present invention, a method of determining the relative positions of a first object and a second object comprises: illuminating said first object so as to produce a shadow of said first object on a surface, said surface being at a known position with respect to said second object; forming a two dimensional image of a scene including said surface; monitoring the distance between the position of said shadow and a reference position in said two dimensional image; and using said distance to derive information relating to the relative positions of said first object and said second object. 
     In one particular method in accordance with the invention said first object is illuminated so as to produce a second shadow of said first object on said surface, and said reference position is defined by the position of said second shadow. 
     The invention also provides apparatus for use in a method of determining the relative positions of a first object with respect to a second object comprising: a light source arranged to illuminate said first object so as to produce a shadow of said first object on a surface, said surface being at a known position with respect to said second object; camera means arranged to produce a two dimensional image of a scene including said surface; and means arranged to provide an indication of the relative position of said first object and said second object in dependence on the distance between the position of said shadow and a reference position in said image. 
     In a preferred form of apparatus in accordance with the invention, the apparatus includes a second light source arranged to illuminate said first object so as to produce a second shadow of said first object on said surface, the position of said second shadow defining said reference position. 
    
    
     One method and apparatus in accordance with the invention will now be described, by way of example only, with reference to the accompanying drawing which is a schematic plan view of the apparatus used in the method. 
    
    
     Referring firstly to FIG. 1, the method to be described is for use in a situation where it is necessary to use a manipulator 1 to grip a remote object 3. The manipulator 1 is carried by a flexible limb 5 consisting of several jointed portions, the limb being capable of extension so as to make the manipulator moveable under the control of a control unit 7. Associated with the control unit 7 are two light sources 9, 11 and a television camera 13 focussed onto a vertical surface 15 to which the object 3 is attached. 
     In use of the apparatus the sources 9, 11 are switched on in turn so as to illuminate the manipulator 1 such that two shadows 17, 19 of the manipulator are in turn cast on the surface 15. As the shapes of the shadows 17, 19 are related to the shape of the manipulator 1, these are easily recognisable in the image of the surface 15 produced by the camera 13, data derived from this image relating to the relative positions of the shadows on the surface 13 then being processed by computational means (not shown) within the control unit 7. As the relative postiions of the light sources 9, 11 are fixed and known, by the application of geometry, the proportion of the distance of the manipulator from the surface 15 to the total distance between the camera 13 and the surface 15 can be determined. As the distance of the manipulator from the control unit 7 can be estimated by the extensions and angles of the various portions of the limb 5, the actual range of the surface 15, and thus the object 3 can then also be estimated. Using this information, the limb 5 may then be extended until the manipulator 1 lies in the vertical plane including the object 3. As the shadows 17, 19 will move as the manipulator moves towards the surface 15, updated information relating to the relative positions of the object 3 and the manipulator is available. When the manipulator 1 lies in the vertical plane containing the object 3, further adjustment of the position of the manipulator until it lies adjacent to the object 3 is achievable by use of information derived from the two dimensional image of the object 3, and manipulator 1 produced by the camera 13. 
     It will be appreciated that whilst the particular method described above illustrates the principle behind the invention, many more methods in accordance with the invention are possible. For example, instead of calculating the proportion of the distance of the manipulator 1 from the surface 15 to the distance between the camera and the surface by geometrical considerations, the fact that the shadows 17, 19 will tend to move towards each other as the manipulator 1 moves towards the surface 15 may be used. When the manipulator reaches the surface 15, the shadows 17, 19 will merge, thus indicating that the manipulator 1 lies in the same plane as the object 3. 
     It will also be appreciated that any number of light sources may be used to give an equivalent number of shadows, generally more light sources, and thus shadows, affording greater accuracy. In one particular adaptation of the method, however, only one light source need be used, and the camera is arranged to monitor the displacement between the manipulator 1 and the single shadow cast on the surface 15. As the manipulator moves towards the surface 15, the manipulator and its own shadow in appropriate circumstances will tend to move towards each other. 
     It will further be appreciated that whilst it is particularly convenient for the surface 15 to be in the vertical plane containing the object 3, as long as the distance between the object 3 and the surface 15 is known, any convenient surface may be used.