[0001]     This is a Continuation of application Ser. No. 10/501,475 filed Jul. 14, 2004 (now U.S. Pat. No. 7,168,290), which in turn is a National Stage of PCT/GB03/00175 filed Jan. 16, 2003. The disclosure of the prior application is hereby incorporated by reference herein in its entirety. 
     
    
     BACKGROUND  
       [0002]     The present invention relates to a method of and apparatus for aligning the components of an optical measuring system preparatory to using them in a measuring operation.  
         [0003]     One known type of optical measuring system consists of two or more housings, at least one of which is to be fixed to the bed of the machine and another one of which is to be carried by the arm or spindle of the machine. Either one or both of the machine bed and machine spindle is movable. One of the housings contains one or more light sources and detectors, and will be referred to hereinafter as the “source housing” while the other housing contains reflectors, and will be referred to hereinafter as the “reflector housing”. Usually the source housing is maintained in a fixed position on the bed of the machine and the reflector housing is mounted on a part of the machine moveable with respect to the machine bed e.g. the machine spindle.  
         [0004]     Aligning the optical components is often a time-consuming process which involves firstly the alignment of the source housing so that the beam or beams generated are directed along, or parallel to, one or more of the X, Y and Z axes of the machine. Then the reflectors have to be aligned with the beam or beams so that the reflected beams are directed back onto the detectors. Depending on the type of detectors being used the alignment may have to be accurate to within a few arc seconds.  
       SUMMARY  
       [0005]     A first aspect of the invention provides a measuring system for calibrating a machine, the measuring system comprising: 
        a base attachable to a surface of the machine:     a housing mountable on the base;     wherein at least one surface of the base and at least one surface of the housing are each provided with a complementary part of a mounting device, such that when the two parts of the mounting device are connected together, the housing may be aligned in any of a plurality of predetermined directions.        
 
         [0009]     Preferably the measuring system has at least two housings, comprising: 
        a base attachable to a first surface of the machine on which a first housing may be mounted;     a second housing attachable to a second surface on the machine, said first and second surfaces of the machine being moveable relative to one another;     said first and second housings each being provided with a complementary part of a first mounting device, such that when the two parts of the first mounting device are connected together, the housings are mutually aligned;     wherein at least one surface of the base and at least one surface of the first housing are each provided with a complementary part of a second mounting device, such that when the two parts of the second mounting device are connected together, the first and second housings may be aligned in any of a plurality of predetermined directions.        
 
         [0014]     The complementary parts of the second mounting device may comprise a set of cooperating elements on the base and the first housing. A subset of cooperating elements used to align the first housing in a first direction may also form a subset of cooperating elements used to align the first housing in a second direction.  
         [0015]     Preferably the second housing is mounted onto the second surface of the machine via a connecting device and wherein a plurality of surfaces on the second housing and at least one surface on the connecting device are each provided with a complementary part of a third mounting device, such that the second housing may be attached to the connecting device when orientated in any of the plurality of predetermined directions  
         [0016]     The complementary parts of the third mounting device may be arranged such that once the first and second housings have been aligned using the first mounting device, and the first housing and base have been aligned using the second mounting device, the second housing and the connecting device may be connected without realignment of the first and second housing relative to one another being required.  
         [0017]     Preferably the geometric combination of the first and second housings and the connecting device is such that the axes along which the first and second housings may be aligned intercept at a common point. This may be such that the housing mounted on the moving part of the machine starts in the same position in X, Y and Z whatever the orientation of the first and second housings. Alternatively it may be such that the housing mounted on the moving part of the machine is moved through the common point of interception, whatever the orientation of the first and second housings.  
         [0018]     A cable leads to the first housing and the cable may be provided with a cable mounting device, and the at least one surface on the cable mounting device and a plurality of surfaces on the base are each provided with complementary parts of a fourth mounting device such that the cable mounting device may be mounted on the base at different locations such that at each orientation of the first housing, the cable transmits an equal force on the housing. The cable mounting device may be provided with a plurality of angled faces, wherein two or more faces of the cable mounting device are provided with said complementary part of the fourth mounting device, such that different faces of the cable mounting device may be attached to base for different orientations of the first housing, such that the cable transmits an equal force on the housing for each orientation of the housing.  
         [0019]     A second aspect of the invention provides a platform for supporting a housing, the housing and platform being provided with complementary parts of a mounting device which define the position of the housing when mounted on the platform, comprising: 
        a fixed surface of the platform on which the housing may be supported and on which part of said mounting device is located;     a lifting mechanism moveable between upper and lower positions relative to said fixed surface;     whereby in its lower position, the lifting mechanism allows the complementary parts of the mounting device of the housing and the fixed surface to be in contact with one another and in its upper position, the lifting mechanism causes the complementary parts of the mounting device of the housing and the fixed surface to at least partly break contact with one another.        
 
         [0023]     Preferably the lifting mechanism comprises a movable surface of the platform which may be raised and lowered; 
        whereby when the housing is placed on the moveable surface of the platform, the moveable surface may be lowered to place the housing onto the fixed surface such that the complementary parts of the mounting device are connected or raised to disconnect the complementary parts of the mounting device.        
 
         [0025]     The movable surface and the housing may be provided with complementary parts of a second mounting device such that the complementary parts of the first mounting device on the housing and fixed surface are thereby pre-aligned.  
         [0026]     In a first embodiment, rotation of the movable surface in a first direction causes said movable surface to be raised and wherein rotation of the movable surface in a second opposite direction causes said movable surface to be lowered.  
         [0027]     In a second embodiment the movable surface is mounted on a spring whereby rotation of a cam raises or lowers the spring and thus the movable surface.  
         [0028]     A third aspect of the invention provides apparatus for adjusting the angle of an object about an axis mounted on a surface comprising: 
        an upper plate onto which the object is mounted and a lower plate which in turn is mounted onto the surface;     a track located on the inner surface of one of the upper and lower plates;     a ball located between the upper and lower plates, the ball being in contact with the at least one track in the upper or lower plates;     wherein the track is arranged such that when the ball is moved in a first direction, the ball is raised and causes the plates to move apart and wherein when the ball is moved in a second opposite direction, the ball is lowered and causes the plates to move together.        
 
         [0033]     The track may comprise a pair of non-parallel rollers. Alternatively, the track may comprise a pair of parallel rollers which are positioned at an angle from the plane of the upper or lower plate in which they are located, or a pair of rollers and wherein each roller in the pair of rollers is tapered.  
         [0034]     Preferably the other of the upper and lower plates is provided with at least one element which is in contact with the ball. The at least one element may comprise a pair of parallel rollers. Alternatively, the at least one element may comprise a plane surface.  
         [0035]     Preferably one of the at least one element and the track in the upper plate is part of a mount for the object and the other of the track and the at least one element of the lower plate is part of a mount for the surface, thereby forming a direct path from the object to the surface through the tracks, balls and elements.  
         [0036]     In a preferred embodiment, several sets of balls and tracks are provided so that the angle of the upper plate may be adjusted about several axes. In addition, a track and ball may be provided between adjacent substantially vertical surfaces of the upper and lower plates such that the upper plate may be rotated about the axis perpendicular to the plane of the lower plate.  
         [0037]     Preferably the apparatus is provided with at least two tracks and balls to adjust the angle of the upper plate relative to the plane of the lower plate and one track and ball to adjust the angle of the upper plate about the axis perpendicular to the plane of the lower plate, wherein the tracks used to adjust the angle of the upper plate relative to the plane of the lower plate are located in the lower plate so that during rotation of the upper plate, the elements in the upper plate may slide or rotate over the balls and thereby allow the upper plate to be rotated independently of the adjustment of the angle of the upper plate relative to the plane of the lower plate.  
         [0038]     Two sets of tracks and rollers and a pivot may be provided to allow adjustment of the angle of the upper plate relative to the plane of the lower plate. In addition, a third set of track and roller may be provided to provide rotation of the upper plate about an axis perpendicular to the plane of the lower plate.  
         [0039]     Three sets of tracks and rollers may be provided to allow adjustment of the angle of the upper plate relative to the plane of the lower plate and in addition allow adjustment of the height of the upper plate relative to the lower plate. A fourth set of tracks and rollers may be provided to provide rotation of the upper plate about an axis perpendicular to the plane of the lower plate. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0040]     The invention will now be described, by way of example only, and with reference to the following drawings in which:  
         [0041]      FIG. 1  is a diagrammatic elevation of the component of a prior art optical measuring system;  
         [0042]      FIG. 2  is a side view of an adjustable connector according to the invention;  
         [0043]      1   FIGS. 3A-3E  are views of the source and reflector housings aligned along the X, Y, -X, -Y and Z axis directions;  
         [0044]      FIGS. 4A and 4B  show the geometric combination of the source and reflector housings aligned along the Y and Z directions;  
         [0045]      FIGS. 4C and 4D  illustrate the interception of axes for different orientations of the housings;  
         [0046]      FIG. 4E  illustrates the errors along the X direction for  FIG. 4D ;  
         [0047]      FIG. 5A  is a plan view of the optical measuring system aligned with the Z axis;  
         [0048]      FIG. 5B  is a side view of the optical measuring system aligned with the Z axis;  
         [0049]      FIG. 5C  is a plan view of the optical measuring system aligned with the X axis;  
         [0050]      FIG. 5D  is a side view of the optical measuring system aligned with the X axis;  
         [0051]      FIGS. 6A and 6B  show plan and side views of a first embodiment of the controlled lowering platform;  
         [0052]      FIGS. 6C and 6D  show plan and side views of a second embodiment of the controlled lowering platform;  
         [0053]      FIGS. 6E-6G  show plan, side and perspective views of a third embodiment of the controlled lowering platform;  
         [0054]      FIG. 6H  illustrates a side view of the controlled lowering platform;  
         [0055]      FIGS. 7-9  illustrate plan, side and perspective views of the base plate;  
         [0056]      FIG. 10  is a cross section of the first tilt adjustment device;  
         [0057]      FIG. 11  is a cross section of the second tilt adjustment device;  
         [0058]      FIG. 12  is a cross section of the third location;  
         [0059]      FIG. 13  is a cross section of the rotation adjustment device;  
         [0060]      FIG. 14  is a schematic illustration of relative movement of the upper plate of the base plate;  
         [0061]      FIG. 15  is a plan and side view of non-parallel rollers;  
         [0062]      FIG. 16  is a plan and side view of parallel angled rollers;  
         [0063]      FIG. 17  is a plan and side view of tapered rollers;  
         [0064]      FIG. 18  is a plan view of a biasing spring used in the base plate of  FIGS. 7-9 ;  
         [0065]      FIG. 19  is a side view of the biasing spring of  FIG. 18 ;  
         [0066]      FIG. 20  is a perspective view of the biasing spring of  FIG. 18 ; and  
         [0067]      FIG. 21  is a schematic illustration of the arrangement of kinematic elements in a prior art base plate. 
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0068]     Referring to the drawings,  FIG. 1  shows a prior art embodiment of an optical measuring system for mounting on a machine as disclosed in co-pending application PCT/GB01/03096 filed on 11 Jul. 2001.  
         [0069]     The optical measuring system includes a base plate  10 , a source housing  20  and a reflector housing  22 , all of which need to be properly aligned with one or more of the machine axes. The base plate  10  is connected to the bed of the machine by screws  12 ,  14 .  
         [0070]     The source housing  20  may contain an autocollimator formed in optical sequence by, a light source  24 , a beam splitter  26 , a collimating lens  28  through which a collimated light beam passes out of the housing, and a detector  30  which receives a return light beam from the reflector  32  in the reflector housing  22  via the beam splitter  26 .  
         [0071]     The source housing  20  also includes a kinematic seat in the form of three cooperating pairs of male and female elements, for example a ball and V-groove  16 , suitably spaced and arranged in a triangular array, for example spaced at 120° apart. The seating elements  16  cooperate with three V-shaped grooves (not shown) on the base plate to form a conventional kinematic seat for repeatable positioning of the housing on the base plate.  
         [0072]     The source housing has a further kinematic seat  18  on its front face (i.e. the face which is orthogonal to the beam direction) on which the reflector housing may be seated. The light source and the reflector are aligned during the manufacturing stage to ensure that when the reflector housing is seated in the kinematic seat  18  on the front fact of the housing, the light beam and reflector are properly aligned.  
         [0073]     It can be seen therefore that once the source housing  20  is correctly aligned to direct a light beam along one of the machine axes, eg the X-axis, the reflector housing  22  can be seated on the kinematic seat  18  on the front face of the source housing  20 , and will automatically be aligned with the beam from the light source  24 . Magnets  33  may be used to urge the two housings  20 ,  22  together at the kinematic seat  18 .  
         [0074]     In order to take care of any mis-match in position between the machine spindle  34  and the reflector housing  22  when the two are to be connected together, the reflector housing  22  is provided with a limited amount of compliance by using an adjustable connector by means of which the housing  22  can be connected to the spindle  34  of the machine. The adjustable connector has a ball  36  which is to be seated in a socket  38  on the machine spindle. The ball  36  is adjustably supported in a retaining device  40  which, in turn is connected to the reflector housing  22 , by any suitable means.  
         [0075]     A preferred embodiment of the adjustable connector will now be described with reference to  FIG. 2 . The socket  38  of the machine spindle comprises a cylindrical bore which houses ball  36  of the adjustable connector. The retaining device  40  also comprises a cylindrical bore  42  and is mounted on the reflector housing  22 , preferably by kinematic mounts  52 .  
         [0076]     The ball  36  of the adjustable connector is connected by a stem  46  to a further ball  48  which lies inside the bore  42  of the retaining device  40 .  
         [0077]     The Balls  36  and  48  of the adjustable connector may only have a part spherical surface at the portion of the ball in contact with the surface of the cylindrical bore.  
         [0078]     The ball  36  can be adjusted through a limited angle to enable it to be engaged in the socket  38  of the machine spindle. The ball is retained in socket  38  in known manner by providing magnets (not shown) in the ball  36 , the socket  38 , or both.  
         [0079]     Two slits  54 ,  56  extends from opposite ends of the adjustable connector along its longitudinal axis to just short of its center, leaving just a small bridging portion  57  connecting the two halves of the adjustable connector together. A locking screw  58  is provided in the socket  38  of the machine spindle which when tightened pushes against the ball  36 , thus fixing the ball  36  within the socket  38  and also pushing the two halves of the ball  36  together. The bridging portion  57  of the adjustable connector acts as a hinge and as the two halves of ball  36  are pushed together, the two halves of ball  48  are pushed apart and against the sides of the cylindrical bore  42 , fixing it in position.  
         [0080]     This connector thus has the advantage that one actuation locks both balls.  
         [0081]     Once the source housing  20  has been aligned with an axis of the machine, the reflector housing  22  attached to the machine spindle can be brought up to the source housing  20 . With the locking screw loosened, the adjustable connector will be free enough to rotate so that the reflector housing  22  will seat in the kinematic seat  18 . By this means automatic alignment of the source housing  20  and the reflector housing  22  can be ensured. Once seated in the kinematic seat  18  the locking screw is tightened to maintain the orientation of the housing  22 .  
         [0082]     It is desirable to align the source housing  20  with other machine axes. In the above-described example, where the source housing is mounted on a base plate, the source housing may have other sets of kinematic elements on its lower surface or on other ones of its orthogonal faces. By this means it can be rotated through 90° in different planes and be re-seated on the kinematic seat on the base plate in different orientations with the light beam from the source directed along different ones of the machine axes. The reflector housing will continue to seat in the same kinematic seat  18  on the source housing so that it will also be aligned with the different axes.  
         [0083]     FIGS.  3 A-D show the plan view of the source housing  20  and reflector housing  22  on the base plate  10 .  FIG. 3   a  shows the light beam aligned with the X axis,  FIG. 3   b  shows the light beam aligned with the Y axis,  FIG. 3   c  shows the light beam aligned with the -X axis and  FIG. 3   d  shows the light beam aligned with the -Y axis.  FIG. 3   e  shows a side view with the light beam aligned with the Z axis.  100681  A set of kinematics elements are provided on the base plate and source housing to define each of the X, Y, Z, -X, -Y directions. Each set of kinematics are not necessarily independent, with balls and rollers or V-shaped grooves from one set also forming part of another set.  
         [0084]     Alternatively a block in the form of a cube or a cuboid may be used instead of a base plate. Such a block would be provided with kinematic seats on various ones of its orthogonal faces so that, by using a single kinematic seat on the source housing, it can be oriented in different directions by engaging its kinematic seat with any one of those on the block. Also in this case the reflector housing will continue to use the same kinematic seat on the source housing.  
         [0085]     For each orientation of the source and reflector housing  20 ,  22 , the reflector housing  22  must be mounted on the retaining device  40  of the adjustable connector in a different position. The location of the reflector housing  22  on the retaining device  40  for each orientation is defined by a respective kinematic seat. A different set of kinematic elements is thus provided between the reflector housing  22  and the retaining device  40  for each orientation of the reflector housing. As before, each set of kinematics may share elements with another set. This enables the orientation of the reflector housing  22  to be changed without adjustment of the adjustable connector in the machine spindle.  
         [0086]     Once the source and reflector housings  20 ,  22  have been aligned for the first axis using the kinematic seat  18 , the orientations of the kinematic elements between the source housing  20  and the base plate  10  and between the reflector housing  22  and the retaining device  40  means that for subsequent axes, realignment of the source and reflector housings  20 ,  22  on the kinematic seat  18  is not required.  
         [0087]     For calibration of large machines, it is desirable to start with the source housing  20  in the middle of the machine and first move the reflector housing  22  along one axis (e.g. X axis) and then turn the source and reflector housings  20 ,  22  around 180° and move the reflector housing along that axis in the opposite direction (e.g. -X axis). It is thus desirable for the source housing  20  and base plate  10  to have kinematics defining the -X and -Y directions.  
         [0088]     There are thus five sets of kinematics between the source housing  20  and the base plate  10  and between the reflector housing  22  and the retaining device  40  defining the X, Y, Z, -X and -Y directions, although each set is not necessarily independent from the other sets.  
         [0089]     As illustrated in  FIGS. 4A and 4B , the geometric combination of the source housing  20  and the reflector housing  22  allow the same co-ordinate start position for calibration of x, y and z axis after initial set up.  
         [0090]      FIG. 4A  shows the source housing  20  being aligned along the Y axis and  FIG. 4B  shows the source housing  20  aligned along the Z axis. In both cases the distance a between the quill  34  and the reflector housing  22  is the same. Thus when calibrating in different orientations (i.e. along the X, Y or Z axes), the start point is always the same.  
         [0091]     As illustrated in  FIG. 4C , the geometric combination of the source housing  20  and the reflector housing  22  is such that there is an interception of the axes (X, Y, Z) along which the source and reflector housings are to be aligned. Preferably the co-ordinate start position is at this interception O (as described above). However it is also possible to have non-common start positions, a 1 , b 1 , c 1  for each orientation, as long as there is an interception of the axes and the distance xa, xb, xc between each start point a 1 , b 1 , c 1  and the interception 0 is known. (Although these distances do not need to be known accurately). In this case, it is preferable that the start points are on the far side of the interception from the direction of travel, as illustrated in  FIG. 4C , as this results in no loss of information during movement of the reflector housing. As illustrated in  FIG. 4D , this would not be the case if the start positions a 2 , b 2 , c 2  are on the other side of the interception  0  as there will be no data between the interception and the start positions a 2 , b 2 , c 2  which will introduce errors.  FIG. 4E  shows the measurement data along the X axis for the arrangement of  FIG. 4D . There is no information between the origin O and the start point a 2 .  
         [0092]     The optical source may be located remotely from the source housing, particularly as heat from the optical source may cause distortion on the housing. An optical fiber may thus be used to channel the light from the light source to the source housing.  
         [0093]     The source housing is provided with a supply cable which houses the optical fibers, electrical signals and supplies. This is shown in FIGS.  5 A-D.  FIGS. 5A and 5B  show the plan and side view respectively of the source housing  20  aligned with the Z axis.  FIGS. 5C and 5D  show the plan and side view respectively of the source housing aligned with the X axis. The supply cable  60  transmits forces due to its bending onto the source housing  20  which may result in the source housing not sitting squarely on the kinematic seat  16 . It is therefore desirable to minimise the force due to the bending of the cable  60 .  
         [0094]     The supply cable  60  is provided with a cable mounting block  62  which may be attached to the base plate  10 . The mounting block can be clipped into various locations on the base plate  10  depending upon the orientation of the source housing  20 . The position of the cable mounting block  62  on the base plate is defined by a location seat which could be kinematic and is held in position by magnets (not shown).  
         [0095]     The cable mounting block  62  has a plurality of angled faces, such that in different orientations of the source housing  20 , different faces of the cable mounting block may be clipped onto the base plate  10 , by way of a respective seat, e.g. kinematic seat on that face.  
         [0096]     The position of the location seats and the angles of the faces of the cable mounting block  62  ensures that minimal and equal forces from the cable are transmitted to the source housing for each orientation of the source housing.  
         [0097]     In the present embodiment the base plate is provided with a controlled lowering platform so that the source block is lowered onto the kinematics in controlled way. Use of a controlled lowering platform has the advantage that the source housing is gently lowered onto the kinematics ensuring an accurate repeatable location. It also minimises damage to the kinematic elements.  
         [0098]      FIG. 6A  shows a first embodiment of the controlled lowering platform. A lifting platform  64  is provided onto which the source housing  20  may be placed. This is separated from the base plate by a rotatable disc  66 . Ball bearings  68  are located between the disc  66  and the base plate  10  and between the disc  66  and the lifting platform  64  to allow rotation of the disc  66 . The surface of the disc  66  is provided with tapered grooves into which the ball bearings  68  are seated. When the ball bearings are seated in the wide portion of the groove, the lifting platform  64  is in its lowered position. However as the disc  66  is rotated, the groove presents its narrower portion to the ball bearing  68  resulting in the lifting platform  64  being raised. By moving the disc  66  in the opposite direction, the lifting platform  64  may gently be lowered.  
         [0099]     A damper  70  is provided to smooth the movement of the lifting platform  64 . The damper  70  may be provided on a lever  72  which is used to rotate the disc  66 . Alternatively the damper  70  may be located between the base plate  10  and the lifting platform  64 , as shown in  FIG. 6B .  
         [0100]     A second embodiment of the controlled lowering platform is illustrated in  FIGS. 6C and 6D . In this embodiment, the lifting platform  64  is provided with a downwardly dependent cylinder  74  with a threaded outer surface. The central disc is replaced by a ring  76  with a threaded inner surface. Rotation of the ring  76 , using the lever  72  will result in the lifting platform  64  being raised by rotation in one direction and the lifting platform being lowered in the opposite direction.  
         [0101]     As in the previous embodiment, a damper  70  may be provided on the lever  72  as shown in  FIG. 6D  or between the base plate  10  and the lifting platform  64 , as shown in  FIG. 6D .  
         [0102]     A third embodiment of the controlled lowering platform is illustrated in  FIGS. 6E, 6F  and  6 G. In this embodiment the lifting platform  64  is supported by a pair of parallel springs  80 ,  82 . Both springs  80 ,  82  are connected to a fixed part of the platform (not shown) at points  84 ,  86 ,  88  on their outer surfaces and to the moveable platform  64  at their inner surfaces.  
         [0103]     A rod  89  is provided with a cam  94  at one end which abuts the upper spring  80  and a lever  96  at its other end. The rod  89  is rotatable about bearings  90 ,  92  in the fixed surface of the platform (not shown). Rotation of the lever  96  causes rotation of the rod  89  and cam  94 . As the cam  94  abuts the upper spring  80 , it raises or lowers the inner surface of the upper spring  80  as it rotates and thereby also raises and lowers the lifting platform  64  which is attached to the inner surface of the spring  80 .  
         [0104]     Use of a pair of parallel springs  80 ,  82  results in parallel movement of the lifting platform  64 , so that even though the cam is located on one side of the spring  80 , the lifting platform  64  is raised and lowered without tilting.  
         [0105]     In all three embodiments, whether the lifting platform has a rotational or linear movement, the mechanisms allow the movement of the lifting platform to be highly repeatable so that each time the source housing is lowered onto the kinematics, it is lowered accurately to the same position.  
         [0106]     The lifting platform of the controlled lowering platform may also be provided with a set of location elements  97 , with a corresponding set of location elements  98  on the source housing  20 , as illustrated in  FIG. 6H . These location elements  97 ,  98  act to correctly position the source housing  20  on the lifting platform  64  so that when the platform  64  is lowered, the kinematic elements  93  on the source housing  20  are pre-aligned with kinematic elements  95  on the base plate  10  and the source housing  20  may therefore be lowered correctly onto the base plate kinematics. These location elements  97 ,  98  on the lifting platform  94  and source housing  20  may be less accurate than the location kinematics  93 ,  95  between the source housing  20  and base plate  10 . The location elements  97 ,  98  on the source housing and lifting platform thereby receive most of the wear and thus protect the kinematic elements  93 ,  95  on the source housing  20  and base plate  10 .  
         [0107]     As described earlier, the source housing is mounted on a base plate which is in turn mounted onto the machine table. The base plate must be aligned with the X-Y plane and this is normally achieved by mounting it on an accurately horizontal machine bed. However if the machine bed is not accurately horizontal, then adjustment of the base plate is required.  
         [0108]     FIGS.  7  to  20  illustrate a base plate provided with an adjustment mechanism which allows for tilt adjustment about both X and Y axes and rotation about the Z axis. The adjustable base plate will now be described in more detail with reference to these figures.  
         [0109]      FIGS. 7, 8  and  9  show top, side and isomeric views of the base plate  10  respectively. The base plate comprises a lower plate  110  and an upper plate  112  which is moveable relative to the lower plate. The upper and lower plates may be connected by means (not shown) which allow relative movement between them, such as magnetic or spring means.  
         [0110]     First and second tilt adjusters  100 ,  102  are provided on the base plate to enable adjustment of the tilt of the upper plate about the X and Y axes respectively.  FIG. 10  shows a cross section of the first tilt adjuster  100 . A pair of rollers  114 ,  116  is provided in the upper plate  112  and a pair of rollers  118 ,  120  is provided in the lower plate  110 . A ball  122  is located between the upper and lower plates and is in contact with both pairs of rollers. The pair of rollers  114 ,  116  located in the upper plate  112  are parallel to one another. However the pair of rollers  118 ,  120  located in the lower plate  110  are non parallel. If the ball  122  is moved along the rollers  118 ,  120  towards the narrower end, the ball  122  will be pushed upwards and will in turn push the upper plate  112  upwards. If the ball  122  is moved in the opposite direction towards the wider end, the ball  122  will be lowered and in turn the upper plate  112  will be lowered. As shown in  FIGS. 7 and 9 , an adjustment screw  124  is provided to alter the position of the ball  122  within the first adjustment device  100 .  
         [0111]     As shown in  FIG. 10 , the pair of rollers  114 ,  116  in the upper plate  112  provide a kinematic seating for a kinematic element  128  of the source housing  20  and an element  130  is located in contact with and in a fixed position relative to the pair of rollers  118 ,  120  in the lower plate  110  to connect with the machine table  11 .  
         [0112]      FIG. 11  shows a cross section of the second tilt adjuster  102 . As before, each of the upper and lower plates  112 ,  110  is provided with a pair of rollers  214 ,  216  and  218 ,  220  respectively, which locates kinematic elements  228  and  230  in contact with the source housing  20  and machine table  11  respectively. As before, element  230  is in contact with and in a fixed position relative to rollers  218 ,  220 . However, the second tilt adjuster  102  differs from the first  100  in that a plate  232  is provided beneath the pair of parallel rollers  214 ,  216  in the upper plate  112 . The ball  222  between the upper and lower plates  112 ,  110  is thus in contact with the pair of rollers  218 ,  220  beneath it and the plate  232  above it. As before, adjustment of the position of ball  222  using the adjustment screw  126  raises or lowers the upper plate  112  directly above it.  
         [0113]     At a third location  104 , another ball  322  is provided between the upper and lower plates  112 ,  110  sandwiched between upper and lower pairs of rollers.  FIG. 12  shows a cross section of this arrangement. As before both of the upper and lower plates  110 ,  112  are provided with a pair of rollers,  314 ,  316  and  318 ,  320  respectively, and the ball  322  is in contact with both pairs of rollers. Ball  322  is in a fixed position with respect to rollers  314 ,  316  and ball  220  is in a fixed position with respect to rollers  318 ,  320 . In this case both pairs of rollers are parallel and the ball  322  is not provided with an adjustment screw.  
         [0114]     To adjust the tilt of the base plate  10  about the X axis, the adjustment screw  124  of the first tilt adjuster  100  is turned. This will either push the ball  122  and thus the upper plate  112  upwards or downwards, depending on the direction it is turned. As the adjustment screw  124  of the first tilt adjuster  100  is turned, the upper plate  112  will pivot about the ball  222  of the second tilt adjuster  102  and the ball  322  in the third location  104 .  
         [0115]     To adjust the tilt of the base plate  10  about the Y axis, the adjustment screw  126  of the second tilt adjuster  102  is turned. As described above, this will either push the ball  222  and thus the upper plate  110  upwards or downwards. As the adjustment screw  126  of the second tilt adjuster  102  is turned, the upper plate  110  will pivot about the ball  122  of the first tilt adjuster  100  and the ball  322  in the third location  104 .  
         [0116]     The base plate  10  also enables rotation of its upper plate  112  about the Z axis. A rotation adjustment device  106  is provided for this purpose. A cross section of the rotation adjustment device  106  is shown in  FIG. 13 .  
         [0117]     At the rotation adjustment device  106 , the upper plate  112  is provided with a cut out  140  and a portion  142  of the lower plate  110  extends upwards into the space provided by the cut out  140 . In this manner the upper and lower plates  112 ,  110  are provided with adjacent substantially vertical walls  144 ,  146 . As shown in  FIG. 13 , the substantially vertical wall  144  of the lower plate  110  is provided with a pair of rollers  150 ,  152 . The substantially vertical wall  146  of the upper plate  112  is provided with a plate  154 . A ball  156  is in contact with both the pair of rollers  150 ,  152  and the plate  154 . As in the first and second tilt adjusters  100 ,  102 , the rollers  150 ,  152  are not parallel. If the ball  156  is moved along the rollers  150 ,  152  towards the narrower end, the ball  156  will be pushed away from the wall  144  of the lower plate  10  and will in turn push away the wall  146  of the upper plate  112 . The upper plate  112  is thereby rotated relative to the lower plate  110 . By moving the ball  156  in the opposite direction, i.e. towards the wider end, the upper plate  112  will rotate relative to the lower plate  110  in the opposite direction. As before, an adjustment screw  127  is used to alter the position of the ball  156  on the rollers  150 ,  152 .  
         [0118]     When the upper plate  112  is rotated, it interacts with the balls and rollers in the first and second tilt adjustment devices  100 ,  102  and the third location  104  in the following way. At the first tilt adjustment device  100 , the parallel rollers  114 ,  116  in the upper plate  112  may slide over the ball  122  in the direction of the nominal center line of the rollers, or may rotate about the ball  122 . At the second tilt adjustment device  102 , the plate  232  of the upper plate  112  slides over the ball  222  and therefore does not constrain rotational movement of the upper plate  112 . At the third location  104 , parallel rollers  314 ,  316  in the upper plate slide over the ball  322  in the direction of the nominal center line of the rollers or the rollers rotate about the ball  322 .  FIG. 14  shows the movement of the upper plate  112  about each of the balls  122 ,  222 ,  322  of the first and second tilt adjustment devices  100 ,  102  and the third location  104  during rotation.  
         [0119]     As the balls  122 ,  222  of the first and second tilt adjustment devices  100 ,  102  remain stationary with respect to the rollers  118 ,  120 ,  218 ,  220  in the lower plate  110  and ball  322  of the third location  104  remains stationary with respect to the rollers  314 ,  316  in the upper plate  112 , rotation of the upper plate  112  has no effect on the tilt adjustment of the upper plate.  
         [0120]     In the above embodiment, the rollers  118 ,  120  in the lower plate  110  of the first and second tilt adjustment devices  100 ,  102  and the rollers  150 ,  152  of the rotation adjustment device  106  are non parallel such that one end of each pair is closer together than the other end. This causes the ball as it is moved along the rollers to be raised as it approaches the ends closer together or to be lowered as it approaches the ends further apart.  
         [0121]     By varying the angle of the rollers about their nominal center line, it is possible to vary the amount of the height lift of the ball for a given distance travelled along the rollers. This has the advantage that it is therefore possible to adjust the sensitivity of the base plate by altering the angle of the rollers.  
         [0122]      FIG. 15  shows a plan view of the non-parallel rollers  118 ,  120 . This effect can be achieved by alternative means, for example as shown in  FIG. 16 , parallel rollers  160 ,  162  are set at an angle from the lower plate  110 , so that the ball  164  is moved up and down a slope as it is moved along the rollers.  FIG. 17  shows a pair of tapered parallel rollers  164 ,  166  with one end of each roller being wider than the other end.  
         [0123]     In an alternative embodiment of the invention, the ball  322  and pairs of parallel rollers  314 ,  316 ,  318 ,  320  located at the third location  104  are replaced with an additional tilt adjustment device, of the same type as the first tilt adjustment device. This enables the height of the upper plate  112  along the Z axis to be altered, as now the height can be individually adjusted about all three tilt adjustment points.  
         [0124]     In each of the tilt adjustment devices and the rotation adjustment device, the balls are biased towards the adjustment device.  FIGS. 18, 19  and  20  illustrates the plan, end and perspective views respectively of a spring used to bias the ball towards the adjustment device. The spring  170  comprises a slide  172  which is located between the upper and lower plates  112 ,  110 . A surrounding plate  174  is provided which defines the boundaries of movement of the slide  172 . The surrounding plate is attached to the bottom of the upper plate  112 . Both the slide  172  and surrounding plate  174  may be made by a chemi-etch process from the same sheet which ensures that they are the same thickness. This ensures good tolerances which provides a good slide mechanism.  
         [0125]     One end of the slide  174  is provided with three tabs  176 ,  178 ,  180 . One of the tabs  180  protrudes upwards from the middle of the end of the slide  174  and abuts a ball  182  of an adjustment device. The remaining two tabs  176 ,  178  protrudes downwards from either side of the end of the slide  174  and abut springs  184 ,  186  located in channels  188 , 190  in the upper plate  112 . The springs  184 ,  186  extend between one end of the channels  188 ,  190  and the protruding tabs  176 ,  178  and thereby exert a force to bias the slide  172  towards the adjustment device  192 . As the slide  172  is pushed towards the adjustment device  192  by the springs  184 ,  186 , the tab  180  pushes the ball  182  towards the adjustment device  192 .  
         [0126]     An advantage of the adjustable base plate of the present invention is that the balls and rollers are in-line between the table, base plate and source housing, i.e. there is a direct path through the kinematic elements. For example, as shown in  FIG. 10 , the first tilt adjustment device  100  has a direct path from source housing  20  through kinematic element  128 , rollers  114 ,  116 , ball  112 , rollers  118 ,  120 , element  130  to table  11 . As shown in  FIG. 11 , the second tilt adjustment device  102  has a direct path from source housing  20  through kinematic element  228 , rollers  214 ,  216 , plate  232 , ball  222 , rollers  218 ,  220 , element  230  to table  11 .  FIG. 21  illustrates a prior art arrangement in which the kinematics between each part are offset. This has the disadvantage that any distortion of the base plate, e.g. due to thermal bowing, will cause a lever effect on the housing. In the present invention with a direct path through the balls and rollers, distortion of the plate has no effect on the position of the housing. The balls and rollers are made of a hard material such as steel or tungsten carbide.
A measuring system for calibrating a machine in which a base ( 10) is attachable to a surface of the machine and a housing ( 20) is mountable on the base, wherein at least one surface of the base and at least one surface of the housing are each provided with a complementary part of a mounting device, such that when the two parts of the mounting device are connected together, the housing may be aligned in any of a plurality of predetermined directions. The base includes a lifting mechanism ( 64) which when lowered allows the complementary parts of the mounting device ( 93, 95) of the housing ( 20) and the base ( 10) to be in contact and when raised causes them to at least partly break contact with one another. The level of the base ( 10) may be adjusted using a system of members and tapered rollers.