Abstract:
There is disclosed: a measurement scale suitable for use with scale reader for determining displacement; various tracks all for holding the scale and all having a channel into which the scale is magnetically attracted; and a scale tensioner which in use tensions the scale in situ by release of a preload, prior to the scale being secured to a substrate, the tensioner being removable thereafter.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a track arrangement for carrying a measurement scale for use in scale, a scale therefor, and to apparatus and a method for tensioning a measurement scale.  
         DESCRIPTION OF THE PRIOR ART SUMMARY OF THE INVENTION  
         [0002]    A known form of opto-electronic scale reading apparatus for measuring relative displacement of two members comprises a length of scale on one of the members, having scale marks defining a periodic pattern, a read head provided on the other member, means for illuminating the length of scale, periodic diffraction means for interacting with light from the scale marks to produce interference fringes having movement relative to the read head and detecting means in the read head responsive to the fringes to produce a measure of the displacement.  
           [0003]    An example of such apparatus is disclosed in EP-A-0 207 121 and also U.S. Pat. No. 4,974,962. Typically, the length of scale could be of spring steel with a surface layer of copper. U.S. Pat. No. 4,926,566 discloses a method of producing a length of scale, in the form of a flexible tape produced by rolling, the pitch of the scale marks being 20 μm or 40 μm for example.  
           [0004]    Conventionally the length of scale may be carried by being received by a track, for example comprising an aluminium extrusion, the lower part of the track being attached to a surface, for example by screws or bolts, and the upper part of the track being formed to receive the length of scale—see FIG. 1 which is a section through a length of track  1  which is attached to a surface  2  by screws  3  spaced apart along its length and lengths of clamping strip  4 , the upper part of the track  1  receiving a length of scale  5  which has been slid into it.  
           [0005]    The upper part of the track  1  has scale holding elements  1   a  for keeping the scale in place.  
           [0006]    However, such an arrangement can be difficult to make and to install. The small ride-height tolerance of a typical read head means that the track  1  requires accurate manufacture. The necessary clearance for the sliding fit of the scale  5  in the track  1  demands that the scale be manufactured even more accurately to compensate for this clearance.  
           [0007]    It is not desirable to clamp the scale  5  securely to the track  1  because there may be relative thermally induced movement between the scale and track, particularly if dissimilar metals are used. For this reason the prior art track shown in FIG. 1 allows sliding movement of the scale. The scale will produce more accurate readings if it is held at a fixed position, both lengthwise and laterally but should not be clamped to the track, and should not be subjected to any deviations in the forces holding it in position.  
           [0008]    According to one aspect of the present invention there is provided an elongate track comprising a channel, the channel being adapted for holding a measurement scale therein, wherein the track is adapted for attracting magnetically the scale to the channel of the track.  
           [0009]    The track could have magnetic material (for example a length of flexible magnetic material such as ferrite rubber) for holding the scale magnetically. In this case, the length of magnetic material could be in the channel of the track, the scale being above the length of magnetic material, for example sitting on edge portions (which are, for example, below the upper surface of the track) along respective sides of the channel.  
           [0010]    The track could be attached to the surface by screws passed through holes in it. Such holes could be at the bottom of the channel or in the track alongside such a channel.  
           [0011]    The track could be machined or an extrusion, for example an aluminium extrusion.  
           [0012]    Alternatively the track could be a length of magnetic material for example ferrite rubber, having the channel formed integrally therein.  
           [0013]    The invention extends to a track and scale therefor.  
           [0014]    It has been found that the scale may lift from a track or similar support under conditions where it is under compression. The present invention according to a second independent aspect relates to a scale tensioner, an embodiment of which prevents lifting of a scale.  
           [0015]    One scale tensioning device is described in U.S. Pat. No. 4,559,707 (Heidenhain). However this and other scale tensioners are directed to correcting errors in the scale&#39;s pitch rather than to prevent lifting.  
           [0016]    The constructional details of these prior art tensioners differ from the proposed invention which provides a measurement scale tensioning device comprising a preloadable resilient member operable to urge the scale into tension.  
           [0017]    The device may include a resilient member loading part operable to preload the member and operable to release the load.  
           [0018]    The invention according to yet another aspect provides a method of tensioning a measurement scale, comprising the following steps:  
           [0019]    providing a measurement scale, and scale tensioner;  
           [0020]    preloading the scale tensioner;  
           [0021]    mounting the scale to a substrate;  
           [0022]    securing one end of the scale to the substrate and clamping the tensioner to the substrate adjacent the other end of the scale;  
           [0023]    releasing the preload in the tensioner to cause a tension in the scale; and  
           [0024]    securing the said other end of the scale to the substrate whilst maintaining the tension in the scale.  
           [0025]    Preferably the method includes the step of providing a track to house the scale.  
           [0026]    The invention extends to a method for tensioning and if necessary repeating the tensioning. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:— 
         [0028]    [0028]FIG. 1 is a section through a known arrangement for carrying a length of scale by a length of track;  
         [0029]    [0029]FIG. 2 is a plan view of a length of track forming a first embodiment of the invention according to the first aspect of the invention;  
         [0030]    [0030]FIG. 3 is an enlarged view of part of the length of track of FIG. 2;  
         [0031]    [0031]FIG. 4 is a section through the length of track and showing a screw through a hole in it;  
         [0032]    [0032]FIG. 5 is a section through the length of track and showing a length of scale held on it;  
         [0033]    [0033]FIG. 6 is a section through a wider form of track;  
         [0034]    [0034]FIG. 7 shows a use of the wider form of track;  
         [0035]    [0035]FIG. 8 is a plan view of a length of track forming a further embodiment of the invention;  
         [0036]    [0036]FIG. 9 is an enlarged view of part of the track of FIG. 8;  
         [0037]    [0037]FIG. 10 a ) is a section through A-A of FIG. 9, FIG. 10 b  being an enlarged view of part of FIG. 10 a );  
         [0038]    [0038]FIG. 11 shows means for fixing lengths of track end to end;  
         [0039]    [0039]FIG. 12 is a sectional view of another type of track forming another embodiment of the invention;  
         [0040]    [0040]FIG. 13 is a sectional view of yet another type of track forming yet another embodiment of the invention;  
         [0041]    [0041]FIGS. 14&amp;15 show a plan and side view respectively of a scale tensioning device according to the second aspect of the invention;  
         [0042]    [0042]FIG. 16 is an enlarged partial view in the direction of arrow “X” in FIG. 14; and  
         [0043]    [0043]FIG. 17 is an isometric view of the tensioning device, scale and scale track shown in FIGS. 14,15 and  16 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0044]    Referring to FIGS.  2 - 5 , a length of aluminium extrusion provides a length of track  6  attachable to a surface by screws  7  passed through holes  8  spaced apart (for example with a pitch of 10 cm) along the length of track  6 . The length of track  6  is formed with a channel  9  running centrally along its length, the holes  8  being at the bottom of the channel  9 . Also, along each side of the channel  9  is an edge portion  10  below the upper surface of the length of track  6 .  
         [0045]    With lengths of track  6  attached to a surface by screws  7 , between each pair of holes  8  there is laid a strip  11  of flexible magnetic material, held on to the bottom of the channel  9  by a strip  12  of double-sided adhesive tape. By way of example, the strips  11  could be strips of ferrite rubber such as that provided by Magnet Applications Limited, of Northbridge Road, Berkhamsted, Hertfordshire HP4 1EH, England. Then, a length of scale  5  (for example for use in opto-electronic scale reading apparatus) is laid on to the edges  10  and held magnetically in place on the track  6  by the strips  11  of flexible magnetic material.  
         [0046]    [0046]FIG. 6 shows a section through a wider length of track  6  for use in the arrangement of FIG. 7 in which a read head carried by a sprung read head carriage  13  runs on the length of track  6 . A wheel  14  is fixed via a bracket to the read head carriage  13  and the assembly sprung towards the scale  5 . The wheel runs on the length of track  6 , maintaining the correct scale to read head gap substantially irrespective of undulations and steps in the surface to which the length of track  6  is attached.  
         [0047]    In the above embodiments, the length of track  6  is formed with a channel  9  running centrally. Referring to FIGS. 8, 9,  10   a  and  10   b  an alternative form of length of track  6   a  comprises an aluminium extrusion formed with holes  8   a  spaced apart along its length for attaching the track  6   a  to a surface by screws, the holes being offset from the longitudinal centre line of the length of track  6   a.  Also offset from the centre line is a channel  9   a  along each side of which there is an edge  10   a.  In use, the length of track  6   a  is attached to a surface, a strip of flexible magnetic material is held on to the bottom of channel  9   a  by a strip of double-sided adhesive tape and a length of scale is laid on to the edges  10   a  and held magnetically in place by the strip of flexible magnetic material. Such a structure may be used, like with the FIG. 7 embodiment, with a sprung read head carriage to which a wheel is attached, the wheel running on the upper surface of the length of track  6  alongside the channel  9   a.    
         [0048]    Such lengths of track  6  or  6   a  may be placed end to end with a small (˜0.5 mm) clearance. This is satisfactory unless the surface to which they are attached is uneven. A solution is to make each end of such a length of track as shown in FIG. 11 so two ends plug together by virtue of dowels  15  and holes  16 . In practice this could be achieved with two keyhole slots open at the bottom, a suitable dowel-pin being fixed in one of the holes. This enhancement would be particularly useful when used in conjunction with a sprung read head carriage.  
         [0049]    The scale may take the form shown in FIG. 12. In this embodiment an aluminium extrusion  20 , having the profile shown, is used as a scale supporting track. The extrusion  20  has a channel  22  for accommodating heads of securement screws etc which pass through holes  24 , shown in hidden detail. A further channel  26  is illustrated. Like the other embodiments this channel  26  accommodates a scale  5  and a magnetic strip  11  for holding the scale in place. The strip is held in place by means of adhesive.  
         [0050]    It will be noted that there is a slight concavity  28  to the underside of the track. The purpose of the concavity is to ensure that the underside stays in register with the surface to which it is mounted, and does not form a convexity. A convexity would allow the track to rock on the surface, and thereby reduce control of the height of the scale.  
         [0051]    In the above embodiments, the magnetic force holding the length of scale in place should not be greater than necessary since it is useful to minimise sliding frictional force between scale and track. This is achieved by the choice of air gap between the scale and the magnetic material.  
         [0052]    [0052]FIG. 13 shows yet another embodiment of the track. In this embodiment a flexible magnetic strip  30  is used to hold the scale  5 . The strip is formed from ferrite rubber of the type mentioned above. A channel  32  is formed in the track  30  for holding the scale  5  and provides lateral support for the scale. This track may be held magnetically to a ferrous substrate e.g. a cast iron machine tool bed. In use the track may be temporarily positioned for, say commissioning of the machine and then removed.  
         [0053]    A variant (not shown) of the track shown in FIG. 13 has a channel slightly wider than the scale to ensure that any radii at the internal corners of the channel do not lift the scale and distort its height.  
         [0054]    The tracks shown in FIGS. 12 and 13 may be employed in a manner similar to manner of use of the other tracks described above.  
         [0055]    It will be appreciated that the present invention may be used with scale and scale reading apparatus other than the opto-electronic type—for example scale and reading apparatus which operates by means of magnetic or capacitive detection can be used.  
         [0056]    Other modifications and variants will be apparent to the skilled addressee. For example the magnetic materials mentioned above might be replaced by electromagnetic elements causing attraction of the scale to the track  
         [0057]    The scale may be magnetised or may have a magnetic strip affixed thereto for co-operation with a magnetic or magnetised part of the track. The scale may gain its magnetic attraction from the substrate to which the track is attached, e.g. the scale may be magnetic and in use is positioned in close proximity (within the channel) to a ferrous substrate like cast iron.  
         [0058]    Referring to FIGS.  14 , 15 , 16  and  17  there is shown a scale tensioner which may be used e.g. to tension any of the scales described above. The tensioner is illustrated with a track similar to that shown in FIGS.  8 , 9 , 10   a &amp; 10   b,  although a track of any type might be used and in its broadest sense the invention according to the second aspect need not use a track. Tension in the scale prevents the possibility of compression and lifting of the scale.  
         [0059]    There is shown a length of track  6  and a scale  5 . The scale is secured at each end to a substrate  66  e.g. a machine tool bed. Two clamps, one for each end of the scale, each have two parts  42  and  44 . The scale is sandwiched and held between these two parts. The two clamps are secured to the substrate  66  by means of screws. The track  6  illustrated is secured to the substrate also, by means of screws extending through holes  46  in the track  6 .  
         [0060]    The tensioner comprises a main body  40  having a resilient coil spring  54  housed therein. The spring abuts a plunger  56 . The spring  54  is compressible within the body  40  to provide a preload and is held in a compressed state by means of an eccentric pin  52  acting to prevent movement of the plunger  56 . The pin  52  is rotatable with shaft  50 . The shaft  50  has a slot  62  for manual turning of the shaft e.g. with a screwdriver.  
         [0061]    The body  40  is securable adjustably using clamp screw  68  to the support  60  which is in turn securable to the substrate via one of the holes  46  normally used to secure the track to the substrate  66 , using bolt  70 . Adjustment of the position of the body  40  relative to the support  60  is achieved by means of a slot  48  in the body through which the clamp screw  68  may pass.  
         [0062]    In use the scale and track are mounted to the substrate  66 , and the two end clamps  42  and  44  are attached to the scale. One end clamp is secured to the substrate (holding the scale) and the other is loosely attached to the substrate (again holding the scale). Support  60  is secured to the substrate via the track mounting holes  46  and the tensioner body  40  is attached to the support and slid toward the loose end clamp i.e. the clamp shown in the Figures. The body  40  in its preloaded state is then tightened to the support.  
         [0063]    The body is initially in the condition shown in FIG. 16 i.e. pin  52  holds back the plunger  56  against the force of the spring  54 . Slot  62  in the shaft  50  is turned through 180° which moves the pin in an eccentric manner away from the plunger  56 . This action releases the tension in the compressed spring and forces the plunger against clamp part  42  (at the abutment point  64  shown in FIG. 16).  
         [0064]    The clamp part  42  is attached to the scale  5  so the scale will be forced into tension. The loose clamp can be tightened to the substrate and the body  40  and support  60  can be removed. The scale will remain in tension. The now vacant track securement holes can be used to secure the track.  
         [0065]    The preload in the spring is adjustable e.g. to give a correctly calibrated tensioning force. A screw  66  is used to adjust the tension in the spring to give the correct tension in the scale when the plunger is released. It is envisaged that the spring will produce a tension force in the order of 10 to 40 N.  
         [0066]    The process of scale tensioning may need to be repeated, particularly when a long scale is being tensioned. Long scales will stretch further than short ones when subjected to the same tension, so the stroke of the pin  52  may be insufficient to induce the required tension in the scale.  
         [0067]    In such circumstances the spring  54  can be compressed again by turning shaft  50  and the body  40  can be repositioned to abut once more clamp part  42  at abutment  64 . The scale clamp is loosened. The preload in the spring  54  is released by turning the shaft  50  and the scale is thereby tensioned once more.  
         [0068]    The clamp is then tightened to the substrate.  
         [0069]    The process may be repeated until no more extension is observed when the spring is released.  
         [0070]    The period of the marking etc used on a scale under tension may need to be shorter than those on a non tensioned scale because the scale may stretch. A scale with a finer pitch may therefore be provided for use with this tensioner.  
         [0071]    Other tensioner modifications and variants will be apparent to the skilled addressee e.g. the spring  54  may be replaced with a gas operated piston, in which case its preload may come from compression of entrapped gas behind the piston, or from a pressurised supply of gas. In either case the piston can be held back under preload by a restraint mechanism e.g. a pin similar to the pin  52 .