Abstract:
A linear guidance device adapted for use in a machine tool such as a hexapod includes a rail having an open region which extends, either completely or partially, in a direction parallel to the longitudinal axis of the rail, and a ball screw having a nut which is fixed to the rail and which either completely or partially penetrates the open region of the rail.

Description:
This application is a continuation of 09/004,944 filed Jan. 9, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention is generally directed to a linear guidance device that can be used in machine tools. The device is particularly adapted for use with hexapod systems. However, the device can also be used with other types of systems, if desired. 
     European Patent No. 0 489 857 shows an example of a typical hexapod system. Such hexapod systems typically include linear guidance devices having a saddle (or slide) which is mounted for translational movement along a rail. However, such devices typically have the drawback of being highly unbalanced (asymmetric) and relatively bulky. 
     Accordingly, it is an object of the present invention to provide a linear guidance device that can be used in a machine tool such as a hexapod and which is better balanced. 
     It is also an object of the present invention to provide a linear guidance device that can be used in a machine tool such as a hexapod and which is less bulky. 
     SUMMARY OF THE INVENTION 
     These and other objects which will become apparent are achieved in accordance with the present invention by providing a linear guidance device that can be used in a machine tool such as a hexapod and which comprises, in combination, a rail having an open region which extends, either completely or partially, in a direction parallel to the longitudinal axis of the rail, and a ball screw having a nut which is fixed to the rail and which either completely or partially penetrates the open region of the rail. The nut may be fixed directly to the rail, or using other components. 
     In a first alternative embodiment, one end of the ball screw is longitudinally immobilized and rotationally driven. The saddle supported by the rail is also immobilized so that the saddle is prevented from rotating. In operation, rotation of the ball screw causes the nut, and as a result, the rail secured to the nut, to be displaced longitudinally. The advantage of such an arrangement is that a better working symmetry is obtained, with linear guidance, while at the same time achieving results which are at least identical to those of known devices, and with a less bulky device. 
     In an alternative embodiment, the reading head of a measurement rule is fixed to the nut of the ball screw. 
     In another alternative embodiment, the ball screw is driven in translation, and the nut is driven only in rotation. 
     The present invention will be better understood with reference to the detailed description which is provided below, together with the following drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal cross-sectional view of a linear guidance device produced in accordance with the present invention. 
     FIG. 2 is a longitudinal cross-sectional view of an alternative embodiment linear guidance device, for measuring the displacement of an associated screw using a laser beam. 
     FIG. 3 is a longitudinal cross-sectional view of another alternative embodiment linear guidance device, for measuring the displacement of an associated screw using a measuring rule. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a linear guidance device  1  which is generally comprised of a linear rail  2 , a saddle  3  that can move relative to the rail  2  and which is guided by or on the rail  2 , and a ball screw  4  which operates through an associated nut S. 
     The rail  2  defines an open region  6 , the axis of which is parallel to the longitudinal axis of the rail  2 . These two axes may be the same, or may be different, as desired. The saddle  3  is prevented from rotating with respect to the rail  2 . The nut  5  of the ball screw  4  is fixed to the rail  2 , either directly or indirectly, by an appropriate fastener  7 . The outer end of the ball screw  4  (relative to the open region  6 ) is immobilized longitudinally. 
     In operation, the ball screw  4  causes displacement of the nut  5 , and in turn, the rail  2 . The saddle  3  will remain longitudinally stationary with respect to the rail  2 , and acts as a slideway. 
     A rule (not shown in FIG. 1) is generally provided for purposes of measuring movements of the linear guidance device, and the leg of the hexapod with which the linear guidance device is associated. The corresponding head for reading the rule (also not shown in FIG. 1) can be fixed either directly or indirectly to the nut  5  of the ball screw  4 . 
     Referring now to FIG. 2, an alternative embodiment linear guidance device is shown in which the ball screw is caused to translate. To this end, the ball screw  8  is associated with the two cardan joints  9 ,  10  of the leg of a hexapod, in this way allowing the length of the leg to be varied. 
     To this end, the nut  11  of the ball screw  8  is rotated by a motor  12  using a driving pulley  13  and a driven pulley  14  which are connected by a transmission belt  15 . The nut  11  is mounted in a barrel  19  by a journal  16  and rolling bearings  17 ,  18 . The barrel  19  forms a cage which is secured to a portion  20  of the corresponding hexapod leg and which is stationary in terms of translation. 
     FIG. 3 shows a structure which is substantially similar to the structure shown in FIG.  2 . However, the linear guidance device shown in FIG. 3 is further configured to solve a problem which can be experienced with known devices, which is that the ball screw (in addition to its main translational movement) can tend to experience a slightly “parasitic” secondary movement of oscillation about its axis, which can in turn result in a slightly wavy or “whiplash” movement. In practice, such movement has been found to make measurement of the actual displacement of the screw, and as a result, the hexapod leg, very difficult. 
     For the device shown in FIG. 2, and in accordance with the present invention, these (actual) measurements are obtained by making provisions for the ball screw  8  to be partially or totally bored with a blind hole  21 . A mirror  22  is placed in the closed end of the bore  21 . An optical fibre  23  is positioned between a laser optical unit  24  which is secured to the barrel  19 , and a laser source and measuring device  25 . This provides a compact and reliable system for accurately measuring the displacement of the leg. The optical fibre  23  can be replaced by any functionally equivalent system (e.g., a mirror with an angular return path, etc.), if desired. 
     For the alternative device shown in FIG. 3, an oscillating assembly  27  is provided. In the example shown, the oscillating assembly  27  includes appropriate bearings (e.g., ball-bearings, roller-bearings or needle-bearings) arranged at the free end  26  of the ball screw. The resulting assembly is connected by a mechanical link  29  to the saddle  28  of a linear guidance system  30 . The saddle  28  and the linear guidance system  30  are in turn connected to a head  31  for reading a measuring rule  32 . 
     It will be understood that various changes in the details, materials and arrangement of parts which have been herein described and illustrated in order to explain the nature of this invention may be made by those skilled in the art within the principle and scope of the invention as expressed in the following claims.