Patent Publication Number: US-2005135914-A1

Title: Parallel positioning mechanism, especially for machining and/or manipulation and/or measuring

Description:
FIELD OF THE TECHNOLOGY  
      The invention relates to a parallel positioning mechanism, especially for machining and/or manipulation and/or measuring, consisting of a platform for the carrying and/or manipulation with, for instance, a tool, a workpiece, a measuring device or the like, wherein the platform is connected with the machine frame by means of at least one positioning arm.  
     CURRENT STATE OF THE ART  
      In the recent past, there has been proposed and constructed a multitude of parallel positioning mechanisms, primarily for the use in machine tools, for the manipulation of robots and the like. Manipulators and robots were primary devices in which parallel positioning mechanisms have been implemented. Their advantages when compared with the mechanisms used in serial robots are, above all, increased rigidity and improved dynamics. The higher rigidity is given by the very essence of the parallel arms as a latticework construction. Improved dynamics is to be found in the fact that the drives can be mounted on the frame and they need not be moved with the movable arms as they are in serial robots, from which there results a lower weight of the moving machine parts. A disadvantage of the existing positioning mechanisms is a more frequent incidence of singular positions, which limit the working space. In the singular position, the mechanism has excessive degrees of freedom and loses its rigidity.  
      Another disadvantage of many parallel positioning mechanisms is that the parallel arms are connected to the frame of the machine or, at the other end, to the platform carrying the tool, by means of a spherical joint, which has a lower rigidity due to the point contact of force transmission, or the joint is configured as a universal joint with limited movability.  
     ESSENCE OF THE INVENTION  
      The aforementioned disadvantages and drawbacks are, to a great extent, removed in a parallel positioning mechanism according to the invention, the essence of which resides in that the platform is joint-connected with at least one sliding guide via at least three rotatable joints of which at least two are arranged on a positioning arm for the connection with the sliding guide, wherein the axes of rotation of the rotatable joints are mutually parallel. The platform is joint-connected with at least one sliding guide via at least one positioning arm and further a rotatable joint for the direct connection with the sliding guide, wherein this direct connection can be replaced by means of at least three further positioning arms.  
      To advantage, the sliding guides extend along straight lines.  
      The platform may be connected with one sliding guide and through one positioning arm with this or another sliding guide.  
      Alternatively, the platform may be connected with one sliding guide and through two positioning arms with this or another sliding guide.  
      In another implementation, the platform may be joint-connected through four positioniong arms with at least two sliding guides.  
      In another implementation, the platform may be joint-connected through four positioning arms with four sliding guides.  
      The projections of the axes of rotation of the rotatable joints of the platform into a plane perpendicular to them form a trapezoid.  
      The sliding guides are parallel with one or two straight lines, or they are in mutually intersecting relationship.  
      The positioning arms are connected with the sliding guide through positionable carriages that are kinematically connected with driving motors through ball screws with nuts or by a linear drive.  
      The platform is provided with a translatable chuck, or a translatable work support is arranged in front of or on the base of the machine.  
      The sliding guides are arranged in a frame of the machine, which may be displaceable with respect to the base of the machine.  
      The construction of the parallel positioning mechanism according to the invention by a direct connection of the platform with the sliding guide through a single positioning arm or even by the connection of the platform with the sliding guide(s) by using excess, that is four, positioning arms for three degrees of freedom of movement, wherein the angular position in a plane can be held permanently constant, means an important increase in the size of the working space and simultaneously the achievement of an improved homogeneity of mechanical properties of rigidity and dynamics. The use of rotatable joints for all joints contributes to the increase in rigidity.  
      The direct securing of the platform to the sliding guide by means of the rotatable joint and by means of the single positioning arm, or the use of excess positioning arms for the connection of the platform with the sliding guide(s) reduces the occurrence of singular positions. Excess positioning arms significantly reduce inhomogeneity in the rigidity and dynamic properties in the working space of the machine. 
    
    
     OVERVIEW OF THE FIGURES OF THE DRAWING  
      The parallel positioning mechanism according to the invention is diagrammatically illustrated in the accompanying drawing, wherein  
       FIG. 1  shows the connection of the platform with a single sliding guide,  
       FIG. 2  shows the connection of the platforms with sliding guides with the use of a single positioning arm,  
       FIG. 3  shows the connection of the platform with the sliding guides utilizing two positioning arms,  
       FIG. 4  shows, in an isometric view, the connection of the platform with sliding guides by means of four positioning arms,  
       FIG. 5  shows in a front elevational view the positioning mechanism illustrated in  FIG. 4  but with a different arrangement of the sliding guides, and  
       FIG. 6  shows another alternative implementation, in which the platform is connected with fours intersecting sliding guides by means of four positioning arms.  
    
    
     EXAMPLES OF THE IMPLEMENTATION OF THE INVENTION  
      A parallel positioning mechanism for machining, for instance on machining centers, milling machines, for manipulation, for instance in industrial robots, with utilization, for instance, even for measuring and the like, consists of a base  1  of the machine, which carries a frame  2  of the machine. The frame  2  of the machine is arranged rigidly or slidably with respect to the base  1  of the machine, while sliding guides  4  are provided therein.  
      In  FIG. 1 , a platform  3  is connected with a single sliding guide  4  by means of three rotatable joints  11 , of which two are arranged on a single positioning arm  6 , whereas the third rotatable joint  11  connects the platform  3  with the sliding guide  4  directly. The rotatable joints  11  used in this implementation, as well as all other joints  11  used in all of the following implementations, are constituted by joints rotatable about a single axis, and are always mutually parallel.  
      As is evident from  FIG. 2 , two sliding guides  4  extend along straight lines that are intersecting; however, they may also be parallel. The platform  3  is arranged between the two sliding guides  4  and is connected with one of the sliding guides  4  by means of one positioning arm  6 , which is provided at each of its ends with a roratable joint  11 , one for a rotatable connection with the platform  3  and the other for the connection with the first sliding guide  4 , and that by means of a positionable carriage  5  which is movable in the sliding guide  4 . The platform  3  is further connected directly, via a further rotatable joint  11 , with the second sliding guide  4 . This rotatable joint  11  is also arranged on a positionable carriage  5 , which is movable in this second sliding guide  4 .  
      In  FIG. 3 , there is illustrated a similar parallel positioning mechanism to that evident from  FIG. 2 , with the platform  3  being connected with the first sliding guide  4  by means of two positioning arms  6 , of which each is provided with two rotatable joints  11 , one for the connection with the platform  3  and the other for connection with the sliding guide  4  via positioning carriages  5 . The connection of the platform  3  with the second sliding guide is similar to the implementation according to  FIG. 2 . It is also possible, withing the framework of the invention, to use a single sliding guide  4  in the implementation according to  FIG. 3  as well.  
      In  FIG. 4 , there is visible the implementation of a parallel positioning mechanism with four positioning arms  6 , wherein two of the positioning arms  6  each provided with rotatable joints  11  at each of its ends connect the platform  3  with one sliding guide  4  and the two other positioning arms  6  connect the platform  3  with another sliding guide  4  that is arranged at the opposite side of the platform  3  than the one sliding guide  4 . The sliding guides  4  arranged at each side of the platform  3  are formed in each instance by two guiding brackets, on which the positionable carriages  5  are movable. Each of the positionable carriages  5  is guided on both of the guiding brackets, and it is connected by means of the rotatable joint  11  with one of the positioning arms  6  in each instance. The positioning arms  6  that are guided on the first sliding guide  4 , and on the second sliding guide  4 , are arranged one underneath the other. The movement of the positionable carriages  5  is accomplished by ball screws  12  with nuts driven by driving motors  10 . In the alternative, the movement of he positionable carriages  5  can be solved as a drive with a linear drive (not shown). The primary part of the drive is then rigidly connected with the frame  2  and the secondary part is connected with the positionable carriages  5 .  
      The exact position of the positionable carriages  5  with rtespect to the frame  2  of the machine is measured by means of a measuring system  14 . The positionable carriages  5  conduct linear, straigt-line, movements on the sliding guides  4  on the frame  2  of the machine. This movement is thren transmitted through the rotatable joints  11  to the positioning arms  6  which are connected by the rotatable joint  11  with the platform  3 . The positioning arms  6  configured as a rigid spatial prism are thermally stabilized or alternatively they are manufactured from materials with a minimum longitudinal thermal expansion. The rotatable joints  11  of the positioning carriages  5  and the platform  3  are constituted by pre-stressed taper roller bearings with high radial and axial rigidity and strength.  
      The platform  3  carries a chuck  7  with a tool  8 , the translational movement of which is achieved by means of a linear rolling guide and a sliding block  13 . The movement of the sliding block  13  with the chuck  7  is accmplkished similarly to the drive of the positioning carriages  5 , also by means of a rotary driving motor and a roller screw (not shown). The sliding block  13  carries the drive for the Z-axis and the associated measuring system  14  and forms the translational movement of the chuck  7  with the tool  8  along the Z-axis of the machine. The sliding block  13  is mounted in the platform  3  on linear rolling guides.  
      Between the platform  3  of the machine and the frame  2  of the machine, there is arranged a counterbalancing system with a pneumatic cylinder  15  and a pressurized air container  9 ; by its force influence, it eliminates that of the weight of the platform  3  and of the parts contained thereon.  
      The rotary motors  10  control, via transmissions, spherical roller screws  12  with nuts, and with the aid of the measuring systems  14 , the translational movements of the positionable carriages  5  on the sliding guides  4  of the frame  4  of the machine. In  FIG. 4 , there is shown only the measuring system  14  for the measuing of the position of the positionable carriages  5  connected with one of the sliding guides  4 ; a symmetrically arranged measuring system for the measuring of the position of the positionalbe carriages  5  connected with the other sliding guide  4  is not shown.  
      The movement of the positionable carriages  5  is transformed via the positionable arms  6  and the rotatable joint  6  into the desired movement of the platform  3  in the X-Y-plane. The platform  3  carries the sliding block  13  with the chuck  7  and the tool  8 . Their translational movement along the Z-axis is accomplished by the displacement of the sliding block  13  with respect to the platform  3 . The tool  8  may be a machining tool, or a gripping tool for manipulation, or a measuring sensor for measuring. The translatable mounting of the chuch  7  may be replaced by the arrangement of a translatable working support in front of or on the base  1  of the machine (not illustrated); in the alternative, it is possible to use both the translatable chuck  7 , and the translatable working support.  
      In  FIG. 4 , the sliding guides  4  are parallel to a single straight line. It is also possible to use an arrangement in which each of the four positioning arms  6  is guided on an individual sliding guide  4 , or where three sliding guides  4  are provided for the four positioning arms  6 .  
      In  FIGS. 2, 3  and  5 , there is illustrated an alternative implementation of the arrangement of the sliding guides  4 , in which these guides are parallel to two stright lines, whereas  FIG. 8  shows the most general arrangement of the sliding guides  4 , where all of the sliding guides  4  extend along generic directions; in this particular case, two of the sliding guides  4  are mutually parallel and at the same in intersecting relatinships with the other two sliding guides  4 . There is also possible an arrangement of the four sliding guides where all of the sliding guides are in intersecting relationships with respect to each other.  
      In the framework of the invention, it is also possible to use sliding guides that are extending along courses deviating from straight lines; a necessary condition, though, is the assurance of the parallelism of the axes of the rotatable joints.