Patent Publication Number: US-6336375-B1

Title: Coordinate positioning machine

Description:
This is a division of application Ser. No. 09/161,284, filed Sep. 28, 1998, now U.S. Pat. No. 6,145,405 which in turn is a continuation of application Ser. No. 08/685,097, filed Jul. 22, 1996, now U.S. Pat. No. 5,813,287, which in turn is a Continuation-in-Part of application Ser. No. 08/396,721, filed Mar. 1, 1995, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The invention relates to a coordinate positioning machine such as a machine tool, inspection robot, or coordinate measuring machine. Coordinate positioning machines include a table for supporting an object upon which the machine is operating, and an arm movable relative to the table, typically with three linear degrees of freedom, which carries an operating module such as a cutting tool, an inspection probe, or a welding arm, for example. 
     2. Description of Related Art 
     Conventional coordinate positioning machines support the movable arm either, in the case of a robot, by a plurality of serially mounted rotatable joints, or, in the case of a machine tool and coordinate measuring machine, on a plurality of serially mounted linear guideways. In each case the serial mounting of the movable arm results in different inertial loads on the machine when the movable arm is displaced in different directions, due to the differing number of moving machine parts which must be displaced to enable such movement. Additionally, any force applied to the movable arm, for example via the operating module, will result in bending moments being applied to at least part of the structure which supports the arm. 
     In an alternative form of coordinate positioning machine, the movable arm is supported by a plurality of members, each of which is connected to the mechanical earth of the machine, such as the table, for example. Machines of this type are known from e.g. International Patent Application Nos. WO91/03145 (Kearney &amp; Trecker) and WO92/17313 (Geodetic Machines), in European Patent Application No. 534585 (Ingersoll), and U.S. Pat. No. 4,732,525, and typically include a movable arm, supported relative to a fixed, or “earthed” structure by means of a plurality of telescopic struts. Movement of the movable arm is achieved by extension and, where appropriate, contraction of one or more of the struts. A further type of coordinate positioning machine is shown in U.S. Pat. No. 4,976,582. 
     SUMMARY OF THE INVENTION 
     The present invention provides a coordinate positioning machine having: a fixed structure; an arm, supported for movement relative to the fixed structure, upon which an operating module may be mounted; the arm being supported relative to the fixed structure by three telescopic struts, each having a motor which is actuable to increase or decrease the length of the corresponding strut; the struts being universally pivotally connected at one end to said arm, and at the other end to said fixed structure, the arm thereby possessing three rotational degrees of freedom for any given combination of lengths of the three struts; constraining means acting between the fixed structure and the arm, for constraining movement of the arm with each of said three rotational degrees of freedom to within predetermined limits, while simultaneously permitting translation of said arm with three linear degrees of freedom, and including at least one passive device which eliminates one of said rotational degrees of freedom. 
     In one preferred embodiment, the constraining means is entirely passive, and constrains movement of the arm with one of said rotational degrees of freedom to within predetermined limits, while eliminating movement of the arm with the remaining two rotational degrees of freedom. In a further preferred embodiment, the constraining means is entirely passive, and eliminates movement of the arms with all three rotational degrees of freedom. 
     Measurement of the displacement of the arm with the available degrees of freedom may be detected, to the extent desired, in any convenient manner. When rotational movement of the arm is constrained to within predetermined limits, detection of rotational displacement may be necessary depending upon the function which the machine is desired to perform. Linear displacement may, for example, be detected by laser triangulation, by transducers provided within the struts, or by the provision of a corresponding number of unpowered, or “passive” telescopic struts, universally pivotally connected to the arm and the fixed structure, and containing transducers. 
     One advantage of a machine according to the present invention is that of a simplified construction, due to a reduction in the number of telescopic struts employed. A further advantage relates to the comparative ease and simplicity of controlling movement of the arm in real time, due to the simple geometry of the device, i.e. movement of one plane (defined by the three points of connection of the three struts at one end) relative to another plane (defined by the three points of connection of the three struts at the other end). These advantages are not however essential for the performance of the invention, nor are they necessarily the only advantages of one or more of the embodiments described. 
     In an alternative embodiment two additional telescopic struts are provided, each of which is connected between a mechanical earth and a point on the movable arm remote from the mounting point of the three supporting struts, the two additional struts controlling movement of the arm about two rotational axes, thereby converting the machine to a five axis machine. 
     The fixed structure of the machine may be provided by a frame rigidly connected to a table of the machine from which the supporting struts are suspended. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will w be described, by way of example, and with reference to the accompanying drawings in which: 
     FIG. 1 shows a plan view of a first embodiment of the present invention; 
     FIG. 2 shows a section on the line II—II in FIG. 1; 
     FIG. 3 shows a detail of FIGS. 1 and 2; 
     FIG. 4 is a plan view of a second embodiment of the present invention; 
     FIG. 5 is a schematic perspective view of a detail of FIG. 4; 
     FIG.  6 A and FIG. 6B are perspective views of a modification to the embodiment of FIGS. 4 and 5; 
     FIG. 7 is a plan view of a third embodiment of the present invention; 
     FIG. 8 is a sectional view illustrating a modification of the embodiment of FIG. 6; 
     FIG. 9 is a perspective view of a fourth embodiment of the present invention; 
     FIGS. 10A-D illustrate the operation of a first part of the constraint of FIG. 8; 
     FIGS. 11A-D illustrate the operation of a second part of the constraint of FIG. 8; 
     FIG. 12 is a perspective view of an alternative to the embodiments of FIGS. 9 to  11 ; 
     FIG. 13 is a perspective view of a fifth embodiment of the present invention; and 
     FIG. 14 is a plan view on XII—XII in FIG.  11 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to FIGS. 1 and 2, a coordinate positioning machine, which in the present example is a machine tool, includes an arm or movable structure  10  in the form of a spindle, movable relative to a table  12 . The spindle  10  is suspended from a rigid triangular frame  14  by means of three powered telescopic supporting struts  16 , which extend from the apexes of the triangular frame  14  to the spindle casing  18  (in which the spindle shaft  18 A is journalled). The struts also contain transducers (not shown) which measure their length; the transducers may be provided for example by opto-electronic or magnetic encoders, LVDT&#39;s, or laser interferometers. The supporting frame  14  is rigidly mounted to the table  12  by a suitable structure which has been omitted here for clarity. Both structures, however, are part of the “mechanical earth” of the machine, and this is indicated throughout the specification by the usual symbol. The spindle shaft  18 A carries an operating module in the form of a cutting tool T, for machining workpieces (although other operating modules may be used, such as touch trigger and analogue probes). Preferably, the geometry of the machine is such that the axes S of each of the supporting struts  16  intersect at the tool tip. 
     The connections of the supporting struts  16  to the frame  14  and the spindle casing  18  preferably permit universal pivotal motion of the struts  16  relative to the frame  14  and casing  18 . Preferably, the connections provide substantially friction free movement, and may comprise magnets and fluid bearings. Alternatively, flexible linkages may be used. Suitable connections are disclosed in our co-pending International Patent Application No. PCT/GB94/02593. Translation of the spindle  10  is provided by expansion and/or contraction of the telescoping-supporting struts  16 ; e.g., a simultaneous equal contraction of all of the supporting struts  16  will cause the spindle  10  to move in a direction indicated in FIG. 2 as the Z direction, with other combinations of expansion and contraction providing movements in the X and Y directions respectively as desired. 
     Because the spindle  10  is suspended by only three telescopic supporting struts  16 , the spindle may, for a given combination of strut lengths, rotate about three perpendicular axes relative to the table  12  by virtue of the universal pivotal mounting of the struts  16  relative to the frame  14  and spindle casing  18 . Movement of the spindle with each of these three degrees of rotational freedom is eliminated by the provision of an anti-rotation device having three mechanical linkages  20 ,  22 ,  24  which prevent rotation about the X, Y and Z axes, respectively. Each of the linkages is passive, i.e. has no motor or other actuator. One such linkage is illustrated in more detail in FIG.  3 . Referring now to FIG. 3, an individual linkage includes a substantially rigid planar member  30 , mounted to a mechanical earth of the machine, such as the supporting frame  14  or the table  12 . The rigid member  30  has a region of relative weakness  32  at its base, which serves as a hinge to enable tilting of the upper part of the member  30  about an axis Al. The upper end of the member  30  is connected to the spindle casing  18  by means of two elongate rods  34 , which are flexible in bending but rigid in tension and compression. 
     The operation of an individual linkage will now be described. Translational movement of the spindle casing  18  along the axis A 3  is permitted by tilting of the member  30  about axis Al, while the resultant changing angle between the rods  34 , rigid member  30  and the spindle casing  18  is accommodated by flexing of the rods  34 . Translational movement of the spindle casing  18  in directions parallel to either axis Al or axis A 2  is permitted by flexing of the rods  34  in a manner similar to that of a pair of parallel leaf springs. Rotation of the spindle casing  18  about an axis parallel either to the axis A 1  or to the axis A 2  is permitted by flexing of the rods  34 . The rigidity of the rods  34  to tension and compression, together with the relative rigidity of the member  30  prevents rotation of the spindle casing  18  about an axis parallel to axis A 3 . An individual linkage  20 ,  22 ,  24  thus permits linear movement of the spindle casing  18  in three perpendicular directions, together with rotation thereof about two perpendicular axes, while preventing rotation about a third axis. 
     Referring again to FIGS. 1 and 2 it can be seen that the combined action of all three linkages  20 ,  22 ,  24  eliminates all rotational movement of the spindle  10  relative to the table  12 , while permitting linear movement thereof due to telescoping of the struts  16 . 
     In a modification of the linkage shown in FIG. 3 the planar member  30  is totally rigid, and a mechanical low-friction hinge is provided in the place of the area of weakness  32 . Additionally the elongate rods  34  are replaced by stiff rods, universally pivotally connected to both the mechanical earth and the spindle casing  18 . The choice of flexural or pivoting linkages depends upon a number of factors, and particularly upon the range of travel of the spindle casing  18  over which constraint, and in this particular embodiment, elimination of rotational movement of the spindle is required. Flexural linkages have the advantage of being friction and backlash free, but have the disadvantage that they are only operable over a short range; pivoting linkages are operable over a large distance, but suffer from friction and backlash. 
     A second embodiment of the present invention will now be described with references to FIGS. 4 and 5. A coordinate positioning machine in the form of a machine tool includes a supporting frame  114  rigidly mounted to a table (not shown). A movable arm in the form of a spindle  110  is suspended from the frame  114  by means of three powered telescopic struts (not shown). The struts are universally pivotally connected to the frame  114  and spindle  110 , thereby allowing, for any given combination of strut lengths, rotational movement of the spindle with three degrees of freedom. Movement of the spindle with two of these rotational degrees of freedom is eliminated, and movement of the spindle with a third degree of rotational freedom is constrained to within predetermined limits by an anti-rotation device provided by a linkage  140 . 
     The linkage  140  includes a torsionally rigid box  150 , mounted by a hinge to a mechanical earth  152 , thereby to enable pivoting of the box  150  about axis B 1 . Box  150  includes top and bottom kite-shaped sub-frames  154 ,  156  interconnected by four vertical rods  158 A, B, C, D and two angularly extending stanchions  160 . Two triangular wishbone frames  162  are connected at their apexes to the upper and lower ends of vertical rod  158 A by means of a suitable mounting providing universal pivotal motion thereof. The ends of the wishbone frames  162  remote from the rod  158 A are connected to the spindle casing  118 . The linkage thus functions in a manner similar to an elbow joint, with translation of the spindle  110  in the X and Y directions being accommodated by pivoting of the torsion box  150  about axis B 1  and/or pivoting of the wishbone frames  162  about axis B 2 . Translation of the spindle  110  in the Z direction is accommodated by pivoting of the wishbone frames  162  as illustrated with dashed lines in FIG.  5 . 
     The mechanical earth to which the rigid box  150  is mounted is provided at the upper end of the box by the supporting frame  114 , and at its lower end by a table (not shown). When the spindle  110  occupies a position centrally within the frame  114 , the rigid torsion box  150  will extend underneath one of the spars  114 A of the frame  114 , while the wishbones  162  will extend substantially perpendicularly thereto. The arcuate motions permitted by the hinged mounting of the rigid torsion box  150  and wishbone frames  162  are illustrated in FIG. 4, and have reference numbers C 1  and C 2 , respectively. 
     This anti-rotation device has the advantage of relative simplicity when compared with the device having three individual linkages  20 ,  22 ,  24  illustrated in FIGS. 1-3. However, rotation of the wishbone frames  162  through a given angle, enabling movement of the spindle to occur along the arcuate path C 2  will cause a corresponding rotation of the spindle casing  118  about an axis parallel to the Z axis. This will be insignificant compared with the relatively rapid rotation of the spindle shaft  118 A relative to the casing  118 . (The limits to within which rotation of the spindle casing is confined are defined by the permissible range of rotation of the wishbones  162  relative to the frame  114 ). 
     If it is desired to use this construction for a coordinate measuring machine, for example, where the permissible rotation may be significant, it may be necessary to measure the extent of rotation of the arm  110  by providing transducers which determine the angular displacement about the axes B 1  and B 2 . 
     Referring now to FIGS. 6A and 6B, in a modification to the embodiments of FIGS. 4 and 5, the stanchions  160  of torsion box  150  are removed. In their place, a pyramidal, torsionally resistant sub-frame  170  is inserted in the plan of vertical rods  158 B,C. The sub-frame is universally pivotally mounted via ball joints  172  to the top and bottom kite-shaped sub-frames  154 ,  156  and therefore enables relative linear movement between sub-frames  154 ,  156  and relative rotation about hinges defined by the points of connection between sub-frames  154  and  170 , and  156  and  170  respectively provided by ball joints  172 , but eliminates all other relative rotation and translation. Lower sub-frame  156  is universally pivotally mounted to a two-axis linear stage  180 , movable with two linear degrees of freedom relative to the mechanical earth, on which upper sub-frame  154  is universally pivotally mounted. Referring specifically to FIG. 6B, movement of the two-axis stage will result in a change in the angle of orientation of the spindle casing  118 . This modification has a number of applications. For example, the two-axis stage  180  may be used only during set-up of e.g. a machine tool, to ensure that the axis of the spindle casing  118  lies orthogonal to the plane of the table relative to which spindle casing  118  in movable. Alternatively, the stage  180  may be employed during normal operation of the machine, e.g. during machining, to provide two-axis rotational orientation of the tool. Alternatively, the two-axis stage  180  may be employed to ensure that the spindle casing  118  is correctly oriented relative to any given workpiece to be machined or inspected. For example, in the case of a machining operation, a workpiece may be probed in advance in order to determine the plane of, e.g. its upper surface, and subsequently the stage  180  may be actuated to adjust the axis of the spindle casing  118 , such that the plane of the workpiece surface and spindle casing axis are orthogonal. 
     A third embodiment of the present invention will now be described with reference to FIGS. 7 and 8. As with previous embodiments the machine includes a spindle  210  supported relative to a table  212  and a frame  214  by three powered telescopic universally pivotally connected struts  216 . Movement of the spindle  210  with the resultant three rotational degrees of freedom is constrained by a combination of a single passive linkage  220 , of the type illustrated in FIG. 3, and a pair of powered auxiliary telescopic constraining struts  280 ,  282 . The passive linkage  220  eliminates rotational movement of the spindle casing  218  about the Z axis. Rotational of the spindle  210  about X and Y axes is controlled by the two auxiliary struts  280 ,  282 , which are connected at one end to a mechanical earth, and at the other end to an elongate pillar  286 , whose lower end is rigidly connected to the spindle casing  218 . Telescoping expansion and contraction of the auxiliary struts  280 ,  282  thus causes rotation of the spindle  210  about the X and Y axes. In the embodiment of FIG. 7, the auxiliary struts  280 ,  282  are pivotally connected to a mechanical earth distinct from the frame  214 , and extend in substantially perpendicular directions. In the alternative embodiment of FIG. 8, however, the auxiliary struts  280 ,  282  are pivotally mounted to a mechanical earth provided by two of the apexes of the supporting frame  214 . The provision of the auxiliary struts  280 ,  282  has the effect of converting a machine having three linear degrees of freedom to a machine with five degrees of freedom, three of which are linear and two of which are rotational. 
     A fourth embodiment will now be described with reference to FIGS. 9 to  11 . The machine includes an arm  310  movable relative to a table (not shown), a supporting frame  314 , and three powered telescopic struts  316 , universally pivotally connected at their ends to the arm  310  and the supporting structure  314 , thereby supporting the arm  310  in a manner which, for a given combination of strut lengths, permits rotation of the arm  310  with three degrees of freedom. 
     Constraint of each of these three rotational degrees freedom is provided by a passive anti-rotation device  340 , which eliminates each of these degrees of rotational freedom while permitting three dimensional translation of the arm under the action of the powered struts  316 . The anti-rotation device  340  acts between a mechanical earth and the movable arm  310 , and includes a linkage provided by an extensible torsion box  350 , which prevents rotation of the arm  310  about two mutually orthogonal axes A, B together with an auxiliary rotational constraining linkage  360 , which prevents rotation of the arm  310  about a third axis, C, extending orthogonal to both axes A and B. 
     Referring now to FIG. 10A, the extensible torsion box  350  includes two door members  352  each of which is mounted to the mechanically earthed structure at a common hinge  354 . The movable arm  310  is connected to the door members  352  at two points spaced apart in the Z-direction by means of two pairs of V-shaped connecting rods  356 A,  356 B. Each of the pairs of connecting rods  356 A,  356 B are universally pivotally connected (e.g. by ball joints) to the movable arm at their apex, with the pairs of connecting rods  356 A,  356 B lying vertically in register with each other. The ends of the pairs of rods  356 A,  356 B remote from the point of connection with the arm  310  are universally pivotally connected to the corners of free swinging edges on door members  352 . 
     Referring now additionally to FIGS. 10B-D, the extensible torsion box  350  permits linear movement of the movable arm  310  in one or more of the directions X, Y, and Z, while simultaneously preventing rotation of the arm about rotational axes A and B. Movement in any one of the X, Y and Z directions is enabled by a combination of pivoting movements of the elements  352 , and  356 A,  356 B relative to each other, and the mechanical earth. For example, referring now to FIG. 10B, translational movement of the movable arm  310  in the X direction is enabled by inward or outward pivoting movement of the door members  352  about hinge  354 , pivoting of the connecting rods  356 A,  356 B relative to their points of connection with the movable arm  310  and the door members  352 , and additionally a swinging movement of the door members  352 , and additionally a swinging movement of the door members  352  about hinge  354 . Referring now to FIGS. 10C and D, translational movement in either the Z or the Y direction is provided by a combination of either simultaneous inward or outward pivoting of the door members  352  relative to each other and about hinge  354 , and pivoting movement of the connecting rods  356 A,  356 B about their points of connection with the movable arm  310  and the door members  352 . 
     The universal pivotal connection of the rods  356 A,  356 B to the movable arm  310  enables arcuate translational motion of the movable arm  310  about the axis C (on which the points of connection of the rods  356 A,  356 B with the arm  310  lie) and, as a result, a corresponding limited degree of rotation of the arm  310 . This minor rotational freedom of the movable arm  310  is constrained by the linkage  360  illustrated in FIGS. 11A-D. Referring now to FIG. 11A, the linkage  360  includes a first pair of fixed rods  362  which are rigidly connected at one end to the mechanical earth, and are universally pivotally connected at the other by ball joints (not shown) to a further rigid door member  364 . The further door member  364  is thus hinged for movement relative to the earth about an axis D. A pair of movable rods  366  are universally pivotally connected to the lower end of further door member  364  and the movable arm  310 . The rigidity of the rods  362 ,  366  and further door member  364  prevents rotation of the arm about the axis C, while the universal pivotal connection between the arm  310 , movable rods  366 , further door member  364 , and fixed rods  362  allows translational motion in the X, Y and Z directions as illustrated in FIGS. 11A-D. 
     All rigid members of the constraining device  340  which are universally pivotally mounted, may be replaced by flexural elements, as appropriate. 
     Referring now to FIG. 12, in an alternative to the embodiment described in FIGS. 9-11, constraint of all three rotational degrees of freedom of the spindle casing  318  is provided by an anti-rotation device which includes a linkage having upper and lower kite-shaped sub-frames  374 ,  376  and an intermediate, torsionally resistant pyramidal sub-frame  380 , upon which upper and lower sub-frames  374 ,  376  are pivotally mounted at ball joints  382 . Upper and lower sub-frames  374 ,  376  may be pivotally mounted upon ball joints  392 , provided on the mechanical earth, or fixed structure, the tension springs  378  enabling easy “snap-on” mounting. Also mounted to the mechanical earth or fixed structure are a pair of elongate substantially rigid struts  384 ,  386 ; the ends of the sub-frames  374 ,  376  and the struts  384 ,  386 , distal to the mechanical earth are universally pivotally mounted, via ball joints  390 , to an intermediate linkage member  388 . The intermediate linkage member  388  is connected to the spindle casing  318  via two rigid door members  394 ,  396 , each of which is mounted, via hinges,  397 A, B and  398 A, B to the intermediate linkage member  388  and spindle casing  318 , respectively. In use, translational movement of the spindle casing  318  is permitted in the XY plane by pivoting of the various elements of the constraint about hinges illustrated with the reference numerals E, F, G, H, I, J, K while translation of the spindle  318  in the Z direction is provided by pivoting of sub-frame  374 ,  376  and linkages  384 ,  386  about hinges illustrated by the reference numerals P, Q, R, S. 
     A fifth embodiment of the present invention will now be described with reference to FIGS. 13-14. The machine of FIG. 13 has a movable arm  410 , table  412 , fixed supporting structure  414 , and three powered telescopic struts  416 , universally pivotally connected at their ends to the supporting structure  414  and the arm  410 . 
     Rotation of the arm  410  about the Z axis is eliminated, and rotation about the X and Y axis is constrained to within predetermined limits by means of motion control linkages  470 ,  480 . Motion control linkage  470  is connected at one end to a mechanical earth, and at the other to the arm  410 , by means of ball joints  472 ,  474  respectively. Motion control linkage  480  has the form of a fork which extends at right angles to motion control linkage  470 , and is universally pivotally connected at one end to a mechanical earth by a ball joint  482 , and at the other to diametrically opposing sides of the arm  410  by means of bearings  484 . (In an alternative embodiment, ball joints may be used.) 
     For a given static setting of telescopic struts  416 , motion control struts  470 ,  480  prevent rotation of the spindle  410 . Upon actuation of one or more of the telescopic struts  416 , the free end of the arm  410  may be positioned in a desired location by a combination of translation of the arm  410 , together with arcuate movement thereof as a result of the action of motion control struts  470 ,  480 . For example, simultaneous expansion or contraction of the telescopic struts  416  will result in linear movement of the arm  410  in the Z direction, in combination with an arcuate movement of the upper and lower ends of the arm  410 , resulting from the consequential pivoting of motion control struts  470 ,  480 . Similarly, movement of the telescopic struts  416  to translate the arm  410  in the X direction will additionally result in rotation of the arm  410  about an axis parallel to the Y axis, at a point defined by the position of the ball joint  474  and bearings  484 ; movement of the telescopic struts  416  to execute a translation of the arm  410  in the Y direction will result additionally in an arcuate pivoting of the arm  410  about an axis parallel to the X axis and a point defined by ball joint  474  and bearings  484 . 
     Because translational movement of the arm  410  will inevitably result in a change in the orientation of its free end, a two axis robot-wrist will preferably be mounted upon the arm  410 , and more preferably a three-axis wrist; this enabling machining of a part at a plurality of angles and orientations. 
     Machines actuated by three telescopic struts are easier to control than prior art machines with six struts. This is because the mathematics involved in controlling the motion of the movable arm is simpler. In particular, the algorithms required to determine the position of the arms are now based only upon three linear measurements, rather than six.