Abstract:
A counterbalance mechanism for use with a multijoint manually positionable measuring arm of a three dimensional coordinate measurement system provides a reversible and adjustable counterbalancing force to offset the weight of the arm and facilitate its movement. The counterbalance mechanism of the present invention acts within the plane of the joint minimizing the moment arm created by the joint and allowing for low overhung loads transmitted from the arm to the base.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 09/444,467 filed Nov. 22, 1999 and also claim the benefit of provisional Nos. 60/138,989 filed Jun. 14, 1999 and 60/111,419 filed Dec. 8, 1998, the entire content of which are incorporated herein by reference. 
    
    
     BACKGROUND OF INVENTION 
     1. Field of Invention 
     This invention relates generally to three dimensional coordinate measuring machines (CMM&#39;s). More specifically this invention relates to an arm and counterbalance mechanism for use on a CMM which provides increased reliability and adjustability. 
     2. Description of the Prior Art 
     It is well known in the art to utilize a CMM to measure objects in a space in terms of their X, Y, and Z coordinates commonly referring to length, width and height, respectively. 
     Advancement in the art has led to lightweight portable CMM&#39;s well suited for general industrial applications. Such a CMM is disclosed in U.S. Pat. No. 5,402,582 which is commonly assigned to the assignee hereof and incorporated herein by reference. 
     One of the above mentioned advancements in the art of portable CMM&#39;s is a light weight multi-jointed manually positionable measuring arm, shown generally in FIG. 1 at  10 . 
     Measuring arm  10  is comprised of a plurality of transfer housings  12  (with each transfer housing comprising a joint and defining one degree of rotational freedom) and extension members  14  attached to each other with adjacent transfer housings being disposed at right angles to define a movable arm  10  preferably having multiple degrees of freedom. At one end of arm  10  is attached a base  20 . At the end of arm  10  opposite base  20  is attached a probe  15 . 
     Referring to FIG. 2, the measuring arm  10  of the prior art further comprises a torsional spring  16  positioned in a joint  22  near base  20  of measuring arm  10 . The torsional spring  16  provides a counter balance force to offset the weight of the arm and ease manipulation thereof by an operator. An air piston shock absorber  18  is mounted on base  20  of arm  10  in intimate contact with joint  22  such that piston  18  is fully compressed when arm  10  is in rest position, as is shown. Piston  18  is fully decompressed and awaiting retraction of arm  10  when said arm is fully extended. Air piston  18  absorbs the shock load accompanying the spring coiled retraction of arm  10  by exerting a force opposite to said retraction. 
     The base  20  of CMM arm  10  of the prior art is typically mounted in the horizontal plane. Referring again to FIG. 2, the recoiled torsional spring  16  generates a compensating torque at the base  20  of the arm  10  in a direction  24  to considerably reduce the weight of the arm  10 , said weight acting in a direction  26  when arm  10  is extended. Such alignment allows for a counterbalanced use of the arm  10  when base  20  is mounted in the horizontal plane as described herein above. However, there are many applications of CMM&#39;s where it is advantageous to mount the arm perpendicular to or inverted to the above discussed original mounted horizontal plane. For instance, it is often desired in the art to mount the arm  10  to a wall or to a ceiling to facilitate a particular use of the CMM. This mounting naturally changes the direction  26  of the weight of the aim  10  relative to said arm. The compensating torque  24 , however, created by torsional spring  16  remains the same. Thus, the effect of the arm&#39;s spring coiled counterbalancing mechanism is diminished. Without the aid of the counterbalancing mechanism, use of the arm  10  may be cumbersome. 
     Prior art CMM arms, as discussed above, do not readily allow multiple applications requiring changeability of a single CMM. For instance, a single CMM may be used by a variety of operators who may require different counterbalancing forces to effect a proper movement of the machine. Different end probes may be required for various application and alternative mountings may be necessary. The CMM arms of the prior art do not readily allow adjustability of the counterbalancing mechanism to compensate for the change in forces acting upon the arm associated with use of various mountings and end probes. 
     The positioning of the torsional spring counterbalance of the prior art CMM arm causes a high overhung load. As discussed herein above the counterbalance mechanism is positioned in a joint near the base of the arm. Such positioning creates a substantial moment arm from the neutral axis of joint previous to the mechanism. The majority of the weight of the measuring arm acts on this moment arm and creates a considerable load on the joint and on the base thereby reducing operability and increasing stress on the base assembly of the CMM arm. 
     Thus the need has arisen for a CMM arm with a mechanism which allows for the counterbalanced use of the arm in a variety of mountings, with a variety of end probes which prevents overhang stress on the base of the arm and provides ease in changeability. 
     SUMMARY OF THE INVENTION 
     The above discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by the present invention. A novel counterbalance mechanism for use with a multi-joint manually positionable measuring arm of a three dimensional coordinate measurement system provides a reversible and adjustable counterbalancing force to offset the weight of the arm and facilitate its movement. An exemplary counterbalance device comprises a ratchet mechanism to select the direction of counterbalance assistance allowing for the mounting of the arm on a horizontal plane, a wall or a ceiling. In one embodiment, the exemplary counterbalance mechanism further comprises a compression spring which adjustably biases a cable and a system of intermeshing gears to provide varying levels of counterbalancing force. Alternatively, a second embodiment of the exemplary counterbalance mechanism comprises a cam assembly around which a member is rotated and counterbalanced by an internal compression spring. The counterbalance mechanism of the present invention acts within the plane of the joint minimizing the moment arm created by the joint and allowing for low overhung loads transmitted from the arm to the base. The mechanism of the present invention is readily adjustable to accommodate a variety of uses and mounting positions. 
     The above discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings wherein like elements are numbered alike in the several figures: 
     FIG. 1 is a side elevational view of a conventional CMM arm; 
     FIG. 2 is a side elevational view of a conventional counterbalancing mechanism; 
     FIG. 3 is an isometric view of an arm of a three dimension coordinate measurement system incorporating a counterbalance mechanism of the present invention; 
     FIG. 4 is a side elevational view in partial section of an exemplary counterbalance mechanism; 
     FIG. 5 is an isometric view of a cable coil gear and cable; 
     FIG. 6 depicts a cross-sectional side elevational view of a counter balance mechanism in an alternative embodiment; 
     FIG. 7 is a cross-sectional side elevational view of the counter balance mechanism of FIG.6; 
     FIG. 8 is a cross-sectional side elevation view of the counter balance mechanism of FIG. 6; 
     FIGS. 9-11 are various views of a support bracket; 
     FIGS. 12 and 13 are views of a barrel; 
     FIGS. 14-15 are cross-sectional views of a spacer; 
     FIGS. 16-18 are various views of a barrel support bracket; 
     FIGS. 19-20 are various views of a barrel guide; 
     FIG. 21 is a side view of a rod; 
     FIGS. 22-23 are various views of a plunger; 
     FIG. 24 is a side view of a cam roller; and 
     FIGS. 25-27 are various views of a cam. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An exemplary multi jointed manually operated coordinate measuring machine (CMM) arm is shown generally at  100  in FIG.  3 . Arm  100  is substantially comprised of a base  102 , an extension assembly  104 , and a probe end  106 . Arm  100  further comprises a first transfer housing  108 , a second transfer housing  110 , a third transfer housing  112 , fourth transfer housing  114 , a fifth transfer housing  116 , and a sixth transfer housing  118 . Base  102  is shaped substantially as a cylindrical solid and may be mounted to a horizontal surface, a wall, or a ceiling, as is discussed more fully herein below. Extension assembly  104  includes a first extension member  122  and a second extension member  124  of substantially similar lengths. Probe end  106  includes a buttons  120  to facilitate CMM usage. First extension member  122  is positioned between second transfer housing  110  and third transfer housing  112 . Second extension member  124  is positioned between fourth transfer housing  114  and fifth transfer housing  116 . 
     A counter balance mechanism  200 , as will be described more fully herein below, is disposed within transfer housing  110 . It will be appreciated that the components of arm  100  accumulate into a total weight of approximately 15 pounds. The counterbalance mechanism of the present invention partially offsets the effects of gravity on the arm by reacting the forces within transfer housing  110  and transmitting them through base  102 . Such redistribution of the forces of gravity acting on the arm  100  reduces fatigue of the operator and wear at the transfer housings. 
     Referring now to FIG. 4, transfer housings  108  and  110  are shown incorporating a counterbalance mechanism  200  in accordance with an embodiment of the present invention. Counterbalance mechanism  200  generally includes a ratchet gear  221 , a transmission gear  222 , a cable coil gear  223 , a compression spring  224 , a cable  225 , and roller guides  226  and  227 . 
     Ratchet gear  221  is rotatably disposed in transfer housing  108 , meshing with transmission gear  222 . Transmission gear  222  is rotatably disposed in transfer housing  110  in meshing engagement with cable coil gear  223  which is also rotatably disposed in transfer housing  110 . Compression spring  224  is disposed within first extension  122  and is separated from the gear arrangement of counterbalance mechanism  200  by a barrier member  224 ′ which is mounted perpendicular to the axis of compression spring  224  and adjacent to the base of said spring. Roller guides  226  and  227  are mounted proximate to each other in transfer housing  110  adjacent to barrier member  224 ′ opposite compression spring  224 . Cable  225  extends, at one end, through roller guides  226  and  227  into transfer housing  110  where it is fixed to cable coil gear  223 . At a second end, cable  225  extends from roller guides  226  and  227  through barrier member  224 ′ and through the axis of compression spring  224 . Referring now to FIG. 5, cable  225  includes an adjustable threaded end  230  and a ball end  230 ′. Ball end  230 ′ of cable  225  is secured within a slot  231  of a disk  232 . Disk  232  is rigidly connected to cable coil gear  223  by a cable spindle  233 . Referring again to FIG. 4, disk  232  and cable coil gear  223  are rotatably supported within transfer housing  110  on axle  234 . Threaded end  230  of cable  225  is secured to the end of compression spring  224 , opposite barrier member  224 ′, by use of a washer  236  and a nut  235 . When cable spindle gear  223  is rotated about axis  234  cable  225  winds around cable spindle  233  in a clockwise or counterclockwise direction depending upon the direction of rotation of cable spindle gear  223 . The winding of cable  225  around cable spindle  233  shortens the extended length of cable  225  thereby creating a tension. This tension in cable  2 : 25  is transferred to spring  224  via washer  236  and nut  235  hence resulting in the compression of spring  224 . The spring  224  exerts a counter force opposite the direction of compression. It is this counter force which counterbalances arm  100  as will be discussed in further detail herein. 
     Counterbalance mechanism  200  further includes a direction locking key  228  and a pawl  229 . Locking key  228  is coupled to pawl  229  which engages ratchet gear  221  to selectively permit rotation of the ratchet gear  221 . Ratchet gear  221  includes a drive aperture  221 ′, shown in this particular embodiment as adaptable to receive a standard square drive tool end (not shown). 
     Still referring to FIG. 4, the force of gravity acting on arm  100  is shown, for purposes of illustration, as acting in the direction of arrow  39 . Pawl  229  is positionable in one of two positions, as discussed above, to allow adjustment of ratchet gear  221 . A first position of pawl  229 , that illustrated in FIG. 4, allows ratchet gear  221  to rotate only in the direction of arrow  237 . When transfer housing  110  is rotated relative to transfer housing  108  with pawl  229  positioned as described above, cable  225  is wound onto spindle  233  of FIG. 5 against roller guide  227  thereby compressing spring  224 . The compression in spring  224  causes tension in cable  225  which in turn produces a torque in cable coil gear  223 . The torque in cable coil gear  223  is transferred to transmission gear  222  in the direction represented by arrow  240 . As noted above, the position of pawl  229  prevents movement of ratchet gear  221  in a direction opposite to that of arrow  237  thus the torque experienced in transmission gear  223  is reacted into base  102  by ratchet gear  221 . The torque produced in transmission gear  222  in the direction of arrow  240  causes a moment about center of rotation  242  of arm  100  in the direction represented by an arrow  243  thereby counteracting a moment  244  produced by the force of gravity  39 . 
     It will be appreciated that sufficient torque is achieved through the adjustment of ratchet gear  221  to offset the weight of arm  100  thereby providing a counterbalance for the system. It will also be appreciated that the amount of torque provided by counterbalance mechanism  200  is easily varied by rotation of ratchet gear  221  and can be adjusted to allow varying degrees of counterbalancing force for different operators and end effectors. 
     A further feature of the present invention is the ability to reverse the direction of torque provided by the counterbalance mechanism  200 . The utility of this feature is realized when the force of gravity acts in a direction opposite to that shown in FIG. 4 relative to base  102 . As described in the background section hereof, applications exist wherein base  102  is mounted to vertical walls and also to ceiling structures. Counterbalance mechanism  200  allows for the reversal in direction of the torque applied by transmission gear  222  by positioning direction locking key  228  opposite that shown in FIG.  4 . With direction locking key  228  positioned accordingly, pawl  229  permits rotation of ratchet gear  221  only in the direction indicated by arrow  238 . When transfer housing  110  is rotated relative to transfer housing  108  with pawl  229  positioned as described, cable  225  is wound against roller guide  226  onto spindle  233  in the opposite direction to that described herein above creating a tension which compresses spring  225  via washer  236  and nut  235 . Spring  225  exerts a counter force to the compression which causes a torque in transmission gear  222  in the direction represented by arrow  241 . The torque produced by transmission gear  222  in direction  241  causes a moment about the rotation center  242  of arm  100  in the direction indicated by arrow  244 . Such moment serves to counterbalance a force of gravity acting upon arm  100  in a direction opposite that of arrow  39 . Pawl  229  may be positioned in a variety of settings so as to create an appropriate counterbalancing force for a specific mounting and/or use of CMM arm  100 . 
     Referring again to FIG. 4, another feature of the present invention is shown with regard to the small moment arm  245  created by transfer housing  110 . As discussed herein above counterbalance mechanisms of the prior art create a relatively large moment arm and shift the axis of rotation of the arm substantially away from the base. The present invention minimizes the overhung load transferred to base  102  by the weight of arm  100  by disposing counterbalance mechanism  200  in line with center of rotation  242 . The distance represented by small moment arm  245  is limited by the size of transmission gear  222  and ratchet gear  221  which are proportioned to provide adequate counterbalance forces. The result is a minimum exertion of overhung force upon base  102 . 
     FIGS. 6-25 depict an alternate embodiment of the invention. FIG. 6 shows a cross-sectional side elevation view of a counter balance mechanism  400  attached to a transfer housing  500 . Transfer housing  500  includes a first transfer housing  502  and a second transfer housing  504 . First transfer housing  502  rotates relative to second transfer housing  504 . The counter balance mechanism  400  includes a bracket  402  and ai cam follower assembly  404 . Bracket  402  is secured to first transfer housing half  502 . Cam follower assembly  404  is secured to bracket  402  through a barrel support bracket  412  and a barrel guide  414  as described in detail further herein. 
     Cam follower assembly  404  includes a barrel  408  and a plunger  422 . The barrel  408  is substantially tubular in shape and contains an outer wall  408 A which defines an inner portion  408 B. A rod  418  is mounted in barrel  408  such that rod  418  traverses through the center of inner portion  408 B, parallel to outer wall  408 A. A compression spring  410  is disposed around rod  418  in inner portion  408 B of barrel  408 . A spacer  416 , as further described herein, travels along rod  418  to compress compression spring  410 . One end of rod  418  is secured to the barrel  408  by a nut  420  which engages threads on rod  418 . The other end of the rod  418  is connected to plunger  422 . Plunger  422  includes a cam roller  424  mounted to plunger  422  opposite rod  418 . Cam roller  424  contacts a cam  406  and enables cam follower assembly  404  to rotate about cam  406 . As the cam roller  424  moves over the surface of cam  406 , plunger  422  moves the spacer  416  upwards and downwards within barrel  408 . As will be discussed in detail further herein, this compresses and releases compression spring  410 . Controlling the surface of the cam  406  allows a counter balancing force to be generated based on rotation of the transfer housing. 
     FIG. 7 is a cross-sectional side elevation view of the cam follower assembly  404  and cam  406 . Cam  406  includes a cam surface  407 , a detent  426 , and an opening  494 . As the cam follower assembly  404  is moved, the cam roller  424  travels over the cam surface  407 . At areas of greater radius of cam  406 , plunger  422  is directed into inner portion  408 B of barrel  408  forcing spacer to travel on rod  418  away from cam  406 , thereby compressing compression spring  410 . The force created by compression spring  410  counter balances the movement of cam follower assembly  404  by seeking to rotate cam follower assembly to a position which creates the least compression in spring  410 . Detent  426  defines a rest position for the cam follower in which spring  410  is in its least compressed state. The counter balancing force generated by the compression of spring  410  is directed at returning cam follower assembly to detent  426 . Traversing the edge of detent  426  requires compression of spring  410  and thus the force of spring  410  helps to maintain the arm in the rest position of detent  426 . 
     FIG. 8 shows a cross-sectional side elevation view of counter balance mechanism  400  in two different positions with cam roller  424  in two different positions on the surface of cam  406 . As can be seen from FIG. 8, the travel of the cam roller  424  over the cam  406  creates a compression of distance x of the compression spring  410  depending upon the positioning of cam roller  424  along the cam surface  407  of cam  406 . When the compression spring  410  is compressed, it tends to drive the cam roller  424  to a location on cam  406  where the spring  410  is less compressed, i.e. an area of reduced radius on cam  406 . Accordingly, effects of gravity or other forces acting on the arm can be overcome by using a cam surface specifically shaped to compress spring  410  to counteract said forces. 
     FIGS. 9-11 are various views of bracket  402 . The bracket  402  has a housing opening  440  for receiving the first transfer housing  502  and may be secured to the first transfer housing  502  through use of a variety of fasteners. The bracket  402  also includes a barrel support bracket opening  442  for receiving barrel support bracket  412 . The bracket further includes a barrel guide opening  444  which receives barrel guide  414  when securing cam follower assembly  404  to bracket  402 . 
     FIG. 12 is a side elevational view of the barrel  408  of a cam follower assembly  404  and FIG. 13 is a cross-sectional side elevational view of barrel  408 . The barrel  408  is hollow, cylindrical structure comprising an outer wall  408 A which defines an inner portion  408 B. Compression spring  410  is received in inner portion  408 B of barrel  408 . Outer wall  408 A of the barrel  408  includes a groove  450  for receiving a raised ring  464  on barrel support bracket  412  as described in detail herein with reference to FIG.  16 . 
     FIG. 14 is a side elevational view of the spacer  416  and FIG. 15 is a cross-sectional view of spacer  116  along axis  413  shown in FIG.  14 . The spacer  416  is generally cylindrical and has an opening  452  formed there through for receiving rod  418 . The spacer  416  may be manufactured in varying lengths L to provide for enhanced compression of spring  410 . For example, to pre-load the spring  410 , the length L of spacer  416  can be increased. Alternatively, to reduce the compression of spring  410 , the length L is reduced. In this manner, the counter balancing device can be adjusted to different coordinate measuring machine arms and different arm applications. 
     FIGS. 16-18 are various views of the barrel support bracket  419 . The barrel support bracket  419  includes a rectangular base  460  and a circular portion  462 . The inside surface of circular portion  462  includes a raised ring  464 . The barrel support bracket  408  may be formed in two halves, a first half  419 A and a second half  419 B, to ease in mounting. The rectangular base  460  of first half  419 A is inserted into opening  442  on bracket  402  and fixed with fasteners. Barrel  408  is placed inside circular portion  462  such that groove  450  engages circular ring  464 . The rectangular base  460  of second half  419 B of barrel support bracket  419  is inserted into opening  442  on bracket  402  such that circular ring  464  engages groove  450 . Second half  419 B is then secured to bracket  402  by use of fasteners. 
     FIG. 19 is a top view of barrel guide  414  and FIG. 20 is a cross sectional view of barrel guide  414 . The barrel guide  414  includes a guide base  466  and a circular portion  468  rigidly attached to the guide base  466 . The guide base  466  is substantially rectangular in shape and is mounted in opening  444  in bracket  402 . The circular portion  468  includes an opening  470  which receives plunger  422 . 
     FIG. 21 is a side elevational view of rod  418 . Rod  418  is substantially cylindrical in shape and is of sufficient length to extend from nut  420  through inner portion  408 B of barrel  408  and into plunger  422 . Both ends of rod  418  are threaded to enable the secure fitting of rod  418  into plunger  422  and nut  420  at opposite ends of rod  418 . 
     FIGS. 22 and 23 show various elevational views of) lunger  422 . Plunger  422  includes a rod end  422 A and a cam end  422 B. Rod end  422 A includes an internally threaded opening  475  which mates with threads on one end of rod  418 . Cam end  422 B of plunger  422  includes a pair of arms  476 , each having a concavity  478  formed therein. Located within each concavity  478  is a mounting hole  479  which receives cam roller  424 . FIG. 24 depicts the cam roller  424  which is placed in mounting holes  479 . The cam roller  424  includes cut outs  480  which may receive C-clips to secure the cam roller  424  to arms  476 . 
     FIGS. 25-27 are views of an exemplary cam  406 . An opening  494  is formed through cam  406  for mounting the cam  406  on second transfer housing  504 . The cam  406  includes a center of rotation  490  and an outside surface  407 . The distance from the center of rotation  490  to the surface  407  varies as a function of angular position. By specifying the surface  407  on cam  406 , the amount of compression of spring  410  can be controlled at different angular positions along the cam surface. The cam  406  also includes a rest position detent  426  as described above. 
     According to the present invention, an adjustable counterbalance mechanism for a multi-jointed coordinate measurement machine arm is provided. The mechanism creates a counter balancing force which opposes other forces acting upon the arm thereby increasing maneuverability of said arm and ensuring safety of usage. The mechanism also diminishes the effects of overhung forces by reducing the moment arm created at joints corresponding to the placement of said mechanism. Further, the present invention allows for rapid and easy adjustment of the forces produced by the counterbalancing mechanism to facilitate various CMM applications involving a variety of mountings, a variety of end probes, and a variety of individual users. 
     While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.