Abstract:
A linkage mechanism for a pick and place robot includes two rotatable drives mounted on a base and connected to a platform by parallelogram linkages, one assigned to each drive. Each parallelogram linkage may include a series of parallelogram linkages connected to one another by a bell crank that couples drive linkages with location linkages to assure proper positioning of the mechanism. This arrangements permits movement of the platform in two dimensions by selective rotation of the drives.

Description:
BACKGROUND  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a translational parallel robot mechanism of the type typically referred to as a pick and place robot.  
         [0003]     2. Description of the Related Art  
         [0004]     Pick and place robot mechanisms are typically utilized in light industries, such as the electronics industry and packaging industries, where such robotic mechanisms are required for accurate repetitive performance of simple operations that occur many times over. Such an operation may, for example, involve the repeated picking up of items at a first location with a gripping mechanism, moving a gripped item to a second location, and releasing said item at the second location. The gripping mechanism is typically mounted to a movable platform.  
         [0005]     In order to meet performance criteria that is typically required of a robot mechanism of this type, one requirement may be that the platform is moved at a relatively high speed with two degrees of freedom in a movement plane without altering the posture of platform. Optionally the mechanism as a whole may also move in a relatively slow or stepwise manner and in a direction normal to the movement plane of the platform. Conventional robotic arm assemblies have difficulty in meeting these requirements with precision of movement and may lack synchronicity of movement due to different drive mechanisms being in place to accomplish these goals.  
       SUMMARY  
       [0006]     The present robotic mechanism overcomes the problems outlined above and advances the art by facilitating additional range of motion including at least two translational degrees of freedom in a mechanism that may be operated quickly and accurately.  
         [0007]     In one embodiment, there is provided a mechanism that includes a base and a platform that is movable relative to the base in a plane extending through the base and platform. The base is provided with two rotatable drive members connected to the platform via respective linkages. Each linkage includes a drive link having a first end fixed for rotation with a respective drive member and a driven link having a first end that is pivotally connected to the platform. The respective second ends of the drive and driven links are pivotally connected together, wherein one of the linkages further includes a location means that operable to maintain the platform in a predetermined orientation with respect to the base during movement thereof, the location means may include, for example, a bell crank that is provided at the pivotal connection between the drive and driven links, a first location link pivotally connected between one arm of the bell crank and the base, and a second location link pivotally connected between the other arm of the bell crank and the platform.  
         [0008]     Such an arrangement permits the platform to be moved in two dimensions by selective rotation of the drive members, either in the same direction or in opposite directions, and to different extents, while maintaining the platform in a predetermined orientation.  
         [0009]     The linkage having the location means may advantageously be provided with two spaced driven links mounted on common pivotal connections to the bell crank and platform respectively. It will be appreciated that the provision of twin driven links enhances the rigidity of the mechanism as a whole. Bracing means may be provided between the driven links. Such bracing means may comprise one or more members extending between the driven links. The members may extend substantially normal to the longitudinal direction of the links. Alternatively the members may extend diagonally between the links. The bracing means may preferably comprise a combination of both types of members.  
         [0010]     In a preferred embodiment both linkages are provided with a location means having the configuration hereinbefore described.  
         [0011]     The base may be arranged so as to be movable in a direction that is substantially normal to the movement plane of the platform. Such movement may be effected by the mounting of the base on an appropriate structure providing movement in a single degree of freedom, for example, as a linear drive mechanism.  
         [0012]     The first location link extending between the bell crank and the base may be provided with a spherical connection means, such as ball joints or rose joints, at each end thereof. Spherical connection means may further be provided between the each end of the second location link and the bell crank and platform respectively. The connection between the or each driven link and the platform may also be effected by a spherical joint.  
         [0013]     In a preferred embodiment, the drive and driven links are inboard of the location means. The drive links and the first location link(s) are preferably of equal length. The driven link(s) and second location link(s) are preferably of equal length. In the preferred embodiment one arm of each bell crank is longer than the other arm, and preferably the longer arm is connected to a respective driven link.  
         [0014]     In one preferred embodiment, the axes of the drive members are parallel and perpendicular to the movement plane of the platform. The drive members may be driven by respective motors located on the same side of the linkage. A line passing through the axes of the drive members is preferably parallel to a line passing through said second ends, and preferably also to a line passing through said first ends of the driven links.  
         [0015]     The axes of the drive members are preferably spaced apart by less than the length of a drive link.  
         [0016]     In one preferred embodiment, the driven links are longer than the drive links. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     Embodiments of the present invention will now be described with reference to the accompanying drawings in which:  
         [0018]      FIG. 1  shows an end view of a mechanism according to a first embodiment of the present invention, the mechanism being shown in a centralized position;  
         [0019]      FIG. 2  shows a side view of the mechanism of  FIG. 1 ;  
         [0020]      FIG. 3  shows a perspective view of the mechanism of  FIG. 1 ;  
         [0021]      FIGS. 4   a  and  4   b  show an end view of the mechanism of  FIG. 1 , the mechanism being shown in opposite laterally displaced positions;  
         [0022]      FIG. 5  shows a perspective view of the mechanism of  FIG. 1  mounted on a traversing assembly;  
         [0023]      FIG. 6  shows an end view of a mechanism according to a second embodiment of the present invention;  
         [0024]      FIG. 7  shows a perspective view of a mechanism according to a third embodiment of the present invention; and  
         [0025]      FIG. 8  shows a plan view of a linkage of the mechanism of  FIG. 7 . 
     
    
     DETAILED DESCRIPTION  
       [0026]     Referring now to  FIGS. 1 through 5 , there is shown a mechanism generally designated  10 . It will be noted that the centrally positioned mechanism  10  is symmetrical about a plane indicated by broken line  18  passing through both the base  12  and working element or platform,  14 , as shown in  FIG. 1 . In the following discussion, the plane  18  forms a basis for mirror image where, for example, linkage arrangements  16  on the left side of  FIG. 1  have identical counterparts on the right side designated as  16 ′. The mechanism  10  comprises a base  12 , a movable platform  14  and two kinematic chains or linkage arrangements  16  (and  16 ′) extending between the base  12  and the platform  14 . The base  12  is provided with two rotatable drive shafts  20 ,  20 ′ which are operable to move the platform  14  relative to the base  12  via the linkage arrangements  16 ,  16 ′. The drive shafts  20 ,  20 ′ extend from an appropriate drive source, for example, synchronous servo motors  17 ,  17 ′ mounted on the base  12 . The platform  14  may be configured so as to accommodate a range of attachments  19  including, for example, a gripper, a sensor or a tool.  
         [0027]     The linkage arrangement  16  includes a pair of upper links  22 ,  24  of equal length, a bell crank assembly  26  and three lower links  28 ,  30 ,  32  of equal length. It will be recognized by those skilled in the art that the links  22 , 24 , 28 , 30 , 32  and bell crank assembly  26  of each linkage arrangement  16  are arranged so as to provide upper and lower parallelogram linkages between the base  12  and the platform. The links  22 ,  24 , 28 ,  30 ,  32 , each are formed as elongate members, for example, having eyes at each end for connectivity. The upper links  22 ,  24  include a drive link  22  which is fixed for rotation with a drive shaft  20 , and an upper location link  24  which is pivotally fixed to the base  12  via a pin  13 . Each bell crank assembly  26  includes a spaced pair of substantially “L” shaped bell crank members  34  each having a pair of legs  36 , 38  extending from a common root  40 . Through holes  42 , 44 , 46  are provided at the root  40  and at the end of each leg  36 ,  38  respectively. The through holes  42 , 44 , 46  accommodate pins  48 , 50 , 52  extending between the spaced bell crank members  34 . The pins  48 , 50 , 52  enable ends of the links  22 , 24 , 28 , 30 , 32  to be pivotally connected to the bell crank assembly  26 . As will be readily observed from the embodiment of  FIG. 1  the upper links  22 , 24  are connected to the bell crank assembly  26  between the bell crank members  34 , with the drive link  22  being connected at the root  40  of the bell crank members  34  and the location link  24  connected to the end of one of the leg pairs  36 .  
         [0028]     The lower links  28 , 30 , 32  of each linkage arrangement  16  comprise a lower location link  28  and two driven links  30 , 32 . The location link  28  is pivotally connected between the other of the leg pairs  38  of the ball crank assembly  26  and a fork  54  of the platform  14 . The location link  28  is pivotally connected to the fork  54  by a pin  56 . The driven links  30 , 32  extend from the root  40  of the bell crank  26  assembly to the platform  14 . It will be noted that the driven links  30 , 32  are provided outboard of the bell crank assembly  26  and, hence, may be mounted on a common pin  50  to the drive link  22 . The inner links  30 ,  32  are further mounted on a common pin  58  to the platform  14 .  
         [0029]     Movement of the platform  14  relative to the base  12  is achieved by rotation of the drive shafts  20  and drive links  22 . Rotation of the drive shafts  20  in opposite directions causes the platform  14  to be moved either towards or away from the base  12  as indicated by arrow  60  on  FIG. 1 . Rotation of the drive shafts  20  in the same direction causes the platform  14  to be moved laterally with respect to the base as shown in  FIGS. 4   a  and  4   b . It will thus be appreciated that the platform  14  has 2 degrees of freedom within a rotational plane passing through the base  12  and platform  14 . This motion is achieved with two drive shafts  20 ,  20 ′ having only a single degree of freedom, i.e. rotation. The linkage arrangements  16  imbue the mechanism  10  with both static and dynamic stiffness, especially in the direction normal to the aforementioned plane of motion.  
         [0030]     In the above described embodiment of the mechanism  10 , the various pivotal connections are achieved by pins and eyes of the links. It has been found however that certain of these pivotal connections may be replaced by spherical joints without affecting the operation of the mechanism  10 . In particular the respective connections of the upper location link  24  to the base  12  and bell crank assembly  26 , the lower location link  28  to the bell crank assembly  26  and the platform  14 , and the connection of driven links  30 , 32  to the platform  14 , e.g., at joint  19 , may comprise spherical joints. The use of such spherical joints enables the elimination of reaction forces due to misalignment, for example due to wear or manufacturing tolerances, without adversely affecting the stiffness and mobility of the mechanism  10  as a whole.  
         [0031]     The mechanism  10  may further be arranged so as to be movable in a direction normal to that of the plane of motion of the platform  14 . This may be effected by the mounting of the base  12  to a movement device such as a single degree of freedom linear drive  62 . The linear drive  62  comprises a motor  64 , guide members  66 , end plates  68  and a lead screw  70 . The guide members  66  extend between the end plates  68  and through apertures of the base  12 . The lead screw  70  is rotatable by the motor and passes through a captive nut provided in the base  12 . It will be understood that rotation of the screw  70  causes the base  12 , and hence the mechanism  10  as a whole, to move along the guide embers as indicated by arrow  72 . It will be appreciated that other movement devices know in the art may be utilized to move the mechanism  10 . The mechanism may, for example, be fitted to a traveller which is movable by interaction with a driven endless belt.  
         [0032]     Referring now to  FIGS. 7 and 8  there is shown a second embodiment of a mechanism according to the present invention and generally designated  74 . The mechanism  74  is for the most part similar to that described with reference to FIGS.  1  to  5  and hence common features are identified with like reference numerals. The mechanism  74  of  FIGS. 7 and 8  differs from that of FIGS.  1  to  5  in that bracing means  76  are provided between the driven links  30 , 32 . The bracing means  76  include transverse brace members  78  and diagonal brace members  80  (e.g. brace members  80   a ,  80   b ,  80   c  and  60   d  as shown in  FIG. 8 ). In the embodiment shown there are provided three transverse members  78  positioned respectively at each end and midway between the driven links  30 , 32 , and four diagonal members  80  provided in two pairs. It will be understood that other arrangements of the bracing members  78 , 80  are possible. The inclusion of the bracing members  78 , 80  has been found to increase the static and dynamic stiffness of the mechanism normal to the motion plane of the platform  14  with little overall increase in the weight of the mechanism  74 .  
         [0033]     In the aforementioned embodiments the mechanisms  10 , 74  have each been provided with pairs of inner lower links  30 , 32 . It will be appreciated that a mechanism in accordance with the present invention may be provided with a single inner lower link on each side and still function effectively. The use of two inner lower links is preferred however for the structural rigidity it imparts to the mechanism as a whole.  
         [0034]     Referring finally to  FIG. 6  there is shown a third embodiment of a mechanism according to the present invention generally designated  82 . As before, features common to the previously described embodiments are identified with like reference numerals. The mechanism  82  differs from those previously described in that it is provided on one side with a linkage arrangement  16  of the previously described type and on the other side with a simplified linkage arrangement  84 . The simplified arrangement  84  comprises a drive link  86  fixed for rotation with a drive shaft  20  and a driven link  88  pivotally connected to the platform  14 . The drive and driven links  86 , 88  are joined by a common pinned link or hinge  90 . It will be readily understood that the more complicated linkage arrangement  16  maintains the required orientation of the platform  12  relative to the base  12 , while the simplified linkage arrangement  84  enables motive forces from the drive shaft  20  with which it is associated to be applied to the platform  14 .