Abstract:
A robot arm includes a circular rack, an elongated first mounting member attached to the rack, and a second mounting member attached to the first mounting member. The first mounting member extends in a radius direction of the rack and moves in a circular trace along the rack. The second mounting member slides relative to the first mounting member along the radius direction, thereby defining a polar coordinate system in a plane defined by the rack to allow the second mounting member to flexibly locate at any desired positions in the plane.

Description:
BACKGROUND 
     1. Technical Field 
     The disclosure relates to robot arms, and particularly, to a robot arm moving in a polar coordinate system. 
     2. Description of Related Art 
     Robot arms are widely used in test procedures. These typical robot arms move linearly along three axes of a three-dimensional coordinate system. However, they are relatively complex in structure and not flexible. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded, isometric view of an exemplary embodiment of a robot arm. 
         FIG. 2  is an assembled view of the exemplary robot arm of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , an exemplary embodiment of a robot arm  1  includes a circular rack  10  and a sliding assembly  19  sliding along the rack  10 . The sliding assembly  19  includes a first driving mechanism  50  attached to the rack  10  via a first mounting member  30 , a second driving mechanism  60  attached to the first mounting member  30  via a second mounting member  56 , a third driving mechanism  40  attached to the first mounting member  30  via a third mounting member  20 , and a fourth driving mechanism  70  coupled to the second driving mechanism  60  via a fourth mounting member  71 . 
     The rack  10  includes a first circle portion  14 , and a second circle portion  16  detachably connecting to the first circle portion  14 . The first circle portion  14  includes an entry  12  defined between two opposite end portions thereof. The first circle portion  14  includes two receiving grooves  144  respectively defined in the two opposite end portions thereof. The second circle portion  16  includes a pair of tips  164  respectively extending from two opposite end portions thereof, for engaging in the receiving grooves  144  of the first circle portion  14 , respectively. Pluralities of teeth  142  and  162  are respectively formed on bottom surfaces of the first circle portion  14  and the second circle portion  16 . A plurality of C-shaped fixing brackets  18  is fixed to an inside of the first circle portion  14 . 
     The first mounting member  30  includes a connecting plate  32 , a frame  35  formed at an end of the connecting plate  32 , and a retaining plate  34  perpendicularly formed at the other end of the connecting plate  32 . The frame  35  includes two horizontal sidewalls  36  and two vertical end walls  37 . An elongated sliding slot  364  is defined in each of the two horizontal sidewalls  36 . A plurality of fixing holes  362  is defined in the lower horizontal sidewall  36  beside the sliding slot  364 . A through hole  372  is defined in each of the vertical end walls  37 . 
     The second mounting member  56  includes a U-shaped bracket  58 , a sliding block  57 , and a connecting board  59 . Two sliding bars  574  respectively extend from two opposite ends of the sliding block  57 . A threaded hole  572  is defined in the sliding block  57 . The bracket  58  includes a base wall  584 , and a pair of sidewalls  586  perpendicular to the base wall  584 . Two through holes  582  are defined the base wall  584 . Two through holes  592  are defined in the connecting board  59 . 
     The third mounting member  20  is generally a cubic hollow sleeve and includes a pair of first sidewalls  21 , and a pair of second sidewalls  24  perpendicular to the first sidewalls  21 . A through hole  242  is defined in each of the second sidewalls  24 . An avoiding slot  27  is defined in one of the second sidewalls  24  close to one of the first sidewalls  21 . A size of the avoiding slot  27  is slightly greater than a thickness of a wall of each of the fixing brackets  18  of the rack  10 . A plurality of mounting holes  212  is defined in the one of the first sidewalls  21 , corresponding to the fixing holes  362  of the first mounting member  30 . 
     The first driving mechanism  50  includes a motor  52 , a threaded pole  54 , and a coupling member  90 . 
     The second driving mechanism  60  includes a motor  62 , an active gear  64 , two passive gears  66  each with a central hole, and a pair of threaded poles  67 . 
     The third driving mechanism  40  includes a motor  42 , a shaft  44 , a coupling member  80 , and a gear  46 . The gear  46  includes a central hole  462  defined therein. 
     The fourth driving mechanism  70  includes a motor  74 . The fourth mounting member  71  includes a bracket  72  for holding the motor  74  therein. Two extension plates  722  are formed at a side of the bracket  72 . Two threaded holes  724  are defined in each extension plate  722 . Two clamps  76  are coupled to the motor  74 . 
     In this embodiment, the motors  42 ,  52 ,  62 ,  74  are servomotors. 
     referring to  FIGS. 1 and 2 , in assembly, the third mounting member  20  is fixed around the first circle portion  14  of the rack via the entry  12 . The wall of each of the support brackets  18  is capable of being received in the avoiding slot  27  of the third mounting member  20 . The second circle portion  16  is attached to the first circle portion  14  with the tips  164  engaging with the corresponding receiving grooves  144 , to fill the entry  12 . The gear  46  of the third driving mechanism  40  is received in the third mounting member  20  and meshes with the teeth  142  and  162  of the rack  10 . The shaft  44  of the third driving mechanism  40  fixedly extends through the central hole  462  of the gear  46 . Two ends of the shaft  44  pivotably extend through the through holes  242  of the third mounting member  20 , respectively. Thus, the third driving mechanism  40  is attached to the third mounting member  20 . 
     The first mounting member  30  is attached to the third mounting member  20 , with a plurality of fasteners (not shown) extending through the fixing holes  362  of the frame  35  to engage in the mounting holes  212  of the third mounting member  20 . Therefore, the first mounting member  30  is mounted to the rack  10  and extends in a radial direction of the rack  10 . The motors  42  and  52  are attached to the retaining plate  34  of the first mounting member  30 , respectively located at the lower portion and the upper portion of the connecting plate  32 . One end of the shaft  44  is coupled to the motor  42  via the coupling member  80 . 
     The second mounting member  56  is movably mounted to the first mounting member  30 . The sliding block  57  is received in the frame  35 , with the two sliding bars  574  slidably received in the sliding slots  364  of the frame  35 . The threaded pole  54  extends through one of the through holes  372  of the frame  35 , the threaded hole  572  of the sliding block  57 , and the other one of the through holes  372  of the frame  35  to be coupled to the motor  52  via the coupling member  90 . 
     The bracket  58  is attached to the upper sliding bar  574  of the sliding block  57  and the connecting board  59  is attached to the lower sliding bar  574 . The active gear  64  is coupled to the motor  62 . The motor  62  is attached to the sidewalls  586  of the bracket  58 . The two threaded poles  67  engage in the threaded holes  724  of the fourth mounting member  71 , threadedly extend through the through holes  592  of the connecting board  59  and the through holes  582  of the bracket  58 , and fixedly engage in the central holes of the two passive gears  66 . The two passive gears  66  mesh with the active gear  64 . The motor  74  is received in the bracket  72  of the fourth mounting member  71 . 
     In use, the exemplary robot arm  1  is fixed to a worktable by a plurality of fasteners extending through the fixing brackets  18  of the rack  10  to engage with the worktable. In the embodiment, the two clamps  76  respectively fix two probes  92 , e.g. an anode and a cathode. An article to be tested such as a printed circuit board (PCB, not shown) is placed on the worktable. The motor  42  drives the gear  46  to rotate via the shaft  44 . The gear  46  meshes with the teeth  142  and  162  of the rack  10 . Thus, the combined first and first mounting members  20  and  30  are driven to move in a circle along the rack  10 . The motor  52  drives the threaded pole  54  to rotate in the threaded hole  572  of the sliding block  57  of the second mounting member  56 . Thus, the second mounting member  56  slides along the sliding slots  364  of the frame  35  in a X-Y plane defined by the rack  10 . The first and third driving mechanisms  50  and  40  form a polar coordinate system in the X-Y plane. The motor  62  drives the active gear  64  to rotate and the active gear  64  meshes with the two passive gears  66 . Thus, the threaded poles  67  rotatably engage in the threaded holes  724  of the fourth mounting member  71  to move the fourth mounting member  71  to slide in a Z-direction (namely a direction perpendicular to the plane defined by the rack  10 ). The clamps  76  and the probes  92  can move to any desired positions to conveniently test the PCB. The motor  74  drives the clamps  76  to adjust a distance between the probes  92 . 
     In other embodiments, the second driving mechanism  60  can include one threaded pole  76  directly coupled to the motor  62  to drive the fourth mounting member  71 . 
     In other embodiments, more than one sliding assembly  19  capable of being slidably attached to the rack  10  can be included. 
     It is to be understood, however, that even though numerous characteristics and advantages of the disclosure have been set forth in the foregoing description, together with details of the structure and function of the disclosure, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the disclosure to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.