Abstract:
A joint may have a multi-stage planetary gearbox between the stationary housing and the rotary housing. To accommodate different gear ratios, the rotary housing may be joined to the stationary housing by a releasable attachment. This allows portions of the planetary gearbox to be replaced so that, for instance, the last stage may be chosen as either a simple or compound differential planetary stage. To allow for different capacities, a quotient of a sum of all teeth of a sun gear of a stage and of the ring gear with which the planetary gears of the stage mesh to both the number three and the number four yields an integer. In this way, the stage may be provided with either three or four planetary gears. The gearbox may have a ring gear common to a plurality of simple planetary stages. Where the final stage is a simple planetary stage, the carrier may be provided with a flange extending around, and bearing mounted to, the common ring gear. To reduce weight and increase robustness, the planetary gears of a stage are retained on their carrier by a bumper ring provided between carriers. An angle sensor may be provided between the stationary and rotary housings.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a planetary gearbox and a robotic joint having a planetary gearbox. 
     Designing robotic joints for harsh remote environments, such as outer space, presents a number of design challenges. For example, typically, there are strict energy budgets. Consequently, the energy requirements for a motor motivating a joint must be as small as possible. On the other hand, the portion of the robot below the joint may have a significant inertial mass so that a large torque is required to drive it according to specifications. In order to meet these conflicting design criteria, a high ratio gearbox is typically provided between the motor and the lower portion of the robot. 
     Because of the cost of delivering robotic components to a remote environment, these components are designed to be as small and lightweight as possible. A light weight also reduces inertial forces of the robot, in use. These requirements extend to the joint gearbox: it too should be as small and lightweight as possible. The gearbox must also be able to withstand the temperatures of the harsh environment; for space or planetary applications, this means withstanding very low, or very high, temperatures. 
     A robot typically has a number of joints, each powered by a motor. Each joint may have different torque and input to output ratio requirements. This suggests different gearboxes for different joints; this provides further design complexity and increased manufacturing costs. 
     Accordingly, a need remains for a gearbox which may provide a high capacity and a high ratio and which may be made so as to be lightweight, miniature, and robust. Such a gearbox would be suitable for use in a robot deployed in a harsh remote environment. A need also remains for such a gearbox designed to accommodate different ratios and capacities in order to minimize complexity and manufacturing cost. 
     SUMMARY OF INVENTION 
     A joint may have a multi-stage planetary gearbox between the stationary housing and the rotary housing. To accommodate different gear ratios, the rotary housing may be joined to the stationary housing by a releasable attachment. This allows portions of the planetary gearbox to be replaced so that, for instance, the last stage may be chosen as either a simple or compound differential planetary stage. To allow for different capacities, a quotient of a sum of all teeth of a sun gear of a stage and of the ring gear with which the planetary gears of the stage mesh to both the number three and the number four yields an integer. In this way, the stage may be provided with either three or four planetary gears. The gearbox may have a ring gear common to a plurality of simple planetary stages. Where the final stage is a simple planetary stage, the carrier may be provided with a flange extending around, and bearing mounted to, the common ring gear. To reduce weight and increase robustness, the planetary gears of a stage are retained on their carrier by a bumper ring provided between carriers. An angle sensor may be provided between the stationary and rotary housings. 
     According to the present invention, there is provided a joint comprising: a stationary housing; a rotary housing joined to said stationary housing by a releasable attachment, said releasable attachment permitting said rotary housing to rotate relative to said stationary housing; a motor carried by said stationary housing; a multi-stage planetary gearbox having a first stage sun gear motivated by said motor, said gearbox terminating at said rotary housing, such that, by releasing said releasable attachment, said rotary housing, and at least a portion of said multi-stage planetary gearbox, may be removed. 
     According to another aspect of the invention, there is provided a planetary gearbox having a sun gear meshing with three or four planetary gears where a quotient of a sum of all teeth of said sun gear and of a ring gear with which said planetary gears mesh to both the number three and the number four yields an integer value. 
     According to a further aspect of the invention, there is provided a planetary gearbox having a sun gear meshing with a given number of planetary gears where a quotient of a sum of all teeth of said sun gear and of a ring gear with which said planetary gears mesh to both of two adjacent integer values, where one of said adjacent integer values is said given number, yields an integer value. 
     According to another aspect of the invention, there is provided a planetary gearbox having a plurality of simple planetary stages with a common ring gear such that said common ring gear, in unison with planetary and sun gears of said gearbox, provides radial stability to said planetary and sun gears without need for an axle. 
     According to a further aspect of the invention, there is provided a planetary gearbox, comprising: a bumper ring extending between a first carrier for a sun gear and a second carrier for planetary gears meshing with said sun gear, said bumper ring being rotatable at least with respect to said second carrier, said bumper ring overlapping with said planetary gears in order to retain said planetary gears on said second carrier. 
     According to a yet further aspect of the invention, there is provided a planetary gearbox, comprising: a bumper ring overlapping with an end face of all planetary gears meshing with a single sun gear so as to retain said planetary gears. 
     According to a yet further aspect of the invention, there is provided a planetary gearbox having a plurality of simple planetary stages with a common ring gear, a final stage of said simple planetary stages having a carrier with a flange extending around, and bearing mounted to, said common ring gear. 
     Other features and advantages of the invention will become apparent to those of ordinary skill in the art upon review of the following description in conjunction with the following figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the figures which illustrate example embodiments of the invention, 
     FIG. 1 is a side view of a portion of a robot having a joint with a planetary gearbox in accordance with this invention, 
     FIG. 2 is an exploded view of the robotic joint of FIG. 1, 
     FIG. 3 is a cross-sectional view of the robotic joint of FIG. 2, 
     FIG. 4 is a schematic view of the robotic joint of FIG. 2, 
     FIG. 5 is a cross-sectional view of a robotic joint according to another embodiment of this invention, and 
     FIG. 6 is a schematic view of the robotic joint of FIG.  5 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows an upper arm (or boom)  12  of a robot joined to a lower arm (or boom)  14  by a robotic joint  10 . Turning to FIGS. 2 and 3, the robotic joint  10  has a stationary housing  20  with a boom-to-joint interface  22  by way of which the stationary housing is affixed to the robotic upper arm. The joint  10  also has a rotary housing  24  with a boom-to-joint interface  26  by way of which the housing is affixed to the robotic lower arm. Joint motor  30  is mounted within stationary housing  20  by way of bolts  33  which are received by the motor. An end plate  34  is mounted to the stationary housing and covers the motor. 
     The shaft  36  of the motor has a square cross-section; this shaft receives spacer  37 , sun gear  140 , and spacer  42 . A bumper ring  38  surrounds the shaft. Sun gear  140  is part of the first stage of the planetary gearbox  16  and meshes with the three planetary gears  120  of the first stage. (For clarity, only two of these three gears  120  are shown.) Each of these first stage planetary gears  120  is supported by a bushing  122  on a pin  124  projecting from a first stage carrier  130 . As will be apparent from FIG. 3, the bumper ring  38  abuts a face  39  of stationary housing  20  and overlaps with an end face of planetary gears  120  in order to retain the planetary gears  120  on their carrier  130 . A sun gear  240  is machined on the back of the first stage carrier  130  and a bumper ring  138  surrounds sun gear  240 . 
     Sun gear  240  is part of the second stage of the gearbox and meshes with the three planetary gears  220  of the second stage. Each of these second stage planetary gears  220  is supported by a bushing  222  on a pin  224  projecting from a second stage carrier  230 . As will be apparent from FIG. 3, the bumper ring  138  overlaps with an end face of planetary gears  220 . A third stage sun gear  340  extends from the back of the second stage carrier  230  and a bumper ring  238  surrounds sun gear  340 . 
     In like fashion, the third stage sun gear  340  meshes with the third stage planetary gears  320  which are bush mounted to the third stage carrier  330 . Bumper ring  238  overlaps with planetary gears  320 . A sun gear  440  extends from the back of the third stage carrier and a bumper ring  338  surrounds sun gear  340 . 
     Similarly, sun gear  440  meshes with the fourth stage planetary gears  420  which are bush mounted to the fourth stage carrier  430 . Bumper ring  338  overlaps with planetary gears  420 . A sun gear  540  extends from the back of the fourth stage carrier. 
     From the foregoing, it will be apparent that the first three stages of the planetary gearbox  16  are identical. The fourth stage is also identical except that its gears  420 ,  440  are longer than those of the preceding stages. Each of the first four stages is a simple planetary stage. 
     The fifth stage sun gear  540  extending from the back of the fourth stage carrier meshes with the larger diameter end  552  of compound differential planetary gears  520 . Each of the compound differential planetary gears  520  is supported by the fifth stage carrier  530  on a pair of needle bearings  551 ,  553  carried by a shaft  556 . The needle bearings are positioned on the shaft by washers  558 ,  560  and spacer  562 . The shaft  556  passes through openings  568  in the ends of the carrier  530  and is held in place by a circlip  570  which attaches to a circumferential groove in the shaft. There are four compound differential planetary gears  520 . 
     The fifth stage carrier  530  is radially supported by shaft  572  which mounts through opening  62  in rotary housing  24  and is held in place by circlip  574  which attaches to a circumferential groove in the shaft  572 . The carrier rides on needle bearings  576 ,  578 ,  580  carried by the shaft  556 , which needle bearings are spaced by spacer  582  washer  584 , and flange  586  on shaft  572 . The fifth stage is a compound differential stage. 
     The first four stages of the planetary gearbox  16 , and a portion of the fifth stage, are received within a common annulus  50  which functions as a common ring gear. Common ring gear  50  is affixed to stationary housing  20  by dowels  32  and bolts  35 . Each of the first four sets of planetary gears  120 ,  220 ,  320 , and  420  meshes with the reduced diameter toothed section  52  of common ring gear  50 . Further, the larger diameter end  552  of compound differential planetary gears  520  meshes with the enlarged diameter toothed section  54  of common ring gear  50 . 
     The smaller diameter end  554  of compound differential planetary gears  520  meshes with ring gear  60 . Ring gear  60  is mounted by pins  64  within rotary housing  24 . 
     A pair of bearings  66 ,  68  between common ring gear  50  and rotary housing  24  supports the housing  24  for rotation on the common ring gear. The bearings  66 ,  68  are spaced by an outer sleeve  70  and inner sleeve  72 . Sleeves  70  and  72  are compressed between the bearings  66 ,  68  so that the outer sleeve moves with the outer rings of the bearings and the inner sleeve moves with the inner rings of the bearings. The bearings  66 ,  68  are positioned by spacer  76  which butts up against a retaining ring  75 . As seen in FIG. 3, the (gapped) retaining ring  75  snaps into a notch, proximate the outer lip of the rotary housing  24 . A sealing gasket  74  is supported within an interior groove of stationary housing  20  and bears against the rotary housing  24 . 
     Optionally, common ring gear  50  is provided with an externally toothed section  56  and the teeth of this externally toothed section mesh with a gear  80  of an angle sensor  82  supported within rotary housing  24 . With the angle sensor carried within the rotary housing at the boom-to-joint interface  26 , the joint remains streamlined. 
     It will be noted from the foregoing that the planetary gears of each of the first four stages are not positively mounted on their carrier. Instead, the planetary gears of a stage are kept on their bushing support by a bumper ring. For stages two to four, this ring  138 ,  238 ,  338  is positioned between the planetary gears and the carrier of the preceding stage. For the first stage, this ring  38  is positioned between the planetary gears and the face  36  of the stationary housing  20 . While bumper rings  138 ,  238 ,  338  are not positively mounted, their freedom to radially shift is limited due to their outside diameter being only slightly less than the inside diameter of toothed section  52  of common ring gear  50 . Thus, any such shifting would not eliminate the overlap of the bumper with the planetary gears and so is not problematic. (Bumper ring  38  is constrained from radial shifting by a lip  41  of stationary housing  20 .) It will also be noted that bumper rings  138 ,  238 ,  338  are between two planetary stages and abut the rotating carrier of one stage and the more slowly circulating pins which carry the planetary gears of the next stage. The resulting tendency to wear may be resisted by an appropriate choice of material for fabrication of the bumpers and also by a low compressive force applied to the bumpers by the sandwiching carrier and pins. A suitable enduring, low-friction material for each of the bumper rings  38 ,  138 ,  238 , and  338  is SP3 VESPEL (a trade-mark of DuPont). 
     The bumper ring may allow the planetary gears it retains a limited amount of axial freedom. However, a planetary gear which migrated toward its bumper would be stopped once the end face of the gear butted up against the bumper. 
     It will also be noted from the foregoing that carriers  130 ,  230 ,  330 , and  430  do not ride on an axle. Instead, the common ring gear  50 , in unison with the planetary gears and sun gears, provides radial stability to these carriers. 
     In operation, with reference to FIG. 4 along with FIGS. 2 and 3, when motor  30 , which is fixed to stationary housing  20 , rotates shaft  36 , sun gear  140  rotates. This causes each of planetary gears  120  to rotate about its own axis. However, since these planetary gears mesh with teeth on common ring gear  50 , when each rotates on its own axis, it “walks” around the inside circumference of ring gear  50 , thereby causing carrier  130  to rotate. Sun gear  240  rotates with carrier  130  and motivates the planetary gears  220  of the second stage of the gearbox to rotate and circulate. The third and fourth stages operate similarly. The sun gear  540  extending from the fourth stage carrier  430  causes the compound differential planetary gears  520  of the fifth stage to rotate about their own axes so that their larger diameter ends  552  walk around the toothed section  54  of common ring gear  50 . In so doing, the smaller diameter ends  554  of the compound differential planetary gears cause ring gear  60  to rotate about its axis. However, ring gear  60  is mounted to rotary housing  24 . In the result, rotary housing  24  rotates with respect to stationary housing  20 . This changes the orientation of the upper arm  12  with respect to lower arm  14 . 
     With the rotary housing rotating with respect to the stationary housing, the gear  80  of the optional angle sensor  82  carried by rotary housing  24  will rotate as the gear  80  walks around the optional toothed section  56  of common ring gear  50 . In consequence, the angle sensor may provide an indication of the angular position of the rotary housing  24  with respect to the stationary housing  20 . 
     As is well understood by those skilled in the art, each of the planetary gears of a given stage has an identical number of teeth. Each stage of the planetary gearbox has a carrier rotating at a lower speed and at a higher torque than the previous stage which is closer to motor  30 . As is well understood by those skilled in the art, the gear ratio provided by a simple planetary stage is a function of the number of teeth on the sun gear of the stage and the number of teeth of the ring gear for the stage. Also as well understood by those skilled in the art, the gear ratio provided by a compound differential planetary stage is a function of the number of teeth on the sun gear of the stage, the number of teeth of each of the two ring gears for the stage, and the number of teeth at each end of the planetary gears for the stage. 
     The planetary gearbox  16  has been described with each of the first four simple planetary stages having three planetary gears and the fifth, compound differential stage, having four planetary gears. For the gears of a (simple or compound) planetary stage to mesh properly, the sum of the number of teeth of the sun gear plus the number of teeth of the ring gear divided by the number of planetary gears of the stage must yield an integer. 
     With the subject gearbox  16 , the noted sum is chosen so that an integer value results where the number of planetary gears is either three or four. Thus, the number of planetary gears for any of the stages may be chosen at either three or four. The fourth planetary gear in a stage increases the capacity of the stage, but at the cost of a higher weight and greater inertia for the gearbox. Because of the lower torques in the first stages of the gearbox, it is normally only necessary to consider a fourth planetary gear for the fifth stage, or for the fourth and fifth stages of the gearbox. 
     It will be appreciated that by simply removing retaining ring  75  from the notch in rotary housing  24 , rotary housing  24  may be axially slid off. Thus, the retaining ring acts as a releasable attachment between the rotary and stationary housings. Once the rotary housing is removed, carrier  530  may be axially slid off and then, subsequently, each of carriers  430 ,  330 ,  230 , and  130  may be removed in turn. This permits substitution of a different carrier holding a different number of planetary gears (e.g., a carrier holding four planetary gears rather than three planetary gears). 
     A suitable choice for the number of teeth for each of the sun gears  140 ,  240 ,  340 ,  440  of the first four stages is twenty-four. A suitable choice for the number of teeth of smaller diameter toothed section  52  of the common ring gear  50  is eighty-four. These choices set the number of teeth for each planetary gear of each of the first four stages at thirty teeth. A suitable choice for the number of teeth of sun gear  540  is also twenty-four. Additionally, a suitable choice for the number of teeth for larger diameter toothed section  54  of common ring gear  50  is also eighty-four. A suitable number of teeth for the larger diameter end  552  of compound differential planetary gear  520  is thirty and for the smaller diameter end  554  of the gear twenty-four. A suitable number of teeth for ring gear  60  is seventy-eight. 
     It will be appreciated that the sum of the number of teeth of the sun gear of a stage plus the number of teeth of the ring gear divided by the number of planetary gears of the stage could be chosen so as to yield an integer where the number of planetary gears is one of two other values. Usually, the sum will be chosen so that two adjacent integer values yield an integer. Thus, for example, the sum could yield an integer where the stage has either four or five planetary gears. In this way, a gearbox may be designed to have other, changeable, capacities. 
     Each of the planetary gears and sun gears, along with the common ring gear  50  and ring gear  60  may be fabricated of steel. Stationary housing  20  and rotary housing  24  may be fabricated of titanium or beryllium aluminum to provide a weight advantage. 
     In another embodiment, seen in FIGS. 5 and 6, the first four stages of the gearbox  1016  of joint  1000  are identical to the first four stages of the embodiment of FIGS. 1 to  4  and like parts have been given like reference numerals. The fifth stage, however, is a further simple planetary stage rather than being a compound differential stage. More particularly, a sun gear  1540  extends from the back of the carrier  430  of the fourth stage and meshes with planetary gears  1520  of the fifth stage. The fifth stage planetary gears are carried by bearings  1522  supported on pins  1524  of fifth stage carrier  1530 . The fifth stage carrier has a flange  1024  which acts as the rotary housing for the joint. Thus, the fifth stage carrier does double duty as the housing, thus reducing the weight of the gearbox. The fifth stage planetary gears mesh with toothed section  54  of the common ring gear  50 . 
     The pins  1524  are separate components from the carrier  1530  and are held to the carrier by circlips  1574 . 
     The number of teeth chosen for the fifth stage sun gear  1540  is the same as that for the fifth stage sun gear  540  of the compound differential stage in the embodiment of FIGS. 1 to  4 . In view of this, and the fact that common ring gear  50  is common to both embodiments, the fifth stage of gearbox  1016  may have either three or four planetary gears  1520 . 
     The operation of the first four stages of the embodiment of FIGS. 5 and 6 is as described in conjunction with the first four stages of the embodiment of FIGS. 1 to  4 . When the fourth stage carrier  430  of gearbox  1016  is motivated to rotate, sun gear  1540  also rotates causing planetary gears  1520  to walk around section  54  of common ring gear  50 . With the planetary gears  1520  circulating, carrier  1530 , with its depending flange  1024 , rotates. 
     If retaining clip  75  is removed, carrier  1530  may be axially slid off. This allows progressive disassembly of the gearbox  1016  so that a different carrier with a different number of planetary gears may be substituted in any of the stages. Furthermore, by removing the simple planetary fifth stage, the compound differential planetary fifth stage of FIGS. 1 to  4  may be substituted. This then allows for manufacture of a gearbox which has a common input end and one of two output ends thereby reducing the manufacturing cost for two different gearboxes. In this regard, as will be apparent from the foregoing, the gearbox when provided with a compound differential last stage will have a higher ratio than when provided with a simple planetary last stage. Thus, the gearbox  16  will be used in higher torque applications. Typically, gearbox  16  will be used in a joint (such as a shoulder joint) where the portion of the robot below the joint is of a larger weight and gearbox  1016  will be used in a joint (such as a wrist joint) where the portion of the robot below the joint is of a smaller weight. 
     As will be apparent to those skilled in the art, gearbox  16  and gearbox  1016  are scaleable by simply changing the diametral pitch of the gears. A larger, higher torque, gearbox, or a smaller, lower torque, gearbox can therefore be manufactured as required. 
     While planetary gearbox  16  has been described as being part of a robotic joint, it will be apparent to those skilled in the art that the gearbox  16  will have application to a wide variety of other mechanisms requiring a gearbox. 
     Other modifications within the spirit of the invention will be apparent to those skilled in the art and, therefore, the invention is defined in the claims.