Abstract:
Described in this application is a zero-backlash preload gear assembly. The assembly is specifically described for use with a positioning system in order to significantly reduce or eliminate mechanical error and improve tolerances. Furthermore, the assembly allows for infinite positioning of a work surface in order to facilitate improved efficiency of an automated system.

Description:
FIELD OF THE INVENTION 
     The present invention relates to positioning apparatus and methods and mechanisms to eliminate or substantially reduce backlash in gearing control systems in a positioning apparatus. 
     BACKGROUND OF THE INVENTION 
     Automated servo-driven robots are increasingly becoming a staple in manufacturing settings. From welding to assembly, these machines are capable of performing repetitive tasks with relatively low per-unit expenses. In order to achieve the optimum quality work from such a robot, materials must be positioned at proper positions with only minor variation, defined as manufacturing tolerances. 
     It is common practice to mount material onto a circular platform having one or more indexed positions. This allows a worker to position materials on one station while the robot is performing the selected operation, for example welding. By providing hard stops between the platform and base to which the table is mounted, the indexed positions can be placed within the appropriate tolerance for the robot to provide function. However, the “hard stop” setup allows only two indexed positions. For more complicated processes requiring several stations, multiple tables must be set up, increasing expense. 
     Applicant&#39;s previous patent, U.S. Pat. No. 5,704,601 to Mangelsen et al., issued Jan. 6, 1998, herein incorporated by reference in its entirety, provided a fluid pressure-actuated cylinder acting as the actuating member, which allowed swift acceleration and deceleration of the rotating table, thereby avoiding jarring the parts or causing damage as the stops are contacted. 
     According to another patent, U.S. Pat. No. 6,375,178 to Schilb et al. issued Apr. 23, 2002, herein incorporated by reference in its entirety, dual cylinders may be used to avoid the null points identified in the &#39;601 patent, as well as provide two positions by selectively pressurizing the cylinders to engage hard stops. The dual cylinder design also allows the system to be slowed before impacting the hard stops, thereby avoiding jarring of parts. 
     Therefore, there is a need in the art for a low-cost, high-reliability and accurate positioning system which is rotatable and provides infinitely variable positions without additional costs for each position. 
     There is also realized in the art a need for a low-cost, high-reliability, and accurate positioning system coupled with a robot which minimizes floor space while maintaining full use of the automated robot. 
     There is further realized in the art a need for a positioning system with infinite variability while maintaining a high degree of accuracy and predictability. 
     These problems are sought to be overcome in the present invention. 
     BRIEF SUMMARY OF THE INVENTION 
     According to one embodiment, the invention relates to an infinitely variable positioning system with a center-mounted welding robot. The positioning system is preferably driven by a servo motor which translates force through a drive gear to a driven gear connected to the table. Positioned adjacent the drive and driven gear are idler gears which eliminate backlash, thereby ensuring accuracy and reliability of the system without necessitating increased per-position expense or secondary positioning apparatuses, such as hard stops or braking mechanisms. The idler gears are positioned on slots allowing them to move radially about the drive and driven gears, thereby ensuring constant engagement with the drive and driven gears. The driven gear is preferably a hollow gear, allowing electronic equipment to be passed through the rotating gear. Such an arrangement allows the robotic welder to be positioned in the center of the workspace, thereby allowing full range of operation of the robot while reducing floor space requirements. 
     Described is a gear set comprising a pair of idler gears meshing with a drive and driven gear. The idler gears reduce backlash or lost motion in the gear set by engaging in an interference with the drive and driven gears and are held in position with an adjustment member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a top view of the table and robot assembly according to one embodiment of the present invention. 
         FIG. 1B  is a side view of the table and robot assembly of  FIG. 1A . 
         FIG. 2A  is a front perspective view of the gear assembly according to one embodiment of the present invention. 
         FIG. 2B  is a back perspective view of the gear assembly of  FIG. 2A . 
         FIG. 3  is a front view of the assembly with the drive gear rotating counterclockwise. 
         FIG. 3A  is an enlarged view of the meshing teeth of the drive and first idler gears in operation according to  FIG. 3 . 
         FIG. 3B  is an enlarged view of the meshing teeth of the driven and second idler gears in operation according to  FIG. 3 . 
         FIG. 3C  is an enlarged view of the meshing teeth of the drive and driven gears in operation according to  FIG. 3 . 
         FIG. 3D  is an enlarged view of the meshing teeth of the first and second idler gears in operation according to  FIG. 3 . 
         FIG. 4  is a front view of the assembly of  FIG. 2A  with the drive gear rotating clockwise. 
         FIG. 4A  is an enlarged view of the meshing teeth of the drive and first idler gears in operation according to  FIG. 4 . 
         FIG. 4B  is an enlarged view of the meshing teeth of the driven and second idler gears in operation according to  FIG. 4 . 
         FIG. 4C  is an enlarged view of the meshing teeth of the drive and driven gears in operation according to  FIG. 4 . 
         FIG. 4D  is an enlarged view of the meshing teeth of the first and second idler gears in operation according to  FIG. 4 . 
         FIG. 5  is an enlarged perspective view of the idler assembly. 
         FIG. 6  is an enlarged front view of the idler assembly. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The zero-backlash preload gear assembly and positioning device  10  will now be described in detail according to the preferred embodiment with reference to the attached figures where numerals relate to their like in the following description. 
     The terms “leading edge” and “trailing edge” of the teeth of a gear will be used within this description. The term “leading edge” and “trailing edge” are intended to indicate the faces of an individual gear tooth. Each term is described with reference to the direction of movement of the gear; for example if a gear is being rotated in a clockwise direction, then the “leading edge” is the clockwise-ward face of the gear tooth, while the counter-clockwise (also known as “contra-clockwise”, “anticlockwise”, or abbreviated “CCW”) face of the gear tooth represents the “trailing edge.” In the case of gear interaction, force will always be transferred from the “leading edge” of a tooth on a drive gear to the “trailing edge” of a tooth on a driven gear (the terms “drive gear” and “driven gear” in this circumstance are used generically and without reference to the specific structure as described below). 
     The term “lost motion” may also be used to describe certain aspects of the invention. This term means, within the context of two or more meshing gears, movement of a drive gear without corresponding movement of a driven gear. Such an occurrence is caused by the teeth of a drive gear being smaller than the tooth gap on a driven gear. Lost motion may occur in one of three circumstances. First, if the drive gear is stopped after some period of driving a driven gear, rotational inertia of the driven gear will cause the driven gear to continue to rotate until the leading edge of the driven gear impacts the trailing edge of the drive gear. Second, if the drive gear is reversed from a first direction to a second direction, the driven gear will continue to rotate in its first direction until contacted by the returning leading edge of the drive gear. Third, a worker loading a part may push or pull on the positioner. The first circumstance results in the driven gear continuing to rotate while the drive gear is stationary. The second circumstance results in the driven gear remaining stationary or moving in a non-preferred direction while the drive gear is in operation. The third circumstance results in movement of the table according to the actions of the worker. All of these situations are undesirable as increasing noise and error and reducing the life of the gears. 
     The term “work surface” may also be used to describe certain aspects of the invention. This term means, without limitation, a table (oriented horizontal or vertical), part, fixture, or another machine, any of which require a high degree of accuracy in positioning. The work surface may be held in a steady position during an industrial operation, or may be movable according to the range of motion of the positioning device. While the term “table” is used throughout this description, this is but one embodiment of the invention. A work surface may be substituted for a table in any of the described embodiments. 
     The zero-backlash preload gear system  20  is preferably designed for use in a positioning system for an assembly  10  consisting of a welding or other automated robot  12  and table  14 . 
     According to one embodiment, as illustrated in  FIGS. 1A and 1B , the robot  12  is mounted to the center of the table  14 . Such an arrangement reduces the required amount of floor space required for the robot to operate while increasing its usable range of operation. 
     As can be appreciated by those skilled in the art, an automated robot  12  is mounted on a first axis of rotation, and therefore has a circular operating reach. Prior art systems which mount a robot adjacent the work table  14  are only able to utilize a small portion of the robot&#39;s  12  operating reach where the circular reach of the robot intersects the table space. Therefore, it is preferable to combine a circular table  14  with a center-mounted robot  12  having a circular reach to maximize usefulness of the robot  12 . Such a system is described with more particularity in U.S. Pat. No. 5,083,070, herein incorporated by reference in its entirety. 
     The &#39;070 patent describes the invention as being used with a “rotatable index table.” The present invention provides an improved rotatable index table  12  which is capable of an infinite number of indexed positions  16  while avoiding positioning errors due to backlash. Backlash is defined as the play in gears caused by tooth spaces exceeding the thickness of an engaging tooth. 
     The zero-backlash preload gear system  20  which drives the table  12  is shown in an assembled configuration in  FIGS. 2A and 2B .  FIG. 2A  shows the drive gear  24  and driven gear  26  arranged according to the preferred embodiment while  FIG. 2B  shows the mounting system for the driven gear  26  and the servo motor  32  which provides the indexing function. The entire gear system  20  is secured to a mounting plate  22 . 
     Servo motors are well known in the art. When a coded signal is sent to a servo motor the motor moves to a predetermined angular position. The motor holds this position so long as the signal is maintained. For example, if a 1.5 ms electrical pulse is sent to the servo every 20 ms, the servo will remain at its neutral or 90° position. A 1.75 ms pulse causes the motor to move to its 180° position while a 1.25 ms pulse would cause the motor to move to its 0° position. These examples are only used by way of illustration, as each manufacturer may provide their own operating parameters, technical specifications, and accuracy. Servo motors may also have rotational positions in excess of 360°, requiring one or more rotations of the motor in response to a received signal. 
     The drive gear  24  is rotated about an axle (not shown) passing through the mounting plate  22 , and is secured by means commonly known in the art, such as a bearing. The driven gear  26  is shown as a hollow gear having an open center, through which power and electronics may be passed, thereby allowing the robot to be mounted through the center of the table  14 . The driven gear  26  is attached to the mounting plate by, for example, a bearing  28  secured to the mounting plate  22 . Positioned about the circumference of the driven gear  26  are a number of mounting holes  30  to which the tabletop  14  is securely fastened. As the driven gear  26  is rotated to a variety of positions by the drive gear  24  and servo motor  32 , the rotation is transferred to the tabletop  14 , cycling through the indexed positions  16 . 
     Also attached to the mounting plate  22  is an idler assembly  40  which eliminates backlash in the system. The idler assembly  40  primarily includes an adjustment member  42 , a first idler gear  44  meshing with the drive gear  24  and a second idler gear  46  meshing with the driven gear  26 . The first  44  and second  46  idler gears mesh with one another. An idler plate  48  covers the idler gears  44 ,  46 . 
       FIG. 3  shows a front view of the entire gear assembly  20  with the idler plate  48  removed so that the interaction of the gears may be demonstrated. According to the embodiment shown, a compression force  38  is applied to the adjustment member  42 , thereby subtly shifting the base plate  52  direction towards the driven gear  26 . This causes the idler gears  44 ,  46  to likewise shift and contact the drive and driven gears  24 ,  26 . As the drive gear  24  is rotated in a counter-clockwise direction, the leading edge of the teeth of the drive gear  24  make contact with the trailing edge of the teeth of the first idler gear  44  (see  FIG. 3A ); while the trailing edge of the teeth of the drive gear  26  make contact with the leading edge of the teeth of the driven gear  26  (see  FIG. 3C ). As shown in  FIGS. 3B and 3D , the leading edge of the teeth of the first idler gear  44  contact the trailing edge of the teeth of the second idler gear  46 ; and the leading edge of the teeth of the second idler gear  46  contact the trailing edge of the teeth of the driven gear  26 . 
     It is to be understood by those skilled in the art that according to this embodiment, when a compression force  38  is applied to the adjustment member  42  and the drive gear  24  is rotated in a counterclockwise direction, power is transferred from the drive gear  24  through the idler gears  44 ,  46  to the driven gear  26 . 
     FIGS.  4  and  4 A-D show the arrangement of  FIG. 3  where the drive gear  24  is rotated in a clockwise direction. As can be appreciated by those skilled in the art, when such an arrangement is used, power is transferred directly from the drive gear  24  to the driven gear  26 . 
     Based on this arrangement, it should be understood that when the drive gear  24  is reversed in direction, with a constant force  38  on the adjustment member  42 , there is no period in which a leading edge of the teeth of the drive gear  24  is not in contact with either the driven gear  26  or the first idler gear  44 . Similarly, there is no period where a trailing edge of the teeth of the driven gear  26  is not in contact with either the drive gear  24  or second idler gear  46 , thereby eliminating the effects of backlash in the system. 
     Those skilled in the art will appreciate that when a tensile force (not shown) is applied to the adjustment member  42 , the above-described arrangement is reversed. When the drive gear is rotated counter-clockwise, power is transferred directly from the drive  24  to the driven gear  26 . When the drive gear  24  is rotated clockwise, power is transferred through the idler gears  44 ,  46  to the driven gear  26 . 
     According to the above-described arrangement, the rotational orientation of the table  14  may be adjusted in either a clockwise or counterclockwise direction without large error in the positioning of the indexed locations  16 . For example, the difference between the tooth gap in one gear and the tooth thickness of another gear may be 0.005″ for a gear having a pitch diameter of 20″. The gear is then attached to a table having a diameter of 80″, translating into 0.02″ error. Therefore, it is shown that the error of a small amount of backlash in a gearing system is compounded to create larger errors in the positioning system. By eliminating or substantially reducing the error due to backlash, tighter manufacturing tolerances may be held on the worked parts. 
     The idler assembly  40 , best shown in  FIG. 5 , is also an important component in the operation of the positioning gear assembly  20 . Generally the idler assembly  40  consists of a base plate  52 , a pair of idler gears  44 ,  46 , a pair of idler plates  48 , spacers  50 , and the adjustment member  42 . The idler gears  44 ,  46  are connected at their axes by the base plate  52  so that there is a fixed distance between their respective axes ensuring constant meshing between them. The connected idler gears  44 ,  46  are then each constrained by slots  54 ,  56  in the idler plates  48  about their respective axes to move in an arc  54 ,  56 . The first idler gear  44  is limited to movement in a first arc  54  which is centered about the axis of the drive gear  24 . The second idler gear  46  is limited to movement in a second arc  56  which is centered about the axis of the driven gear  26 . 
     This arrangement serves dual purposes. First, since the first and second idler gears  44 ,  46  are constrained to have a fixed distance between centers, their movement relative one another can be predicted. Therefore, the idler gears  44 ,  46  will not separate or jam one another. Second, by constraining the first and second idler gears  44 ,  46  to arcs  54 ,  56  centered on the drive and driven gears  24 ,  26 , it is ensured that the idler gears  44 ,  46  remain in constant contact with the drive and driven gears  24 ,  26 . This setup is important to ensure constant meshing of teeth between adjacent gears even as wear of the teeth occurs. 
     The adjustment member  42  is connected to base plate  52  to transmit the necessary force to ensure contact between the teeth of the idler gears  44 ,  46  and the drive and driven gears  24 ,  26 . Even when the drive  44  and driven  46  gears are new, variations in manufacturing will result in some amount of backlash between the gears. The adjustment member  42  transfers a force  38 , either compressive or tensile, onto the base plate  52  and thereby to the idler gears  44 ,  46 . 
     When a force  38  is applied to the adjustment member  42 , the first idler gear  44  is forced into contact with either the leading or trailing edge of the drive gear  24  (depending on whether the force is tensile or compressive and whether the drive gear is operated in a clockwise or counterclockwise direction). Once contact is made, the first idler gear  44  is rotated slightly to contact the teeth of the second idler gear  46 . This causes the second idler gear  46  to be slightly rotated as well, until it contacts the teeth of the driven gear  26 , which also rotates to contact the drive gear  24 . At this point, all gears are in contact with at least two other gears, and whichever direction the drive gear  24  is rotated the leading edge of at least one tooth will be in contact with either the first idler gear  44  or the driven gear  24 . Providing a larger-than-necessary force  38  to the adjustment member  42  allows the system to self-correct during operation of the gear system  20 . 
     The adjustment member  42  may take one of several forms. For example, and without limitation, the adjustment member  42  may comprise a spring assembly, the spring may be either a tension or compression spring, providing a constant force against the idler base plate  52 . Alternatively, the adjustment member  42  may be a screw, acting against the idler assembly  40  and an external wall. The screw may be accessible without disassembling the gear assembly  20  or positioning assembly  10 , thereby allowing for compensation without excessive downtime. The system may also comprise a piston, magnet, biasing member, applied load, or any other structure commonly known to those in the art which is capable of providing a linear force. 
     The above description is illustrative of one embodiment of the invention. Other options and alternatives may be used. 
     While the gears shown in the attached figures are circular gears, it can be appreciated that the invention would also work with other types of gears, such as a rack and pinion, elliptical gears, incomplete gears, gears having a varying radius, linear gears, and generally any gear combination where backlash is present and sought to be reduced. 
     Additionally, the driven gear  26  has been shown and described as a hollow gear. It should be apparent that the hollow gear is shown according to one preferred embodiment, and the invention is not limited to the driven gear  26  being hollow. A solid gear is within the scope of the invention. 
     Additionally, the idler gears are described as moving along an arc centered at the drive and driven gears. In practicing the invention, it may be found that the amount of movement of the idler gears is minimal, and therefore little benefit is achieved by moving the idler gears in an arc. It may be preferable to allow the idler gears to move along a line, thereby reducing expense in producing the parts while maintaining a similar tolerance for adjustment of the idler gears. 
     Additionally, the system described may be used in series with other such systems. A gear train, either stacked or arranged linearly, may be positioned with an idler gear assembly between any two adjacent gears. This arrangement would reduce error compounded through the gear train. 
     While the device described above has been generally identified as providing positions of infinitely variability, those skilled in the art will appreciate that the limitations of servo motors impose practical limitations on the actual number and location of positions on the table. Additionally, practical mechanical and tolerance restrictions impose additional limitations on the number of actual positions which may be utilized. However, it should be understood to those in the art that the above described gear system is capable of such infinite variability absent these external limitations. 
     The invention has been shown and described above with the preferred embodiments, and it is understood that many modifications, substitutions, and additions may be made which are within the intended spirit and scope of the invention. From the foregoing, it can be seen that the present invention accomplishes at least all of its stated objectives.