This application is based on and claims benefit from Provisional patent application Ser. No. 60/243,108 filed Oct. 24, 2000. 
     The present invention relates to robot arm drive systems, particularly systems for handling wafers in the semiconductor manufacturing industry. 
    
    
     BACKGROUND 
     Current robot arm systems include drive systems for independently controlling the extension of the arm, rotation of the arm, rotation of the grasping tool on the end of the arm and other movements in various different angular directions as are necessary to provide the necessary flexibility to perform numerous different maneuvers on, and manipulations of, items held by the robot hand. Certain robot systems also include telescoping mechanisms to extend the arm or to elevate the arm and related assemblies. Typically the operative end of the arm is equipped with an end effector or mechanical hand for holding or grasping an item such as a silicon wafer, computer hard disc, compact disk or other flat, touch sensitive product. The most common end effector uses one or more vacuum outlets which contact the surface of the wafer. However, more recently edge gripping systems are coming into use. 
     U.S. Pat. No. 4,897,015 to Abbe et al. describes a rotary-to-linear motion robot arm that uses a first motor to control a multi-linkage robot arm to produce straight line radial motion from motor-driven rotary motion. An additional motor may be coupled to the robot arm for operation independent of that of the first motor to angularly move the multi-linkage robot arm without radial motion. Because they independently produce radial motion and angular motion, the first and second motors produce useful robot arm movement when either one of them is operating. The robot arm of the Abbe et al. patent extends and retracts an end effector (or a hand) along a straight line path by means of a mechanism that pivotally couples in a fixed relationship a first arm portion (or aft arm) and a second arm portion (or forearm) so that they move in predetermined directions in response to rotation of the aft arm. To achieve angular displacement of the hand, a theta. drive motor rotates the entire robot arm structure. 
     U.S. Pat. No. 5,007,784 to Genov et al. describes a robot arm with an end effector structure that has two oppositely extending hands, each of which is capable of picking up and transporting a specimen. The end effector structure has a central portion that is centrally pivotally mounted about the distal end of a second link or forearm. 
     The robot arm structures of the Abbe et al. and Genov et al. patents operate similarly, in that each of the end effector structures picks up and transports specimens by using one motor to extend and retract a hand and another, different motor, to rotate the entire robot arm structure, to allow the hand to extend and retract at different ones of a restricted number of angular positions. 
     U.S. Pat. No. 5,741,113 to Bacchi et al is directed to a multiple link robot arm mechanism that uses two motors capable of synchronized operation to permit movement of the robot arm hand along a curvilinear path as the extension of the hand changes. A first motor rotates a forearm about an elbow axis that extends through distal and proximal ends of the aftarm and forearm arm respectively, and a second motor rotates the aftarm about a shoulder axis that extends through a proximal end of the aftarm. A mechanical linkage couples the aftarm and the forearm. The mechanical linkage forms an active drive link and a passive drive link. The active drive link operatively connects the first motor and the forearm to cause the forearm to rotate about the elbow axis in response to the first motor. The passive drive link operatively connects the forearm and the hand to cause the hand to rotate about a wrist axis in response to rotation of the forearm about the elbow axis. The wrist axis extends through distal and proximal ends of the forearm and hand, respectively. 
     Two major problems exist with currently available robots. First, robots with motors below the base plate perform only highly restricted radial, angular moves. Secondly, robots capable of X, Y, Z and Theta movement carry the drive motors above the base mounting flange and at the arm joints, resulting in a unit with a high inertia and sluggish response as well as a tendency to wear and, as a result, generate particles in the vicinity of the arm. 
     SUMMARY OF THE INVENTION 
     The current invention provides an arrangement, which allows completely independent movement of each arm element and also keeps all of the drive motors below the base plate of the system. This provides a much cleaner, streamlined arrangement, which is better balanced, and provides for a much more structurally rigid assembly. The new motor mounting arrangement allows for easy tensioning of drive belts as well as easy and rapid replacement of drive motors or belts if they should happen to malfunction. 
     The design incorporates multiple concentric spindles with a common central axis, all of the spindles being mounted on a single frame. Each spindle is separately driven and each controls a different linkage or arm portion. Because of this concentric assembly the drive assembly is easier to assemble, requires fewer parts and costs less to build, has a lower inertia, wear is reduced, the operation of the robot is quieter and it responds more quickly to signals from the electronic control unit. 
     Another unique aspect of the invention is the mechanical arrangement, which provides a multi-turn rotational hard stop for each of the nested spindles. 
     A still further aspect of the invention is a belt tensioning mechanism in the arm which delivers symmetrical outward forces on the belts eliminating creep of drive belts seen in prior art systems. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cutaway side view of spindle and drive assembly showing a single motor drive. 
     FIG. 2 is an enlarged top view taken along line  2 — 2  of FIG. 1 showing the drive frame with motor mounting cleats (without motor). 
     FIG. 3 is a front view showing a single motor drive set between the frame and a cleat. 
     FIG. 4 a partially cutaway view of the fully assembled system showing the assembly of FIG. 1 in its full-up position, mounted to a table top and enclosed in a half-shell frame. 
     FIG. 5 is an enlarged cutaway view of the spindle assembly with a portion of the arm housing covering the drive pulleys on the top of the spindles. 
     FIG. 6 is a top view of the robot arm with the arm top cover removed, showing the pulley and drive system within the robot arm. 
     FIG. 7 is a side cutaway view of FIG. 6 showing the pulley and drive system within the robot arm. 
     FIG. 8 is a cutaway view of the tab ring sub assembly. 
     FIGS. 9,  10 ,  11  and  12  show four different rotational positions of rings with tab extensions mounted on the shaft of a drive motor and a spindle to facilitate the hard stop arrangement. 
     FIG. 13 is a cutaway view of the fully assembled system of FIG. 4 with the assembly in its full down position. 
     FIG. 14 is a cross sectional view of the half shell assembly taken along line  14 — 14  of FIG.  4 . 
     FIG. 15 shows the rail assembly that is mounted in the half shell frame. 
     FIG. 16 is a front view showing the Z-lift motor with pulley. 
     FIG. 17 is a front view of the nut, pulley and gimble, which cooperates with the screw to raise and lower the Z-body. 
     FIGS. 18 and 19 are side and top perspective views respectively of the gimble of FIG.  17 . 
     FIGS. 20,  21  and  22  are side, top and bottom views respectively of the Z-pulley. 
     FIGS. 23 and 24 are top perspective views, assembled and expanded, respectively, of the forearm. 
     FIGS. 25 and 26 are top perspective views, assembled and expanded, respectively, of the aftarm. 
     FIG. 27 is a top perspective view of the forearm and aftarm with the top covers removed to show the assembled internal components. 
     FIGS. 28 and 29 are expanded perspective side views of the motor gear head and lift motor assembly, respectively. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, the robot drive assembly  10 , comprises a set of concentrically nested spindles  12 ,  14 ,  16  the lower portion of which is mounted in a body (the z-body)  18 . The lower surface of the z-body is a motor mount flange  20  into which the drive motor assemblies  22  are placed and secured. The motor assemblies  22  drive the spindles  12 ,  14 ,  16 , as described below. Referring to FIGS. 6 and 7, a robot arm  24  comprises a lower arm or aftarm  26  and a forearm  28  with movable elbow joint  30  and wrist joint  32 . The robot arm  24  is attached to the top of the outer spindle  12 . The robot drive assembly  10  is placed in the half shell frame assembly  34 , which includes means for raising and lowering the assembly  10  and mounting the assembly  10  to a table or platform  36 . 
     FIG. 1 shows the spindle and drive assembly. The spindles  12 ,  14 ,  16 , comprising hollow tubes, are concentrically placed about a central axis  38  such that each spindle  12 ,  14 ,  16  can be independently rotated around said central axis  38 . A first spindle, designated the shoulder spindle  12 , provides controlled rotary movement to the robot arm  24  at the shoulder end  40  of the lower or aftarm  26 . A second spindle, designated the elbow spindle  14 , provides controlled rotary movement to the robot forearm  38  at the elbow  30  of the robot arm  24 , the elbow  30  being at the junction of the lower arm  26  and the forearm  28 . A third spindle, designated the wrist spindle  16 , provides controlled rotary movement to the wrist joint  32  of the robot arm  24 , the wrist  32  being at the distal end of the forearm  28 . 
     The portion of the nested spindles  12 ,  14 ,  16  near there lower end are surrounded by a z-body  18 , with its motor mount flange  20 . While the spindles are free to rotate around the central axis, they are mechanically constrained by the z-body  18  and bearings  42  so that they are not free to telescope in relationship to each other. Independent rotational motion of the spindles  12 ,  14 ,  16  is made possible by bearing rings (ball bearing or roller bearings)  42  attached at the top and bottom between each of the spindle pairs. Bearings  42  are also present at the top and bottom of the z-body in contact with the outer wall of the shoulder spindle  12 . 
     Attached to the bottom of each spindle  12 ,  14 ,  16  is a pulley for receiving a drive belt, the drive belt transmitting rotational motion to the pulley, and in turn to the spindle, the rotational motion being generated by several individual drive motors  22 , one for each spindle. Also, attached to the top of the elbow and wrist spindle  14 ,  16  are pulleys for transmitting the rotational movement of each spindle to the appropriate portion of the robot arm  24 . More specifically, mounted on the bottom of the shoulder spindle  12  is a lower shoulder pulley  44 , mounted on the bottom of the elbow spindle  14  is a lower elbow pulley  48  and mounted on the bottom of the wrist spindle  16  is a lower wrist pulley  46 . Likewise, mounted on the top of the of the elbow spindle  14  is an upper elbow pulley  52  and mounted on the top of the wrist spindle  16  is an upper wrist pulley  50 . 
     FIG. 2 is a top view of the z body motor mount  20  taken along line  2 — 2  of FIG.  1 . Motor mount flange is substantially flat with four frame cutouts or openings  54  for receiving and retaining the three drive motors  22  and the z travel motor  182 . In particular, the unique detail  60  of the mating surfaces in each of four cutouts  54  in the z-body  18 , and the motor assembly mounting annular ring or mounting extension  56  on each motor assembly  22 ,  182  holds the motor assembly&#39;s central axis parallel to the spindle axis  38  regardless of the belt tension, even with the mounting cleat  68  loose. This feature permits belt tensioning with the cleat  68  loose. The belt tension setting remains at the same value when the cleat is locked into place. As shown in FIG. 2, FIG.  3  and FIG. 16, the motor assembly mounting annular ring, also referred to as positioning collar  56 , has two features, a quarter toroidal top, and a flat surface perpendicular to the motor axis on the bottom which rest in mounting grooves  60  in the vertical wall surface of the non-circular openings  54 . The cleat  68  is secured against the positioning collar  56 , locking the drive motors  22 ,  182  in desired positions. The arrangement of the motor shaft  62  off center in the positioning collar  56  provides the operator the opportunity to loosen the cleat  68  and rotate the positioning collar in the non-circular opening  54 , thus moving the motor shaft, and the pulley attached thereto, so that the belt connecting the motor pulley to the spindle pulley or Z-drive can be readily tensioned. Resecuring the cleat  68  in position then locks the motor with the shaft in the new desired location to properly tension the drive belt on the respective pulley. A critical feature of the geometry described above is that lessening the cleat  68  to tension a belt by rotating the motor assembly maintains the axis of the motor assembly. Therefore the motor axis stays parallel to the spindle axis allowing the belt to run true on its pulleys and the reset tension in the belt is not altered by subsequently tightening the cleat  68 . This feature is further facilitated by the shape of the cleat  68  and the way it is attached to the z-body. The cleat  68  is restrained by one of its mounting surfaces and a tab on the opposite end of the cleat  68  that is in intimate contact with the z-body. With the bolts loosened on the cleat  68 , it is not permitted to move except to rotate about a pin parallel to the motor axis. 
     With reference to FIGS. 1,  3  and  28  it can be seen that three identical drive motor assemblies  22  with their mounting collars  56  can be provided for constructing the assembly. It should be noted that FIG. 2 shows only one drive motor installed. However, one will recognize that three drive motors  22  are required to drive the three spindles. In addition, a Z-drive motor assembly  182  is shown. This is possible because the positioning collar  56 , in the embodiment shown, is repeated three times on the motor assembly located so that use of the proper mounting ring  56  positions the motor pulley in line with the appropriate spindle pulley  44 ,  46 ,  48 , FIG. 1 shows the drive motor assembly  22  mounted using the lower mounting extension  56  so the motor pulley  66  is aligned with the shoulder pulley. FIG. 1 also shows a drive belt  80  connecting the motor pulley  66  and lower shoulder pulley  44 . FIG. 3 shows the middle mounting extension  56  held between the motor mounting grooves  60  and the cleat  68 , which is the arrangement for driving the wrist spindle  16 . If the upper mounting extension  56  were held between the mounting frame  20  and the cleat  68  then the drive motor  22  would be aligned for driving the elbow spindle  14 . 
     The preferred drive motor  23  incorporates a harmonic drive gear reduction to provide the desired rotational speed. More specifically a preferred drive motor  23  is a servomotor with an absolute digital encoder attached to the motor shaft  62 , the motor being available from numerous different suppliers. These drive motors  23  are in turn connected to motor controllers operating by a computer based control system. 
     FIGS. 14 and 15 show the base frame assembly  34  consisting of a flange  78  and the half shell  70  brazed into a single ridged assembly  34 . Two bearing rails  72  are attached rigidly to the base frame assembly  34 . Riding vertically on the bearing rails  72  are three carriages, or rail hangers  74  one on the first rail and two on the second rail. These three carriages  74  are attached rigidly to the robot drive assembly  10  and facilitates the robot drive assemblies&#39; vertical movement. 
     The Z lift motor assembly  182  shown in FIGS. 16 and 29 drives the robot drive assembly  10  in the vertical direction. Referring to FIGS. 17-22, the Z-drive motor  184 , through pulley  66  and an attached drive belt (not shown), causes a nut  186  that is mounted on a fixed vertical screw  188  to rotate and in turn cause the robot drive assembly  10  to raise vertically along the rail  72 . The z motion is further facilitated by a fixed gimbal (not shown) attached to the top of the vertical lead screw  188 , and a lower gimbal  192  shown in perspective and cutaway view in FIGS. 17,  18  and  19  between the z drive pully  194 , shown in three views in FIGS. 20-22, and the nut  186 . The bottom end of the lead screw  188  is unsupported. This unique configuration permits smooth and non-binding operation of the vertical lift with misalignment of the vertical rails  72  and the lead screw  188 . This non-critical alignment feature permits loose tolerances in a host of features in various parts within the robot, and eliminates the need for precise alignment of the lead screw during assembly of the robot, 
     Also shown in FIGS. 14 and 15 is a top flange  78  used to mount the whole assembly to a table top or platform. 
     The robot arm  24 , as shown in FIGS. 5-7 and  23 - 27 , is attached to the top of the shoulder spindle  12  by bolts placed through clamping ring  82  into the arm base on the shoulder spindle  12 . This clamping configuration allows total freedom in orienting the robot arm  24  in any orientation relative to the spindle  12 . When the screws are tightened, the arm  24  holds rigidly to the outer (shoulder) spindle  12 . When clamped, rotation of the shoulder spindle  12  causes the robot arm to also rotate in a plane perpendicular to the central axis  38  of the spindles. The shoulder end of the robot arm  24  serves as a protective cover over the upper wrist and upper elbow pulleys  50 ,  52  and drive belts  92 ,  96  attached thereto. The upper elbow pulley  52  is shown at the left end of FIGS. 6 and 26. However, the upper wrist pulley  50 , which is directly below the upper elbow pulley  52 , and of substantially the same outer diameter, can not be seen in FIG.  6 . The upper elbow/wrist pulley  84  is shown in FIG.  6  and both the upper elbow/wrist pulley  84 , the lower elbow/wrist pulley  86  and the wrist pulley  90  are shown in FIGS. 7 and 24. The upper elbow/wrist pulley  84  and the lower elbow/wrist pulley  86  are operatively connected so that rotation of one of said pulleys causes the other pulley to rotate a similar or proportionate amount. The assembled robot arm  24  with top  190  removed is best shown in FIG.  27 . 
     The wrist joint pulley  90  appears on the right hand side of both FIGS. 6 and 7 and in FIG. 24. A first wrist drive belt  96  connects the upper wrist pulley  50  to the lower elbow/wrist pulley  86  and a second wrist drive belt  94  connects the upper elbow/wrist pulley  84  to the wrist joint pulley  90 . As a result, rotation of the wrist spindle  16  by drive motor  22  connected thereto is transmitted through the first wrist belt  96  and second wrist belt  94  causing an operating tool (not shown), such as an end effector, attached to wrist joint pulley  90  to also be swiveled in a controlled manner and in a plane perpendicular to the central axis  38  of the spindles. 
     As shown in FIGS. 5,  7  and  26  an elbow belt  92  is mounted on both the upper elbow pulley  52  and the elbow joint pulley  88 . In a like manner, rotation of the elbow spindle  14  by drive motor  22  connected thereto is transmitted through the elbow belt  92  causing the forearm  28  to be swiveled in a controlled manner in a plane perpendicular to the central axis  38  of the spindles. 
     Prior art devices exhibit long term drift problems caused by belt creep. Their belts are typically tensioned by a single roller applied to an outer surface of the belt at a location midway between drive pulleys, pushing the belt toward the center line between the drive pulleys. This places uneven forces on the belt causing the belts to stretch unevenly, resulting in unwanted rotation of the effected part. In contrast, the present system uses belt tensioner assemblies  98 , comprising a tensioner frame  100  with a pair of rollers  102  mounted at the ends of the tensioner frame. The tensioner frames  100  are pivotally mounted on the center line between the respective pulleys with both rollers  102  contacting opposite inner surfaces of the belt. Rotating the belt tensioner  98  around its central mount causes the rollers  102  to tension the belt in an outward direction, providing less stress on the belt structure and evenly balanced belt strain yielding stable arm positioning. FIGS. 6,  24 ,  26  and  27  show three belt tensioner assemblies  98 , a first belt tensioner tightening assembly  98  tightening the first wrist belt  96 , a second belt tensioner assembly  98  tightening the second wrist belt  94  and a third belt tensioner tightening the forearm drive belt  92 . 
     Prior art devices that limit the rotational travel of the arms typically are limited to rotations less than 360 degrees. This restriction leaves a spot in the robot&#39;s arm travel that can not be reached (a blind spot). Described below is a method used in the present invention to create hard stops that allow more than one revolution of each element of the robot arm. FIGS. 8-12 show a tab ring pully subassembly  104  that mounts to the drive motor shaft and serves to receive a drive belt. As the shaft in the motor rotates the motor tab  106  travels a circular path. There is a similar tab mounted on the lower pulley on each of the spindles (the spindle tab  108 ). The motor tab  106  extends from a tab ring  105  bolted to the pulley  44 ,  46 ,  48  after it is rotated to a desired position on the pully. A tapered compress ring  107  is placed inside the tab ring. Tightening the mounting bolt causes a locking fit to the tab ring  105 . This combination of a motor tab  106  and a spindle tab  108  on each paired drive and spindle cooperate to establish a hard stop on the rotation of that particular spindle to prevent the robot aft arm, forearm, or wrist from inadvertently rotating too far and damaging internal plumbing and/or cabling inside the robot. As typically configured, because of gear ratios (or different pulley sizes) on the motor shaft and spindle the motor shaft will rotate three times while a spindle will rotate twice. The tab ring assembly  104  allows the motor tab  106  to be readily adjusted and then locked into place. This adjustment facilitates even contact between tabs. 
     The movement of the tabs is explained by reference to FIGS. 9-12 which show a 3/2 motor pulley to spindle pulley ratio. While this arrangement applies to each motor assembly  22 , for illustration purposes the motor drive assembly  22 , shown in FIG. 1, driving the shoulder spindle  12  is shown. 
     FIG. 9 shows the tabs  106 ,  108  in contact at one extreme of the rotation of a spindle. In FIG. 10 the motor shaft has made one full rotation while the spindle has rotated 240°. FIG. 11 shows the motor tab after  2  full rotations and a spindle rotation of 480° (1⅓ turns). FIG. 12 shows the tabs in a hard stop position after the motor rotates almost 3 turns and the spindle rotates almost  2  full turns. By changing the gear (pulley) ratio and/or adding additional tabs  106 ,  108  to the motor and/or spindle and/or changing the tab widths any degree of rotation between hard stops can be selected. This arrangement facilitates the ability to control the degree of rotation of the components of the robot arm. For example, referring to FIG. 10, a second tab added to the spindle 120° ahead of the first tab would restrict the spindle to 240° of rotation for one turn of the motor. 
     While the above assembly can be constructed from various different materials, one skilled in the art can readily choose these materials for the particular ambient environment in which the robot would operate and the various stresses applied to each piece by the design and operation of the assembly. Preferred material of construction is typically aluminum. However, various different materials may be used for operation under various different atmospheric conditions. However, it may be desirable to plate on a protective metal layer or to construct some or all of the components from different materials such as stainless steel for operation in an acid environment, in a vacuum, under extreme temperatures, or in a wet or otherwise hostile environment. 
     Also, while the above description refers to pulleys and belts, it is contemplated that various different drive train assemblies can be used. For example, the invention contemplates the use of intermeshing gears and gears and drive chains in addition to toothed pulleys or timing belts, as described above. 
     One skilled in the art will also recognize that the Figures include numerous unlabeled components which are generally used in assembling mechanical devices including frames, mounting brackets, nuts, bolts, washers, spacing, locking or retaining rings, adapters, shims, clamps, bearings, housings, plates, etc. These components are not labeled so as not to unnecessary clutter the drawings with labeling of standard assembly components. 
     It is evident from the foregoing that there are many additional embodiments of the present invention which, while not expressly described herein, are within the scope of this invention and may suggest themselves to one of ordinary skill in the art. It is therefore intended that the invention be limited solely by the appended claims.
A robot drive assembly for moving a working tool in x, y, z and theta directions comprising three independent, coaxially nested tubes, each tube being driven around a common central axis by drive belts attached to separate drive motors located in a mounting flange associated with the outermost tube. The motors, and the tubes which they drive, provide horizontal rotary motion to a robot arm attached to the upper end of the outer tube and the wrist and elbow of that arm. A fourth motor controls vertical motion of the whole assembly. The robot system also includes motor position adjustment structure and belt tension structure designed for ease of use and to eliminate movement of tensioned components once locked in position.