Patent Publication Number: US-11640919-B2

Title: Robot having arm with unequal link lengths

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional patent application of application Ser. No. 15/846,401 filed Dec. 19, 2017, which is a divisional application of U.S. application Ser. No. 15/017,970 filed Feb. 8, 2016, now U.S. Pat. No. 10,224,232, which claims priority under 35 USC 119(e) to U.S. provisional patent application No. 62/112,820 filed Feb. 6, 2015, and is a continuation-in-part application of U.S. patent application Ser. No. 14/827,506 filed Aug. 17, 2015, now U.S. Pat. No. 9,840,004, which is a continuation of U.S. patent application Ser. No. 13/833,732 filed Mar. 15, 2013, now U.S. Pat. No. 9,149,936, which claims priority under 35 USC 119(e) on U.S. Provisional Patent Application No. 61/754,125 filed Jan. 18, 2013 and U.S. Provisional Patent Application No. 61/762,063 filed Feb. 7, 2013 which are hereby incorporated by reference in their entireties. 
    
    
     BACKGROUND 
     Technical Field 
     The disclosed embodiment relates to a robot having an arm with unequal link lengths and more particularly to a robot having one or more arms with unequal link lengths, each supporting one or more substrates. 
     Brief Description of Prior Developments 
     Vacuum, atmospheric and controlled environment processing for applications such as associated with manufacturing of semiconductor, LED, Solar, MEMS or other devices utilize robotics and other forms of automation to transport substrates and carriers associated with substrates to and from storage locations, processing locations or other locations. Such transport of substrates may be moving individual substrates, groups of substrates with single arms transporting one or more substrates or with multiple arms, each transporting one or more substrate. Much of the manufacturing, for example, as associated with semiconductor manufacturing is done in a clean or vacuum environment where footprint and volume are at a premium. Further, much of the automated transport is conducted where minimization of transport times results in reduction of cycle time and increased throughput and utilization of the associated equipment. Accordingly, there is a desire to provide substrate transport automation that requires minimum footprint and workspace volume for a given range of transport applications with minimized transport times. 
     SUMMARY 
     The following summary is merely intended to be exemplary. The summary is not intended to limit the claims. 
     In accordance with one aspect of the exemplary embodiment, a transport apparatus has at least one drive; a first robot arm having a first upper arm, a first forearm and a first end effector. The first upper arm is connected to the at least one drive at a first axis of rotation. A second robot arm has a second upper arm, a second forearm and a second end effector. The second upper arm is connected to the at least one drive at a second axis of rotation which is spaced from the first axis of rotation. The first and second robot arms are configured to locate the end effectors in first retracted positions for stacking substrates located on the end effectors at least partially one above the another. The first and second robot arms are configured to extend the end effectors from the first retracted positions in a first direction along parallel first paths located at least partially directly one above the other. The first and second robot arms are configured to extend the end effectors in at least one second direction along second paths spaced from one another which are not located above one another. The first upper arm and the first forearm have different effective lengths. The second upper arm and the second forearm have different effective lengths. 
     In accordance with another aspect of the exemplary embodiment, a method is provided comprising providing a first robot arm comprising a first upper arm, a first forearm and a first end effector, where the first upper arm and the first forearm have different effective lengths; providing a second robot arm comprising a second upper arm, a second forearm and a second end effector, where the second upper arm and the second forearm have different effective lengths; connecting the first upper arm to at least one drive at a first axis of rotation; and connecting the second upper arm to the at least one drive at a second axis of rotation which is spaced from the first axis of rotation, where the first and second robot arms are configured to locate the end effectors in first retracted positions for stacking substrates located on the end effectors at least partially one above the another, where the first and second robot arms are configured to extend the end effectors from the first retracted positions in a first direction along parallel first paths at least partially located directly one above the other, and where the first and second robot arms are configured to extend the end effectors in at least one second direction along second paths spaced from one another which are not located above one another. 
     In accordance with another aspect of the exemplary embodiment, a method is provided comprising locating a first end effector and a second end effector of first and second respective robot arms at first retracted positions for stacking substrates located on the end effectors at least partially one above the another, where the first robot arm comprising a first upper arm, a first forearm and the first end effector, where the first upper arm is connected to at least one drive at a first axis of rotation, and where the second robot arm comprises a second upper arm, a second forearm and the second end effector, where the second upper arm is connected to the at least one drive at a second axis of rotation which is spaced from the first axis of rotation; moving the first and second robot arms to move the end effectors from the first retracted positions in a first direction along parallel first paths located at least partially directly one above the other; and moving the first and second robot arms to move the end effectors to extend the end effectors in at least one second direction along second paths spaced from one another which are not located above one another. 
     In accordance with another aspect of the exemplary embodiment, a transport apparatus has a first robot arm comprising a first upper arm, a first forearm and a first end effector; a second robot arm comprising a second upper arm, a second forearm and a second end effector; and a drive connected to the first and second robot arms, where the first upper arm is connected to the drive at a first axis of rotation, where the second upper arm is connected to the drive at a second axis of rotation which is spaced from the first axis of rotation, where the drive comprises only three motors for rotating first and second upper arms, where the first and second robot arms are configured to locate the end effectors in first retracted positions for stacking substrates located on the end effectors at least partially one above the another, where the first and second robot arms are configured to extend the end effectors from the first retracted positions in a first direction along parallel first paths located at least partially directly one above the other, and where the first and second robot arms are configured to extend the end effectors in at least one second direction along second paths spaced from one another which are not located above one another. 
     In accordance with another aspect of the exemplary embodiment, a method comprises locating a first end effector and a second end effector of first and second respective robot arms at first retracted positions for stacking substrates located on the end effectors at least partially one above the another, where the first robot arm comprising a first upper arm, a first forearm and the first end effector, where the first upper arm is connected to a drive at a first axis of rotation, and where the second robot arm comprises a second upper arm, a second forearm and the second end effector, where the second upper arm is connected to the drive at a second axis of rotation which is spaced from the first axis of rotation; moving the first and second robot arms to move the end effectors from the first retracted positions in a first direction along parallel first paths located at least partially directly one above the other; moving the first and second robot arms to move the end effectors to extend the end effectors in at least one second direction along second paths spaced from one another which are not located above one another; rotating the first and second robot arms together about a third axis of rotation which is spaced from the first and second axes of rotation, where the moving from the first retracted positions in the first direction, the moving to extend the end effectors in the at least one second direction, and the rotating is with use of only three motors of the drive. 
     In accordance with another aspect of the exemplary embodiment, a method comprises providing a first robot arm comprising a first upper arm, a first forearm and a first end effector; providing a second robot arm comprising a second upper arm, a second forearm and a second end effector; connecting the first upper arm to a drive at a first axis of rotation; and connecting the second upper arm to the drive at a second axis of rotation which is spaced from the first axis of rotation, where the first and second robot arms are configured to locate the end effectors in first retracted positions for stacking substrates located on the end effectors at least partially one above the another, where the first and second robot arms are configured to be rotated to extend the end effectors from the first retracted positions in a first direction along parallel first paths at least partially located directly one above the other, and where the first and second robot arms are configured to be rotated to extend the end effectors in at least one second direction along second paths spaced from one another which are not located above one another, where the drive comprises only three motors for rotating the first and second robot arms to extend the end effectors and for rotating the first and second robot arms about a third axis of rotation spaced from the first and second axes of rotation. 
     In accordance with another aspect of the exemplary embodiment, an apparatus comprises a first robot arm comprising a first upper arm, a first forearm and a first end effector; a second robot arm comprising a second upper arm, a second forearm and a second end effector; and a drive connected to the first and second robot arms, where the first upper arm is connected to the drive at a first axis of rotation, where the second upper arm is connected to the drive at a second axis of rotation which is spaced from the first axis of rotation, where the drive comprises five motors for rotating first and second upper arms, where a first one of the motors is connected to the first and second robot arms to rotate the first and second arms about a third axis of rotation spaced from the first and second axes of rotation, where second and third ones of the motors are connected to the first robot arm to rotate the first upper arm and the first forearm respectively, and where fourth and fifth ones of the motors are connected to the second robot arm to rotate the second upper arm and the second forearm, respectively, independently from the first robot arm, where the first and second robot arms are configured to locate the end effectors in first retracted positions for stacking substrates located on the end effectors at least partially one above the another, where the first and second robot arms are configured to extend the end effectors from the first retracted positions in a first direction along parallel first paths located at least partially directly one above the other, and where the first and second robot arms are configured to extend the end effectors in at least one second direction along second paths spaced from one another which are not located above one another. 
     In accordance with another aspect of the exemplary embodiment, a method comprises locating a first end effector and a second end effector of first and second respective robot arms at first retracted positions for stacking substrates located on the end effectors at least partially one above the another, where the first robot arm comprising a first upper arm, a first forearm and the first end effector, where the first upper arm is connected to a drive at a first axis of rotation, and where the second robot arm comprises a second upper arm, a second forearm and the second end effector, where the second upper arm is connected to the drive at a second axis of rotation which is spaced from the first axis of rotation; moving the first and second robot arms to move the end effectors from the first retracted positions in a first direction along parallel first paths located at least partially directly one above the other; moving the first and second robot arms to move the end effectors to extend the end effectors in at least one second direction along second paths spaced from one another which are not located above one another; rotating the first and second robot arms together about a third axis of rotation which is spaced from the first and second axes of rotation, where the moving from the first retracted positions in the first direction, the moving to extend the end effectors in the at least one second direction, and the rotating is with use of five motors of the drive, where a first one of the motors is connected to the first and second robot arms to rotate the first and second arms about the third axis of rotation, where second and third ones of the motors are connected to the first robot arm to rotate the first upper arm and the first forearm respectively, and where fourth and fifth ones of the robot arms are connected to the second robot arm to rotate the second upper arm and the second forearm respectively independently from the first robot arm. 
     In accordance with another aspect of the exemplary embodiment, a method comprises providing a first robot arm comprising a first upper arm, a first forearm and a first end effector; providing a second robot arm comprising a second upper arm, a second forearm and a second end effector; connecting the first upper arm to a drive at a first axis of rotation; and connecting the second upper arm to the drive at a second axis of rotation which is spaced from the first axis of rotation, where the first and second robot arms are configured to locate the end effectors in first retracted positions for stacking substrates located on the end effectors at least partially one above the another, where the first and second robot arms are configured to be rotated to extend the end effectors from the first retracted positions in a first direction along parallel first paths at least partially located directly one above the other, and where the first and second robot arms are configured to be rotated to extend the end effectors in at least one second direction along second paths spaced from one another which are not located above one another, where the drive comprises five motors for rotating the first and second robot arms to extend the end effectors and for rotating the first and second robot arms about a third axis of rotation spaced from the first and second axes of rotation, where a first one of the motors is connected to the first and second robot arms to rotate the first and second arms about the third axis of rotation, where second and third ones of the motors are connected to the first robot arm to rotate the first upper arm and the first forearm respectively, and where fourth and fifth ones of the robot arms are connected to the second robot arm to rotate the second upper arm and the second forearm respectively independently from the first robot arm. 
     In accordance with another aspect of the exemplary embodiment, an apparatus comprises a first robot arm comprising a first upper arm, a first forearm and a first end effector; a second robot arm comprising a second upper arm, a second forearm and a second end effector; and a drive connected to the first and second robot arms, where the first upper arm is connected to the drive at a first axis of rotation, where the second upper arm is connected to the drive at a second axis of rotation which is spaced from the first axis of rotation, where the drive comprises four motors for rotating first and second upper arms, where a first one of the motors is connected to the first upper arm, where a second one of the motors is connected to the second upper arm, where a third one of the motors is connected to the first forearm, where a fourth one of the motors is connected to the second forearm, where the third and fourth motors are aligned in a common axis spaced from the first and second axis, where the first motor is aligned in the first axis and where the second motor is aligned in the second axis, where the first and second robot arms are configured to locate the end effectors in first retracted positions for stacking substrates located on the end effectors at least partially one above the another, where the first and second robot arms are configured to extend the end effectors from the first retracted positions in a first direction along parallel first paths located at least partially directly one above the other, and where the first and second robot arms are configured to extend the end effectors in at least one second direction along second paths spaced from one another which are not located above one another. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and other features are explained in the following description, taken in connection with the accompanying drawings, wherein: 
         FIG.  1 A  is a top view of a transport apparatus; 
         FIG.  1 B  is a side view of a transport apparatus; 
         FIG.  2 A  is a top partial schematic view of a transport apparatus; 
         FIG.  2 B  is a side section partial schematic view of a transport apparatus; 
         FIG.  3 A  is a top view of a transport apparatus; 
         FIG.  3 B  is a top view of a transport apparatus; 
         FIG.  3 C  is a top view of a transport apparatus; 
         FIG.  4    is a graphical plot; 
         FIG.  5 A  is a top view of a transport apparatus; 
         FIG.  5 B  is a side view of a transport apparatus; 
         FIG.  6 A  is a top partial schematic view of a transport apparatus; 
         FIG.  6 B  is a side section partial schematic view of a transport apparatus; 
         FIG.  7 A  is a top view of a transport apparatus; 
         FIG.  7 B  is a top view of a transport apparatus; 
         FIG.  7 C  is a top view of a transport apparatus; 
         FIG.  8    is a graphical plot; 
         FIG.  9    is a side section partial schematic view of a transport apparatus; 
         FIG.  10 A  is a top view of a transport apparatus; 
         FIG.  10 B  is a side view of a transport apparatus; 
         FIG.  11 A  is a top view of a transport apparatus; 
         FIG.  11 B  is a side view of a transport apparatus; 
         FIG.  12    is a side section partial schematic view of a transport apparatus; 
         FIG.  13    is a side section partial schematic view of a transport apparatus; 
         FIG.  14 A  is a top view of a transport apparatus; 
         FIG.  14 B  is a top view of a transport apparatus; 
         FIG.  14 C  is a top view of a transport apparatus; 
         FIG.  15 A  is a top view of a transport apparatus; 
         FIG.  15 B  is a side view of a transport apparatus; 
         FIG.  16 A  is a top view of a transport apparatus; 
         FIG.  16 B  is a side view of a transport apparatus; 
         FIG.  17 A  is a top view of a transport apparatus; 
         FIG.  17 B  is a side view of a transport apparatus; 
         FIG.  18    is a side section partial schematic view of a transport apparatus; 
         FIG.  19    is a side section partial schematic view of a transport apparatus; 
         FIG.  20 A  is a top view of a transport apparatus; 
         FIG.  20 B  is a top view of a transport apparatus; 
         FIG.  20 C  is a top view of a transport apparatus; 
         FIG.  21 A  is a top view of a transport apparatus; 
         FIG.  21 B  is a side view of a transport apparatus; 
         FIG.  22 A  is a top view of a transport apparatus; 
         FIG.  22 B  is a side view of a transport apparatus; 
         FIG.  23    is a side section partial schematic view of a transport apparatus; 
         FIG.  24 A  is a top view of a transport apparatus; 
         FIG.  24 B  is a top view of a transport apparatus; 
         FIG.  24 C  is a top view of a transport apparatus; 
         FIG.  25 A  is a top view of a transport apparatus; 
         FIG.  25 B  is a side view of a transport apparatus; 
         FIG.  26 A  is a top view of a transport apparatus; 
         FIG.  26 B  is a top view of a transport apparatus; 
         FIG.  26 C  is a top view of a transport apparatus; 
         FIG.  27 A  is a top view of a transport apparatus; 
         FIG.  27 B  is a side view of a transport apparatus; 
         FIG.  28 A  is a top view of a transport apparatus; 
         FIG.  28 B  is a side view of a transport apparatus; 
         FIG.  29 A  is a top view of a transport apparatus; 
         FIG.  29 B  is a top view of a transport apparatus; 
         FIG.  29 C  is a top view of a transport apparatus; 
         FIG.  30 A  is a top view of a transport apparatus; 
         FIG.  30 B  is a side view of a transport apparatus; 
         FIG.  31 A  is a top view of a transport apparatus; 
         FIG.  31 B  is a side view of a transport apparatus; 
         FIG.  32 A  is a top view of a transport apparatus; 
         FIG.  32 B  is a top view of a transport apparatus; 
         FIG.  32 C  is a top view of a transport apparatus; 
         FIG.  32 D  is a top view of a transport apparatus; 
         FIG.  33 A  is a top view of a transport apparatus; 
         FIG.  33 B  is a side view of a transport apparatus; 
         FIG.  34 A  is a top view of a transport apparatus; 
         FIG.  34 B  is a top view of a transport apparatus; 
         FIG.  34 C  is a top view of a transport apparatus; 
         FIG.  35 A  is a top view of a transport apparatus; 
         FIG.  35 B  is a side view of a transport apparatus; 
         FIG.  36    is a top view of a transport apparatus; 
         FIG.  37 A  is a top view of a transport apparatus; 
         FIG.  37 B  is a side view of a transport apparatus; 
         FIG.  38 A  is a top view of a transport apparatus; 
         FIG.  38 B  is a side view of a transport apparatus; 
         FIG.  39    is a top view of a transport apparatus; 
         FIG.  40 A  is a top view of a transport apparatus; 
         FIG.  40 B  is a side view of a transport apparatus; 
         FIG.  41    is a top view of a transport apparatus; 
         FIG.  42    is a top view of a transport apparatus; 
         FIG.  43 A  is a top view of a transport apparatus; 
         FIG.  43 B  is a side view of a transport apparatus; 
         FIG.  44    is a top view of a transport apparatus; 
         FIG.  45    is a top view of a transport apparatus; 
         FIG.  46 A  is a top view of a transport apparatus; 
         FIG.  46 B  is a side view of a transport apparatus; 
         FIG.  47 A  is a top view of a transport apparatus; 
         FIG.  47 B  is a side view of a transport apparatus; 
         FIG.  48    is a top view of a transport apparatus; 
         FIG.  49    is a top view of a transport apparatus; 
         FIG.  50 A  is a top view of a transport apparatus; 
         FIG.  50 B  is a side view of a transport apparatus; 
         FIG.  51    is a top view of a transport apparatus; 
         FIG.  52 A  is a top view of a transport apparatus; 
         FIG.  52 B  is a side view of a transport apparatus; 
         FIG.  53    is a top view of a transport apparatus; 
         FIG.  54 A  is a top view of a transport apparatus; 
         FIG.  54 B  is a side view of a transport apparatus; 
         FIG.  55 A  is a top view of a transport apparatus; 
         FIG.  55 B  is a top view of a transport apparatus; 
         FIG.  55 C  is a top view of a transport apparatus; 
         FIG.  56 A  is a top view of a transport apparatus; 
         FIG.  56 B  is a side view of a transport apparatus; 
         FIG.  57 A  is a top view of a transport apparatus; 
         FIG.  57 B  is a top view of a transport apparatus; 
         FIG.  57 C  is a top view of a transport apparatus; 
         FIG.  58 A  is a top view of a transport apparatus; 
         FIG.  58 B  is a side view of a transport apparatus; 
         FIG.  59 A  is a top view of a transport apparatus; 
         FIG.  59 B  is a top view of a transport apparatus; 
         FIG.  59 C  is a top view of a transport apparatus; 
         FIG.  60 A  is a top view of a transport apparatus; 
         FIG.  60 B  is a side view of a transport apparatus; 
         FIG.  61 A  is a top view of a transport apparatus; 
         FIG.  61 B  is a top view of a transport apparatus; 
         FIG.  61 C  is a top view of a transport apparatus; 
         FIG.  62    is a top view of a transport apparatus; 
         FIG.  63    is a diagram illustrating exemplary pulleys; 
         FIG.  64    is a top view of a transport apparatus; 
         FIG.  65    is a top view of a transport apparatus; 
         FIG.  66 A  is a top view of a transport apparatus; 
         FIG.  66 B  is a isometric view of a transport apparatus; 
         FIG.  66 C  is an end view of a transport apparatus; 
         FIG.  66 D  is a side view of a transport apparatus; 
         FIG.  67 A  is a top view of a transport apparatus; 
         FIG.  67 B  is a isometric view of a transport apparatus; 
         FIG.  67 C  is an end view of a transport apparatus; 
         FIG.  67 D  is a side view of a transport apparatus; 
         FIG.  68 A  is a top view of a transport apparatus; 
         FIG.  68 B  is a top view of a transport apparatus; 
         FIG.  69    A-F are top views of a transport apparatus; 
         FIG.  70    A-F are top views of a transport apparatus; 
         FIG.  71    A-E are top views of a transport apparatus; 
         FIG.  72    A-B are top and side views of a transport apparatus; 
         FIG.  72    C-D are top and side views of a transport apparatus; 
         FIG.  73    A-B are top and side views of a transport apparatus; 
         FIG.  73    C-D are top and side views of a transport apparatus; 
         FIG.  74 A  is a top view of a transport apparatus; 
         FIG.  74 B  is a top view of a transport apparatus; 
         FIG.  75    A-F are top views of a transport apparatus; 
         FIG.  76 A  is a top view of a transport apparatus; 
         FIG.  76 B  is a top view of a transport apparatus; 
         FIG.  76 C  is a top view of a transport apparatus; 
         FIG.  76 D  is a top view of a transport apparatus; 
         FIG.  77    A-B are top and side views of a transport apparatus; 
         FIG.  77    C-D are top and side views of a transport apparatus; 
         FIG.  78    A-B are top and side views of a transport apparatus; 
         FIG.  79 A  is a top view of a transport apparatus; 
         FIG.  79 B  is a top view of a transport apparatus; 
         FIG.  80 A  is a top view of a transport apparatus; and 
         FIG.  80 B  is a top view of a transport apparatus. 
     
    
    
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENT 
     Aside from the embodiment disclosed below, the disclosed embodiment is capable of other embodiments and of being practiced or being carried out in various ways. Thus, it is to be understood that the disclosed embodiment is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. If only one embodiment is described herein, the claims hereof are not to be limited to that embodiment. Moreover, the claims hereof are not to be read restrictively unless there is clear and convincing evidence manifesting a certain exclusion, restriction, or disclaimer. 
     Referring now to  FIGS.  1 A and  1 B , there is shown top and side views respectively of robot  10  having drive  12  and arm  14 . Arm  14  is shown in a retracted position. Arm  14  has upper arm or first link  16  rotateable about a central axis of rotation  18  of drive  12 . Arm  14  further has forearm or second link  20  rotatable about an elbow axis of rotation  22 . Arm  14  further has end effector or third link  24  rotatable about a wrist axis of rotation  26 . End effector  24  supports substrate  28 . As will be described, arm  14  is configured to cooperate with drive  12  such that substrate  28  is transported along a radial path  30  that may coincide with (as seen in  FIG.  1 A ) or a path, for example, path  34 ,  36  or otherwise parallel to a linear path  32  that coincides with the central axis of rotation  18  of drive  12 . In the embodiment shown, the joint-to-joint length of forearm or second link  20  is larger than the joint-to-joint length of the upper arm or first link  16 . In the embodiment shown, the lateral offset  38  of the end-effector or third link  24  corresponds to the difference of the joint-to-joint lengths of the forearm  20  and upper arm  14 . As will be described in greater detail below, the lateral offset  38  is maintained substantially constant during extension and retraction of arm  14  such that substrate  28  is moved along a linear path without rotation of substrate  28  or end effector  24  with respect to the linear path. This is accomplished with structure internal to arm  14  as will be described without the use of an additional controlled axis to control rotation of end effector  24  at wrist  26  with respect to forearm  20 . In one aspect of the disclosed embodiment, with respect to  FIG.  1 A , the center of mass of the third link or end effector  24  may reside at the wrist centerline or axis of rotation  26 . Alternately, the center of mass of the third link or end effector  24  may reside along path  40  offset  38  from the central axis of rotation  18 . In this manner, the disturbance to the bands that constrain end effector  24  with respect to links  16 ,  18  may be minimized due to a moment applied as a result of the mass being offset otherwise during extension and retraction of the arm. Here, the center of mass may be determined with or without the substrate or may be in between. Alternately, the center of mass of the third link or end effector  24  may reside at any suitable location. In the embodiment shown, substrate transport apparatus  10  transports substrate  28  with moveable arm assembly  14  coupled to drive section  12  on central axis of rotation  18 . Substrate support  24  is coupled to the arm assembly  14  on wrist axis of rotation  26  where arm assembly  14  rotates about central axis of rotation  18  during extension and retraction as will be seen with respect to  FIGS.  3 A-C . Wrist axis of rotation  26  moves along wrist path  40  parallel to and offset  38  or otherwise from radial path, for example, path  30 ,  34  or  36  relative to the central axis of rotation  18  during extension and retraction. Substrate support  24  similarly moves parallel to radial path  30  during extension and retraction without rotation. As will be described in greater detail in other aspects of the disclosed embodiment, the principles and structure that constrain the end effector to move in a substantially purely radial motion may be applied where the length of the fore arm is shorter than that of the upper arm. Further, the features may be applied where more than one substrate is being handled by the end effector. Further, the features may be applied where a second arm is used in connection with the drive handling one or more additional substrates. Accordingly, all such variations may be embraced. 
     Referring also to  FIGS.  2 A and  2 B , there are shown partial schematic top and side views respectively of system  10  showing the internal arrangements used to drive the individual links of arm  14  shown in  FIGS.  1 A and  1 B . Drive  12  has first and second motors  52 ,  54  with corresponding first and second encoders  56 ,  58  coupled to housing  60  and respectively driving first and second shafts  62 ,  64 . Here shaft  62  may be coupled to pulley  66  and shaft  64  may be coupled to upper arm  64  where shafts  62 ,  64  may be concentric or otherwise disposed. In alternate aspects, any suitable drive may be provided. Housing  60  may be in communication with chamber  68  where bellows  70 , chamber  68  and an internal portion of housing  60  isolate a vacuum environment  72  from an atmospheric environment  74 . Housing  60  may slide in a z direction as a carriage on slides  76  where a lead screw or other suitable vertical or linear z drive  78  may be provided to selectively move housing  60  and arm  14  coupled there to in a z  80  direction. In the embodiment shown, upper arm  16  is driven by motor  54  about the central axis of rotation  18 . Similarly, forearm is driven by motor  52  through a band drive having pulleys  66 ,  82  and bands  84 ,  86  such as conventional circular pulleys and bands. In alternate aspects, any suitable structure may be provided to drive forearm  20  with respect to upper arm  16 . The ratio between pulleys  66  and  82  may be 1:1, 2:1 or any suitable ratio. Third link  24  with the end-effector may be constrained by a band drive having pulley  88  grounded with respect to link  16 , pulley  90  grounded with respect to end effector or third link  24  and bands  92 ,  94  constraining pulley  88  and pulley  90 . As will be described, the ratio between pulleys  88 ,  90  may not be constant in order for third link  24  to track a radial path without rotation during extension and retraction of aim  14 . This may be accomplished where pulleys  88 ,  90  may be one or more non circular pulleys, such as two non-circular pulleys or where one of pulley  88 ,  90  may be circular and the other being non circular. Alternately, any suitable coupling or linkage may be provided to constrain the path of third link or end effector  24  as described. In the embodiment shown, at least one non-circular pulley compensates for the effects of the unequal lengths of upper aim  16  and forearm  20  so that the end-effector  24  points radially  30  regardless of the position of the first two links  16 ,  20 . The embodiment will be described with respect to pulley  90  being non circular and pulley  88  being circular. Alternately, pulley  88  may be non-circular and pulley  90  circular. Alternately, pulleys  88  and  92  may be non-circular or any suitable coupling may be provided to constrain the links of arm  14  as described. By way of example, non-circular pulleys or sprockets are described in U.S. Pat. No. 4,865,577 issued on Sep. 12, 1989 and entitled Noncircular Drive which is hereby incorporated by reference herein in its entirety. Alternately, any suitable coupling may be provided to constrain the links of arm  14  as described, for example, any suitable variable ratio drive or coupling, linkage gears or sprockets, cams or otherwise used alone or in combination with a suitable linkage or other coupling. In the embodiment shown, elbow pulley  88  is coupled to upper arm  16  and is shown round or circular where wrist pulley  90  coupled to wrist or third link  24  is shown non circular. The wrist pulley shape is non-circular and may have symmetry about a line  96  perpendicular to the radial trajectory  30  which also may coincide with or be parallel to the line between the two pulleys  88 ,  90  when the forearm  20  and upper arm  16  are lined up over each other with the wrist axis  26  closest to shoulder axis  18 , for example as seen in  FIG.  3 B . The shape of pulley  90  is such that bands  92 ,  94  stay tight as arm  14  extends and retracts establishing points of tangency  98 ,  100  on opposing sides of pulley  90  having changing radial distances  102 ,  104  from the wrist axis of rotation  26 . For example, at the orientation shown in  FIG.  3 B , each of the points of tangency  98 ,  100  of the two bands on the pulley is at an equal radial distance  102 ,  104  from the wrist axis of rotation  26 . This will be further described with respect to  FIG.  4    showing respective ratios. In order for arm  14  to rotate, both drive shafts  62 ,  64  of the robot need to move in the direction of rotation of the arm by the same amount. In order for the end-effector  24  to extend and retract radially along a straight-line path, the two drive shafts  62 ,  64  need to move in a coordinated manner, for example, in accordance with the exemplary inverse kinematic equations presented later in this section. Here, a substrate transport apparatus  10  is adapted to transport substrate  28 . Forearm  20  is rotatably coupled to upper arm  16  and rotatable about elbow axis  22  being offset from central axis  18  by an upper arm link length. End effector  24  is rotatably coupled to forearm  20  and rotatable about wrist axis  26  offset from the elbow axis  22  by a forearm link length. Wrist pulley  90  is fixed to the end effector  24  and coupled to elbow pulley  88  with band  92 ,  94 . Here, the forearm link length is different than the upper arm link length and the end effector is constrained with respect to the upper arm by the elbow pulley, the wrist pulley and the band such that the substrate moves along a linear radial path  30  with respect to the central axis  18 . Here, substrate support  24  coupled to the upper arm  16  with a substrate support coupling  92  and driven about the wrist axis of rotation  26  by relative movement between the forearm  20  and the upper arm  16  about the elbow axis of rotation  22 .  FIGS.  3 A,  3 B and  3 C  illustrate extension motion of the robot of  FIGS.  1  and  2   .  FIG.  3 A  shows the top view of the robot  10  with the arm  14  in its retracted position.  FIG.  3 B  depicts the arm  14  partially extended with the forearm  20  aligned on top of the upper arm  16 , illustrating that the lateral offset  38  of the end-effector corresponds to the difference of the joint-to-joint lengths of the forearm  20  and upper arm  16 .  FIG.  3 C  shows the arm  14  in an extended position although not full extension. 
     Exemplary direct kinematics may be provided. In alternate aspects, any suitable direct kinematics may be provided to correspond to alternative structure. The following exemplary equations may be used to determine the position of the end-effector as a function of the position of the motors:
 
 x   2   =l   1  cos θ 1   +I   2  cos θ 2   (1.1)
 
 y   2   =l   1  sin θ 1   +l   2  sin θ 2   (1.2)
 
 R   2 =sqrt( x   2   2   +y   2   2 )  (1.3)
 
 T   2   =a  tan 2( y   2   ,x   2 )  (1.4)
 
α 3   =a  sin( d   3   /R   2 ) where  d   3   =l   2   −l   1   (1.5)
 
α 12 =θ 1 −θ 2   (1.6)
 
If α 12   π: R =sqrt( R   2   2   −d   3   2 )+ l   3   , T=T   2 +α 3 , else  R =−sqrt( R   2   2   −d   3   2 )+ l   3   , T=T   2 −α 3 +π  (1.7)
 
     Exemplary inverse kinematics may be provided. In alternate aspects, any suitable inverse kinematics may be provided to correspond to alternative structure. The following exemplary equations may be utilized to determine the position of the motors to achieve a specified position of the end-effector:
 
 x   3   =R  cos  T   (1.8)
 
 y   3   =R  sin  T   (1.9)
 
 x   2   =x   3   −l   3  cos  T+d   3  sin  T   (1.10)
 
 y   2   =y   3   −l   3  sin  T−d   3  cos  T   (1.11)
 
 R   2 =sqrt( x   2   2   +y   2   2 )  (1.12)
 
 T   2   =a  tan 2( y   2   ,x   2 )  (1.13)
 
α 1   =a  cos(( R   2   2   +l   1   2   −l   2   2 )/(2 R   2   l   1 ))  (1.14)
 
α 2   =a  cos(( R   2   2   −l   1   2   +l   2   2 )/(2 R   2    l   2 ))  (1.15)
 
If  R&gt;l   3 : θ 1   =T   2 +α 1 ,θ 2   =T   2 −α 2 , else: θ 1   =T   2 −α 1 , θ 2   =T   2 +α 2   (1.16)
 
     The following nomenclature may be used in the kinematic equations:
         d 3 =lateral offset of end-effector (m)   l 1 =join-to-joint length of first link (m)   l 2 =joint-to-joint length of second link (m)   l 3 =length of third link with end-effector, measured from wrist joint to reference point on end-effector (m)   R=radial position of end-effector (m)   R 2 =radial coordinate of wrist joint (m)   T=angular position of end-effector (rad)   T 2 =angular coordinate of wrist joint (rad)   x 2 =x-coordinate of wrist joint (m)   x 3 =x-coordinate of end-effector (m)   y 2 =y-coordinate of wrist joint (m)   y 3 =y-coordinate of end-effector (m)   θ 1 =angular position of drive shaft coupled to first link (rad)   θ 2 =angular position of drive shaft coupled to second link (rad).       

     The above exemplary kinematic equations may be used to design a suitable drive, for example, a band drive that constraints the orientation of the third link  24  so that the end-effector  24  points radially  30  regardless of the position of the first two links  16 ,  20  of the arm  14 . 
     Referring to  FIG.  4   , there is shown a plot  120  of the transmission ratio r 31    122  of the band drive that constraints the orientation of the third link as a function of normalized extension of the arm measured from the center of the robot to the root of the end-effector, i.e., (R−l 3 )/l 1 . The transmission ratio r 31  is defined as a ratio of the angular velocity of the pulley attached to the third link, ω 32 , over the angular velocity of the pulley attached to the first link, ω 12 , both defined relative to the second link. The figure graphs the transmission ratio r 31  for different l 2 /l 1  (from 0.5 to 1.0 with increment of 0.1, and from 1.0 to 2.0 with increment of 0.2). The profile of the non-circular pulley(s) may be calculated to achieve the transmission ratio r 31  in accordance with  FIG.  4   , for example, the profile depicted in  FIGS.  2 A,  54 A and  54 B . 
     In the disclosed embodiment, a longer reach may be obtained compared to an equal-link arm with the same containment volume with the use of one or more with non-circular pulley(s) or other suitable device to constrain the end effector motion. In alternate aspects, the first link may be driven by a motor either directly or via any kind of coupling or transmission arrangement. Here, any suitable transmission ratio can be used. Alternately, the band drive that actuates the second link may be substituted by any other arrangement with an equivalent functionality, such as a belt drive, cable drive, gear drive, linkage-based mechanism or any combination of the above. Similarly, the band drive that constrains the third link may be substituted by any other suitable arrangement, such as a belt drive, cable drive, non-circular gears, linkage-based mechanism or any combination of the above. Here, the end-effector may but does not need to point radially. For example, the end effector may be positioned with respect to the third link with any suitable offset and point in any suitable direction. Further, in alternate aspects, the third link may carry more than one end-effector or substrate. Any suitable number of end-effectors and/or material holders can be carried by the third link. Further, in alternate aspects, the joint-to-joint length of the forearm can be smaller than the joint-to-joint length of the upper arm, for example, as seen represented by l 2 /l 1 &lt;1 in  FIG.  4    and as seen and described with respect to  FIGS.  25 - 34  and  43 - 53   . 
     Referring now to  FIGS.  5 A and  5 B , there are shown top and side views respectively of robot  150  incorporating some features of robot  10 . Robot  150  is shown having drive  12  with arm  152  shown in a retracted position. Arm  152  has features similar to that of arm  14  except as described herein. By way of example, the joint-to-joint length of the forearm or second link  158  is larger than the joint-to-joint length of the upper arm or first link  154 . Similarly, the lateral offset  168  of the end-effector or third link  162  corresponds to the difference of the joint-to-joint lengths of the forearm  158  and upper arm  154 . Referring also to  FIGS.  6 A and  6 B , there is shown drive  150  with the internal arrangements used to drive the individual links of the arm. In the embodiment shown, upper arm  154  is driven by one motor through shaft  64  as described with respect to arm  14  of  FIGS.  1  and  2   . Similarly, end effector or third link  162  is constrained with respect upper arm  154  by a non-circular pulley arrangement as described with respect to arm  14  of  FIGS.  1  and  2   . The exemplary difference between arm  152  and arm  14  is seen where forearm  158  is coupled via a band arrangement with at least one non-circular pulley to shaft  62  and another motor of drive  12 . Here, the coupling or band arrangement may have features as described herein or as described with respect to pulley drive  88 ,  90  of  FIGS.  1  and  2   . The coupling or band arrangement has non circular pulley  202  coupled to shaft  62  of drive  12  and is rotateable about axis  18  with shaft  62 . The band arrangement of arm  152  further has circular pulley  204  coupled to upper arm link  158  and rotatable about elbow axis  156 . Circular pulley  204  is coupled to non-circular pulley  202  via bands  206 ,  208  where bands  206 ,  208  may be kept tight by virtue of the profile of non-circular pulley  202 . In alternate aspects, any combination of pulleys or other suitable transmission may be provided. Pulleys  202  and  204  and bands  206 ,  208  cooperate such that rotation of upper arm  154  relative to pulley  202  (for example, holding pulley  202  stationary while rotating upper arm  154 ) causes wrist joint  160  to extend and retract along a straight line parallel to the desired radial path  180  of the end-effector and offset  168  from the path  180 . Here, third link  162  with the end-effector is constrained by a band drive as described with respect to arm  14 , for example, with at least one non-circular pulley so that the end-effector points radially  180  regardless of the position of the first two links  154 ,  158 . Here, any suitable coupling may be provided to constrain the links of arm  14  as described, for example, one or more suitable variable ratio drive or coupling, linkage gears or sprockets, cams or otherwise used alone or in combination with a suitable linkage or other coupling. In the embodiment shown, elbow pulley  204  is coupled to fore arm  158  and is shown round or circular where shoulder pulley  202  coupled to shaft  62  is shown non circular. The shaft pulley shape is non-circular and may have symmetry about a line  218  perpendicular to the radial trajectory  180  which also may coincide with or be parallel to the line between the two pulleys  202 ,  204  when the forearm  158  and upper arm  154  are lined up over each other with the wrist axis  160  closest to shoulder axis  18 , for example as seen in  FIG.  7 B . The shape of pulley  202  is such that bands  206 ,  208  stay tight as arm  152  extends and retracts establishing points of tangency  210 ,  212  on opposing sides of pulley  202  having changing radial distances  214 ,  216  from the shoulder axis of rotation  18 . For example, at the orientation shown in  FIG.  7 B , each of the points of tangency  210 ,  212  of the two bands on the pulley is at an equal radial distance  214 ,  216  from the shoulder axis of rotation  18 . This will be further described with respect to  FIG.  8    showing respective ratios. In order for arm  152  to rotate, both drive shafts  62 ,  64  of the robot need to move in the direction of rotation of the arm by the same amount. In order for the end-effector  162  to extend and retract radially along a straight-line path, the two drive shafts  62 ,  64  need to move in a coordinated manner, for example, in accordance with the exemplary inverse kinematic equations presented later in this section, for example, the drive shaft coupled to the upper arm needs to move according to the inverse kinematic equations presented below while the other motor is kept stationary.  FIGS.  7 A,  7 B and  7 C  illustrate extension motion of robot  150  of  FIGS.  5  and  6   .  FIG.  7 A  shows the top view of the robot with the arm  152  in its retracted position.  FIG.  7 B  depicts the arm partially extended with the forearm aligned on top of the upper arm, illustrating that the lateral offset  168  of the end-effector  162  that corresponds to the difference of the joint-to-joint lengths of the forearm  158  and upper arm  154 .  FIG.  7 C  shows the arm in an extended position although not full extension. 
     Exemplary direct kinematics may be provided. In alternate aspects, any suitable direct kinematics may be provided to correspond to alternative structure. The following exemplary equations may be used to determine the position of the end-effector as a function of the position of the motors:
 
 d   1   =l   1  sin(θ 1 −θ 2 )  (2.1)
 
If (θ 1 −θ 2 )&lt;π/2: θ 2l =θ 2   −l   2   a  sin( d   1   +d   3 )/ l   2 ), else θ 2l =θ 2   +l   2    a  sin(( d   1   +d   3 )/ l   2 )+π  (2.2)
 
 x   2   =l   1  cos θ 1   +l   2  cos θ 2l   (2.3)
 
 y   2   =l   1  sin θ 1   +l   2  sin θ 2l   (2.4)
 
 R   2 =sqrt( x   2   2   +y   2   2 )  (2.5)
 
 T   2   =a  tan 2( y   2   ,x   2 )  (2.6)
 
If (θ 1 −θ 2 )&lt;π/2:  R =sqrt( R   2   2   −d   3   2 )+ l   3   ,T=θ   2 , else  R =−sqrt( R   2   2   −d   3   2 )+ l   3   , T=θ   2   (2.7)
 
     Exemplary inverse kinematics may be provided. In alternate aspects, any suitable inverse kinematics may be provided to correspond to alternative structure. The following exemplary equations may be utilized to determine the position of the motors to achieve a specified position of the end-effector:
 
 x   3   =R  cos  T   (2.8)
 
 y   3   =R  sin  T   (2.9)
 
 x   2   =x   3   −l   3  cos  T+d   3  sin  T   (2.10)
 
 y   2   =y   3   −l   3  sin  T−d   3  cos  T   (2.11)
 
 R   2 =sqrt( x   2   2   +y   2   2 )  (2.12)
 
 T   2   =a  tan 2( y   2   ,x   2 )  (2.13)
 
α 1   =a  cos(( R   2   2   +l   1   2   −l   2   2 )/(2 R   2    l   1 ))  (2.14)
 
If  R&gt;l   3 : θ 1   =T   2 +α 1 ,θ 2   =T , else: θ 1   =T   2 −α 1 , θ 2   =T   (2.15)
 
     The following nomenclature is used in the kinematic equations:
 
 d   3 =lateral offset of end-effector ( m )
 
 l   1 =join-to-joint length of first link ( m )
 
 l   2 =joint-to-joint length of second link ( m )
 
 l   3 =length of third link with end-effector, measured from wrist joint to reference point on end-effector ( m )
 
 R =radial position of end-effector ( m )
 
 R   2 =radial coordinate of wrist joint ( m )
 
 T =angular position of end-effector (rad)
 
 T   2 =angular coordinate of wrist joint (rad)
 
 x   2   =x -coordinate of wrist joint ( m )
 
 x   3   =x -coordinate of end-effector ( m )
 
 y   2   =y -coordinate of wrist joint ( m )
 
 y   3   =y -coordinate of end-effector ( m )
 
θ 1 =angular position of drive shaft coupled to first link (rad)
 
θ 2 =angular position of drive shaft coupled to second link (rad).
 
     The above kinematic equations may be used to design the band drive that controls the second link  158  so that rotation of the upper arm  154  causes the wrist joint  160  to extend and retract along a straight line parallel to the desired radial path  180  of the end-effector  162 . 
     Referring now to  FIG.  8   , there is shown a graph  270  that shows the transmission ratio r 20    272  of the band drive that drives the second link as a function of normalized extension of the arm measured from the center of the robot to the root of the end-effector, i.e., (R−l 3 )/l 1 . The transmission ratio r 20  is defined as a ratio of the angular velocity of the pulley attached to the second link, ω 21 , over the angular velocity of the pulley attached to the second motor, cool, both defined relative to the first link. The figure graphs the transmission ratio r 20  for different l 2 /l 1 . 
     The profile of the non-circular pulley(s) for the band drive that drives the second link is calculated to achieve the transmission ratio r 20    272  in accordance with  FIG.  8   . An example pulley profile is depicted in  FIG.  6 A  and as will be described with respect to  FIGS.  55 A and  55 B . 
     The transmission ratio r 31  of the band drive that constraints the orientation of the third link  168  may be the same as depicted in  FIG.  4    for the embodiment of  FIGS.  1  and  2   . The transmission ratio r 31  is defined as a ratio of the angular velocity of the pulley attached to the third link, ω 32 , over the angular velocity of the pulley attached to the first link, ω 12 , both defined relative to the second link. The figure graphs the transmission ratio r 31  for different l 2 /l 1  (from 0.5 to 1.0 with increment of 0.1, and from 1.0 to 2.0 with increment of 0.2). The profile of the non-circular pulley(s) for the band drive that constrains the third link  162  may be calculated to achieve the transmission ratio r 31  in accordance with  FIG.  4   . An example pulley profile is depicted in  FIG.  6 A . 
     In the embodiment shown, a longer reach may be obtained as compared to an equal-link arm with the same containment volume while using non-circular pulleys or other suitable mechanism to constrain the end effector as described. As compared to the embodiment disclosed in  FIGS.  1  and  2   , one more band drive with non-circular pulleys may be in place of conventional one at shoulder axis  18 . In alternate aspects, the first link may be driven by a motor either directly or via any kind of coupling or transmission arrangement, for example, any suitable transmission ratio may be used. Alternately, the band drives that actuate the second link and constrain the third link may be substituted by any other arrangement with an equivalent functionality, such as a belt drive, cable drive, non-circular gears, linkage-based mechanism or any combination of the above. Further, the third link may be constrained to keep the end-effector radial via a conventional two stage band arrangement that synchronizes the third link to the pulley driven by the second motor, as illustrated in  FIG.  9   . Alternatively, the two stage band arrangement may be substituted by any other suitable arrangement, such as a belt drive, cable drive, gear drive, linkage-based mechanism or any combination of the above. In addition, the end-effector may but does not need to point radially. For example, the end effector may be positioned with respect to the third link with any suitable offset and point in any suitable direction. In alternate aspects, the third link may carry more than one end-effector or substrate. Here, any suitable number of end-effectors and/or material holders can be carried by the third link. Further, the joint-to-joint length of the forearm may be smaller than the joint-to-joint length of the upper arm, for example, as represented by l 2 /l 1 &lt;1 in  FIG.  8   . 
     Referring now to  FIG.  9   , there is shown an alternative robot  300  where the third link may be constrained to keep the end-effector radial via a conventional two stage band arrangement that synchronizes the third link to the pulley driven by the second motor. Robot  300  is shown having drive  12  and arm  302 . Arm  302  may have upper arm or first link  304  coupled to shaft  64  and rotatable about central or shoulder axis  18 . Arm  302  has forearm or second link  308  rotateably coupled to upper arm  304  at elbow axis  306 . Links  304 ,  308  may have unequal lengths as previously described. Third link or end effector  312  is rotatably coupled to the second link or forearm  308  at wrist axis  310  where end effector  312  may transport a substrate  28  along a radial path without rotation with links  304 ,  308  having unequal link lengths as previously described. In the embodiment shown, shaft  62  is coupled to two pulleys,  314 ,  316  where pulley  314  may be circular and where pulley  316  may be non-circular. Here, circular pulley  314  constrains the third link  312  to keep the end-effector  312  radial via a conventional two stage  318 ,  320  circular band arrangement that synchronizes the third link  312  to the pulley driven by shaft  314 . The two stage arrangement  318 ,  320  has pulley  314  coupled by bands  322  to elbow pulley  324  that is coupled to elbow pulley  326  where elbow pulley  326  is coupled to wrist pulley  328  via bands  330 . Forearm  308  may further have elbow pulley  332  that may be circular and coupled to shoulder pulley  316  through bands  334  where shoulder pulley may be non-circular and coupled to pulley  314  and shaft  62 . 
     The disclosed embodiment may be further embodied with respect to robots having robot drives with additional axis and where the arms coupled to the robot drive may have independently operable additional end effectors capable of carrying one or more substrates. By way of example, arms with two independently operable arms linkages or “dual arm” configurations may be provided where each independently operable arm may have an end effector adapted to support one, two or any suitable number of substrates. Here and as will be described below, each independently operable arm may have first and second links having different link lengths and where the end effector and supported substrate coupled to the links operate and track as described above. Here, a substrate transport apparatus may transport first and second substrates and having first and second independently moveable arm assemblies coupled to a drive section on a common axis of rotation. First and second substrate supports are coupled to the first and second arm assemblies respectively on first and second wrist axis of rotation. One or both of the first and second arm assemblies rotate about the common axis of rotation during extension and retraction. The first and second wrist axis of rotation move along first and second wrist paths parallel to and offset from a radial path relative to the common axis of rotation during extension and retraction. The first and second substrate supports move parallel to the radial path during extension and retraction without rotation. Variations on the disclosed embodiment having multiple and independently operable arms are provided below where in alternate aspects any suitable combination of features may be provided. 
     Referring now to  FIGS.  10 A and  10 B , there are shown top and side views respectively of robot  350  with a dual arm arrangement. Robot  350  has arm  352  having a common upper arm  354  and independently operable forearms  356 ,  358  each having respective end effectors  360 ,  362 . In the embodiment shown, both linkages are shown in their retracted positions. The lateral offset of the end-effectors  366  corresponds to the difference of the joint-to-joint lengths of the forearm  354  and upper arms  356 ,  358 . In the embodiment shown, the upper arms may have the same length and being longer than the forearm. Further, end effectors  360 ,  362  are positioned above forearms  356 ,  358 . Referring now to  FIGS.  11 A and  11 B  show top and side views respectively of a robot  375  with the arm in an alternative configuration. In the embodiment shown, arm  377  may have features as described with respect to  FIGS.  10 A and  10 B  with both linkages are shown in their retracted positions. In this configuration, the third link with the end-effector  382  of the upper linkage is suspended underneath the forearm  380  to reduce vertical spacing between the two end-effectors  382 ,  384 . Here, a similar effect may be achieved by stepping  368  the top end-effector  360  of the configuration of  FIGS.  10 A and  10 B  down. Referring also to  FIGS.  12  and  13    there is shown the internal arrangements of robots  350 ,  375  respectively used to drive the individual links of the arms of  FIGS.  10  and  11   , respectively. In the embodiment shown, drive  390  may have first second and third driving motors  392 ,  394 ,  396  that may be rotor stator arrangements driving concentric shafts  398 ,  400 ,  402  respectively and having position encoders  404 ,  406 ,  408  respectively. Z drive  410  may drive the motors in a vertical direction where the motors may be contained partially or completely within housing  412  and where bellows  414  seals an internal volume of housing  412  to chamber  416  and where the internal volume and an interior of chamber  416  may operate within an isolated environment such as vacuum or otherwise. In the embodiment shown, the common upper arm  354  is driven by one motor  396 . Each of the two forearms  356 ,  358  pivot on a common axis  420  at the elbow of upper arm  354  and are driven independently by motors  394 ,  396  respectively through band drives  422 ,  424  respectively that may have conventional pulleys. The third links with the end-effectors  360 ,  362  are constrained by band drives  426 ,  428  respectively, each with at least one non-circular pulley, which compensate for the effects of the unequal lengths of the upper arms and forearms. Here, the band drives in each of the linkages may be designed using the methodology described for  FIGS.  1  and  2    and where the kinematic equations presented for  FIGS.  1  and  2    may also be used for each of the two linkages of the dual arm. In order for the arm to rotate, all three drive shafts  398 ,  400 ,  402  of the robot need to move in the direction of rotation of the arm by the same amount. In order for one of the end-effectors to extend and retract radially along a straight-line path, the drive shaft of the common upper arm and the driveshaft coupled to the forearm associated with the active end effector need to move in a coordinated manner in accordance with the inverse kinematic equations for  FIGS.  1  and  2   . At the same time, the driveshaft coupled to the other forearm needs to rotate in synch with the drive shaft of the common upper arm in order for the inactive end-effector to remain retracted. Referring also to  FIGS.  14 A,  14 B and  14 C  there is shown the arm of  FIGS.  11 A and  11 B  as the upper and lower linkages extend. Here, the inactive linkage  356 ,  360  rotates while the active linkage  358 ,  362  extends. By way of example, the upper linkage  358 ,  362  rotates as the lower linkage  356 ,  360  extends, and the lower linkage  356 ,  360  rotates as the upper linkage  358 ,  362  extends. In the disclosed embodiment of  FIGS.  10  and  11   , set up and control may be simplified where the arm arrangement may be used on a coaxial drive with no dynamic seals while providing a longer reach compared to equal-link length arms with the same containment volume. Here, no bridge is used to support any of the end-effectors. In the embodiment shown, the inactive aim rotates while the active one extends. One of the wrist joints travels above the lower end-effector (closer to wafer than in an equal-link arrangement). 
     Referring now to  FIGS.  15 A and  15 B , there are shown top and side views respectively of robot  450  with a dual arm arrangement. Robot  450  has arm  452  having a common upper arm  454  and independently operable forearms  456 ,  458  each having respective end effectors  460 ,  462 . In the embodiment shown, both linkages are shown in their retracted positions. The lateral offset of the end-effectors  466  corresponds to the difference of the joint-to-joint lengths of the forearm  454  and upper arms  456 ,  458 . In the embodiment shown, the upper arms may have the same length and being longer than the forearm. Further, end effectors  460 ,  462  are positioned above forearms  456 ,  458 . Referring also to  FIGS.  16 A and  16 B  show the top and side views of the robot  475  with the arm in an alternative configuration. Again, both linkages are shown in their retracted positions. In this configuration, the third link and the end-effector  482  of the left linkage is suspended underneath the forearm  480  to reduce vertical spacing between the two end-effectors  482 ,  484 . A similar effect can be achieved by stepping  468  the top end-effector of the configuration of  FIGS.  15 A and  15 B  down. Alternatively, a bridge can be used to support one of the end-effectors. The combined upper arm link  454  may be a single piece as depicted in  FIGS.  15  and  16    or it can be formed by two or more sections  470 ,  472 , as shown in the example of  FIGS.  17 A and  17 B . Here, a two-section design may be provided as lighter and using less material, with the left  472  and right  470  sections may be identical components. Here, a two piece design may also have provisions for adjustment of the angular offset between the left and right sections, which may be convenient when different retracted positions need to be supported. Referring also to  FIGS.  18  and  19   , there is shown the internal arrangements used to drive the individual links of the arm of  FIGS.  15  and  16   , respectively. The combined upper arm  554  is shown driven by one motor with shaft  402 . Each of the two forearms  456 ,  458  is driven independently by one motor each via shafts  400 ,  398  respectively through band drives  490 ,  492  with conventional pulleys. Here, links  456 ,  458  rotate on separate axis&#39;  494 ,  496  respectively. The third links with the end-effectors  460 ,  462  are constrained by band drives  498 ,  500  respectively, each with at least one non-circular pulley, which compensate for the effects of the unequal lengths of the upper arms and forearms. Here, band drives  498 ,  500  in each of the linkages  456 ,  460  and  458 ,  462  are designed using the methodology described for  FIGS.  1  and  2   . Here, the kinematic equations presented for  FIGS.  1  and  2    may also be used for each of the two linkages  456 ,  460  and  458 ,  462  of the dual arm. In order for the arm  452  to rotate, all three drive shafts  398 ,  400 ,  402  of the robot need to move in the direction of rotation of the arm by the same amount. In order for one of the end-effectors to extend and retract radially along a straight-line path, the drive shaft of the common upper arm and the driveshaft coupled to the forearm associated with the active end effector need to move in a coordinated manner in accordance with the inverse kinematic equations presented with respect to  FIGS.  1  and  2   . At the same time, the driveshaft coupled to the other forearm needs to rotate in synch with the drive shaft of the common upper arm in order for the inactive end-effector to remain retracted. Referring also to  FIGS.  20 A,  20 B and  20 C , there is shown the arm of  FIGS.  16 A and  16 B  as the left  458 ,  462  and right  456 ,  460  linkages extend. Note that the inactive linkage  456 ,  460  rotates while the active linkage  458 ,  462  extends. Here, the right linkage  456 ,  460  rotates as the left linkage  458 , 462  extends, and the left linkage  458 ,  462  rotates as the right linkage  456 ,  460  extends. The embodiment shown leverages the benefits of a solid link design being easy to set up and control and the coaxial drive, for example, with no dynamic seals while providing a longer reach compared to equal-link arms with the same containment volume. Here, no bridge is used to support any of the end-effectors. Here, the inactive arm rotates while the active one extends. One of the wrist joints travels above the lower end-effector, closer to the wafer than in an equal-link arrangement. This can be avoided by using a bridge (not shown) to support the top end-effector. In this case, the unsupported length of the bridge may be longer compared to an equal-link arm design. Further, the retract angle may be more difficult to change compared to the configuration with common elbow joint, for example, as seen in  FIGS.  10  and  11    and independent dual arm, for example, as seen in  FIGS.  21  and  22   . 
     Referring now to  FIGS.  21 A and  21 B , there is shown top and side views respectively of robot  520  with independent dual arms  522 ,  524 . In the embodiment shown, both linkages  522 ,  524  are shown in their retracted positions. Arm  522  has independently operable upper arm  526 , forearm  528  and third link with end effector  530 . Arm  524  has independently operable upper arm  532 , forearm  534  and third link with end effector  536 . In the embodiment shown, forearms  528 ,  534  are shown longer than upper arms  526 ,  532  where end effectors  530 ,  536  are positioned above forearms  528 ,  534  respectively. Referring also to  FIGS.  22 A and  22 B  show the top and side views of robot  550  with features similar to that of robot  520  with the arm in an alternative configuration and with both linkages shown in their retracted positions. In this configuration, the third link and the end-effector  552  of the left linkage is suspended underneath the forearm  554  to reduce vertical spacing between the two end-effectors. A similar effect can be achieved by stepping the top end-effector of the configuration of  FIG.  21    down. Alternatively, a bridge can be used to support one of the end-effectors. In  FIGS.  21  and  22   , the right upper arm  532  is located below the left upper arm  526 . Alternatively, the left upper may be located above the right upper arm, for example, where one linkage can be nested within the other. Referring also to  FIG.  23   , there is shown the internal arrangements used to drive the individual links of the arm of  FIGS.  21 A and  21 B . Here, for graphical clarity, to avoid overlap of components, the elevations of the links are adjusted. Each of the two upper arms  526 ,  532  is driven independently by one motor each through shafts  398 ,  402  respectively. The forearms  528 ,  534  are coupled via band arrangements  570 ,  572 , each with at least one non-circular pulley, to a third motor via shaft  400 . The third links  530 ,  536  with the end-effectors are constrained by band drives  574 ,  576 , each with at least one non-circular pulley. The band drives are designed so that rotation of one of the upper arms  526 ,  532  causes the corresponding linkage  528 ,  530  and  534 ,  536  respectively to extend and retract along a straight line while the other linkage remains stationary. The band drives in each of the linkages may be designed using the methodology described with respect to  FIGS.  5  and  6    where the kinematic equations presented for  FIGS.  5  and  6    can also be used for each of the two linkages of the dual arm. In order for the arm to rotate, all three drive shafts  398 ,  400 ,  402  of the robot need to move in the direction of rotation of the arm by the same amount. In order for one of the end-effectors to extend and retract radially along a straight-line path, the drive shaft of the upper arm associated with the active end-effector needs to be rotated according to the inverse kinematic equations for  FIGS.  5  and  6    and the other two drive shafts need to be kept stationary. Referring also to  FIGS.  24 A,  24 B and  24 C , there is shown the arm of  FIG.  22    as the left  522  and right  524  linkages extend. Note that the inactive linkage  524  remains stationary while the active linkage  522  extends. That is, the left linkage  522  does not move while the right linkage  524  extends, and the right linkage  524  does not move when the left linkage  522  extends. The embodiment shown provides a longer reach compared to equal-link arm design with the same containment volume. Here, no bridge is used to support any of the end-effectors and the inactive linkage remains stationary while the active one extends potentially leading to higher throughput as active linkage may extend or retract faster with no load. The embodiment shown may be more complex than shown in  FIGS.  15  and  16    with two more band drives with non-circular pulleys in place of conventional ones. One of the wrist joints travels above the lower end-effector as seen in  FIG.  24   . This can be avoided by using a bridge (not shown) to support the top end-effector. In this case, the unsupported length of the bridge is longer compared to an equal-link arm design. 
     Referring now to  FIGS.  25 A and  25 B , there are shown top and side views respectively of robot  600  with arm  602 . In the embodiment shown, both linkages are shown in their retracted positions. The lateral offset of the end-effectors  604  corresponds to the difference of the joint-to-joint lengths of the upper arm  606  and forearms  608 ,  612  where in this embodiment, forearms  608 ,  612  are shorter than the common upper arm  606 . The internal arrangements used to drive the individual links of the arm may be similar to  FIGS.  10 - 13   , for example as in  FIG.  13    however the forearms in this instance are shorter than the common upper arm. Here, the common upper arm is driven by one motor. Each of the two forearms is driven independently by one motor through a band drive with conventional pulleys. The third links  614 ,  616  with the end-effectors are constrained by band drives, each with at least one non-circular pulley, which compensate for the effects of the unequal lengths of the upper arms and forearms. The band drives in each of the linkages may be designed using the methodology described for  FIGS.  1  and  2   . The kinematic equations presented for  FIGS.  1  and  2    may also be used for each of the two linkages of the dual arm. Referring also to  FIGS.  26 A,  26 B and  26 C , there is shown the arm of  FIGS.  25 A and  25 B  as the upper linkage  612 ,  616  extends. The lateral offset  604  of the end-effector corresponds to the difference of the joint-to-joint lengths of the upper arm and forearm, and the wrist joint travels along a straight line offset with respect to the trajectory of the center of the wafer by this difference. Note that the inactive linkage  608 ,  614  rotates while the active linkage  612 ,  616  extends. For instance, the upper linkage rotates as the lower linkage extends, and the lower linkage rotates as the upper linkage extends. Here,  FIG.  26 A  depicts the arm with both linkages in the retracted positions.  FIG.  26 B  shows the upper linkage  612 ,  616  partially extended in a position where the wrist joint of the upper linkage is closest to the wafer carried by the lower linkage. It is observed that the wrist joint of the upper linkage does not travel over the wafer (however, it moves in a plane above the wafer).  FIG.  26 C  depicts farther extension of the upper linkage  612 ,  616 . The embodiment shown may provide ease of to set up and control, and may be used on a coaxial or tri axial drive with no dynamic seals or other suitable drive. Here, no bridge may be used to support any of the end-effectors. The wrist joint of the upper linkage does not travel over the wafer on the lower end-effector, which is the case for an equal-link design (however, it moves in a plane above the wafer on the lower end-effector). Here, the inactive arm rotates while the active one extends. The elbow joint may be more complex which may translate to a larger swing radius or shorter reach. Here, the arm may be taller than that shown in  FIGS.  30  and  31    and  FIG.  33    due to the overlapping forearms  608 ,  612 . 
     Referring now to  FIGS.  27 A and  27 B , there is shown top and side views respectively of robot  630  with arm  632 . Arm  630  may have features similar to that disclosed with respect to  FIGS.  15 - 19    except the forearms  636 ,  640  are shown with shorter link length than the upper arm  636 . Both linkages are shown in their retracted positions. The lateral offset  634  of the end-effectors  642 ,  646  corresponds to the difference of the joint-to-joint lengths of the upper arm  636  and forearms  638 ,  640 . The combined upper arm link  636  may be a single piece as depicted in  FIGS.  27 A and  27 B  or it can be formed by two or more sections  636 ′,  636 ″, as shown in the example of  FIGS.  28 A and  28 B . A two-section design may be lighter with less material and where left  636 ′ and right  636 ″ sections may be identical components. Allowances for adjustment of the angular offset between the left  636 ′ and right  636 ″ sections may be provided, for example, where different retracted positions need to be supported. The internal arrangements used to drive the individual links of the arm  632  may be similar to that in  FIGS.  15 - 19   , for example, as seen  FIG.  19   . The common upper arm  636  is driven by one motor. Each of the two forearms  638 ,  640  is driven independently by one motor through a band drive with conventional pulleys. The third links with the end-effectors  642 ,  646  may be constrained by band drives, each with at least one non-circular pulley, which compensate for the effects of the unequal lengths of the upper arm  636  and forearms  638 ,  640 . The band drives in each of the linkages may be designed using the methodology described for  FIGS.  1  and  2   . The kinematic equations presented for  FIGS.  1  and  2    may also be used for each of the two linkages of the dual arm. Referring also to  FIGS.  29 A,  29 B and  29 C , there is shown the arm of  FIGS.  27 A and  27 B  as the right, upper linkage  640 ,  646  extends. The lateral offset  634  of the end-effector corresponds to the difference of the joint-to-joint lengths of the upper arm and forearm, and the wrist joint travels along a straight line offset with respect to the trajectory of the center of the wafer by this difference. Here, the inactive linkage  638 ,  642  rotates while the active linkage  640 ,  646  extends. For instance, the upper linkage rotates as the lower linkage extends, and the lower linkage rotates as the upper linkage extends. In  FIGS.  29 A,  29 B and  29 C ,  FIG.  29 A  depict the arm with both linkages in the retracted positions.  FIG.  29 B  shows the right upper linkage  640 ,  646  partially extended in a position where the wrist joint of the right upper linkage  640 ,  646  is closest to the wafer carried by the left lower linkage  638 ,  642 . Here the wrist joint of the right upper  640 ,  646  linkage does not travel over the wafer however, it moves in a plane above the wafer.  FIG.  29 C  depicts farther extension of the right upper linkage  640 ,  646 . The embodiment shown leverages the benefits of a solid link design, ease of set up and control and the coaxial drive, for example, no dynamic seals. No bridge is used to support any of the end-effectors. The wrist joint of the upper linkage does not travel over the wafer on the lower end-effector, which is the case for an equal-link design however, it moves in a plane above the wafer on the lower end-effector. The inactive arm  638 ,  642  rotates while the active arm  640 ,  646  extends. The retract angle is more difficult to change compared to the configuration with common elbow joint, for example as seen in  FIGS.  25 A and  25 B  and independent dual arm, for example, as seen in  FIGS.  33 A and  33 B . Further, the arm is shown taller than  FIGS.  30  and  31    and  FIGS.  33 A and  33 B  as forearm  640  is shown at a higher elevation than forearm  638 . 
     Referring now to  FIGS.  30 A and  30 B , there is shown the top and side views respectively of robot  660  with arm  662 . Arm  662  may have features as described with respect to  FIGS.  27 - 29    however employing a bridge and with the two forearms at the same elevation as will be described. Both linkages are shown in their retracted positions. The lateral offset  664  of the end-effectors corresponds to the difference of the joint-to-joint lengths of the upper arm  66  and forearms  668 ,  670 . The combined upper arm link  666  can be a single piece as depicted in  FIGS.  30 A and  30 B  or it can be formed by two or more sections  666 ′,  666 ″, as shown in the example of  FIGS.  31 A and  31 B . The internal arrangements used to drive the individual links of the arm may be identical to that shown for  FIGS.  15 - 19    but where the forearms  668 ,  670  are shorter than the upper arm  666 . The common upper arm  666  is driven by one motor. Each of the two forearms  668 ,  670  is driven independently by one motor through a band drive with conventional pulleys. The third links with the end-effectors  672 ,  674  are constrained by band drives, each with at least one non-circular pulley, which compensate for the effects of the unequal lengths of the upper arms and forearms. The band drives in each of the linkages may be designed using the methodology described for  FIGS.  1  and  2   . The kinematic equations presented for  FIGS.  1  and  2    can also be used for each of the two linkages of the dual aim. Third link and end effector  674  has a bridge  680  that has an upper end effector portion  682 , a side offset support portion  684  offset from the wrist axis between link  670  and link  674  and further has a lower support portion  686  coupling the wrist axis to the offset support portion  684 . Bridge  680  allows forearms  668  and  670  to be packaged at the same level while providing clearance for the interleaved portions of third link and end effector  672  (which may include the wafer) and the bridge  680  as can be seen below with respect to  FIG.  32   . Bridge  680  further provides an arrangement where any moving parts, for example, associated with the two wrist joints, reside below the wafer surface during transport. Referring also to  FIGS.  32 A,  32 B,  32 C and  32 D , there is shown the top view of the robot min of  FIGS.  30 A and  30 B  as the right linkage  670 ,  674  extends. The lateral offset  664  of the end-effector corresponds to the difference of the joint-to-joint lengths of the upper arm  666  and forearm  670 , and the wrist joint  690  travels along a straight line offset with respect to the trajectory of the center of the wafer  692  by this difference. Note that the inactive linkage  668 ,  672  rotates while the active linkage  670 ,  674  extends. For instance, the upper linkage rotates as the lower linkage extends, and the lower linkage rotates as the upper linkage extends. In  FIGS.  32 A,  32 B,  32 C and  32 D ,  FIG.  32 A  depicts the arm with both linkages in the retracted positions.  FIG.  32 B  shows the right linkage  670 ,  674  partially extended in a position that corresponds to the worst-case clearance (or is close to the worst-case clearance) between the bridge  680  of the right linkage  670 ,  674  and the end-effector  672  of the left linkage  668 ,  672 .  FIG.  32 C  shows the right linkage  670 ,  674  partially extended in a position when the forearm  670  is aligned with the upper arm  666 . The lateral offset of the end-effector corresponds to the difference of the joint-to-joint lengths of the upper arm and forearm. The wrist joint  690  axis travels along a straight line offset with respect to the trajectory of the center of the wafer  692  by this difference.  FIG.  32 D  depicts farther extension of the right linkage  670 ,  674 . The embodiment shown combines the benefits of the side-by-side dual scara arrangement, for example, slim profile, resulting in a shallow chamber with a small volume, the solid link design and the coaxial drive. The bridge  680  on the right linkage  670 ,  674  is much lower and its unsupported length between vertical member  684  and wrist  690  is shorter than in a prior art coaxial dual scara arm and all of the joints are below the end-effectors. Here, the inactive arm  668 ,  672  rotates while the active arm  670 ,  674  extends. As will be described below, in other aspects of the disclosed embodiment, and arm which does not exhibit this behavior may be provided with a different band drives with non-circular pulleys in place of the conventional ones disclosed here. Alternatively, the bridge that supports the top end-effector may be eliminated by utilizing an arrangement similar to those described for  FIGS.  25 A and  25 B  and  FIGS.  27  and  28    above. 
     Referring now to  FIGS.  33 A and  33 B , there is shown top and side views respectively of robot  700  with arm  702 . Arm  702  may have features similar to that of the arm shown in  FIGS.  21 - 23    but with forearm lengths shorter than the upper arm lengths and employing a bridge as described with respect to bridge  680  by way of example and with the forearms located at the same elevation. Both linkages are shown in their retracted positions. In  FIGS.  33 A and  33 B , the right upper arm  708  is located above the left upper arm  706 . Alternatively, the left upper  706  may be located above the right upper arm  708 . Similarly, the third link and end-effector  716  of the right linkage  712 ,  716  feature a bridge that extends over the third link and end-effector  714  of the left linkage  710 ,  714 . Alternatively, the third link and end-effector  714  of the left linkage  710 ,  714  may feature a bridge that may extend over the third link and end-effector  716  of the right linkage  712 ,  716 . The internal arrangements used to drive the individual links of the arm may be similar to the embodiment shown in  FIGS.  21 - 23   . Each of the two upper mins  706 ,  708  is driven independently by one motor. The forearms  710 ,  712  are coupled via band arrangements, each with at least one non-circular pulley, to a third motor. The third links  714 ,  716  with the end-effectors are constrained by band drives, each with at least one non-circular pulley. The band drives are designed so that rotation of one of the upper arms  706 ,  708  causes the corresponding linkage to extend and retract along a straight line while the other linkage remains stationary. The band drives in each of the linkages are designed using the methodology described for the embodiment shown in  FIGS.  5  and  6   . The kinematic equations presented for the embodiment shown in  FIGS.  5  and  6    can also be used for each of the two linkages of the dual arm. Referring also to  FIGS.  34 A,  34 B and  34 C , there is shown the arm of  FIGS.  33 A and  33 B  as the right linkage  708 ,  712 ,  716  extends. Here, the inactive linkage  706 ,  710 ,  714  remains stationary while the active linkage  712 ,  716  extends. That is, the left linkage does not move while the right linkage extends, and the right linkage does not move when the left linkage extends. The embodiment shown combines the benefits of the side-by-side dual scara arrangement, for example, slim profile, resulting in a shallow chamber with a small volume and the coaxial drive. The bridge on the right linkage is much lower and its unsupported length is shorter than in the existing coaxial dual scara arms and all of the joints are below the end-effectors. The inactive linkage remains stationary while the active one extends potentially leading to higher throughput as active linkage may extend or retract faster with no load. Alternatively, the bridge that supports the top end-effector may be eliminated by utilizing an arrangement similar to those described for  FIGS.  25 ,  27  and  28   . 
     Referring now to  FIGS.  35 A and  35 B , there is shown top and side views of robot  730  with arm  732  with both linkages shown in their retracted positions. Each linkage has a dual-holder end-effector  740 ,  742 , each supporting two substrates offset from each other for a total of 4 substrates supportable. The internal arrangements used to drive the individual links of the arm  732  may be identical to  FIGS.  10  and  11   , for example,  FIG.  13   . The common upper arm  734  is driven by one motor. Each of the two forearms  73736 ,  738  is driven independently by one motor through a band drive with conventional pulleys. The third links with the end-effectors  740 ,  742  are constrained by band drives, each with at least one non-circular pulley, which compensate for the effects of the unequal lengths of the upper arms and forearms. The embodiment shown has forearms longer than the upper arm. Alternately, they may be shorter. The band drives in each of the linkages are designed using the methodology described for  FIGS.  1  and  2   . The kinematic equations presented for  FIGS.  1  and  2    may also be used for each of the two linkages of the dual arm. Referring also to  FIG.  36   , there is shown the arm of  FIGS.  35 A and  35 B  as one linkage  738 ,  742  extends. Note that the inactive linkage  736 ,  740  rotates while the active linkage  738 ,  742  extends. For instance, the upper linkage rotates as the lower linkage extends, and the lower linkage rotates as the upper linkage extends. Compared to  FIGS.  37  and  38   , end-effector does not need to be shaped to avoid interference with opposite elbow. 
     Referring now to  FIGS.  37 A and  37 B , there is shown top and side views respectively of robot with arm  750 . Both linkages are shown in their retracted positions with each linkage having a dual-holder end-effector  758 ,  760 . The combined upper arm link  752  can be a single piece as depicted in  FIGS.  37 A and  37 B  or it can be formed by two or more sections  752 ′,  752 ″, as shown in the example of  FIGS.  38 A and  38 B . The internal arrangements used to drive the individual links of the arm may be identical to  FIGS.  15 - 19   , for example,  FIG.  19   . The combined upper arms  752  are driven by one motor. Each of the two forearms  754 ,  756  is driven independently by one motor through a band drive with conventional pulleys. The third links  758 ,  760  with the end-effectors are constrained by band drives, each with at least one non-circular pulley, which compensate for the effects of the unequal lengths of the upper arms and forearms. The embodiment shown has forearms longer than the upper arm. Alternately, they may be shorter. The band drives in each of the linkages are designed using the methodology described for  FIGS.  1  and  2   . The kinematic equations presented for  FIGS.  1  and  2    may also be used for each of the two linkages of the dual arm. In order for the arm to rotate, all three drive shafts of the robot need to move in the direction of rotation of the arm by the same amount. In order for one of the end-effector assemblies to extend and retract radially along a straight-line path, the drive shaft of the common upper arm and the driveshaft coupled to the forearm associated with the active linkage need to move in a coordinated manner in accordance with the inverse kinematic equations for  FIGS.  1  and  2   . At the same time, the driveshaft coupled to the other forearm needs to rotate in synch with the drive shaft of the common upper arm in order for the inactive linkage to remain retracted. Referring also to  FIG.  39   , there is shown the arm of  FIGS.  37 A and  37 B  as one linkage  756 ,  760  extends. Here, the inactive linkage  754 ,  758  rotates while the active linkage extends. For instance, the right linkage rotates as the left linkage extends, and the left linkage rotates as the right linkage extends. The embodiment shown has no bridge. The upper wrist travels over one of the wafers on the lower end-effector. Here, the arm and end-effectors need to be designed so that the top elbow clears the lower end-effector. 
     Referring now to  FIGS.  40 A and  40 B , there is shown top and side views respectively of robot  750  with arm  752 . Both linkages are shown in their retracted positions where each linkage has a dual-holder end-effector  792 ,  794 . The internal arrangements used to drive the individual links of the arm may be identical to  FIGS.  21 - 23   . Each of the two upper arms  784 ,  786  is driven independently by one motor. The forearms  788 ,  790  are coupled via band arrangements, each with at least one non-circular pulley, to a third motor. The third links with the end-effectors  792 ,  794  are constrained by band drives, each with at least one non-circular pulley. The band drives are designed so that rotation of one of the upper arms causes the corresponding linkage to extend and retract along a straight line while the other linkage remains stationary. The embodiment shown has forearms longer than the upper arm. Alternately, they may be shorter. The band drives in each of the linkages are designed using the methodology described for  FIGS.  5  and  6   . The kinematic equations presented for  FIGS.  5  and  6    can also be used for each of the two linkages of the dual arm. In order for the arm to rotate, all three drive shafts of the robot need to move in the direction of rotation of the arm by the same amount. In order for one of the end-effector assemblies to extend and retract radially along a straight-line path, the drive shaft of the upper arm associated with the active linkage needs to be rotated according to the inverse kinematic equations for  FIGS.  5  and  6   , and the other two drive shafts need to be kept stationary. Referring also to  FIG.  41   , there is shown the arm of  FIGS.  40 A and  40 B  as one linkage  784 ,  788 ,  794  extends. Note that the inactive linkage  786 ,  790 ,  792  may remain stationary while the active linkage  794 ,  788 ,  794  extends. That is, the left linkage does not move while the right linkage extends, and the right linkage does not move when the left linkage extends. Alternately, the left and right linkages may be moved at the same time radially independently, for example as seen in  FIG.  42    where the right linkage extends slightly independently as compared to  FIG.  41   . The motion of the elbow of the upper linkage may be limited due to potential interference with a wafer on the lower end-effector, which may limit the reach of the robot as illustrated in  FIG.  41   . This limitation may be mitigated by extending the lower linkage slightly to provide additional clearance and achieve full reach as shown in  FIG.  42   . The embodiment shown has no bridge. The wrist of the upper linkage may travel above a wafer on the lower end-effector. 
     Referring now to  FIGS.  43 A and  43 B , there is shown top and side views respectively of robot  810  with arm  812 . Both linkages are shown in their retracted positions with each linkage having a dual-holder end-effector  820 ,  822 . The internal arrangements used to drive the individual links of the arm may be identical to  FIGS.  10 - 13   . The common upper arm  814  is driven by one motor. Each of the two forearms  816 ,  818  is driven independently by one motor through a band drive with conventional pulleys. The third links with the end-effectors  820 ,  822  are constrained by band drives, each with at least one non-circular pulley, which compensate for the effects of the unequal lengths of the upper arms and forearms. In the embodiment shown, the forearms are shorter than the upper arm; alternately they may be longer. The band drives in each of the linkages are designed using the methodology described for  FIGS.  1  and  2   . The kinematic equations presented for  FIGS.  1  and  2    may also be used for each of the two linkages of the dual arm. Referring also to  FIGS.  44  and  45   , there is shown the arm of  FIGS.  43 A and  43 B  as the upper linkage  818 ,  822  extends. Note that the inactive linkage  816 ,  820  rotates while the active linkage  818 ,  822  extends. For instance, the upper linkage rotates as the lower linkage extends, and the lower linkage rotates as the upper linkage extends.  FIGS.  44  and  45    illustrate that the wrist joint  824  of the upper linkage  818 ,  822  does not travel over the wafers  826  carried by the lower linkage  816 ,  820  of the arm. The embodiment shown has no bridge. Compared to  FIGS.  46  and  47   , the end-effector does not need to be shaped to avoid interference with opposite elbow. 
     Referring now to  FIGS.  46 A and  46 B , there is shown top and side views respectively of robot  840  with arm  842 . Both linkages are shown in their retracted positions where each linkage has a dual-holder end-effector  850 ,  852 . The combined upper arm link  844  can be a single piece as depicted in  FIGS.  46 A and  46 B  or it can be formed by two or more sections  844 ′,  844 ″, as shown in the example of  FIGS.  47 A and  47 B . The internal arrangements used to drive the individual links of the arm may be identical to  FIGS.  15 - 19   , for example  FIG.  19   . The combined upper arms  844  are driven by one motor. Each of the two forearms  846 ,  848  is driven independently by one motor through a band drive with conventional pulleys. The third links with the end-effectors  850 ,  852  are constrained by band drives, each with at least one non-circular pulley, which compensate for the effects of the unequal lengths of the upper arms and forearms. In the embodiment shown, the forearms are shorter than the upper arm; alternately they may be longer. The band drives in each of the linkages are designed using the methodology described for  FIGS.  1  and  2   . The kinematic equations presented for  FIGS.  1  and  2    may also be used for each of the two linkages of the dual arm. In order for the min to rotate, all three drive shafts of the robot need to move in the direction of rotation of the arm by the same amount. In order for one of the end-effector assemblies to extend and retract radially along a straight-line path, the drive shaft of the common upper arm  844  and the driveshaft coupled to the forearm associated with the active linkage need to move in a coordinated manner in accordance with the inverse kinematic equations for  FIGS.  1  and  2   . At the same time, the driveshaft coupled to the other forearm needs to rotate in synch with the drive shaft of the common upper arm in order for the inactive linkage to remain retracted. Referring also to  FIGS.  48  and  49   , there is shown the aim of  FIGS.  46 A and  46 B  as the upper linkage  848 ,  852  extends. Here, the inactive linkage  846 ,  850  rotates while the active linkage  848 ,  852  extends. For instance, the upper linkage rotates as the lower linkage extends, and the lower linkage rotates as the upper linkage extends.  FIGS.  48  and  49    illustrate that the wrist joint  854  of the upper linkage does not travel over the wafers  856  carried by the lower linkage of the arm. The embodiment shown has no bridge and the wrist joint of the upper linkage does not travel over a wafer carried by the lower linkage. Here, the inactive arm rotates less, allowing for a higher speed of motion when active arm extends or retracts with no load. 
     Referring now to  FIGS.  50 A and  50 B , there is shown top and side views of robot  870  with arm  872 . Both linkages are shown in their retracted positions where each linkage has a dual-holder end-effector  880 ,  882 . The combined upper arm link  974  can be a single piece as depicted in  FIGS.  50 A and  50 B  or it can be formed by two or more sections, as shown in the example of  FIGS.  47 A and  47 B . The internal arrangements used to drive the individual links of the arm may be identical to  FIGS.  15 - 19   , for example,  FIG.  18   . The combined upper arms  874  are driven by one motor. Each of the two forearms  876 ,  878  is driven independently by one motor through a band drive with conventional pulleys. The third links with the end-effectors are constrained by band drives, each with at least one non-circular pulley, which compensate for the effects of the unequal lengths of the upper arms and forearms. In the embodiment shown, the forearms are shorter than the upper arm; alternately they may be longer. The band drives in each of the linkages may be designed using the methodology described for  FIGS.  1  and  2   . The kinematic equations presented for  FIGS.  1  and  2    may also be used for each of the two linkages of the dual arm. In order for the arm to rotate, all three drive shafts of the robot need to move in the direction of rotation of the arm by the same amount. In order for one of the end-effector assemblies to extend and retract radially along a straight-line path, the drive shaft of the common upper arm  874  and the driveshaft coupled to the forearm associated with the active linkage need to move in a coordinated manner in accordance with the inverse kinematic equations for  FIGS.  1  and  2   . At the same time, the driveshaft coupled to the other forearm needs to rotate in synch with the drive shaft of the common upper arm  874  in order for the inactive linkage to remain retracted. Referring also to  FIG.  51   , there is shown the arm of  FIGS.  50 A and  50 B  with one linkage  878 ,  882  extended. Here, the inactive linkage  876 ,  880  rotates while the active linkage  878 ,  882  extends. For instance, the upper linkage rotates as the lower linkage extends, and the lower linkage rotates as the upper linkage extends. The embodiment shown has short forearm links that may be stiffer with shorter short bands and where the forearms are located side-by-side facilitating a shallow chamber. Here, the short links may cause more rotation of inactive arm compared to  FIGS.  46  and  47    which may be addressed by longer upper arms. Bridge  884  is provided where the arm and end-effectors may be designed so that the bridge  884  clears the inactive end-effector  880  during an extension move. Here, the base of the end-effector features an angled shape  886  as shown. 
     Referring now to  FIGS.  52 A and  52 B , there is shown top and side views respectively of robot  900  with arm  902 . Both linkages are shown in their retracted positions with each linkage having a dual-holder end-effector. The internal arrangements used to drive the individual links of the arm may be identical to  FIGS.  21 - 23   . Each of the two upper arms  904 ,  906  is driven independently by one motor. The forearms  908 ,  910  are coupled via band arrangements, each with at least one non-circular pulley, to a third motor. The third links with the end-effectors  912 ,  914  are constrained by band drives, each with at least one non-circular pulley. The band drives are designed so that rotation of one of the upper arms  904 ,  906  causes the corresponding linkage to extend and retract along a straight line while the other linkage remains stationary. In the embodiment shown, the forearms are shorter than the upper arm; alternately they may be longer. The band drives in each of the linkages are designed using the methodology described for  FIGS.  5 - 6   . The kinematic equations presented for  FIG.  5 - 6    may also be used for each of the two linkages of the dual arm. In order for the arm to rotate, all three drive shafts of the robot need to move in the direction of rotation of the arm by the same amount. In order for one of the end-effector assemblies to extend and retract radially along a straight-line path, the drive shaft of the upper arm associated with the active linkage needs to be rotated according to the inverse kinematic equations for  FIGS.  5 - 6   , and the other two drive shafts need to be kept stationary. Referring also to  FIG.  53   , there is shown the arm of  FIGS.  52 A and  52 B  with one linkage  906 ,  910 ,  914  extended. Note that the inactive linkage  904 ,  908 ,  912  remains stationary while the active linkage  906 ,  910 ,  914  extends with bridge  916 . That is, the left linkage need not move while the right linkage extends, and the right linkage need not move when the left linkage extends although they may be moved radially independently. The embodiment shown has shorter links that may be stiffer with short bands and side-by-side forearms facilitating a shallow chamber. Alternately, the forearms may be longer than upper arms in the configuration with a bridge. 
     Referring now to  FIGS.  54 - 55    there is shown a coupled dual arm  930  with opposing end effectors  938 ,  940 .  FIGS.  54 A and  54 B  show respectively the top and side views of the robot with the arm. Both linkages are shown in their retracted positions where the lateral offset of the end-effectors corresponds to the difference of the joint-to-joint lengths of the upper arm  932  and forearms  934 ,  936 . The combined upper arm link  932  can be a single piece as depicted in  FIG.  54    or it can be formed by two or more sections. By way of example, a two-section design may be lighter where less material, and left and right sections may be identical components. The internal arrangements used to drive the individual links of the arm may be based on that shown with respect to  FIGS.  18  and  19    or otherwise. The common upper arm  932  is driven by one motor. Each of the two forearms  934 ,  936  is driven independently by one motor through a band drive with conventional pulleys. The third links with the end-effectors  938 ,  940  are constrained by band drives, each with at least one non-circular pulley, which compensate for the effects of the unequal lengths of the upper arms  934 ,  936  and forearm  932 . The band drives in each of the linkages are designed using the methodology described with respect to  FIG.  1    or otherwise. The kinematic equations presented for  FIG.  1    can also be used for each of the two linkages of the dual arm.  FIGS.  55 A- 55 C  shows the arm of  FIG.  54    as the first  934 ,  938  and second  936 ,  940  linkages extend from the retracted position. The lateral offset of the end-effector corresponds to the difference of the joint-to-joint lengths of the upper arm  934 ,  936  and forearm  932 , and the wrist joint  942 ,  944  travels along a straight line offset with respect to the trajectory of the center of the wafer by this difference. Note that the inactive linkage rotates while the active linkage extends. For instance, the second linkage rotates as the first linkage extends, and the first linkage rotates as the second linkage extends.  FIG.  55 A  depicts the arm with both linkages in the retracted positions.  FIG.  55 B  shows the first linkage  934 ,  938  extended.  FIG.  55 C  depicts the second linkage  936 ,  940  extended. The arm shown has a low profile as the forearms travel in the same plane and the end-effectors travel in the same plane, allowing for a shallow vacuum chamber with a small volume. Since the retracted position of the wrist of one linkage is constrained by the wrist of the other linkage, the containment radius of the arm may be large, making the arm particularly suitable for applications with a large number of process modules where the diameter of the chamber is dictated by the size of the slot valves. Due to its low profile, the arm may replace a frogleg-type arm with opposing end-effectors. In the embodiment shown, the forearms are shorter than the upper arm; alternately they may be longer, for example, where the forearms are in different elevations and overlapping. 
     Referring to  FIGS.  56 - 57   , there is shown an independent dual arm  960  with opposing end effectors  970 ,  972 .  FIGS.  56 A and  56 B  show the top and side views of the robot with the arm. Both linkages are shown in their retracted positions. In  FIG.  56   , the upper arm  962  of the first linkage is located above the upper arm  964  of the second linkage. Alternatively, the upper arm of the second linkage may be located above the upper arm of the first linkage. The internal arrangements used to drive the individual links of the aim may be based on  FIG.  23    or otherwise. Here, each of the two upper arms  962 ,  964  may be driven independently by one motor. The forearms  966 ,  968  are coupled via band arrangements, each with at least one non-circular pulley, to a third motor. The third links with the end-effectors  970 ,  972  are constrained by band drives, each with at least one non-circular pulley. The band drives are designed so that rotation of one of the upper arms causes the corresponding linkage to extend and retract along a straight line while the other linkage remains stationary. The band drives in each of the linkages are designed using the methodology described for  FIG.  5   . The kinematic equations presented for  FIG.  5    can also be used for each of the two linkages of the dual arm.  FIGS.  57 A- 57 C  show the arm of  FIG.  56    as the first  962 ,  966 ,  970  and second  964 ,  968 ,  972  linkages extend from the retracted position. Here, that the inactive linkage remains (but not need do so) stationary while the active linkage extends. That is, the second linkage does not move while the first linkage extends, and the first linkage does not move when the second linkage extends. The arm has a low profile as the forearms travel in the same plane and the end-effectors travel in the same plane, allowing for a shallow vacuum chamber with a small volume. Since the retracted position of the wrist of one linkage is constrained by the wrist of the other linkage, the containment radius of the arm is large, making the arm particularly suitable for applications with a large number of process modules where the diameter of the chamber is dictated by the size of the slot valves. Due to its low profile, the arm can replace a frogleg-type arm with opposing end-effectors. In the embodiment shown, the forearms are shorter than the upper arm; alternately they may be longer, for example, where the forearms are in different elevations and overlapping. 
     Referring now to  FIG.  58   , there is shown a coupled dual arm  990  with angularly offset end effectors  998 ,  1000 .  FIGS.  58 A and  58 B  show the top and side views of the robot with the arm. Both linkages are shown in their retracted positions. The lateral offset  1002 ,  1004  of the end-effectors corresponds to the difference of the joint-to-joint lengths of the upper arm  994 ,  996  and forearm  992 . The combined upper arm link  992  can be a single piece as depicted in  FIG.  59    or it can be formed by two or more sections. The internal arrangements used to drive the individual links of the arm are based on  FIGS.  18  and  19    or otherwise. Here, the common upper arm  992  may be driven by one motor. Each of the two forearms  994 ,  996  may be driven independently by one motor through a band drive with conventional pulleys. The third links with the end-effectors  998 ,  1000  are constrained by band drives, each with at least one non-circular pulley, which compensate for the effects of the unequal lengths of the upper arms and forearms. The band drives in each of the linkages are designed using the methodology described for  FIG.  1    or otherwise. The kinematic equations presented for  FIG.  1    can also be used for each of the two linkages of the dual arm. Referring also to  FIGS.  59 A-C , there is shown the arm of  FIG.  58    as the left  994 ,  998  and right  996 ,  1000  linkages extend. The lateral offset  1002 ,  1004  of the end-effector corresponds to the difference of the joint-to-joint lengths of the upper arm and forearm, and the wrist joint travels along a straight line offset with respect to the trajectory of the center of the wafer by this difference. Here, the inactive linkage rotates while the active linkage extends. For instance, the right linkage rotates as the left linkage extends, and the left linkage rotates as the right linkage extends.  FIG.  59 A  depicts the arm with both linkages in the retracted positions.  FIG.  59 B  shows the left linkage  994 ,  998  extended.  FIG.  59 C  depicts the right linkage  996 ,  1000  extended. Here, the inactive arm rotates while the active one extends. In the embodiment shown, the forearm is are shorter than the upper arm; alternately they may be longer, for example, where the forearms are in different elevations and overlapping. In the embodiment shown, the end effectors may be 90 degrees apart; alternately any separation angle may be provided. 
     Referring now to  FIG.  60   , there is shown and independent dual arm  1030  with angularly offset end effectors  1040 ,  1042 . Here,  FIGS.  60 A and  60 B  show the top and side views of the robot with the arm. Both linkages are shown in their retracted positions. In  FIG.  60   , the right upper arm  1034  is located below the left upper arm  1032 . Alternatively, the left upper may be located below the right upper arm. The internal arrangements used to drive the individual links of the arm may be based on  FIG.  23   . Each of the two upper arms  1032 ,  1034  may be driven independently by one motor each. The forearms are coupled via band arrangements, each with at least one non-circular pulley, to a third motor. The third links with the end-effectors  1040 ,  1042  are constrained by band drives, each with at least one non-circular pulley. The band drives are designed so that rotation of one of the upper arms  1032 ,  1034  causes the corresponding linkage to extend and retract along a straight line while the other linkage remains stationary. The band drives in each of the linkages are designed using the methodology described for  FIG.  5    or otherwise. The kinematic equations presented for  FIG.  5    can also be used for each of the two linkages of the dual arm.  FIG.  61 A- 61 C  shows the arm of  FIG.  60    as the left  1032 ,  1036 ,  1040  and then the right  1034 ,  1038 ,  1042  linkage extends. Here, the inactive linkage remains (but need not do so) stationary while the active linkage extends. That is, the left linkage does not move while the right linkage extends, and the right linkage does not move when the left linkage extends. Here, the inactive linkage remains stationary while the active one extends. In the embodiment shown, the forearms are shorter than the upper aim; alternately they may be longer, for example, where the forearms are in different elevations and overlapping. In the embodiment shown, the end effectors may be 90 degrees apart; alternately any separation angle may be provided. 
     By way of example with respect to  FIG.  62    or otherwise, the third link and end-effector  1060 ,  1062 , each of which may be referred to as a third-link assembly, may be designed so that the center of mass  1064 ,  1066  is on or close to the straight-line trajectory of the wrist joint  1068 ,  1070  respectively as the corresponding linkage of the arm extends and retracts. This reduces the moment due to the inertial force acting at the center of mass of the third-link assembly and the reaction force at the wrist joint, thus reducing the load on the band arrangement that constraints the third-link assembly. Here, the third-link assembly may further be designed so that its center of mass is on one side of the wrist joint trajectory when payload is present and on the other side of the trajectory when no payload is present. Alternatively, the third-link assembly may be designed so that its center of mass is substantially on the wrist joint trajectory when payload is present as the best straight-line tracking performance is typically required with the payload on, as illustrated in  FIG.  62   . In  FIG.  62 ,  1 L  is the straight-line trajectory of the center of the wrist joint of the left linkage,  2 L is the center  1070  of the wrist joint of the left linkage,  3 L is the center of mass  1066  of the third-link assembly of the left linkage,  4 L is the force acting on the third-link assembly of the left linkage as the left linkage accelerates at the beginning of an extend move (or decelerates at the end of a retract move), and  5 L is the inertial force acting at the center of mass of the third-link assembly of the left linkage as the left linkage accelerates at the beginning of an extend move (or decelerates at the end of a retract move). Similarly,  1 R is the straight-line trajectory of the center of the wrist joint of the right linkage,  2 R is the center  1068  of the wrist joint of the right linkage,  3 R is the center of mass  1064  of the third-link assembly of the right linkage,  4 R is the force acting on the third-link assembly of the right linkage as the right linkage decelerates at the end of an extend move (or accelerates at the beginning of a retract move), and  5 R is the inertial force acting at the center of mass of the third-link assembly of the right linkage as the right linkage decelerates at the end of an extend move (or accelerates at the beginning of a retract move). In the embodiment shown, dual wafer end effectors are provided. In alternate aspects, any suitable end effector and arm or link geometry may be provided. 
     In alternate aspects, the upper arms in any of the aspects of the embodiment can be driven by a motor either directly or via any kind of coupling or transmission arrangement. Any transmission ratio may be used. Alternately, the band drives that actuate the second link and constrain the third link can be substituted by any other arrangement of equivalent functionality, such as a belt drive, cable drive, circular and non-circular gears, linkage-based mechanisms or any combination of the above. Alternately, for example, in the dual and quad arm aspects of the embodiment, the third link of each linkage can be constrained to keep the end-effector radial via a conventional two stage band arrangement that synchronizes the third link to the pulley driven by the second motor, similarly to the single arm concept of  FIG.  9   . Alternatively, the two stage band arrangement can be substituted by any other suitable arrangement, such as a belt drive, cable drive, gear drive, linkage-based mechanism or any combination of the above. Alternately, the upper arms in the dual and quad arm aspects of the embodiment may not be arranged in a coaxial manner. They can have separate shoulder joints. The two linkages of the dual and quad arms do not need to have the same length of the upper arms and the same length of the forearms. The length of the upper arm of one linkage may be different from the length of the upper arm of the other linkage, and the length of the forearm of one linkage may be different from the length of the forearm of the other linkage. The forearm-to-upper-arm ratios can also be different for the two linkages. In the dual and quad arm aspects of the embodiment that have different elevations of the links of the left and right linkages, the left and right linkages can be interchanged. The two linkages of the dual and quad arms do not need to extend along the same direction. The arms can be configured so that each linkage extends in a different direction. The two linkages in any of the aspects of the embodiment may consist of more or less than three links (first link=upper arm, second link=forearm, third link=link with end-effector). In the dual and quad arm aspects of the embodiment, each linkage may have a different number of links. In the single arm aspects of the embodiment, the third link can carry more than one end-effector. Any suitable number of end-effectors and/or material holders can be carried by the third link. Similarly, in the dual arm aspects of the embodiment, each linkage can carry any suitable number of end-effectors. In either case, the end-effectors can be positioned in the same plane, stacked above each other, arranged in a combination of the two or arranged in any other suitable manner. Further, for dual arm configurations, each arm may be independently operable, for example, independently in rotation, extension and/or z (vertical), for example, as described with respect to pending U.S. patent application Having Ser. No. 13/670,004 entitled “Robot System with Independent Arms” having filing date Nov. 6, 2012 which is herein incorporated by reference in its entirety. Accordingly all such modifications, combinations and variations are embraced. 
     Referring now to  FIG.  63   , there is shown a graphical representation  1100  of exemplary pulleys. The exemplary pulley profiles may be for an arm with unequal link lengths as will be described. By way of example, the graph  1100  may show profiles for a wrist pulley where the elbow pulley is circular. Here, the following example design was used for the figure: Re/l2=0.2 where Re is the radius of the elbow pulley and l2 is the joint-to-joint length of the forearm. Alternately, any suitable ratio may be provided. For the purpose of clarity, the graph shows extreme design cases in comparison with a pulley for an equal-link arm. The most outer profile  1110  is for l2/l1=2, where l2 is the joint-to-joint length of the forearm and l1 is the joint-to-joint length of the upper arm, for example, this case represents a longer forearm. The middle profile  1112  is for l2/l1=1, for example, a case with equal link lengths. The most inner profile  1114  is for l2/l1=0.5, for example, this case represents a shorter forearm. In the embodiment shown, a polar coordinate system  1120  is used. Here, the radial distance is normalized with respect to the radius of the elbow pulley, for example, expressed as a multiple of the radius of the elbow pulley. In other words, Rw/Re is shown, where Rw represents polar coordinates of the wrist pulley with Re representing the elbow pulley. The angular coordinates are in deg, and the zero points along the direction  1122  of the end-effector, for example, the end-effector points to the right with respect to the figure. 
     Referring now to  FIGS.  64  and  65   , there is shown two additional configurations of the arm with unequal link lengths  1140  and  1150 . Arm  1140  is shown with a forearm  1144  longer than upper arm  1142  where the single arm configuration may utilize the features as disclosed with respect to  FIGS.  1 - 4  and  5 - 8    or otherwise. In the embodiment shown, two end-effectors  1146 ,  1148  supporting respective substrates  1150 ,  1152  are connected rigidly to each other and pointing in opposing directions. The substrates travel in a radial path that coincides with the center  1156  of robot  1140  and offset  1154  from the wrist as shown. Similarly, arm  1160  is shown with a forearm  1164  shorter than upper  1162  where the single arm configuration may utilize the features as disclosed with respect to  FIGS.  1 - 4  and  5 - 8    or otherwise. In the embodiment shown, two end-effectors  1166 ,  1168  supporting respective substrates  1170 ,  1172  are connected rigidly to each other and pointing in opposing directions. The substrates travel in a radial path that coincides with the center  1176  of robot  1160  and offset  1174  from the wrist as shown. Here, the features of the disclosed embodiments may be similarly shared with any of the other disclosed embodiments. 
     Referring now to  FIGS.  66  and  67   , the disclosed describes a dual-arm robot  1310  with stacked and side-by-side end-effector configurations. The device may be used in combination with transport mechanisms and devices as disclosed in United States Publication No. 2013/0071218 published Mar. 21, 2103 based on U.S. patent application Ser. No. 13/618,117 filed Sep. 14, 2012 and entitled “Low Variability Robot” or U.S. patent application Ser. No. 14/601,455 filed Jan. 21, 2015 and entitled “Substrate Transport Platform” both of which are hereby incorporated by reference herein in their entirety. Alternately, the embodiment may be used in any suitable device or applications. The disclosed device may provide a robot  1310  with two end-effectors which (i) has a small footprint so that it can move and rotate in a narrow tunnel, (ii) can access the same station with both end-effectors either independently or simultaneously, and (iii) can access side-by-side offset stations either independently or simultaneously. 
     An example embodiment of the robot  1310  is depicted diagrammatically in  FIGS.  66 A- 66 D and  67 A- 67 D . The robot may consist of a robot drive unit  1312  with a pivoting base  1314  about axis  1334  and a robot arm  1316 . The robot arm  1316  may feature two linkages, i.e., a left linkage  1318  and a right linkage  1320 .  FIGS.  66 A- 66 D  show the robot with both linkages retracted,  FIGS.  67 A- 67 D  show the robot with the left linkage  1318  extended. 
     The left linkage  1318  may consist of a left upper arm  1322 , a left forearm  1324  and a left end-effector  1326 . The left upper arm  1322  may be coupled to the base via a rotary joint or axis  1336 , the left forearm  1324  may be coupled to the left upper arm  1322  by another rotary joint or axis  1338 , and the left end-effector  1326  may be coupled to the left forearm  1324  by yet another rotary joint or axis  1340 . 
     Similarly, the right linkage  1320  may consist of a right upper arm  1328 , a right forearm  1330  and a right end-effector  1332 . The right upper arm  1328  may be coupled to the base via a rotary joint or axis  1342 , the right forearm  1330  may be coupled to the right upper arm  1328  by another rotary joint or axis  1344 , and the right end-effector  1332  may be coupled to the right forearm  1330  by yet another rotary joint or axis  1346 . 
     The joint-to-joint length of the left forearm may be longer than the joint-to-joint length of the left upper arm. Alternatively, the joint-to-joint length of the left forearm may be equal to the joint-to-joint length of the left upper arm. In yet another alternative, the left forearm and left upper arm may have any other suitable lengths. 
     Similarly, the joint-to-joint length of the right forearm may be longer than the joint-to-joint length of the right upper aim. Alternatively, the joint-to-joint length of the right forearm may be equal to the joint-to-joint length of the right upper arm. In yet another alternative, the right forearm and right upper arm may have any other suitable lengths. 
     In the example of  FIGS.  66 A- 66 D and  67 A- 67 D , the joint-to-joint lengths of the left and right upper arms and left and right forearms are shown the same. Similarly, the dimensions of the left and right end-effectors, including the lengths and lateral offsets, are shown the same. However, the linkages may feature any suitable dimensions of the upper arms, forearms and end-effectors. 
     In order for the two end-effectors to be able to access simultaneously side-by-side offset stations, the distance between the joints that couple the left and right upper arms to the base may be selected to satisfy the following relationship:
 
 D= 2 d 0  (1)
 
     where D=center-to-center distance between side-by-side offset stations (m), and d0=distance between joints that couple left and right upper arms to base (m). 
     In addition, in order for the two end-effectors to be able to access the same station simultaneously, the dimensions of the linkages may be selected to satisfy the following relationship:
 
 d 0= I 2 L− 11 L+d 3 L+ 12 R− 11 R+d 3 R   (2)
 
     The following nomenclature is used in Equation (2) above: d3L=lateral offset of left end-effector (m), d3R=lateral offset of right end-effector (m), 11L=join-to-joint length of left upper arm (m), 11R=join-to-joint length of right upper arm (m), 12L=joint-to-joint length of left forearm (m), and 12R=joint-to-joint length of right forearm (m). 
     When the robot arm is symmetric, i.e., the left linkage and the right linkage have the same dimensions, Equation (2) may be simplified as follows:
 
 d 0=2( l 2 −l 1+ d 3)  (3)
 
     where d3=lateral offset of end-effectors (m), 11=join-to-joint length of upper arms (m), and 12=joint-to-joint length of forearms (m). 
       FIGS.  68 A and  68 B  illustrate diagrammatically an example arrangement  1398 ,  1438  that may be used to drive the base and individual links, i.e., upper arms, forearms and end-effectors, of the robot. As depicted in  FIGS.  68 A and  68 B , the base may be driven by a drive shaft  1400 ,  1448 , for example, T 0 . 
     The left upper arm  1402 ,  1454  may be actuated by drive shaft T 1 L  1420 ,  1440 . The left forearm  1406 ,  1456  may be coupled via a band arrangement  84  with at least one non-circular pulley to another drive shaft, T 2 L  1422 ,  1442 . The band arrangement may be designed so that rotation of the left upper arn causes the left wrist joint, i.e., the joint that couples the left end-effector to the left forearm, to extend and retract along a straight line parallel to the desired straight-line path of the left end-effector. 
     The left end-effector  1410  may be constrained by another band arrangement  92  with at least one non-circular pulley, which compensates for the effects of the unequal lengths of the left upper arm and left forearm so that the left end-effector may travel along a straight line while maintaining the desired orientation. 
     Alternatively, if 11L=12L, conventional pulleys may be utilized, as shown in  FIG.  68 B . In this embodiment, the band arrangement that couples the left forearm to shaft T 2 L is designed so that the diameter of the pulley coupled to shaft T 2 L is twice the diameter of the pulley coupled to the left forearm. The band arrangement that constrains the left end-effector is designed so that the diameter of the pulley attached to the left upper arm is half of the diameter of the pulley attached to the left end-effector. 
     Similarly, the right upper arm  1404 ,  1450  may be actuated by drive shaft T 1 R  1424 ,  1444 . The right forearm  1408 ,  1452  may be coupled via a band arrangement  86  with at least one non-circular pulley to another drive shaft, T 2 R  1426 ,  1446 . The band arrangement may be designed so that rotation of the right upper thin causes the right wrist joint, i.e., the joint that couples the right end-effector to the right forearm, to extend and retract along a straight line parallel to the desired straight-line path of the right end-effector  1412 . 
     The right end-effector may  1412  be constrained by another band arrangement  94  with at least one non-circular pulley, which compensates for the effects of the unequal lengths of the right upper arm and right forearm so that the left end-effector may travel along a straight line while maintaining the desired orientation. 
     Alternatively, if 11R=12R, conventional pulleys may be utilized, as shown in  FIG.  68 B . In this embodiment, the band arrangement that couples the right forearm to shaft T 2 R is designed so that the diameter of the pulley coupled to shaft T 2 R is twice the diameter of the pulley coupled to the right forearm. The band arrangement that constrains the right end-effector is designed so that the diameter of the pulley attached to the right upper arm is half of the diameter of the pulley attached to the right end-effector. 
     In order for the entire robot arm to rotate, all drive shafts, i.e., T 0 , T 1 L, T 2 L, T 1 R and T 2 R, need to move in the desired direction of rotation of the arm by the same amount with respect to a fixed reference frame (or drive shaft T 0  needs to move while the other drive shafts may be viewed as stationary with respect to the base). This is depicted diagrammatically in  FIGS.  69 A through  69 C . In this particular example, the entire robot arm rotates in the counterclockwise direction by 180 deg. 
     In order for the left end-effector to extend and retract along a straight-line path, drive shaft T 1 L needs to move by an angle determined based on the inverse kinematic equations of the left linkage while shafts T 0  and T 2 L are kept stationary. The robot  1500  with left and right arms  1502 ,  1504  with the left end-effector extended from the initial position of  FIG.  69 A  is shown diagrammatically in  FIG.  69 D . 
     Similarly, in order for the right end-effector to extend and retract along a straight-line path, drive shaft T 1 R needs to move by an angle determined based on the inverse kinematic equations of the right linkage while shafts T 0  and T 2 R are kept stationary. The robot with the right end-effector extended from the initial position of  FIG.  69 A  is depicted diagrammatically in  FIG.  69 E . 
     Both left and right end-effectors of the robot may be extended and retracted simultaneously along a straight-line path by rotating drive shafts T 1 L and T 1 R in the opposite directions and, if the left and right linkages feature the same dimensions, by the same amount. The robot with both left and right end-effectors extended from the initial position of  FIG.  69 A  is shown diagrammatically in  FIG.  69 F . 
     The motion described above with respect to  FIGS.  69 D- 69 F  allows the robot to extend/retract the end-effectors to/from the same station either independently or simultaneously. Therefore, the robot is capable of picking/placing material, such as semiconductor wafers, from/to the same station independently or simultaneously with both end-effectors along a straight line path  1510 . 
     The left and right linkages  1502 ,  1504  may also be rotated individually. In order for the left linkage to rotate, drive shafts T 1 L and T 2 L need to move in the desired direction of rotation by the same amount. Similarly, in order for the right linkage to rotate, drive shafts T 1 R and T 2 R need to move in the desired direction of rotation by the same amount. 
     When the left and right linkages rotate individually by 180 deg, the left end-effector and right end-effector become laterally offset, as depicted in the example diagrams shown in  FIGS.  70 A- 70 C . In this particular example, the left linkage  1502  rotates in the clockwise direction and the right linkage  1504  rotates simultaneously in the counterclockwise direction (preventing the risk of collision of the left and right wrist joints). However, the left and right linkages may rotate independently in sequence, in the same direction or in any other suitable manner. 
     As a result of the individual rotations of the left and right linkages described above, provided that the dimensions of the robot meet the conditions of Equations (1) and (2), the arm becomes reconfigured such that the centers of the left and right end-effectors are laterally offset by distance D. 
     In case that the above end-effector offset reconfiguration by individual rotations of the left and right linkages precedes or follows a rotation of the entire arm, the moves may be conveniently blended to minimize the overall duration. 
     Once in the position of the diagram of  FIG.  70 C , the left end-effector may again be extended and retracted along a straight-line path  1512  by moving drive shaft T 1 L while holding shafts T 0  and T 2 L stationary. Similarly, the right end-effector may be extended and retracted along a straight-line path by moving drive shaft T 1 R while holding shafts T 0  and T 2 R stationary. And, finally, both left and right end-effectors of the robot may be extended and retracted simultaneously along straight-line paths by rotating drive shafts T 1 L and T 1 R in opposite directions and, if the left and right linkages feature the same dimensions, by the same amount. 
     The robot with the left end-effector extended from the initial position of  FIG.  70 C  is shown diagrammatically in  FIG.  70 D ; the robot with the right end-effector extended from the initial position of  FIG.  70 C  is depicted diagrammatically in  FIG.  70 E ; and the robot with both left and right end-effectors extended from the initial position of  FIG.  70 C  is shown diagrammatically in  FIG.  70 F . 
     The motion described above with respect to  FIGS.  70 E- 70 F  allows the robot to extend/retract the end-effectors to/from two side-by-side offset stations. Therefore, the robot is capable of picking/placing material, such as semiconductor wafers, from/to two side-by-side offset stations either independently or simultaneously. 
     In case that the access paths to the side-by-side offset stations are not parallel, for example, path  1514  or  1516  in  FIG.  71   , the robot may individually rotate the left and right linkages so that the directions of their extension/retraction paths align with the access paths to the stations. An example of such a scenario is illustrated diagrammatically in the diagrams of  FIGS.  71 A- 71 C . Assuming the initial position of diagram  71 A, the left and right linkages may be rotated to reconfigure the arm so that the end-effectors are laterally and angularly offset as depicted in diagram  71 B. In this particular example, the angular offset between the left and right end-effectors is 30 deg. From the retracted position of diagram  71 B, the left linkages may be extended, either independently or simultaneously, as shown in diagram  71 C. 
     The robot may also access stations 180 deg apart, either independently or simultaneously, as depicted in the example diagrams  71 D and  71 E. In this particular example, assuming the starting position of diagram  71 A, the left and right linkages may first be rotated to the configuration of diagram  71 D, and then the left end-effector and/or the right end-effector may be extended, either independently or simultaneously, as shown in diagram  71 E. 
     While both left and right linkages are shown extended in the diagram  FIG.  71 E , in alternate aspects only one of the two linkages may extend. Here, the reach of the linkages (measured from the center of the robot, which is represented by the axis of drive shaft T 0 ) is longer in the configuration shown in diagram  71 E and, therefore, this configuration may be utilized for stations located further away from the robot. 
     The robot may be driven using three- to five-axis drive arrangement, depending on the number of degrees of freedom required in a particular application. 
     A 3-axis drive arrangement may include three independently controlled motors, M 0 , M 1  and M 2 , as illustrated by the two examples  1600 ,  1700  of  FIGS.  72 A and  72 B  and  FIGS.  72 C and  72 D . 
     In  FIGS.  72 A- 72 D , diagrams  72 A and  72 B show the top and side views, respectively, of an example arrangement  1600  of the robot drive unit and arm base  1618  where motor M 0  is directly coupled to shaft T 0   1602 , which actuates the base  1618 , motor M 1   1604  is directly attached to shaft T 1 L  1610 , driving the left upper anti, and motor M 2   1606  is directly attached to shaft T 2 R  1616 , which is coupled to the right forearm. Furthermore, two belt arrangements  1620 ,  1622  are utilized so that shafts T 1 L  1610  and T 1 R  1614  rotate in opposite directions than shafts T 2 L  1612  and T 2 R  1616 , respectively. This is achieved via a crossover band arrangement  1620  between shafts T 1 L and T 1 R, and, similarly, by another crossover band arrangement  1622  between shafts T 2 L and T 2 R. 
     Alternatively, drive  1700  may have motors M 0   1702 , M 1   1704  and M 2   1706  arranged in the drive unit, and motion may be transmitted from motors M 1  and M 2  to shafts T 1 L  1710 , T 1 R  1714  and T 2 L  1712 , T 2 R  1716 , respectively, using band drives  1720 ,  1722 , as illustrated in the example of diagrams  72 C and  72 D. 
     In yet another alternative, any suitable combination of direct coupling and band arrangements between the motors and drive shafts may be employed. In general, any suitable means of transmission of motion between the motors and drive shafts, which provides the desired motion relationship, may be used. 
     When a 3-axis drive arrangement according to the examples of  FIG.  72 A- 72 D  is utilized, the robot may perform all operations defined in  FIGS.  69 - 71    except for independent extensions and retractions of the left and right linkages (diagrams D and E in  FIGS.  69  and  70   ). 
     A 4-axis drive arrangement may include four independently controlled motors, as illustrated in the examples  1800 ,  1900  of the diagrams  FIGS.  73 A and  73 B . Diagrams  73 A and  73 B show the top and side views of the robot drive unit and arm base  1802 . Motors M 0   1804 , M 1 L  1808  and M 1 R  1810  may be utilized to actuate shafts T 0   1804 , T 1 L  1808  and T 1 R  1810 , respectively, in an independent manner. Motor M 2   1806  may be used to actuate shafts T 2 L  1812  and T 2 R  1814  so that the two shafts rotate in opposite directions. In the particular example of the diagrams in  FIGS.  73 A and  73 B , this is achieved via a straight band arrangement  1820  between a pulley coupled to motor M 2  and shaft T 2 L, and a crossover band arrangement  1822  between another pulley coupled to motor M 2  and shaft T 2 R. 
     Alternatively, any combination of direct coupling and band arrangements or any other suitable means of transmission of motion between the motor and drive shafts, which facilitates independent actuation of shafts T 0 , T 1 L and T 1 R and coupled actuation of shafts T 2 L and T 2 R, may be employed. 
     When such a 4-axis drive arrangement is utilized, the robot may perform all operations according to  FIGS.  69 - 71   , including independent extensions and retractions of the left and right linkages. 
     A 5-axis drive arrangement  1900  may include five independently controlled motors, M 0   1904 , M 1 L  1906 , M 2 L  1908 , M 1 R  1910  and M 2 R  1912 , that may be coupled to drive shafts T 0 , T 1 L, T 2 L, T 1 R and T 2 R directly, as depicted in the example of the diagrams in  FIGS.  73 C and  73 D , where diagram  73 C illustrates the top view and diagram  73 D shows the side view of the drive unit  1900  and base  1902 ; via band drives by extending the example of the diagrams in  FIGS.  72 C and  72 D ; using a combination of direct coupling and band arrangements, or in any other suitable manner that may facilitate transmission of motion form the motors to the drive shafts. 
     When a 5-axis drive arrangement is utilized, the robot may perform all operations according to  FIGS.  69  to  71   . In addition, the left and right linkages can be operated in a completely independent manner, including independent rotations, which cannot be supported with 3-axis and 4-axis drive arrangements. 
     Another example internal arrangement of the base and linkages of the robot  2010  of  FIG.  66    is depicted diagrammatically in  FIG.  74 A . Again, the base  2012  may be driven by drive shaft T 0 . 
     The left  2014  upper aim may be actuated by drive shaft T 1 L. The left forearm may be driven by another drive shaft, T 2 L, through a band arrangement  84  with conventional pulleys. The left end-effector may be constrained by another band arrangement  92  with at least one non-circular pulley, which compensates for the effects of the unequal lengths of the left upper arm and left forearm so that the left end-effector may travel along a straight line while maintaining the desired orientation. Alternatively, if 11L=12L, conventional pulleys may be utilized, as shown in  FIG.  74 B  with arm  2030  having base  2032 , left arm  2034  and right arm  2036 . 
     Similarly, the right  2016  upper arm may be actuated by drive shaft T 1 R. The right forearm may be driven by another drive shaft, T 2 R, through a band arrangement  86  with conventional pulleys. The right end-effector may be constrained by another band arrangement  94  with at least one non-circular pulley, which compensates for the effects of the unequal lengths of the right upper arm and right forearm so that the right end-effector may travel along a straight line while maintaining the desired orientation. Alternatively, if 11R=12R, conventional pulleys may be utilized, as shown in  FIG.  74 B . 
     In order for the entire robot arm to rotate, all drive shafts, i.e., T 0 , T 1 L, T 2 L, T 1 R and T 2 R, need to move in the desired direction of rotation of the arm by the same amount with respect to a fixed reference frame (or drive shaft T 0  needs to move while the other drive shafts are stationary with respect to the base). 
     In order for the left end-effector to extend and retract along a straight-line path, drive shafts T 1 L and T 2 L need to move in a coordinated manner in accordance with the inverse kinematic equations of the left linkage. Similarly, in order for the right end-effector to extend and retract along a straight-line path, drive shafts T 1 R and T 2 R need to move in a coordinated manner in accordance with the inverse kinematic equations of the right linkage. Example kinematic equations can be found above. 
     Both end-effectors of the robot may be extended and retracted along a straight-line path by rotating drive shafts T 1 L, T 2 L and T 1 R, T 2 R simultaneously in a manner described above for independent extension of the left and right end-effectors. 
     The left and right linkages may also be rotated individually. In order for the left linkage to rotate, drive shafts T 1 L and T 2 L need to move in the desired direction of rotation by the same amount. Similarly, in order for the right linkage to rotate, drive shafts T 1 R and T 2 R need to move in the desired direction of rotation by the same amount. Similarly to  FIGS.  68 A and  68 B , when the left and right linkages rotate individually by 180 deg, the left end-effector and right end-effector become laterally offset, see diagrams  70 A through  70 C. 
     Considering the above motion capabilities, the robot with the internal arrangement according to  FIGS.  74 A and  74 B  may perform the same operations as, as outlined in  FIGS.  69 - 71   . 
     The base and linkages with the internal arrangements of  FIGS.  74 A and  74 B  may be driven by the 3-axis and 5-axis drive arrangements of  FIGS.  72  and  73 C,  73 D  respectively. 
     Another example embodiment of the robot  2100  is depicted in the diagrams of  FIGS.  75 A and  75 B . Diagram ( 75 A shows a top view of the robot with both linkages retracted, diagram  75 B depicts the robot with both end-effectors extended. 
     An example internal arrangement of the robot is illustrated diagrammatically  2330  in  FIG.  76 A . In the figure, base  2332  with linkages  2334 ,  2336  with equal length of the upper arms and forearms and circular pulleys are shown; however, unequal lengths and non-circular pulleys may be utilized. 
     The robot may be actuated by the drive arrangements described earlier with reference to  FIGS.  72  and  73   . 
     An alternative internal arrangement of the robot of diagrams  75 A and  75 B is shown diagrammatically  2360  in  FIG.  76 B . In the figure, base  2362  and linkages  2364 ,  2366  with equal length of the upper arms and forearms and with circular pulleys are shown; however, unequal lengths and non-circular pulleys may be utilized. 
     The robot may be actuated by the drive arrangements according to  FIGS.  72   . and  73 C,  73 D 
     Yet another example embodiment of the robot  2200  is depicted in the diagrams of  FIG.  75 C and  75 D . Diagram  75 C shows a top view of the robot with both linkages retracted, diagram  75 D depicts the robot with both end-effectors extended. Diagrams  75 C and  75 D show the linkages of the robot in a left handed configuration. Alternatively, the linkages may be configured in a right-handed arrangement, as shown in diagrams  75 E and  75 F with robot  2300 . 
     An example internal arrangement of the embodiments according to diagrams  75 C and  75 D is illustrated diagrammatically  2390  in  FIG.  76 C . Similarly, an example internal arrangement of the embodiment according to diagrams  75 E and  75 F is illustrated diagrammatically  2430  in  FIG.  76 D . In  FIGS.  76 C and  76 D , linkages  2394 ,  2396 ,  2434 ,  2436  with equal length of the upper arms and forearms and with circular pulleys are shown; however, unequal lengths and non-circular pulleys may be utilized. 
     The robot may be actuated by the drive arrangements according to  FIGS.  77 A- 77 D,  78 A- 78 B and  73 C and  73 D . In  FIGS.  77 A and  77 B , drive  2500  has base  2504  driven by motor M 0   2502 . M 1   2506  drives T 1   l    2510  while M 2   2508  drives T 2   r    2516  with T 1   l    2510  and t 1   r    2514  constrained by a band and T 2   l    2512  and T 2   r    2516  constrained by a band. In  FIGS.  77 C and  77 D , drive  2560  has base  2562  driven by motor M 0   2564 . M 1   2566  drives T 1   l    2570  while M 2   2568  drives T 2   r    2576  with T 1   l    2570  and t 1   r    2574  constrained by a band and T 2   l    2572  and T 2   r    2576  constrained by a band. In  FIGS.  78 A and  78 B , drive  2700  has base  2702  driven by motor M 0   2704 . M 1   l    2706  drives T 1   l  while M 1   r    2708  drives T 1   r  and M 2   2710  drives T 2   r    2714  and T 2   l    2712  by a band. 
     When a 3-axis drive arrangement, for instance, according to the examples of  FIG.  77    is utilized, the robot may perform all operations defined in  FIGS.  69  and  70    except for independent extensions and retractions of the left and right linkages (diagrams D and E in  FIGS.  69  and  70   ). It may not perform simultaneous extensions and retractions along nonparallel and opposing paths of  FIG.  71   . 
     When a 4-axis drive arrangement, such as the example of  FIG.  78   , is used, the robot may perform all operations according to  FIGS.  69  and  70   , including independent extensions and retractions of the left and right linkages. It may not perform simultaneous extensions and retractions along nonparallel and opposing paths of  FIG.  71   . 
     When a 5-axis drive arrangement is utilized, the robot may perform all operations according to  FIGS.  69  to  71   . In addition, the left and right linkages can be operated in a completely independent manner, including independent rotations, which cannot be supported with 3-axis and 4-axis drive arrangements. 
     The disclosed shows a favorable reach-to-containment ratio. In combination with the 3-axis driving arrangement of  FIGS.  77 A and  77 B , it also offers a low profile and low complexity. In addition, in combination with a 4-axis drive arrangement, the disclosed supports independent extension of left and right linkages. 
     Alternative internal arrangement of the example embodiments of the diagrams of  FIGS.  75 A- 75 D  are shown diagrammatically  2800 ,  2830  in  FIGS.  79 A and  79 B  respectively. In the figures, base  2802 ,  2832  with linkages  2804 ,  2806 ,  2834 ,  2836  with equal length of the upper arms and forearms and with circular pulleys are shown; however, unequal lengths and non-circular pulleys may be utilized. 
     The robot may be actuated by the drive arrangements in accordance with  FIGS.  77  and  73 C and  73 D . 
     Although the left and right linkages are shown in the figures with the same dimensions, the left linkage may have different dimensions than the right linkage, and the drive unit may be configured to reflect the differences in the dimensions. 
     The robot arm may be designed so that some of its links, such as the upper arms and/or forearms, are below one or both of the end-effectors and other links are above one or both of the end-effectors. 
     When the terms band arrangements and band drives are used, they refer generally to the means of transmitting motion, force and/or torque, including bands, belts, cables, gears or any other suitable arrangement. 
     While the motors of the robot are shown as attached directly to the shafts, pulleys and other driven components in the figures throughout the text, the motors may be coupled to the driven components through additional bands, belts, cables, gears or any other suitable arrangement that can transmit motion, force and/or torque. 
     Although the motors of the robot are depicted in the drive unit or base in the figures throughout the text, the motors may be located within the robot aria, e.g., as part of the upper arm(s) or forearm(s), or integrated into the rotary joints of the robot. 
     The drive unit of the robot may further include a vertical lift mechanism to adjust elevation of the entire robot arm. Alternatively, the drive unit may comprise two vertical lift mechanisms, one of the left linkage and the other for the right linkage, to adjust the elevation of the left and right linkages independently. Here, the end effectors may be stacked or set at the same level or otherwise be independently positioned in a z axis. 
     In an alternative embodiment, any number and any type of suitable mechanisms may be used within the robot drive and/or the robot arm to control the elevation of the left and right end-effectors of the robot. 
     The robot may further include a traverser mechanism that may allow the robot, e.g., to move along the tunnel in which it is installed. 
     In another embodiment, the robot may be designed to operate in an upside-down configuration, e.g., with support provided from the top rather than from the bottom. 
     The robot may be combined with another robot of the same or similar type, e.g., in an upside-down configuration, to provide a system with four end-effector, which can support fast material exchange. 
     The robot may be design for operation in special environments, e.g., in vacuum, which may include the use of static and/or dynamic seals and other means of isolating some of the components of the robot from the environment in which it operates. 
       FIG.  80 A  shows a system  2900  with a robot. The robot drive unit  2904  may be configured to be movable with respect to the stationary part  2902  of the system as indicated by the arrow  2906 ,  2908 . As an example, the robot drive unit may be on rails, linear bearings, magnetic bearings or may be coupled to the stationary part of the system in any suitable manner that allows the robot drive unit to move with respect to the stationary part of the system. As an example, the robot drive unit may be actuated by an electric linear motor with windings in the drive unit, by an electric linear motor with windings in the stationary part of the system, via a magnetic coupling, using a pneumatic or hydraulic actuator, via a ball-screw, via a cable or belt, or utilizing any other suitable arrangement that may actuate the robot drive unit with respect to the stationary part of the system. As described in the original write-up, the robot drive unit may include a pivoting base and a robot arm. In the diagram (a), the pivoting base is actuated with respect to the robot drive unit, as indicated by the arrow. 
       FIG.  80 B  shows system  3000  with an arrangement where the pivoting base  3004  is actuated directly with respect to the stationary part  3002  of the system as indicated by the arrows  3006 ,  3008  on the sides of the pivoting base. When both sides of the pivoting base are actuated in sync by the same amount in the same direction, the entire robot translates in the corresponding direction. When the sides of the pivoting base are actuated in sync by the same amount in the opposite directions, the pivoting base rotates while its center remains stationary. Any combination of translation and rotation may be achieved by actuating the sides of the pivoting base accordingly. As an example, the base may be actuated by an electric linear motor with windings in the pivoting base, by an electric linear motor with windings in the stationary part of the system, via magnetic couplings, via ball-screws, via cables or belts, or utilizing any other suitable arrangement that may actuate the pivoting base with respect to the stationary part of the system. 
     In accordance with one aspect of the exemplary embodiment, an apparatus comprises at least one drive; a first robot arm comprising a first upper arm, a first forearm and a first end effector, where the first upper arm is connected to the at least one drive at a first axis of rotation; and a second robot arm comprising a second upper arm, a second forearm and a second end effector, where the second upper arm is connected to the at least one drive at a second axis of rotation which is spaced from the first axis of rotation; where the first and second robot arms are configured to locate the end effectors in first retracted positions for stacking substrates located on the end effectors at least partially one above the another, where the first and second robot arms are configured to extend the end effectors from the first retracted positions in a first direction along parallel first paths located at least partially directly one above the other, and where the first and second robot arms are configured to extend the end effectors in at least one second direction along second paths spaced from one another which are not located above one another, where the first upper arm and the first forearm have different effective lengths, and where the second upper arm and the second forearm have different effective lengths. 
     In accordance with another aspect, the apparatus comprises at least one non-circular pulley and a first band connecting the at least one drive to the first forearm at a first joint between the first upper arm and the first forearm. 
     In accordance with another aspect, the apparatus comprises a second band connecting the first end effector, at a wrist joint of the first end effector to the first forearm, to the first joint. 
     In accordance with another aspect, the apparatus comprises where the first and second end effectors each have a general L shape. 
     In accordance with another aspect, the apparatus comprises a first circular pulley and a first band connecting the at least one drive to a second circular pulley at a first joint between the first upper arm and the first forearm, where the first and second pulleys have different diameters. 
     In accordance with another aspect, the apparatus comprises where the first paths are along a straight line from the first retracted positions. 
     In accordance with another aspect, the apparatus comprises where the first and second robot arms are configured to provide second retracted positions to locate the end effectors such that the substrates located on the end effectors are not stacked one above the another. 
     In accordance with another aspect, the apparatus comprises a controller configured to controller the at least one drive to move the first and second robot arms substantially simultaneously from the first retracted positions along the first paths and move the first and second robot arms individually or simultaneously along the second paths. 
     In accordance with another aspect, a method comprises providing a first robot arm comprising a first upper arm, a first forearm and a first end effector, where the first upper arm and the first forearm have different effective lengths; providing a second robot arm comprising a second upper attic, a second forearm and a second end effector, where the second upper arm and the second forearm have different effective lengths; connecting the first upper arm to at least one drive at a first axis of rotation; and connecting the second upper arm to the at least one drive at a second axis of rotation which is spaced from the first axis of rotation, where the first and second robot arms are configured to locate the end effectors in first retracted positions for stacking substrates located on the end effectors at least partially one above the another, where the first and second robot arms are configured to extend the end effectors from the first retracted positions in a first direction along parallel first paths at least partially located directly one above the other, and where the first and second robot arms are configured to extend the end effectors in at least one second direction along second paths spaced from one another which are not located above one another. 
     In accordance with another aspect, the method comprises at least one non-circular pulley at the first axis of rotation and a first band connecting the at least one drive to the first forearm at a first joint between the first upper arm and the first forearm. 
     In accordance with another aspect, the method comprises a second band connecting the first end effector, at a wrist joint of the first end effector to the first forearm, to the first joint. 
     In accordance with another aspect, the method comprises a first circular pulley and a first band connecting the at least one drive to a second circular pulley at a first joint between the first upper arm and the first forearm, where the first and second pulleys have different diameters. 
     In accordance with another aspect, the method comprises where the first and second robot arms are configured to provide the first paths along a straight line from the first retracted positions. 
     In accordance with another aspect, the method comprises where the first and second arms are configured to provide second retracted positions to locate the end effectors such that the substrates located on the end effectors are not stacked one above the another. 
     In accordance with another aspect, the method comprises connecting a controller to the at least one drive configured to controller the at least one drive to move the first and second robot arms substantially simultaneously from the first retracted positions along the first paths and move the first and second arms individually or simultaneously along the second paths. 
     In accordance with another aspect, a method comprises locating a first end effector and a second end effector of first and second respective robot arms at first retracted positions for stacking substrates located on the end effectors at least partially one above the another, where the first robot arm comprising a first upper arm, a first forearm and the first end effector, where the first upper arm is connected to at least one drive at a first axis of rotation, and where the second robot arm comprises a second upper arm, a second forearm and the second end effector, where the second upper arm is connected to the at least one drive at a second axis of rotation which is spaced from the first axis of rotation; moving the first and second robot arms to move the end effectors from the first retracted positions in a first direction along parallel first paths located at least partially directly one above the other; and moving the first and second robot arms to move the end effectors to extend the end effectors in at least one second direction along second paths spaced from one another which are not located above one another. 
     In accordance with another aspect, the method comprises where moving the first and second robot arms comprises at least one non-circular pulley and a first band connecting the at least one drive to the first forearm at a first joint between the first upper arm and the first forearm. 
     In accordance with another aspect, the method comprises where moving the first and second robot arms comprises a second band connecting the first end effector, at a wrist joint of the first end effector to the first forearm, to the first joint. 
     In accordance with another aspect, the method comprises where moving the first and second robot arms comprises a first circular pulley and a first band connecting the at least one drive to a second circular pulley at a first joint between the first upper arm and the first forearm, where the first and second pulleys have different diameters. 
     In accordance with another aspect, the method comprises a controller controlling the at least one drive to move the first and second robot arms substantially simultaneously from the first retracted positions along the first paths and move the first and second robot arms individually or simultaneously along the second paths. 
     In accordance with another aspect, an apparatus comprises a first robot arm comprising a first upper arm, a first forearm and a first end effector; a second robot arm comprising a second upper arm, a second forearm and a second end effector; and a drive connected to the first and second robot arms, where the first upper arm is connected to the drive at a first axis of rotation, where the second upper arm is connected to the drive at a second axis of rotation which is spaced from the first axis of rotation, where the drive comprises only three motors for rotating first and second upper arms, where the first and second robot arms are configured to locate the end effectors in first retracted positions for stacking substrates located on the end effectors at least partially one above the another, where the first and second robot arms are configured to extend the end effectors from the first retracted positions in a first direction along parallel first paths located at least partially directly one above the other, and where the first and second robot arms are configured to extend the end effectors in at least one second direction along second paths spaced from one another which are not located above one another. 
     In accordance with another aspect, the apparatus comprises where the first upper arm and the first forearm have different effective lengths, and where the second upper arm and the second forearm have different effective lengths. 
     In accordance with another aspect, the apparatus comprises at least one non-circular pulley and a first band connecting the drive to the first forearm at a first joint between the first upper arm and the first forearm. 
     In accordance with another aspect, the apparatus comprises a second band connecting the first end effector, at a wrist joint of the first end effector to the first forearm, to the first joint. 
     In accordance with another aspect, the apparatus comprises where the first and second end effectors each have a general L shape. 
     In accordance with another aspect, the apparatus comprises a first circular pulley and a first band connecting the drive to a second circular pulley at a first joint between the first upper arm and the first forearm, where the first and second pulleys have different diameters. 
     In accordance with another aspect, the apparatus comprises where the first paths are along a straight line from the first retracted positions. 
     In accordance with another aspect, the apparatus comprises where the first and second robot arms are configured to provide second retracted positions to locate the end effectors such that the substrates located on the end effectors are not stacked one above the another. 
     In accordance with another aspect, the apparatus comprises a controller configured to control the drive to move the first and second robot arms substantially simultaneously from the first retracted positions along the first paths and move the first and second robot arms individually or simultaneously along the second paths. 
     In accordance with another aspect, the apparatus comprises where the three motors are aligned in a common axis. 
     In accordance with another aspect, the apparatus comprises where the three motors are located in three respective spaced axes. 
     In accordance with another aspect, the apparatus comprises a z-axis motor connected to the drive to move the drive and the first and second robot arms vertically. 
     In accordance with another aspect, a method comprises locating a first end effector and a second end effector of first and second respective robot arms at first retracted positions for stacking substrates located on the end effectors at least partially one above the another, where the first robot arm comprising a first upper arm, a first forearm and the first end effector, where the first upper arm is connected to a drive at a first axis of rotation, and where the second robot arm comprises a second upper arm, a second forearm and the second end effector, where the second upper arm is connected to the drive at a second axis of rotation which is spaced from the first axis of rotation; moving the first and second robot arms to move the end effectors from the first retracted positions in a first direction along parallel first paths located at least partially directly one above the other; moving the first and second robot arms to move the end effectors to extend the end effectors in at least one second direction along second paths spaced from one another which are not located above one another; rotating the first and second robot arms together about a third axis of rotation which is spaced from the first and second axes of rotation, where the moving from the first retracted positions in the first direction, the moving to extend the end effectors in the at least one second direction, and the rotating is with use of only three motors of the drive. 
     In accordance with another aspect, the method comprises where moving the first and second robot arms comprises at least one non-circular pulley and a first band connecting the drive to the first forearm at a first joint between the first upper arm and the first forearm. 
     In accordance with another aspect, the method comprises where moving the first and second robot arms comprises a second band connecting the first end effector, at a wrist joint of the first end effector to the first forearm, to the first joint. 
     In accordance with another aspect, the method comprises where moving the first and second robot arms comprises a first circular pulley and a first band connecting the drive to a second circular pulley at a first joint between the first upper arm and the first forearm, where the first and second pulleys have different diameters. 
     In accordance with another aspect, the method comprises where further comprising a controller controlling the motors of the drive to move the first and second robot arms substantially simultaneously from the first retracted positions along the first paths and move the first and second robot arms individually or simultaneously along the second paths. 
     In accordance with another aspect, a method comprises providing a first robot arm comprising a first upper arm, a first forearm and a first end effector; providing a second robot arm comprising a second upper aim, a second forearm and a second end effector; connecting the first upper arm to a drive at a first axis of rotation; and connecting the second upper arm to the drive at a second axis of rotation which is spaced from the first axis of rotation, where the first and second robot arms are configured to locate the end effectors in first retracted positions for stacking substrates located on the end effectors at least partially one above the another, where the first and second robot arms are configured to be rotated to extend the end effectors from the first retracted positions in a first direction along parallel first paths at least partially located directly one above the other, and where the first and second robot arms are configured to be rotated to extend the end effectors in at least one second direction along second paths spaced from one another which are not located above one another, where the drive comprises only three motors for rotating the first and second robot arms to extend the end effectors and for rotating the first and second robot arms about a third axis of rotation spaced from the first and second axes of rotation. 
     In accordance with another aspect, the method comprises where the first robot arm is provided with the first upper arm and the first forearm have different effective lengths, and where the second robot arm is provided with the second upper arm and the second forearm have different effective lengths. 
     In accordance with another aspect, the method comprises at least one non-circular pulley at the first axis of rotation and a first band connecting the drive to the first forearm at a first joint between the first upper mill and the first forearm. 
     In accordance with another aspect, the method comprises a second band connecting the first end effector, at a wrist joint of the first end effector to the first forearm, to the first joint. 
     In accordance with another aspect, the method comprises a first circular pulley and a first band connecting the drive to a second circular pulley at a first joint between the first upper arm and the first forearm, where the first and second pulleys have different diameters. 
     In accordance with another aspect, the method comprises where the first and second robot arms are configured to provide the first paths along a straight line from the first retracted positions. 
     In accordance with another aspect, the method comprises where the first and second arms are configured to provide second retracted positions to locate the end effectors such that the substrates located on the end effectors are not stacked one above the another. 
     In accordance with another aspect, the method comprises connecting a controller to the drive configured to controller the drive to move the first and second robot arms substantially simultaneously from the first retracted positions along the first paths and move the first and second arms individually or simultaneously along the second paths. 
     In accordance with another aspect, an apparatus comprises a first robot arm comprising a first upper arm, a first forearm and a first end effector; a second robot arm comprising a second upper arm, a second forearm and a second end effector; and a drive connected to the first and second robot arms, where the first upper arm is connected to the drive at a first axis of rotation, where the second upper arm is connected to the drive at a second axis of rotation which is spaced from the first axis of rotation, where the drive comprises five motors for rotating first and second upper arms, where a first one of the motors is connected to the first and second robot alms to rotate the first and second arms about a third axis of rotation spaced from the first and second axes of rotation, where second and third ones of the motors are connected to the first robot arm to rotate the first upper arm and the first forearm respectively, and where fourth and fifth ones of the motors are connected to the second robot arm to rotate the second upper arm and the second forearm, respectively, independently from the first robot arm, where the first and second robot arms are configured to locate the end effectors in first retracted positions for stacking substrates located on the end effectors at least partially one above the another, where the first and second robot arms are configured to extend the end effectors from the first retracted positions in a first direction along parallel first paths located at least partially directly one above the other, and where the first and second robot arms are configured to extend the end effectors in at least one second direction along second paths spaced from one another which are not located above one another. 
     In accordance with another aspect, the apparatus comprises where the first upper arm and the first forearm have different effective lengths, and where the second upper arm and the second forearm have different effective lengths. 
     In accordance with another aspect, the apparatus comprises at least one non-circular pulley and a first band connecting the drive to the first forearm at a first joint between the first upper arm and the first forearm. 
     In accordance with another aspect, the apparatus comprises a second band connecting the first end effector, at a wrist joint of the first end effector to the first forearm, to the first joint. 
     In accordance with another aspect, the apparatus comprises where the first and second end effectors each have a general L shape. 
     In accordance with another aspect, the apparatus comprises a first circular pulley and a first band connecting the drive to a second circular pulley at a first joint between the first upper arm and the first forearm, where the first and second pulleys have different diameters. 
     In accordance with another aspect, the apparatus comprises where the first paths are along a straight line from the first retracted positions. 
     In accordance with another aspect, the apparatus comprises where the first and second robot arms are configured to provide second retracted positions to locate the end effectors such that the substrates located on the end effectors are not stacked one above the another. 
     In accordance with another aspect, the apparatus comprises a controller configured to controller the drive to move the first and second robot arms substantially simultaneously from the first retracted positions along the first paths and move the first and second robot anus individually or simultaneously along the second paths. 
     In accordance with another aspect, the apparatus comprises a z-axis motor connected to the drive to move the drive and the first and second robot arms vertically. 
     In accordance with another aspect, a method comprises locating a first end effector and a second end effector of first and second respective robot arms at first retracted positions for stacking substrates located on the end effectors at least partially one above the another, where the first robot arm comprising a first upper arm, a first forearm and the first end effector, where the first upper arm is connected to a drive at a first axis of rotation, and where the second robot arm comprises a second upper arm, a second forearm and the second end effector, where the second upper arm is connected to the drive at a second axis of rotation which is spaced from the first axis of rotation; moving the first and second robot arms to move the end effectors from the first retracted positions in a first direction along parallel first paths located at least partially directly one above the other; moving the first and second robot arms to move the end effectors to extend the end effectors in at least one second direction along second paths spaced from one another which are not located above one another; rotating the first and second robot arms together about a third axis of rotation which is spaced from the first and second axes of rotation, where the moving from the first retracted positions in the first direction, the moving to extend the end effectors in the at least one second direction, and the rotating is with use of five motors of the drive, where a first one of the motors is connected to the first and second robot arms to rotate the first and second arms about the third axis of rotation, where second and third ones of the motors are connected to the first robot arm to rotate the first upper arm and the first forearm respectively, and where fourth and fifth ones of the robot arms are connected to the second robot arm to rotate the second upper arm and the second forearm respectively independently from the first robot arm. 
     In accordance with another aspect, a method or apparatus comprises where the first motor is aligned in the third axis, the second and third motors are aligned with each other in the first axis and the fourth and fifth motors are aligned with each other in the second axis. 
     In accordance with another aspect, a method comprises providing a first robot arm comprising a first upper arm, a first forearm and a first end effector; providing a second robot arm comprising a second upper arm, a second forearm and a second end effector; connecting the first upper arm to a drive at a first axis of rotation; and connecting the second upper arm to the drive at a second axis of rotation which is spaced from the first axis of rotation, where the first and second robot arms are configured to locate the end effectors in first retracted positions for stacking substrates located on the end effectors at least partially one above the another, where the first and second robot arms are configured to be rotated to extend the end effectors from the first retracted positions in a first direction along parallel first paths at least partially located directly one above the other, and where the first and second robot arms are configured to be rotated to extend the end effectors in at least one second direction along second paths spaced from one another which are not located above one another, where the drive comprises five motors for rotating the first and second robot alms to extend the end effectors and for rotating the first and second robot arms about a third axis of rotation spaced from the first and second axes of rotation, where a first one of the motors is connected to the first and second robot arms to rotate the first and second arms about the third axis of rotation, where second and third ones of the motors are connected to the first robot arm to rotate the first upper arm and the first forearm respectively, and where fourth and fifth ones of the robot arms are connected to the second robot arm to rotate the second upper arm and the second forearm respectively independently from the first robot arm. 
     In accordance with another aspect, an apparatus comprises a first robot arm comprising a first upper arm, a first forearm and a first end effector; a second robot arm comprising a second upper arm, a second forearm and a second end effector; and a drive connected to the first and second robot arms, where the first upper arm is connected to the drive at a first axis of rotation, where the second upper arm is connected to the drive at a second axis of rotation which is spaced from the first axis of rotation, where the drive comprises four motors for rotating first and second upper aims, where a first one of the motors is connected to the first upper arm, where a second one of the motors is connected to the second upper arm, where a third one of the motors is connected to the first forearm, where a fourth one of the motors is connected to the second forearm, where the third and fourth motors are aligned in a common axis spaced from the first and second axis, where the first motor is aligned in the first axis and where the second motor is aligned in the second axis, where the first and second robot arms are configured to locate the end effectors in first retracted positions for stacking substrates located on the end effectors at least partially one above the another, where the first and second robot arms are configured to extend the end effectors from the first retracted positions in a first direction along parallel first paths located at least partially directly one above the other, and where the first and second robot arms are configured to extend the end effectors in at least one second direction along second paths spaced from one another which are not located above one another. 
     In one example embodiment an apparatus is provided comprising at least one processor; and at least one non-transitory memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to: locate a first end effector and a second end effector of first and second respective robot arms at first retracted positions for stacking substrates located on the end effectors at least partially one above the another, where the first robot arm comprising a first upper arm, a first forearm and the first end effector, where the first upper arm is connected to a drive at a first axis of rotation, and where the second robot arm comprises a second upper arm, a second forearm and the second end effector, where the second upper arm is connected to the drive at a second axis of rotation which is spaced from the first axis of rotation; move the first and second robot arms to move the end effectors from the first retracted positions in a first direction along parallel first paths located at least partially directly one above the other; move the first and second robot arms to move the end effectors to extend the end effectors in at least one second direction along second paths spaced from one another which are not located above one another; rotate the first and second robot arms together about a third axis of rotation which is spaced from the first and second axes of rotation, where the moving from the first retracted positions in the first direction, the moving to extend the end effectors in the at least one second direction, and the rotating is with use of only three motors of the drive. 
     In accordance with one example embodiment, an apparatus is provided comprising non-transitory program storage device readable by a machine, tangibly embodying a program of instructions executable by the machine for performing operations, the operations comprising: locating a first end effector and a second end effector of first and second respective robot arms at first retracted positions for stacking substrates located on the end effectors at least partially one above the another, where the first robot arm comprising a first upper arm, a first forearm and the first end effector, where the first upper arm is connected to a drive at a first axis of rotation, and where the second robot arm comprises a second upper arm, a second forearm and the second end effector, where the second upper arm is connected to the drive at a second axis of rotation which is spaced from the first axis of rotation; moving the first and second robot arms to move the end effectors from the first retracted positions in a first direction along parallel first paths located at least partially directly one above the other; moving the first and second robot arms to move the end effectors to extend the end effectors in at least one second direction along second paths spaced from one another which are not located above one another; rotating the first and second robot arms together about a third axis of rotation which is spaced from the first and second axes of rotation, where the moving from the first retracted positions in the first direction, the moving to extend the end effectors in the at least one second direction, and the rotating is with use of only three motors of the drive. 
     In one example embodiment an apparatus is provided comprising at least one processor; and at least one non-transitory memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to: locate a first end effector and a second end effector of first and second respective robot arms at first retracted positions for stacking substrates located on the end effectors at least partially one above the another, where the first robot arm comprising a first upper arm, a first forearm and the first end effector, where the first upper arm is connected to a drive at a first axis of rotation, and where the second robot arm comprises a second upper arm, a second forearm and the second end effector, where the second upper arm is connected to the drive at a second axis of rotation which is spaced from the first axis of rotation; move the first and second robot arms to move the end effectors from the first retracted positions in a first direction along parallel first paths located at least partially directly one above the other; move the first and second robot arms to move the end effectors to extend the end effectors in at least one second direction along second paths spaced from one another which are not located above one another; rotate the first and second robot arms together about a third axis of rotation which is spaced from the first and second axes of rotation, where the moving from the first retracted positions in the first direction, the moving to extend the end effectors in the at least one second direction, and the rotating is with use of five motors of the drive, where a first one of the motors is connected to the first and second robot awls to rotate the first and second arms about the third axis of rotation, where second and third ones of the motors are connected to the first robot arm to rotate the first upper arm and the first forearm respectively, and where fourth and fifth ones of the robot arms are connected to the second robot arm to rotate the second upper arm and the second forearm respectively independently from the first robot arm. 
     In accordance with one example embodiment, an apparatus is provided comprising non-transitory program storage device readable by a machine, tangibly embodying a program of instructions executable by the machine for performing operations, the operations comprising: locating a first end effector and a second end effector of first and second respective robot arms at first retracted positions for stacking substrates located on the end effectors at least partially one above the another, where the first robot arm comprising a first upper arm, a first forearm and the first end effector, where the first upper arm is connected to a drive at a first axis of rotation, and where the second robot arm comprises a second upper arm, a second forearm and the second end effector, where the second upper arm is connected to the drive at a second axis of rotation which is spaced from the first axis of rotation; moving the first and second robot arms to move the end effectors from the first retracted positions in a first direction along parallel first paths located at least partially directly one above the other; moving the first and second robot arms to move the end effectors to extend the end effectors in at least one second direction along second paths spaced from one another which are not located above one another; rotating the first and second robot arms together about a third axis of rotation which is spaced from the first and second axes of rotation, where the moving from the first retracted positions in the first direction, the moving to extend the end effectors in the at least one second direction, and the rotating is with use of five motors of the drive, where a first one of the motors is connected to the first and second robot arms to rotate the first and second arms about the third axis of rotation, where second and third ones of the motors are connected to the first robot arm to rotate the first upper arm and the first forearm respectively, and where fourth and fifth ones of the robot arms are connected to the second robot arm to rotate the second upper arm and the second forearm respectively independently from the first robot aim. 
     Any combination of one or more computer readable medium(s) may be utilized as the memory. The computer readable medium may be a computer readable signal medium or a non-transitory computer readable storage medium. A non-transitory computer readable storage medium does not include propagating signals and may be, for example, but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. More specific examples (a non-exhaustive list) of the computer readable storage medium would include the following: an electrical connection having one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing. 
     It should be seen that the foregoing description is only illustrative. Various alternatives and modifications can be devised by those skilled in the art. Accordingly, the present embodiment is intended to embrace all such alternatives, modifications, and variances. For example, features recited in the various dependent claims could be combined with each other in any suitable combination(s). In addition, features from different embodiments described above could be selectively combined into a new embodiment. Accordingly, the description is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.