Patent Publication Number: US-2020298402-A1

Title: Multi-active-axis, non-exoskeletal rehabilitation device

Description:
REFERENCE TO PENDING PRIOR PATENT APPLICATIONS 
     This patent application: 
     (i) is a continuation-in-part of pending prior U.S. patent application Ser. No. 14/500,810, filed Sep. 29, 2014 by Barrett Technology, Inc. and William T. Townsend et al. for MULTI-ACTIVE-AXIS, NON-EXOSKELETAL REHABILITATION DEVICE (Attorney&#39;s Docket No. BARRETT-5), which patent application claims benefit of prior U.S. Provisional Patent Application Ser. No. 61/883,367, filed Sep. 27, 2013 by Barrett Technology, Inc. and William T. Townsend et al. for THREE-ACTIVE-AXIS REHABILITATION DEVICE (Attorney&#39;s Docket No. BARRETT-5 PROV); 
     (ii) claims benefit of pending prior U.S. Provisional Patent Application Ser. No. 62/235,276, filed Sep. 30, 2015 by Barrett Technology, Inc. and Alexander Jenko et al. for MULTI-ACTIVE-AXIS, NON-EXOSKELETAL REHABILITATION DEVICE (Attorney&#39;s Docket No. BARRETT-8 PROV); and 
     (iii) claims benefit of pending prior U.S. Provisional Patent Application Ser. No. 62/340,832, filed May 24, 2016 by Barrett Technology, LLC and William T. Townsend et al. for MULTI-ACTIVE-AXIS, NON-EXOSKELETAL REHABILITATION DEVICE (Attorney&#39;s Docket No. BARRETT-10 PROV). 
     The four (4) above-identified which patent applications are hereby incorporated herein by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with Government support under Agreement No. HR0011-12-9-0012 awarded by DARPA. The Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to devices for the rehabilitation of disabled persons with a neurological injury, such as stroke or spinal-cord injury, or otherwise impaired anatomical extremities. 
     BACKGROUND OF THE INVENTION 
     A new and exciting branch of physical and occupational therapies is therapy assisted by a computer-directed robotic arm or device (sometimes also called a “manipulator” to distinguish it from the human arm that may engage it, in certain embodiments). These robotic systems leverage plasticity in the brain, which literally rewires the brain. Recent science has demonstrated that dosage (i.e., the amount of time engaged in therapy) is an essential element in order to benefit from this effect. The potential benefits of using a manipulator system for tasks such as post-stroke rehabilitative therapy, which typically involves moving a patient&#39;s limb(s) through a series of repeated motions, are significant. There exist some types of therapy, such as error-augmentation therapy, that simply cannot be implemented effectively by a human therapist. Furthermore, computer-directed therapy can engage the patient in games, thereby making the experience more enjoyable and encouraging longer and more intense therapy sessions, which are known to benefit patients. Finally, the therapist is able to work with more patients, e.g., the therapist is able to work with multiple patients simultaneously, the therapist is able to offer patients increased therapy duration (higher dosage) since the session is no longer constrained by the therapist&#39;s physical endurance or schedule, and the therapist is able to work more consecutive therapy sessions since the number of consecutive therapy sessions is no longer constrained by the therapist&#39;s physical endurance or schedule. 
     A useful way to categorize robotic rehabilitation systems is by the number of degrees of freedom, or DOFs, that they have. Generally speaking, for mechanical systems, the degrees of freedom (DOFs) can be thought of as the different motions permitted by the mechanical system. By way of example but not limitation, the motion of a ship at sea has six degrees of freedom (DOFs): (1) moving up and down, (2) moving left and right, (3) moving forward and backward, (4) swiveling left and right (yawing), (5) tilting forward and backward (pitching), and (6) pivoting side to side (rolling). The majority of commercial robotic rehabilitation systems fall into one of two broad categories: low-DOF systems (typically one to three DOFs) which are positioned in front of the patient, and high-DOF exoskeletal systems (typically six or more DOFs) which are wrapped around the patient&#39;s limb, typically an arm or leg. Note that these exoskeletons also need the ability to adjust the link lengths of the manipulator in order to accommodate the differing geometries of specific patients. Generally speaking, an exoskeletal system can be thought of as an external skeleton mounted to the body, where the external skeleton has struts and joints corresponding to the bones and joints of the natural body. The current approaches for both categories (i.e., low-DOF systems and high-DOF exoskeletal systems) exhibit significant shortcomings, which have contributed to limited realization of the potential of robotic rehabilitation therapies. 
     Low-DOF systems are usually less expensive than high-DOF systems, but they typically also have a smaller range of motion. Some low-DOF systems, such as the InMotion ARM™ Therapy System of Interactive Motion Technologies of Watertown, Mass., USA, or the KINARM End-Point Robot™ system of BKIN Technologies of Kingston, Ontario, Canada, are limited to only planar movements, greatly reducing the number of rehabilitation tasks that the systems can be used for. Those low-DOF systems which are not limited to planar movements must typically contend with issues such as avoiding blocking a patient&#39;s line of sight, like the DeXtreme™ system of BioXtreme of Rehovot, Israel; providing an extremely limited range of motion, such as with the ReoGO® system of Motorika Medical Ltd of Mount Laurel, N.J., USA; and insufficiently supporting a patient&#39;s limb (which can be critically important where the patient lacks the ability to support their own limb). Most of these systems occupy space in front of the patient, impinging on the patient&#39;s workspace, increasing the overall footprint needed for a single rehabilitation “station” and consuming valuable space within rehabilitation clinics. 
     High-DOF exoskeletal systems, such as the Armeo®Power system of Hocoma AG of Volketswil, Switzerland, the Armeo®Spring system of Hocoma AG of Volketswil, Switzerland, and the 8+2 DOF exoskeletal rehabilitation system disclosed in U.S. Pat. No. 8,317,730, are typically significantly more complex, and consequently generally more expensive, than comparable low-DOF systems. While such high-DOF exoskeletal systems usually offer greater ranges of motion than low-DOF systems, their mechanical complexity also makes them bulky, and they typically wrap around the patient&#39;s limb, making the high-DOF exoskeletal systems feel threatening and uncomfortable to patients. Furthermore, human joints do not conform to axes separated by links the way robots joints do, and the anatomy of every human is different, with different bone lengths and different joint geometries. Even with the high number of axes present in high-DOF exoskeletal systems, fine-tuning an exoskeleton system&#39;s joint locations and link lengths to attempt to follow those of the patient takes considerable time, and even then the high-DOF exoskeletal system frequently over-constrains the human&#39;s limb, potentially causing more harm than good. 
     Finally, there are a handful of currently-available devices which do not fit in either of the two categories listed above: for example, high-DOF non-exoskeletal devices, or low-DOF exoskeletal devices. To date, these devices have generally suffered the weaknesses of both categories, without leveraging the strengths of either. A particularly notable example is the KINARM Exoskeleton Robot™ of BKIN Technologies of Kingston, Ontario, Canada, which is an exoskeletal rehabilitation device designed for bi-manual and uni-manual upper-extremity rehabilitation and experimentation in humans and non-human primates. Like the KINARM End-Point Robot™ of BKIN Technologies of Kingston, Ontario, Canada (see above), the KINARM Exoskeletal Robot™ system provides only two degrees of freedom for each limb, limiting the range of rehabilitation exercises that it can conduct. Meanwhile, by implementing an exoskeletal design, the KINARM Exoskeletal Robot™ device can provide some additional support to the patient&#39;s limb, but at the cost of significant increases in device size, cost, complexity and set-up time. 
     While robot-assisted physical and occupational therapy offers tremendous promise to many groups of patients, the prior art has yet to match that promise. As the previous examples have shown, current therapy devices are either too simplistic and limited, allowing only the most rudimentary exercises and frequently interfering with the patient in the process; or too complex and cumbersome, making the devices expensive, intimidating to patients, and difficult for therapists to use. Thus there remains a need for a novel device and method that can provide patients and therapists with the ability to perform sophisticated 2-D and 3-D rehabilitation exercises, in a simple, unobtrusive and welcoming form factor, at a relatively low price. 
     SUMMARY OF THE INVENTION 
     The present invention bridges the categories of low-DOF systems and high-DOF exoskeletal systems, offering the usability, mechanical simplicity and corresponding affordability of a low-DOF system, as well as the reduced footprint, range of motion, and improved support ability of a high-DOF exoskeletal system. 
     More particularly, the present invention comprises a relatively low number of active (powered) DOFs—in the preferred embodiment, three active DOFs, although the novel features of the invention can be implemented in systems with other numbers of DOFs—which reduces the device&#39;s cost and complexity to well below that of high-DOF exoskeletal systems. However, because of the innovative positional and orientational relationship of the system to the patient—unique among non-exoskeletal systems to date, as explained further below—the device of the present invention enjoys advantages that have previously been limited to high-DOF exoskeletal systems, such as more optimal torque-position relationships, better workspace overlap with the patient and a greater range of motion. 
     In addition, it has been discovered that a novel implementation of a cabled differential (with the differential input being used as a pitch axis and the differential output being used as a yaw axis relative to the distal links of the device) permits the mass and bulk of the power drives (e.g., motors) to be shifted to the base of the system, away from the patient&#39;s workspace and view. Through the combination of these two major innovations—the orientation and position of the device relative to the patient, and the implementation of a cabled differential with special kinematics—as well as other innovations, the present invention provides a unique rehabilitation device that fills a need in the rehabilitation market and is capable of a wide variety of rehabilitation tasks. 
     Significantly, the present invention enables a new method for bi-manual rehabilitation—a new class of rehabilitative therapy where multiple limbs, usually arms, are rehabilitated simultaneously—in which rehabilitative exercises can be conducted in three dimensions, by using two similar devices, simultaneously and in a coordinated fashion, on two different limbs of the patient. 
     In one preferred form of the invention, there is provided a non-exoskeletal rehabilitation device, with as few as 2 active degrees of freedom, wherein the device is oriented and positioned such that its frame of reference (i.e., its “reference frame”) is oriented generally similarly to the reference frame of the patient, and motions of the patient&#39;s endpoint are mimicked by motions of the device&#39;s endpoint. 
     In another preferred form of the invention, there is provided a non-exoskeletal rehabilitation device, with as few as 2 active degrees of freedom, of which 2 degrees are linked through a cabled differential. 
     In another preferred form of the invention, there is provided a method for bi-manual rehabilitation, wherein the method utilizes a pair of rehabilitation devices, wherein each rehabilitation device is designed to be capable of inducing motion in three or more degrees of freedom, is easily reconfigurable to allow both right-handed and left-handed usage, and is located relative to the patient such that two devices may be used simultaneously without interfering with each other. 
     In another preferred form of the invention, there is provided a robotic device for operation in association with an appendage of a user, wherein the appendage of the user has an endpoint, the robotic device comprising: 
     a base; and 
     a robotic arm attached to the base and having an endpoint, the robotic arm having at least two active degrees of freedom relative to the base and being configured so that when the base is appropriately positioned relative to a user, the reference frame of the robotic device is oriented generally similarly to the reference frame of the user and motions of the endpoint of the appendage of the user are mimicked by motions of the endpoint of the robotic arm. 
     In another preferred form of the invention, there is provided a method for operating a robotic device in association with an appendage of a user, wherein the appendage of the user has an endpoint, the method comprising: 
     providing a robotic device comprising:
         a base; and   a robotic arm attached to the base and having an endpoint, the robotic arm having at least two active degrees of freedom relative to the base and being configured so that when the base is appropriately positioned relative to a user, the reference frame of the robotic device is oriented generally similarly to the reference frame of the user and motions of the endpoint of the appendage of the user are mimicked by motions of the endpoint of the robotic arm;       

     positioning the base relative to the user so that the reference frame of the robotic device is oriented generally similarly to the reference frame of the user, and attaching the appendage of the user to the robotic arm; and 
     moving at least one of the endpoint of the appendage of the user and the endpoint of the robotic arm. 
     In another preferred form of the invention, there is provided a robotic device comprising: 
     a base; 
     an arm having a first end and a second end, the first end of the arm being mounted to the base and the second end of the arm being configured to receive an endpoint device; 
     an endpoint device configured to be mounted to the second end of the arm and being configured for engagement by a limb of a user; and a controller mounted to at least one of the base and the arm for controlling operation of the arm; 
     wherein the endpoint device comprises a user-presence sensing unit for detecting engagement of the endpoint device by a limb of a user and advising the controller of the same. 
     In another preferred form of the invention, there is provided a robotic device comprising: 
     a base; 
     an arm having a first end and a second end, the first end of the arm being mounted to the base and the second end of the arm being configured to receive an endpoint device; 
     an endpoint device configured to be mounted to the second end of the arm and being configured for engagement by a limb of a user; and 
     a controller mounted to at least one of the base and the arm for controlling operation of the arm; 
     wherein the endpoint device is mountable to the second end of the arm using a modular connection which provides mechanical mounting of the endpoint device to the second end of the arm and electrical communication between the endpoint device and the arm. 
     In another preferred form of the invention, there is provided a robotic device comprising: 
     a base; 
     an arm having a first end and a second end, the first end of the arm being mounted to the base and the second end of the arm being configured to receive an endpoint device; 
     an endpoint device configured to be mounted to the second end of the arm and being configured for engagement by a limb of a user; and 
     a controller mounted to at least one of the base and the arm for controlling operation of the arm; 
     wherein the endpoint device is adjustable relative to the second end of the arm along a pitch axis and a yaw axis. 
     In another preferred form of the invention, there is provided a robotic device comprising: 
     a base; 
     an arm having a first end and a second end, the first end of the arm being mounted to the base and the second end of the arm being configured to receive an endpoint device; 
     an endpoint device configured to be mounted to the second end of the arm and being configured for engagement by a limb of a user; and 
     a controller mounted to at least one of the base and the arm for controlling operation of the arm; 
     wherein the controller is configured to compensate for the effects of gravity when the endpoint device is engaged by a limb of a user. 
     In another preferred form of the invention, there is provided a method for providing rehabilitation therapy to a user, the method comprising: 
     providing a robotic device comprising: 
     a base; 
     an arm having a first end and a second end, the first end of the arm being mounted to the base and the second end of the arm being configured to receive an endpoint device;
         an endpoint device configured to be mounted to the second end of the arm and being configured for engagement by a limb of a user; and   a controller mounted to at least one of the base and the arm for controlling operation of the arm;   wherein the endpoint device comprises a user-presence sensing unit for detecting engagement of the endpoint device by a limb of a user and advising the controller of the same; and       

     operating the robotic device. 
     In another preferred form of the invention, there is provided a method for providing rehabilitation therapy to a user, the method comprising: 
     providing a robotic device comprising:
         a base;   an arm having a first end and a second end, the first end of the arm being mounted to the base and the second end of the arm being configured to receive an endpoint device;   an endpoint device configured to be mounted to the second end of the arm and being configured for engagement by a limb of a user; and   a controller mounted to at least one of the base and the arm for controlling operation of the arm;   wherein the endpoint device is mountable to the second end of the arm using a modular connection which provides mechanical mounting of the endpoint device to the second end of the arm and electrical communication between the endpoint device and the arm; and       

     operating the robotic device. 
     In another preferred form of the invention, there is provided a method for providing rehabilitation therapy to a user, the method comprising: 
     providing a robotic device comprising:
         a base;   an arm having a first end and a second end, the first end of the arm being mounted to the base and the second end of the arm being configured to receive an endpoint device;   an endpoint device configured to be mounted to the second end of the arm and being configured for engagement by a limb of a user; and   a controller mounted to at least one of the base and the arm for controlling operation of the arm;   wherein the endpoint device is adjustable relative to the second end of the arm along a pitch axis and a yaw axis; and       

     operating the robotic device. 
     In another preferred form of the invention, there is provided a method for providing rehabilitation therapy to a user, the method comprising: 
     providing a robotic device comprising:
         a base;   an arm having a first end and a second end, the first end of the arm being mounted to the base and the second end of the arm being configured to receive an endpoint device;   an endpoint device configured to be mounted to the second end of the arm and being configured for engagement by a limb of a user; and   a controller mounted to at least one of the base and the arm for controlling operation of the arm;   wherein the controller is configured to compensate for the effects of gravity when the endpoint device is engaged by a limb of a user; and       

     operating the robotic device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects and features of the present invention will be more fully disclosed or rendered obvious by the following detailed description of the preferred embodiments of the invention, which is to be considered together with the accompanying drawings wherein like numbers refer to like parts, and further wherein: 
         FIGS. 1 and 2  are schematic front perspective views showing one preferred form of robotic device formed in accordance with the present invention; 
         FIGS. 3 and 4  are schematic top views showing the robotic device of  FIGS. 1 and 2 ; 
         FIGS. 5A, 5B and 5C  are schematic front perspective views showing how the robotic device of  FIGS. 1 and 2  may use a “stacked down”, “stacked flat” or “stacked up” construction; 
         FIGS. 6 and 7  are schematic views showing details of selected portions of the robotic device of  FIGS. 1 and 2 ; 
         FIGS. 8A, 8B and 8C  are schematic views showing the pitch-yaw configuration of the robotic device of  FIGS. 1 and 2  in comparison to the roll-pitch and pitch-roll configurations of prior art devices; 
         FIG. 9  is a schematic top view showing how the robotic device of the present invention may be switched from right-handed use to left-handed use; 
         FIG. 10  is a schematic view showing two robotic devices being used for bi-manual rehabilitation; 
         FIG. 11  is a schematic view showing how the robotic device may communicate with an external controller; 
         FIG. 12  shows how a pair of robotic devices may communicate with an external controller, which in turn facilitates communication between the devices; 
         FIGS. 13, 13A, 14 and 15  are schematic views showing one preferred endpoint device for the robotic device of the present invention; 
         FIG. 15A  is a schematic view showing the robotic device being used by a patient in a sitting position; 
         FIG. 15B  is a schematic view showing the robotic device being used by a patient in a standing position; 
         FIG. 16  is a schematic view showing another preferred endpoint device for the robotic device of the present invention; 
         FIG. 17  is a schematic view showing another preferred endpoint device for the robotic device of the present invention; 
         FIG. 18  is a schematic view showing another preferred endpoint device for the robotic device of the present invention; 
         FIG. 19  is a schematic view showing details of the construction of the endpoint device of  FIG. 16 ; 
         FIG. 20  is a schematic view showing another preferred endpoint device for the robotic device of the present invention; 
         FIGS. 21-26  are schematic views showing how the robotic device may be changed from left-handed use to right-handed use; 
         FIGS. 27-29  are schematic views showing still another construction for an endpoint device; and 
         FIGS. 30-32  are schematic views showing still another construction for an endpoint device. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The Novel, Multi-Active-Axis Non-Exoskeletal Robotic Device in General 
     Looking first at  FIG. 1 , there is shown a novel multi-active-axis, non-exoskeletal robotic device  5  that is suitable for various robotic-assisted therapies and other applications. Robotic device  5  generally comprises a base  100 , an inner link  105 , an outer link  110 , and a coupling element  115  for coupling outer link  110  to a patient, commonly to a limb of the patient (e.g., as shown in  FIG. 1 , the patient&#39;s arm  120 ). 
     The preferred embodiment shown in  FIG. 1  has three degrees of freedom, although it will be appreciated by one skilled in the art that the present invention may comprise fewer or greater numbers of degrees of freedom. Three degrees of freedom theoretically provide the ability to access all positions in Cartesian space, subject to the kinematic limitations of the device, such as joint limits, link lengths, and transmission ranges. To produce those three degrees of freedom, robotic device  5  comprises three revolute joints, shown in  FIG. 1  as joint J 1  providing pitch around an axis  125 , joint J 2  providing yaw around an axis  130  and joint J 3  providing yaw around an axis  135 . In the preferred embodiment, these joints are implemented as follows. Joint J 1  is a pitch joint, and consists of a segment  138  which rotates inside a generally U-shaped frame  140 . Joint J 2  is a yaw joint, and consists of a second segment  145  attached perpendicularly to segment  138 . This segment  145  contains a third segment  150 , which rotates inside segment  145 . In the preferred embodiment, these two joints (i.e., joint J 1  and joint J 2 ) are linked through a cabled differential as will hereinafter be discussed. Joint J 3  is also a yaw joint, and is separated from joint J 2  by inner link  105 . As will hereinafter be discussed, a cable transmission connects the motor that actuates joint J 3  (and which is located coaxially to the axis  130  of joint J 2 , as will hereinafter be discussed) to the output of joint J 3 ; this cable transmission runs through inner link  105 . It should be noted that while this particular embodiment has been found to be preferable, the present invention may also be implemented in alternative embodiments including but not limited to:
         devices with alternative kinematics—for example, three joints in a yaw-pitch-yaw arrangement (as opposed to the pitch-yaw-yaw arrangement of  FIG. 1 );   devices using other types of joints, such as prismatic joints (i.e., slider joints); and   devices that implement other drive technologies, such as gear drivetrains, belts, hydraulic drives, etc.       

     To provide additional degrees of freedom, different endpoint attachments may be provided at the location of the coupling element  115 , to permit different degrees of control over the patient&#39;s limb orientation, or to provide additional therapeutic modalities. By way of example but not limitation, different endpoint attachments may comprise a single-DOF endpoint attachment for performing linear rehabilitation exercises; or a three-DOF endpoint attachment to enable more complex motions, by enabling control over the orientation of the patient&#39;s limb; or an actively-controlled multi-DOF endpoint attachment. By reducing the number of degrees of freedom in the core of the robotic device to three in the preferred implementation (i.e., the robotic device  5  shown in  FIG. 1 ), the design of the robotic device is vastly simplified, reducing cost while maintaining the device&#39;s ability to provide a wide range of rehabilitative services including three-dimensional rehabilitative therapies. 
     Looking next at  FIGS. 1 and 6 , further details of the construction of the preferred embodiment of the present invention are shown. The preferred embodiment of the robotic device consists of the following four kinematic frames (i.e., the kinematic frames of reference for various points on the robotic device):
         1) The ground kinematic frame, consisting of all components that are generally static when the device is in use;   2) The joint J 1  kinematic frame, consisting of all non-transmission components that rotate exclusively about axis  125  of joint J 1 ;   3) The joint J 2  kinematic frame, consisting of all non-transmission components that may rotate exclusively about axis  125  of joint J 1  and axis  130  of joint J 2 ; and   4) The joint J 3  kinematic frame, consisting of all non-transmission components that may rotate about axis  125  of joint J 1 , axis  130  of joint J 2  and axis  135  of joint J 3 .       

     In this definition of kinematic frames, transmission components are excluded to simplify definition: a pulley within a transmission may be located away from a given joint, but rotate with that joint. Similarly, some pulleys in the system may be caused to rotate by the motion of more than one axis—for example, when they are part of a cabled differential, such as is employed in the preferred form of the present invention. 
     In the preferred embodiment, joints J 1  and J 2  are implemented through the use of a cabled differential transmission, designed similarly to that disclosed in U.S. Pat. No. 4,903,536, issued Feb. 27, 1990 to Massachusetts Institute of Technology and J. Kenneth Salisbury, Jr. et al. for COMPACT CABLE TRANSMISSION WITH CABLE DIFFERENTIAL, which patent is hereby incorporated herein by reference. 
     As described in U.S. Pat. No. 4,903,536, a cabled differential is a novel implementation of a differential transmission, in which two input pulleys (e.g., pulleys  505  in the robotic device  5  shown in  FIG. 6 ) with a common axis of rotation are coupled to a common output pulley, (e.g., pulley  540  in the robotic device  5  shown in  FIGS. 1 and 6 ) which is affixed to a spider or carrier (e.g., carrier  541  in the robotic device  5  shown in  FIGS. 1 and 6 ). This carrier is able to rotate about the common axis of rotation of the two input pulleys independently of those pulleys. The common output pulley, meanwhile, is able to rotate about an axis perpendicular to, and coincident with, the common axis of rotation of the two input pulleys. The two input pulleys are coupled to the output pulley such that a differential relationship is established between the three, wherein the rotation of the output pulley (e.g., pulley  540  in robotic device  5  shown in  FIGS. 1 and 6 ) is proportional to the sum of the rotations of the two input pulleys (e.g., pulleys  505  in robotic device  5  shown in  FIGS. 1 and 6 ), and the rotation of the carrier (e.g., carrier  541  in robotic device  5  shown in  FIGS. 1 and 6 ) is proportional to the difference of the rotations of the two input pulleys. In robotic device  5  shown in  FIGS. 1 and 6 , the rotation of the carrier of the differential is used to produce motion of the system about one axis of rotation (in the preferred embodiment, about axis  125  of joint J 1 ), and the rotation of the output of the differential transmission (i.e., the rotation of output pulley  540 ) is used to produce motion of the system about a second axis of rotation (in the preferred embodiment, about axis  130  of joint J 2 ). The use of a cabled differential enables these two motions to be produced by motors, which are affixed to lower kinematic frames (in the case of the preferred embodiment, to the ground kinematic frame, consisting of all components that are generally static when the device is in use). This dramatically decreases the moving mass of the device, thereby improving the dynamic performance and feel of the device. In the preferred implementation, this cabled differential transmission consists of two motors  500 , input pulleys  505 , output pulley  540 , etc., as hereinafter discussed. 
     Stated another way, as described in U.S. Pat. No. 4,903,536, the cabled differential is a novel implementation of a differential transmission, in which two input pulleys (e.g., pulleys  505  in robotic device  5  shown in  FIG. 6 ) with a common axis of rotation are coupled to a third common output pulley (e.g., pulley  540  in robotic device  5  shown in  FIG. 6 ), which rotates about an axis perpendicular to the input pulley axis, and is affixed to a carrier (e.g., carrier  541  in robotic device  5  shown in  FIG. 6 ) that rotates about the input pulley axis (i.e., axis  125  in robotic device  5  shown in  FIG. 6 ). The two input pulleys are coupled to the output pulley such that a differential relationship is established between the three, wherein the rotation of the output pulley is proportional to the sum of the rotations of the two input pulleys, and the rotation of the carrier is proportional to the difference of the rotations of the two input pulleys. This mechanism produces rotations about two axes (e.g., axis  125  of joint J 1  and axis  130  of joint J 2 ), while allowing the motors producing those motions to be affixed to lower kinematic frames, thereby decreasing the moving mass of the device and improving dynamic performance and feel. In the preferred implementation, this transmission consists of two motors  500 , two input pulleys  505 , output pulley  540 , etc., as hereinafter discussed. 
     In other words, as described in U.S. Pat. No. 4,903,536, the cabled transmission is a novel implementation of a differential transmission, wherein two input pulleys (e.g., pulleys  505  in robotic device  5  shown in  FIG. 6 ) are connected to a third common output pulley (e.g., pulley  540  in robotic device  5  shown in  FIG. 6 ) such that the rotation of the output pulley is proportional to the sum of the rotations of the two input pulleys, and the rotation of the differential carrier (e.g., carrier  541  in robotic device  5  shown in  FIG. 6 ) is proportional to the difference of the rotations of the two input pulleys. In the preferred implementation, this transmission consists of two motors  500 , two input pulleys  505 , output pulley  540 , etc., as hereinafter discussed. 
     As seen in  FIG. 6 , the cabled differential transmission preferably comprises two motors  500  which are affixed to the ground kinematic frame (e.g., base  502 ), which are coupled to input pulleys  505  through lengths of cable  571  and  572  —commonly wire rope, but alternatively natural fiber, synthetic fiber, or some other construction generally recognized as a form of cable—that are attached to the pinions  510  of motors  500 , wrapped in opposite directions but with the same chirality about pinions  510 , and terminated on the outer diameters  515  of input pulleys  505 . These input pulleys  505  rotate about axis  125  of joint J 1 , but their rotation may produce rotation of the device about axis  125  of joint J 1 , axis  130  of joint J 2 , or both axes simultaneously, due to the properties of the cable differential; furthermore, these input pulleys  505  are fixed to neither the aforementioned joint J 1  kinematic frame nor the aforementioned joint J 2  kinematic frame. As per U.S. Pat. No. 4,903,536, these input pulleys  505  include both large outer diameters  515 , as well as a series of substantially smaller stepped outer diameters  520 ,  525 ,  530  and  535 . These smaller stepped outer diameters  520 ,  525 ,  530  and  535  are coupled through further lengths of cable to output pulley  540 , which comprises a series of stepped outer diameters  545 ,  550 ,  555 , and  560 , which are substantially larger than the steps  520 ,  525 ,  530  and  535  they are coupled to on input pulleys  505 . This output pulley  540  rotates about axis  130  of joint J 2 , and is fixed to the joint J 2  kinematic frame. It has been found that it can be useful to make the range of motion of joint J 2  symmetric about a plane coincident with joint J 2  and perpendicular to joint J 1 , as this facilitates switching the device&#39;s chirality as described below. 
     By implementing this set of diametral relationships in the series of pulleys (i.e., input pulleys  505  and output pulley  540 ), progressively higher transmission ratios are achieved through the cabled transmission. In the preferred embodiment, a transmission ratio of 8.51:1 is implemented between motor pinions  510  and input pulleys  505 , and a transmission ratio of 1.79:1 is implemented between input pulleys  505  and output pulley  540 , generating a maximum transmission ratio between motor pinions  510  and output pulley  540  of 15.26:1. Throughout this cabled transmission, and all cabled transmissions of the present invention, care is taken to ensure that the ratio between the diameter of a given cable and the smallest diameter that it bends over is kept at 1:15 or smaller. Larger ratios, occurring when the cable is bent over smaller diameters, are known to significantly reduce cable fatigue life. 
     Still looking now at  FIG. 6 , distal to output pulley  540  is another cable transmission, comprising a motor  565 , coupled from its motor pinion  570  through cables  576 ,  577  to intermediate pulleys  575 , which are in turn coupled through cables  578 ,  579  to an output pulley  580 . These transmission cables are contained inside inner link  105 , which is fixed to the aforementioned joint J 2  kinematic frame. In this additional cable transmission, no differential element is implemented. In keeping with the cable transmission design taught in U.S. Pat. No. 4,903,536, the first stage of the cable transmission between motor pinion  570  and intermediate pulleys  575  is designed to be a high-speed, lower-tension transmission stage that traverses a greater distance; while the second stage of the cable transmission, between intermediate pulleys  575  and output pulley  580 , is designed to be a low-speed, higher-tension transmission stage that traverses a very short distance. In this cable transmission, intermediate pulleys  575 , output pulley  580  and the joint axis  135  of joint J 3  are substantially distal to motor  565 , a design which is accomplished by implementing a long cable run between motor pinion  570  and intermediate pulleys  575 . 
     As described in U.S. Pat. No. 4,903,536, this design has the benefit of moving the mass of motor  565  toward base  502  of robotic device  5 , reducing the inertia of the system. In the preferred implementation, the motor&#39;s mass is positioned coaxial to axis  130  of joint J 2 , and as close as possible to axis  125  of joint J 1 , thereby reducing inertia about both axes. This design is particularly valuable in the preferred implementation shown, since the mass of motor  565  is moved close to both axis  130  of joint J 2  and axis  125  of joint J 1 , thereby reducing inertia about both axes. A transmission ratio of 1.89:1 is preferably implemented between motor pinion  570  and intermediate pulleys  575 , and a transmission ratio of 5.06:1 is preferably implemented between intermediate pulleys  575  and output pulley  580 , yielding a maximum transmission ratio between motor pinion  575  and output pulley  580  of 9.55:1. 
     All transmission ratios listed here have been optimized based on a range of factors, including:
         device link lengths;   device component inertias and moments about axes;   the intended position of the device relative to the patient;   motor instantaneous peak and sustained torque limits;   motor controller output current capacity, and motor current capacity;   desired ability of device to overpower patient/be overpowered by patient; and   expected peak output force of patient.       

     This optimization process is extensive and at least partially qualitative; it is not reproduced here, since both the optimization process and its outcome will change significantly as the above factors change. Based on data gathered from a number of sources and internal experimentation, these forces are estimated to be:
         push/pull away from/towards patient&#39;s body: 45 N   up/down in front of patient: 15 N   left/right laterally in front of patient: 17 N It should be noted that generous factors of safety have been applied to these estimates.       

     Beyond output pulley  580  of joint J 3 , there is generally an outer link  110  ( FIGS. 1, 6 and 7 ). Outer link  110  is connected to output pulley  580  ( FIGS. 6 and 7 ) of joint J 3  by a mechanism  590  that allows the position of outer link  110  to be adjusted relative to output pulley  580  of joint J 3 . Mechanism  590  ( FIG. 7 ), which in a preferred embodiment allows the position of outer link  110  to be moved by some number of degrees (e.g., 172.5 degrees) about axis  135  of joint J 3  relative to output pulley  580  of joint J 3 , facilitates reversing the chirality of the robotic device, the importance and method of which is described herein. In the preferred embodiment, mechanism  590  is implemented by means of clamping two tabs  591  against a central hub  592  (which is shown in  FIG. 7  in cutaway) by means of a toggle lock  593  (e.g., like those commonly found on the forks of bicycles). The contacting faces of tabs  591  and central hub  592  are tapered as shown in  FIG. 7 , to both locate the parts in directions transverse to the direction of force application, and to increase the amount of torque that the clamped parts can resist. It has been found that it is important to ensure that the taper (at the contacting faces of tabs  591  and central hub  592 ) is a non-locking type, so that the system does not jam. Mechanism  590  allows outer link  110  to be flipped across a plane coincident to axis  135  of joint J 3 , rather than rotated around axis  135  of joint J 3 . While this initially seems like a minor distinction, when implemented with certain types of endpoint attachments, utilizing a mechanism that flips, rather than rotates, can significantly reduce the time required to reverse the chirality of the robotic device. There are also other components of the sort well known in the art of robotic arms that are not shown here which are used to ensure that mechanism  590  reaches its desired position, and that the mechanism&#39;s position does not shift during operation. By way of example but not limitation, these components may include limit switches, magnets, latches, etc. of the sort well known to a person skilled in the art of robotic arms. There is also a separate mechanism that allows outer link  110  to be removed from mechanism  590 , which facilitates switching between different types of endpoint attachments. In the preferred construction shown in  FIG. 7 , this is implemented through a latch  594 , which firmly clamps outer link  110  inside a tubular member  595  which is firmly attached to tabs  591 . This latch  594  is engaged when the robotic device is in use, but may be released to allow outer link  110  to be removed. 
     Robotic device  5  also comprises an onboard controller and/or an external controller for controlling operation of robotic device  5 . The onboard controller and/or external controller are of the sort which will be apparent to those skilled in the art in view of the present disclosure. By way of example but not limitation,  FIGS. 1 and 2  show an onboard controller  596  for controlling operation of robotic device  5 . Onboard controller  596  may sometimes be referred to herein as an “internal controller”.  FIG. 11  shows how an external controller  597  may be used to control operation of robotic device  5  and/or to receive feedback from robotic device  5  (where robotic device  5  may or may not also have an onboard controller). 
     There may also be other components that are included robotic device  5  which are well known in the art of robotic devices but are not shown or delineated here for the purposes of preserving clarity of the inventive subject matter, including but not limited to: electrical systems to actuate the motors (e.g., motors  500  and  565 ) of the robotic device; other computer or other control hardware for controlling operation of the robotic device; additional support structures for the robotic device (e.g., a mounting platform); covers and other safety or aesthetic components of the robotic device; and structures, interfaces and/or other devices for the patient (e.g., devices to position the patient relative to the robotic device, a video screen for the patient to view while interacting with the robotic device, a patient support such as, but not limited to, a wheelchair for the patient to sit on while using the robotic device, etc.). 
     Some specific innovative aspects of the present invention will hereinafter be discussed in further detail. 
     Non-Exoskeletal Device 
     As discussed above, robotic device  5  is a non-exoskeletal rehabilitation device. Exoskeletal rehabilitation devices are generally understood as those having some or all of the following characteristics:
         joint axes that pierce/are coaxial to the patient&#39;s limb joint axes, typically with each patient joint matched to at least one device joint; and   device components that capture each of the patient&#39;s limbs that are being rehabilitated, typically firmly constraining each limb segment to a corresponding segment of the arm of the robotic device.       

     In  FIG. 1 , a simplified representation of the joint axes of a patient&#39;s shoulder are shown: the abduction and adduction axis  600 , the flexion and extension axis  605 , and the internal and external rotation axis  610 . Also shown in  FIG. 1  is the axis  615  of the patient&#39;s elbow joint. As  FIG. 1  shows, joint axes J 1 , J 2  and J 3  of robotic device  5  are, by design, non-coaxial with the patient&#39;s joint axes  600 ,  605 ,  610  and  615 . Furthermore, in the preferred embodiment, the patient&#39;s limb  120  is only connected to, or captured by, robotic device  5  at the coupling element  115 . In other embodiments of the present invention, there may be multiple coupling points between the patient and the robotic device, which may partially or completely enclose the patient&#39;s limb; however, the majority of the structure of the robotic device of the present invention is not capturing the patient&#39;s limb. 
     Because the aforementioned two “conditions” of an exoskeletal system are not met (i.e., the joint axes J 1 , J 2  and J 3  of the robotic device are not intended to be coaxial with the patient&#39;s joint axes  600 ,  605 ,  610  and  615 , and because the segments of the patient&#39;s limb are not secured to corresponding segments of the arm of the robotic device), the robotic device of the present invention is not an exoskeletal rehabilitation device. While there are many non-exoskeletal rehabilitation devices currently in existence, the non-exoskeletal design of the present device is a critical characteristic distinguishing it from the prior art, since the device incorporates many of the beneficial characteristics of exoskeletal devices while avoiding the cost and complexity that are innate to exoskeletal designs. 
     Kinematic Relationship of Robotic Device and Patient 
       FIGS. 2 and 3  show a coordinate reference frame  160  for the patient (consisting of an up axis  161 , a forward axis  162  and a right axis  163 ), as well as a coordinate reference frame  170  for robotic device  5  (consisting of an up axis  171 , a forward axis  172  and a right axis  173 ). The locations and orientations of these reference frames  160 ,  170  defines a kinematic relationship between (i) robotic device  5  and its links  105 ,  110 , and (ii) the patient and their limb: robotic device  5  is designed such that its motions mimic those of the patient, in that a given motion of the patient&#39;s endpoint in reference frame  160  of the patient will be matched by a generally similar motion of the device&#39;s endpoint in reference frame  170  of robotic device  5 . This relationship is important to the definition of many of the innovative aspects of robotic device  5 , as shown below. 
     Before further explaining this concept, it is helpful to provide some terminology. The “patient reference frame” (or PRF)  160  and the “device reference frame” (or DRF)  170 , as used here, are located and oriented by constant physical characteristics of the patient and robotic device  5 . As shown in  FIGS. 2 and 3 , the origin of PRF  160  is defined at the base of the patient&#39;s limb which is coupled to the robotic device, and is considered fixed in space. The “up” vector  161 , which is treated as a “Z” vector in a right-handed coordinate system, is defined to point from this origin in the commonly accepted “up” direction (i.e., against the direction of gravity). The “forward” vector  162  is likewise defined in the commonly accepted “forward” direction (i.e., in front of the patient). More precisely, it is treated as a “Y” vector in a right-handed coordinate system, and is defined as the component of the vector pointing from the origin to the center of the limb&#39;s workspace which is perpendicular to the “up” vector. Finally, the “right” vector  163  points to the right of the patient. Rigorously defined, it is treated as an “X” vector in a right-handed coordinate system, and is consequently defined by the other two vectors. Thus, a reference frame  160  is defined for the patient which is located and oriented entirely by constant physical characteristics and features. While this coordinate frame definition has been executed in  FIGS. 2 and 3  for a patient&#39;s arm, this definition method can easily be extended to other limbs, such as a leg. 
     A similar reference frame is defined for the robotic device. The origin is placed at the centroid of the base of robotic device  5 , which must also be fixed in space. The “forward” vector  172  is defined as the component of the vector pointing from the origin to the geometric centroid of the device&#39;s workspace. The “up” vector  171  and the “right” vector  173  may be defined in arbitrary directions, so long as they meet the following conditions:
         1) they are mutually perpendicular;   2) they are both perpendicular to “forward” vector  172 ;   3) they meet the definition of a right-handed coordinate system wherein “up” vector  171  is treated as a Z vector, “right” vector  173  is treated as an X vector, and “forward” vector  172  is treated as a Y vector; and   4) preferably, but not necessarily, “up” vector  171  is oriented as closely as possible to the commonly accepted “up” direction (i.e., against the direction of gravity).       

     In some cases, such as with the ReoGO® arm rehabilitation system of Motorika Medical Ltd. of Mount Laurel, N.J., USA, the aforementioned condition “4)” cannot be satisfied because the device&#39;s “forward” vector already points in the generally accepted “up” direction; consequently, the “up” vector may be defined arbitrarily subject to the three previous conditions. This case is further detailed below. 
     When existing rehabilitation devices are separated into exoskeletal and non-exoskeletal devices as per the description above, a further distinction between these two groups becomes apparent based on this definition of reference frames. In exoskeletal devices, the robotic device and the patient operate with their reference frames (as defined above) oriented generally similarly, i.e., “up”, “right” and “forward” correspond to generally the same directions for both the patient and the robotic device, with the misalignment between any pair of directions in the PRF (patient reference frame) and DRF (device reference frame), respectively, preferably no greater than 60 degrees (i.e., the “forward” direction in the DRF will deviate no more than 60 degrees from the “forward” direction in the PRF), and preferably no greater than 45 degrees. Meanwhile, to date, a non-exoskeletal device in which the device reference frame and the patient reference frame are generally oriented similarly in this way has not been created. Devices available today are oriented relative to the patient in a number of different ways, including the following:
         The DRF may be rotated 180° around the “up” axis relative to the PRF so that the device “faces” towards the patient, or rotated 90° around the “up” axis so that the device “faces” perpendicular to the patient: for example, in the InMotion ARM™ system of Interactive Motion Technologies of Watertown, Mass., USA; the HapticMaster™ haptic system of Moog Incorporated of East Aurora, N.Y., USA; the DeXtreme™ arm of BioXtreme of Rehovot, Israel; or the KINARM End-Point Robot™ of BKIN Technologies of Kingston, Ontario, Canada. In the case of the DeXtreme™ arm, for example, the device is designed to be used while situated in front of the patient. Its workspace, which is generally shaped like an acute segment of a right cylinder radiating from the device&#39;s base, likewise faces toward the patient. When a coordinate reference frame is generated for the device&#39;s workspace as outlined above, the “forward” direction for the device—which points from the centroid of the base of the device to the centroid of the device&#39;s workspace—will be found to point toward the patient. Consequently, the device reference frame is not oriented similarly to the patient reference frame.   Alternatively, the DRF may be rotated 90° about the “right” axis relative to the PRF such that the device&#39;s “forward” axis is parallel to the patient&#39;s “up” axis; or other combinations. One example is the ReoGO® arm rehabilitation system of Motorika Medical Ltd of Mount Laurel, N.J., USA, where the device&#39;s base sits underneath the patient&#39;s arm undergoing rehabilitation, and its primary link extends up to the patient&#39;s arm. Its workspace is generally conical, with the tip of the cone located at the centroid of the base of the device. When a coordinate reference frame is generated for the device as outlined above, the “forward” vector of the device reference frame will be found to have the same direction as the “up” vector in the patient reference frame. Consequently, the device reference frame is not oriented similarly to that of the patient reference frame.   Finally, devices like the ArmAssist™ device of Tecnalia® of Donostia-San Sebastián, Spain may not have a definable DRF. The ArmAssist™ device is a small mobile platform which is designed to sit on a tabletop in front of the patient. The patient&#39;s arm is attached to the device, which then moves around the tabletop to provide rehabilitative therapy. Since the ArmAssist™ device is fully mobile, a fixed origin cannot be defined for it as per the method outlined above, and it is not relevant to this discussion.       

     The robotic device of the present invention is the first non-exoskeletal device which is designed to operate with its reference frame  170  oriented generally similarly to the reference frame  160  of the patient. This innovation allows the robotic device to leverage advantages that are otherwise limited to exoskeletal devices, including:
         Reduced interference with the patient&#39;s line-of-sight or body, since the robotic device does not need to sit in front of/to the side of the patient.   More optimal position-torque relationships between patient and device, since the moment arms between the device and patient endpoints and their joints are directly proportional to one another, rather than inversely proportional to one another as in other devices. For example, when the device&#39;s links are extended, the patient&#39;s limb undergoing rehabilitation will generally be extended as well. While the device is not able to exert as much force at its endpoint as it can when the endpoint is closer to the device&#39;s joints, the patient&#39;s force output capacity will likewise be reduced. Similarly, when the patient&#39;s limb is contracted and the force output is maximized, the device&#39;s endpoint will be closer to its joints, and its endpoint output force capacity will also be maximized.   Better workspace overlap between the patient and the device, since the device&#39;s links extend from its base in the same general direction that the patient&#39;s limb extends from the body.       

     Like an exoskeletal device, robotic device  5  generally mimics the movements of the patient&#39;s limb, in that the endpoint of the device tracks the patient&#39;s limb, and a given motion in reference frame  160  of the patient produces motion in a generally similar direction in the device&#39;s reference frame  170 . For example, if the patient moves their limb to the right in the patient&#39;s reference frame  160 , the device&#39;s links will generally move to the right in the device&#39;s reference frame  170 , as shown in  FIG. 4 . However, unlike an exoskeletal device, the individual links and joints of the robotic device do not necessarily mimic the motions of individual segments or joints of the patient&#39;s limb, even though the endpoint of the robotic device does track the patient&#39;s endpoint. As shown in  FIG. 4 , in the preferred embodiment, motions in front of the patient cause both the patient&#39;s limbs and links  105 ,  110  of robotic device  5  to extend; by contrast, in  FIG. 4 , motions to the far right of the patient cause the patient&#39;s limb to straighten while links  105 ,  110  of robotic device  5  bend. By operating without this constraint (i.e., that the individual links and joints of the robotic device do not necessarily mimic the motions of the individual segments or joints of the patient&#39;s limb), robotic device  5  avoids many of the weaknesses inherent in exoskeletal devices, particularly the bulk, complexity, cost and set-up time associated with directly replicating the kinematics of a limb. 
     Because of the need for this distinction between the robotic device of the present invention and exoskeletal devices (i.e., that a relationship cannot easily be defined between the patient&#39;s limb and the links of robotic device  5 ), it is necessary to define the relationship between the robotic device and the patient as a function of the bases, endpoints and orientations of the robotic device and the patient. By defining device and patient reference frames in this manner, the previous statement that “robotic device  5  is designed such that its motions mimic those of the patient, in that a given motion of the patient&#39;s endpoint in reference frame  160  of the patient will be matched by a generally similar motion of the device&#39;s endpoint in reference frame  170  of robotic device  5 ” is satisfied only when robotic device  5  is oriented relative to the patient as described herein. A series of simple logical tests have been developed to aid in determining whether a device meets the criteria outlined above. For these tests, the device is assumed to be in its typical operating position and configuration relative to the patient, and a PRF is defined for the patient&#39;s limb undergoing rehabilitation as described above.
         1) Is the device an exoskeletal rehabilitation device, as defined previously?
           a. YES: Device does not meet criteria—criteria are only applicable to non-exoskeletal devices.   b. NO: Continue.   
           2) Can an origin that is fixed relative to the world reference frame and located at the centroid of the base of the device be defined?
           a. YES: Continue.   b. NO: Device does not meet criteria—criteria are not applicable to mobile devices.   
           3) Consider the device&#39;s workspace, and find the geometric centroid of that workspace. Can a “forward”, or Y, vector be defined between the geometric centroid of the device&#39;s workspace and the device&#39;s origin?
           a. YES: Continue.   b. NO: Device does not meet criteria.   
           4) Can the “up”, or Z, vector and the “right”, or X, vector be defined as outlined above relative to the “forward”, or Y, vector?
           a. YES: Continue.   b. NO: Device does not meet criteria—it is likely designed for a significantly different rehabilitation paradigm than the device disclosed here.   
           5) Are the workspaces of the device and patient oriented generally similarly, in that the “right”, or X, “forward”, or Y, and “up”, or Z, vectors of both coordinate reference frames have generally the same direction, with a deviation of less than a selected number of degrees between any pair of vectors? (In the preferred embodiment, this is preferably less than 60 degrees, and more preferably less than 45 degrees.)
           a. YES: Continue.   b. NO: The device does not meet the criteria outlined—it is positioned differently relative to the patient than the device outlined here.   
           6) Are motions of the patient&#39;s endpoint mimicked or tracked by similar motions of the device&#39;s endpoint?
           a. YES: The device meets the criteria outlined.   b. NO: The device does not meet the criteria outlined.
 
To date, no device with more than 2 degrees of freedom, other than the system described herein, has been found that successfully passes this series of tests.
   
               

     Stated another way, generally similar orientation between the patient and the device can be examined by identifying a “forward” direction for both the user and the device. In the patient&#39;s case, the “forward” direction can be defined as the general direction from the base of the patient&#39;s arm undergoing rehabilitation, along the patient&#39;s limb, towards the patient&#39;s endpoint when it is at the position most commonly accessed during use of the device. In the device&#39;s case, the “forward” direction can be defined as the general direction from the base of the device, along the device&#39;s links and joints, towards the device&#39;s endpoint when it is at the position most commonly accessed during use of the device. If the “forward” direction of the device and the “forward” direction of the patient are generally parallel (e.g., preferably with less than 60 degrees of deviation, and more preferably with less than 45 degrees of deviation), then the device and the user can be said to be generally similarly oriented. 
     General Location of System 
     One preferred embodiment of the present invention is shown in  FIGS. 3 and 4 , where robotic device  5  is positioned to the side of, and slightly behind, the patient (in this case, with axis  125  of joint J 1  behind, or coincident to, the patient&#39;s coronal plane). In this embodiment, reference frame  170  of robotic device  5  and reference frame  160  of the patient are oriented generally similarly to one another, as described above. Robotic device  5  is kept out of the patient&#39;s workspace and line of sight, making it both physically and visually unobtrusive. The workspaces of the robotic device and the patient overlap to a high degree. The range of motion allowed by this positioning is still quite large, as shown in  FIG. 4 , and approaches or exceeds that allowed by high-DOF exoskeletal systems. 
     It should be noted that while this arrangement (i.e., with robotic device  5  positioned to the side of, and slightly behind, the patient) has been found to be preferable for certain rehabilitative therapies, there are other embodiments in which robotic device  5  is positioned differently relative to the patient which may be better suited to other applications, such as use as a haptic input/control device, or other rehabilitative activities. For example, in the case of advanced-stage arm rehabilitation, in situations where the patient is reaching up and away from the device, it may prove optimal to place the robotic device slightly in front of the patient. 
     Link Stacking Order 
     Looking next at  FIGS. 5A, 5B and 5C , several novel implementations of the system are shown wherein the device&#39;s links  105 ,  110  are ordered in different directions to facilitate different activities. By way of example but not limitation,  FIG. 5A  shows a configuration referred to as the “stacked-down” configuration, in which outer link  110  of robotic device  5  is attached to the underside of inner link  105  of robotic device  5 , allowing the device to reach from above the patient, downwards, to their limb (attached via coupling element  115 ).  FIG. 5C  shows a configuration referred to as the “stacked-up” configuration, in which outer link  110  of robotic device  5  is attached to the top side of inner link  105  of robotic device  5 , allowing the device to reach from below the patient, upwards, to their limb (attached via coupling element  115 ). Both implementations may prove optimal in different situations. The “stacked-down” variant is less likely to interfere with the patient&#39;s arm during rehabilitation activity because of its position above the patient&#39;s workspace, and may prove more useful for high-functioning rehabilitation patients who require expanded workspace. Conversely, the “stacked-up” variant is better able to support a patient&#39;s arm, and is less likely to interfere with the patient&#39;s visual workspace; it is better suited for low-functioning patients.  FIG. 5B  shows a configuration referred to as the “stacked-flat” configuration, in which outer link  110  of robotic device  5  is attached to the bottom side of inner link  105  of robotic device  5 , and coupling element  115  is attached to the top side of outer link  110 , allowing the device to reach the patient so that the forearm of the patient is approximately flat with inner link  105 . 
     Cabled Differential, with Alternative Configurations 
       FIG. 6  illustrates an important aspect of the present invention, i.e., the use of a cabled differential (see, for example, U.S. Pat. No. 4,903,536) in a rehabilitation device. The preferred embodiment of robotic device  5  comprises three revolute joints J 1 , J 2  and J 3 , implemented in a pitch-yaw-yaw configuration ( FIG. 1 ), with the first two joints (i.e., J 1  and J 2 ) linked in a cabled differential as shown in  FIG. 6 . As shown in  FIG. 6 , the use of a cabled differential allows a motor that would normally be mounted on a higher-level kinematic frame to be moved down to a lower-level frame. For example, in the preferred embodiment shown in  FIG. 6 , motors  500  that cause rotation about joint J 1  and joint J 2  are moved from the aforementioned joint J 1  kinematic frame (which rotates about axis  125  of joint J 1 ) down to the aforementioned ground kinematic frame (the ground frame; co-located with base  100  in  FIG. 1 ). This significantly reduces the inertia that motors  500  are required to move, which improves the performance of the robotic device and reduces its cost by permitting smaller motors  500  to be used. Although this is implemented in the preferred embodiment at the base of the robotic device, the principle behind this design is valid anywhere along a device&#39;s kinematic chain. This is a particularly important innovation in the context of a rehabilitation device because of its ability to reduce the device&#39;s cost, which must be kept low to ensure the commercial success of the device. This configuration also allows the exclusive use of rotary joints (instead of prismatic joints), which greatly simplifies the design of the device. Lower inertia also improves the safety of the device by lowering the momentum of the device. Finally, this innovation also maximizes usability by allowing the visual bulk of the device to be shifted away from the patient&#39;s line of sight towards the base of the device. While this concept is executed as part of a rehabilitation device with three degrees of freedom in the preferred embodiment, it is clearly applicable to other rehabilitation devices with as few as two degrees of freedom. 
     Furthermore, in the preferred embodiment shown in  FIGS. 1 and 6 , the implementation of a cabled differential with the input and output axes (i.e., the axes of input pulleys  505  and output pulley  540 ) both perpendicular to the distal link axis (i.e., the axis along inner link  105 ) provides the benefits of a cabled differential while allowing the unique pitch-yaw kinematic arrangement that makes this device so well suited to rehabilitation use. Previous implementations of cabled differentials have either been arranged in a pitch-roll configuration such as in the Barrett WAM product of Barrett Technology, Inc. of Newton, Mass. as shown at  700  in  FIG. 8C , or in a roll-pitch configuration such as in the Barrett WAM wrist product as shown at  720  in  FIG. 8B . In both of these implementations (i.e., the pitch-roll configuration  700  of  FIG. 8C  and the roll-pitch configuration  720  of  FIG. 8B ), either the distal link (i.e., the link beyond the differential in the kinematic chain) or the proximal link (i.e., the link before the differential in the kinematic chain) is permanently coaxial with one of the two differential rotational axes. In the case of the pitch-roll configuration  700  of  FIG. 8C , outer link  710  is always coaxial to the differential output axis  705 ; in the roll-pitch configuration  720  of  FIG. 8B , inner link  725  is always coaxial to the differential input axis  730 . 
     To date, however, the cabled differential has not been used in a configuration where neither of the differential axes is coaxial to one of the links. This configuration has been successfully implemented in the preferred embodiment of the present invention, as seen in both  FIG. 6  (see the pitch-yaw configuration of joints J 1  and J 2  relative to the inner link of robotic device  5 ) and in  FIG. 8A , where the novel pitch-yaw configuration  740  is shown. This new implementation of the cabled differential enables innovative kinematic configurations like that used in the present invention. 
     Bi-Manual, Multi-Dimensional Rehabilitation Exercises and Device Design 
       FIG. 9  shows how the preferred embodiment of robotic device  5  is optimal for the purposes of switching from right-handed use to left-handed use. Robotic device  5  is essentially symmetric across a plane parallel to the patient&#39;s mid-sagittal plane and coincident with axis  130  of joint J 2 . By simply ensuring that the range of joint J 2  is symmetric about the previously-described plane, and enabling outer link  110  to be reversed about axis  135  of joint J 3  such that its range of motion is symmetric about the previously-described plane in either position, the device&#39;s chirality can easily be reversed, enabling it to be used on either the right side of the patient&#39;s body or the left side of the patient&#39;s body, as seen in  FIG. 9 . 
     Finally,  FIG. 10  illustrates how the innate symmetry and reversible chirality of robotic device  5  combine with its unique working position/orientation and small size to allow two units of the robotic device to be used simultaneously for three-dimensional bi-manual rehabilitation. In bi-manual rehabilitation, the afflicted limb is paired with a non-afflicted limb in rehabilitation activities, including cooperative tasks, such as using both limbs to lift an object; and instructive tasks, where the healthy limb “drives” the afflicted limb. The value of bi-manual rehabilitation (particularly in the context of rehabilitation from a neuromuscular injury such as a stroke, which can make execution of neurologically complex tasks like coordinated movement between limbs on opposite sides of the body exceedingly difficult) was theorized as early as 1951, and has gained significant traction over the past 20 years. See “Bimanual Training After Stroke: Are Two Hands Better Than One?” Rose, Dorian K. and Winstein, Carolee J. Topics in Stroke Rehabilitation; 2004 Fall; 11(4): 20-30. Robotic rehabilitation devices are extremely well suited to this type of therapy, due to their ability to precisely control the motion of the patient&#39;s limbs and coordinate with other rehabilitation devices. 
     In an exemplary implementation shown in  FIG. 10 , a first robotic device  5  is connected to the patient&#39;s afflicted right arm, while a second robotic device  5  is connected to a more functional left arm. The robotic devices are linked to each other through some type of common controller (e.g., as seen in  FIG. 12 , an external controller  597  that communicates with the onboard controllers of both robotic devices  5 , while facilitating communication between the two devices), which coordinates the rehabilitation therapy. While this example is demonstrated using images of the preferred embodiment of the robotic device, it may be understood that the essential concept of bi-manual rehabilitation may be implemented with any variety of devices, even if those devices are dissimilar to one another and/or to the preferred embodiment of robotic device  5 . However, there are significant advantages to using two similar robotic devices  5  for bi-manual rehabilitation, which are disclosed below, and which lead to a novel method for bi-manual rehabilitation. 
     The robotic device  5  described here is the first non-planar rehabilitation device to be purpose-designed for this type of dual-device, simultaneous use in a three-dimensional bi-manual system. As described earlier, the robotic device&#39;s innate symmetry allows its chirality to be easily reversed, allowing the same robotic device design to be used for rehabilitation of both right and left limbs. Furthermore, the device&#39;s small footprint facilitates simultaneous use of two systems, as shown in  FIG. 10 . While other devices, such as the Armeo™Power system of Hocoma AG of Volketswil, Switzerland, are similarly reversible, the size of these systems and their position relative to the patient precludes their use in a bi-manual rehabilitation system, since the bases of the two systems would interfere. There are also some devices that have been deliberately designed for bi-manual rehabilitation, such as the KINARM Exoskeleton™ and End-Point™ robots of BKIN Technologies of Kingston, Ontario, Canada. However, as mentioned above, these devices are deliberately limited to planar (i.e., two-dimensional) rehabilitative therapies, significantly impacting their utility for patients. 
     There exists one known example of a system that is nominally capable of performing limited 3-dimensional bi-manual rehabilitation therapies with only uni-manual actuation, i.e., the 3 rd -generation Mirror-Image Motion Enabler (MIME) rehabilitation robot, developed as a collaborative project between the Department of Veterans Affairs and Stanford University in 1999. See “Development of robots for rehabilitation therapy: The Palo Alto VA/Stanford experience.” Burgar et. al. Journal of Rehabilitation Research and Development. Vol. 37 No. 6, November/December 2000, pp. 663-673. The 3 rd -generation MIME robot consists of a PUMA-560 industrial robot affixed to the patient&#39;s afflicted limb, and a passive six-axis MicroScribe™ digitizer affixed to a splint, which is in turn coupled to the patient&#39;s healthy limb. In the system&#39;s bi-manual mode, motions of the healthy limb are detected by the digitizer and passed to the robotic arm, which moves the afflicted limb such that its motions mirror those of the healthy limb. While this system can execute a limited set of bi-manual rehabilitation therapies, it is fundamentally limited by the uni-directional flow of information within the system: information can be passed from the healthy limb to the afflicted limb, but not from the afflicted limb back to the healthy limb to the healthy limb, since the digitizer is passive and does not have motors or other mechanisms with which to exert forces on the patient&#39;s healthy limb. 
     In the implementation described herein, the use of two similar, active robotic devices  5 —in the preferred implementation, with similar kinematics, joint ranges, force output limits and static and dynamic performance characteristics —enables bi-directional information flow (i.e., bi-directional information flow wherein both devices send, receive and respond to information from the other device), creating a bi-manual rehabilitation system that is capable of monitoring the position of both the afflicted and healthy limbs, moving the patient&#39;s afflicted limb in three dimensions and potentially controlling its orientation simultaneously, and optionally providing simultaneous force feedback, support or other force inputs to the healthy limb. For example, the robotic device connected to the patient&#39;s healthy limb can be used to “drive” the robotic device connected to the patient&#39;s afflicted limb, while simultaneously supporting the healthy limb to prevent fatigue, and providing force feedback to the healthy limb as required by the therapy. In this respect it has been found that the cable drives used in the preferred implementation of the present invention are particularly well suited to this type of use, because of the high mechanical bandwidth of cable drive transmissions; however, alternative embodiments could be implemented using alternative mechanical drive systems. Regardless of the specific implementation, this bi-directional information flow—when executed between two similar devices with the facilitating characteristics described here—allows the device to be used for a far wider range of three-dimensional bi-manual rehabilitative therapies than prior art systems and enables the method disclosed herein. 
     User Interface Endpoint Device and Left-Hand to Right-Hand Flipping Mechanism 
     In the foregoing sections, robotic device  5  was described as having a coupling element  115  for coupling outer link  110  to a patient, commonly to a limb of a patient, with outer link  110  being detachably connected to the remainder of the robotic device at the aforementioned mechanism  590  ( FIGS. 6 and 7 ), e.g., via latch  594  ( FIG. 7 ). Coupling element  115  and outer link  110  can be thought of as together constituting a user interface endpoint device (i.e., an “endpoint”) for robotic device  5 , i.e., the portion of robotic device  5  that physically contacts the patient. In the following section, different possible embodiments of endpoints, all of which are modular and “swappable” on robotic device  5 , are described. Different types of endpoints are important to allow patients with different functional capabilities, and different therapeutic goals, to use the system. 
       FIGS. 13, 13A, 14 and 15  show a cradle endpoint  800  for use by the right-hand of a patient. Cradle endpoint  800  generally comprises a cradle  805  for receiving a limb (e.g., the forearm) of a patient, straps  810  for securing the limb to cradle  805 , a connector  815  for connecting cradle  805  to outer link  110 , and the aforementioned outer link  110 . Cradle endpoint  800  preferably also comprises a ball grip  820  for gripping by the patient (e.g., the hand of a patient). With cradle endpoint  800 , the patient grabs the ball and straps their forearm to the cradle. Cradle endpoint  800  is intended to be used by patients with moderate or severe functional impairments, or by users that want to rest the weight of their arm on the system during use. If desired, a monitor  825  may be provided adjacent to robotic device  5  for providing the patient with visual feedback while using robotic device  5 . By way of example but not limitation, cradle endpoint  800  may provide haptic feedback to the patient and monitor  825  may provide visual feedback to the patient, and the system may also provide audible feedback. 
     Note that in  FIGS. 13 and 13A , robotic system  5  is shown mounted to a movable base  100 , i.e., a base  100  which is mounted on wheels (or casters)  826  which may be free-wheeling or driven by onboard controller  596  (which may be contained in its own housing, e.g., in the manner shown in  FIG. 13 ). 
     Note also that in this form of the invention, U-shaped frame  140  may be supported above base  100  via a telescoping assembly  827  which allows the height of U-shaped frame  140  (and hence the height of the robotic arm) to be adjusted relative to base  100 . This feature is highly advantageous, since it facilitates the use of robotic device  5  with patients who are both sitting ( FIG. 15A ) and standing ( FIG. 15B ). In one preferred form of the invention, telescoping assembly  827  comprises a rigid and strong linear actuator (not shown) that can extend approximately 0.5 meter in height. An electric motor (not shown) raises and lowers the top of telescoping assembly  827  (and hence raises and lowers the robotic arm mounted to the top of the telescoping assembly). This height adjustment is important for people of different heights and for different wheelchair types. By way of example but not limitation, lower-functioning patients who are wheelchair-bound can use the device near the lower end of the vertical travel. Higher-functioning patients who are re-learning to amble can use the device near the upper end of the vertical travel and engage with exercises that gently challenge balance, e.g., in an enjoyable game atmosphere. 
     Of course, the vertical height adjustment could be done by other means well known in the art, such as a manual foot-pumping hydraulic lift. 
       FIG. 16  shows the same cradle endpoint  800 , except reconfigured for use by the left-hand of a patient. 
       FIG. 17  shows a ball endpoint  800 B. Ball endpoint  800 B is substantially the same as cradle endpoint  800 A, except that cradle  805 A and straps  810 A are omitted. With ball endpoint  800 B, ball grip  820 B is simply “grabbed” by the user. Ball endpoint  800 B is intended to be used by relatively healthy users, for example, high-functioning stroke patients. Ball endpoint  800 B can also be used as a haptic-input device for healthy users for gaming or use with computer programs. Also contemplated is the possibility to secure the user&#39;s hand to the ball with an ace bandage (not shown) or a built-in strap/webbing system (not shown). 
       FIG. 18  shows a cradle endpoint with hand-grip assist  800 C. Cradle endpoint with hand-grip assist  800 C is substantially the same as cradle endpoint  800 A except that ball grip  820 A is replaced by an actuated or spring-based hand-grip  820 C. In this form of the invention, the user slips their hand into hand-grip  820 C and straps their forearm to cradle  805 C using straps  810 C. Cradle endpoint with hand-grip assist  800 C is similar to cradle endpoint  800 A described above, with the added functionality of an actuated or spring-based device that provides assistance to the user to open and/or close their hand. 
     Novel attributes of these endpoint devices are listed below and described in further detail in the sections that follow: 
     A. single yaw-axis coincident with point-of-interest; 
     B. flexible arm support (cradle); 
     C. adjustable pitch angle; 
     D. off-axis rotatable hand support; 
     E. hand-presence sensing; 
     F. modular endpoint; 
     G. endpoint-presence sensing; 
     H. endpoint-type sensing; 
     I. gravity compensation algorithms; and 
     J. changing handedness. 
     A. Single Yaw Axis Coincident with Point-of-Interest 
     In one preferred form of the invention, the endpoint device comprises a single yaw axis which is coincident with a point-of-interest (e.g., the user&#39;s hand). By way of example but not limitation, and looking now at  FIG. 19 , cradle endpoint  800  comprises a single passive degree-of-freedom (yaw) that is coincident with the point-of-interest (i.e., ball grip  820  which is grasped by the user&#39;s hand). Note that cradle  805  and ball grip  820  both rotate about a yaw axis  830 . Note also that connector  815  comprises a first portion  835  for connection to outer link  110 , and a second portion  840  for connection to cradle  805  and ball grip  820 , with first portion  835  being connected to outer link  110  so as to provide rotation about a pitch axis  845 . 
     B. Flexible Arm Support (Cradle) 
     Another aspect of the present invention is the ability to provide a flexible connection between a forearm support (e.g., cradle  805 ) and the rest of the endpoint device. In this way the endpoint device is able to support the weight of the arm, but allows the user to outstretch their arm without uncomfortable pressure from the rear strap  810 . By way of example but not limitation, and looking now at  FIG. 20 , there is shown a cradle endpoint  800  that comprises a leaf spring  850  which enables flexibility and allows a user&#39;s arm to lift up during certain three-dimensional motions. Hard stops  855  support the weight of the user&#39;s arm when the cradle is perpendicular to yaw axis  830 . 
     C. Adjustable Pitch Angle 
     Another aspect of the present invention is the provision of an adjustable pitch angle that: 1) enables left-hand to right-hand switching, and 2) enables small angular adjustments depending on user size, the workspace of interest, and the type of exercise. By way of example but not limitation, and looking now at  FIG. 20 , it will be seen that a pitch angle adjustment knob  860  may allow the configuration of first portion  835  to be adjusted relative to outer link  110 . 
     D. Off-Axis Rotatable Hand Support 
     Still another aspect of the present invention is the provision of an off-axis-rotatable hand grip (e.g., ball grip) that enhances comfort while allowing for different hand sizes. By way of example but not limitation, and looking now at  FIG. 20 , ball grip  820  can be rotated about yaw axis  830 . Note that in this form of the invention, the mounting shaft  865  for ball grip  820  is disposed “off-axis” from the center of ball grip  820 . This “off-axis” mounting allows the ball grip to be rotated manually for comfort—for a small hand, the ball grip can be rotated so that the bulk of the ball grip (i.e., the fatter section) is oriented away from the palm of the user, while for a larger hand, the ball grip can be rotated so that the bulk of the ball grip is oriented towards the palm of the user. 
     E. Hand-Presence Sensing 
     Another feature of the present invention is the inclusion of an electronic hand-presence sensing system. More particularly, in one preferred form of the invention, a capacitive sensing system is provided which detects the presence of the user&#39;s limb on the endpoint device and signals the robotic device that a person&#39;s limb is (or is not) present on the endpoint device. This is a safety and functionality feature and is particularly important for some endpoint devices, e.g., ball endpoint  800 B ( FIG. 17 ) in which the user&#39;s arm is not necessarily strapped to the endpoint—if the user lets go of the endpoint device, the capacitive sensing system detects this and the robotic device can pause (“soft-stop”). Even in the case where straps are used, the patient may still slip off of the device. Once the user re-engages the endpoint device (e.g., grabs the ball grip again), the capacitive sensing system detects this and the robotic device continues working. 
     The status of the presence of the user is preferably made clear to the patient and therapist immediately by lighting up ball grip  820  (or another status light, not shown, provided on the endpoint device or elsewhere on robotic device  5 ) in one of several colors to report status, such as green when the patient engages the device and the device is active, or yellow to indicate that the system is ready to go and awaiting the patient or user. The system may also use audible sounds to help identify or confirm the status of the presence of the user. 
     By way of example but not limitation, cradle endpoint  800  may have its ball grip  820  configured with a capacitive sensing system which communicates with onboard controller  596  of robotic device  5 . Such capacitive sensing systems are well known in the sensor art and are easily adaptable to ball grip  820 . In accordance with the present invention, when the user grips ball grip  820 , the capacitive sensing system associated with ball grip  820  detects user engagement and advises onboard controller  596  of robotic device  5  that the user is engaged with the endpoint device. Robotic device  5  may then proceed with the therapeutic regime programmed into onboard controller  596  of robotic device  5 . However, if the user lets go of ball grip  820 , the capacitive sensing system associated with ball grip  820  detects user disengagement and advises onboard controller  596  of robotic device  5  that the user is no longer engaged with the endpoint device. Robotic device  5  may then suspend the therapeutic regime programmed into onboard controller  596  of robotic device  5 . 
     F. Modular Endpoint 
     Another aspect of the present invention is the ability to easily “swap out” different endpoints on robotic device  5  and to have electrical connections occur automatically when the mechanical connection between the new endpoint and the robotic device is made. In one preferred form of the invention, this is accomplished with a mechanical latch (e.g., a mechanical latch such as one manufactured by SouthCo of Concordville, Pa.), custom-designed nested tubes, and a floating electrical connector system (e.g., a “Molex Mini-Fit Blindmate” system such as one manufactured by Molex of Lisle, Ill.) which together provide mechanical and electrical connections which are able to account for mechanical misalignment without stressing the electrical connections. 
     G. Endpoint-Presence Sensing 
     In one preferred form of the invention, a mechanical switch is provided on robotic device  5  that detects the presence (or absence) of an endpoint device. Alternatively, an electrical switch may also be provided to detect the presence (or absence) of an endpoint device. Such mechanical and electrical switches are well known in the sensor art and are easily adaptable to the portion of robotic device  5 , which receives outer link  110  of the endpoint devices. Endpoint-presence sensing is important for system safety—if the endpoint should become disconnected from robotic device  5  during operation of robotic device  5 , the robotic device  5  can go into a safe (“motionless”) mode until the endpoint is re-attached (or another endpoint is attached in its place). 
     H. Endpoint-Type Sensing 
     An important aspect of the modularity of the endpoints is that robotic device  5  is configured so that it can automatically sense and recognize the type of endpoint that is installed on the robotic device. This allows robotic device  5  to automatically adjust its operating parameters according to the particular endpoint which is mounted to the robotic device, e.g., it allows robotic device  5  to adjust various operating parameters such as the kinematics related to endpoint location, gravity-assist calculations (see below), etc. By way of example but not limitation, outer link  110  of each endpoint can comprise an encoded element representative of the type of endpoint and the portion of robotic device  5  which receives outer link  110  can comprise a reader element—when an endpoint is mounted to robotic device  5 , the reader element on robotic device  5  reads the encoded element on the mounted endpoint and the reader element appropriately advises onboard controller  596  for robotic device  5 . 
     I. Gravity Compensation Algorithms 
     In one preferred form of the invention, gravity compensation means are provided to make the user&#39;s limb feel weightless. This is done by applying an upward bias to the endpoint device which can offset the weight of the user&#39;s limb, thereby effectively rendering the user&#39;s limb “weightless”. Such gravity compensation may be achieved by having onboard controller  596  read the torque levels on motors  500  and  565  when a user&#39;s limb is engaging the endpoint device and then energizing motors  500  and  565  so as to apply an offsetting torque to the motors, whereby to offset the weight of the user&#39;s limb. Gravity compensation is important inasmuch as it allows a user to use the system for extended periods of time without tiring. However, this can be complex inasmuch as the weight of different people&#39;s limbs are different and because the weight of a single person&#39;s limb changes as he/she moves the limb to different locations and activates/adjusts different muscle groups. To this end, the gravity compensation means of the present invention includes various apparatus/algorithms/procedures which involve: 
     1) strapping a user&#39;s limb to an endpoint device, having the user move the endpoint of their limb to a predetermined number of points, relaxing at each point, and having the robotic device record the motor-torques (e.g., the loads imposed on motors  500  and  565 ) at each point; 
     2) taking the data as described in step  1 ) above from multiple users and taking an average of the data; 
     3) taking the data as described in step  1 ) above from multiple users and creating different user profiles based on body/limb size; 
     4) using the results of the above steps to create an easily-adjustable gain factor that increases and decreases the gravity-assistance forces provided by robotic device  5  so as to render the user&#39;s limb substantially weightless as it moves through a prescribed physical therapy regime; and 
     5) using the results of the above steps so that a new user (with no calibration record) needs to relax his/her limb in only a small set of data points (e.g., 1 to 5 data points) and the system then maps that user to a useful gravity-compensation profile using the reduced set of data points. 
     Note that onboard controller  596  may be configured to compensate for the effects of gravity when the endpoint device is engaged by a limb of a user in a single step, or onboard controller  596  may be configured to compensate for the effects of gravity in a series of incremental steps. This latter approach can be advantageous in some circumstances since the gradual application of gravity compensation avoids any surprise to the user. Note also that onboard controller  596  can apply the gravity compensation automatically or onboard controller  596  can apply the gravity compensation under the guidance of an operator (e.g., a therapist). 
     J. Changing Handedness 
     Robotic device  5  is configured so that it has the ability to easily flip from a right-hand to a left-hand configuration, e.g., using a cam-latch (similar to those found on front bicycle wheels) such as the aforementioned cam-latch  594  which allows outer link  110  of a given endpoint device to be quickly and easily attached to/detached from the remainder of robotic device  5 . Furthermore, robotic device has knowledge of the “handedness” of a given endpoint device due to the aforementioned automatic endpoint sensing switches. This allows robotic device  5  to automatically alter the software in its onboard controller  596  to account for the different kinematics of different endpoint devices. The various endpoint devices have been designed to accommodate this flipping and can be used in both right-hand and left-hand configurations. 
     To change from left-handed use to right-handed use, or vice versa, requires three 180-degree flips about three axes. 
     By way of example but not limitation, and looking now at  FIGS. 21-26 , the process of changing from left-handed use to right-handed use will now be described. First, lever  593  is released ( FIG. 21 ) to unclamp the extra joint located near the elbow joint J 3 . This action allows the entire arm beyond the elbow of the device to be flipped 180 degrees ( FIG. 22 ), then that freedom is re-secured ( FIG. 23 ) using lever  593 . Next, there is a second 180-degree flip ( FIG. 24  and  FIG. 25 ) by loosening, flipping and then tightening the thumbscrew  860 . Finally, there is the last 180-degree flip ( FIG. 26 ) where the cradle is rotated 180 degrees. Note that there is no mechanical lock for this last flip because the rotation of this joint is passive. 
     To change back from right-handed use to left-handed use, the flips are performed in the same order, but reversing the directions of the flips. 
     Accommodating Pronation/Supination of the Forearm/Wrist 
     In some situations it may be important to allow pronation/supination of the user&#39;s forearm/wrist while the user&#39;s forearm is strapped to cradle  805 . Pronation/supination is the twist/rotation of the wrist about the longitudinal axis of the forearm. 
     To that end, in one form of the invention, and looking now at  FIGS. 27-29 , a pair of Kaydon-style ring bearings  905  are used to support cradle  805  above a cradle support  910  (not shown in  FIGS. 27-29 ), which is in turn connected to outer link  110  (not shown in  FIGS. 27-29 ). Kaydon-style ring bearings  905  are large enough (e.g., 150 mm) to accommodate pronation/supination of the forearm/wrist of the 95th-percentile male hand and arm while the user&#39;s forearm is strapped to cradle  805 . An encoder  915  is used to track user position and communicate the same to onboard controller  956  of robotic device  5 . 
     Alternatively, other arcuate bearings of the sort well known in the bearing art may also be used. 
     However, the use of such Kaydon-style ring bearings and other arcuate bearings can increase the cost of the endpoint device. 
     Therefore, in another preferred embodiment of the present invention, and looking now at  FIGS. 30-32 , a 4-bar linkage  920  is used to support cradle  805  above a cradle support  925 , with cradle support  925  being connected to connector  815  (not shown in  FIGS. 30-32 ), which is in turn connected to outer link  110  (not shown in  FIGS. 30-32 ). Cradle support  925  and linkage  920  are located beneath the cradle, completely hidden from the view of the user. This approach enables about 90 degrees of wrist pronation/supination and lowers fabrication costs by avoiding the use of ring bearings. Also, with this approach, the patient or user can more easily get into and out of the endpoint device. Furthermore, there is no limitation on the size of the user&#39;s hand and forearm as there might be the case with the ring bearings. An encoder  930  is used to track user position and communicate the same to onboard controller  956  of robotic device  5 . 
     Providing Game-Based Physical Therapy and Occupational Therapy, and Providing Activity-Based Physical Therapy and Occupational Therapy, with the Robotic Device 
     In the foregoing disclosure, there is disclosed a novel multi-active-axis, non-exoskeletal robotic device for providing physical therapy and occupational therapy (sometimes collectively referred to herein as “physical therapy/occupational therapy” and/or simply “therapy”) to a patient. 
     A. Game-Based Therapy 
     In one form of the invention, the robotic device is configured to provide game-based rehabilitation. In this form of the invention, the patient views a two-dimensional (2D) or three-dimensional (3D) scene using a computer screen, a projector, glasses, goggles, or similar means. The 2D or 3D scene depicts a game which the patient “plays” by moving their limb (which is connected to the robotic device) so as to cause corresponding movement of a virtual object (or virtual character) within the 2D or 3D scene. As the patient endeavors to appropriately move their limb so as to cause appropriate movement of the virtual object (or virtual character) within the 2D or 3D scene of the game, the patient “effortlessly” participates in the therapy process. This form of the invention is a powerful tool, since it promotes increased engagement of the patient in the therapy process, and thereby yields higher “dosages” of the physical therapy or occupational therapy, which is known to be an essential element in successful recovery from stroke and many other injuries and diseases. 
     If desired, the 2D or 3D scene may take another non-game form, i.e., the 2D or 3D scene may be a non-game graphical or textual display, with the patient endeavoring to appropriately move their limb (which is connected to the robotic device) so as to cause appropriate movement of a virtual object within a graphical or textual display. This non-game approach, while less engaging for the patient than the game-based physical therapy or occupational therapy described above, is nonetheless capable of providing a valuable assessment measure. 
     In both of the foregoing forms of the invention, the patient is essentially endeavoring to appropriately move their limb (which is connected to the endpoint of the robotic device) so as to cause corresponding appropriate movement of a virtual object (or virtual character) on a computer screen, projector, glasses, goggles or similar means. 
     B. Activity-Based Therapy 
     While the foregoing approaches provide excellent therapy for the patient, they do not lend themselves to Activity Based Training (ABT). With ABT, the patient learns to accomplish an important daily activity, e.g., feeding themselves with a spoon. 
     To this end, in another form of the present invention, the robotic device is configured so that the therapist guides (e.g., manually assists) the patient in moving their limb (which is connected to the robotic device) through a desired motion (e.g., feeding themselves with a spoon). As this occurs, the robotic device “memorizes” the desired motion (i.e., by recording the movements of the various segments of the robotic device), and then the robotic device thereafter assists the patient in repeating the desired motion, e.g., by helping carry the weight of the patient&#39;s limb and by restricting motion of the patient&#39;s limb to the desired path. Thus, with the robotic device operating in this activity-based mode, the patient is manipulating a real object in real space (and is not manipulating a virtual object on a computer screen, as with the game-based physical therapy). 
     However, it should be appreciated that the robotic device is also configured so that activity-based therapy may be provided without requiring physical intervention from the therapist, as it may be sufficient for the robotic device to simply suspend some fraction of the weight of the patient&#39;s limb, thereby allowing the patient to succeed at a given activity. The robotic device may also be provided with pre-conceived therapy modalities that go beyond just simply limb suspension, such as a generalized pre-defined path along which the patient movement is constrained, so that the robotic device acts in the sense of a guide. 
     Additional Applications for the Present Invention 
     In the preceding description, the present invention is generally discussed in the context of its application for a rehabilitation device. However, it will be appreciated that the present invention may also be utilized in other applications, such as applications requiring high-fidelity force feedback. By way of example but not limitation, these applications may include use as an input/haptic feedback device for electronic games, as a controller for other mechanical devices such as industrial robotic arms and/or construction machines, or as a device for sensing position, i.e., as a digitizer or coordinate-measuring device. 
     Modifications of the Preferred Embodiments 
     It should be understood that many additional changes in the details, materials, steps and arrangements of parts, which have been herein described and illustrated in order to explain the nature of the present invention, may be made by those skilled in the art while still remaining within the principles and scope of the invention.