Abstract:
A method and system for controlling an intelligent assist device, robot, or other powered system that is partially or fully directed by the intent of a human operator. The method and system includes measuring a force or motion imparted by the human operator to a control. The measurement is determined with respect to the motion of the machine at a point of the machine that allows the benefit of collocation of sensing and actuation, or of approximation of such collocation. The system includes a support attached to the machine at the beneficial point, and a plurality of sensors to measure the force, torque, or motion imparted by the human operator to the support with respect to the beneficial point. The method and system provide improved stability of control of the system, and improved responsiveness to the intent of the human operator.

Description:
RELATED APPLICATION 
     This application claims priority to U.S. Provisional Application 60/291,702, which was filed on May 17, 2001, and which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     This present invention relates in general to the field of programmable robotic manipulators, and assist devices that can interact with human operators. 
     BACKGROUND OF THE INVENTION 
     An Intelligent Assist Device (IAD) is in a class of computer-controlled machines that interacts with a human operator to assist in moving a payload. An IAD may provide a human operator with a variety of types of assistance, including supporting payload weight, helping to overcome friction or other resistive forces, helping to guide and direct the payload motion, or moving the payload without human guidance. 
     Currently, a great deal of assembly and material handling work is done with the help of an x-y overhead rail system. Two exemplary types of x-y overhead rail systems include powered overhead bridge cranes for large loads usually running on I-beams, and unpowered overhead rail systems for smaller loads (e.g., up to a few hundred pounds), running on low-friction enclosed rails. Bridge cranes are typically slow and are usually controlled via pushbuttons. Enclosed rail systems are typically moved by direct application of the user&#39;s force to the payload. Enclosed rail systems are usually faster and more pleasant to use than bridge cranes, and often allow greater operator dexterity. 
     Getting the payload moving in an x-y overhead rail system is usually done by forward pushing, therefore using the large muscles of the lower body, which are not easily injured. However, controlling the motion of the moving payload can lead to a greater problem, as it requires pulling sideways with respect to the payload&#39;s direction of motion, generally using the smaller and more easily injured muscles of the upper body and back. Stopping the motion of the moving payload can also be problematic as it involves muscles of the upper body and back. 
     Furthermore, although low-friction x-y overhead designs are used, both the friction and the inertia are greater in the direction in which the payload has to carry with it the whole bridge rail than in the direction in which the payload simply moves along the bridge rail. Thus, anisotropy produces an unintuitive and undesirable response of the payload to applied user forces and often results in the user experiencing a continuous sideways “tugging” as the payload moves, in order to keep it on track. Both steering and anisotropy contribute to ergonomic strain, lower productivity, and a changeover to slow bridge cranes at an unnecessarily low payload weight threshold. 
     An IAD attempts to combine the powered assistance currently available with bridge cranes, and also the quick and intuitive operator interface available with unpowered rail systems. Ideally, IAD systems also improve upon the performance of unpowered rail systems and provide greater dexterity and speed than powered bridge cranes. However, although current IAD systems attempt to combine the best of overhead systems, many systems present another set of undesirable limitations. 
     FIG. 1 illustrates an exemplary approach that has been developed for IAD systems in which the payload is hung from a “rigid descender”. The rigid descender, also known as a “manipulator arm,” is manufactured by a variety of companies including Creative Ergonomic Systems (Sterling Heights, Mich.), Jomat Industries (Romeo, Mich.), Protomark (Clinton Twp., Mich.), Knight Industries (Auburn Hills, Mich.), and Scaglia S.p.A. (Brembilla, Italy). They are used instead of cables to reach under an overhang (e.g., place a component inside an automobile body, where the roof of the automobile acts as an overhang), to accommodate a shifting center of gravity, or if a high level of rigidity is needed for any other reason depending on the particular application. 
     In this type of system, a multi-axis force sensor  202  is used to measure operator-applied forces and torques. These measurements are used as an indicator of intent. Sensor  202  may be mounted directly to rigid descender  201  as shown, or to end-effector  203 , or to some other location convenient for the operator. Not only can horizontal motion intent be measured in this way, but with a sensor such as the ATI F/T sensor system manufactured by ATI Industrial Automation (Apex, N.C.) or the Cobotics, Inc. multi-axis intent sensor, vertical motion intent as well as intent in the roll, pitch and yaw axes, can be measured. These measurements may, in principle, be used to drive corresponding powered axes. There must, of course, be drive systems associated with those axes (the only drive units shown in FIG. 2 are motorized drive units  204  for horizontal motion). 
     The operator must grasp an intent sensor  202  rather than either the part itself (not pictured) or the end-effector  203 , but there is typically a high degree of correspondence between the motion of the intent sensor and the motion of the part. The operator simply pushes and the part follows. There is also an ergonomic benefit to this approach, in that the forces required are typically much lower than those required in an unpowered system. 
     However, there are undesirable limitations such as the absence of motorized drive units for axes of motion other than horizontal. For example, rotation about the vertical (yaw) is almost always a requirement when rigid descenders are used, but currently there is not a commercially available drive unit for this particular rotation. Another undesirable limitation is that many commercially available rigid arms incorporate pivoted joints that permit yaw motion and are neither powered nor outfitted with angle sensors. As a consequence, it is not possible to establish the orientation of the intent sensor with respect to the overhead bridge rail without retrofitting those arms with joint angle sensors. This retrofit process is expensive and unique to each particular arm design. 
     Yet another limitation is that this approach involves “non-collocated” control. Non-collocation refers to the presence of significant structural flexibility between the point of sensing (e.g., at the intent sensor) and the point of actuation (e.g., at the bridge and runway rails). It is well-known that non-collocated systems are difficult to control and prone to instability. Generally speaking, the control system bandwidth or system response is limited by the lowest-frequency structural mode, which is typically in the 1 Hz range. As a consequence, the responsiveness of the control system may not be dramatically better than that of the unassisted rigid arm. Accordingly, the ergonomic benefits may be limited. 
     A serial manipulator may be described as a kinematic chain, extending from a ground reference frame, through various links, supports, and through powered or unpowered joints, finally to a payload at the end of the chain. A mounting location of an intent sensor may be referred to as more proximal as it is closer to the ground of the kinematic chain, and more distal as it is closer to the payload of the kinematic chain. Similar terminology may be used for a parallel kinematic mechanism, and for hybrid serial/parallel mechanisms. 
     The prior art shows a number of undesirable limitations including limited ergonomic benefit, especially for larger payloads. Some systems exhibit potentially unstable behavior, and the inability to adapt easily to the many styles of commercially-available descenders, including cable, chain, and various rigid arm designs. Moreover, many systems lack a standard motorized drive unit for yaw rotation in rigid descenders. 
     SUMMARY OF THE INVENTION 
     A method and system is provided to measure the motion intent of a human operator in one or more axes. According to the exemplary embodiments, the method and system are implemented on an Intelligent Assist Device (IAD). 
     According to an aspect of the present embodiment, a system provides intuitive control, preferably such that the human operator has direct and intuitive control over motion speed and direction of a payload. In the exemplary embodiment, the operator can push or twist the a sensor in a particular direction and the IAD would move in that direction. Preferably, the harder the operator pushes, the faster the IAD should move. The sensor is preferably located “upstream” on the system to prevent low frequency vibrational modes from interfering with the sensor signals. 
     According to another aspect of the present embodiment, a system facilitates ergonomic benefit, even for large loads (e.g., ¼ ton or larger). In the exemplary embodiment, the human operator can provide modest forces and torques to operate the IAD, and these forces and torques preferably do not scale significantly with payload inertia. 
     Another aspect of the present embodiment, a system makes possible collocation of intent sensing and actuation for horizontal motions as well as yaw rotation. In the exemplary embodiment, a minimal structural compliance exists between the point of intent sensing and the point at which the motorized drive units act. 
     Yet another aspect of the present embodiment, the system facilitates compatibility with a wide range of commercially-available descenders. In the exemplary embodiment, existing intent sensors are improved by being compatible with both cable/chain based and rigid descender based systems. Preferably, various types of rigid descenders (e.g., manipulator arms) can be accommodated. 
     According to an aspect of the present embodiment, a standard motorized drive unit is provided for yaw rotation that may be used with a wide range of commercially-available descenders. 
     The exemplary embodiments provide flexible ergonomic benefit, because the system preferably involves collocated control, which can make the system easier to control. Moreover, the exemplary embodiments promote stable behavior, including the ability to adapt easily to the many styles of commercially-available descenders, including cable, chain, and various rigid arm designs. Also, the exemplary embodiments can include a standard motorized drive unit for yaw rotation. According to the teachings described herein, the method and apparatus can be utilized by other devices that can use the measured motion intent, if so desired. 
    
    
     The foregoing and other objects, features and advantages of the automated execution system having firm participation will be apparent from the following more particular description of preferred embodiments as illustrated in the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of an approach to intent sensing for an Intelligent Assist Device, in which operator-applied forces are measured by a multi-axis force sensor; 
     FIG. 2 is a diagram of an exemplary four-axis guide stick suspended from a bridge crane with three powered axes; 
     FIG. 3 is an diagram that further illustrates the system of FIG. 2, but includes a rigid descender; 
     FIG. 4 is yet another diagram that illustrates the system of FIG. 2, but includes a cable-based descender; 
     FIG. 5 is a diagram further illustrating the four-axis guide stick; 
     FIG. 6 is a diagram illustrating an exemplary powered rotate drive unit that provides yaw rotation; 
     FIG. 7 is a view from the opposite side of the powered rotate drive unit of FIG. 6 with some components removed for clarity; and 
     FIG. 8 is a view of the powered rotate drive unit of FIG. 6 from underneath. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The exemplary embodiments disclose a system and method for controlling an assist device or robotic manipulator that can interact with a human operator. The exemplary embodiments have been implemented in an Intelligent Assist Device (IAD) system that includes a support for controlling the movement of a payload. More specifically, the exemplary embodiments have been implemented using a rigid physical support or “guide-stick” for controlling the movement of the payload. Exemplary assist devices or IADs are illustrated and described in commonly owned U.S. patent application Ser. No. 09/781,683, the contents of which are incorporated by reference herein; in commonly owned U.S. Provisional Patent Application Serial No. 60/267,790, the contents of which are incorporated by reference herein; and in commonly owned U.S. patent application Ser. No. 09/246,826. 
     It should be understood, however, that the present embodiments may utilize other types of support structures such as supports that are rigid, semi-rigid, or devices that do not use a physical support, described more below, for controlling the movement of the IAD and the payload carried by the IAD. Therefore, details regarding the IAD, the payload carried by the IAD, and the types of supports for controlling the payload are provided as an example, and are not necessary to the invention unless otherwise specified. 
     FIG. 2 is an illustration of an exemplary guide-stick intent sensor and powered bridge crane. The viewpoint in this illustration is that of an observer standing underneath the bridge crane and looking upward. This exemplary embodiment of the guide stick contains sensors sufficient to control up to four degrees of freedom of the IAD including motion along two horizontal axes (one axis is aligned with runway rails  301  and the other with the bridge rails  302 ), rotation about the vertical axis, and vertical motion. The bridge crane provides drive units for motion along the bridge rail, motion along the runway rails, and rotation about the vertical (yaw rotation). The drive unit for yaw rotation is further described in FIGS. 6-8. A drive unit for vertical motion would normally be part of the descender, which is not shown in FIG. 3 for clarity. It should be emphasized that, while the preferred embodiment contains sensing for four degrees of freedom, implementations having either more or fewer degrees of freedom are possible. 
     The runway rail  301  and bridge rails  302  are structural components that may take a number of different forms, including I-beam and enclosed track profiles of known type. In a preferred embodiment, RAD4800A enclosed track aluminum rail from Knight Industries (Auburn Hills, Mich.) is used. Runway rails  301  are fixed rigidly to ground, typically via steel supports or overhead steel beams, not shown. Carriage frame  303  rolls along bridge rails  302 , and both bridge rails  302  rolls along runway rails  301 . 
     In a preferred embodiment, two motorized drive units  304  drive carriage frame  303  along bridge rail  302 , and motorized drive units  305  at either end of bridge rail  302  drive the bridge rail along runway rails  301 . Of course, it is not necessary to use two motorized drive units per axis. For many applications, a single unit per axis will suffice, and for other applications with very large loads, more than two units per axis may be needed. Nor is it necessary to use the same number of drive units on the bridge rails as on the runways. In a preferred embodiment, motorized drive units  304  and  305  are iTrollies available from Cobotics Inc. (Evanston, Ill.), which includes onboard computer and interface electronics for control purposes. 
     It should be noted that, while the preferred embodiment of the invention is based upon a bridge crane architecture, other well-known crane architectures may be used. Specifically, it is possible to use a monorail, jib crane, articulated jib crane, or gantry crane without altering the intent of the invention. 
     Also in a preferred embodiment, powered rotate assembly  700  is rigidly mounted to carriage frame  303 . Mounting flange  307  is connected rotatably to carriage frame  303 . The rotation axis of mounting flange  307  is vertical (perpendicular to the plane in which runways rails  301  lie). As described further below, powered rotate assembly  700  is used to drive rotations of mounting flange  307 . A standard manipulator arm or other descender may be secured to mounting flange  307 . 
     The bridge rail system described thus far is capable of powered motion in three axes: translation along runways  301 , translation along bridge rails  302 , and rotation of mounting flange  307 . This bridge rail system can be expected to exhibit high structural stiffness as compared to a typical descender, be it a cable, chain, or rigid manipulator arm. Thus, in order to achieve “collocation”, as discussed above, it is desirable to minimize the structural flexibility that separates the output of these powered axes (namely mounting flange  307 ) from the location of operator intent sensing. This is achieved with strut  306  and support or “guide stick” assembly  600 . Strut  306  is a stiff beam that is mounted directly to mounting flange  307 . Guide stick assembly  600  is mounted to strut  306  and is, as a result, effectively collocated with the drive units  304 ,  305  and  700 . 
     Strut  306  provides a plurality of mounting holes where guide stick assembly  600  may be attached. Four different mounting locations are illustrated, but of course, the number may be larger or smaller. The mounting location of guide stick assembly  600  may also be made continuously adjustable by a number of well-known techniques. 
     In another exemplary embodiment, it is possible to mount guide stick assembly  600  directly to carriage frame  303 . In this case, the guide stick assembly remains effectively collocated with drive units  304  and  305 , but becomes decoupled from drive unit  700 . Similarly the guide stick assembly  600  may be mounted to the bridge rails  302  so that it remains collocated with drive units  304 , but becomes decoupled from drive units  305  and  700 . It may even be mounted to runway rails  301  or elsewhere to an unmoving frame so that it is decoupled from all drive units. In this case, the operator is able to remain standing in one location while commanding the motion of the various drive units. 
     FIG. 3 illustrates the manner in which a rigid manipulator arm  401  may be attached to the powered bridge rail system of FIG.  2 . Although obscured in the figure for clarity, manipulator arm  401  is rigidly connected to mounting flange  307 . The orientation of strut  306  and the mounting location of guide stick assembly  600  along strut  306  may be adjusted in order to present handle  604  in a location that is convenient for the operator. Handle  604  is located close to end effector  402 . It is important to note that the manipulator arm  401  may be replaced by a variety of commercially available manipulator arms, as well as other rigid descenders, without altering the functionality of the present embodiment. 
     FIG. 4 illustrates another exemplary embodiment, in which a cable or chain-based system may be attached to the powered bridge rail system. In this case, there is no need for powered rotation, because the rotational drive cannot be transmitted via a cable or chain. Therefore, powered rotate assembly  700  has been removed and replaced with a passive carriage assembly  501 . Mounted to carriage assembly  501  are drive unit  502  (in a preferred embodiment, the iTrolley manufactured by Cobotics, Inc.) and passive trolley  503 , which permit rolling along a pair of bridge rails (not shown). 
     An exemplary pneumatic balancer  504  is shown mounted on carriage assembly  501 , although many other lifting devices might be used as well. Pneumatic balancer  504  employs air pressure to reel cable  505  either in or out. An end-effector or payload is normally suspended from hook  506 . It is also possible to eliminate one bridge rail and mount pneumatic balancer  504  or other lifting device immediately beneath the one remaining bridge rail. 
     Guide stick assembly  600  is preferably mounted directly to carriage assembly  501 . This configuration effectively collocates guide stick assembly  600  with drive unit  502  and the drive units for bridge rail motion (equivalent to drive units  305 ). The mounting location of guide stick assembly  600  may be adjusted in order to present handle  604  in a location that is convenient for the operator. 
     Also illustrated in FIG. 4 is an exemplary cable angle sensor  507  that detects the magnitude and orientation of any departure of the cable angle from the vertical. The cable angle sensor can be, for example, a non-contact cable angle sensor manufactured by Cobotics Inc. (Evanston, Ill.) although many other sensors could be used as well. The sensor may optionally be used in conjunction with guide stick assembly  600 . In a preferred embodiment, commands for drive units (e.g.,  502 ,  303  and  304 ) are based on a weighted sum of signals from both guide stick assembly  600  and cable angle sensor  507 , although many other combinations of signals are possible. The use of cable angle sensor  507  in conjunction with guide stick assembly  600  allows an operator to control movement of the IAD by either pushing on the guide stick or by pushing on the payload directly. In addition, the use of cable angle sensor  507  may reduce payload oscillations that might otherwise occur. In another embodiment, the readings from cable angle sensor  507  can be used for diagnostic or other purposes instead of for contributing to the motion commands issued to the drive units. 
     FIG. 5 illustrates the preferred embodiment of the guide stick assembly. In this embodiment, bracket  601  provides attachment to strut  306  or strut  503 . Three axis industrial joystick  602  is rigidly mounted to the bottom of bracket  601 . In a preferred embodiment, joystick  602  is model J50 manufactured by ETI-Systems (Carlsbad, Calif.), having rotational axes  607 ,  608  and  609 . The J50 joystick employs conductive plastic potentiometers to measure rotation about each of these axes, but of course many other types of sensor might be used as well. In a preferred embodiment, the J50 joystick is outfitted with spring-return detents in each axis so that, when no operator force is applied to the joystick, it will return to a repeatable home position in each axis. 
     While joystick  602  has been presented here as an exemplary angle sensing device, many other techniques may also be used to measure guidestick  603  angle. For example, the noncontact cable angle sensor manufactured by Cobotics Inc. (Evanston, Ill.) may be used to measure rotations about axes  607  and  608 . This solution would alone suffice if there is no need to measure operator intent in rotation about the vertical axis  609  (e.g., if the payload is suspended from a cable or chain as illustrated in FIG.  4 ). This solution could be combined with a simple rotary joint and angle sensor if operator intent is needed for rotation about the vertical. 
     Another alternative to joystick  602  is a sensing device that detects torques rather than angles. For instance, the Forcestick Controller P/N 55965-2 manufactured by Orbit International Corporation (Hauppauge, N.Y.) may be used to measure torques applied to axes  607  and  608  without allowing significant angular deflection about these axes. 
     Rigid connecting rod  603  is preferably used to connect handle  604  to joystick  602 . In a preferred embodiment, handle  604  is the in-line slide handle manufactured by Cobotics Inc. (Evanston, Ill.). The operator may grasp this handle and indicate motion intent by creating twists about axes  607 ,  608  and  609 . In addition, because the Cobotics Inc. in-line slide handle can detect forces applied in a vertical direction (i.e., along axis  609 ) this configuration may be used to measure operator motion intent in the vertical direction. This information would normally be relayed to the device responsible for vertical motion. Wires for this purpose are not shown in FIG.  5 . The vertical motion device may be any lifting device capable of receiving an electronic signal, including electric hoists, pneumatic balancers, pneumatic cylinders such as cylinder  403  in FIG. 3, and a variety of other devices. 
     In a preferred embodiment, momentary switch  605  may be configured as an “operator present” switch. As such, an operator must continually depress this switch in order to enable drive unit movement. This technique ensures that the IAD will not respond to accidental displacement of the guide stick  603 . Of course, there are many other ways to detect operator presence, including pressure sensors, capacitive sensors, and optical sensors. 
     In a preferred embodiment, switch  606  may be used to stop and restart the IAD. Preferably, the alphanumeric display  610  indicates IAD status. 
     Many modifications can be made to the preferred embodiment of the support or guide stick without altering its basic function. For instance, rigid rod  603  may be replaced with a rod that is slightly flexible, or even with one that is very flexible (e.g., a cable or chain). Handle  604  may be replaced with other handle configurations, or it may even be eliminated. Additional switches, controls and displays may be added to the handle assembly, or may be mounted at any point on rod  603 . For instance, it may be desirable in some applications to include controls for opening and closing a gripper. As another example, it may be desirable to include controls for other axes of motion, such as roll and pitch. 
     Moreover, the support may not be physical, but could be replaced by a sensor such as an optical sensor (not shown). In this embodiment, the optical sensor would preferably be placed on strut ( 306  in FIG.  2 ). Then, the optical sensor could detect movement with of the operator or device held by the operator. For example, forward movement of the operator would initiate movement of the payload in the forward direction. Such operator movement would be referenced via the optical sensor. 
     In any event, according to the exemplary embodiment, drive unit speeds are related to guide stick measurements via the following algorithm. For the purposes of this algorithm, it is assumed that guide stick axis  607  is aligned with strut  306  as illustrated in FIG.  3 . It is also assumed that θ is the angle of strut  306  with respect to bridge rails  302 : θ is zero when strut  306  and bridge rails  302  align. Finally, it is assumed that L is the horizontal distance from the axis  609  to the center of mounting flange  307 . ν B , ν R , and ω are the velocity commands for drive units  304 ,  305  and  700 , respectively. α 607 , α 608 , α 609  are the angle measurements obtained from axes  607 ,  608  and  609  of the guide stick, respectively. The algorithm is:          [           v   B               v   R             ω         ]     =       [           cos                 θ             -   sin                   θ         0             sin                 θ           cos                 θ         0           0       L       1         ]                [             G   1          α   607                   G   1          α   608                   G   2          α   609             ]                            
     where G 1  and G 2  are parameters that may be adjusted for sensitivity of response. Many modifications to this algorithm are possible. For instance, the parameter L may be adjusted or even set to zero. The guide stick angle measurements may first be passed through a deadband function so that small angles are ignored, and these measurements may also be filtered to remove measurement noise. The velocity commands may also be made to depend on the time derivatives and time integrals of the angle measurements, resulting in a form of PID control. 
     FIG. 6 illustrates the exemplary drive unit  700 . The FIG. 6 is from the viewpoint of an observer above and to the side of the assembly. FIG. 7 is a view from the opposite side of drive unit  700  with carriage frame  303  removed for clarity. FIG. 8 is a view of drive unit  700  from underneath. 
     Drive unit  700  comprises a number of components mounted to carriage frame  303 . Bracket  701  is mounted rigidly to carriage frame  303 . A subassembly comprising motor  702 , right angle gearbox  703 , tensioning bracket  704 , pulley  705  and clutch  706  is mounted slidably on bracket  701 . The output shaft of motor  702  causes the input shaft of gearbox  703  to rotate which causes the output shaft of gearbox  703  to rotate. The output shaft of gearbox  703  causes clutch  706  to rotate. When engaged, clutch  706  causes pulley  705  to rotate. 
     In a preferred embodiment, motor  702  is a model IB346-T manufactured by MCG, Inc. (Eden Prairie, Minn.), gearbox  703  is a model K202AB-0560-MQ10 manufactured by Stober Drives, Inc. (Maysville, Ky.) and having a gear ratio of 56:1, clutch  706  is a model 5AH30P manufactured by Matrix International, Ltd. (Elk Grove Village, Ill.), and pulley  705  is a model 8MX-36S-62-PB manufactured by Gates Rubber (Denver, Colo.) having 36 teeth and modified to mount on clutch  706 . Of course, a variety of other industrial components may be used instead. In particular the pneumatic clutch may be replaced with an electrically actuated clutch, a pin clutch, or a number of other possible designs. 
     Pulley  705  engages pulley  707  via a toothed belt (not shown). In a preferred embodiment, pulley  707  is a model 8MX-140S-62 having 140 teeth and the toothed belt is a model 8MGT-1280, both manufactured by Gates Rubber (Denver, Colo.). The toothed belt is tensioned by turning threaded rod  708  which engages tensioning bracket  704 . This action draws the entire subassembly comprising elements  702 - 706  away from pulley  707 . Said subassembly is mounted to bracket  701 , but allowed to slide in slots  801 . 
     Pulley  707  is connected rigidly to mounting flange  307  by drive shaft  802 . Mounting flange  307  is connected rotatably to flange  901  which is rigidly mounted to carriage frame  303 . This arrangement allows motor  702  to drive rotation of mounting flange  303  when clutch  706  is engaged, and for mounting flange  307  to rotate freely when clutch  706  is disengaged. 
     The rotational angle of mounting flange  307  (which is used in the algorithm discussed above) is measured by potentiometer  803 . Sprocket  804  mounted on the shaft of potentiometer  803  is connected to sprocket  805  mounted on drive shaft  802  by means of a no-slip belt (not shown). In a preferred embodiment, potentiometer  803  is a model MW22-10 turn manufactured by ETI Systems (Carlsbad, Calif.), and sprockets  804  and  805  are models FC5-24 and FC5-112 respectively, manufactured by Pic Design (Middlebury, Conn.). The non-slip belt is a model FA-144 also manufactured by Pic Design. Of course, a variety of other techniques known in the art may also be used to measure angle. 
     Electronics enclosure  709  contains electronics including a power supply (TPS80-45 manufactured by Total Power International, Lowell, Mass.), computer model Jumptec MOPS/586 manufactured by Jumptec (Deggendorf, Germany) and motor amplifier model B25A20AC-01 manufactured by Advanced Motion Controls (Camarillo, Calif.) necessary for the control of drive unit  700 . 
     The exemplary embodiments provide flexible ergonomic benefit, because the system preferably involves collocated control, which can make the system easier to control. Moreover, the exemplary embodiments promote stable behavior, including the ability to adapt easily to the many styles of commercially-available descenders, including cable, chain, and various rigid arm designs. Also, the exemplary embodiments can include a standard motorized drive unit for yaw rotation. Moreover, the control system bandwidth or system response is increased because the lowest-frequency structural mode where the sensor is attached is in a higher frequency range. As a consequence, the responsiveness of the control system, which is proportional to the frequency, may be significantly improved compared to that of prior art systems. 
     According to the teachings described herein, the method and apparatus can be utilized by other devices that can use the measured motion intent, if so desired. 
     It should be understood that the programs, processes, methods and systems described herein are not related or limited to any particular type of support device, assist device or machine, unless indicated otherwise. Various types of general purpose or specialized assist or robotic systems may be used in accordance with the teachings described herein. 
     In view of the wide variety of embodiments to which the principles of the present embodiments can be applied, it should be understood that the illustrated embodiments are exemplary only, and should not be taken as limiting the scope of the present invention. For example, more or fewer elements may be used in the diagrams.