Patent Publication Number: US-8994643-B2

Title: Force reflecting haptic interface

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 10/697,963, filed Oct. 30, 2003, the entire disclosure of which is hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to a man/machine interface and, more specifically, to a force reflecting haptic interface. 
     BACKGROUND 
     Force reflecting haptic interfaces and associated computer hardware and software are used in a variety of systems to provide tactile sensory feedback to a user in addition to conventional visual feedback, thereby affording an enhanced man/machine interface. These systems are becoming more prevalent in such diverse areas as surgical technique training, industrial design and modeling, and personal entertainment. 
     Two examples of haptic interfaces for use in a desktop environment are disclosed in U.S. Pat. Nos. 5,587,937 and 6,417,638, the disclosures of which are hereby incorporated herein by reference in their entireties. Generally, haptic interfaces define a user reference point located, for example, proximate or within a volume of a user connection element such as a finger thimble or stylus configured to be donned or grasped by a user. Disposed between the user connection element and a spatial or reference ground are a series of mechanical transmission elements such as gimbals, linkages, and frames configured to permit substantially unrestricted movement of the connection element within a predetermined work volume of the haptic interface when in an unpowered state. 
     Based on the configuration and orientation of the transmission elements, multiple independent degrees of freedom may be provided. Depending on the particular application for the interface, each degree of freedom may be powered and/or tracked, or free, being neither powered nor tracked. For example, a degree of freedom may be powered by a motor or other actuator so that, under appropriate conditions, the interface can resist, balance, or overcome a user input force along that degree of freedom. The powered axis may be active, with force being varied as a function of system conditions, or passive, such as when a constant resistance or drag force is applied. Alternatively or additionally, a degree of freedom can be tracked using an encoder, potentiometer, or other measurement device so that, in combination with other tracked degrees of freedom, the spatial location of the reference point within the work volume can be determined relative to ground. Lastly, a degree of freedom may be free, such that a user is free to move along the degree of freedom substantially without restriction and without tracking within the limits of the range of motion. The interface, in combination with appropriate computer hardware and software, can be used to provide haptic feedback in a virtual reality environment or link a user to an actual manipulator located, for example, in a remote or hazardous environment. 
     Significant challenges exist in designing a force reflecting haptic interface with appropriate operational and response characteristics. For example, it is desirable that the haptic interface have low friction and weight balance such that a user&#39;s movements will not be unduly resisted and the user will not become fatigued merely by moving the connection element within the work volume. It is also desirable that the haptic interface have a high degree of resolution and be highly responsive so as to replicate, as closely as possible, an actual haptic experience. Compact size, low cost, and the interchangeability of various input interfaces are also beneficial attributes from the standpoint of commercial acceptance and appeal. 
     Nevertheless, the complex technology involved in a force reflecting haptic interface has hampered efforts to reduce size and cost. The architecture of such a device requires unit sizes that are larger than desirable, often because of such factors as motor placement, weight counter-balancing measures, and component size characteristics. Such large unit sizes often drive commercial costs higher, as do the unique components that are required in such a device. The complex technology has also limited the interchangeability of input interfaces, thus requiring the acquisition of a custom device with a specific input interface for each different application. 
     These limitations on size and cost are presently an unfortunate bar to many markets to which a force reflecting haptic interface is well-suited. For example, lower cost would make such an interface available to consumers, for use with their home personal computers. Use of a haptic interface as a peripheral device would effectively widen the bandwidth of human interaction with the computer, by providing an interface that incorporates a sense of touch, well beyond the standard two-dimensional interaction of sight and sound. 
     There is, therefore, a need for a force reflecting haptic interface with enhanced functionality that is compact in size and of relatively low cost, so as to be available to a broad consumer market. 
     SUMMARY OF THE INVENTION 
     In one aspect, the invention relates to a force reflecting haptic interface including at least three degrees of freedom and a user interface. The user interface includes a nose section and a user connection section detachably coupled to the nose section. The nose section is interchangeable with alternative user connection sections. 
     In various embodiments of the foregoing aspect of the invention, the user connection section can be a stylus, a pistol grip, a roller ball, a mouse, a joystick, and/or a steering device. In addition, the user connection section can be coupled to the nose section by a jack and chuck arrangement and the user connection section can decouple from the nose section upon application of a load greater than a threshold load value. 
     In some embodiments, the user interface further includes a first user input and, optionally, a second user input. In additional embodiments, the first user input and/or the second user input is customizable by a user. The user input can be a switch or push-button. Either the first user input or the second user input or both can modify a function of the user interface. In various embodiments, the user interface is adapted to function as a force feedback device, a computer mouse, and/or a digitizer. 
     The user interface includes a housing. In one embodiment, the housing is made up of multiple components that interlock so as to provide structural integrity and component retention without requiring a fastener. Additionally, the force reflecting haptic interface can include a yoke assembly coupled to the nose section of the user interface. In one embodiment, the yoke assembly includes two hinged halves adapted to capture a pair of projections extending from the nose section. Each projection is adapted to mate with a bearing and at least one of the projections is adapted to mate with a sensor for outputting a signal representative of a position of the user interface relative to the yoke assembly. 
     In further embodiments, the user interface includes a sensor for outputting a signal representative of a position of the user connection section relative to the nose section. In addition, the user interface may include a docking station. The docking station includes a projection disposed on one of the user interface and a housing of the haptic interface and a mating recess formed in the other of the user interface and the housing. Further, the docking station may include a sensor for indicating mating of the projection in the recess. In another aspect, the invention relates to a force reflecting haptic interface including at least three degrees of freedom and a multiple use user interface. The user interface is adapted to support a first function and a second function. In one embodiment, the user interface is further adapted to support a third function. In various embodiments of this aspect of the invention, the first function is as a force feedback device, the second function is as a computer mouse, and the third function is as a digitizer. In one embodiment, the user interface is switchable between the first function and the second function, and the third function is enabled independently from the first function and the second function. 
     In another aspect, the invention relates to a docking station for a force reflecting haptic interface including a housing and a user interface. The docking station includes a mating structure and a switch disposed proximate the mating structure. In some embodiments, the mating structure includes a receptacle formed in the housing and the switch is actuatable by insertion of at least a portion of the user interface into the receptacle. Upon actuation of the switch, the haptic interface is set to a home position. 
     In further embodiments, the docking station includes a retainer for retaining the user interface in the docking station, and the retainer can include a spring loaded projection disposed on one of the user interface and the docking station and a mating recess for receiving the projection disposed on the other of the user interface and the docking station. In addition, the docking station can include an indicator. The indicator can be a visual indicator and can indicate at least one of a fault condition and a status. 
     In another aspect, the invention relates to a force reflecting haptic interface including at least three degrees of freedom. The haptic interface includes a direct drive assembly having a first actuator for driving a first rotary element and a coaxial transfer drive assembly having a second actuator for driving a second rotary element. The direct drive assembly and the transfer drive assembly are disposed on opposite sides of at least one of the first rotary element and the second rotary element. 
     In various embodiments of the foregoing aspect of the invention, the direct drive assembly and the transfer drive assembly each include a rotary element or other type of drive element, the respective rotary elements disposed in an opposed coaxial configuration. The force reflecting haptic interface can further include a reflective encoder disposed on one end of at least one of the first actuator and the second actuator and/or a threaded capstan disposed on a shaft of at least one of the first actuator and the second actuator. 
     In one embodiment, the force reflecting haptic interface includes a base for housing electrical components. The base can include ballast to at least partially, and typically fully, offset forces arising during use of the haptic interface. In one embodiment, the ballast can include a plurality of plates. Further, the force reflecting haptic interface can include an electrical interface in accordance with IEEE 1394. In some embodiments, the force reflecting haptic interface includes an external non-structural housing, wherein the housing can include two halves mounted in opposition on a shaft passing through an axis of rotation of a rotary element. In various embodiments, the force reflecting haptic interface includes a spring for balancing at least one cantilevered rotary element without requiring a counterweight. The spring may be a torsion spring disposed about an axis of rotation of the rotary element. 
     In another aspect, the invention relates to a force reflecting haptic interface including at least three degrees of freedom and an internal temperature monitoring system without requiring a temperature sensor. In one embodiment, the temperature monitoring system includes circuitry for measuring duration and magnitude of current drawn by an actuator powering at least one of the degrees of freedom. Further, the system calculates a temperature inside the interface based on the measured duration and magnitude. In one embodiment, the system disables at least a portion of the interface if the calculated temperature exceeds a threshold temperature value. 
     In another aspect, the invention relates to a method of monitoring an internal temperature of a force reflecting haptic interface. The method includes the steps of measuring magnitude of current drawn by an actuator within the interface, measuring duration of the current drawn, and calculating a temperature based upon the magnitude and duration measurements. In one embodiment, the method includes an additional step of disabling at least a portion of the interface if the calculated temperature exceeds a threshold temperature value. 
     These and other objects, along with advantages and features of the present invention herein disclosed, will become apparent through reference to the following description, the accompanying drawings, and the claims. Furthermore, it is to be understood that the features of the various embodiments described herein are not mutually exclusive and can exist in various combinations and permutations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which: 
         FIG. 1A  is a schematic perspective side view of a force reflecting haptic interface in accordance with one embodiment of the invention; 
         FIG. 1B  is a schematic perspective rear view of the force reflecting haptic interface of  FIG. 1A ; 
         FIG. 1C  is another schematic perspective rear view of the force reflecting haptic interface of  FIG. 1A  with external housing components removed; 
         FIG. 2A  is a schematic perspective view of one embodiment of a user interface for a haptic interface in accordance with one embodiment of the invention; 
         FIG. 2B  is a schematic perspective partial sectional view of a user connection end of the user interface of  FIG. 2A ; 
         FIGS. 2C-2D  are schematic perspective partial sectional and exploded views of a nose end of the user interface of  FIG. 2A ; 
         FIG. 3A  is a schematic perspective view of a yoke arm assembly for a haptic interface in accordance with one embodiment of the invention; 
         FIG. 3B  is a schematic perspective view of the hinged yoke of the yoke arm assembly of  FIG. 3A ; 
         FIG. 3C  is a schematic perspective partial sectional view of a portion of the yoke arm assembly of  FIG. 3A  and the nose; 
         FIG. 4A  is a partially exploded schematic perspective view of an embodiment of a docking station and user interface for use in a haptic interface in accordance with one embodiment of the invention; 
         FIG. 4B  is a schematic front view of the docking station of  FIG. 4A ; 
         FIG. 4C  is a schematic cross-sectional side view of the docking station and user interface of  FIG. 4A ; 
         FIG. 5  is a rear schematic perspective view of an embodiment of an internal drive system for use in a haptic interface in accordance with one embodiment of the invention; 
         FIG. 6A  is a schematic perspective view of an embodiment of a transfer drive for powering a third articulation of a haptic interface in accordance with one embodiment of the invention; 
         FIG. 6B  is a schematic diagram of an automatic cable tensioning device employed to drive the third articulation of the haptic interface in accordance with one embodiment of the invention; 
         FIG. 7  is a schematic side view of an embodiment of an actuator assembly for use in the haptic interface in accordance with one embodiment of the invention; 
         FIG. 8A  is a schematic diagram of an automatic cable tensioning device useful in a haptic interface in accordance with one embodiment of the invention; 
         FIG. 8B  is a schematic side view of an actuator capstan for use in a cable drive in accordance with one embodiment of the invention; 
         FIG. 8C  is a schematic plan view of an automatic cable tensioning device employed to drive a first articulation of the haptic interface in accordance with one embodiment of the invention; 
         FIG. 9  is a schematic plan view of an automatic cable tensioning device employed to drive a second articulation of a haptic interface in accordance with one embodiment of the invention; 
         FIG. 10  is a schematic plan view of an automatic cable tensioning device employed to drive a transfer drive element of a third articulation of a haptic interface in accordance with one embodiment of the invention; 
         FIGS. 11A-11C  are flowcharts of an algorithm for controlling and monitoring force and internal temperature of a haptic interface in accordance with one embodiment of the invention; 
         FIG. 12  is a schematic diagram of an IEEE 1394 compliant interface board useful in a haptic interface in accordance with one embodiment of the invention; and 
         FIG. 13  is a schematic perspective view of a wrist rest to be used with a haptic interface in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1A  is a schematic perspective view of a six degree of freedom force reflecting haptic interface  10  in accordance with one embodiment of the present invention. Various features and functions of the inventions can be utilized, with advantage, in interfaces with different configurations, different kinematics, and greater or fewer degrees of freedom. The interface  10  includes a base  12  defining a reference ground, six joints or articulations, and six structural elements. A first powered tracked rotary element  14  is supported by the base  12  to define a first articulation  16  with an axis of rotation “A” having a substantially vertical orientation. A second powered tracked rotary element  18  ( FIG. 5 ) is mounted on the first powered tracked rotary element  14  to define a second articulation  20  ( FIG. 5 ) with an axis of rotation “B” having a substantially perpendicular orientation relative to the first axis, A. A third powered tracked rotary element  22  is mounted on a generally outwardly radially disposed cantilevered extension  24  (in the form of a thigh) of the second element  18  to define a third articulation  26  having an axis of rotation “C” that is substantially parallel to the second axis, B. A fourth free rotary element  28  is mounted on a generally outwardly radially disposed extension  30  (in the form of a shin) of the third element  22  to define a fourth articulation  32  having an axis of rotation “D” that is substantially perpendicular to the third axis, C. A fifth free rotary element  34  in the form of a nose is mounted on a generally outwardly radially disposed extension  36  (in the form of a yoke) of the fourth element  28  to define a fifth articulation  38  having an axis of rotation “E” that is substantially perpendicular to the fourth axis, D. A sixth free rotary user connection element  40  in the form of a stylus configured to be grasped by a user is mounted on a generally outwardly radially disposed extension  42  of the fifth element  34  to define a sixth articulation  44  having an axis of rotation “F” that is substantially perpendicular to the fifth axis, E. When not in use, the nose  34  is secured conveniently within a docking station  46  located on the base  12  of the haptic interface  10 . A generally spherical upper housing  56  encloses the internal components, protecting them from damage and contaminants. Interfaces employing more or less than six axes are contemplated and, in any embodiment of the haptic interface, any of the axes may be powered (i.e., controlled by a motor assembly) or free. 
       FIG. 1B  is a rear schematic perspective view of the force reflecting haptic interface  10  of  FIG. 1A . The haptic interface  10  has at least two connection ports formed in the housing  12 . The electrical connections employ a ferrite bead  54  to offer RF shielding, parasitic suppression, and RF decoupling. A power connection  50  supplies electrical power to the interface  10  to operate internal components, including control circuitry, sensors, actuators and display elements. In one embodiment of the force reflecting haptic interface  10  in accordance with the invention, the interface  10  also includes at least two Institute of Electrical and Electronics Engineers (IEEE) 1394 port connections  52   a ,  52   b , such as the FIREWIRE® brand sold by Apple Computer, Inc. Specifically, the interface  10  has both a PHY interface connection (which manages physical interface, CRC checking, pass-through operations, and speed negotiations) and a LINK controller connection (which formats data, and manages isochronous transfers), creating a two-channel interface. IEEE 1394 connections provide many advantages over prior peripheral connection methods. For example, a IEEE 1394 connection transfers data much faster than conventional parallel or serial connections, or even higher speed Universal Serial Bus (USB) connections. The dual-connection embodiment depicted in  FIG. 1B  enables the haptic interface  10  to operate at 100, 200, and 400 Mbs bus speeds, which is useful for high levels of data transfer in real-time. Such speeds are possible because the connections provide dedicated time slots for data transfer regardless of other operations. In addition, the IEEE 1394 connection automatically recognizes the presence of the peripheral device, without the need for additional installation software. 
       FIG. 1C  depicts a rear schematic perspective view of the haptic interface  10  with the exterior housings removed. Base  12  is sized to accommodate control circuitry, such as a pair of computer boards,  58   a ,  58   b , in this embodiment arranged substantially vertically within the base  12  and substantially parallel to each other. In this embodiment, power board  58   a  generally controls the power to the haptic interface  10 , while the IEEE 1394 interface board  58   b  controls complex force feedback, sensing, and other functions. Also contained in the base is the motor assembly  401  for the first powered tracked rotary element  14 , and a number of steel plates  59  for ballast, to at least partially offset forces arising during use of the haptic interface  10 . Rubberized or suction cup feet disposed on an underside of the interface  10  help stabilize the interface  10  and prevent it from sliding on smooth surfaces. At least one stop within the base  12  prevents over-rotation of rotary element  14 . 
       FIG. 2A  depicts a user interface  60  of the haptic interface  10 . In one embodiment, the user interface  60  consists of a nose end  34  and a user connection section, such as a stylus  40 . In a particular embodiment, the housing of both the nose  34  and stylus  40  are of split construction, for both ease of assembly and component construction, although a single housing component for either the nose  34  and/or stylus  40  is contemplated. 
     In the embodiment shown in  FIG. 2A , two pieces of the stylus housing  62  are removable. Alternatively, the entire housing  62  of the stylus  40  may be split, allowing for its complete removal. In one embodiment, the housing  62  is made in four main pieces to allow for faster assembly. The removable parts of the housing  62  may be secured with a compression ring  64 , may snap together, or use other means of joining. The rear portion of the stylus  40  is ergonomically designed and slips on the end of a connector shaft  90  ( FIG. 2C ) as a sleeve. Two user inputs  66   a ,  66   b  are depicted in this embodiment as buttons, but switches, toggles, rollers, or other devices may be used. First input  66   a  and second input  66   b  allow the user to control various functions of the stylus  40  and interface  10 . In one embodiment, the first input  66   a  operates as a standard ON/OFF toggle for the force feedback function of the interface  10 , while the second input  66   b  incorporates other system features, although either, both, or neither of the inputs  66   a ,  66   b  may be customizable by the user. In one embodiment, for example, pressing the second input  66   b  allows the user interface  60  and interface  10  to operate in a manner similar to that of a computer mouse without force feedback. Other alternative features of the second input  66   b  include, but are not limited to, a PAUSE or SLEEP control, force feedback toggle, digitizer control, spatial position reset, or any other option as desired by the user or required for a particular application. Moreover, either button may be used to toggle the haptic interface between two different functions. 
     The nose housing  68  and stylus housing  62  meet at or near the compression ring  64 . In one embodiment (as shown in  FIG. 2A ) the nose housing  68  is separable into two pieces. Once assembled, the nose housing  68  forms at least two projections  70  that engage bearings  120  of a mating yoke  36 . At or near the end of at least one of these projections  70  is a potentiometer blade  72 , which drives a potentiometer  130  located within the yoke  36  ( FIG. 3B ). The outside surface of the nose housing  68  can incorporate at least one recess  74  to engage a spring-loaded projection  154  ( FIG. 4A ) within the docking station  46  on the base  12  of the haptic interface  10 . The recess  74  may be oval, triangular, arcuate, or any other shape that allows for proper engagement with the spring-loaded projection  154 . A shaped tip  76  protrudes from or near the tapered end of the nose  34 . In one embodiment, the tip  76  may be used for precise tracing of the contours of a physical model or drawing and recording the accompanying data in computer memory, when the haptic interface  10  is used as a digitizer using the inputs  66   a ,  66   b . Although the tapered end of a plastic housing  68  may itself be used for this purpose, a hardened metal tip can be used, as it will more effectively withstand wear. In an alternative embodiment, when functioning as a digitizer, the shaped tip  76  may employ a manual or spring-loaded switch, optical technology, or any other technology known in the art. 
     Turning now to  FIG. 2B , the stylus  40  is shown with a portion of the housing  62  removed. Within the stylus  40 , a snap-type connection  80  serves as a connection element for the two halves for the user interface  60 . In this embodiment, a four-jaw snap barrel chuck with an O-ring  82  is used, but any snap-type connection that can properly join with a connector  96  on a connector shaft  90  of the nose  34  may be employed. The O-ring  82  keeps the jaws in a collapsed mode, thus allowing a connector to be trapped therein. The snap-type connection  80  used in the stylus  40  serves at least several purposes. First, the snap-type connection  80  allows for simple changeover of a variety of user connection elements for various applications. Instead of the stylus  40  shown in the figures, pistol-grip, ball, mouse, joystick, steering wheel, or other connections may be employed. Such an arrangement also allows for easy repair or replacement of the user interface  60 , should it become damaged. Second, the release characteristics of the snap-type connection  80  prevent damage to the haptic interface  10  if the stylus  40  is aggressively pulled. Generally, the maximum range of force typically applied to the stylus  40  during use is approximately three-quarters to one pound. A breakaway force of about five times the usage force will prevent damage to the haptic interface  10 . Moreover, employing a snap-type connection  80  allows the user interface  60  to maintain structural integrity without the need for additional screws or other fasteners. At a point at or near the compression ring  64 , a groove  84  is located and sized to mate with a guide  98  of the connector shaft  90  ( FIG. 2C ). The groove  84  can either be formed within a portion of the housing  62 , or may be formed by a gap where the two removable portions of the housing  62  join. 
     Referring to  FIGS. 2C and 2D , the nose  34  of the interface  60  is shown with a portion of the housing  68  removed. Moreover,  FIG. 2D  depicts a partially exploded nose  34  of the user interface  60 . Within the housing  68 , bearing seats or rests  86  provide a location for at least one set of bearings  88 . In one embodiment, use of a plurality of bearings  88  to support the connector shaft  90  eliminates undesirable play in the user interface  60 . A first stop  92  located within the housing  68  engages a second stop  94  on the connector shaft  90  to prevent over-rotation of the connector shaft  90 . The first stop  92  also prevents the bearings  88  from moving axially within the nose  34 , which could result in damage to the connector shaft  90  and nose  34 . 
     Contained partially within the nose housing  68  is the connector shaft  90 . The distal end of the shaft  90  is a conical connector  96  that serves as the joining element between the nose  34  and stylus  40 . In one embodiment, a conical connector similar to an audio device jack is employed, but diamond, tapered cylinder, and other non-conical shapes also may be used, provided they mechanically interlock with the snap-type connection  80  in the stylus  40 . A guide  98  extends radially outward from the connector shaft  90  and is sized to mate with the groove  84  on the stylus  40 . The connector shaft  90  is supported by at least one set of bearings  88 , which allows for low-friction rotation of the shaft  90  within the housing  68 . Ball, needle, roller, or other types of bearings may be used. A potentiometer retainer  100  also prevents non-rotational motion of the shaft  90 , by securing the bearings  88 , as well as a potentiometer  104 . Generally, potentiometers are of the type that employ a floating central disk, similar to those of the 251 Series, manufactured by CTS Corp., are used in the haptic interface  10 , although other sensors for outputting a signal representative of position may be employed. At a location within the housing  68 , a potentiometer blade  102  joins the connector shaft  90  at or near its terminus. A plurality of wires (not visible) exit the potentiometer  104  and are routed via the interior portions of the nose  34 , yoke  36 , shin  30 , and thigh  24  to the computer boards within the base  12  of the haptic interface  10 . Finally, the digitizing tip  76  is secured within the tapered end of the housing  68 . 
     As a user grips and rotates the stylus  40 , the rotational force is directed via the groove  84  to the guide  98 . As the guide  98  is a part of the connector shaft  90 , the shaft rotates about the F axis. This movement of the connector shaft  90  rotates the potentiometer blade  102 , which in turn drives the potentiometer  104 . Electronic output signals are then directed through the wiring back to the computer boards of the haptic interface  10 . 
       FIG. 3A  depicts a yoke arm assembly  110  of the haptic interface  10 . In one embodiment, the yoke arm  110  consists of two main parts, the shin  30  and the yoke  36 . The shin  30  and yoke  36  are joined at or near the midpoint of the yoke arm  110 . At this connection point, a shin band  116 , integral to housing  114 , contains a stop (not shown) which engages stop  124  ( FIG. 3B ). The other end (opposite the connection point  116 ) of the shin  30  rotatably connects to the thigh  24  of the haptic interface  10 , and the opposite end of the yoke  36  rotatably connects to the nose  34  of the user interface  60 . In one embodiment, both the shin  30  and yoke  36  are of split construction, for ease of assembly and component construction. For example, the split design allows for the component parts to be designed so they positively clamp the bearings  120  with pressure at all times, such that there is no play or sloppiness during use. By ensuring essentially zero backlash, very high system resolution and responsiveness can be achieved. 
       FIG. 3B  depicts the yoke  36  of the yoke arm assembly  110 . As can be seen, the branches of the yoke  36  are joined by two hinge pins  118  that allow for easy assembly of the housing  114  and eliminate the need for screws or other fasteners. In an alternative embodiment, a molded flexible joint may be used in lieu of hinge pins  118 . The use of the split housing  114  also allows the bearings  120  to be clamped with positive pressure at all times to eliminate play and backlash in the device. Use of a yoke  36  in this embodiment instead of a cantilever connection eliminates looseness and play in the device, which would be otherwise felt by the user, without the need for other mechanical reinforcements. Each branch of the yoke  36  contains at least one bearing  120  that joins one of the projections  70  on the nose  34 . The bearings  120  provide low-friction rotational movement of the projections  70  within each branch of the yoke  36 . The yoke  36  joins the yoke shaft  122  at a point at or near the shin band  116 . Extending radially outward from the yoke shaft  122  is a first stop  124 , designed to prevent over-rotation of the yoke shaft  122  by contacting a corresponding second stop on the inside circumference of the shin band  116 . The terminus of the yoke shaft  122  joins a blade  126  which drives a potentiometer  136  contained within the shin  30  of the yoke arm assembly  110 . A groove  123  is sized to receive a retaining ring to prevent the axial movement of the yoke shaft  122 . 
     Turning now to  FIG. 3C , the yoke arm assembly  110  is shown with portions of the housings  114 ,  112  of both the shin  30  and yoke  36  removed. At least one branch of the yoke  36  also contains a potentiometer retainer  128  and a potentiometer  130 , which are arranged such that the retainer  128  is between the bearing  120  and the potentiometer  130 . A plurality of wires (not shown) exit the potentiometer  130  and are routed via the interior portions of the yoke  36 , shin  30 , and thigh  24 , to the computer boards within the base  12  of the haptic interface  10 . The yoke shaft  122  extends from the yoke  36  into the shin  30 , and rotates about the D axis within the housing  112 , supported by at least one set of bearings  132 . In one embodiment, use of a plurality of positively clamped bearings  120  to support the yoke shaft  122  eliminates undesirable play in the yoke arm assembly  110 . Additionally, a retention ring  133  prevents axial movement of the yoke shaft  122 . A potentiometer retainer  134  prevents non-rotational motion of the yoke shaft  122  by securing the bearings  132  and also prevents movement of the potentiometer  136 . A plurality of wires (not shown) exit the potentiometer  136  and are routed via the interior portions of the yoke  36 , shin  30 , and thigh  24 , to the main computer board within the base  12  of the haptic interface  10 . 
     As a user manipulates the stylus  40 , certain forces are transferred to the nose  34 , causing the projections  70  to rotate within the bearings  120  of the yoke  36 . This movement of the projections  70  in turn rotates the potentiometer blade  72  about the E axis, which drives the potentiometer  130 . Electronic output signals are then directed through the wiring back to the computer boards of the haptic interface  10 . Similarly, as the user manipulates the stylus  40 , certain forces are transmitted via the nose  34  and projections  70  to the yoke  36 , causing the yoke  36  to rotate. As the yoke  36  is joined to the yoke shaft  122 , the shaft  122  rotates about the D axis. This movement of the yoke shaft  122  rotates the potentiometer blade  126 , which in turn drives the potentiometer  136 . Electronic output signals are then directed through the wiring back to the computer boards of the haptic interface  10 . 
       FIG. 4A  depicts a partially exploded docking station  46  used in the haptic interface  10 . The docking station  46  is secured within the base  12  of the haptic interface  10 , and serves as a resting point and home position for the nose  34  of the interface  60 . A tapered barrel  150  of the docking station  46  is configured to receive the nose  34 . A spring loaded projection  154  within the barrel  150  mates with the recess  74  on the nose  34 , thereby retaining the interface  60  within the docking station  46 . Alternatively, other types of retaining mechanisms such as magnets or compression rings may be employed. Also, other embodiments of the present invention may incorporate a male docking station  46  with a female connection on the nose  34  of the user interface  60 . 
     Now referencing  FIGS. 4B and 4C , a schematic view directed down the barrel  150  of the docking station  46  and a cross-sectional side schematic view of the docking station  46  are depicted, respectively. In addition to functioning as a rest position for the nose  34 , elements disposed within the tapered barrel  150  of the docking station  46  can serve other functions of the haptic interface  10 . A switch  152  is located on the inner circumference of the barrel  150  and detects the presence of the nose  34  and recalibrates the position of the interface  10  to home. Thus, a user may reset the spatial position of the entire interface  10  to a zero position or a user-defined home position, as required. Other embodiments of the haptic interface  10  allow the user to reset the spatial position of the interface  10  by manually pressing an input on the user interface  60 , without the need for docking the interface  60 . In one embodiment, an LED  156  is located at the base of the tapered barrel  150 . The LED  156  may signal a variety of diagnostic functions and/or errors by emitting various colors of different characteristics. For example, the LED  156  may blink to remind the user to dock the nose  34  at the completion of a program. A red strobe emission may be used to indicate a diagnostic problem with the haptic interface  10  or stylus  40 . Also, a steady green light, for example, may indicate that the haptic interface  10  is functioning properly. In one embodiment, the LED  156  is a blue neon pipe. Any of various combinations of light colors and emission patterns can be used to signal status or prompt the user. As one alternative to the LED  156  in the base of the tapered barrel  150 , the barrel  150  itself may be constructed of a clear plastic material. By energizing LEDs  156  installed proximate this clear barrel  150 , the entire barrel  150  would emit light, which could be more visible to a user and be more visually appealing. 
       FIG. 5  depicts a rear schematic perspective view of the internal drive system of one embodiment of the haptic interface  10 . The base  12  supports the first powered tracked rotary element  14  to define a first articulation  16  about the axis A having a substantially vertical orientation. A vertically oriented first actuator  401  ( FIG. 1C ) drives a vertically oriented first threaded capstan  413  that in turn manipulates a first cable  453  ( FIG. 8C ). The first cable is secured at, at least two points  455   a ,  455   b  ( FIG. 8C ), to the horizontally oriented first powered tracked rotary element  14 , and thus rotates the first element  14  about the A axis. For a more detailed description of first motor assembly  401  and its operation, refer to  FIGS. 7 and 8C  and accompanying text. Mounted on the first powered tracked rotary element  14  are a second powered tracked rotary element  18  and a rotary transfer drive element  164  of the third powered tracked rotary element  22  and their associated motor assemblies  501 ,  601 . 
     Both the second powered tracked rotary element  18  and the rotary transfer drive element  164  operate in a manner similar to that of the first powered tracked rotary element  14 . For a more detailed description of the second motor assembly  501  and its operation, refer to  FIGS. 7 and 9  and accompanying text.  FIGS. 7 and 10  provide a more detailed description of the third motor assembly  601  and its operation. 
     The orientations of the second powered tracked rotary element  18  and rotary transfer drive element  164  and their associated motor assemblies  501 ,  601  allow for a very compact configuration and a reduction in overall size of the haptic interface  10 . As can be seen from  FIG. 5 , the second motor assembly  501  and third motor assembly  601  are installed horizontally, substantially parallel to each other, in a balanced configuration about the A axis. This arrangement imparts rotational forces about the B axis on either side of the geometric center A, thus relatively evenly loading the interface  10  on both sides. In one embodiment, the torque from one rotary element is transferred through a central shaft upon which the other element rests. Similarly, the second rotary element  18  and rotary transfer drive element  164  are installed on opposite ends of the first rotary element  14 . This particular arrangement eliminates the requirement for a large housing to enclose the internal components and simplifies access for repair. The balanced arrangement also more evenly distributes the overall inertia of the motors within the device, thus improving stability of the haptic interface  10  as opposed to a cantilevered arrangement. A single assembly shaft  172 , installed substantially horizontal and parallel to the second and third motor assemblies  501 ,  601 , and in line with the B axis, secures the second rotary element  18  and rotary transfer drive element  164 , allowing for easy assembly. Also, an assembly rod  173  runs through shaft  172  and secures the spherical housing  56  to the interface  10 ; thus the need for a number of screws or other fasteners penetrating the housing  56  is eliminated. 
     The particular embodiment of the haptic interface  10  shown in  FIG. 5  also utilizes at least one torsion spring  160  on the B axis of the interface  10 . During use, the weight of the thigh  24 , shin  30 , yoke  36 , nose  34 , and stylus  40  tend to oppose many of the manipulations of the user. Naturally, due to the force of gravity, the weight of those elements induces rotation about the B axis. Such rotations are felt by the user as a sluggishness or resistance when using the haptic interface  10 . In an effort to overcome these forces caused by the weight of the extension elements, previous haptic interfaces utilized bulky counterweights attached to the rotary elements. These bulky weights, however, increase the size and weight of the haptic interface. The haptic interface  10  of the present invention, however, utilizes solely the torsion spring  160 , to offset the forces imposed on rotary element  18  without the need for any counterweight. 
       FIG. 6A  depicts an embodiment of the transfer drive  162  useful in the haptic interface  10 . Although the rotary transfer drive element  164  rotates about the B axis, the rotary transfer drive element  164  defines a third articulation  26  having an axis C, located on the outwardly radially disposed extension  24  of the second element  18 . As a capstan  613  of the rotary transfer drive element  164  rotates, the element  164  rotates a transfer drive shaft  166  aligned with the second axis B, converting rotary motion to linear motion of first  168   a  and second  168   b  transfer drive rods disposed along the radial extension  24  of the second element  18 . The first  168   a  and second  168   b  drive rods terminate in looped braided steel cable ends which are hooked onto a raised ground tab  165  of the third rotary element  22 . 
     Accordingly, the second transfer drive rod  168   b  is directly grounded through looped cable ends to each of the transfer drive shaft  166  and the third rotary element  22 ; whereas, the first drive rod  168   a  is directly grounded through a looped cable end to a raised ground tab  167  of the third rotary element  22  and indirectly grounded with a single cable to the transfer drive shaft through a clutch post  757  ( FIG. 6B ) and spring  759 . The drive rods  168   a ,  168   b  minimize cable lengths and therefore enhance the stiffness and rigidity of the transfer drive  162 . The cables are used solely at the grounding points, with one cable  753  end of the first drive rod  168   a  being routed through the automatic cable tensioning device  751  depicted in  FIG. 8A  to substantially eliminate backlash in the third axis drive. 
     In  FIG. 6B , one embodiment of a cable tensioning device  351  described later with respect to  FIG. 8A  is employed in the transfer drive  162  of the haptic interface  10 , defined generally here as cable tensioning device  751 . Depicted is the circular transfer drive shaft  166 . The cable  753  (from the terminus of the first drive rod  168   a ) is fixed to shaft  166  at a first ground location  755   a  and circumscribes the shaft  166  in a clockwise direction. The cable  753  wraps around a clutch post  757 , and thereafter, is attached to a spring  759  in tension, which is grounded to a radial extension  170  of shaft  166  at ground  755   b . Tabs, slots, and other guide features may be provided in the shaft  166  to facilitate routing and retention of the cable  753  in the proper location and orientation throughout the range of motion of the shaft  166 . The tension achieved with the automatic cable tensioning device  751  also provides added stiffness and rigidity in the drive system. 
       FIG. 7  shows a schematic representation of a typical actuator assembly  301  used in one embodiment the haptic interface  10 . In order to track the location of the powered axes A-C, each actuator  303  is fitted with an encoder board  305  at the base of the actuator  303 . An emitter/detector optical encoder chip  307  is secured on or within the board  305 . Rotation of the actuator shaft  311   a  is tracked by mounting a reflective encoder disk  309  on an actuator shaft extension  311   b  extending from the actuator  303  remote from the capstan  313 . By incorporating a reflective encoder disk  309 , in lieu of a common non-reflective disk, the overall volume of the actuator assembly  301  is reduced, since a non-reflective disk requires the use of an emitter/detector pair that straddles an edge of the disk  309 . 
     In an embodiment of the haptic interface  10 , the emitter/detector  307  is a single unit mounted at the end of the actuator  303 , directing pulses to, and receiving pulses from, the reflective encoder disk  309 . As the actuator  303  causes the disk  309  to rotate, or as the disk  309  rotates due to user movement of the manipulation device  10 , the emitter/detector  307  outputs pulses that are in turn reflected by the disk  309 , allowing the angular orientation of the articulation to be determined. Three of these actuator assemblies  301  are used in the haptic interface  10 , one for each of the powered articulations  16 ,  20 ,  26 ; however, more or less actuator assemblies may be employed depending on the number of powered axes. 
     The actuator assembly  301  uses components readily available in the market. In one embodiment, the actuator  303  is a D.C. motor. Generally, a reflective encoder disk similar to the 8000 Series manufactured by Agilent Technologies is utilized. The capstan  313  and reflective encoder disk  309  may be secured to the actuator shaft  311   a  and extension  311   b  by a variety of means, such as mechanical connections or press fit connections employing heat expansion and cooling. In a particular embodiment of the present invention, however, the capstan  313  and disk  309  are secured using a strong bonding adhesive, such as one marketed under the name Loctite®, manufactured by Henkel Consumer Adhesives, Inc., to reduce the overall size of the assembly  301 . 
     Instead of using mechanical linkages, gears, or other force transmission components, the interface  10  employs three dedicated actuators (described above) fitted with capstans and corresponding cables to power rotary axes A-C. Cable drives provide good force transmission characteristics with low weight; however, backlash can be a problem, especially in high precision, high resolution haptic interfaces. Backlash or play in a rotary mechanical transmission, such as those employed in the interface  10 , is most evident when direction of rotation is reversed. One method of reducing backlash is to provide a manual adjustment feature to adjust the position of one or both of the cable ends relative to ground so that slack in the cable can be reduced. Further, the cable can be preloaded in tension so that there is minimal slippage between the cable and the actuator capstan as the capstan rotates; however, as the cable stretches and the components of the mechanism wear over time, cable tension is reduced and must be periodically adjusted to prevent slippage. Additionally, cable tension is difficult to measure and excessive tensioning can lead to deformation of the structural elements and accelerated, premature wear in the articulation bearings. 
       FIG. 8A  is a schematic diagram of an automatic cable tensioning device  351  that overcomes many of the limitations of known cable drives and is useful in the powered axes of the haptic interface  10 . The tensioning device  351  automatically loads the cable  353  to a predetermined tension and maintains that level of tension over time, even in the event of cable stretching and component wear. The tensioning device  351  includes a cable  353  fixed at proximal and distal ends directly or indirectly to a ground surface, shown generally at  355   a ,  355   b . A non-rotating clutch post  357 , also fixed to ground, is located along the cable path. A spring  359  is disposed along the cable path between the clutch post  357  and ground  355   b . Lastly, the actuator capstan  313  is provided along the cable path between the clutch post  357  and ground  355   a  on the side opposite the spring  359 . As depicted in  FIG. 8A , the cable  353  extends from ground  355   a , circumscribes both the actuator capstan  313  and the clutch post  357  at least once each, and is connected to the spring  359  that is in tension and connected to ground  355   b.    
     A non-rotating post, such as the clutch post  357 , may be used to amplify or multiply an applied cable tension to resist or offset tension applied to the cable  353  downstream of the post  357 . As is known by those skilled in the art, the amplification factor is a function of post diameter, wrap angle of the cable around the post, and the coefficient of friction between the cable and the post. Accordingly, for a given spring tension, as wrap angle and/or friction increases, a larger downstream cable force can be offset or resisted. 
     In a static state, the tension induced in the cable  353  by the spring  359  causes the cable  353  to be pulled to the right, eliminating any slack or looseness in the cable  353 , cable tension being a function of the spring constant, k, and the linear displacement, x, of the spring ends from a rest state. In operation, as the actuator capstan  313  rotates in a clockwise direction, as depicted, tension is applied to the portion of the cable  353  between the capstan  313  and ground  355   a  and the capstan  313  moves to the left relative to ground  355   a . Any looseness or slack in the cable  353  to the right of the capstan  313  is automatically taken up by the spring  359 , the cable  353  sliding around the clutch post  357  whenever the spring force overcomes the frictional drag of the cable  353  around the clutch post  357 . 
     Alternatively, when the capstan  313  rotates in a counter-clockwise direction, the capstan  313  applies tension to the cable  353  portion between the capstan  313  and the clutch post  357 . As long as the spring tension enhanced by the clutch post effect exceeds the tension induced by the capstan  313 , the cable  353  will be effectively locked to the clutch post  357  and will not slip around the post  357 . The spring  359  will be effectively isolated from the capstan loading. Accordingly, the tensioning device  351  automatically self-adjusts and maintains cable tension at a predetermined magnitude, taking up any slack when the capstan  313  rotates in a first direction and locking when the capstan  313  rotates in a second direction. 
     Referring to  FIG. 8B , an enlarged view of an actuator capstan  313  for use in one embodiment of the haptic interface  10  is shown. While the capstan  313  may be a uniform cylinder, in one embodiment, the capstan  313  includes a helical channel  315  formed along an exterior surface thereof. The helical channel  315  may include a generous radius without sharp edges, which could cut through the cable  353  or otherwise reduce cable life. The helical channel  315  nests and routes the cable  353 , preventing overlapping or tangling of the cable  353  on the capstan  313 . In one embodiment, a nylon coated cable  353  is used to prevent slippage upon the capstan  313  and to protect the cable  353  from damage to ensure a long life. A variety of cable materials can be used including, but not limited to, tungsten, stainless steel, uncoated steel, or another form of coated steel. Also, the number of wraps the cable makes around the capstan is dependant on capstan and cable size, anticipated loads, and other related considerations. 
     Turning now to  FIG. 8C , the cable tensioning device  351  described above is employed in the first articulation  16  of the haptic interface  10 , defined generally here as cable tensioning device  451 . Depicted is a generally D-shaped hub portion of the first element  14 . A cable  453  is fixed to the first powered tracked rotary element  14  at a first ground location  455   a  and circumscribes the element  14  in a counterclockwise direction. The cable  453  wraps an actuator capstan  413  disposed substantially tangentially to the circumference of the element  14  before wrapping several times around a clutch post  457 . Thereafter, the cable  453  is attached to a spring  459  in tension, which is grounded, to the element  14  at ground  455   b . Since the actuator is fixed in the housing  12  of the interface  10 , as the actuator rotates the capstan  413 , the first element  14  is caused to rotate about first axis A. Tabs, slots, and other guide features may be provided in the element  14  to facilitate routing and retention of the cable  453  in the proper location and orientation throughout the range of motion of the element  14 . 
     As can be seen in  FIG. 8C , the hub portion of the first rotary element  14  is generally D-shaped. Alternatively, a circular or partially circular element  14  is contemplated. The rotary element  14  is supported at a centrally located axis shaft on the A axis. The first rotary element  14  may be either of a segmented construction, as shown, or solid, perforated, or any other construction, as required. If required, a support surface for the other rotary elements and their associated motors may be secured to the first rotary element  14 . Moreover, the A axis shaft may be hollow or include a groove to accommodate any of the control or power wiring of the haptic interface  10 . Alternatively, openings may be formed within first rotary element  14  for this purpose. 
     Turning now to  FIG. 9 , the cable tensioning device  351  described above is employed in the second articulation  20  of the haptic interface  10 , defined generally here as cable tensioning device  551 . Depicted is a generally D-shaped hub portion of the second element  18 . A cable  553  is fixed to the element  18  at a first ground location  555   a  and circumscribes the element  18  in a counterclockwise direction. The cable  553  wraps an actuator capstan  513  disposed substantially tangentially to the circumference of the element  18  before wrapping several times around a clutch post  557 . Thereafter, the cable  553  is routed through a recess  561  in the rotary element  18  and attached to a spring  559  in tension, which is grounded to the element  18  at ground  555   b . As will be apparent to one of ordinary skill in the art, tabs, slots, and other guide features may be provided in the outer circumference of element  18  to facilitate routing and retention of the cable  553  in the proper location and orientation throughout the range of motion of the element  18 . 
     In one embodiment of the haptic interface  10 , the second rotary element  18  is penetrated by at least two control wire conduits  563   a ,  563   b . These conduits  563   a ,  563   b  provide a location for the power and control wiring and generally restrict the wires movement as the element  18  rotates. The rotary element  18  rotates about a centrally located B axis shaft that may be smooth, include grooves or tabs, or be threaded as required. As an alternative to the D-shaped element shown in the  FIG. 9 , a circular rotary element may be employed. Use of a D-shaped element  18 , however, can reduce the overall size of the haptic interface  10 . 
     Similarly, in  FIG. 10 , the cable tensioning device  351  described above is employed in the third articulation  26  of one embodiment of the haptic interface  10 , defined generally here as cable tensioning device  651 . Depicted is a generally D-shaped hub portion of the rotary transfer drive element  164 . A cable  653  is fixed to the element  164  at a first ground location  655   a  and circumscribes the element  164  in a counterclockwise direction. The cable  653  wraps an actuator capstan  613  disposed substantially tangentially to the circumference of the element  164  before wrapping several times around a clutch post  657 . Thereafter, the cable  653  is routed through a recess  661  in the rotary transfer drive element  164  and attached to a spring  659  in tension which is grounded to the element  164  at ground  655   b . As will be apparent to one of ordinary skill in the art, tabs, slots, and other guide features may be provided in the outer circumference of rotary transfer drive element  164  to facilitate routing and retention of the cable  653  in the proper location and orientation throughout the range of motion of the element  164 . 
     The rotary transfer drive element  164  rotates freely about axis B. Rotational force is transferred to third articulation  26  by transfer drive shaft  166  and associated components depicted in more detail in  FIGS. 6A-6B  and described in the accompanying text. As an alternative to the D-shaped element shown in the  FIG. 10 , a circular rotary element may be employed. Use of a D-shaped element  164 , however, can reduce the overall size of the haptic interface  10 . 
     During use of the haptic interface  10 , the three powered tracked rotary elements  14 ,  18 , and  22  may be either “powered” or “free.” When powered, the actuators are energized and can control the rotation of the respective rotary elements, directing the elements to either resist or force the movements of the interface user depending on the application. This powered setting is useful for force feedback situations, such as simulating surgical techniques, providing feedback during computer game play, etc. In the free setting, the actuators are not energized and the rotary elements are subject to the forces of the interface user. Such a setting is useful for digitizing drawings or objects directly into a computer program, using the user interface as a personal computer mouse, drafting computer-aided design (CAD) images, etc. Any number of the three rotary elements may be in either powered or free mode for any particular application, or may switch between the two modes when certain criteria are met. 
     Light weight, low cost, high stiffness, and high strength are preferred characteristics for the moveable portions of the haptic interface. For these reasons, injection molded 40% carbon fiber filled nylon or similar compositions may be selected for the structural elements such as second element  18 , second element extension  24 , third element  22 , third element extension  30 , fifth element  34 , and sixth element  40 . Other glass and carbon fiber filled, injection molded plastics may be used as well. Moreover, in one embodiment, the external gripping surfaces of the stylus housing  62  are treated with an anti-slip coating or paint to prevent the stylus  40  from slipping from the user&#39;s grasp. Alternatively, the external surfaces may be physically textured or knurled as required. In one embodiment, the haptic interface  10  may be used in conjunction with a wrist rest  700  as depicted in  FIG. 13 . An example of such a wrist rest  700  is disclosed in U.S. Pat. No. 6,417,638. All internal components may be manufactured from plastics, metal, or any combination of such materials. Desirable characteristics for the base  12  and spherical housing  56  of the haptic interface  10  also include low cost, high strength, and high stiffness; however, because the base structure may also serve as a heat sink for the internal electronics, it is desirable that the base structure be thermally conductive. 
       FIG. 11A  depicts an algorithm  800  employed in one embodiment of the interface controller for measuring and controlling the forces generated by the haptic interface  10 . Signals from the actuators and/or potentiometers first update  802  the force reading stored in memory. Next, a new force is computed  804 , and the electrical current corresponding to that computed force is sent  806  to one of the actuators, to either rotate the associated element or resist such a rotation. The algorithm  800  then awaits a responsive signal  808  from the actuators and/or potentiometers (due to user manipulation) and updates the stored force reading  802  accordingly. This algorithm continues to operate during an entire program, translating and tracking electrical signals to allow the interface user to interact with a computer application program. 
     Temperature sensing devices are required in consumer products to prevent overheating and possible injury to users and to prevent damage to a device&#39;s internal components. Generally, thermocouples are used to measure temperatures of internal motors and other components to meet this requirement. One embodiment of the haptic interface  10  in accordance with the present invention, however, uses a computer algorithm to monitor temperature within the device, in the absence of any thermocouple or other sensor that directly reads internal temperature. A flowchart of such an temperature calculating algorithm  810  (a subroutine of control algorithm  800  described above) is depicted in  FIG. 11B . Generally, the algorithm  810  use time and actuator current usage to estimate temperature. As electrical current is sent to an actuator to generate a force  806  upon a rotary element (to either rotate the element or resist such a rotation), the algorithm  810  measures the current delivered to the motor and the total length of delivery time. The algorithm  810  then computes the estimated internal actuator temperature based on the amount of time the current has been delivered to the actuator, thereby updating its thermal model  812 . 
     If the result of the update is an internal temperature less than about 80° C.  814 , the force is applied to the rotary element  816 . If, however, the internal temperature exceeds about 80° C.  818 , the force is disabled  820 , and delivery of current to the actuator is terminated. Under the latter condition, the temperature data is cached  822  for application in a temperature error algorithm  830  (described below), and a temperature error message  824  is delivered to the user. This error may take the form of a notation within the associated computer program to be displayed on a computer screen and/or will result in a visible change in the LED in the haptic interface docking station to indicate a system error. The temperature limit can be adjusted, as required, for any given application or to prevent damage to internal device components. A threshold temperature of 49° C., for example, can be set to cause shutdown of the interface  10  before any damage occurs to the actuators or other internal components. 
     A flowchart for the temperature error algorithm  830  is depicted in  FIG. 11C . Upon updating the force  802  stored in the control algorithm  800 , the subroutine temperature error algorithm  830  determines the consequences of a possible temperature error. A determination that no temperature error has occurred  832  causes a bypass of steps  834  and  836 . If however, the temperature calculating algorithm  810  determines that an error has occurred  824 , the temperature error algorithm  830  reads the cached temperature data  834 , stored in the cache temperature  822  step of the temperature calculating algorithm  810 . The algorithm  830  then computes any thermal decay  836  of interface components due to the excessive temperature. Information regarding decay, and how it will affect future interface performance, is stored  838  and taken into account in any later kinematics calculations  840 . Thus, as interface performance is impacted by temperature errors, the interface  10  can compensate, as required, to continue to deliver an accurate force-reproduction experience for the user. 
     Turning now to  FIG. 12 , a schematic representation of the IEEE 1394 compliant interface board  58   b  of one embodiment of the haptic interface  10  is depicted. The board  58   b  controls various types of electromechanical interface  900  and digital  902  functions. The board  58   b  is powered by the haptic interface power supply  906 . The electromechanical interface functions  900  of the board  58   b  ultimately control the function of the various components  904  (described in more detail above) of the haptic interface  10 . Specifically, current drivers  910  control the function of the three actuators. The current drivers  904  consist of three channels, for permanent magnet D.C. servomotors. The drivers  904  operate at a maximum continuous output of 14.4 Watts per channel, plus or minus 18 volts mA. The maximum output for the three channels is 25 Watts. The drivers  904  also have 12 bits of resolution at 1 kHz bandwidth. Encoder counters  912  consist of three channels. The counters  912  can receive a rate of pulses up to 500 kHz and 16 bits of resolution. The analog potentiometer inputs  914  also have three channels, and typically recognize 0-5 volt signals from 5K Ohm potentiometers. The inputs also have 10 bits of resolution and 1 kHz of filtering with a 3-dB cutoff. Digital input/output  916  consists of four output channels and eight input channels. The digital input/output operates with debounced TTL in and TTL out with sufficient current to drive any LEDs. 
     The digital functions  902  of the board  58   b  communicate with the various electromechanical interface functions  900  and the program host computer  908  via the IEEE 1394 connection  926 . Local feedback and safety logic function  918  performs several functions. These include, but are not limited to, velocity based positive feedback to compensate for back-emf of motor and friction, velocity threshold shutdown, current shutdown if threshold exceeded, and watchdog shutdown if not updated within a certain time. Also, a 32-bit read-only serial number interface  920  identifies the haptic interface  10  to the host computer. The digital functions  902  also include 32 bit read-write volatile  922  and non-volatile  924  registers. Additionally, the electronics of the interface may include an 8031 microprocessor, FLASH memory, Programmable Logic Device (PLD) and PLD-based delta sigma A/D converters, and a four-layer printed circuit card. 
     The microprocessor negotiates with the host computer, manages system initialization and isochronous data transfers during operation, loads the PLD configuration, and manages the FLASH memory read/write operations (to allow remote updates of the 8031 program, the PLD configuration, and system constants). The PLD implements three 16-bit quadrature encoder interfaces, encoder speed detection, power fail and over current safety logic, motor enablement monitoring, 512 byte stack RAM bank to supplement 8031 memory, FIFO interface to IEEE 1394 connection link controller isochronous data mover port, control for three nine-bit accurate delta-sigma potentiometer A/D converters, three ten-bit PWM generators to set motor currents, triangle wave frequency generator, and power supply sync frequency generator. The power board  58   a  ( FIG. 1C ) includes a power supply, safety circuitry, three PWN amplifiers, PWM-based D/A converters, and a two-layer printed circuit card. 
     While there have been described herein what are to be considered exemplary and preferred embodiments of the present invention, other modifications of the invention will become apparent to those skilled in the art from the teachings herein. The particular methods of manufacture and geometries disclosed herein are exemplary in nature and are not to be considered limiting. It is therefore desired to be secured in the appended claims all such modifications as fall within the spirit and scope of the invention. Accordingly, what is desired to be secured by Letters Patent is the invention as defined and differentiated in the following claims.