Abstract:
A replaceable instrument mechanism for a haptic device comprises a coupler having a coupler body adapted to be connected to an output end of the haptic device. A force transmission mechanism is connected to the coupler body, has a movable connector displaceable along one degree-of-freedom with respect to the coupler body, and is connected to a force feedback system of the haptic device so as to receive force feedback from the haptic device and impart the force feedback to the movable connector. An instrument has an instrument body having a connector end releasably secured to the second end of the coupler body so as to be displaceable with the coupler. A handle portion is manually actuatable in one degree-of-freedom with respect to the instrument body to simulate an operation performed with the instrument. A member is associated with the handle portion so as to move by actuation of the handle portion, and releasably connected to the movable connector of the force transmission mechanism so as to transmit force feedback from the movable connector to the handle portion in response to actuation of the handle portion as detected by the haptic device.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This patent application claims priority on U.S. Provisional Patent Application No. 60/689,066, filed on Jun. 10, 2005. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to force feedback haptic devices (a.k.a., hand controllers), particularly to six-degree-of-freedom hand controllers with interchangeable instruments involving revolute or prismatic jointed handles.  
         [0004]     2. Background Art  
         [0005]     Force-reflecting master hand controllers have been used to drive robotic systems, and to provide an interface to a computer system that allows the user to input spatial position and to feel forces in response to his or her movement.  
         [0006]     In U.S. Pat. No. 6,593,907, issued on Jul. 15, 2003 to Demers et al., a tendon-driven hand controller that provides powered translation in three degrees of freedom, and powered rotation in three degrees of freedom is described.  
         [0007]     The six-degree-of-freedom (6-DOF) hand controller is especially useful in surgical simulation. A virtual force may be presented as if it were acting at any location on a handle held by the operator. The force that is felt can therefore mimic the feeling of surgical instruments as they are used in an operation.  
         [0008]     To this point, it has been possible to simulate instruments with single handles, such as a scalpel, but many instruments involve their own degree-of-freedom mechanisms, such as two members connected by a revolute joint. Scissors, forceps, clamps and rongeurs fall under this category. Moreover, the varieties of such instruments number in the tens of thousands, as they are created for cutting and manipulating tissue in many different surgical procedures.  
         [0009]     In spite of the large variety of instruments, a limited number of handles for hand controllers have been designed for these instruments, since the human hand has a limited number of shapes and sizes.  
         [0010]     There do exist surgical simulators with fixed handles and a limited number of degrees of freedom. Rosenberg, in U.S. Pat. No. 5,731,804, issued on Mar. 24, 1998, describes a 4-DOF hand controller with a gripper handle. This device is intended to simulate an endoscopic surgical instrument, in which the working surfaces are at the end of long shafts inserted through holes in the patient&#39;s body. The simulated endoscopic surgical instrument however appears to be permanently part of the hand controller.  
         [0011]     Likewise, U.S. Pat. No. 6,684,129, issued to Salisbury, Jr. et al. on Jan. 27, 2004, discloses the operation of the master controller of the robotic surgical system from Intuitive Surgical Inc. The master controller has six degrees of freedom, plus one degree for activation of a gripper. The handle, however, is fixed to the device, and designed to enable the operator to manipulate the various tools of the surgical system.  
       SUMMARY OF INVENTION  
       [0012]     It is therefore an aim of the present invention to provide an haptic device addressing issues associated with the prior art.  
         [0013]     It is also an aim of the present invention to provide a novel replaceable instrument mechanism.  
         [0014]     Therefore, in accordance with the present invention, there is provided a replaceable instrument mechanism for a haptic device, comprising: a coupler having: a coupler body with a first end and a second end, the first end adapted to be connected to an output end of the haptic device so as to be displaceable along the degrees-of-freedom of the haptic device; and a force transmission mechanism connected to the coupler body, the force transmission mechanism having a movable connector displaceable along at least one degree-of-freedom with respect to the coupler body, the force transmission mechanism adapted to be connected to a force feedback system of the haptic device so as to receive force feedback from the haptic device and impart the force feedback to the movable connector; at least one instrument having: an instrument body having a connector end releasably secured to the second end of the coupler body so as to be displaceable with the coupler; a handle portion manually actuatable in at least one degree-of-freedom with respect to the instrument body to simulate an operation performed with the instrument; and a member associated with the handle portion so as to move by actuation of the handle portion, and releasably connected to the movable connector of the force transmission mechanism so as to transmit force feedback from the movable connector to the handle portion in response to actuation of the handle portion as detected by the haptic device.  
         [0015]     Therefore, in accordance with the present invention, there is provided a scissors mechanism comprising a fixed shaft and a sliding shaft, the fixed shaft having a coupler at one end and a fixed handle at the other end with a loop or a bar for the part of the hand near or at the thumb, the sliding shaft having a coupler at one end and a revolute joint at the other end, the revolute joint attaching a moving handle, the handle also being attached by a revolute joint to the end of the fixed shaft near the fixed handle, so that moving the movable handle moves the sliding shaft by a lever action.  
         [0016]     Also in accordance with the present invention, there is provided a mechanism for coupling the scissors mechanism to a platform, comprising a coupler to attach the fixed shaft fixedly to the platform, and a second coupler for coupling the sliding shaft to a driven slider, the driven slider being a member connected to the platform by a prismatic joint, having a coupler at one end and a driving means somewhere else along its length. In a preferred embodiment, the driving means is a revolute joint attached to a scissors drive pulley at a radius r, the scissors drive pulley being attached to the platform by a revolute joint at the centre of the scissors drive pulley.  
         [0017]     Also in accordance with the present invention, there is provided a mechanism for uncoupling the fixed shaft from the hand controller, and for uncoupling the sliding shaft from the driven slider. In a preferred embodiment, the coupling/uncoupling mechanism of both the fixed shaft and the sliding shafts each comprise a spring-loaded lever that presses a latch member into a recess in the side of the shaft, the lever being attached at its midpoint to the moveable base by a revolute joint, having a button end and a latch end, with a tension spring under the button end pushing the button end away from the platform, such that the latch end presses the latch member into the recess in the shaft, and such that a user may press the button end to release the latch mechanism, freeing the shaft so that it may be withdrawn from the platform.  
         [0018]     Also in accordance with the present invention, there is provided a mechanism for actuating the driven slider, comprising a tendon routed around a configuration of idler pulleys that brings it from a capstan attached to the shaft of a rotary actuator on a fixed base through six joints of the 6-DOF hand controller (comprising two joints in a shoulder, an elbow joint, and pitch, yaw and roll joints in a spherical wrist) to the platform on the sixth and last joint in the hand controller, consisting of a shoulder pulley, an elbow pulley, a yaw idler pulley, and a roll-routing configuration of pulleys, consisting of a pair of inward idlers that accepts the tendon from the yaw pulley and redirects the tendon tangentially to the roll of the platform, a pair of cross over idlers that allows the tendon halves to cross over at the roll joint, and a pair of outward idlers that directs the tendon from a direction tangential to the roll of the platform to the direction of the scissors drive pulley.  
         [0019]     Also in accordance with the present invention, there is provided a mechanism for sensing the angle of the scissors opening, consisting of either an angle sensor on the scissors joint, or an angle sensor on the scissors drive pulley, or a linear sensor measuring the movement of the slider shaft relative to the fixed shaft.  
         [0020]     While the preferred embodiment of the platform and the tendon routing is a 6-DOF device as just described, the interchangeable handles and the platform may be mounted on a hand controller having any number of degrees of freedom, from a device attached to a fixed base to a multi-degree-of-freedom device. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     Reference will now be made to the accompanying drawings, showing by way of illustration a preferred embodiment of the present invention and in which:  
         [0022]      FIGS. 1   a  to  1   c  are schematic views of handles of an instrument known to surgeons as a rongeur, showing its attachment to a coupler of a replaceable instrument mechanism in accordance with an embodiment of the present invention;  
         [0023]      FIGS. 2   a  to  2 C are schematic views of handles of a pair of scissors or forceps, showing its attachment to the coupler of the replaceable instrument mechanism of  FIG. 1   a;    
         [0024]      FIGS. 3   a  to  3   c  are schematic views of handle of a thumb forceps, showing its attachment to the coupler of the replaceable instrument mechanism of  FIG. 1   a;    
         [0025]      FIGS. 4   a  and  4   b  are enlarged views of the coupler of the replaceable instrument mechanism, showing coupling for fixed and sliding shafts;  
         [0026]      FIGS. 4   c  and  4   d  are enlarged views of coupling ends of instruments connectable to the coupler, showing fixed and sliding shafts;  
         [0027]      FIG. 5   a  is a perspective schematic view of a coupling mechanism, with a slider shaft contained inside a fixed shaft;  
         [0028]      FIG. 5   b  is a perspective schematic view of an alternative coupling mechanism, with a slider shaft on top of a fixed shaft;  
         [0029]      FIG. 5   c  is a schematic assembly view of a latching mechanism for securing the sliding shaft and the fixed shaft into place;  
         [0030]      FIG. 6   a  is a schematic view of a drive mechanism of the replaceable instrument mechanism;  
         [0031]      FIG. 6   b  is a schematic view of a four-bar mechanism comparable to the drive mechanism of  FIG. 6   a,  showing links and revolute joints;  
         [0032]      FIG. 6   c  is a schematic view of the four-bar mechanism, showing symbolic designations for the links and angles formed by the links;  
         [0033]      FIG. 6   d  is a schematic view of the four-bar mechanism, showing free body diagrams of each link and the forces acting upon them;  
         [0034]      FIG. 7  is an enlarged perspective view of a pulley assembly for transferring power from a base to the platform through a roll joint;  
         [0035]      FIG. 8  is a front view of the pulley assembly of  FIG. 7 ;  
         [0036]      FIG. 9   a  is a schematic view of the pulley assembly of  FIG. 7 , showing the tendon path after a roll;  
         [0037]      FIG. 9   b  is a schematic view of the pulley assembly of  FIG. 7 , showing possible rotation of the drive pulley if the tendon path is held at one end;  
         [0038]      FIG. 10   a  is a bottom schematic view of the pulley assembly of  FIG. 7 , showing the tendon path crossing over between idler pulleys;  
         [0039]      FIG. 10   b  is a schematic view of a mechanism of the pulley assembly of  FIG. 7  for allowing two tendons to cross over;  
         [0040]      FIG. 10   c  is a bottom schematic view of an alternative embodiment of the pulley assembly, showing a tendon path that avoids crossing over between idler pulleys;  
         [0041]      FIG. 10   d  is a top schematic view of the pulley assembly of  FIG. 10   c,  showing a tendon path that avoids crossing over between idler pulleys;  
         [0042]      FIG. 11   a  is a schematic view of a mechanism with revolute jointed handles connected directly to the drive pulley;  
         [0043]      FIG. 11   b  is a schematic view of a mechanism with a fixed handle connected fixedly to the fixed coupler, and another mechanism with a plunger;  
         [0044]      FIG. 11   c  is a schematic view of a mechanism with a plunger;  
         [0045]      FIG. 12  is a schematic representation of a processing system used with the manipulator of  FIG. 1 ;  
         [0046]      FIG. 13  is a perspective view of the distal stage of a Freedom 6S hand controller with the replaceable instrument mechanism of the present invention;  
         [0047]      FIG. 14  is an overall perspective view the Freedom 6S hand controller with the replaceable instrument mechanism of  FIG. 13 ;  
         [0048]      FIG. 15  is a perspective view of the distal stage of the Freedom 6S hand controller with an alternative embodiment of the replaceable instrument mechanism; and  
         [0049]      FIG. 16  is an exploded view of an instrument used in the replaceable instrument mechanism.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0050]     The illustrated embodiments show haptic devices in the form of serial controllers, with a serial mechanism supporting a replaceable instrument mechanism. The replaceable instrument mechanism may include motors which are generally lightweight. The controller preferably has a balanced design, which permits the motors to apply all their power to the handle mechanism, rather than consuming energy to overcome an unbalanced gravitational load. This leaves the natural weight of the instrument itself, allowing a realistic simulation of instruments that may include surgical instruments. The embodiments illustrate a replaceable instrument mechanism, permitting the user to change handles to maintain a feel for a variety of scissors and scissors-like instruments.  
         [0051]     The hand controllers used in connection with computers allow for a user to move the handle mechanism of the instrument to activate, for example, a virtual forceps in a synchronous motion. The hand controllers preferably produce a feedback force on the instrument to be reflected to the user&#39;s hand when the virtual forceps comes into contact with an obstacle.  
         [0052]     Rongeur and Coupler. Referring to  FIGS. 1   a  to  1   c,  a replaceable instrument mechanism for haptic interface according to an embodiment is generally shown at  10 . The mechanism  10  generally consists of an instrument with a coupler  12 , the coupler  12  being the interface between the hand controller. A top view of a rongeur  14  is shown in  FIG. 1   a,  the rongeur  14  being one variety of the replaceable instrument. In  FIGS. 1   b  and  1   c,  a side view of the rongeur  14  of the replaceable instrument mechanism is generally shown, again in combination with a coupler  12 .  
         [0053]     The rongeur scissors  14  consists of a forward handle  22 , a back handle  24 , a sliding shaft  26 , and a fixed shaft  28 . The sliding shaft  26  is joined to the fixed shaft  28  by a prismatic joint, generally shown as  21  that binds the two shafts together, but allows them to slide relative to each other.  
         [0054]     The fixed shaft  28  has a first end  30  (i.e., connector end)designed to fasten to a matching fixed shaft coupler  66  (an end of the coupler body) in the coupler  12 . Its second end is fixedly attached to the back handle  24 .  
         [0055]     The sliding shaft  26  is a member relating the handle portion of the instrument  14  to the coupler  12  to transmit an actuation. The sliding shaft  26  has a first end  20  and a second end. Its first end  20  is designed to fasten to a matching sliding shaft coupler  16  in the coupler  12 . Its second end is attached by a revolute joint  34  to the forward handle  22 . A second revolute joint  32  attaches the forward handle  22  to the second end of the fixed shaft  28 , in such a way that pulling the forward handle  22  toward the back handle  24  results in the sliding shaft  26  sliding forward relative to the fixed shaft  28 . Accordingly, a scissors-like motion of the handles  22  and  24  will result in a reciprocating translational motion of the shaft  26  with respect to the shaft  28 .  
         [0056]     The coupler, generally shown at  12 , and in a more detailed view in  FIG. 4   a , has a coupler body secured to an output end of the hand controller and a force transmission mechanism to transmit forces between the hand controller and the instrument  14 . The coupler body has the fixed shaft coupler  66 , whereas the force transmission mechanism has the drive pulley  36 , and the slider  18 . The drive pulley  36  is connected by a revolute joint  60  at its centre to a platform  120  of the coupler body. The slider  18  is joined by a prismatic joint to the platform  120 . Likewise, the fixed shaft coupler  66  is fixedly attached to the platform  120 . The platform  120  is connected by a revolute roll joint  122  to a five-degree-of-freedom platform  121  representing the output end of the hand controller.  
         [0057]     Referring to  FIGS. 1   a  to  1   c  and  4   a  to  4   d , the slider  18  has an end  16  and a side extension  64 . The slider end  16  has a sliding shaft coupler, into which the end  20  of the sliding shaft  26  of the scissors can be inserted and locked into place. The extension  64  has a first end and a second end. Its first end is fixedly attached to the side of the slider  18 . Its second end is attached to the drive pulley  36  by a prismatic-revolute joint  62 , at a non-zero distance from the centre revolute joint  60  of the drive pulley  36 . The prismatic revolute joint  62  allows both rotation and sliding in a radial slot  37  made along a radius of the drive pulley  36 . Thus, when the pulley  36  turns, the slider  18  moves in its prismatic joint relative to the platform  120 . Likewise, when the slider  18  moves relative to the platform  120 , the pulley  36  turns. At the same time, the extension  64  moves relative to the drive pulley  36 , both rotating about joint  62  and moving in slot  61 , since the distance of joint  62  from the revolute joint  60  at the centre of the pulley  36  varies as the pulley  36  rotates about its centre.  
         [0058]     Those skilled in the art will recognize that the slot  61  would be unnecessary if the prismatic joint  21  on the removable scissors  14  enabled the slider  26  to separate from the fixed shaft  28 , moving in this case laterally as the drive pulley  36  rotates. Then joint  62  would be a purely revolute joint.  
         [0059]     Referring to  FIGS. 1   b  and  4   b , the fixed shaft coupler  66  has an end  67 , into which the end  30  of the fixed shaft  28  of the instrument can be inserted and locked into place. Thus when both the slider shaft  26  is locked into the slider  18  and the fixed shaft  28  is locked into the fixed coupler  66 , as in  FIG. 1   c,  the sliding motion of the sliding shaft  26  relative to the fixed shaft  28  is conveyed to the sliding motion of the slider  18  relative to the platform  120 . This in turn is conveyed to a rotation of the drive pulley  36  about the revolute joint  60 , as explained previously. Additionally, the sliding motion of the sliding shaft  26  relative to the fixed shaft  28  is activated by the motion of the forward handle  22  relative to the back handle  24 . Thus relative motion of these handles  22  and  24  is matched to a rotation of the drive pulley  36 .  
         [0060]     Referring to  FIG. 1   c,  rotation of the drive shaft pulley  36  is measured by an angular sensor  68 . In the exemplary embodiment, the angle sensors are contactless magneto-restrictive sensors that offer a minimum of rotational friction. By sensing near the handle, the hand controller has maximum stability when the computer program simulates contact with a virtual rigid body.  
         [0061]     Mosquito Forceps and Coupler. Other types of instruments may be connected by similar means to the coupler  12  of  FIGS. 4   a  and  4   b .  FIGS. 2   a  to  2   c  show a pair of mosquito forceps  46 . Like the rongeur  14 , the forceps  46  have a fixed shaft  28 , a sliding shaft  26 , a fixed handle  24  and a moveable handle  22 . Unlike the rongeur  14 , the forceps  46  are shown oriented horizontally in the top view of  FIG. 2   a , whereas the rongeur  14  is oriented vertically in this view ( FIG. 1   a ). Therefore, the sliding shaft  26  is on the same horizontal level as the fixed shaft  28 , rather than being vertically above it, as in the rongeur  14 . The forceps are therefore connected to the coupler  12  at right angles to the orientation of the rongeur  14 .  
         [0062]     The force transmission mechanism of the coupler  12  in  FIGS. 2   a  to  2   c  has the drive pulley  36 , the slider  18 , and the vertical fixed shaft coupler  66 , whereas the coupler body has a horizontal fixed shaft coupler  70 . The vertical fixed shaft coupler  66  accepts the fixed shaft  28  of vertically oriented instruments such as the rongeur  14  ( FIGS. 1   a  to  1   c ), while the horizontal fixed shaft coupler  70  is additionally provided to accept the fixed shaft  28  of horizontally oriented instruments such as the forceps  46 . The sliding shaft coupler  16  therefore accepts the sliding shaft  26  of either vertically or horizontally aligned instruments.  
         [0063]     The sliding shaft  26  is joined to the fixed shaft  28  by a prismatic joint, generally shown as a bracket  21  that binds the two shafts together, but allows them to slide relative to each other. The fixed shaft  28  has a first end  42  and a second end. Its first end  42  is designed to fasten to the matching fixed shaft coupler  70  in the coupler  12 . Its second end is fixedly attached to the fixed handle  24 . The sliding shaft  26  has a first end  40  and a second end. Its first end  40  is designed to fasten to a matching sliding shaft coupler  16  in the coupler  12  ( FIGS. 4   a  and  4   b ). Its second end is attached by a revolute joint  84  to one end of link  82 . The other end of the link  82  is attached by a revolute joint  86  to an extension  80  of the moveable handle  22 . A second revolute joint  32  attaches the moveable handle  22  to the second end of the fixed shaft  28 , in such a way that pulling the moveable handle  22  toward the fixed handle  24  results in the sliding shaft  26  sliding forward relative to the fixed shaft  28 .  
         [0064]     Referring to  FIGS. 2   a  to  2   c , in the case of the forceps shown at  46 , a latch  44  is optionally provided that will latch the handles at one position, so that the handles may be squeezed more tightly together but cannot be pushed apart without releasing the latch.  
         [0065]     Thumb Forceps and Coupler.  FIGS. 3   a  to  3   c  show a third exemplary instrument handle modified to allow attachment to the hand controller via the coupler  12  ( FIGS. 4   a  and  4   b ). This is a thumb forceps, shown generally at  50 . Like the rongeur  14  ( FIGS. 1   a  to  1   c ), this device  50  has a fixed shaft  28 , a sliding shaft  26 , a fixed handle stub  90  and a moveable handle stub  91 . The thumb forceps  50  is therefore connected to the coupler  12  in the same orientation as the rongeur  14 , with end  20  of sliding shaft  26  mating with coupler  16 , and end  30  of fixed shaft  28  mating with coupler  66  ( FIGS. 4   a  and  4   b ).  
         [0066]     The thumb forceps  50  consists of a top leaf  94  and a bottom leaf  95 . The two leaves are made of spring material, so that if they are bent they tend to return to their original shape in elastic fashion. The top leaf  94  has a first end and a second end. Likewise, the bottom leaf  95  has a first end and a second end. The first ends of both pieces are bent to the outside, so the first end of the top leaf  94  is bent upward, while the first end of the bottom leaf  95  is bent downward. The first end of the top leaf  94  is fixedly attached to the first end of the bottom leaf  95  at junction  96 , so that if a user squeezes the second ends together using the thumb and forefinger, they will offer some resistance and push back on the user&#39;s thumb and forefinger.  
         [0067]     The second end of the top leaf  94  is attached by a rotary joint  92  to the fixed handle stub  90 . The fixed handle stub  90  is fixedly attached to one end of the fixed shaft  28 . The second end of the bottom leaf  95  is attached by a rotary joint  93  to the moveable handle stub  91 .  
         [0068]     The sliding shaft  26  has the first end  40  and a second end. Its first end  40  is designed to fasten to a matching sliding shaft coupler  16  in the coupler  12 , as has been described previously ( FIGS. 4   a  and  4   b ). Its second end is attached by a revolute joint  84  to one end of link  82 . The other end of the link  82  is attached by a revolute joint  86  to an extension  80  of the moveable handle stub  91 . A second revolute joint  32  attaches the moveable handle stub  91  to the second end of the fixed shaft  28  via link  82 , in such a way that moving the moveable handle stub  91  toward the fixed handle stub  90  results in the sliding shaft  26  sliding forward relative to the fixed shaft  28 .  
         [0069]     Because of the attachment of the second end of the bottom and top leaves  94  and  95  to the fixed and moveable handle stubs  90  and  91 , respectively, then squeezing the second ends of the leaves  94  and  95  together will result in the moveable handle stub  91  being pulled toward the fixed handle stub  90 , and thus the slider shaft  26  moving relative to the fixed slider shaft  28 . When the thumb forceps  50  is docked into the coupler  12 , then squeezing the leaves  94  and  95  of the thumb forceps  50  will result in rotation of the drive wheel  36 . Likewise, rotation of the drive wheel  36  will result in the first ends of the leaves  94  and  95  of the thumb forceps  50  being moved apart or toward one another.  
         [0070]     Alternative Coupling Mechanisms. The coupling between the instruments, shown generally at  14 / 46 / 50  in  FIGS. 4   c  and  4   d , may be made in different ways.  FIG. 5   a  shows a central slider  76  inside a fixed outer casing  74 . The alternative coupling is symmetric in rotation, including the latch notch  72 . The fixed couplers  66  and  70  of the coupler  12  ( FIGS. 4   a  and  4   b ) would then be replaced by a single coupler arranged around the slider coupler  16  (which receives the slider  76 ) and designed to accept the fixed outer casing  74 .  
         [0071]      FIG. 5   b  shows the end  20  of the sliding shaft  26  and the end  30  of the fixed shaft  28 . The shape of the prismatic joint  21  between the shafts  26  and  28  ensures that they will not easily pry apart, while moving with respect to the joint  21 .  
         [0072]     In both  FIG. 5   a  and  FIG. 5   b , the notch  72  may be seen in the sliding shaft  26  and in the fixed shaft.  FIG. 5   c  shows an exemplary mechanism for latching the shaft ends  20  and  30  into place in the coupler  12 . Sliding shaft end  20  is inserted into sliding coupler  18 , and fixed shaft end  30  is inserted into fixed coupler  66 . As mentioned previously, both the sliding shaft end  20  and the fixed shaft end  30  have the notches  72 .  
         [0073]     Exemplary details are given for the sliding shaft coupling. A plunger  140  fits into the notch  72  of the sliding shaft end  20 . The plunger has a first end and a second end. The first end of the plunger passes through a hole  141  in the wall of the coupler  18 . It is shaped so that it will not fall completely through the hole  141 . When the sliding shaft end  20  is removed, the first end of the plunger  140  goes into the cavity left by the shaft. When the sliding shaft end  20  is reinserted, its slanted shape pushes the plunger  140  upward.  
         [0074]     A similar mechanism would hold for the inside slider  76 /outside shell  74  mechanism. In this case, the inside slider  76  would project further into the receiving coupler  12 , so that the latch could be set onto the inside slider  76  and outside slider  74  at different locations in the receiving coupler  12 . Alternately, an elongated hole could be provided in the outside case  74  by which a narrower plunger  140  could penetrate through the outside shell and latch onto the inside slider  76 , but still permit the slider to move linearly in its prismatic joint  21 .  
         [0075]     A flat plate  142 , seen from the side in  FIG. 5   c , is joined at its centre to a support  145  by a revolute joint  144 . The support  145  is fixedly attached to the sliding coupler  18 . The flat plate  142  has a first end and a second end. The first end is attached by a revolute joint  143  to the second end of the plunger  140 . The second end of the plate  142  is pushed upward by a spring  146 . Thus the flat plate  142  acts as a lever, pushing the plunger  140  into the hole  141  and securing the sliding shaft end  20  in place by pressing into the notch  72 .  
         [0076]     A similar device secures the fixed shaft end  30 . When a user wants to remove the removable scissors or like instrument, pressure is applied manually on the second ends of both plates  142 , thereby pulling the plungers out of their positions and releasing the shaft ends  20  and  30 .  
         [0077]     Means for Imparting Force to the Scissors. An example of means for imparting force to the removable scissors is shown in  FIG. 6   a . A rotary motor  101  has a capstan  100  attached to its output shaft. A tendon  102  connects the capstan  100  to the drive pulley  36 . The tendon  102  may pass over numerous idlers, although not shown. The capstan  100  has a radius r 1 , and the drive pulley  36  has a radius r 2 .  
         [0078]     If the motor exerts a torque τ 1 , then the tendon  102  has a tension F 1 =τ 1 /r 1 . The outside of the pulley  36  at radius r 2  is therefore pulled with a force F 1 , giving a torque τ 2 =r 2 ×F 1 . The torque r 2  is different from the original motor torque τ 1  by a factor r 2 /r 1 , since τ 2 =(r 2 /r 1 )×τ 1  by substitution of the first equation for F 1  into the second equation for τ 2 . In an exemplary case, the capstan  100  has a radius of 5 mm, and the drive pulley  36  has a radius of 20 mm, so the torque at the drive pulley  36  is four times the torque at the motor capstan  100 .  
         [0079]     The relation between the instrument (e.g., rongeur  14 , forceps  46 / 50 ) and the coupler  12  is for all these instruments, but will be described with respect to the rongeur  14  by way of an example to simplify the description. Referring to  FIGS. 6   a  and  6   b , the drive pulley  36  is connected to the moveable scissors handle  22  by the sliding shaft  26 . The fixed shaft  28  supports the fixed scissors handle  24 . The handles are joined at a revolute joint  32 , represented symbolically at the center of a solid disk  104  in  FIG. 6   a . In this representation, the moveable handle  22  is fixedly attached to the solid disk  104 , so that when the handle  22  moves, the disk  104  rotates about its center  32 . As described above, the first end of the sliding shaft  26  is connected to the drive pulley  36  by the revolute joint  60 , and the second end of the sliding shaft  26  is connected to the solid disk  104  by a revolute joint  34 . The distance between joint  34  and the disk centre joint  32  is a radius r 4 . The distance between the centre of the ring of the handle  22  and the disk centre joint  32  is a radius r 5 .  
         [0080]     The distance between the revolute joint  60  and the drive pulley  36  centre is a radius r 3 . The drive pulley  36  has a radius r 2 .  
         [0081]     The shafts  26  and  28 , the drive pulley  36  and the solid disk  104  form a four-bar mechanism. It is redrawn in  FIG. 6   b  to emphasize its four-bar design, and again in  FIG. 6   c  to mark the corresponding links with symbols. The four-bar is shown as a parallelogram with corners A, B, C and D, but the equations can be generalized to unequal link lengths. The radii r 2 , r 3 , r 4  and r 5  are marked on the diagram. Radius r 5  is shown going upward from joint  32  through joint  34 , as if the moveable handle  22  were on the other side of the device from its location in  FIG. 6   a . The two positions are equivalent, since the handle is fixedly attached to the assumed disk  104 , and the angle of attachment is of no consequence as far as the kinematics are concerned.  
         [0082]     The parallel four-bar has four links. Link a 1  is the frame link, and is assumed to be grounded, or fixedly attached to the earth. This corresponds to the fixed shaft  28  of the replaceable instrument. Link a 2  is the input link, with the length r 3  between joints  60  and  62  link a 4  is the output link, with length r 4  between joints  32  and  34 . Link a 3  is the connecting link, corresponding to the sliding shaft  26 . Angle ψ between link a 1  and link a 2  is the input angle. Angle α between link a 2  and link a 3  is the connecting angle. Angle β between link a 3  and link a 4  is the transmission angle. Angle φ between link a 4  and link a 1  is the output angle.  
         [0083]     The analysis is made by considering each moveable link as a free body.  FIG. 6   d  shows the forces acting on the input link a 2 , the connecting link a 3  and the output link a 4 . Force F is at right angles to link a 2 , since it originates in a tendon wrapped around the pulley and emerging tangentially to its circumference. To emphasize this, it is marked F ⊥  on the diagram. In addition, any force component parallel to the link would not cause a movement, since such movement is constrained by the revolute joint  60  at end A of the link a 2 . Similarly, the output force is taken as component F ⊥ ′ normal to the output link a 4 . The force F ⊥ ′ is shown acting on the link, as required by free body analysis.  
         [0084]     Connecting link a 3  has two forces acting on it, +Fc at the left end, and −Fc at the right end (taking the right direction to be positive, and the left direction to be negative). The forces are equal, because the link is in static equilibrium, moving neither to the right nor to the left. There are no force components normal to the link, because the link is in static equilibrium, and does not rotate clockwise or anti-clockwise. So the two forces, +Fc and −Fc, are equal and directed along the link.  
         [0085]     Consider the input link a 2  as a free body. Force F ⊥  pushes from the left at a distance r 2  from point A (the revolute joint between the input link a 2  and the frame link a 1 ). Force F c  from the link pushes from the right, and a reaction force F A  pushes from point A. The torque around point A must add to zero, so taking a sum of torque about A, 
 
 r   2    F   ⊥   −r   3    F   C  sin α=0 
 
 The term r 3  sin α takes into account the lever arm of the force Fc from the connecting rod. In the same way, the torque about point D on the output link a 4  must add up to zero. The external force F′ pushes back on the link, giving it a negative direction, while force Fc pushes from the left. 
 
− r   5    F   ⊥   ′+r   4    F   c  sin β=0 
 
 Combining the equations to eliminate the connecting link force Fc, we find  
         F   ⊥   ′     =           F   ⊥     ·   r     ⁢           ⁢     2   ·   r     ⁢           ⁢     4   ·   sin     ⁢           ⁢   β       r   ⁢           ⁢     3   ·   r     ⁢           ⁢     5   ·   sin     ⁢           ⁢   α           
 
 This may be related to the motor torque by inserting F ⊥ =τ/r 1  from an earlier analysis of the pulley system, to give,  
         F   ⊥   ′     =         τ1   ·   r     ⁢           ⁢     2   ·   r     ⁢           ⁢     4   ·   sin     ⁢           ⁢   β       r   ⁢           ⁢     1   ·   r     ⁢           ⁢     3   ·   r     ⁢           ⁢     5   ·   sin     ⁢           ⁢   α           
 
         [0086]     In the special case of a parallelogram r 2 =r 3 =r 4 =r 5 , and sin β=sin(π−α)=sin α, and F ⊥ ′=F ⊥ . The output force from a parallel four-bar is equal to the input force. For other configurations, the equation holds, and the links can be configured to set the desired force on the scissors. Thus a correct choice of radii r 2 , r 3 , r 4  and r 5  gives the optimal force and angular range of motion for simulation forces felt at the scissors handles while executing cutting or clamping procedures.  
         [0087]     One restriction that is placed on a four-bar assembly is that the connecting angle α and the transmission angle β should not be less than 45°. Below these angles, coupling of motion through the connecting link is inefficient, and the four-bar can lock up, with either the output or the input link unable to move. This restricts the linear motion of the slider to the distance between the ends of an arc described by a point on the pulley at a given radius r, while the pulley rotates between ψ=45° and ψ=45°+90°. This distance is given by 2 r sin 45°=r√2. If 5 mm of motion the sliding shaft is desired, then the radii r 3  and r 4  are approximately 3 mm or more.  
         [0088]     In our exemplary case, we have τ 1 =50 mN·m, r 1 =5 mm, r 2 =20 mm, r 3 =3 mm, r 4 =10 mm, r 5 =40 mm, and we assume sin β=sin α, giving F ⊥ ′=17 N. This is comparable to the standard male gripping strength of 50 N. The torque is quoted for a 10 W Maxon RE025 motor. Larger motors would give greater torque.  
         [0089]     The replaceable instrument mechanism generally shown at  10  is mounted on a moveable platform. As described previously, the coupler, generally shown at  12  in  FIGS. 1   a  to  4   b , has the drive pulley  36 , and the slider  18  as part of the force transmission mechanism, and the fixed shaft coupler  66  has part of the coupler body. The drive pulley  36  is connected by the revolute joint  60  at its centre to the platform  120 . The slider  18  is joined by a prismatic joint to the platform  120 . Likewise, the fixed shaft coupler  66  is fixedly attached to the platform  120 . The platform  120  is connected by the revolute roll joint  122  to the platform  121 , which is part of the haptic device.  
         [0090]      FIG. 7  shows an isometric view of a pulley assembly  123  allowing to route the tendon  102  from the platform  121  over the roll joint  122  to the drive pulley  36 .  FIG. 10   a  shows another view of the pulley assembly  123 . Seven pulleys are provided to make the transfer. The pulleys are configured so that turning the platform  120  in roll around axis  124  relative to the moveable platform  121  will not result in a change to the overall length of the tendon  102 . The pulley set consists of a large pulley  112  generally in the same plane as the drive pulley  36  when the roll joint  122  is in its home position, as shown in  FIG. 7 . An inner set of roll idlers  114  and  115 , an outer set of roll idlers  116  and  117 , and a set of crossover idlers  130  and  132  complete the pulley assembly  123 .  
         [0091]     The axes of rotation of idlers  114  and  115  are generally orthogonal to the axis of rotation  125  of the large pulley  112 , and to the roll axis  124 . Likewise, the axes of rotation of idlers  116  and  117  are generally orthogonal to the axis of rotation  126  of the scissors pulley  36 , and to the roll axis  124 .  
         [0092]     Reference will now generally be made to left-side parts and right-side parts, as shown in  FIG. 7 . It is pointed out that  FIG. 10   a  is a view from the underside of the assembly  123 , so right and left are reversed in this view when compared to  FIG. 7 .  
         [0093]     The tendon  102  has a left tendon half  106  and a right tendon half  108 . The left tendon half  106  is routed by a one-quarter turn around the large pulley  112 , over the left inner roll idler  114 , under the left crossover idler  130  ( FIG. 10   a ), back up over the right outer idler  117 , and around the right side of the drive pulley  36 . From the left side of the drive pulley  36 , the tendon right half  108  is routed over the left outer idler  116 , under the right crossover idler  132  ( FIG. 10   a ), over the right inner idler  115 , and by a three-quarter turn around the large pulley  112 . The idlers are arranged so that the tendon  102  arrives at each pulley in a direction tangential to the surface of the pulley, and generally in the plane of the pulley.  
         [0094]      FIG. 8  shows a view of the assembly along the roll axis  124  from the direction of the instrument. The platform  120  is shown in a position rolled about 45° counterclockwise from the home position. The tendon  102  can be seen routed around idlers  112 ,  114 ,  115 ,  116  and  117 , and the drive pulley  36 . Idlers  116  and  117 , and the drive pulley  36  are shown rotated in roll about axis  124 , since they are attached to the platform  120 . Idlers  112 ,  114  and  115  are shown in the same position as in  FIG. 7 , since they are attached to the moveable platform  121 .  
         [0095]      FIG. 9   a  shows the tendon path in stylized form. The large idler  112  is stationary in this frame, and the drive pulley  36 , being mounted on the platform  120 , is shown rotated in roll.  
         [0096]      FIG. 9   b  shows the overall tendon path as a continuous loop from the right side of the large pulley  112 , around the roll joint  122 , around the drive pulley  36 , around the roll joint  122  again, and back to the left side of the large pulley  112 . The drive pulley  36  has an axis of rotation  126  that is normal to the roll axis  124 . If the tendon ends at the large pulley  112  are held, then rotating the platform  120  about the roll axis  124  will result in rotation of the drive pulley  36  about its axis  126 .  
         [0097]     On the other hand, holding the drive pulley  36  from rotating about its axis  126  will result in movement of the tendon  102  about the large pulley  112 . Therefore the entire path of tendon  102  over its idler pulleys  112 ,  114 ,  115 ,  116 ,  117 ,  130 ,  132  and other idlers not shown along the tendon path between the capstan  100  and the large pulley  112 , must have low friction. In addition, the motor  101  and all pulleys and idlers must have low inertia. To the extent that these conditions are not met, the instrument will tend to open or close as the platform  120  is rotated about the roll axis  122 .  
         [0098]     In the exemplary system, care has been taken to ensure that all idlers have low friction, and that the motor  101  and all pulleys and idlers have low inertia. For example, friction in the roll joint without the instrument (but with tendon  102 , together with the motor  101  and the guide pulleys along the path between the roll joint and the motor) is under 5 mN·m of torque.  
         [0099]     The crossover pulleys  130  and  132  are shown in  FIG. 10   b . In this exemplary embodiment, left crossover pulley  130  is an open-frame pulley, while the right crossover pulley  132  is a closed-frame pulley. In the assembly, the right crossover pulley  132  is mounted inside the open-frame left crossover pulley  130 . The left tendon half  106  ( FIG. 10   a ) is routed under the left crossover idler  130 , while the right tendon half  108  ( FIG. 10   a ) is routed under the right crossover idler  132  and through the centre of the left crossover idler  130 . Thus, the tendon halves  106  and  108  can cross over under the roll joint  122  without touching each other.  
         [0100]     It is possible to route the tendons so that no crossover is necessary.  FIG. 10   c  shows an alternative configuration of the pulley assembly  123  from a similar viewpoint as for  FIG. 10   a,  while  FIG. 10   d  shows the alternative tendon routing from a similar viewpoint as for  FIG. 7 . Thus  FIG. 10   a  and  FIG. 10   c  show alternative configurations of the pulley assembly  123 .  
         [0101]     In the alternative configuration, the crossover pulleys  130  and  132  are replaced by a single roll idler  180  with an axis of rotation coincident with the roll axis  124 . As in  FIG. 10   a,  the axes of rotation of idlers  114  and  115  are generally orthogonal to the axis of rotation of the large pulley  112 , and to the roll axis  124 . Unlike  FIG. 10   a,  however, the axes of rotation of idlers  116  and  117  are generally parallel to the axis of rotation  126  of the drive pulley  36 , and orthogonal to the roll axis  124 . This does not change the consideration of the independence of the scissors angle opening of the roll angle, as discussed in conjunction with  FIGS. 9   a  and  9   b.    
         [0102]     The left half  106  of tendon  102  is routed by a one-quarter turn around the large pulley  112 , over the left inner roll idler  114 , then under the single roll idler  180 , across its top surface and onto left outer idler  116 , thence around the left side of the drive pulley  36 . From the right side of the drive pulley  36 , the tendon half  108  is routed around the right outer idler  117 , across and under the single roll idler  180 , then up over the right inner idler  115 , and by a three-quarter turn around the large idler  112 .  
         [0103]     From the point of view of distance along the roll axis  124 , the left outer idler  116  is placed closer to the large idler  112  than the right outer idler  117 . Similarly, the left inner idler  114  is placed closer to the large idler  112  than the right inner idler  115 . The tendon half  106  is fed from the left inner idler  114  around the roll idler  180  to the left outer idler  116 . Similarly, the tendon half  108  is fed from the right inner idler  115  around the roll idler  180  to the right outer idler  117 . Because the left idlers  114  and  116  are closer to the large idler  112  than the right idlers  115  and  117 , the wrapping of the left tendon  106  around the roll idler  180  is closer to the large idler  112  than the wrapping of the right tendon  108 . Thus, the two wrappings are separated. In this way, the tendon paths wind in a helical fashion about the roll idler  180 , but do not touch each other.  
         [0104]     Moreover, to ensure free rotation of the platform  120  about roll joint  122 , the left outer idler  116 , although it is closer to the large idler  112  than the right outer idler  117 , it must be farther from the large idler  112  than the right inner idler  115 . On rotation clockwise about roll axis  124 , tendon  106  eventually touches tendon  108 , preventing further rotation. The same is true of counterclockwise rotation. The interference between strings is estimated to occur at 125° clockwise rotation, or 235° counterclockwise rotation. If the idlers were not separated in the manner just described, idlers  115  and  116  themselves would touch upon rotation of the platform  120  about the roll axis  124 . This would give a more restricted range of roll angles, since idlers  115  and  116  subtend a larger angle in roll than the narrow tendons.  
         [0105]     We note that the tendon routing configuration  123  shown in  FIG. 10   a  also has a restricted range of roll, since tendon  108  passing through the inner idler  132  would touch the inside surface of the outer idler  130  if platform  120  is rotated counterclockwise by less than 90°. If platform  120  is rotated counterclockwise by 180°, then tendon  106  would become disengaged from its idlers. For this reason, the alternative tendon routing shown in  FIGS. 10   c  and  10   d  offers a wider range of rotation of the platform  120  about the roll axis  124 .  
         [0106]     It would be clear to those conversant with the art that the description of the mechanism in terms of “right” and “left” do not preclude the possibility of interchanging the left and right sides of the assembly.  
         [0107]     The mechanism  10  described above presents several advantages. Various types of instruments can be attached to the coupler and removed, so that the operator feels the shape and texture of each instrument in ways reminiscent of the corresponding instrument as it is used in surgery.  
         [0108]     Because in a preferred embodiment the removable instrument is mounted on the sixth joint of a six-degree-of-freedom hand controller, a high degree of movement is permitted In this way, many different surgical procedures can be duplicated, with the instrument oriented in the hand of the surgeon in ways appropriate for each surgical procedure.  
         [0109]     In the exemplary case of the hand controller, the device is counterbalanced, so that the only weight that the surgeon feels is the weight of the instrument itself. Because of the counterweights, the five-degree-of-freedom moveable platform  121  is balanced in a gravitational field. Accordingly, the coupler  12  maintains any position without assistance when no motion is transmitted by the instrument. This reduces the load on the motors, which can put their energy into positioning rather than holding a position.  
         [0110]     Another embodiment of the moveable platform  121  is shown at  168  in  FIG. 14 . The instrument of the replaceable instruments mechanism  10  is shown in place in an apparatus  168 , at the end of a balanced arm. The replaceable instrument mechanism  10  may itself be counterbalanced partially by the addition of counterweights  162  to the distal stage  164 , as shown in  FIG. 13 . The counterweights  162  will serve to balance the weight of the replaceable mechanism  10  about the pitch axis  166  of the distal stage  164 . The force transmission mechanism in this example has the pulley  36  mounted directly to the pivot of the scissors-like handle.  
         [0111]     A further embodiment of the removeable instrument mechanism  10  is illustrated in  FIG. 15 . In this embodiment, idlers  114  and  115  are shown supported by brackets  186  fixedly attached to the distal stage  164 . Similarly, idlers  116  and  117  are supported by brackets  188  fixedly attached to the platform  120 . The large idler  112  may be seen in its location in the distal stage  164 . Likewise, the drive pulley  36  is seen in its location in the platform  120 . The fixed scissors handle  24  is fixedly attached to the platform  120 , while the moveable scissors handle  22  is fixedly attached to the drive pulley  36 . In this embodiment, the scissors handles  22  and  24  can each be detached and reattached from the assembly by couplings  182  and  184 , respectively These are prismatic joints that may be, for example, in the form of the dovetail joint shown in  FIG. 5   b.    
         [0112]      FIG. 16  shows the handle coupling in more detail. The handle  24  is fixedly attached to a member  190  by means of a dovetail joint  184 . The member  190  has its body fixedly attached to the platform  120 , and is formed into a receptacle half  192  of the dovetail joint  184 .  
         [0113]     The handle  24  has a first end and a second end. Its first end has a finger ring  198 , into which a user may insert a finger. Its second end is formed into an insertion half  194  of the dovetail joint  184  that is complementary to the dovetail joint  192  in member  190 , so that the ends  192  and  194  may be slid together by prismatic motion, so that the two form the complete joint  184 . The joint  184  is further secured by a screw  196 . A hole  193  with threads that match the screw  196  is made in the middle of end  192 . A hole  195  through which the screw  196  passes is made in the middle of joint  194 . The screw  196  is then inserted into the hole  195 , and screwed into the threaded hole  193 , thereby holding the dovetail joint  184  securely closed. One skilled in the art would recognize that the joint  184  could equally well have been secured in the opposite direction, with threads in hole  195  and hole  193  widened to allow the screw  196  to pass.  
         [0114]     A rotational sensor  68  mounted in the coupler  12  permits the opening angle of the scissors to be determined from the rotational angle of the drive pulley  36 . Since this angle is determined close to the hand of the user, there is minimal time lag in sensing the angle, as there would be if the angle sensor  68  were mounted near the drive capstan  100 . This permits the simulation of contact with virtual rigid bodies.  
         [0115]     Preloaded bearings in each joint, including the roll joint, the idler rotational joints, and the joints of the removable instruments, allow response with reduced backlash and a minimum of friction.  
         [0116]     Alternative means for attachment of the instrument to the device are possible.  FIG. 11   a  shows an exemplary embodiment with the handles  24  and  22  separately attached, with the fixed handle  24  plugged into the platform  120  and the moveable handle  22  attached directly to the drive pulley  36  by means of a temporary coupler, rather than attached by means of sliding member  18   
         [0117]     In a second alternative embodiment shown in  FIG. 11   b,  the fixed handle  24  alone is connected to the platform  120  by way of, for example, the vertical fixed shaft coupler  66  ( FIGS. 4   a  and  4   b ) or the horizontal fixed shaft coupler  70 , or even by way of the moveable shaft coupler  16 . In this way, non-jointed handles of different types can be attached to the hand controller by way of the same coupling mechanism that is used to temporarily attach revolute jointed handle mechanisms.  
         [0118]     Again alternatively, as shown in  FIG. 11   c,  a handle that has only a prismatic joint, such as a plunger mechanism  170  operating in a casing  172 , can be operated with sliding shaft  26  fixedly attached to the plunger  170 , and fixed shaft  28  fixedly attached to the casing  172 , and the assembly attached to the receiver on the platform  120  by way of the moveable shaft coupler  16  and one of the fixed shaft couplers  66  or  70 , and both sensed in linear position and activated by way of the sensor  68  and the drive shaft  18 . This could most conveniently be coupled using the inside slider/outside shell mechanism pictured at  74  and  76  in  FIG. 5   a.    
         [0119]     By making use of a magneto-resistance effect angle sensor connected to a 16-bit analog to digital converter, the mechanism can deliver an angular resolution of some  7  seconds of arc over a 120 degree range of motion of the drive pulley  36 , without the weight, size and expense penalties incurred by optical encoders. This angular range is well suited for the opening angle of a typical hand-held surgical instrument, where ranges of motion of only 20 degrees are common. Alternatively, a precise linear sensor sensing the movement of the sliding drive shaft  18  relative to the fixed coupler  66  or  70  could be used in the same way with a 16-bit analogue to digital converter.  
         [0120]      FIG. 12  illustrates a processing system for coupling the hand controller device to a computer. The angle sensor signal  152  is conditioned to provide a clean signal of the instrument motion.  
         [0121]     In operation, the user chooses the instrument from a set of choices, illustrated by  14 ,  46  and  50 , and plugs the instrument into the haptic device through the coupler  12 . The user then grasps the handles  22  and  24  of the instrument, and moves them toward or away from each other, while at the same time positioning the instrument with three translational degrees of freedom and three rotational degrees of freedom, according to a preferred embodiment. Movements of the moveable handle  22  relative to the fixed handle  24  are measured by the rotational sensor  68  attached to the drive pulley  36  on the platform  120 .  
         [0122]     In the processing system of  FIG. 12 , the voltages representing angle sensor signals  152  of the sensor  68 , together with other angle sensors on the hand controller, are passed to a computer  150  through a signal conditioner  154  and an analogue to digital converter  156 . In the signal conditioner  154 , the signals  152  are amplified to the full voltage range of the A/D converter  156  and filtered with a 100 Hz low pass filter to remove noise.  
         [0123]     In a preferred embodiment, a program in the computer  150  accepts the angle measurements  152  and moves a virtual instrument synchronously with the motion of the mechanism  10 . If desired, the computer program computes the required force to be reflected to the users hand, when, for example, the virtual probe touches a virtual surface. The program uses kinematics algorithms to convert this required force to a required motor torque, then to a voltage known to produce that torque which is fed to a digital to analogue converter  158 . The output of the D/A converter  158  is fed to a voltage to current converter  160  connected to the motor  101 . The current applied to the motor  101  then produces the required torque.  
         [0124]     In a preferred embodiment, the motor  101  is a 10-Watt DC motor from Maxon, Model 118746, with precious-metal brushes, although the 20-Watt Model 118752 with carbon brushes may also be used, with its increased power but slightly greater commutation noise. The D/A converter  158  is a PCI-6208 converter from Adlink, while the voltage to current converter  160  for each motor is a model PA12A converter from Apex. The rotational sensors  68  is a magneto-resistance sensor from Midori America Corporation, Model CP-2UPX. The A/D converter  156  for up to eight sensors is a KPCI-3107 converter from Keithley.  
         [0125]     The embodiments of the invention described above are intended to be exemplary. Those skilled in the art will therefore appreciate that the foregoing description is illustrative only, and that various alternatives and modifications can be devised without departing from the spirit of the present invention. Accordingly, the present is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.