Abstract:
The present invention is a wrist mechanism and a method for making robotic devices in which the transmission of motion, force and/or torque around a revolute joint can be accomplished without coupling. This construction allows mounting the actuators on the base or lower elements of a mechanism, so that only linkage elements move the end-effector. Thus reducing inertia of the moving elements and increasing performance of the device. The decoupled motion of the end-effector or links is achieved by routing their transmission cables around idler pulleys placed parallel to the joint rotation axis on an optimal position such any stretch on the transmission cable is minimized. In particular, this construction can be use for robotic surgical tools that have two independently driven jaws, decoupled and orthogonal from its articulating wrist.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority from commonly owned U.S. Provisional Patent Application No. 60/376,964 titled Articulated Wrist And Tool With Decoupled Motion Transmission, presently pending, which is hereby incorporated by reference in its entirety. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    Minimally invasive surgical robotic systems are being developed to increase surgeon&#39;s dexterity when working within an internal surgical location. Such minimally invasive procedures are conducted by inserting surgical instruments through small incisions on the skin of the patient. The use of dexterous surgical instruments to manipulate tissues and suture would facilitate the performance of an endoscopic surgical procedure. A manually or robotically actuated surgical device that can articulate as well as actuate reliably would minimize patient risk and operating time.  
           [0003]    Current minimally invasive surgical tools have many drawbacks. Most endoscopic tools have rigid shafts and are constrained to approach the workspace from the direction of a small incision. In addition, many robotic devices often have their motors or actuators mounted on the adjacent links. When these motors are mounted on moving links, their weight often reduces the performance of the device. Sometimes the volume that motors occupy near the end-effector can interfere with the access to a workspace. Furthermore, the construction and length of many endoscopic instruments reduces the surgeon&#39;s ability to feel the force exerted by tissues and organs on the end-effector. The limited dexterity and sensitivity provided by endoscopic tools is a major obstacle to the improvement and expansion of minimally invasive surgery.  
           [0004]    Surgical tools that increase the surgeons&#39; dexterity are needed to improve minimally invasive surgery procedures. It is desirable to provide surgical instruments having a wrist with two or tree degrees-of-freedom. It is further desirable to provide a wrist mechanism that has low friction in order to provide the surgeon with sensitivity feedback of the contact forces exerted by the surgical tools. Furthermore, it is desirable to minimize the stretch in a cable operatively coupling a portion of the tool that is inserted through the incision with power and control mechanisms that remain outside the incision.  
         SUMMARY OF INVENTION  
         [0005]    The present invention provides a wrist in which the transmission of motion, force and/or torque around a revolute joint can be accomplished without coupling. The motion of its adjacent link does not affect the relative motion of the decoupled element. This construction allows mounting the actuators on other elements of a mechanism, so that only linkage elements move the end-effector. Thus reducing inertia of the moving elements and increasing performance of the device. The decoupled motion of the end-effector or links is achieved by routing their transmission cables around idler pulleys placed parallel to the joint rotation axis on an optimal position such that any stretch on the transmission cable is minimized. This construction may be use for robotic surgical tools that have two independently driven jaws, decoupled and orthogonal from its articulating wrist. This surgical device may also be used in grasping, cutting, suturing or alike operations.  
           [0006]    The wrist includes a base that may be mounted to an arm or other structure that locates the wrist away from a drive mechanism. The wrist also includes a link having an end-effector portion that may be pivotally attached to an end-effector to allow the end-effector to pivot about a link axis, and a base portion that is pivotally attached to the base and operable to pivot about a wrist axis of the base. The wrist also includes an idler having a diameter and rotatably attached to the base such that the idler is operable to rotate about an idler axis that is offset from the wrist axis. A cable operatively couples the end-effector to a drive mechanism to allow a user to operate the end-effector. The offset of the idler axis is calculated to minimize stretch in the cable as the link pivots about the wrist axis. Thus, the cable is less likely to develop a permanent stretch over multiple uses, which can cause backlash and adversely affect the sensitivity or “feel” a user often requires while operating the end-effector. Furthermore, when a user pivots the end-effector relative to the link and pivots the end-effector relative to the wrist, one of the motions does not affect the other motion, i.e., the motions are independent of each other.  
           [0007]    The offset that provides the minimal stretch in the cable over a desired range of motion, such as pivoting the link 180°, or 120° depends on the desired range of motion, the idler diameter and the ratio of the idler diameter to the distance between the link axis and the wrist axis. For example, the offset that provides the minimal stretch in the cable over a 180° range of motion with an idler pulley diameter of 5 millimeters (mm) and a 12.5 mm distance between the link axis and the wrist axis is 1.48 mm. For the same range of motion but other idler diameters and other distances between the link axis and the wrist axis the offset can be calculated from the following ratios  
           [0008]    D w /D j  C s /D i    
           [0009]    Infinite 0.2854  
           [0010]    1000 0.2859  
           [0011]    50 0.2863  
           [0012]    10 0.2903  
           [0013]    5 0.2959  
           [0014]    3 .3049  
           [0015]    Where D w  is twice the distance between the link axis and the wrist axis (diameter); D i  is the diameter of the idler and Cs is the offset distance. So for a given ratio of the distance between the link and wrist axes to the diameter of the idler pulley, the offset can be calculated by multiplying the diameter of the idler by the number provided in the chart.  
           [0016]    In one embodiment of the wrist, the idler axis is parallel or substantially parallel to the wrist axis and the link axis is perpendicular or substantially perpendicular to the wrist axis. In addition, the wrist may include at least two idlers. For example, the wrist may include eight idlers in four groups of two, with the two idlers in each group concentric with each other and each idler is operable to pivot about one of four respective idler axes that are each offset from the wrist axis. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0017]    [0017]FIG. 1 is a perspective view of a wrist mechanism according to an embodiment of the invention;  
         [0018]    [0018]FIG. 2 is a front view of the wrist mechanism shown in FIG. 1 according to an embodiment of the invention;  
         [0019]    [0019]FIG. 3 is a side view of the wrist mechanism shown in FIG. 1 according to an embodiment of the invention;  
         [0020]    [0020]FIG. 4 is an exploded perspective view of the wrist mechanism of FIGS.  1 - 3  according to an embodiment of the invention;  
         [0021]    [0021]FIG. 5 is a schematic top view of a wrist mechanism with an end-effector pivoted 90° about a wrist axis and illustrates a path of a cable used to pivot the end-effector, according to an embodiment of the invention;  
         [0022]    [0022]FIG. 6 is a schematic front view of a wrist mechanism without idler pulleys and illustrates the lack of cable stretch as the end-effector pivots about a wrist axis;  
         [0023]    [0023]FIG. 7 is a schematic front view of a wrist mechanism with idler pulleys located to minimize cable stretch as the end-effector pivots about a wrist axis, according to an embodiment of the invention;  
         [0024]    [0024]FIG. 8 is a schematic front view of a wrist mechanism with idler pulleys located in another position to minimize cable stretch as the end-effector pivots about a wrist axis, according to an embodiment of the invention;  
         [0025]    [0025]FIG. 9 is a perspective view of a tool incorporating the wrist mechanism of FIGS.  1 - 3  according to an embodiment of the invention;  
         [0026]    [0026]FIG. 10 is a cross-sectional view of the tool in FIG. 9 showing cables disposed within a tube, according to an embodiment of the invention.  
         [0027]    [0027]FIG. 11 is a detail view of the driver mechanism in FIG. 9;  
         [0028]    [0028]FIG. 12 is a perspective view of a tool incorporating the wrist mechanism of FIGS.  1 - 3  according to another embodiment of the invention;  
         [0029]    [0029]FIG. 13 is a detail view of the driver mechanism in FIG. 12;  
         [0030]    [0030]FIG. 14 is an illustration of an alternative grasper configuration with the jaws and wrist transmission cable routed through an intermediate joint.  
         [0031]    [0031]FIG. 15 is a detail view of a joint of the mechanism in FIG. 14. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0032]    [0032]FIGS. 1 through 4 illustrate a wrist and tool  10 . The end-effector consists of two jaws  40 . 1  and  40 . 2 , each jaw is rigidly coupled to their respective drive pulleys  42 . 1  and  42 . 2 . Both jaws rotate around α on a common shoulder screw  41  that is mounted to the jaws base  30 . An orthogonal wrist rotation β occurs when the jaws base rotates with its pulley  33  around pin  31 .  
         [0033]    [0033]FIG. 4 shows an exploded view of the wrist mechanism  10 . The jaw  40 . 1  has a ring  40 . 1  a machined around its rotation axis. Each drive pulley  42 . 1  and  42 . 2  mounts on its jaw ring with its respective cable attached. Each jaw  40 . 1  and  40 . 2 , pulley  42 . 1  and  42 . 2 , and the cable  50 . 1  and  50 . 2  (not shown in FIG. 4) is rigidly coupled together with its setscrew  43 .  
         [0034]    Each jaw  40 . 1  and  40 . 2  can rotate independently around the shoulder screw  41 , α axis. When the two jaws rotate on the same direction, one wrist articulation motion is accomplished. And when the jaws rotate on opposite direction, the jaws will open or closed. The jaws motions are driven through their cables  50 . 1  and  50 . 2 . These cable are routed through guiding idler pulleys  22  from their distal drive mechanism.  
         [0035]    The jaw base  30  mounts to its pulley  33  with two pins  32 . The setscrew  34  couples rigidly the jaw base  30  and pulley  33  with its cable  50 . 3  (not shown in FIG. 4). The jaw base subassembly is then mounted on the center slot of the wrist base  20  with pin  31 . This subassembly pivots on the jaw base center hole  30   a , β axis. The pulley center hole  33   a  is larger than the base center hole  30   a  to avoid binding when the setscrew  34  presses down the cable and pulley.  
         [0036]    There are four pairs of idler pulleys  22 , two pairs per jaw; they mount between the two lateral slots of the wrist base  20  with pins  21 . These idlers guide the jaw cables  50 . 1  and  50 . 2  from the jaw pulleys  42 . 1  and  42 . 2  to the drive mechanism. Note that there are four pins  21  for the idlers, two pins per side. Pins  21  cannot go through the whole wrist base  20  because they would interfere with the jaw base  30  and it pulley  33 .  
         [0037]    [0037]FIGS. 5 through 8 illustrate schematic wrist drawings to determine an optimal geometry such that the length of the transmission cable  50 . 1  remains approximately constant for β between −90° and 90°. FIG. 5 shows a schematic top view of the wrist, the pulley cable  50 . 1  is shown with thick line with wrist articulation β rotated 90° with respect to its tube axis. The cable detaches from the jaw pulley  42 . 1  at A and A′. Also the cable detached from the idler pulleys  22 . 1  and  22 . 5  at O and O′.  
         [0038]    [0038]FIG. 6 is a schematic front view of the wrist, it shows the cable routing at three different angles β: AO when β is 90°; BO when β is 0°; and CO when β is −45°. If cable  50 . 1  turns on a sharp corner, no idlers  22 , then the cable length would be constant at any angle β:  
         AO=BO=CO  
         [0039]    But sharp or small round edges would wear the transmission element (cable, belt, etc.) fast and increase friction. Also transmission elements required a minimum turning radius to increase their useful life. Therefore idler pulleys are required to guide the cable.  
         [0040]    [0040]FIG. 7 is a schematic front view of the wrist as FIG. 6, but there are idler pulleys  22 . 1  and  22 . 5  located such that their center is 0.50 Ø i  below the center of β axis. For this case:  
           AO= 0.5(Ø w +Ø i (π−1))  
           BO= 0.5(Ø w +Ø i )  
         [0041]    The difference is:  
           BO−AO=Ø   i (1−π/4)=0.2146 Ø i    
         [0042]    [0042]FIG. 8 is a schematic front view of the wrist as FIG. 7, but the idler pulleys  22  have been shifted upward 0.215Ø i , such that their centers are 0.285Ø i  below the center of β axis. When Ø i &lt;&lt;Ø w  and C s =0.285Ø i , we get:  
         AO≈BO≈CO  
         [0043]    The cable length remains approximately constant for any angle of β between −90° and +90°. The optimal offset C s  is sensitive to the Ø i /Ø w  ratio. The optimal C s  to minimize cable stretch for a very large Ø i /Ø w  ratio is 0.2854Ø i . For a Ø i /Ø w =100 ratio, the optimal offset C s =0.2859Ø i ; for Ø i /Ø w =50 the optimal C s 6=0.2863Ø i ; for Ø i /Ø w =10 the optimal C s =0.2903Ø i ; for Ø i /Ø w −5 the optimal C s =0.2959Ø i ; and for Ø i /Ø w =3 optimal C s =0.3049Ø i .  
         [0044]    In particular, the embodiment shown in FIG. 1 has the following dimensions (mm): Ø w =25.0; Ø i =5.0; C s =1.48; and the diameter of the jaw pulley is 8 mm. Consequently, Ø w /Ø i =5.0 and C s =0.296Ø i . The maximum cable length, AO=13.98, occurs when β=0°, −90° and 90°. The minimum cable length is 13.69 when β=61°. So the cable stretches 0.29 mm (2.1%) between its minimum and maximum lengths.  
         [0045]    But if the wrist is mounted in embodiments as the ones shown in FIGS. 9 and 10, the drive system is locate distal from the wrist and they are connected through an elongated arm. The total lengths of the cable loops on the embodiment shown on FIGS. 9 and 10 are about 800 mm each. So the 0.29 mm stretch that occurs equally on each side of the jaws pulleys represent only 0.073% of the total cable length, less than one thousand stretch. For most mechanical systems a variation of less than one thousand of the length of their transmission element is negligible. Also, the stretch is equal on each end of the jaws pulleys  42 , consequently the motion of the jaws  40  are decoupled from the wrist motion.  
         [0046]    In general, this invention presents a method for guiding transmission cables around joints while minimizing the stretch in cable. Any mechanism with its particular Ø i /Ø w  ratio, and range of motion β, can be designed using this method for minimizing cable stretch. For example: if the wrist shown in FIG. 1 would have a reduced range of motion, rotate an angle β between −60° and +60°, then cable stretch can be minimized by adjusting C s  as follows; start with the optimal C s =1.48 mm (optimal for ±90° range of motion); shift the idler pulleys  22  upwards a distance equal to the maximum stretch for this range or motion, or 0.29 mm; the new C s =1.19 mm, and the new maximum stretch has been reduce to 0.164 mm; repeat shifting position upward until a minimum is found; the new optimal idlers  22  position is at C s =0.926 mm and the stretch has been reduced to 0.078 mm, only about one quarter the stretch the occurs for a ±90° range of motion.  
         [0047]    [0047]FIG. 9 shows the wrist  10  connected to a driver mechanism, located away from the wrist mechanism, through the tube  80 . The tube  80  mounts to the wrist base with screw  85 , and to the idlers base  81  with setscrew  86 . By locating the driver mechanism away from the wrist mechanism, the cable stretch that occurs at different β angles is spread over the whole length of the cable. Consequently, any small change of cable length that occurs on the wrist is negligible over the whole cable length.  
         [0048]    [0048]FIG. 10 illustrates a section view of the bottom of the wrist; it shows the arrangement of the transmission cables  50 . The cables run through the inside of the tube, which shields the cable.  
         [0049]    [0049]FIG. 11 illustrates a detail view of the driver mechanism. This driver mechanism comprises of pulley hangers  60 . 1 ,  60 . 2  and  70  that slide on a dovetail groove located on the idlers guide  82 . Pulling the hangers with screws mounted on the rear plate  83  sets the cable tension. Crimped loop sleeves  51  are used for joining the two ends of an open cable. To rotate the wrist or jaws around their axes, one pulls their respective cable relative to the tube. In other embodiment the cables can be pulled by actuators or motors powered by electricity, compressed air or hydraulic fluid.  
         [0050]    [0050]FIG. 12 shows the wrist  10  connected to another distal driver mechanism through the tube  80 . The tube  80  mounts to the driver&#39;s base  91  with setscrew  86 . FIG. 13 illustrates a detail view of the drive mechanism; it shows how the jaw cables  50 . 1  and  50 . 2  mount around their driver pulleys  95 . 1  and  95 . 2 . These cables loop a few times around their pulleys  95  (capstan) such that friction allows the motors  97  to properly drive the cables within a limited range of motion, typically wrist and jaws motions between −90° and 90°. The motor mounts  94  slides on dovetail grooves located on the driver guide  92 . Pulling the motor mounts with screws mounted on the rear plate  93  sets the cable tension.  
         [0051]    [0051]FIG. 14 shows an alternative five axes embodiment in which the motors, for driving the wrist  10  links and the arm  102 , are located on a motor housing  101 . The wrist transmission cables are routed through the hollow arms  102  and  103 , are around the joint  100  idler pulleys. FIG. 15 illustrates a detail view of the joint  100 . The wrist cables  50 . 1 ,  50 . 2  and  50 . 3  are guided around joint  100  by their respective idlers, which are location at an optimal position to minimize cable stretch.