Abstract:
A rotatable wrist connecting a gripper tool to the distal end of a continuum robot shaft. The rotatable wrist includes a wrist hub that is non-rotatably connected to the distal end of the shaft. A wrist capstan is rotatably connected to the wrist hub and non-rotatably connected to the gripper. A flexible wire loop extends through the wrist hub and partially contacts the wrist capstan. Linear movement of the flexible wire loop through the shaft of the continuum robot causes rotation of the wrist capstan due to friction between the flexible wire loop and the wrist capstan. The wrist also supports selective detachability and control of roll, pitch and roll, pitch yaw and roll according to different embodiments.

Description:
RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application No. 61/636,001, filed on Apr. 20, 2012 and titled “DEXTEROUS WRISTS FOR SURGICAL INTERVENTION,” the entire contents of which are incorporated herein by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with government support under grant 7R21EB007779-04 awarded by National Institutes of Health. The government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to dexterous gripping devices and surgical wrists. In particular, the invention relates to gripper assemblies with integrated axial rotation capabilities, selective detachability, and roll-yaw-pitch wrist action for use with robotic systems during minimally invasive surgical procedures. 
     SUMMARY 
     In one embodiment, the invention provides a continuum robot including a plurality of controllably bending continuum robot segments, a gripper, and a wrist. The continuum robot has tubular shafts (backbones) that actuate its segments to cause it to bend and also provide an actuation pathway for the gripper and the wrist. The gripper is selectively connectable to the distal end of the continuum robot (hereafter referred to as the end disk). A rotatable wrist connects the gripper to the end disk. The rotatable wrist includes a hub that is selectively connectable to the end disk. A wrist capstan is rotatably connected to the wrist hub and non-rotatably connected to the gripper base. A flexible wire rope enters the wrist hub through one tubular shaft (backbone), wraps around the wrist capstan and then returns though a second tubular shaft (backbone) of the continuum robot. This wire rope makes a closed loop distally connected to the wrist capstan and proximally connected to an actuation unit with a linear actuator and a tensioning idler pulley. Linear movement of the actuator causes linear movement of the flexible wire loop through the shafts of the continuum robot and thus causes rotation of the wrist capstan due to friction between the flexible wire loop and the wrist capstan. 
     In some embodiments, the wrist capstan includes a grooved surface and the flexible wire loop includes a spherical feature that meshes inside a matching grooved surface in the wrist capstan. In some such embodiments, the wire does not make a full turn around the capstan and torque transmission to the capstan relies on the positive lock between the spherical feature and the capstan rather than on friction between the wire rope and the capstan. 
     In some embodiments the wire rope is routed on idler pulleys in the wrist hub. The wire rope enters the wrist hub through one continuum robot shaft, bends on the circumference of a first idler pulley tangentially oriented to the wrist capstan, wraps fully or partially around the capstan, and returns on a second idler pulley in a similar manner into a second continuum robot shaft (backbone). 
     In some embodiments the idler pulleys are replaced by curved surfaces in the wrist hub in order to reduce size and cost. The wire rope then slides on these curved surfaces and wraps around the capstan. The curved surfaces may be treated with friction reducing treatments such as PTFE coatings or hard anodize treatment. The curved surface geometry is uniquely determined such that the first curved surface where the wire rope enters the wrist hub is placed at a height difference compared to the second curved surface where the wire rope exits the wrist hub. This axial height difference is determined by the pitch of the helical path of the wire rope winding around the capstan. 
     In some embodiments, the wrist hub includes a first helical circumferential groove and a second helical circumferential groove in the wrist hub. These grooves replace the function of the idler pulleys and allow transmission of the wire rope from the entry point of the wrist hub along the first helical path to a point of tangency to the wrist capstan and then returning to the second helical groove to the exit shaft in the continuum robot 
     In some embodiments the wrist capstan is made of two parts comprising of a capstan shaft and a capstan ring. The capstan ring is attached to the capstan shaft in a manner that allows transmission of torque but does not allow transmission of axial motion. Such embodiment may include a spline shaft. In this design the capstan is allowed to move axially to conform with the movement of the helically wound wire rope loop. 
     In some embodiments the wrist base (hub) is attached to the end disk of the continuum robot through a revolute articulated joint (herein called pitch axis). Actuation of the wrist is achieved through a wire rope loop that passes through two backbones (shafts) of the continuum robot while bending of the pitch axis is achieved via a push-pull superelastic NiTi wire that passes through a third shaft of the continuum robot or via a wire rope loop that passes through two opposing shafts of the continuum robot. 
     In some embodiments the wrist base (hub) is attached to the end disk of the continuum robot through a universal (Cardan) articulated joint that provides bending in the yaw and pitch axes. Actuation of the wrist (roll axis) is achieved through a wire rope loop that passes through two backbones (shafts) of the continuum robot while bending of the pitch axis is achieved via a push-pull superelastic NiTi wire that passes through a third shaft of the continuum robot. Similarly, bending of the yaw axis is achieved via a push-pull superelastic NiTi wire that passes through a third shaft of the continuum robot. 
     Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded view of a rotatable gripper wrist according to one embodiment. 
         FIG. 2  is a view of a rotatable gripper wrist according to another embodiment. 
         FIG. 3  is a exploded, perspective, and cross-sectional view of a rotatable gripper wrist according to a third embodiment. 
         FIGS. 4A and 4B  are detailed views of two examples of wrist hub components used with a rotatable gripper wrist. 
         FIG. 5  is a perspective view of a wrist and gripper assembly with a pivot (pitch) joint. 
         FIG. 6  is a perspective view of a wrist and gripper assembly with a gimbal (yaw and pitch) joint. 
         FIG. 7  is an exploded with of a rotatable gripper wrist according to a fourth embodiment. 
         FIG. 8A  is a side view of the wrist hub component of the rotatable gripper wrist of  FIG. 7  fitted with a flexible control wire. 
         FIG. 8B  is a perspective view of the bottom of the wrist hub of  FIG. 8A . 
         FIG. 8C  is a side view of the assembled rotatable gripper wrist of  FIG. 7  fitted with the flexible control wire. 
     
    
    
     DETAILED DESCRIPTION 
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. 
     Some surgical tools, such as described in U.S. Pub. No. 2011/0230894, which is incorporated herein by reference, include continuum robots with gripping tools connected to the distal end of the continuum robot. A continuum robot is a snake-like robot with a plurality of segments. The segments are controlled independently to adjust the shape and position of the continuum robot. Although some of these tools include articulated wrists for adjusting the position of the gripper, the existing tools are incapable of producing instrument roll about the gripper axis. This limits implementation of these devices for highly precise manipulations such as micro-surgery since very exact coordinated motion of several degrees of freedom is required. 
       FIG. 1  illustrates a first example of an articulated wrist  11  that is capable of rotating the gripper  13  relative to the shaft (or plurality of segments) of a continuum robot or other device. The example of  FIG. 1  is achieved using micro-planetary gears. The sun gear  15  is actuated through a miniature torsional shaft and the planetary gears  17  amplify this torque and rotate the wrist about its axis. 
       FIG. 2  shows a different construction of a wrist assembly  20  that uses a miniature pulley with wire actuation to achieve rotation of the gripper. As illustrated, the ends of a wire loop  21  each pass across a pulley  22  extending into the shaft of the continuum robot. The wire loop is then positioned around a textured or grooved capstan assembly  23 . As either end of the flexible wire loop  21  is inserted and retracted from the shaft of the continuum robot, the friction between the wire loop  21  and the capstan assembly  23  causes the capstan assembly to rotate relative to the shaft of the continuum robot. This rotation also causes the gripper to rotate. The ends of the wire loop extend through tubular structures in the shaft of the continuum robot called secondary backbones. The wire loop in the example of  FIG. 2  includes a positive-locking, spherical shaped terminal  24  that is crimped on the flexible wire. The terminal  24  causes increased frictions between the flexible wire  21  and the capstan  23 . The flexible wire in this example is a NiTi wire. 
     A plurality of ball bearings  25  are incorporated into the wrist assembly  200  to provide for smooth rotation of the capstan assembly  23  and, as a result, the gripper. The gripper is operated by a wire-based mechanism that extends through a channel  26  in the center of the capstan  23 . 
       FIG. 3  illustrates another example of a rotating wrist assembly. This assembly has two main sub-assemblies: the wrist and the gripper. The wrist base  102  allows the wrist and gripper to be selectively detached from the shaft (e.g., the snake arm) of the continuum robot and also serves as the end disk of a multi-backbone continuum snake robot. The lock nut  101  serves as a means of locking the wrist assembly to the secondary backbones of the snake arm. The hollow screw shaft  103  is threaded into the wrist capstan  108  and is glued to it or attached by press-fit. This screw shaft serves as the shaft hub locking the rotatable wrist capstan  108  to the wrist hub  107 . Once the capstan  108  and the screw shaft  103  are connected they are inserted into a bearing made of the wrist capstan  108 , the bearing balls  106 , the wrist hub  107 , and then locked by the lower bearing brace/lock nut  104 . The wrist hub  107  is coupled to the wrist base  102  using shear pins. 
     The gripper includes a fixed jaw  203 , a moving jaw  204 , a sliding block  202 , and a guiding pin. The gripper attaches to the rotating wrist capstan  108  using shear pins  201 . Actuation of the gripper is achieved using a superelastic NiTi wire that pushes the sliding block  202 , which in turn rotates the moving jaw using a shear pin that passes in the slot openings in the fixed and moving jaws. 
     The example of  FIG. 3  differs from the example of  FIG. 2  in that the pulleys are replaced by a wrist hub  107  with sliding surfaces to guide the flexible wire. Also, the example of  FIG. 3  eliminates the positive locking terminal. As such, the flexible wire loop can be extended and refracted further linearly through the shaft of the continuum robot and the rotation of the wrist is not limited by a physical structure on the wire. Another difference is that the design in  FIG. 3  allows detaching the wrist from the backbones of the snake segment. The end disk of the snake segment, which serves as the wrist hub  102  includes a series of linear grooves allowing for the side insertion of the NiTi backbones of the snake robot. The backbones have enlarged features at their tip that match the grooves in  102 . A rotation of lock disk  101  selectively locks the backbones into the wrist hub  102 . This selective locking functionality allows for easy replacement of wrist modules. 
       FIG. 4A  illustrates the wrist hub  107  in further detail. The wrist hub  107  includes an extrusion  401  that guides the flexible wire and pushes it against the surface of the rotating capstan  108 .  FIG. 4B  shows the wrist hub  107  from a different perspective. 
       FIG. 5  illustrates a pivot joint  300  that can be included to provide an additional degree of freedom to the rotatable gripper of  FIG. 3 . The added capability is achieved using a revolute joint assembly including a base  301  and an output link  302  pivotably connected via a pin. There are at least four holes through the base link that provide access for superelastic NiTi wires that control the rotatable gripper assembly. In one example, two adjacent holes are used to pass either end of the flexible wire loop used to actuate the rotatable wrist and the other two holes are used to actuate the revolute joint using push-pull actuation through a wire rope connected to the output link  302 . In another embodiment, the revolute joint is actuated through superelastic NiTi tubes connected to the output link  302  and passing through guide tubes in the base  301  and the wrist is actuated through wire ropes that pass through the NiTi tubes of the revolute joint. 
       FIG. 6  illustrates an alternative joint assembly  310  for connecting the rotatable gripper to the shaft of the continuum robot. This example provides yet another degree of freedom (both yaw and pitch) in addition to the rotating capabilities. The added degree of freedom is achieved using a Cardan (Hooke) joint assembly. The joint assembly includes a base  311 , a gimbal  312 , and an output link  313 . The gimbal is connected to the base and output links via pins. The base link again has at least four holes. In one example, two opposing holes are used to pass the actuation wires of the yaw degree of freedom while the other two holes are used to pass actuation wires of the pitch direction. The wrist actuation in a design using only four holes in the base  311  would require the use of a rotation tube and a gripper as illustrated in  FIG. 1 . In another embodiment, the base  311  has at least six holes and an additional center hole for actuating the gripper. Two holes are used to pass wires for actuating the pitch axis, two for actuating the yaw, and two to actuate the rotation of the gripper. In such constructions, a hole must also be provided through the center of gimbal  312  to allow the mechanism for actuating the gripper to pass through the joint  310 . In some other constructions, gimbal  312  is replaced with a binary link having two axially offset pivots that are mutually perpendicular. 
       FIG. 7  illustrates another alternative rotatable wrist  700  for a gripper assembly. The wrist includes a snake end disk  741 , a bearing nut  742 , a vented screw  743 , bearing balls  744 , a wrist hub  745 , a capstan  746 , and a cover ring  747 . When connected to the capstan assembly  746 , the bearing nut  742  supports the bottom set of bearing balls  744  and locks the entire wrist structure around the wrist hub  745 . The capstan  746  has locating pins for mounting the gripper jaw. 
       FIGS. 4A and 4B  further illustrate the differences between the wrist hub  107  of the example of  FIG. 3  and the wrist hub B 45  of the example of  FIG. 7 . Wrist hub  107  includes two smooth extrusions  401  to allow routing of the wire rope loop that is used to control the rotation of the capstan and, thereby, the gripper. Wrist hub  745  includes a groove  403  that routes the wire rope to the correct position to wrap around the capstan  746 . As the wire rope is inserted or retracted from the shaft of the continuum robot to control the rotation of the wrist, the wire rope move linearly through the grooves of the wrist hub  745 . 
       FIGS. 8A, 8B, and 8C  show various components of the wrist assembly of  FIG. 7  fitted with a flexible wire loop.  FIG. 8A  shows the wrist hub  745  from the side and illustrates the ends of the wire loop running through the grooves  403  of the wrist hub  745  and extending out of the bottom of the wrist hub  745 .  FIG. 8B  shows the same assembly from the bottom. In  FIG. 8C , the entire rotatable wrist assembly is assembled and attached to the distal end of a continuum robot. The wire loop is visible in the groove  403  of the wrist hub B 75  in  FIG. 8C . 
     Thus, the invention provides, among other things, a rotatable wrist assembly for an articulable gripper tool. Various features and advantages of the invention are set forth in the following claims.