Patent Publication Number: US-2020281665-A1

Title: Electromechanical surgical systems and robotic surgical instruments thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of and priority to U.S. Provisional Patent Application No. 62/303,695 filed Mar. 4, 2016, the entire disclosure of which is incorporated by reference herein 
    
    
     BACKGROUND 
     Robotic surgical systems have been used in minimally invasive medical procedures. Some robotic surgical systems included a console supporting a surgical robotic arm and a surgical instrument including at least one end effector (e.g., forceps or a grasping tool) mounted to the robotic arm. The robotic arm provided mechanical power to the surgical instrument for its operation and movement. Each robotic arm may have included an instrument drive unit having a plurality motors operatively connected to the surgical instrument. 
     One motor of the instrument drive unit was used to rotate a threaded rod of the surgical instrument, which in turn, effected the opening and closing of jaws of the end effector and/or the stapling function of the end effector. The speed at which the threaded rod rotated was directly proportional to the rate at which the jaws of the end effector opened and closed. However, existing instrument drive units do not open and close the jaws of the end effector at a desired speed of operation while also providing sufficient torque for performing the stapling and/or cutting functions. 
     SUMMARY 
     In accordance with an aspect of the present disclosure, a robotic surgical instrument for actuating an electromechanical end effector is provided. The robotic surgical instrument includes a housing, a first input drive, a second input drive, and a shaft assembly. The housing has a proximal end configured to be coupled to an instrument drive unit. The first input drive is rotatably disposed within the housing and configured to be drivingly coupled to a first motor of the instrument drive unit. The second input drive is rotatably disposed within the housing and configured to be drivingly coupled to a second motor of the instrument drive unit. The shaft assembly extends distally from within the housing and includes a shaft and a rod. The shaft has a distal end, and a proximal end operably coupled to the first and second input drives. The rod has a proximal end threadingly coupled to the distal end of the shaft. Rotation of the first and second input drives rotates the shaft to effect axial movement of the rod relative to the shaft. 
     In some embodiments, the shaft of the shaft assembly may define a longitudinal axis, and the first and second input drives may be oriented parallel to and offset from the longitudinal axis. 
     It is contemplated that each of the first and second input drives may include a gear. The shaft of the shaft assembly may also include a gear, which is in operative engagement with the gear of each of the first and second input drives such that the gear of each of the first and second input drives transfers rotational motion to the gear of the shaft. The gear of the shaft and the gear of each of the first and second input drives may be a spur gear. 
     It is envisioned that each of the first and second input drives may include a coupler configured to be drivingly coupled to a respective one of the first motor and the second motor of the instrument drive unit. 
     In some aspects, the proximal end of the rod may be disposed within the distal end of the shaft and may be prevented from rotating as the shaft rotates. 
     In some embodiments, the robotic surgical instrument may further include an end effector operably coupled to a distal end of the rod of the shaft assembly. The end effector may include a pair of opposing jaw members configured to change a size of a gap therebetween and fire staples therefrom upon axial movement of the rod. 
     In another aspect of the present disclosure, an electromechanical surgical system for use with a robotic system is provided. The electromechanical surgical system includes an instrument drive unit including a first motor and a second motor, and a robotic surgical instrument. The robotic surgical instrument includes a housing, a first input drive, a second input drive, and a shaft assembly. The housing has a proximal end configured to be coupled to the instrument drive unit. The first input drive is rotatably disposed within the housing and configured to be drivingly coupled to the first motor of the instrument drive unit. The second input drive is rotatably disposed within the housing and configured to be drivingly coupled to the second motor of the instrument drive unit. The shaft assembly extends distally from within the housing. The shaft assembly includes a shaft, and a rod. The shaft has a distal end, and a proximal end operably coupled to the first and second input drives. The rod has a proximal end threadingly coupled to the distal end of the shaft. Rotation of the first and second input drives by actuation of the first and second motors rotates the shaft to effect axial movement of the rod relative to the shaft. 
     In some embodiments, the shaft of the shaft assembly may define a longitudinal axis, and the first and second input drives of the robotic surgical instrument may be oriented parallel to and offset from the longitudinal axis. 
     It is contemplated that each of the first and second input drives of the robotic surgical instrument may include a gear. The shaft of the shaft assembly may also include a gear, which is in operative engagement with the gear of each of the first and second input drives such that the gear of each of the first and second input drives transfers rotational motion to the gear of the shaft. The gear of the shaft and the gear of each of the first and second input drives may be a spur gear. 
     It is envisioned that each of the first and second input drives of the robotic surgical instrument may include a coupler. The instrument drive unit may include a first drive coupler, and a second drive coupler. The first drive coupler may extend from the first motor and be configured to be drivingly coupled to the coupler of the first input drive of the robotic surgical instrument. The second drive coupler may extend from the second motor and be configured to be drivingly coupled to the coupler of the second input drive of the robotic surgical instrument. 
     In some aspects, the proximal end of the rod may be disposed within the distal end of the shaft and be prevented from rotating as the shaft rotates. 
     In some embodiments, the robotic surgical instrument may further include an end effector operably coupled to a distal end of the rod of the shaft assembly. The end effector may include a pair of opposing jaw members configured to change a size of a gap therebetween and fire staples therefrom upon axial movement of the rod. The electromechanical surgical system may further include a processor configured to actuate the first motor and the second motor of the instrument drive unit to fire staples from the pair of opposing jaw members. The processor may be configured to independently actuate at least one of the first or second motors of the instrument drive unit to move the pair of opposing jaw members. 
     In some aspects, the first motor and the second motor may each be configured to produce a maximum torque T such that upon the concurrent actuation of the first motor and the second motor, the first and second motors together produce a maximum torque 2T. 
     Further details and aspects of exemplary embodiments of the present disclosure are described in more detail below with reference to the appended figures. 
     As used herein, the terms parallel and perpendicular are understood to include relative configurations that are substantially parallel and substantially perpendicular up to about + or −10 degrees from true parallel and true perpendicular. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present disclosure are described herein with reference to the accompanying drawings, wherein: 
         FIG. 1  is a schematic illustration of a robotic surgical system including an electromechanical surgical system in accordance with the present disclosure; 
         FIG. 2  is a perspective view of the electromechanical surgical system of  FIG. 1 , illustrating a robotic surgical instrument and an instrument drive unit being attached to a surgical robotic arm; 
         FIG. 3  is a cross sectional view of the instrument drive unit of  FIG. 2  illustrating a first motor and a second motor; 
         FIG. 4  is a perspective view of the robotic surgical instrument of  FIG. 2 ; 
         FIG. 5  is a cross sectional view, taken along lines  5 - 5  of  FIG. 4 , of the robotic surgical instrument; and 
         FIGS. 6 and 7  are perspective views of a prior art end effector for use with the robotic surgical instrument of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the presently disclosed robotic surgical system including an electromechanical surgical system for actuating an electromechanical end effector and methods thereof are described in detail with reference to the drawings, in which like reference numerals designate identical or corresponding elements in each of the several views. As used herein the term “distal” refers to that portion of the robotic surgical system, instrument drive unit, robotic surgical instrument, electromechanical end effector, or component thereof, that is further from the user, while the term “proximal” refers to that portion of the robotic surgical system, instrument drive unit, robotic surgical instrument, electromechanical end effector, or component thereof, that is closer to the user. 
     As will be described in detail with respect to  FIGS. 1-5 , the present disclosure is directed to a surgical instrument, for example, a robotic surgical instrument for use with a robotic surgical system. The robotic surgical instrument includes an instrument drive unit having at least two motors that together drive the actuation of certain functions of an end effector of the robotic surgical instrument, as will be described in detail below. In some embodiments, a handheld surgical instrument, such as, for example, a handheld surgical stapling apparatus, may be provided that has a plurality of motors that together drive a single firing rod of the handheld surgical stapling apparatus. 
     Referring initially to  FIGS. 1 and 2 , a surgical system, such as, for example, a robotic surgical system  1 , generally includes a plurality of surgical robotic arms  2 ,  3  having a robotic surgical instrument  100  removably attached thereto; a control device  4 ; and an operating console  5  coupled with control device  4 . 
     Operating console  5  includes a display device  6 , which is set up in particular to display three-dimensional images; and manual input devices  7 ,  8 , by means of which a person (not shown), for example a surgeon, is able to telemanipulate robotic arms  2 ,  3  in a first operating mode, as known in principle to a person skilled in the art. Each of the robotic arms  2 ,  3  may be composed of a plurality of members, which are connected through joints. Robotic arms  2 ,  3  may be driven by electric drives (not shown) that are connected to control device  4 . Control device  4  (e.g., a computer) is set up to activate the drives, in particular by means of a computer program, in such a way that robotic arms  2 ,  3 , their instrument drive units  20 , and thus robotic surgical instrument  100  (including electromechanical end effector  200 ,  FIGS. 6 and 7 ) execute a desired movement according to a movement defined by means of manual input devices  7 ,  8 . Control device  4  may also be set up in such a way that it regulates the movement of robotic arms  2 ,  3  and/or of the drives. 
     Robotic surgical system  1  is configured for use on a patient “P” lying on a surgical table “ST” to be treated in a minimally invasive manner by means of a surgical instrument, e.g., robotic surgical instrument  100 . Robotic surgical system  1  may also include more than two robotic arms  2 ,  3 , the additional robotic arms likewise being connected to control device  4  and being telemanipulatable by means of operating console  5 . A surgical instrument, for example, robotic surgical instrument  100  (including electromechanical end effector  200 ,  FIGS. 6 and 7 ), may also be attached to the additional robotic arm. 
     Control device  4  may control a plurality of motors (Motor  1  . . . n) with each motor configured to drive a relative rotation of drive members of robotic surgical instrument  100  to effect operation and/or movement of each electromechanical end effector  200  of robotic surgical instrument  100 . It is contemplated that control device  4  coordinates the activation of the various motors (Motor  1  . . . n) to coordinate a clockwise or counter-clockwise rotation of drive members (not shown) of instrument drive unit  20  in order to coordinate an operation and/or movement of a respective electromechanical end effector  200 . In embodiments, each motor can be configured to actuate a drive rod or a lever arm to effect operation and/or movement of each electromechanical end effector  200  of robotic surgical instrument  100 . 
     For a detailed discussion of the construction and operation of a robotic surgical system, reference may be made to U.S. Pat. No. 8,828,023, entitled “Medical Workstation,” the entire contents of which are incorporated herein by reference. 
     With reference to  FIGS. 2 and 3 , robotic surgical system  1  includes an electromechanical surgical system  30 , which includes robotic arm  2 , instrument drive unit  20 , and robotic surgical instrument  100 . Instrument drive unit  20  of electromechanical surgical system  30  is configured to be coupled to robotic surgical instrument  100 , and robotic surgical instrument  100  is configured to be coupled with or to robotic arm  2 . Instrument drive unit  20  is configured for powering robotic surgical instrument  100 . Instrument drive unit  20  transfers power and actuation forces from its motors, for example, a first motor M 1  and a second motor M 2 , to robotic surgical instrument  100  to ultimately drive movement of components of electromechanical end effector  200  ( FIGS. 6 and 7 ) of robotic surgical instrument  100 , for example, a movement of a knife blade (not shown) and/or a closing and opening of jaw members  202   a ,  202   b  of electromechanical end effector  200 . 
     First motor M 1  may be configured as a master motor, and second motor M 2  may be configured as a slave motor that matches the amount of torque being output by master motor M 1  so that first and second motors M 1 , M 2  operate in synchrony. First and second motors M 1 , M 2  are in communication with one another via a processor “P” that synchronizes first and second motors M 1 , M 2  so that second motor M 2  will produce the same torque as first motor M 1  at any given time to ultimately rotate first and second input drives  108 ,  110  of robotic surgical instrument  100  at the same rate. First and second motors M 1 , M 2  are each configured to produce a maximum torque T, depending on their size and make, such that upon the concurrent actuation of first and second motors M 1 , M 2 , first and second motors M 1 , M 2  together produce a maximum torque 2T. In some embodiments, instrument drive unit  20  may include a plurality of slave motors such that instrument drive unit  20  can produce a torque greater than 2T. 
     In embodiments, when a particular amount of torque is desired to be output by the instrument drive unit  20  (e.g., as determined by a clinician or the control device  4 ), the processor may be configured to cause the second motor M 2  to output a torque that is equal to the difference between the desired torque and the torque output by the first motor M 1  such that the combined torque output by the first and second motors M 1 , M 2  matches the desired torque. In embodiments, the second motor M 2  may be configured to output a constant torque whereas the first motor M 1  may be configured to output an amount of torque that brings the total torque output by the instrument drive unit  20  up to the desired torque. 
     Instrument drive unit  20  includes a plurality of rotatable output shafts  22 ,  24  attached to respective first and second motors M 1 , M 2  such that output shafts  22 ,  24 , are independently rotatable with respect to one another. In some embodiments, instrument drive unit  20  may include more than two motors, for example, three or four motors, that each have a respective output shaft rotatably attached thereto. In embodiments, the first motor M 1  may be the master motor and two or more motors may act as slave motors. Instrument drive unit  20  has a first drive coupler  26  and a second drive coupler  28  non-rotatably attached to respective first and second output shafts  22 ,  24  such that first and second drive couplers  26 ,  28  extend from first and second motors M 1 , M 2 , respectively. First and second drive couplers  26 ,  28  each have a mechanical interface  26   a ,  28   a , for example, a plurality of teeth or a crown gear, configured to drivingly couple to respective first and second input drives  108 ,  110  ( FIG. 4 ) of robotic surgical instrument  100 . As such, actuation of first and second motors M 1 , M 2  effects rotation of first and second input drives  108 ,  110  of robotic surgical instrument  100  at the same rate as one another when robotic surgical instrument  100  is operably engaged to instrument drive unit  20 , as will be described in detail below. 
     Instrument drive unit  20  includes sensors, such as, for example, torque transducers  32 , connected to first and second motors M 1 , M 2 . Torque transducers  32  sense the amount of torque that is being output by motors M 1 , M 2  during their operation. Processor “P” of instrument drive unit  20  is in communication with torque transducers  32  to control the amount of power output by first and/or second motors M 1 , M 2  based on the amount of torque sensed by torque transducers  32 . In particular, when additional torque is required to carry out a certain function of end effector  200 , for example, stapling tissue and/or cutting tissue, processor “P” will activate second motor M 2  (to operate concurrently with first motor M 1 ) and cause second motor M 2  to produce the same torque as first motor M 1 . 
     Further, instrument drive unit  20  includes a sensor (e.g. a pressure sensor) (not shown) able to detect and measure both firing and retraction forces of shaft assembly  120  ( FIG. 5 ) of robotic surgical instrument  100 . Processor “P” is in communication with the pressure sensor and is configured to actuate both first and second motors M 1 , M 2  concurrently when the amount of force sensed by the pressure sensor is indicative of tissue being clamped and ready for stapling. Processor “P” is also configured to actuate only one first motor M 1  when the amount of force sensed by the pressure sensor is indicative of tissue not being clamped between jaws  202   a ,  202   b  of electromechanical end effector  200 . 
     As such, a torque T is output by instrument drive unit  20  for clamping and unclamping tissue disposed between jaws  202   a ,  202   b  of electromechanical end effector  200 , and a torque 2T is output by instrument drive unit  20  for stapling and/or cutting tissue clamped between jaws  202   a ,  202   b  of electromechanical end effector  200 . It is contemplated that torque transducers  32 , the pressure sensors, and/or processor “P” may be disposed in any of the components of electromechanical surgical system  30 . It is contemplated that a clinician may activate first motor M 1 , second motor M 1 , or first and second motors M 1 , M 2  concurrently depending on the desired effect on electromechanical end effector  200 , for example, clamping/unclamping or stapling/cutting. In some embodiments, the instrument drive unit  20  may be configured to output more or less than the torque 2T for stapling and/or cutting tissue. 
     With reference to  FIGS. 4 and 5 , robotic surgical instrument  10  generally includes robotic surgical instrument  100 , and electromechanical end effector  200 , which extends distally from robotic surgical instrument  100 . 
     Robotic surgical instrument  100  includes a housing  102  and a shaft assembly  120  extending distally from within housing  102 . Housing  102  of robotic surgical instrument  100  has a generally cylindrical configuration, and has a proximal end  102   a  configured to be coupled to instrument drive unit  20 , and a distal end  102   b . In embodiments, housing  102  may be any shape suitable for receipt in a distal end  2   a  of robotic arm  2 . Housing  102  defines a cavity  105  that houses various components of robotic surgical instrument  100 . Proximal end  102   a  of housing  102  supports a first input drive  108  and a second input drive  110  each being rotatably disposed within cavity  105  of housing  102  and extending in parallel alignment with a longitudinal axis “X” defined by shaft assembly  120 . In some embodiments, housing  102  may include more than two input drives. First and second input drives  108 ,  110  of robotic surgical instrument  100  are illustrated as being rod-shaped, but it is contemplated that they may take on any other suitable shape. 
     First and second input drives  108 ,  110  of robotic surgical instrument  100  each have a proximal end and a distal end. The proximal end of each of first and second input drives  108 ,  110  includes a proximal coupler  108   a ,  110   a , for example, a crown gear, disposed at proximal end of housing  102   a . Proximal coupler  108   a ,  110   a  of each of first and second input drives  108 ,  110  is configured to be detachably, non-rotatably coupled to mechanical interface  26   a ,  28   a  ( FIG. 3 ) of respective first and second drive couplers  26 ,  28  of instrument drive unit  20 . As such, upon connecting instrument drive unit  20  with housing  102  of robotic surgical instrument  100 , first and second input drives  108 ,  110  of robotic surgical instrument  100  are drivingly coupled to respective first and second motors M 1 , M 2  of instrument drive unit  20 . In some embodiments, proximal couplers  108   a ,  110   a  of robotic surgical instrument  100  may be connected to respective first and second drive couplers  26 ,  28  of instrument drive unit  20  via helical gears, a belt drive assembly, or any other suitable mechanism for transferring rotational motion between first and second input drives  108 ,  110  and instrument drive unit  20 . The distal end of each of the first and second input drives  108 ,  110  includes a distal coupler  108   b ,  110   b , for example, a spur gear. Distal coupler  108   b ,  110   b  of each of first and second input drives  108 ,  110  of robotic surgical instrument  100  is in meshing engagement with a gear  126  of shaft assembly  120  of robotic surgical instrument  100 . 
     Thus, upon the concurrent actuation of first and second motors M 1 , M 2  of instrument drive unit  20 , first and second drive couplers  26 ,  28  of instrument drive unit  20  rotate, resulting in concomitant rotation of first and second input drives  108 ,  110  of robotic surgical instrument  100  via the first and second proximal couplers  108   a ,  110   a  of housing  102 . The rotation of first input drive  108  and/or second input drive  110  of housing  102  of robotic surgical instrument  100  drives a rotation of an inner shaft  124  of shaft assembly  120  to ultimately result in the opening or closing of jaw members  202   a ,  202   b  of electromechanical end effector  200 , the ejection of staples (not shown) from jaw members  202   a ,  202   b , and/or the actuation of a knife blade (not shown) of electromechanical instrument  200 . In some embodiments, distal couplers  108   b ,  110   b  of robotic surgical instrument  100  may be connected to shaft assembly  120  via helical gears, a belt drive assembly, or any other suitable mechanism for transferring rotational motion between first and second input drives  108 ,  110  and shaft assembly  120 . 
     In some embodiments, second input drive  110  is movable between a first position, in which distal coupler  110   b  of second input drive  110  is out of meshing engagement with gear  126  of inner shaft  124 , and a second position, in which distal coupler  110   b  of second input drive  110  is in meshing engagement with gear  126  of inner shaft  124 . As such, when more torque is required to actuate functions of electromechanical end effector  200 , second input drive  110  may be moved from the first position into the second position. When the added torque is not required, second input drive  110  may be moved into the first position. 
     As mentioned above, robotic surgical instrument  100  includes shaft assembly  120 , which extends distally from within housing  102 . Shaft assembly  120  operatively intercouples instrument drive unit  20  with jaw members  202   a ,  202   b  of electromechanical end effector  200  and a staple actuator (not shown) of electromechanical end effector  200 . Shaft assembly  120  generally includes an outer tube or outer shaft  122 , an inner shaft  124 , and a threaded rod  130 . Outer shaft  122  has a proximal end  122   a , and a distal end  122   b , which is mechanically attached to one or both jaw members  202   a ,  202   b  of electromechanical end effector  200 . 
     Inner shaft  124  of shaft assembly  120  has a proximal end  124   a  and a distal end  124   b . Proximal end  124   a  of inner shaft  124  has a gear  126 , for example, a spur gear, in meshing engagement with both distal couplers  108   b ,  110   b  of respective first and second input drives  108 ,  110  of housing  102  such that distal couplers  108   b ,  110   b  of first and second input drives  108 ,  110  transfer rotational motion to gear  126  of inner shaft  124 . Distal end  124   b  of inner shaft  124  defines a threaded bore  128  longitudinally therethrough. Rod  130  of shaft assembly  120  has a threaded outer surface  132  threadingly engaged to threaded bore  128  of inner shaft  124 . Rod  130  of shaft assembly  120  has a non-circular portion (not shown) that is disposed within a correspondingly shaped fixture (not explicitly shown) that prevents rod  130  from rotating. As such, as shaft  124  of shaft assembly  120  rotates, rod  130  of shaft assembly  120  does not rotate therewith, but instead, translates or moves axially relative to shaft  124 . 
     Threaded outer surface  132  of rod  130  has a high thread pitch of approximately 32 threads per inch of length of rod  130 . The high thread pitch of threaded outer surface  132  of rod  130  provides for a high rate of axial movement of rod  130  per revolution of shaft  124 , which ultimately results in a high rate of opening and closing of jaw members  202   a ,  202   b  of electromechanical end effector  200 . 
     Rod  130  extends from distal end  102   b  of housing  102 , through the length of outer shaft  122 , and terminates at jaw members  202   a ,  202   b  of electromechanical end effector  200 . The distal end (not shown) of rod  130  is operably coupled to components of end effector  200  such that axial movement of rod  130  effects an opening or closing of jaw members  202   a ,  202   b  of electromechanical end effector  200  and the operation of the stapling function and cutting function of electromechanical end effector  200 . 
     For a detailed discussion of the construction and operation of end effector  200 , reference may be made to U.S. Pat. No. 6,953,139, filed on Nov. 5, 2004, entitled “SURGICAL STAPLING APPARATUS,” the entire content of which is incorporated herein by reference. 
     In use, to change a size of a gap between jaw members  202   a ,  202   b  of electromechanical end effector  200 , instrument drive unit  20  is operably coupled to robotic surgical instrument  100 . First motor M 1  of instrument drive unit  20  is then activated to drive a rotation of first output shaft  22  of instrument drive unit  20 . Rotation of first output shaft  22  effects rotation of first input drive  108  of robotic surgical instrument  100  via the meshing engagement between mechanical interface  26   a  of first drive coupler  26  of instrument drive unit  20  and proximal coupler  108   a  of first input drive  108  of robotic surgical instrument  100 . Rotation of first input drive  108  of robotic surgical instrument  100  drives either a clockwise or counter-clockwise rotation of inner shaft  124  of shaft assembly  120  via the meshing engagement of distal coupler  108   b  of first input drive  108  and gear  126  of inner shaft  124 . 
     The rotation of inner shaft  124  causes rod  130  of shaft assembly  120  to move axially relative to shaft  124  in a proximal or distal direction. Proximal axial movement of rod  130  relative to shaft  124  actuates a closing of jaw members  202   a ,  202   b  of electromechanical end effector  200 , and distal axial movement of rod  130  relative to shaft  124  actuates an opening of jaw members  202   a ,  202   b  of electromechanical end effector  200 . In some embodiments, distal axial movement of rod  130  may close jaw members  202   a ,  202   b , and proximal axial movement of rod  130  may open jaw members  202   a ,  202   b . As mentioned above, due to the high thread pitch of rod  130 , jaw members  202   a ,  202   b  open and close at a fast rate. 
     With tissue clamped between jaw members  202   a ,  202   b , staples may be ejected from electromechanical end effector  200  into the tissue and the knife blade of electromechanical end effector  200  may be translated through the tissue to carry out a particular surgical procedure. There is an increased resistance to rotation of inner shaft  124  of shaft assembly  120  from the tissue clamped between jaws  202   a ,  202   b , and the increased thread pitch of rod  130 . Thus, to carry out the stapling function and/or cutting function of electromechanical end effector  200 , more torque than what first motor M 1  alone can provide may be required. 
     To staple tissue clamped between jaw members  202   a ,  202   b , a sufficient amount of power is delivered to second motor M 2  of instrument drive unit  20  to cause second motor M 2  to match the torque output by first motor M 1  so that second input drive  110  rotates at the same rate as first input drive  108  and no slip occurs between distal coupler  110   b  of second input drive  110  and gear  126  of inner shaft  124 . Rotation of second input drive  110  of robotic surgical instrument  100  supplements the torque applied to inner shaft  124  of shaft assembly  120  by first input drive  108 . Since the rotation of inner shaft  124  of shaft assembly  120  is being driven by both first and second input drives  108 ,  110 , which is being driven by the activation of first and second motors M 1 , M 2 , any resistance experienced by electromechanical end effector  200  to stapling through the tissue or to movement of the knife blade through the tissue can be overcome by the added torque provided by second motor M 2 . It is contemplated that both first and second motors M 1 , M 2  may be activated to open and close jaw members  202   a ,  202   b  instead of only first motor M 1 . 
     In some embodiments, the shaft assembly may be incorporated into a surgical instrument that uses a capstan/wire spool mechanism for converting rotary motion into linear motion. For example, in this embodiment, the gear  126  of the inner shaft  124  may be configured as a capstan having a wire(s) or cable(s) wrapped thereabout. 
     It will be understood that various modifications may be made to the embodiments disclosed herein. Therefore, the above description should not be construed as limiting, but merely as exemplifications of various embodiments. Those skilled in the art will envision other modifications within the scope and spirit of the claims appended thereto.