Passive preload and capstan drive for surgical instruments

[0001] Robotically (as used herein, the terms “robot” or “robotically” and the like include teleoperation or telerobotic aspects) controlled instruments are well known and often used in minimally invasive medical procedures. FIG. 1 shows an example of an instrument 100 having a structure that is simplified to illustrate basic working principles of some current robotically controlled medical instruments. Instrument 100 includes a tool or end effector 110 at the distal end of a shaft 120. The proximal end of shaft 120 attaches to a transmission or drive mechanism 130 that is sometimes referred to as backend mechanism 130. During a medical procedure, end effector 110 and the distal end of shaft 120 can be inserted through a small incision or a natural orifice of a patient to position end effector 110 at a work site in the patient. End effector 110 as illustrated includes jaws 112 and 114 that may be used at the work site for clamping, gripping, cutting, or other purposes. Other types of end effectors, for example, scalpels, wire loops, and cauterizing instruments, are known and could alternatively be mounted on the distal end of shaft 120. Normally, a surgical instrument would further include a wrist mechanism (not shown) at the distal end of shaft 120 to provide additional degrees of freedom of motion for positioning, orienting, and using end effector 110.

[0002] Tendons 121 and 122, which may be stranded cables, rods, tubes, or similar structures, run from backend mechanism 130 through shaft 120 and attach to end effector 110. A typical surgical instrument would also include additional tendons (not shown) that connect backend mechanism 130 to other structural members of end effector 110 or of a wrist mechanism, so that backend mechanism 130 can manipulate the tendons to operate end effector 110 and/or the wrist mechanism when performing the desired procedure at the work site. FIG. 1 illustrates two tendons 121 and 122 attached to jaw 112 in a pin joint structure, where jaw 112 is mounted for rotation about a pivot pin 116. To enable both clockwise and counterclockwise rotations of jaw 112, tendon 121 acts on jaw 112 at a moment arm about pivot pin 116 such that pulling on tendon 121 causes a torque tending to rotate jaw 112 clockwise in the view of FIG. 1. Similarly, tendon 122 acts at a moment arm such that pulling on tendon 122 causes a torque tending to rotate jaw 112 counterclockwise in the view of FIG. 1. Jaw 112 is thus provided with bi-directional actuation through pulling in a length of one tendon 121 or 122 and simultaneously releasing an identical length of the other tendon 122 or 121. Mechanisms other than pin joints are known or can be devised that provide bi-directional actuation of a distal joint through pulling in a length of one tendon 121 or 122 and releasing an equal and opposite length of the other tendon 122 or 121. For example, U.S. Pat. No. 6,817,974 (filed Jun. 28, 2002) entitled “Surgical Tool Having Positively Positionable Tendon-Actuated Multi-Disk Wrist Support” by Cooper et al. and U.S. Pat. No. 6,394,998 (filed Sep. 17, 1999) entitled “Surgical Tools For Use In Minimally Invasive Telesurgical Applications” by Wallace et al., both of which are incorporated herein by reference, describe some known medical instrument structures in which actuation requires pulling one or more tendons while releasing lengths of one or more other tendons.

[0003] Slack in tendons 121 and 122 can cause malfunctions, for example, by permitting tendons 121 and 122 to derail from guides or pulleys (not shown) that route tendons 121 and 122 through instrument 100. Slack can also cause jumpy or unpredictable motion of the instrument. To avoid creating slack in tendon 121 or 122 when moving jaw 112, backend mechanism 130 operates to release a length of one tendon 121 or 122 while simultaneously reeling in an equal length of the other tendon 122 or 121. Tendons 121 and 122 can be attached to the same capstan 132 but wrapped in opposite directions to provide the desired movements of tendons 121 and 122 when a drive motor (not shown) turns capstan 132. In cases where an end effector has several degrees of freedom that are controlled by several tendons, for instance as described in patent U.S. Pat. Nos. 6,394,998 and 6,817,974, the backend mechanism can include mechanisms other than capstans to perform the function of releasing and reeling in related lengths of tendons in order to avoid slack in the tendons as distal joints are turned. It can be seen that tendons 121 and 122 can be two separate components, or they may be part of a closed loop component with a capstan actuator, such as that disclosed in U.S. Pat. No. 7,316,681 B2 (filed Oct. 4, 2005) entitled “Articulated Surgical Instrument For Performing Minimally Invasive Surgery With Enhanced Dexterity And Sensitivity” by Madhani et al., which is incorporated herein by reference.

[0004] Avoiding slack may also require tendons 121 and 122 to be pre-tensioned, particularly when end effector 110 may be used for pushing and pulling, clamping, gripping, or other actions that encounter resistance. In particular, backend mechanism 130 can apply tension to tendon 122 to cause jaw 112 to clamp down on material between jaws 112 and 114. Increasing tension in tendon 122 causes higher clamping force but also causes tendon 122 to stretch. To prevent the stretching of tendon 122 from causing a corresponding amount of slack in tendon 121, tendon 121 can be preloaded with a tension that stretches tendon 121. For example, assuming tendons 121 and 122 are identical, tendon 121 can be pre-loaded with a tension about equal to or greater than maximum clamping tension used in tendon 122. As a result, tendon 121 starts stretched by the preloaded tension, and when applying a clamping force, the stretching of tendon 122 reduces the tension in tendon 121, allowing tendon 121 to contract without becoming slack.

[0005] The tensions preloaded in tendons can increase the forces that a backend mechanism must apply to operate an instrument. In particular, tension increases friction where tendons may ride on guides or solid surfaces. Also, if shaft 120 is a flexible tube such as might be employed to follow a natural lumen in a patient's body, the preloaded tension causes friction where tendons 121 and 122 contact curved surfaces of shaft 120. In all the above cases, increased friction quickly makes accurate control of the motion of end effector 110 difficult and can lead to imprecise manipulation of, e.g., tissue, sutures, and needles during a surgical procedure. Further, preloading of the correct tension in tendons of a medical instrument can also increase the complexity of the manufacturing process of the medical instrument. Accordingly, medical instruments are desired that can avoid slack in drive tendons without requiring high preloaded tension.

[0006] In accordance with an aspect of the invention, a passive preload system, such as a spring based mechanism, is attached to a tendon that is wrapped around a capstan, so that the passive preload system can control the relaxed tension in the tendon. The capstan can be driven by a motor when the tendon is needed to pull on a structural member of the instrument. For example, when applying clamping pressure or moving the structural member against resistance, capstan friction can produce tensions in the tendon that are many times the passive preload tension. However, when the tendon is not needed to apply force to the member, the capstan can be effectively and quickly made free to spin by zeroing or releasing the torque applied to the capstan by a drive motor, so that the passive preload system controls tendon tension and can provide just enough tension to prevent derailment or other malfunctions of the tendon. It should be noted that when the capstan is freed, the tendon can easily slide on the capstan since the tendon is not permanently attached to the capstan. In other words, forces are exchanged between the capstan and the tendon solely through friction. The reduced tension in relaxed tendons can reduce tendon friction, particularly in instruments with bent or flexible shafts. Two modes of operation of an instrument using a tendon attached to a passive preload system includes a mode where a motor turns a capstan about which the tendon is wrapped and thereby pulls on the tendon, and a mode where the capstan is freed and the tendon tension matches a tension of the passive preload system. A system can quickly switch between these two modes of operation by controlling the torque output by a drive motor and thus has a large bandwidth torque control.

[0007] One specific embodiment of the invention is a surgical instrument. The surgical instrument includes a shaft, a member mounted at a distal end of the shaft, a mechanism attached to a proximal end of the shaft, and a tendon. The member can be a link, or a jaw of an end effector, or a group of links coupled together, and the member is mounted to permit movement of the member relative to the shaft. The mechanism includes a capstan and a passive preload system, such as a spring system. The capstan can normally be free to roll and has a coupling through which a motor can attach to and rotate the capstan. The tendon has a first end attached to the member, runs down the shaft, wraps around the capstan, and has a second end attached to the passive preload system, which controls relaxed tension in the tendon.

[0008] Another specific embodiment of the invention is a robotic surgical system. The robotic surgical system includes a motor and an instrument coupled to the motor by means of a docking port. The robotic surgical system can also include an articulated arm holding the motor pack and the docking port. The instrument generally includes a shaft; a member mounted at a distal end of the shaft, a backend mechanism attached to a proximal end of the shaft, and a tendon. The member is mounted to permit movement of the member relative to the shaft. The backend mechanism includes a capstan and a passive preload system. The capstan is coupled to a drive motor in the docking port, so that the drive motor can rotate the capstan. The tendon has a first end attached to the member, runs down the shaft, wraps around the capstan, and has a second end attached to the passive preload system.

[0009] Yet another embodiment of the invention is a method for operating a surgical instrument containing a movable member, a first tendon attached for movement of the member in one direction, and a second tendon attached for movement of the member in another direction. The method includes: applying a torque to a first capstan about which a first tendon is wrapped; and at the same time, freeing from motor torques a second capstan about which the second tendon is wrapped so that a passive preload system attached to an end of the second tendon extending from the second capstan controls tension in the second tendon.

[0019] Use of the same reference symbols in different figures indicates similar or identical items.

[0020] In accordance with an aspect of the invention, a surgical instrument can avoid tendon slack and maintain a relatively low tension in a relaxed drive tendon by wrapping the tendon around a capstan that is allowed to rotate freely and attaching an end of the tendon to a passive preload system such as a spring system. When operating the surgical instrument, friction between the tendon and the capstan allows a drive motor that turns the capstan to reel in a length of the tendon, and capstan friction can apply a maximum tension to the tendon that depends exponentially on the total angle of wrap the tendon about the capstan. The force from the passive preload system, e.g., spring force from a spring, and the tension in a relaxed tendon can therefore be kept relatively low while still being able to produce the high tensions needed for clamping or other movement of the instrument against resistance. When the motor torque on the capstan is zeroed, the capstan can rotate freely, and the passive preload system can pull in the tendon and prevent the tendon from becoming slack. The low relaxed tensions can decrease the forces needed for manipulation of the instrument, and tendon friction, which can be particularly problematic in medical instruments with curved or flexible shafts, can be reduced.

[0021] FIGS. 2A and 2B respectively show top and side views of a surgical instrument 200 in accordance with an embodiment of the invention. Instrument 200 includes an end effector 210 at a distal end of a shaft 220, which is connected to a backend mechanism 230. End effector 210 in the illustrated embodiment includes jaws 212 and 214, which are both mounted to enable relative movement of jaws 212 and 214. Jaws 212 and 214 can have shapes selected for the particular procedures for which effector 210 is intended. For example, jaws can be shaped to act as forceps of any desired type or could include blades that allow effector 210 to act as scissors. The materials and structure of jaws 212 and 214 can generally be selected for the particular uses of instrument 200, and many suitable effector architectures are known or may be developed.

[0022] Jaw 212 is connected to a first pair of tendons 221 and 222, and jaw 214 is connected to a second pair of tendons 223 and 224. Each pair of tendons 221 and 222 or 223 and 224 may be portions of a continuous tendon having a point that is attached to jaw 212 or 214, for example, by an adhesive or by a crimp residing in a notch in jaw 212 or 214. In a typical embodiment, additional tendons (not shown) would be connected in instrument 200 to a wrist mechanism or joints (not shown) that provide additional degrees of freedom for movement for positioning and orienting end effector 210.

[0023] Tendons 221, 222, 223, and 224 apply force and torques to jaws 212 and 214 when pulled by backend mechanism 230 and can have any structure suitable for that task. For example, tendons can be stranded cables, wires, rods, or tubes made of metal, a polymer, or other material. In an exemplary embodiment, tendons 221, 222, 223, and 224 include portions of stranded cable that are fused to tubes, with the stranded cable being used where significant bending or flexing of the tendons is expected, and the tubes being used elsewhere to reduce stretching of the tendons. In another embodiment, particularly useful when shaft 220 is flexible, tendons 221, 222, 223, and 224 can be made of a woven polymer material and run inside individual sheaths (not shown).

[0024] FIGS. 2A and 2B illustrate the example embodiment where tendons 221, 222, 223, and 224 attach to a pivot joint structure in which relative motion of jaws 212 and 214 corresponds to independent rotations of jaws 212 and 214 about a pivot pin 216. This joint structure is only an example, and other mechanisms for moving members of a surgical instrument in response to tensions applied to tendons 221, 222, 223, and 224 are known and could be employed in alternative embodiments of the invention. For example, a planar, cylindrical, or spherical rolling joint could provide similar freedom of motion to jaws 212 and 214 in end effector 210, or a prismatic linear joint or slide could be employed in end effector 210 to provide a linear bi-directional degree of freedom of motion.

[0025] Shaft 220 is a hollow tube through which tendons 221, 222, 223, and 224 run. Shaft 220 may be rigid or flexible. For example, shaft 220 may be a rigid stainless steel tube if instrument 200 is intended to be inserted through a straight cannula during a medical procedure, but shaft 120 would need to be flexible if instrument 200 is intended to follow the variable path of natural lumens. In a typical configuration, shaft 220 may have a diameter of about 5 mm or 8 mm for use with some existing cannulae and may have a length on the order of several tens of centimeters. In case the shaft 220 is flexible, the tendons can run inside sheaths in a Bowden cable arrangement (i.e., like bicycle cables). As used herein, the term “flexible” includes continuously flexible components (such as a rubber tube, which bends at each point along its length) and constrained series of short, discrete links that allow the links to approximate the movement of a continuously flexible unit (such as a series “snake-like” vertebrae).

[0026] Backend mechanism 230 attaches to the proximal end of shaft 220 and acts as a transmission that converts the rotation of drive motors (e.g., drive motors 242 and 244 in FIG. 2B) into movement of end effector 210. Backend mechanism 230 particularly manipulates tendons 221, 222, 223, and 224 to operate end effector 210 and may additionally be able to rotate or move shaft 220 to change the orientation or position of end effector 210. In the illustrated embodiment, backend mechanism 230 includes one capstan 231, 232, 233, or 234 per tendon 221, 222, 223, or 224. In accordance with an aspect of the current invention, each tendon 221, 222, 223, or 224 wraps for a set wrapping angle (that could be less than a full turn or as large as one or more turns) around the corresponding capstan 231, 232, 233, or 234 and has an end extending from the capstan 231, 232, 233, or 234. Tendons 221, 222, 223, and 224 are not required to be permanently attached to capstans 231, 232, 233, and 234 and may be able to slip relative to the capstans 231, 232, 233, and 234.

[0027] The ends of tendons 221, 222, 223, and 224 attach to passive preload systems, here implemented as linear coiled springs 235, 236, 237, and 238, which can be anchored to a case or chassis of backend mechanism 230. Springs 235, 236, 237, and 238 are biased, e.g., stretched, so that springs 235, 236, 237, and 238 apply a non-zero force to respective attached tendons 221, 222, 223, and 224 throughout the range of motion of surgical instrument 200. With this configuration, when capstans 231, 232, 233, and 234 are free to rotate, the corresponding spring 235, 236, 237, or 238 controls the tension in the associated tendon 231, 232, 233, or 234 and avoids slack by pulling in the required length of tendon 231, 232, 233, or 234. Tendons 221, 222, 223, and 224 do not require a preloaded tension that is higher that the maximum tensions used for operation of surgical instrument 200.

[0028] Each passive preload system more generally can be any structure or system that is able to apply a force to the free end of a tendon while allowing the tendon end to displace. The preload system is passive in that the system does not need to respond to a control or feedback system. Such systems can use linear coil springs as illustrated in FIGS. 2A and 2B or use other spring elements, such as rotary coil springs, leaf springs or compliant members, such as bending beams, cantilever beams, or elastic bands. Further, the spring elements or compliant members can work through extension or compression to apply force directly or indirectly to the end of the attached tendons. Other methods for applying the desired force, such as a system using weights or magnets, might alternatively be employed. In addition to the source of force, the passive preload system may include mechanical elements that direct or control the magnitude of the force applied to the attached tendon.

[0029] End effector 210 can be operated using drive motors which are under the active control of human input (e.g., master control input in a master-slave servo control system) and software executed in a robotically controlled system. In particular, four drive motors, which are provided in a docking port of a control system (not shown), can be respectively coupled to capstans 231, 232, 233, and 234 of FIG. 2A. FIG. 2B shows two drive motors 242 and 244, which are coupled to capstans 232 and 234. The orientations of capstans 231, 232, 233, and 234 in backend mechanism 230 are not required to have particular fixed relations to the position or configuration of instrument 200. Accordingly, any mechanical coupling that allows a motor to rotate or apply a torque to a capstan can be used for the coupling of motors 242 and 244 to capstans 232 and 234 in backend mechanism 230.

[0030] Backend mechanism 230 can be removably attached to a motor pack including motors 242 and 244 through a sterile barrier, such as a sterile sheet of plastic, and the instrument 200 can be designed to be disposable or reusable and sterilizable. When the instrument is detached from the motor pack, the passive preload systems (e.g., springs 235, 236, 237, and 238 in FIG. 2A) still keep tendons 221, 222, 223, and 224 from slacking and allow end effector 210 and shaft 220 (when flexible) to be manually arranged (or posed) in any configuration without damaging backend mechanism 230 or creating slack in any tendons 221, 222, 223, or 224. This “backdriving” capability is generally a desirable property of a surgical instrument, particularly an instrument with a flexible shaft that may be bent or manipulated during instrument insertion while the instrument is not under active control. For example, the flexible shaft can be manually posed, and the tendons within the shaft do not experience undue tension or slack.

[0031] One system for mechanically coupling a motor to a capstan uses a friction connection where a pinion of the motor fits tightly into a bore in the capstan and relies on friction between the pinion and bore. Generally, when the motor rotates the capstan, the tendon wrapped around the capstan will tighten, which can increase the frictional connection when the capstan is sufficiently elastic to shrink in radius in response to the squeezing force from the tendons. FIG. 3A shows an alternative technique for mechanically coupling a drive motor 310 to a capstan 320. With this particular technique, a shaft 312 of drive motor 310 fits into a bore 322 of capstan 320. Shaft 312 has a slot 314, and capstan 320 has a spring loaded lever or other projection 324 that extends into bore 322. Projection 324 can be compressed when shaft 312 is inserted in bore 322, but projection 324 can spring back and engage slot 314 when slot 314 is rotated to become aligned with projection 324. Motor 310 can thus apply a torque to capstan 320. It should be understood that in other implementations, male-female mechanical relationships and their many variations can be reversed (e.g., so that a projection extends from the motor shaft and engages a slot in the capstan).

[0032] Backdriving capabilities of an instrument can also create situations where the orientation of capstan 320 relative to shaft 312 may be unknown when shaft 312 is inserted into bore 322. Regardless of the relative orientation, with the mechanical coupling of FIG. 3A, motor 312 can simply rotate shaft 312 until projection 324 engages slot 314. This ability is particularly important when a backend mechanism 330 includes multiple capstans arranged in an array to engage shafts 312 of motors in a motor pack 340 as illustrated in FIG. 3B, where all of the motor shafts 312 may have different and unknown orientations relative to the bores 322 of associated capstans.

[0033] The drive motors coupled to backend mechanism 230 of FIGS. 2A and 2B are generally operated to either apply a torque to the associated capstan or to allow the associated capstan to turn freely. In a process that controls a pair of tendons that are connected for rotation of a structural member of instrument 200, one drive motor applies a torque to a capstan around which one tendon is wrapped, and the capstan about which the other tendon is wrapped is allowed to freely rotate. For example, to rotate jaw 212 counterclockwise in the view of FIG. 2A, motor 242, which engages capstan 232, is activated and turns capstan 232 in a direction that pulls on tendon 222. The resulting torque that tendon 222 applies to jaw 212 tends to rotate jaw 212 counterclockwise toward jaw 214. The maximum tension that capstan 232 can apply to tendon 222 without tendon 222 slipping on capstan 232 is proportional to the force from spring 236 and increases exponentially with the wrap angle of tendon 222 about capstan 232. Accordingly, with a suitable wrap angle for tendon 222 about capstan 232, the force from spring 236 can be much less than the maximum tension that may need to be applied to tendon 222 when rotating jaw 212 against resistance. While capstan 232 pulls in tendon 222, capstan 231 is free to rotate and pays out a length of tendon 221 corresponding to the rotation of jaw 212. Capstan 221 being free to rotate allows the tension in the portion of tendon 212 between capstan 231 and jaw 212 to equalize with the tension in the portion of tendon 221 between capstan 231 and spring 235. Tendon 221 does not become slack, and the tension in tendon 221, which is controlled by spring 235, can be kept much lower than the maximum anticipated tension applied to tendon 222. When jaw 212 reaches the desired position and no clamping tension is required in tendon 222, both capstans 231 and 232 can be released to rotate freely. Clockwise rotation of jaw 212 can be achieved through operation of a drive motor coupled to capstan 231 to pull on tendon 221 while capstan 232 is left free to rotate. Rotations of jaw 214, or movements of other elements of instrument 200, can also be performed using similar procedures.

[0034] The control of jaws 212 and 214 and any joint in end effector 210 or shaft 220 can be effected in a closed loop by basing the motor control torques for that joint on measurement of the current position of the joint. Such measurements can, for example, be done by using a sensing technology such as the one described in U.S. Pat. App. Pub. No. US 2007/0156019 A1 (filed Jul. 20, 2006), entitled “Robotic Surgery System Including Position Sensors Using Fiber Bragg Gratings” by Larkin et al., and U.S. patent application Ser. No. 12/164,829 (filed Jun. 30, 2008) entitled “Fiber optic shape sensor” by Prisco, both of which are incorporated herein by reference. FIGS. 2A and 2B illustrate an illustrative shape sensing fiber 250 that extends to jaw 212 for sensing of the position of jaw 212. For the control loop, a control system compares a desired joint position with a measured joint position to compute a joint position error. A control or correction torque is then computed based on the current value of the joint position error, for instance using a Proportional Derivative Integral control law. Depending on the control torque sign indicating a clockwise or counterclockwise correction, one of the motors associated with the joint is commanded to apply a torque to its associated capstan while the other motor associated with the joint is commanded to zero its motor torque, thereby letting go of its associated capstan. In this way, the joint is placed in the desired position.

[0035] FIG. 4 illustrates portions of a medical instrument 400 in accordance with an embodiment of the invention using three tendons 421, 422, and 423 to control rotation of a structural member 410 about two perpendicular axes defined by pivots 412 and 414. Member 420 may be a link in a wrist or joint mechanism in an instrument shaft or in an end effector. Most of the mechanical structure associated with the joint and pivots 412 and 414 is omitted to better illustrate the operation of tendons 421, 422, and 423, but such joints can be implemented using known mechanical structures. Tendons 421, 422, and 423, which generally may be of the same type and construction as described above with reference to FIGS. 2A and 2B, attach to member 410 and extend back through a shaft 420 to a backend mechanism including capstans 431, 432, and 433 and passive preload systems 435, 436, and 437. Shaft 420 may generally be the same as shaft 220 described above. Also, in a manner similar to described above, each tendon 421, 422, or 423 wraps around a corresponding capstan 431, 432, or 433 and has an end attached to a corresponding passive preload system 435, 436, or 437. Passive preload systems 435, 436, and 437 can be anchored to the case or chassis of the backend mechanism.

[0036] Controlled rotation of member 410 about pivot 412 relies on tendons 421 and 422 being attached to member 410 at moment arms such that pulling tendon 421 or 422 can cause rotation of member 410 about pivot 412 in a direction that is opposite to the rotation that pulling on tendon 423 can cause. Similarly, rotation of member 410 about pivot 414 relies on tendons 422 and 423 being attached to member 410 at moment arms such that pulling tendon 422 or 423 can cause rotation of member 410 about pivot 414 in a direction that is opposite to the rotation that pulling on tendon 421 can cause. The desired relative anchoring of tendons 421, 422, and 423 can be achieved, for example, by terminating tendons 421, 422, and 423 on rays that extend at a 120° angles relative to each other from a central longitudinal axis of member 410. In operation of instrument 400, passive preload systems 435, 436, and 437 and drive motors 441, 442, and 443 create tensions in tendons 421, 422, and 423, and a control system (not shown) can control drive motors 441, 442, and 443 to balance the torques about pivots 412 and 414 as needed to achieve the desired relative orientation of member 410 or overcome resistance to the movement or stationary position of member 410. Sensors (not shown; see e.g., element 250 in FIGS. 2A and 2B) may be implanted in structural member 410 to provide feedback to the control system operating motors 441, 442, and 443. One illustrative example of a structural member 410 is a part of the parallel motion mechanism disclosed in U.S. Pat. App. Pub. No. US 2008/0065102 A1 (filed Jun. 13, 2007) entitled “Parallel Motion Mechanism” by Cooper, which is incorporated herein by reference.

[0037] The instruments described above employ passive preload systems to maintain a minimum relaxed tension in drive tendons. Passive preload systems as described above are readily implemented employing flexible members or springs, such as linear or rotational coil springs. Most common springs produce forces that are at least approximately described by Hooke's law over a range of forces and deflections. In particular, most springs apply a force that is proportional to the deflection of the spring from its equilibrium length. With Hooke's law springs, the tensions in relaxed tendons of an instrument of the types described above will depend on how far the springs are stretched, which in turn depends on the general configuration of the instrument, including the positions and orientations of the joints and effector in the instrument. This variation may not be significant if the range of motion of the tendons is small compared to the stretch already in the springs for biasing. However, simple Hooke's law springs will tend to create a minimal energy configuration for the instrument, and the instrument will tend to approach that configuration when all of the capstans are free to rotate. In practice, as mentioned above, this effect is small since the spring length change can be designed to be small compared to the spring bias. Further, spring systems that apply constant force can be used to avoid or reduce variations in the tensions in relaxed tendons.

[0038] FIG. 5A shows a surgical instrument 500A in accordance with an embodiment of the invention employing a constant-force spring 535A. Instrument 500A includes an end effector 510, a shaft 520, and a backend mechanism 530, all of which can be substantially the same as described for similar elements above. A tendon 521 attaches to end effector 510, extends back through shaft 520 to backend mechanism 530, wraps around a capstan 531, and attaches to constant-force spring 535A. A constant-force spring is a spring that exerts a constant force over the range of motion of the spring. In the embodiment of FIG. 5A, constant-force spring 535A is a rolled ribbon of spring material that is relaxed when the ribbon is fully rolled up. As the ribbon unrolls, the portion of the ribbon near the roll produces the spring force. This spring force remains nearly constant as the ribbon unrolls because the portion of the ribbon that produces the spring force, i.e., the portion near the roll, has nearly the same shape as the spring unrolls.

[0039] Constant-force spring systems can also be produced or approximated using a spring obeying Hooke's law and one or more cams or variable radius pulleys. FIG. 5B shows a surgical instrument 500B containing the same elements as in surgical instrument 500A, with the exception that constant-force spring 535A is replaced with a constant-force spring system 535B that includes a spring element 538 that obeys Hooke's law and a variable radius cam 539. In particular, cam 539 has an input radius that is different from its output radius, with the ratio of the input radius and the output radius being the cam force transmission ratio at the specific angular position of cam 539. As spring element 538 extends, the input force to cam 539 increases proportionally and cam 539 rotates. The cam profile is designed to reduce the cam force transmission ratio as cam 539 rotates an amount that cancels the increase in input spring force. The spring element 538 can also be implemented, for example, as a linear spring, a rotational spring, or a leaf spring or as a compliant member such as a cantilever beam or elastic band. In one particular embodiment, spring element 538 is a rotational spring that is integrated with the cam 539 on the same cam axis. More details on constant spring forces are described in U.S. Pat. App. Pub. No. US 2004/0035243 A1 (filed May 22, 2003) entitled “Counter Balance System And Method With One Or More Mechanical Arms” by Duval, which is incorporated herein by reference.

[0040] The low tension maintained using techniques described above are particularly advantageous in surgical instruments having flexible shafts. FIG. 6, for example, illustrates a medical instrument 600 having an end effector 610 on the distal end of a shaft 620 that is flexible and bent into a curved, sinuous shape. Such bending may result during a minimally invasive medical procedure when end effector 610 and shaft 620 are guided through a natural lumen, such as a portion of the digestive tract of a patient. The shape of shaft 620 will generally be different during different procedures. A tendon 621 used to operate end effector 610 runs through flexible shaft 620 and therefore follows almost the same path as shaft 620. Tendon 621 may have a surrounding sheath 626 that is more rigid than tendon 621 to aid in guiding tendon 621 through shaft 620 and prevent lateral movement of tendon 621 during actuation of effector 610. Bends in shaft 620 can cause tendon 621 to ride on the solid surface of sheath 626, and frictional forces at these bends increase with the tension in tendon 621. Accordingly, reducing the relaxed tension in tendon 621 when shaft 620 is bent can reduce the force that backend mechanism 630 must be able to exert to operate effector 610. Backend mechanism 630 may thus be able to use small components that may not be robust enough to overcome the higher tendon friction that results from higher tendon tension.

[0041] The above-described embodiments illustrate embodiments of surgical instruments having backend mechanisms in which capstans have rotation axes that are generally perpendicular to the direction of the instruments' shafts. However, many mechanical systems for routing and directing tendons are known and could be employed in a backend mechanism to alter configuration of the capstans. FIG. 7A, for example, illustrates an embodiment of a medical instrument 700 having a backend mechanism 730 containing a capstan 723 that has a rotation axis parallel to a shaft 720 on which an end effector 710 is mounted. In this embodiment, a tendon 721 attached to end effector 710 runs the length of shaft 730 into backend mechanism 730. Tendon 721 then winds around a first cam or pulley 731, wraps around capstan 732, winds around a second cam or pulley 733 and attaches to a spring 735 that is anchored in backend mechanism 730. Cams or pulleys 731 and 733 may be shaped to provide a constant tension in tendon 721 when tendon 721 is relaxed, even if spring 735 obeys Hooke's law.

[0042] FIG. 7B shows that the bores of the capstans for multiple tendons in medical instrument 700 can be arranged on the back (most proximal surface) of backend mechanism 730. A docking port for instrument 700 can provide a matching array of motor shafts that fit into the bores of the capstans 732 for use of instrument 700 when instrument 700 is connected to the control system. As described above, many male-female mechanical mating configurations can be used, as well as many ways to rotationally align the capstans and motor shafts.

[0043] FIG. 8 shows an example of a robotically controlled system 800 capable of using medical instruments in accordance with embodiments of the present invention. System 800, which may, for example, be a da Vinci® Surgical System available from Intuitive Surgical, Inc. includes multiple surgical instruments 810, each of which is mounted in a docking port 820 on a robotic manipulator arm 830. Each instrument 810 generally includes an end effector 812, a shaft 814, and a backend mechanism 816, which may be of any of the types described above. In particular, instruments 810 may have shafts 816 that are rigid or flexible. When shafts 816 are flexible for insertion into and following of a natural lumen, robotically controlled and jointed arms 830 may not be needed, and instruments 810 can be mounted in mounting ports 820 on a cart or other fixed structure. Instruments 810 can be made interchangeable, so that the instruments 810 mounted in docking ports 820 can be selected for a particular medical procedure or changed during a medical procedure to provide the clinical functions needed. Instruments 810 can implement many functions, including but not limited to forceps or graspers, needle drivers, and scissors of many different shapes and sizes. An endoscopic camera, for example, a stereoscopic camera, can also be mounted on an arm 840 to provide visual information, particularly images, of the work site at which instruments 810 are operating.

[0044] Docking ports 820 generally include drive motors that provide mechanical power for operation of instruments 810 and systems for establishment of a sterile barrier between instrument 810 and the rest of robotically control system 800. Some suitable sterile barriers are described in U.S. Pat. No. 6,132,368 (filed Nov. 21, 1997) entitled “Multi-Component Telepresence System And Method” by Cooper, which is incorporated herein by reference. Docking ports 820 may additionally include an electrical interface for communication with instruments 810, for example, to identify the type of instrument in the docking port and to access parameters of the instrument. The electrical interface may also convey measurements such as measurements of the position and orientation of effectors 812 or shaft 814. A computer system 850 can receive the measurements and execute software that controls drive motors in the docking ports 820 as needed to manipulate instruments 810 as directed by a surgeon or other medical personnel using system 800 to perform a surgical procedure.

[0045] Although the invention has been described with reference to particular embodiments, the description is only an example of the invention's application and should not be taken as a limitation. Various other adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as defined by the following claims.