5.4 System Design
5.4.2 Actuation Unit Design
In the course of this work, we have designed and built two separate actuation units for controlling concentric tube robots. The first was a bimanual, or two arm, system that was our initial prototype for investigating the use of concentric tube robots in endonasal skull base surgery. The second was a quadramanual, or four arm, system aimed at providing all four surgical tools we envisioned using during the procedure through a single nostril at once. Both actuation units are described and shown below.
Bimanual System
We designed and built the robotic actuation unit shown in Fig. 5.5 to coordinate the motion of all the tubes (axial rotation and translation at tube bases). In order to actuate these degrees of freedom, there is an individual carrier for each tube which contains two encoded motors (RE 339152, Maxon USA). These were selected based on a desired max- imum torque of 0.25 Nm and translational speed of 4 cm/s for teleoperation (torque and speed requirements were qualitatively determined). On a given carrier, translation is ac- complished by one motor using a worm gear to spin a nut that rides on a stationary lead screw. The rotation mechanism on a given carrier also uses a worm gear to spin the spring collet used to grasp the base of its respective tube. Use of a collet closure system permits easy replacement of tubes or changes in tube diameters as needed.
The carriers are affixed to a Frelon self-lubricating guide block that rides on an alu- minum guide rail. For each three tube robot, three carriers ride on a single guide rail, making up one actuation module. Based on the required workspace, a major design re-
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Figure 5.5: Prototype bimanual concentric tube actuation unit. (1) Concentric tube robot with gripper. (2) Actuation module for one concentric tube robot. (3) Carrier associated with one tube.
(4) Lead screw for translation of the carriers. (5) Collet closure for grasping a tube. (6) Guide rail.
quirement for the prototype actuation unit is the dimension of 35 mm at the nostril entry, which restricts the maximum distance between the concentric tube robots. In this design, we have mirrored two modules and placed them next to each other, which locates the tubes 35 mm apart. The lengths of the lead screws and guide rail can be adjusted in order to accommodate longer relative travel lengths of the tubes.
Quadramanual System
In designing the quadramanual system, a key objective was to keep the base entry points of all four concentric tube manipulators within the envelope imposed by the nostril opening, similar to the design restraint placed on the bimanual system. The quadramanual system is capable of delivering up to four surgical instruments to the skull base through a single nostril. Each surgical instrument is delivered to the site via a manipulator module, with the module controlling a six degree of freedom concentric tube manipulator. The quadra-
Figure 5.6: (a) CAD drawing of the quadramanual system with (b) one arm removed from the main body and (c) an overhead view of all tubes exiting the robot within the anatomical nostril constraint.
(d) Photograph of the complete quadramanual actuation unit showing the modular design and 80/20 backbone. A close-up of the concentric tube robots shows the various end effectors we envision for our endonasal system (ring curette, gripper, suction, and chip tip camera with light source).
manual system consists of four copies of this manipulator module that are mirrored with respect to one another (see Fig. 5.6(a)), allowing the tube bases to exit the robot within the 16 mm by 35 mm opening.
The four manipulator modules are supported by a frame constructed from extruded alu- minum (80/20 Inc., USA, shown in Fig. 5.6(d)). This frame serves the secondary purpose of providing a convenient method for mounting the robot to a support arm in the operating room. Since the individual extruded members use slotted connections, the clearances and alignment between manipulator modules can be easily adjusted. The modular nature of the design also allows for each manipulator module to be removed/replaced without having to disassemble the entire robot.
Each of the four manipulator modules contains three identical single-tube stages (Fig. 5.7).
Each single-tube stage controls one tube in two degrees of freedom (axial translation and axial rotation). The single-tube stage translates via a captive acme nut that rides on a 3/8-8, 4-start acme lead screw actuated remotely by a DC motor (#339152, Maxon USA) with a 53:1 planetary gearhead. Rotation is accomplished by a motor-pulley-belt system mounted to the stage with an effective gear ratio of 689:17 (≈40.53:1), with the same motor as-
Figure 5.7: Single-tube stage assembly. Exploded view with (1) cannula holder assembly, (2) sleeve bearing, (3) thrust bearings, (4) jam nuts, (5) acme nut, (6) rotation motor and bracket, (7) cable carrier mounting holes, (8) cannula stage body, (9) motor timing pulley, (10) timing belt, (11) stage mounting bracket, and (12) carriage.
sembly being used to rotate the tube. These motors were selected because they provide an excellent power/volume ratio, and will be capable, over a wide range of possible tube designs, of drastically exceeding the design specifications. Each stage is mounted to a PTFE-lined sleeve-bearing, which rides on a shared track (i.e., all three single-tube stages in a given manipulator module share the same track).
Each single-tube stage is designed to accommodate a range of tube sizes. In previ- ous concentric tube robot actuation units, adaptation to different size tubes was accom- plished by a collet system, in which, if a new tube diameter was needed, one could replace a commercially available collet with another collet size [186]. While this tube gripping mechanism design is highly flexible, it requires the manipulator modules to hold their re- spective tubes at a distance of at least one collet diameter from one another. Since we wish to minimize this intra-manipulator distance, instead of collets, each tube was permanently bonded to a cylindrical brass housing (4 mm OD). Each brass housing was bonded to an aluminum pulley on one face, and threaded on the other to secure it to the single-tube stage
Figure 5.8: a) Photograph of a single manipulator module, b) a close up view of concentric tube robot tip and c) a single tube stage.
(Fig. 5.7). In the assembled device, the brass housing around the tube is supported inside a low-friction sleeve bearing, and secured axially against front and rear thrust bearings by two M4 jam nuts.
Up to three single-tube stages can be mounted on a single manipulator module in our current embodiment (Fig. 5.8). Staggering the motors on each single-tube stage in the three possible mounting locations in the lateral direction (with respect to the tube axis), implies that the minimum distance between stages is limited only by the thickness of the linear bearings that support the single-tube stage on the track. The front end plate on the single track assembly supports the three lead screws via bronze bushings. The lead screw motors (which accomplish translation of each single-tube stage) can be mounted to the front or rear end plates. As in the bimanual system, the lead screws and guide rails are the only components that need to be changed to alter the available travel of the single-tube stages.
Each motor is coupled to a lead screw using a flexible coupling which supports axial and radial loads exerted on the screw.