Rigid Mechanism with Uniform, Variable Curvature Kaitlin Oliver Butler | Andrew Orekhov | Jake Childs | Caleb Rucker

UTK Women in STEM Symposium April 16, 2016

Background

Results

Recent robotics research has begun looking at slender, elastic manipulators, also known as “continuum” robots, for minimally invasive surgery (MIS). Known for their ability to follow a curved path, many of these robots are already in use today: e.g. tendon-actuated catheters and concentric-tube steerable needles. However, the compliance that gives these manipulators their range of motion also reduces their ability to withstand externally applied forces as well as deliver forces to tissue through the end effector. While rigid instruments can do this, they cannot conform to a non-linear path; thus, operations that require a high force output at the end of a nonlinear path are difficult to perform with currently available MIS tools.

A small-scale prototype was constructed with machine screws and 3Dprinted parts. Two Kevlar thread tendons were routed through holes in the horizontal connection links (as shown in Figure 4) and used to manually actuate the prototype from the end of a tube, demonstrating the use of this design as a wrist. The prototype has a 4x5 mm rectangular lumen for easy passage of flexible instruments. As shown in Figure 5, the prototype was able to sustain an applied external load of 0.5 N in the plane of curvature in various actuated positions. In transverse loading (perpendicular to the plane of curvature), the prototype could hold a load of 0.98 N in the same positions.

Figure 1: A single, module-linking bar forms a secondary mechanism through shared bar sections with the primary mechanism.

Figure 2: According to the Side-Side-Side postulate, a crossed four-bar mechanism forms two congruent triangles.

Design Methods A crossed four-bar mechanism as shown in Figure 2 can be concatenated with one or more crossed mechanisms, sharing a common link (the top bar of one mechanism is the bottom bar of the subsequent one). Figure 1 shows that using an additional bar to link the corresponding crossed bars of two independent mechanism modules creates a secondary mechanism from the module-linking bar and sections of the primary mechanism. An appropriate geometry as shown in Figure 1 forces the primary modules to exhibit the same angular deflection and reduces the mechanism to a single degree of freedom (DOF), α or θ, as shown in Figure 3. This creates a rigid manipulator arm with actuatable curvature. Connecting two parallel mechanism chains to form a square tube from them creates an arm or wrist with a large lumen that can be used as a passage for flexible surgical instruments to the end effector. Actuation tendons can be routed through the intermediate links that connect the two mechanism chains. If more DOF are desired, chains can be concatenated and not connected with the module-linking bar, making them independent of one another.

Figure 3: Multiple mechanisms can be chained in order to form a longer manipulator.

Figure 4: Shown with a penny as reference to scale, the 3D-printed prototype is 10x22 mm and approx. 30 mm long, a size appropriate for MIS.

Figure 6: Shown here is the concept prototype for a 2-DOF, planar device. It can move in both an s-shape as well as in a continuous curve. The circle is a penny for a reference to size.

Figure 7: This is the workspace of the 2-DOF, planar manipulator with 1-DOF rotation at the base added. The chains were assumed to have ±180° of motion

Conclusions/Future Work Figure 5: The device prototype can resist a force of 0.5 N without visible deflection in various actuated positions..

This mechanism can be created at scales appropriate for MIS. Future work will involve the construction of multi-degree-of-freedom devices, as shown in Figure 6, and various analyses with respect to kinematics, workspace (see Figure 7), control, and strength characteristics.