Drive train improvements and performance evaluation of a robotically steered needle

Abstract

The focus of this thesis is on the design and evaluation of a robust drive mechanism intended to robotically steer a thermal ablation electrode. The device is intended to enable more efficient kidney tumor ablation therapy guided by CT scanning. The steering mechanism is based on the concept of substantially straightening a pre-curved Nitinol stylet by retracting it into a concentric outer cannula, and re-deploying it at a different position. A drive mechanism was previously designed to actuate the cannula and stylet motions. While successfully demonstrating the concept, the previous prototype used a screw-spline mechanism that had a key and groove that were difficult to manufacture, assemble and align. To address the issue of the key being misaligned with the spline nut, an alternative, more robust means was considered for constraining the screw-spline from rotating. This was achieved through the design of a new profiled threaded shaft and nut that reduced the part count and simplified manufacturing and assembly. To determine the optimal parameters for the profile shape, an analytical expression was derived that related the tolerance between the nut and shaft to the angular backlash, which was validated using SolidWorks. A deterministic design process was then followed to size the actuators and transmissions for such drive mechanisms given a set of specifications for cannula/stylet force and speed. Specifically, the gear train was designed to ensure sufficient force for deploying a pre-curved needle from a cannula and into a gelatinous solid based on previously collected experimental data. Using this process, Faulhaber AM1524 micro-stepper motors with 15A planetary gearheads were selected that could provide 10 N and 20N of force to both the cannula and stylet (in line with the screws) at a speed of 7 mm/s. Accuracy and repeatability of the drive mechanism were measured in a bench-top experiment using calipers and a protractor and yielded sub-mm values for translation and approximately two degrees of play for rotation. Targeting experiments were also performed in ballistics gelatin and ex-vivo tissue samples where the distal tip of the stylet was recorded with a camera and CT scanner and demonstrated that the device is ready for testing with an integrated thermal ablation system.