Development of a Flexible Actuator and Motion Planning for Endoscopic Robots

Abstract

Endoscopy is a procedure by which a long flexible device called the `endoscope' is inserted into a patient's gastro-intestinal(GI) tract primarily for diagnosis. An endoscope is typically equipped with a camera, fiber-optic lighting system and nozzle for spraying water or pumping air. Most commercial endoscopes are also equipped with a catheter channel for passing instruments (catheters) for specific treatments and diagnostic procedures. The thesis aims at addressing two common issues faced by endoscopists: 1) Actuation and positioning of the catheter tip at a desired location while maintaining a stationary camera focus and 2) Manoeuvring the endoscope inside the stomach while avoiding the curling of scope and perforation of tissue walls. Efficient methods to solve these problems could reduce the procedure time and hence, overall discomfort experienced by the patients. In order to address the first problem, a flexible end-effector for independently actuating the catheter is developed and analysed. The design uses miniaturized pneumatic artificial muscles (MPAMs) for actuating the end-effector. For analysis and implementation, a mathematical model which accurately predicts the pressure-deformation characteristics of MPAM is necessary and hence, a detailed survey on existing models for PAMs as well as MPAMs was conducted. Comparison between static characteristics of PAMs obtained from different phenomenological models in the literature and experiments conducted on the in-house fabricated MPAMs show that the existing models are either inaccurate or inconsistent with changes in fabrication parameters of MPAMs. Hence, a new and improved mathematical model for the pressure-deformation characteristics of MPAM is derived. For MPAMs with less than 2 mm diameter and lengths ranging from 40 mm to 70 mm, it is shown that the developed model could consistently predict the deformation characteristics of the prototype with less than 5% error. An end-effector prototype which uses three MPAMs for actuation is fabricated and tested. The prototype which is 55 mm long with an outer diameter of 8 mm could detect a commercial forceps catheter tip by about 20 mm in different directions. An iterative scheme for the forward kinematics of end-effector which takes into account the static characteristics of MPAMs is also developed. The forward kinematics model could predict the final pose of the end-effector with a maximum error of 2 mm at the tip. An inverse kinematic strategy, using the projection of the workspace of the end-effector is developed and the end-effector actuation is implemented in real-time, taking input from a thumb-stick. The second problem faced in endoscopy is partially addressed by proposing the use of a multi-segmented continuum endoscopic robot. To this end, a new optimization based approach to solve forward kinematics of a single segment of the robot is presented at first. Actuation of the continuum robot in 2D plane is mathematically proven to provide the exact configuration as that obtained from differential geometry based methods. Simulations conducted with different number of segments also validate the same, barring the cumulative errors arising from the numerical solution procedure. The method is extended to 3D and is also verified using numerical simulations. For the multi-segmented robot, a motion planning algorithm to con ne the travel of the robot within the GI tract is developed. Different methods to represent ducts in 2D and 3D are discussed and a tractrix based optimization scheme is developed for each representations. Motion of an endoscope through GI tract is simulated using a GI tract pro le obtained from the CT scan data of human viscus. The proposed method is shown to confine the movement of the endoscope within the tract, while emulating realism.