Design and Control of a Flexible Tube Manipulator for Optical Irradiation of a Superficial Tumor

Abstract

Flexible manipulators have proven their potential in medical applications like minimally invasive surgeries, endoscopies, laparoscopies, colonoscopies, etc. One of the promising areas in medicine where flexible manipulators are required, is cancer treatment through targeted thermal therapy. The treatment method involves irradiation of a targeted superficial tumor in different orientations for prolonged sessions that requires manipulation of a flexible tube-like medical optical fiber bundle by a surgeon. The manual manipulation of the flexible tube makes the treatment process tiring for the surgeon who is accustomed to human error. Thus, an automated solution is required that can actively manipulate the flexible tube around the targeted tumor and thereby, increase the effectiveness of the therapy. This thesis provides the required basis for developing a flexible tube manipulator (FTM) with trajectory control for use in tumor irradiation procedures. To develop the FTM, shape memory alloy (SMA) actuators are selected due to their low weight and size, ease of installation, and excellent biocompatibility. First, the displacement generation capabilities of different SMA actuator configurations like SMA wire-bias spring, antagonistic SMA wires, SMA spring–bias spring and antagonistic SMA springs are investigated. In this direction, numerical implementation of SMA wire-bias spring actuator model is independently carried out by three iterative solvers: Newton-Raphson, implicit differential equation solver, and trust-region-dogleg algorithm and through a comparative evaluation, the implicit differential equation solver (IDES) is found as the most suitable solver. Next, the displacement generation capability of a two-way actuator i.e. antagonistic SMA wire actuator is calculated through the mathematical model. The reported results indicate low displacement generation capabilities of SMA wire actuators. As SMA is a path-dependent material, it is important to include the history of traveled paths in the modeling scheme to bridge the gaps between actual and simulated results. No SMA spring model is found in the literature that considers this effect in SMA spring modeling. Therefore, in the present work, an established loading history based SMA wire phase kinetics model is adapted for the SMA spring which incorporates path history in the form of memory parameters to develop a mathematical model of the SMA spring. Thereafter, the mathematical model of SMA spring normal spring as bias is developed to determine the output displacement. Next, as two-way actuators, the output displacements of SMA spring-SMA wire and antagonistic SMA spring configurations are determined through mathematical modeling. Among all three SMA spring configurations, the antagonistic SMA spring actuator demonstrated the highest output displacement and is selected as the most suitable SMA actuator configuration for FTM. Parametric variations are performed considering variations in both geometrical and material parameters. It is observed that geometrical parameters significantly affect the output force and displacement of antagonistic SMA spring configuration as compared to its material parameters. Further, the effect of arbitrary loading on the performance of the antagonistic SMA actuator is considered as one of the parameters. Higher force and equal displacement generation capabilities are reported under arbitrary thermo-mechanical loading as compared to general thermo-mechanical loading. After developing the actuator model, two pairs of antagonistic SMA spring actuators are selected to bend the tube in two orthogonal planes. The bending mechanics of the flexible tube is determined using a circular arc model and dynamics of FTM is decoupled into dynamics of 4 single-input single-output systems. A complete mathematical model of FTM is then constructed by combining the actuator dynamics with the dynamics of the flexible tube. The parametric variations are performed in simulations considering geometry and effect of pre-strain of SMA spring. The reported results concluded the diameter of the spring wire is the most influencing parameter that considerably affects the performance of the manipulator. To investigate the performance of FTM in trajectory tracking applications, a state space representation of a single SMA spring-based FTM is formulated and a robust controller is applied for tracking different trajectories. The same controller is proposed to develop a model-based control architecture of the complete model of FTM. Due to the unavailability of required sensors and inherent assumptions in the mathematical model, a modeless control scheme is developed and some 2-D shapes like circles, oval, concentric circles, and irregular shapes are considered based on real-life encountered tumor shapes to demonstrate the performance of the proof of concept prototype of FTM. The experimental results demonstrated the capability of FTM to track any arbitrarily shaped tumor trajectory and therefore, confirm the pertinence of FTM in the application concerning optical irradiation of superficial tumors. It is aspired that the modeling and control architecture for SMA actuators developed in this thesis can be utilized in practical applications.