Master of Engineering Science
Electrical and Computer Engineering
Patel, Rajni V.
Unlike conventional manipulators where the robot is actuated at discrete joints, continuum robots are actuated continuously in smooth curves. These robots are often dexterous and compact, allowing them to operate in constrained environments during minimally invasive medical interventions. Since the unconventional robot structure often consists of elastic or flexible materials, the corresponding kinematics formulation is significantly more challenging to derive and simulate. This thesis introduces two different but related continuum robot designs: the concentric tube robot (CTR) and the eccentric tube robot (ETR). These designs utilize multiple pre-curved and superelastic nitinol tubes to actuate the robot. This mechanism also leads to an undesirable behavior called "snapping". Based on Cosserat Rod theory, two separate kinematics models are derived, solved, and simulated for CTR and ETR. Additionally, an ETR prototype is designed and constructed for experimental validation. Compared to the simulation, the measured average tip error is about 3.8% of the robot length.
Summary for Lay Audience
A conventional robot manipulator generally consists of rigid links, which means that the robot is actuated at individual joints connecting the rigid links. Continuum robots, on the other hand, describe a different class of robots where the actuation is achieved by continuously changing the shape of the robot in smooth curves. These continuum structures are commonly seen in nature, like an elephant's trunk, an octopus's arms, and a snake's spine. Continuum robots are usually very flexible with a compact design and a slender shape essential for navigating and performing tasks in a constrained environment. For example, robot-assisted minimally invasive surgery is one of the most popular continuum robot applications discussed in the literature. However, their desirable properties come with challenges, such as the non-intuitive robot kinematics (relation between joint inputs and the resulting robot shape). Unlike a rigid-link manipulator where its kinematics is determined based on robot geometry, the continuum robot kinematics model is derived based on elastic material deformations. In this thesis, two different but related continuum robots are introduced: the concentric tube robot (CTR) and the eccentric tube robot (ETR). These robots consist of very elastic tubes made from nitinol (a nickel-titanium alloy), and the robot actuation relies on elastic tube deformations. Based on the physics of tube bending, separate kinematics models are formulated for the CTR and ETR. More importantly, the corresponding forward kinematics solutions are also derived in the thesis, and they are simulated to visualize robot shapes. Finally, to demonstrate the original design concept of the ETR, its kinematics model is validations experimentally. The detailed design and construction of a manually actuated ETR prototype are presented in the thesis. By comparing with ETR simulations, the position error from experiments was about 3.6 mm on average, equivalent to about 3.8% of the robot's total length.
Wang, Lexuan, "Modeling and Simulation of Concentric and Eccentric Tube Continuum Robots" (2020). Electronic Thesis and Dissertation Repository. 7582.
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