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Publication

Modeling and Constraint-based Control of Continuum Robots. Applications in Robotic Surgery.

Book - Dissertation

A growing interest can be seen in the use of compliant instruments or robots in tasks where interaction with fragile structures in the environment is required. The reason for this interest is that the compliant nature of these devices brings in an inherent level of safety. Flexible robots and instruments would, at least initially, bend away upon contact or impact. These actuated compliant structures are often also referred to as continuum robots. The control of these types of robots is much more involved. The distributed flexibility complicates the derivation of kinematic expressions, as these expressions become a function of the interaction forces that are exchanged with the environment. The work in this thesis treats the problems in the domain of modelling and control of continuum robots. At first instance the modelling principles for the kinematics of the continuum robot are adopted from the Cosserat rod theory. Building upon the Cosserat rod model, a method for calculation of the differential kinematics of continuum robots is developed. This relates to the calculation of the robot Jacobian and compliance matrices which account for the robot actuation but also for the tip applied loads. These Jacobian and compliance matrices are then employed in a constraintbased control framework to allow for concurrent position and force control. Controller synthesis followed hereto the principles of the Task Frame and instantaneous Task Specification using Constraints (iTaSC) formalisms. iTaSC is found particularly suitable due to its ability to express and combine control tasks in a natural way. Control tasks can be formulated as combinations of target positions, velocities or forces expressed in an arbitrary number and type of coordinate frames. In this thesis it has been shown how various artificial constraints can be imposed on the control of the robotic system which features large compliance and undergoes large deformation. Here, force/position control of a robots end-effector is shown to be possible while maintaining a rotational center-of-motion at a more proximal part of the instrument. Especially in Minimal Invasive Surgery this capability is considered highly desirable. In the next stage, a pneumatically driven single-section continuum robot has been designed and developed. Actuated with McKibben muscles, the robot features compliant behaviour in interaction with the environment and possesses two degrees of freedom. The developed continuum robot exhibits large axial and torsional stiffness which allowed for simplification of current state-of-theart kinematic models for antagonistically actuated continuum robots. The shape of the developed continuum robot, when operated in free space, can be described by adopting the constant curvature model. This model, despite its wide popularity, exhibits singularities in case when the robot is straight. In this thesis a singularity-free model for calculation of robot Jacobian has been developed. As a final stage of the thesis, a hybrid rigid/continuum robotic mechanism has been developed by serially combining a pneumatically driven continuum robot and a rigid robot. The motivation for the development of such a robotic system lies in the constraints which arise in the scenarios of minimally invasive surgery. For example, by introducing the instrument through the entry point (trocar), the surgeon is effectively losing two degrees of freedom. Her/his motions are restricted to insertion/retraction, rotation and pivoting about the entry point. By using an instrument which offers two additional degrees of freedom past the trocar point, instrument dexterity can be restored, allowing for complete control over position and orientation of the tool. The performance of the hybrid robotic mechanism was then evaluated for scenarios where following of fast trajectories is required. The envisioned application is here minimally invasive heart surgery. Compensation of the heart-beating motion can allow the surgeon to position and orientate the surgical tools more accurately in presence of a moving environment.
Publication year:2015
Accessibility:Closed