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Tesi etd-06292018-112751

Thesis type
Tesi di laurea magistrale
Magnetic Capsule Levitation: A Novel Dynamic Control Approach
Corso di studi
relatore Prof. Avizzano, Carlo Alberto
relatore Prof. Valdastri, Pietro
controrelatore Prof. Caiti, Andrea
Parole chiave
  • robotic colonoscopy
  • Magnetic capsule
  • Levitation
  • magnetic actuation
Data inizio appello
Secretata d'ufficio
Data di rilascio
Riassunto analitico
Standard colonoscopy has been shown to induce tissue stress which is the primary cause of discomfort during the procedure and often leads to the need for sedation.
In the present work, we discuss a novel approach to force control for the Magnetic Air Capsule (MAC). This magnetically actuated robotic system was introduced for painless colonoscopy, since able to reduce tissue stress. In fact, as the guidance is performed by magnetically actuating the capsule tip, tissue deformation is limited.
The advantages of using permanent magnets, one external (External Permanent Magnet (EPM)) and one inside the capsule, over electromagnetic actuated systems, are the limited size of the platform and energy consumption. This is paid by some lack of controllability, to be balanced with accurate and effective control techniques.
The limitations of previous works, concerning the control of the MAC, are related to the absence of a closed-loop control of the force and control in the z direction. Therefore, force is not ensured to converge to the required value and the imposed coupling force leads the capsule to exert excessive force on the colon wall. The latter turns into being more invasive and reduces smoothness of motion due to an increase in friction, presence of folds and deformation of the wall. So far, a kinematic control approach is used. More precisely a PID control on the capsule position is implemented. The main issue of these solutions is that the capsule moves alongside the colon wall, then being affected by friction and stuckness.
The main goal of the work presented here is the design of a closed-loop controller, which is able to control the pose of a magnetic capsule. In particular, we aim to control the position of the capsule to be in the center of the colon, overcoming the previously mentioned limitations. The main difficulty faced is the fact that magnetic forces acting on the capsule generate unstable equilibria. A possible solution that allows the capsule to maintain this equilibria is applying a dynamic controller on the capsule pose. The particular approach we propose solves the problem of capsule levitation. The study of this problem is quite important in order to guide the capsule inside the colon. In fact, the structure of the colon is composed by many folds preventing a smooth control of the capsule. The dynamic control
was chosen because this approach allows us to take into account all forces that act on the system. In particular: the coupling between magnets is easily expressed in terms of interaction (generalized) forces and levitation arises from the equilibrium of these forces with gravity. Therefore, controlling the forces that act on the system allows us to maintain the capsule in any unstable equilibria and achieve levitation.
In particular, we implemented two different type of controllers. The first one, the Gravity Compensation PID Control (GCPID) takes into consideration only the compensation of the gravity on the z axis. The second one, Adaptive Backstepping Control (ABC), a more complex controller, takes into consideration all the forces acting on the MAC. Some simulations and experiments were performed to prove the strength of the proposed approach.