Underwater Robots - Gianluca Antonelli.pdf

6. Kinematic Control of UVMSs
“. ..mais de toutes les sciences la plus absurde, à mon avis et celle qui est la
plus capable d’é touffer toute espè ce de génie, c’est lagé omé trie. Cette science
ridicule apour objet des surfaces, des lignes et des points qui n’existent pas
dans la nature. Onfait passer en esprit cent mille lignes courbes entre uncercle
et une ligne droite qui le touche, quoique, dans la réalité, on n’ypuisse pas
passer un fétu. La géomé trie, en vérité, n’est qu’une mauvaise plaisanterie.”
Voltaire, “Jeannot et Colin” 1764.
6.1 Introduction
Arobotic system is kinematically redundant when it possesses more degrees
of freedom than those required to execute agiven task. Ageneric manipulation
task is usually given in terms of position/orientation trajectories for
the end effector. In this sense, an Underwater Vehicle-Manipulator System
is always kinematically redundant due to the DOFs provided by the vehicle
itself. However, it is not always efficient touse vehicle thrusters to move the
manipulator end effector because of the difficulty ofcontrolling the vehicle
in hovering. Moreover, due to the different inertia between vehicle and manipulator,
movement ofthe latter is energetically more efficient. On the other
hand, reconfiguration ofthe whole system is required when the manipulator
is working at the boundaries ofits workspace or close to akinematic singularity;
motion ofthe sole manipulator, thus, is not always possible or efficient.
Also, off-line trajectory planning is not always possible in unstructured environments
asincase of underwater autonomous missions.
Whenamanipulationtask hastobeperformed with an UVMS,the system
is usually kept in aconfined space (e.g., underwater structure maintenance).
The vehicle is then used to ensure station keeping. However, motion of the
vehicle can berequired for specific purposes, e.g., inspection of apipeline,
reconfiguration of the system, real-time motioncoordination whileperforming
end-effector trajectory tracking.
According tothe above, aredundancy resolution technique might beuseful
toachieve system coordination insuch away as to guarantee end-effector
tracking accuracy and, at the same time, additional control objectives, e.g.,
G. Antonelli: Underwater Robots, 2nd Edition, STAR 2, pp. 105–140, 2006.
© Springer-Verlag Berlin Heidelberg 2006