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Underwater Robots - Gianluca Antonelli.pdf

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206 8. Interaction Control of UVMSs<br />

scheme proposed in [99], aforce control scheme isanalyzed that does not<br />

require exact dynamic compensation; however, the knowledge ofpart ofthe<br />

dynamic model can always beexploited when available. Extension of the<br />

original scheme to redundant systems is achieved via atask-priority inverse<br />

kinematics redundancy resolution algorithm [78] andsuitable secondary tasks<br />

are defined to exploit all the degrees of freedom of the system.<br />

Force control scheme. Asketch ofthe implemented scheme is provided in<br />

Figure 8.3.Inour case the Inverse Kinematics (i.e., theblock IK in the sketch)<br />

is solved at the differential level allowing to efficiently handle the system redundancy.<br />

The matrix J k ( R B I )isthe nonlinear, configuration dependent,<br />

matrix introduced in (2.58); its inverse, when resorting to the quaternion<br />

attitude representation, isalways defined. The matrix J pos( R B I , q )has been<br />

defined in (2.67). Moreover, astability analysis for the kinematically redundant<br />

case is provided. The Motion Control block can be any suitable motion<br />

control law for UVMS; thus, if apartial knowledge of the system is available,<br />

amodel based control can be applied.<br />

In Section 2.10, the mathematical model of an UVMS in contact with the<br />

environment isgiven.<br />

x p,d, x s,d<br />

˙x c<br />

IK<br />

ζ d<br />

− 1<br />

J k<br />

� +<br />

−<br />

η , q<br />

Fig. 8.3. External Force Control Scheme<br />

8.4.1 Inverse Kinematics<br />

Motion<br />

Control<br />

−<br />

+<br />

J T pos<br />

UVMS<br />

+env.<br />

Let m be the number of degrees of freedom of the mission task. The system<br />

DOFs are 6+n ,where 6are the DOFs of the vehicle and n is the number<br />

ofmanipulator’s joints. When 6+n>mthe system is redundant with<br />

respect to the given task and Inverse Kinematics (IK) algorithms can beapplied<br />

to exploit such redundancy. The IK algorithm implemented is based on<br />

atask-priority approach [78] (see also Section 6.5), that allows to manage<br />

the natural redundancy of the system while avoiding the occurrence of algorithmic<br />

singularities. In this phase we can define different secondary tasks to<br />

PID<br />

+<br />

−<br />

f e<br />

f e,d

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