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Underwater Robots - Gianluca Antonelli.pdf

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7.7 Adaptive Control 157<br />

of gravity and buoyancy compensation, while they are null at steady state in<br />

view of the particular final system configuration. It can be recognized that<br />

control generalized forces are smooth while the task is successfully executed.<br />

[deg]<br />

0.5<br />

0<br />

q 2<br />

q 1<br />

−0.5<br />

0 5 10 15<br />

time [s]<br />

100<br />

80<br />

60<br />

40<br />

20<br />

0<br />

τ q,1<br />

τ q,2<br />

0 5 10 15<br />

Fig. 7.7. Sliding mode control. Left: manipulator joint errors. Right: manipulator<br />

joint torques. The steady state vehicle moment and manipulator torques are null<br />

due tothe restoring force characteristic ofthis specific UVMS<br />

7.7 Adaptive Control<br />

Adaptive Control is awide topic in control theory. The basic idea of adaptive<br />

control is to modify on-line some control gains to adapt the controller to<br />

the plant’s parameters that are supposed to be unknown or slowly varying.<br />

However, the term adaptive control can assume slightly different meanings.<br />

In this work, acontroller is considered adaptive if it includes explicitly on-line<br />

system parameters estimation.<br />

Often, the mathematical model of the system to be controlled is known<br />

but the dynamic parameters are not known ormay depend from aload.<br />

In this case, the adaptive control is mainly based onaPD action plus a<br />

dynamic compensation the parameters of which are updated on-line. This<br />

dynamic compensation can be intended to cancel the system dynamics, thus<br />

achievingdecouplingand linearization of thesystem, or preservethe passivity<br />

properties of the closed loop system [223].<br />

While the first concepts of adaptive control appeared, without success, in<br />

the aircraft control in the early fifties, the first robotic applications appeared<br />

later [223, 267].<br />

Control law. Based on the control law developed in the previous Section,<br />

acontrol law ispresented for the tracking problem of UVMSs. As in the<br />

previous control law, to overcome the occurrence of kinematic singularities,<br />

the control law isexpressed in body-fixed and joint-space coordinates soas

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