Underwater Robots - Gianluca Antonelli.pdf

46 3. Dynamic Control of 6-DOF AUVs
into account the effect ofthe ocean current. The works [34, 35, 125] consider
asix-DOF control problem in which the ocean current iscompensated in
vehicle-fixed coordinates; since the current effects are modeled as an external
disturbance acting on the vehicle, there is no need for additional sensors.
In [145, 243, 244] adifferent approach isproposed for the three-DOF surge
control of the vehicle Phoenix: the current, ormore generally, the sea wave,
is modeled by an Auto-Regressive dynamic model and an extended Kalman
filter is designed to estimate the relative velocity between vehicle and water;
the estimated relative velocity isthen used byasliding-mode controller
to drive the vehicle. Inthis case, additional sensors besides those typically
available on-board are required. The controller developed in [145] has been
also used for the NPS ARIES AUV [201]. Reference [133] reports an algorithm
for the underwater navigation of atorpedo-like vehicle in presence of
unknown current where the current itself is estimated by resorting to arange
measurement from asingle location.
Acommon feature of all the adaptive control laws proposed in the literature
isthat they are designed starting from dynamic models written either
in the earth-fixed frame or in the vehicle-fixed frame. Nevertheless, some hydrodynamic
effects are seen as constant inthe earth-fixed frame (e.g., the
restoring linear force) while some others are constant inthe vehicle-fixed
frame (e.g., the restoring moment).
In this Chapter acomparison among the controllers developed in [13, 121,
122, 125, 280, 320] is shown. The controllers have been designed for 6-DOFs
control of AUVs and they do not need the measurement ofthe ocean current
(when it is taken into account). The analysis will mainly concern the controllers
capacity tocompensate for the persistent dynamic effects, e.g., the
restoringforcesand the ocean current. Foreachcontroller areduced version is
derived and eventually modified so as to achieve null steady state error under
modeling uncertainty and presence ofocean current. It is worth noticing that
the reduced controller is not given by the Authors of the corresponding paper;
this has to be taken into account while observing the simulation results.
The reduced controller will be developed in order to achieve aPDaction
plus the adaptive/integral compensation of the persistent effects, i.e., it can
be considered as the equivalent ofanadaptive PD+gravity compensation for
industrial manipulator. In other words, the velocity and acceleration based
dynamic terms of the model will not be compensated for. Numerical simulations
using the model of ODIN (Omni-Directional Intelligent Navigator) [320],
have been run toverify the theoretical results.
For easy of readings, Table 3.1 reports the label associated with each
controller.