Underwater Robots - Gianluca Antonelli.pdf

[deg]
3
2.5
2
1.5
1
0.5
7.8 Output Feedback Control 163
without adaptation
with adaptation
0
0 20 40 60 80 100
time [s]
Fig. 7.11. Adaptive control. Comparison between adaptive and PD +dynamic
compensation in terms of joint position error absolute values
fromgyroscopic sensors and/or compasses.Velocitymeasurements are usually
obtained from sensors based on the Doppler effect.
In the case of underwater vehicles operating close to off-shore structures,
position and orientation measurements are fairly accurate, while velocity
measurements are poor, especially during slow maneuvers. Hence, itisworth
devising algorithms for position and attitude control that do not require direct
velocity feedback.
Anonlinear observer for vehicle velocity and acceleration has been proposed
in [125, 129], although acombined controller-observer design procedure is
not developed. On the other hand, apassivity-based control law isproposed
in [193], where the velocities are reconstructed via alead filter; however, this
control scheme achieves only regulation of position and orientation variables
for anunderwater vehicle-manipulator system.
In this Section, the problem of outputfeedbacktracking control of UVMSs
is addressed. The output ofthe controlled system is represented by the position
and the attitude of the vehicle, together with the position of the manipulator’s
joints. Remarkably, the unit quaternion isused to express the
orientation of avehicle-fixed frame so as to avoid representation singularities
when expressing the vehicle attitude.
The new control law here discussed isinspired by the work in [45] in that
amodel-based control law isdesigned together with anonlinear observer for
velocity estimation; the two structures are tuned to each other in order to
achieve exponential convergence tozero of both motion tracking and estimation
errors. It must be remarked that differently from the work in [45], where
asimple time-derivative relates position and velocity variables atthe joints,
in the control problem considered in this Chapter, anonlinear mapping exists
between orientation variables (unit quaternion) and angular velocity ofthe