Underwater Robots - Gianluca Antonelli.pdf

202 8. Interaction Control of UVMSs
effectively obtained by closing an external force feedback loop around aposition/velocity
feedback loop [101], since the output of the force controller
becomes the reference input to the standard motion controller of the manipulator.
Nevertheless, many manipulation tasks require simultaneous control of
both the end-effector position and the contact force. This in turn demands
exact knowledge of the environment geometry: the force reference, in fact,
must be consistent with the contact constraints [202].
Afirst strategy is the hybrid force/position control [239]: the force and
position controllers are structurally decoupled according to the analysis of
the geometric constraints to be satisfied during the task execution. These
control schemes require adetailed knowledge of the environment geometry,
and therefore are unsuitable for use in poorly structured environments and
for handling the occurrence of unplanned impacts.
To overcome this problem, the parallel force/position control can be adopted
[79]. In this case, position and force loops are closed in all task-space
directions, while structural properties of the controller ensure that aproperly
assigned force reference value isreached at steady state. Since the two loops
are not decoupled, adrawback of parallel control isthe mutual disturbance
of position and force variables during the transient.
Most force control schemes proposed in the literature donot take into
account the possible presence of kinematic redundancy in the robotic system,
which isalways the case of UVMSs. Reference [171] presents aunified
approachfor motion andforce control, extending the formulation to kinematically
redundant manipulators. Reference [226] proposes two control schemes,
the extended hybrid control and the extended impedance control, and some
experimental results for a3-link manipulator are provided. In [172] the Operational
Space Formulation is experimentally applied to acoordinated task
of two mobile manipulators. Reference [213] presents an hybrid force control,
in which redundancy isused together with the dynamically consistent
pseudo-inverse. Reference [219] proposes an extended hybrid impedance control.
Finally, [211] develops aspatial impedance control for redundant manipulators,
and reports experiments with a7-DOF industrial manipulator. All
the above schemes are based on dynamic compensation. However, while dynamic
model parameters of land-based manipulators are usually well known,
this is not the case of UVMSs for which the application of these schemes is
not straightforward.
In [165] avery specific problem is approach, afloating vehicle, with a
manipulator mounted on board, uses its thrusters and the resorting forces in
order to apply the required force at the end effector.