4 Prerequisites and Conceptual Framework
Out of intense complexities intense simplicities
emerge
Winston Churchill
4.1 Overview
The work-piece based approach calls for imparting full control of the work-piece to the
programmer during a cooperative task. As described in the latter chapter, full control
includes posture of the work-piece and force loading on it in each phase of the given
task. In order to achieve this, tightly coupled cooperative tasks have to be synthesized
and understood. Combining this knowledge with the requirements imposed by the pro-
posed approach on the system architecture, a conceptual framework for implementing
the work-piece based programming approach is presented. This framework represents a
conceptual blueprint for implementing the proposed approach according to generic design
principles.
4.2 Task prerequisites
A comprehensive synthesis of cooperative tasks is introduced in this section. The aim is
to gain a deeper insight into the separate phases and consequently derive the motion and
loading i.e. position and force requirements for each phase. According to Surdilovic et al.
(2001)(Zivanovic & Vukobratovi´c 2006, P. 2-3) nine phases could be identified during
a cooperative task. By omitting the ’planning of the approach’ phase and combining
the ’grasping’ and ’gripping’ phases together, seven phases are left. To cover the phases
in a more task-oriented form, an extra phase ’machining’ will be added to them, while
extending the ’lowering’ phase to include ’assembling’. Thereby, a total of eight phases
will be presented here as shown in Figure 4.1. The synthesis however excludes the case of
manipulators cooperatively assembling a work-piece, where each manipulator is holding
one part of the final work-piece commonly known as dual-arm cooperative assembly (Choi
et al
. 1999).
Phase 1:
Approaching
The manipulators approach their respective grasping locations on the work-piece.
This phase is characterized by a pure positional movement of the manipulator
4 Prerequisites and Conceptual Framework
without interaction with the work-piece hence the need for force control is not
necessary. Due to the shared workspace of the manipulators, mechanisms for
collision detection and avoidance must be taken into account. After reaching
the grasping location, the manipulators’ velocity should be successively reduced.
In this regard, grasping zones could be defined, that place constraints on the
manipulator’s movement once it enters.
Phase 2: Engaging
/Grasping
After entering the grasping zone in the latter phase the manipulators start moving
to their predefined grasping positions where they come for the first time in
contact with the work-piece. During this phase, two aspects have to be taken into
consideration. The first is that only one manipulator at a time should attempt
interacting with the work-piece otherwise unexpected movement of the work-
piece can happen. The second aspect concerns the force
/position constraints
during grasping. If the manipulators possess a relatively good positional accuracy
and the work-piece is exactly positioned at the predefined position, no forces are
bound to arise. Otherwise forces will immediately appear right after grasping,
which represents the usual case given positional discrepancies on the factory
floor. Additionally, force control here should be activated only after the grasping
phase is terminated and deactivated once another manipulator starts its respective
grasping phase. In order to consider all the latter aspects a grasping strategy
should be implemented. This has to include the order in which the manipulators
start and stop their grasping phase which implicitly includes the force and
position constraints on each manipulator.
Phase 3:
Lifting
The main aim of this phase is to allow the manipulators to detect and distribute
the load among themselves. It begins when all manipulators are tightly grasping
the work-piece i.e. no slip or friction is assumed. For synthesis purposes, friction
between the gripper and the work-piece is always assumed non-existent. The
center of gravity of the work-piece could be deduced from the force
/torque
distribution and the absolute positions of the manipulators. Before this phase
ends, load distribution on the manipulators could be carried out according to a
predefined distribution ratio (Zheng & Luh 1988)(Hsu 1989). However, it has
to be taken into consideration that the position correction involuntarily leads to
di
fferent force loading on the work-piece.
Phase 4: Transporting
/Positioning
This phase represents the main body of the cooperative task. Whether transport-
ing heavy loads or precisely positioning the work-piece, the objective is to change
the absolute spatial position and orientation of the work-piece or to track a given
predefined path. The main requirement here is of absolute position
/orientation
accuracy in terms of either positioning (point) or tracking (path).
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