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A straight path never leads anywhere except to the
objective
Andre Gide
3.1 Motivation
Despite the fact that most commercial robot manufacturers o
ffer cooperating robotic
systems in some form, either as an add-on technology or through a dedicated central
controller (ref to section 2.5), their wide spread acceptance in production facilities have yet
to materialize. This is attributed to a multitude of problems concerning their programming
and operation. The discussion in the previous chapter highlights the shortcomings of both
existing research and industrial practices in the field of CIR. Accordingly, these issues can
be summarized as follows:
• The conventional method of programming tightly coupled CIR by off-line program-
ming with on-line adaptation represents a technically complex process and hence
incurs high costs. Furthermore, the complexity increases disproportionately with
the number of robots involved in the planned operation (Reinhart & Zaidan 2009).
• Synchronization schemes in CIR are limited to kinematic signals (position and its
derivatives) (Gerstenberger et al. 2005). Hence no provision for dynamic signals
(force and torques) and consequently no force control is possible.
• Although stresses build up in the work-piece during operation due to inaccurate
positioning and
/or deflection through loading, force measurement and subsequent
control in CIR in an industrial context is rarely investigated (Spiller & Verl
2010)(Reinhart et al. 2010b).
• Even if the dynamic loading on the work-piece is determined, integrating this
information in the architecture comes as an afterthought. With the exception of
a few research works (Schneider & Cannon 1993)(Surdilovic et al. 2001), tight
integration of dynamic loading in programming and throughout task execution is
virtually non-existent in industrial systems.
• Research activities were fixated on proving the validity of control schemes for
cooperating manipulators but hardly investigated the integration of force control in
programming and subsequent operation.
3 Motivation and Objective
• Providing simple programming techniques through integration of intuitive HMI is
seldom discussed in the context of CIR.
• Most experimental test-rigs use research components instead of commercial off-the-
shelf components and hence make it more di
fficult to replicate the performance on
an industrial test-rig.
3.2 Objective
Most CIR programming approaches focus on the robotic manipulators, irregardless of the
type of cooperative task (tightly or loosely coupled). Given that for tightly coupled CIR
only one work-piece is manipulated, it would be beneficial to focus on the work-piece
instead of the manipulators themselves. Hereby the objective of this research can be
derived as follows:
Simplifying the process of programming tightly coupled cooperating in-
dustrial robots by investigating an approach for programming based on
defining the required task of the work-piece and not of the robots. The
approach requires imparting full control of the work-piece to the user.
The idea builds upon and extends the work done by several other researchers albeit with
focus on the programming aspect and not the control aspect solely. Particular inspirations
for this research work are the operational space concept developed by Khatib (1987), the
object impedance for cooperative manipulation developed by Schneider & Cannon (1992)
and the cooperative bilateral teleoperation scheme investigated by Lee & Spong (2005). As
commonly known for robotic manipulators, defining equations of motions is done either in
the generalized coordinates also known as the joint space or in the operational space also
termed the task space (Khatib 1987). In the latter formulation, the manipulator’s motion is
allowed to be defined according to the task at hand and not the manipulator’s structural
characteristics as in the former case. Although the operational space formulation contains
an implicit definition of the manipulator’s dynamics, it specifically moves the focus of the
corresponding control analysis to the forces acting on the end-e
ffector of the manipulator
as seen in Figure 3.1. Similarly, the work-piece based programming approach (WPBA)
moves the focus of programming from the robots to the work-piece. Thereby building
on and extending control concepts pertaining to cooperating manipulators to cover the
programming aspect.
On the rationale for a work-piece based approach
The WPBA encompasses a methodology to define and satisfy practical task objectives
formulated in the work-piece space. This is done by imparting to the user full control over
the work-piece in spatial coordinates irregardless of the manipulator’s configuration. By
extension, since the object lies in the operational space of the task i.e. the work-piece
space, and the cooperative task mainly entails manipulation of the object, the work-piece
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3.2 Objective
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