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A robotic manipulator is frequently used to
move objects or operate on surfaces or both. In moving
a part, it undergoes free-space motion and is an open kinematic chain. While
operating on a surface/part it is a closed kinematic chain
and operates in constrained space. The latter task is referred to
as an interaction or contact task (or process). Fig. 1 below shows
the dual arm performing contact tasks. The process parameters can
be legion, the predominant ones being force and velocity of the
interfacial element, which directly impact the quality/performance
of the task. The term Process should be intrepreted as the
collection of all progressive events that lead to execution of the
interaction task with acceptable performance and optimal input
demands.
Fig 1. Dual Arm Performing Contact Tasks (Left: Preparing
for Drilling; Right:
Sawing)
Qualitative requirements for a contact task,
when converted to numerical specifications in the force and velocity
domains, frequently involves a desired motion plan and reference force
trajectory. Together, these requirements in the force and velocity
domains may be termed as a Process Plan . The mathematical representation of the physics of
interaction between a robot and the environment is referred to as
the Process Model . Such a process model may be
represented in the form of equations between the Process
Parameters.
Alternatively their variation with respect to
one another may be encapsulated within look-up tables determined
experimentally over the entire operational regime of the system.
These look up tables are called Process Performance Maps .
Having developed the process plan and model, the next issue is
to ensure successful completion of the process using a
decision-making (or control) system. This last step is called
Process Control.
Note that the terms motion control and force control have been
used in the literature with the connotation of energy transfer in
the mechanical systems domain. We use Process Control in a
more generic sense to include all parameters (above and beyond the
mechanical domain, such as the thermal domain) that affect task
performance. Hence force/motion control are special cases of process
control.
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