Tech Briefs Magazine - October 2023 - MD-7

effort is unlikely to result in a substantially
better design.
Choosing
Initial Design
Sample:
Choosing the initial design is challenging,
given the enormous number
of possible options. Where to start?
Experienced robotic engineers may
have some intuition for what heuristics
to use to narrow the space a bit. For
example, we might expect every robot
to be allocated approximately the same
number of tasks. However, even this
intuitive heuristic fails to account for
the fact that each task - as well as moving
to that task - can take a different
amount of time. Other heuristics might
involve assigning tasks to robots that are
closer to them, as well as sequencing
tasks to minimize total robot motion.
Evaluating Design Samples: To evaluate
a design, one must determine its performance.
And, to determine its performance,
one must create a motion plan for
the robots. Motion planning is the process
of computing how to get the robots
from task to task without hitting any obstacles
or other robots. Motion planning
is a historically difficult computational
problem, even for a single robot, and choreographing
multiple robots is extremely
difficult. In fact, motion planning takes so
much effort that it is the primary reason
why multi-robot workcell designs tend not
to achieve their performance potentials.
Because the choreography is so difficult,
engineers frequently use " interference
zones " to simplify the task. If two
or more robots can potentially occupy
a part of the workcell, that volume is
considered an interference zone and
requires a robot to obtain a lock before
entering it, such that only one robot
can be in a given interference zone at
a time. While interference zones make
motion planning easier, they sacrifice
considerable performance, because it is
frequently possible that multiple robots
could have operated in an interference
zone without collision.
In addition, because finding a good
motion plan takes so long, it limits
how many samples can be evaluated
in a practical amount of time. With
slow motion planning, engineers can
typically only consider a handful of designs.
Evaluating fewer designs means
that there is less chance of finding an
excellent design.
Choosing Subsequent Design Samples:
Based on the evaluation of the most
Motion Design, October 2023
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recent sample, as well as the evaluation
of any previous samples, engineers must
choose the next sample. Many algorithms
are possible here. Typically, these
algorithms trade off the desire to sample
near to a previous sample that is considered
good, versus the desire to sample
far away so as to avoid getting trapped in
a local optimum.
Optimizing Multi-Robot Workcell
Productivity
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Tech Briefs Magazine - October 2023

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