Distribution of a Swarm of Robots in a Circular Workplace under Gravity

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This Demonstration examines the mean, variance, correlation, and covariance of a very large swarm of robots as they move inside a circular workplace under the influence of gravity pointing in the direction . The swarm is large, whereas the robots are much smaller by comparison. Under gravity, the swarm flows like a liquid, moving to a side of the workplace and filling the area under a chord to height
.
Contributed by: Haoran Zhao and Aaron T. Becker (February 2016)
Open content licensed under CC BY-NC-SA
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Though rectangular boundaries are common in artificial workspaces, biological workspaces are usually rounded. The workspace in this example is a circle centered at with radius 1 and thus area
. For notational simplicity, the swarm is parameterized by the angle of the force of gravity
and the fill level
. The range of possible angles for the angle of the force of gravity
is
. In this range of angles, the swarm always fills the area under a chord, but this shape is rotated around the unit circle.
The area under a chord of a circle is the area of a sector less the area of the triangle originating at the circle center:
,
thus
,
where is the arc length,
is the chord length,
is the radius, and
is the height. Solving for
and
gives
,
.
Therefore, the area under a chord is
.
For a circular workspace with given , the mean of
and
is:
,
.
The variance of and
is:
,
,
and finally,
.
The locus of mean positions is aligned with , and the mean position is at radius
from the center.
The variance
is maximized at
and
, while the covariance is maximized at
and
. For small
values, the correlation approaches
.
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