Accumulation Types let you define parameters for accumulating paths, i.e. paths on which
AGVs will detect proximity and avoid collisions with each other.
You can get to the AGV Accumulation Types page by right-clicking on a Path or Control Point and
choosing AGV Network Properties.
The Accumulation Types Tab
The Accumulation Types tab has the following properties:
Add, remove, rename and re-order Accumulation Types through the combobox and buttons at
the top of the page.
To assign a path an Accumulation Type, click on the path, and in its Quick Properties on
the right, choose the desired type from the Accumulation Type drop-down.
These properties define how two AGVs will detect proximity with each other while on a
common accumulating path.
From Front AGV Edge To Behind AGV Edge - This
defines the AGV edges used to determine proximity. Usually this will be the default:
Front AGV <Trailing Edge> to Behind AGV <Leading Edge>. This will evaluate
the distance from the back of the AGV ahead to the front of the AGV behind. You could
also choose something like Front AGV <Center> to Behind AGV <Center>, which
would evaluate the distance from center to center.
Stop Threshold - This is the proximity distance at
which an AGV will go into a "proximity stop" state, and slow down to a stop.
Use as Target Stop Spacing - If checked, then if
an ahead AGV is stopped, the stop threshold defines the target stop spacing. Stopped
AGVs will thus accumulate with this stop spacing. In other words, an AGV approaching a
stopped AGV will begin decelerating to stop BEFORE the stop threshold (as a proximity
distance) is breached. The AGV will instead start decelerating such that it comes to a
stop with the stop threshold as its spacing behind the ahead AGV. In situations where
both AGVs are still moving, the stop threshold will continue to be used in regular
Added Trailing Time - In addition to direct distance-based
proximity detection, you can also include time-based proximity detection. In other words,
you may want AGVs to follow n seconds behind each other, in addition to (or instead of)
following m meters behind each other. When you add time-based trailing, AGVs will
enforce more spacing between themselves as they travel faster. For example, an added trailing
time of 5 seconds means that, when the AGV is traveling 1 meter per second, it must stay 5 meters
behind the AGV in front of it, whereas when it is traveling at 2 meters per second, it must
stay 10 meters behind, etc.
Resume Threshold - The proximity distance at which
an AGV can resume its travel after going into a "proximity stop" state. The resume
threshold must be greater than the stop threshold.
Plus Time - An optional additional time, started
when the Resume Threshold is reached, that the AGV will wait before resuming from a
"proximity stop" state.
When you define an Accumulation Type for a path, the AGV network will treat intersections
on that path as allocations. Similar to the way AGVs must allocate Control Points and
Control Areas, AGVs must allocate the intersection points on an accumulating path before
proceeding past those intersection points. In the Accumulation Types page you define stop
distances, which are distances before the intersection where the AGV must stop before an
intersection point if the AGV cannot allocate it, as well as clear distances, which are
distances after passing the intersection point where AGVs will release the intersection and
allow other AGVs to claim it. Each of these distances are split out by whether the AGV is
entering, exiting, or on the path. If already on a path or staying on a path, the distances
are split out by the path geometry, i.e. whether or not the intersection branches out away
from the AGV or toward it.
Intersection Stop Point
Here you define the stop distances for an intersection. For each distance, you define the
distance itself as well as an AGV edge, which determine what part of the AGV should stop at
the stop distance. Usually stop distances will be based on the leading edge of the AGV. When
you click in a given field, the diagram on the right will display the distance / edge that
you are defining, to help you figure out what exactly the field defines.
Path Entry - This defines the stop distance and
agv edge associated with entering a path.
On Path Long - This defines the stop distance and
agv edge associated with approaching an intersection when already on a path. It is the
"long" stop distance because it will be applied to intersections that branch out toward
the AGV, and thus require the AGV to stop farther away from the intersection point in
order to provide room for merging AGVs.
On Path Short - This defines the stop distance and
agv edge associated with approaching an intersection when already on a path. It is the
"short" stop distance because it will be applied to intersections that branch out away
from the AGV, hence the AGV can stop closer to the intersection without causing
Prioritize Control Point Lookahead - If checked,
the stop point / allocation point for path transfers will be adjusted when there is a
control point before an intersection, to ensure that the control point lookahead
mechanism will happen before allocation of path transfers. This can prevent gridlock
in some situations.
As an example of using this, let's say you have configured the stop points so that
the AGV's lead edge will stop 1 meter short of a given intersection if it can't allocate
the path transfer. Then you place a control point closer to the intersection point,
let's say at a point where the AGV's lead edge will be 0.5 meters from the intersection.
If you have checked the "Prioritize" box, then the point of allocation for the path
transfers will be adjusted forward, so that, instead of stopping 1 meter short to
allocate path transfers, the AGV will travel to the control point, then lookahead
to allocate the next control point, BEFORE attempting the path transfer allocation.
This can resolve potential gridlock issues, especially since you can manually adjust
things by placing control points and control areas in certain locations, and blocking
will always happen as part of control point/area mutual exclusion.
Intersection Clear Point
Here you define the clear distances for an intersection. Like with stop distance, you
define the distance itself as well as an AGV edge. Usually clear distances will be based on
the trailing edge of the AGV. When you click in a given field, the diagram on the right will
display the distance / edge that you are defining, to help you figure out what exactly the
On Path Long - This defines the clear distance and
agv edge associated with clearing an intersection when still on the path (not exiting).
It is the "long" clear distance because it will be applied to intersections that branch
out toward the clear point, and thus require the AGV to travel farther away from the
intersection point before clearing it.
On Path Short - This defines the clear distance
and agv edge associated with clearing an intersection when still on the path (not
exiting). It is the "short" clear distance because it will be applied to intersections
that branch out away from the clear point, hence the AGV can clear it closer to the
intersection point without causing issues.
Path Exit - This defines the clear distance and
agv edge associated with exiting a path.
There are a few special rules that apply to allocating intersection points.
Pure On-Path Allocations - The AGV network tries
to make basic proximity detection take precedence over intersection stop and clear
points. Thus, if an AGV is already on a path and already has an AGV ahead of it for
which it is detecting proximity, and if it is not exiting the path at that intersection,
then the AGV will be allowed to allocate the intersection point before the ahead AGV has
cleared it. Since it is already detecting proximity on the ahead AGV, it is OK to have a
simultaneous allocation of the intersection point because the basic proximity detection
will already avoid proximity errors.
End-To-End Path Transfers - When transferring from
the end of one path to the beginning of another path, the network still treats that as
an "intersection" between two paths, i.e. it still requires an allocation of
the intersection point. However, again if it can detect that, in transferring to the new
path, the AGV will still be detecting proximity with the same AGV that it was detecting
before the transfer, then it will go ahead and allow the intersection point to be
allocated simultaneously by both AGVs, since the basic proximity detection will already
prevent proximity errors. Note that this only applies to transfers between two paths of
the same Accumulation Type. If the Accumulation Type is different, it will treat it like
a regular intersection allocation.
Troubleshooting AGV Accumulation
When you use accumulation in an AGV system, you may run into deadlock issues. See
Troubleshooting AGV Accumulation for more information on how to remedy these problems.