Documentation HG G-73650ZD | English, Revision 05 | Date: 09.03.2017 | www.goetting-agv.com
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Chapter 2: Basic Principles of Track Guidance
To guide the vehicle it is necessary to determine the vehicle's target position first. At this
position a tangent is applied to the segment and the two points "Virtual Point Front" and
"Virtual Point Rear" are defined (in the previous control unit these points were indicated as
"Länge vorne" und Länge Hinten"). These are the two points that are relevant for the vehicle
control. Here the vectors "Forward Dis.Fix" and "Forward Dis Var" will be added in the seg-
ment's direction of travel. "ForwardDis Var" results from the multiplication of the corre-
sponding parameter and the actual velocity in m/s. All steering angles of the vehicle are
then calculated in such a way that the points "Virtual Point Front" and "Virtual Point Rear"
will move towards the end points of these vectors ("Direction Front" and "Direction Rear").
While the vehicle continues to travel, the direction of travel of the two points "Virtual Point
Front" and "Virtual Point Rear" changes in such a way that the vehicle approaches the target
position.
Figure 22
Guiding an omnidirectional vehicle: Control process
The vehicles indicated by bolder lines display the initial position. The thinly drawn vehicles
reflect the situation after having passed a short section. It is visible that the errors "Deriva-
tion Front" and "Derivation Rear" become smaller.
In principle:
The shorter the vectors "Forward Dis. Fix" and "Forward Dis. Var" are, the larg-
er the vehicle's steering angles will be. As "Forward dis.Var" is speed-dependent, "Forward
Dis. Fix" determines the vehicle control at low speeds.
The positions of "Virtual Point Front" and "Virtual Point Rear" have an impact on the control
system: If the distance to the zero-point of the vehicle is too short, the vehicle will quickly
be guided back to the segment. However, this may have the result that points further away
from the zero point might not return asymptotically to the segment. The points "Virtual Point
Front" and "Virtual Point Rear" will always return asymptotically to the segment (if the vehi-
cle doesn't swerve).
To ensure a damping of the vehicle's swaying movements the angle of "Direction Front" and
"Direction Rear" can be limited. This can be done with the parameters "Approach Lim. Fix"
and "Approach Lim Var". "Approach Lim Var" is speed-dependent as well. The limitation of
the angles is calculated as follows:
Figure 23
Formula: Limitation of angles
Limit
1
v
m
s
-
1
+
------------
-
Approach Lim. Var
Approach Lim. Fix
+
=