Operating Instructions
NAV350
Product description
8013889/ZML0/2017-06-09
© SICK AG · Germany · All rights reserved · Subject to change without notice
33
Chapter
3
3.9.3
Relative position system and absolute position system
The AGV uses two position systems for vehicle control: a relative position system and an ab-
solute position system.
The
relative position system
takes its data from the odometry (incremental encoder). The
odometry supplies new data with a high refresh rate; the vehicle computer can calculate the
position of the vehicle from these data. However, the relative position calculation includes
errors. The error increases the greater the distance covered.
The
absolute position system NAV350
on the other hand supplies absolute position data
on the route. Based on these absolute positions the AGV’s vehicle computer can minimise
the errors from the odometry.
fig. 19
shows schematically the integration of the NAV350 in a complete navigation system:
Fig. 19:
Integration of the NAV350 in a navigation system
Overview of the integration of the NAV350 in the control system for an AGV
The vehicle computer informs the NAV350 of the actual velocity and rate of turn of the AGV
from its odometry (see
fig. 18 on page 32
). To optimise the control, the vehicle computer
transmits the velocity to the NAV350 several times between two position requests. The
NAV350 uses the velocity data for three calculations:
•
From the velocity data an expected position for reflectors in the next measuring cycle is
calculated, in this way the measurement of the reflectors within one head revolution is
corrected (see
fig. 11 on page 25
).
•
Calculated position data are extrapolated to the time of the data transfer.
•
By transferring the velocity several times between two position requests the NAV350
calculates a movement profile so as to take into account large AGV movement changes.
Important
If no velocity data are transferred to the NAV350 by the vehicle’s control system, the
NAV350 calculates internally the velocity between two positions determined. This determi-
nation of the velocity can only be used at low accelaration and is only suitable for test pur-
poses. For practical operation the velocity must be provided by the vehicle’s control system.
The velocity and the rate of turn of the AGV must be transformed to the position of the
NAV350 on the AGV.
