Product description
34
© SICK AG · Germany · All rights reserved · Subject to change without notice
8013889/ZML0/2017-06-09
Operating Instructions
NAV350 Laser positioning sensor
Chapter
3
3.9.4
Digital output
The NAV350 has a digital output that is used for the synchronisation of the internal clock
on the NAV350 and the vehicle computer’s clock. The output supplies a 10 or 128 ms long
pulse depending on the synchronisation method (see section
3.9.5 on page 34
). The pulse
is output dependent on the synchronisation method.
Fig. 20:
Pulse for synchronisation
3.9.5
Synchronisation of the clock in the AGV and the clock in the NAV350
For precise control, it is necessary to synchronise as accurately as possible the navigation
data and landmark data from the NAV350 and the calculations on the vehicle computer.
The internal time stamp in the NAV350 is added to the measured data from the NAV350.
The internal time stamp in the NAV350 is a 32-bit counter that counts up by 1 every 1 ms.
To synchronise the internal time in the NAV350 with the system time in the vehicle’s control
system, the NAV350 provides three options.
1. via telegram
The vehicle computer requests the internal time from the NAV350 using a telegram.
The NAV350 writes its internal time in a telegram and sends it to the vehicle computer.
However, there can be a delay of up to 3 ms on sending the data, as a result there is
certain amount of inaccuracy.
2. via telegram and comparison with the digital output on the NAV350
The vehicle computer request the internal time from the NAV350 using a telegram. The
hardware output supplies a pulse of at least 10 ms in length as soon as the internal
time stamp is written to the telegram. When the telegram is subsequently received by
the vehicle computer, the vehicle computer can add to the time in this telegram the del-
ta between the pulse and the reception. As a result the AGV can determine the actual
time in the NAV350.
3. based on pulses on the hardware output
The parameters for the hardware output are set using a telegram to generate, based on
the 32-bit counter, an output pulse of 128 ms in length in a fixed cycle. The configura-
tion is set using a bit pattern from bit 10 … 20. Depending on the bit set, an output
pulse is generated when this bit overflows in the counter.
A configuration of e.g. 15 bit generates an output pulse every 32,768 ms.
When the pulse arrives, the vehicle computer knows that in the NAV350 the 14 least
significant bits of the counter are 0. Based on the time stamp in the telegram received
subsequently, the vehicle computer can now determine the time that has elapsed since
the telegram and add it to the time in the telegram.
Important
Program the vehicle’s control system such that it reacts to the rising edge of the output
pulse
128 ms
