HEIDENHAIN ND 780 - Handbuch (Seite 143)

ND 780
19
I - 1 F
undamentals of P
o
sitioning
Encoder Reference Marks
Encoders normally contain one or more reference marks, which the
ND 780’s Reference Mark Evaluation feature uses to re-establish
datum positions after a power interruption. There are two main
options available for reference marks; fixed and distance-coded.
Encoders with distance-coded reference marks have marks
separated by a specific encryption pattern that allows the ND 780 to
use any two pair of marks along the length of the encoder to re-
establish the prior datums. This configuration means that the operator
only has to travel a very short distance, any where along the encoder,
to re-establish the datums when the ND 780 is turned back on.
Encoders with fixed reference marks have one, or more marks on
fixed intervals. To re-establish the datums correctly, it is necessary to
use the same exact reference mark, during the Reference Mark
Evaluation routine, that was used when the datum was first
established.
The established datums can not be restored from one
power cycle to the next if the reference marks were not
crossed before the datums were set.