HEIDENHAIN ND 287 User Manual - Manual (Page 20)

20
I Working with the ND 287 position display unit
I – 2 F
undamentals of positioning
Reference marks
Encoders normally contain one or more reference marks (see Fig. I.9)
which the ND 287'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 287 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, anywhere along the encoder,
to re-establish the datums when the ND 287 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.
Fig. I.9 Linear scales – with distance-coded
reference marks (upper illustration) and one
reference mark (lower illustration)
Danger to workpiece!
The established datums cannot be restored from one
power cycle to the next if the reference marks were not
crossed before the datums were set.