Siemens SIMATIC FM 452 Manual page 116

Settings
Incorrect Measurement
If a length measurement / edge detection is incorrect, the FM 452 returns the value
-1 for the length.
A "length measurement" or "edge detection" can pass through zero a maximum of
1
126 times
direction are recognized by the FM 452, an incorrect "length measurement" or
"edge detection" is signaled even when zero passes in the other direction were
detected afterwards.
A length measurement error also occurs when:
The measured length of a rotary axis is greater than 2
The on and off edges are detected simultaneously by the FM 452 (for example,
caused by switch bounce).
Adjustment of the Coordinate System During Length Measurements
Adjustments in the coordinate system influence the measured length in the
following situations:
You are using an incremental encoder or an initiator and you are operating the
FM 452 in the simulation mode.
You are running "set reference point" or "retrigger reference point" during an
active length measurement.
Example
You can use the influences on the measured length listed above as follows:
You have, for example, a system in which a slip always occurs during length
measurement.
With retrigger reference point, you can correct this slip so that correct length
measurement values are then output.
1
Zero pass = rotary axis pass from the "end of rotary axis" value to 0 and vice versa
9-18
in one direction. As soon as more than 126 zero passes in one
31
.
FM 452 Electronic Cam Controller
C79000-G7076-C452-04