Siemens SIMATIC FM 353 Manual page 253

Length measurement
The current actual position value is acquired at both the rising and the subsequent
falling edge of the sensing probe. In addition, the distance (absolute value) actually
traversed between the two edges is determined.
Length measurement On
Measurement input
Process interrupt
Measurement taking place
"Measured length value"
S1 − start of length measurement
S2 − end of length measurement
∆S − Measured length value
∆t − minimum signal length at the digital input: w 2
The measured length is determined as follows under certain supplementary
conditions:
S Retrigger the reference point:
The measured length constitutes the difference of the edge positions.
S Set the actual value: The measured length constitutes the distance actually
traversed.
S Zero offset:
The "Zero offset" function does not change the actual position of the axis and is
thus not relevant for evaluations regarding the "Length measurement" function.
S Rotary axis:
The measured length is the path actually traversed over several rotations of the
rotary axis.
S Direction reversal:
The axis can change the traversing direction within the length to be measured
as often as you want.
− Case 1:
The axis passes the falling edge in the traversing direction of the rising
edge: The measured length is the difference of the edge positions.
− Case 2:
The axis passes the falling edge in the opposite direction to the rising edge:
End of the length measurement with output of an error message.
FM 353 Stepper Drive Positioning Module
6ES7 353-1AH01-8BG0
∆S
∆t
S1 S2
@
FM cycle
Description of Functions
9-53