Siemens SIMATIC S7-300 Manual page 151

7.3.7 Activating the software limit switches, drift compensation and
backlash compensation
Software limit
Carefully run the axis up to its extreme limits as specified for normal opera-
switches tion of the machine. Enter these position actual values into the machine data
MD21/MD22 as software limit switches, and activate them.
Note
If you change the reference-point coordinate later or use Set reference point
for the absolute encoder, you must redefine the positioning values of the
software limit switches.
If you do not need the software limit switches, the input limits –10
[MSR] must be entered in MD21/MD22 (for default values, see Table 5-5).
Drift compensation
If you want to use the drift compensation function in addition to the offset
compensation already described in Section 7.3.2, activate it in the machine
data (please see the function description in Section 9.7, Position control).
Backlash com- With indirect position measurement (for example, with an encoder on the
pensation motor) the free play of mechanical transmission elements during positioning
may cause a position deviation of a machine part (such as a lathe saddle) that
is to be positioned but does not lie in the measured-value feedback loop. As a
rule, a piece of the distance will be "missing" after a reversal of direction.
This backlash amount can be determined as a mean at various axis positions,
and entered in the machine data MD30 and MD31.
You can use the following flow chart to determine backlash and activate
backlash compensation.
FM 354 Servo Drive Positioning Module
Starting up the FM 354
9 9
and 10
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