Siemens SIMATIC S7-300 Manual page 252

Description of Functions
Backlash compensation
Mechanical drive components as a rule have a certain amount of backlash
(free play).
Mechanical reversing backlash can be compensated with MD30. In an indi-
rect measuring system (with the encoder on the motor), the mechanical back-
lash is traveled at each change of direction before any axis movement occurs.
The result is positioning errors.
When the position encoder is situated on the machine part to be positioned
(e.g. on a saddle - direct arrangement), backlash adversely affects the achiev-
able K
drive motor (indirect arrangement), a high K
the cost of position deviations that cannot be detected by the position control-
ler. A backlash amount entered in MD30 is applied as a correction by the
position controller as a function of the traveling direction at a given moment,
thus achieving an approximate compensation for backlash in positioning.
MD31 is used to label the "backlash-free" or "accurate-measurand" traveling
direction of the axis. If MD31 = 0, the "backlash-free" direction is the one
that matches the direction of axis movement when synchronization is re-
corded. Depending on MD18, this will correspond to the following associa-
tion:
MD18 = 0, 2, 4, 8: Plus direction is backlash-free
MD18 = 1, 3, 5, 9: Minus direction is backlash-free
30
31
9-76
–
Backlash
factor. On the other hand, if the position encoder is attached to the
v
MD Designation
Backlash compensation
Directional reference of
backlash
+
Spindle
factor can be achieved, but at
v
Value/Meaning
–1 000 000...+1 000 000
0 = as in reference point approach
(not for absolute encoders)
1 = positive
2 = negative
FM 354 Servo Drive Positioning Module
Unit
(MSR)
–