12 Functional Descriptions 12.93
12.18.13 Start-up
Set position control sampling times
The position control sampling times for the following drive and associated leading drives within
a GI grouping must be set to the same value. This sampling time may however vary from
grouping to grouping (provided the groupings are not chained as a gearbox).
By increasing the position control sampling times for non-critical axes (loader axes, drives not
included in the GI grouping), you can release computing capacity which you require to obtain a
short position control sampling time for the GI grouping.
Drift and tacho compensation
• General
You have various options for compensating non-linearity of the drive or tacho (e.g. drift):
– Drift compensation
– Automatic tacho compensation
• Setting the drift compensation
The drift can be set either manually via machine data MD 272* or 401* or semi-
automatically (for axes only) through selection of the following softkeys (in service display)
Drift comp.
Axis----
(with the axes at standstill). The value calculated is then automatically entered in MD 272*.
• Setting the automatic tacho compensation (for axes only)
This setting can be made only if the drift compensation setting is correct. Select tacho
compensation with NC MD 1804 bit 1. This function is fully automatic, i.e. no further
settings need be made.
The tacho compensation function calculates a direction-dependent compensation value at
constant traversal of the axis; this value can compensate deviations of up to 12 % of the
multgain. The compensation value is injected in parallel to the P feedforward control, i.e.
the tacho compensation function also activates the feedforward control (computing time
requirement!).
If a compensation value cannot be calculated (e.g. axes not traversing constantly, 12 %
limitation, speed less than 1/8 of maximum speed), the last valid value to be calculated is
used. The compensation value is erased when compensation is deselected. Refer to
Section NC machine data, NC MD 1804* bit 1 for further details.
When the link is activated, the currently valid compensation value for the following axis is
frozen. Deviations in synchronism caused by changes to the controlled system can therefore
only be compensated by the I-action component of the compensatory controller.
12–158
© Siemens AG 1992 All Rights Reserved 6FC5197- AA50
SINUMERIK 840C (IA)
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