Rockwell Automation 1784-PM16SE SoftLogix Motion Card Setup and Configuration Manual - Manual (Page 379)

Publication 1784-UM003A-EN-P – June 2003
Motion Object Attributes 371
Tune Acceleration/Deceleration The Tune Acceleration Time and Tune Deceleration attributes return the
measured acceleration and deceleration values for the last run MRAT (Motion
Run Axis Tuning) instruction. These values are used, in the case of an external
torque servo drive configuration, to calculate the Tune Inertia value of the axis,
and are also typically used by a subsequent MAAT (Motion Apply Axis Tune)
to determine the tuned values for the Maximum Acceleration and Maximum
Deceleration attributes.
Tune Inertia The Tune Inertia value represents the total inertia for the axis as calculated
from the measurements made during the last MRAT (Motion Run Axis Tune)
initiated tuning process. In actuality, the units of Tune Inertia are not industry
standard inertia units but rather in terms of percent (%) of rated drive output
per MegaCounts/Sec
2
of feedback input. In this sense it represents the input
gain of torque servo drive. These units represent a more useful description of
the inertia of the system as seen by the servo controller. The Tune Inertia value
is used by the MAAT (Motion Apply Axis Tune) instruction to calculate the
Torque Scaling.
If the Tune Inertia value exceeds 100 %Rated/MegaCounts Per Second
2
,
performance of the digital servo loop may be compromised due to excessive
digitalization noise associated with the velocity estimator. This noise is
amplified by the Torque Scaling gain which is related to the Tune Inertia factor
and passed on to the torque output of the drive. A high Tune Inertia value can,
thus, result in excitation of mechanical resonances and also result in excessive
heating of the motor due to high torque ripple. The only solution to this
problem is to lower the loop bandwidths and optionally apply some output
filtering.
Since the Tune Inertia value represents a measure of the true system inertia,
this situation can occur when driving a high inertia load relative to the motor,
i.e. a high inertia mismatch. But it can also occur when working with a drive
that is undersized for the motor or with a system having low feedback
resolution. In general, the lower the Tune Inertia the better the performance of
the digital servo loops will approximate that of an analog servo system.
GSV/SSV Access Attribute Name Data Type Values
GSV Tune Acceleration REAL Position Units / Sec2
GSV Tune Deceleration REAL Position Units / Sec2
GSV/SSV Access Attribute Name Data Type Values
GSV Tune Inertia REAL
% / MegaCounts Per Sec
2