Rockwell Automation 1784-PM16SE SoftLogix Motion Card Setup and Configuration Manual - Manual (Page 326)

Publication 1784-UM003A-EN-P – June 2003
318 Motion Object Attributes
Tune Inertia When the axis is configured for interface to a external torque servo drive, 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 full-scale servo
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 attribute.
If the Tune Inertia value exceeds 100 %Rated/MegaCounts Per Second
2
,
performance of the digital servo loop may be compromised due to excessive
digitization 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.
The Logix tuning algorithm addresses excessive noise by managing
quantization noise levels. The product of the Tune Inertia (% Rated/MCPS)
and the Velocity Servo Bandwidth (Hertz) can be calculated to directly
determine quantization noise levels. Based on this product, the tuning
algorithm can take action to limit high frequency noise injection to the motor.
For motors with a Tune Inertia Bandwidth product of 1000 or greater, the LP
Filter is applied with a Filter Bandwidth of 5x the Velocity Servo Bandwidth in
Hertz. This limits the amount of phase lag introduced by the LP filter to ~12
degrees which is relatively small when compared to the 30 to 60 degrees of
phase margin for a typical tuned servo system. With a tuned LP filter
Bandwidth value of 200 Hz, you can expect the high frequency quantization
noise in the 1 KHz range to be attenuated roughly by a factor of 5.