Rockwell Automation 1784-PM16SE SoftLogix Motion Card Setup and Configuration Manual - Manual (Page 343)

Publication 1784-UM003A-EN-P – June 2003
Motion Object Attributes 335
Velocity Integral Gain When configured for a torque (current) loop servo drive, every servo update
the current Velocity Error is also accumulated in variable called the Velocity
Integral Error. This value is multiplied by the Velocity Integral Gain to
produce a component to the Servo Output or Torque Command that attempts
to correct for the velocity error. The characteristic of Vel I Gain correction,
however, is that any non-zero Velocity Error will accumulate in time to
generate enough force to make the correction. This attribute of Vel I Gain
makes it invaluable in applications where velocity accuracy is critical. The
higher the Vel I Gain value the faster the axis is driven to the zero Velocity
Error condition. Unfortunately, I Gain control is intrinsically unstable. Too
much I Gain will result in axis oscillation and servo instability.
In certain cases, Vel I Gain control is disabled. One such case is when the
servo output to the axis’ drive is saturated. Continuing integral control
behavior in this case would only exacerbate the situation. Another common
case is when performing certain motion. When the Integrator Hold Enable
attribute is set, the servo loop automatically disables the integrator during
commanded motion.
Due to the destabilizing nature of Integral Gain, it is recommended that
Position Integral Gain and Velocity Integral Gain be considered mutually
exclusive. If Integral Gain is needed for the application use one or the other,
but not both. In general, where static positioning accuracy is required, Velocity
Integral Gain is the better choice.
The typical value for the Velocity Integral Gain is ~15 mSec -1 -Sec -1 .
Velocity Scaling The Velocity Scaling attribute is used to convert the output of the servo loop
into equivalent voltage to an external velocity servo drive. This has the effect
of “normalizing” the units of the servo loop gain parameters so that their
values are not affected by variations in feedback resolution, drive scaling, or
mechanical gear ratios. The Velocity Scaling value is typically established by
servo’s automatic tuning procedure but these values can be calculated if
necessary using the following guidelines.
If the axis is using a velocity servo drive, the software velocity loop in the servo
module is disabled. In this case the Velocity Scaling value can be calculated by
the following formula:
Velocity Scaling = 100 % / (Speed @ 100 %)
GSV/SSV Access Attribute Name Data Type Values
SSV/GSV Velocity Integral Gain REAL 1/mSec-Sec