Rockwell Automation 1784-PM16SE SoftLogix Motion Card Setup and Configuration Manual - Manual (Page 400)

Publication 1784-UM003A-EN-P – June 2003
392 Motion Object Attributes
In certain cases, Pos 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.
While the Pos I Gain, if employed, is typically established by the automatic
servo tuning procedure, the Pos I Gain value may also be set manually. Before
doing this it must be stressed that the Torque Scaling factor for the axis must
be established for the drive system. Refer to Torque Scaling attribute
description for an explanation of how the Torque Scaling factor can be
calculated. Once this is done the Pos I Gain can be computed based on the
current or computed value for the Pos P Gain using the following formula:
Pos I Gain = 0.25 * 0.001 Sec/mSec * (Pos P Gain)
2
Assuming a Pos P Gain value of 100 Sec
-1
this results in a Pos I Gain value of
2.5 ~0.1 mSec
-1
-Sec
-1
Velocity Feedforward Gain Servo Drives require non-zero command input to generate steady-state axis
acceleration or velocity. To provide the non-zero output from the drive to the
motor, a non-zero position or velocity error needs to be present. We call this
dynamic error while moving “following error”. The non-zero following error
condition is a situation you are trying to avoid. Ideally you want zero following
error -- all the time. This could be achieved through use of the position
integral gain controls as described above, but typically the response time of the
integrator action is too slow to be effective. An alternative approach that has
superior dynamic response is to use Velocity and Acceleration Feedforward.
The Velocity Feedforward Gain attribute is used to provide the Velocity
Command output necessary to generate the commanded velocity. It does this
by scaling the current command velocity (derivative of command position) by
the Velocity Feedforward Gain and adding it as an offset to the Velocity
Command generated by the position loop control elements. With this done,
the position loop control elements do not need to generate much of a
contribution to the Velocity Command, hence the Position Error value is
significantly reduced. The Velocity Feedforward Gain allows the following
error of the servo system to be reduced to nearly zero when running at a
constant speed. This is important in applications such as electronic gearing and
synchronization applications where it is necessary that the actual axis position
not significantly lag behind the commanded position at any time.
GSV/SSV Access Attribute Name Data Type Values
SSV/GSV Velocity Feedforward Gain REAL %