Rockwell Automation 1784-PM16SE SoftLogix Motion Card Setup and Configuration Manual - Manual (Page 337)

Publication 1784-UM003A-EN-P – June 2003
Motion Object Attributes 329
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
1784-PM02AE, a non-zero position or velocity error would need to be
present. This dynamic error while moving is called “following error”. We
ideally 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 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. Hence, 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.
The optimal value for Velocity Feedforward Gain is 100% theoretically. In
reality, however, the value may need to be tweaked to accommodate velocity
loops with non-infinite loop gain and other application considerations. One
thing that may force a smaller Velocity Feedforward value is that increasing
amounts of feedforward tends to exacerbate axis overshoot. If necessary, the
Velocity Feedforward Gain may be "tweaked" from the 100% value by running
a simple user program that jogs the axis in the positive direction and monitor
the Position Error of the axis during the jog. Increase the Velocity
Feedforward Gain until the Position Error at constant speed is as small as
possible, but still positive. If the Position Error at constant speed is negative,
the actual position of the axis is ahead of the command position. If this occurs,
decrease the Velocity Feedforward Gain such that the Position Error is again
positive. Note that reasonable maximum velocity, acceleration, and
deceleration values must be entered to jog the axis.
GSV/SSV Access Attribute Name Data Type Values
SSV/GSV Velocity Feedforward Gain REAL %