Rockwell Automation 1784-PM16SE SoftLogix Motion Card Setup and Configuration Manual - Manual (Page 402)

Publication 1784-UM003A-EN-P – June 2003
394 Motion Object Attributes
The optimal value for Acceleration Feedforward is 100% theoretically. In
reality, however, the value may need to be tweaked to accommodate torque
loops with non-infinite loop gain and other application considerations. One
thing that may force a smaller Acceleration Feedforward value is that
increasing amounts of feedforward tends to exacerbate axis overshoot.
If necessary, the Acceleration Feedforward Gain may be "tweaked" from the
100% value by running a simple user program that jogs the axis in the positive
direction and monitors the Position Error of the axis during the jog. Usually
Acceleration Feedforward is used in tandem with Velocity Feedforward to
achieve near zero following error during the entire motion profile. To fine-tune
the Acceleration Feedforward Gain, the Velocity Feedforward Gain must first
be optimized using the procedure described above. While capturing the peak
Position Error during the acceleration phase of the jog profile, increase the
Acceleration Feedforward Gain until the peak Position Error is as small as
possible, but still positive. If the peak Position Error during the acceleration
ramp is negative, the actual position of the axis is ahead of the command
position during the acceleration ramp. If this occurs, decrease the Acceleration
Feedforward Gain such that the Position Error is again positive. To be
thorough the same procedure should be done for the deceleration ramp to
verify that the peak Position Error during deceleration is acceptable. Note that
reasonable maximum velocity, acceleration, and deceleration values must be
entered to jog the axis.
Velocity Proportional Gain The standard RA SERCOS drive’s digital velocity loop provides damping
without the requirement for an analog tachometer. The Velocity Error is
multiplied by the Velocity Proportional Gain to produce a Torque Command
that ultimately attempts to correct for the velocity error, creating the damping
effect. Thus, increasing the Velocity Proportional Gain results in smoother
motion, enhanced acceleration, reduced overshoot, and greater system
stability. The velocity loop also allows higher effective position loop gain
values to be used, however, too much Velocity Proportional Gain leads to high
frequency instability and resonance effects. Note that units for Velocity
Proportional Gain are identical to that of the Position Proportional Gain
making it easy to perform classic calculations to determine damping and
bandwidth.
If you know the desired unity gain bandwidth of the velocity servo in Hertz,
use the following formula to calculate the corresponding P gain.
Vel P Gain = Bandwidth (Hertz) / 6.28
GSV/SSV Access Attribute Name Data Type Values
SSV/GSV Velocity Proportional Gain REAL 1/Sec