Rockwell Automation 1784-PM16SE SoftLogix Motion Card Setup and Configuration Manual - Manual (Page 338)

Publication 1784-UM003A-EN-P – June 2003
330 Motion Object Attributes
Acceleration Feedforward Gain The Acceleration Feedforward Gain attribute is used to provide the Torque
Command output necessary to generate the commanded acceleration. It does
this by scaling the current Command Acceleration by the Acceleration
Feedforward Gain and adding it as an offset to the Servo Output generated by
the servo loop. With this done, the servo loops do not need to generate much
of a contribution to the Servo Output, hence the Position and/or Velocity
Error values are significantly reduced. Hence, when used in conjunction with
the Velocity Feedforward Gain, the Acceleration Feedforward Gain allows the
following error of the servo system during the acceleration and deceleration
phases of motion to be reduced to nearly zero. 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 Acceleration Feedforward 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 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.
Note: that Acceleration Feedforward Gain is not applicable for
applications employing velocity loop servo drives. Such systems would
require the acceleration feed-forward functionality to be located in the
drive itself.
GSV/SSV Access Attribute Name Data Type Values
SSV/GSV Acceleration Feedforward Gain REAL %