Rockwell Automation 1784-PM16SE SoftLogix Motion Card Setup and Configuration Manual - Manual (Page 401)

Publication 1784-UM003A-EN-P – June 2003
Motion Object Attributes 393
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.
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
control effort, hence the Position and/or Velocity Error values are
significantly reduced. 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 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
control effort, hence the Position and/or Velocity Error values are
significantly reduced. 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.
GSV/SSV Access Attribute Name Data Type Values
SSV/GSV Acceleration Feedforward
Gain REAL %