Rockwell Automation 1771-QC , D17716.5.25 SER/B SERVO POS - Manual (Page 17)

Positioning Concepts
Chapter 3
3Ć4
to generate the velocity command. Gain is expressed in ipm/mil (where 1
mil - 0.001 in) or mmpm/mil (where 1 mil = 0.001 mm).
For example, with a velocity of 100 ipm and a gain of 1 ipm/mil, the
following error is:
velocity 100 ipm
2following error =gain = 1 ipm/mil = 100 mil
When you increase the gain, you decrease the following error and
decrease the cycle time of the system. However, the gain that you can use
is limited by the drive, the motor, and the machine; a gain that is too large
causes instability.
Feed Forward
To decrease the following error without increasing the gain, we can add a
feed forward component (Figure 3.4).
Figure 3.4
Velocity Loop, Positioning Loop, and Feed Forwarding
Axis Motion
Motor Tach
-
+
Velocity
Command
Velocity Feedback
Encoder
Incremental Position Feedback
K1
-
+
Axis
Feedrate
Position
Command
Following
Error
D/A
12002
K2
+
+
Velocity Command = K (following Error) - K (Axis Feedrate)
1 2
Position
Feed
Forward