Rockwell Automation 1771-QC , D17716.5.25 SER/B SERVO POS - Manual (Page 15)

Positioning Concepts
Chapter 3
3Ć2
Velocity Loop
Most closed-loop servo positioning installations use a dc motor to power
the leadscrew. To accurately control the velocity of the dc motor, we need
a velocity loop (Figure 3.2).
The velocity loop contains a summing point, an amplifier, and a
tachometer. A tachometer is a precision generator that produces a voltage
signal directly proportional to the angular velocity of the motor shaft. The
output of the tachometer is the velocity feedback signal which is
subtracted from the velocity command signal. The difference is the
velocity error signal that is amplified to provide power for the motor to
run at the commanded velocity.
Figure 3.2
Velocity Loop
Axis Motion
Motor Tach
-
+
Velocity
Error
Velocity
Command
Amplifier
Velocity Feedback
Summing Point
12000
Velocity Error = (Velocity Command Ć Velocity Feedback)
Whenever the velocity deviates from the commanded velocity, the
velocity feedback signal adjusts the velocity error signal until the velocity
matches the velocity command signal.