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Rockwell Automation 1771-QC , D17716.5.25 SER/B SERVO POS User Manual

Page 15

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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.