Rockwell Automation 8520-ARM2 9/Series CNC AMP Reference Manual Documentation Set - Manual (Page 149)

Servo Parameters
Chapter 7
7-3
Servo Feedback Options
Each servo system contains two loops:
velocity loop, used to control motor speed vs commanded motor speed
position loop, used to control axis position vs commanded axis position.
Hardware Type How It Works Feedback Method
Analog uses a drive to close the velocity loop while the CNC closes only the
position loop. This configuration requires a motor--mounted tachometer
(for the velocity loop) and a encoder to return position data to the CNC.
Single Feedback
Digital requires that both the velocity and positioning loop be closed at the CNC Single or Optional Feedback
Tachless Analog closes both the velocity and position loops using the CNC and position
data avoiding the cost of a tachometer for velocity feedback. This
configuration is not available on high--resolution feedback systems.
Single Feedback only with
3-axis modules; Single and
Optional Feedback with 4-axis
modules.