Rockwell Automation 1746-HSRV SLC Servo Control Module User Manual - Manual (Page 81)

Publication 1746-6.1.2 - July 2000
7-2 Setting Up Your SLC Servo Module
Understanding the Theory
of Motion Control The major components of a motion control system are:
•
Machine mechanics
•
Velocity loop
•
Position loop
Machine Mechanics
Machine mechanics are the combined gearing, ball-screws, and
mechanical linkages that convert the motor’s rotary motion into the
axis motion that you want.
Velocity Loop
Velocity loop is a feedback control loop in which the controlled
parameter is encoder velocity. A tachometer is usually used for the
feedback device. Command input from the controller to the drive is a
DC voltage that is proportional to encoder speed (e.g., 1V equals 5
rpm and 5V equals 5000 rpm). Using the tachometer as feedback, a
drive maintains the speed of the encoder at the commanded speed
within its output capabilities.
A typical drive contains adjustments to do the following:
•
Scale the input command voltage to the motor speed.
•
Zero the motor speed for a zero input command.
•
Set the maximum current (torque) to the motor.
•
Control the response of the velocity loop.
Refer to the drive manual for instructions on setting these adjustments.
Position Loop
Position loop is a feedback control loop in which the controlled
parameter is mechanical position. The position loop compares
position feedback with the position command to modify the velocity
output signal to correct for any position error. Encoders are position
measuring devices that provide the SLC Servo Module with precise
actual axis position at all times. Based on motion statements, the SLC
Servo Module computes an axis position and compares it to the actual
axis position. Following error is the difference between the
commanded axis position and the actual axis position. Axis gain (or