Rockwell Automation 8510 AC Spindle Drive System Programming Manual - Manual (Page 42)

Drive Tuning
Chapter 3
3-39
Spindle Orient Tuning Process
During the final deceleration toward the target position, the drive should
not be in torque limit. When the drive is in torque limit, the actual spindle
position will fall behind the commanded spindle position, and the system is
likely to overshoot the target position as the control loops try to recover.
The peak torque required from the drive to decelerate the spindle is directly
related to the total system inertia, the orient position loop gain (Kp), and
the motor speed when the deceleration is initiated.
Please note the following points:
- For any setting of Orient Speed, the required peak torque will increase in
direct proportion to the increase in Kp (Orient Start setting is decreased).
- For any specific value of Kp (ratio of Orient Speed to Orient Start), the
required peak torque will increase in direct proportion to the increase in
value of Orient Speed.
- For any setting of Orient Start, the required peak torque will increase as
the square of any percentage increase in the Orient Speed parameter
value. For example, if the Orient Speed is increased from 100 to 200
rpm, the peak torque will increase to 400% of the previous value.
Increasing Orient Speed without changing Orient Start increases both
the value of Kp and the initial decel speed.
The time to perform the spindle orient is composed of three components:
1. the time to decelerate to the Orient Speed,
2. the time to search for the Orient Start spindle position, and
3. the time for the final deceleration to the target position.
The initial time to decelerate to the Orient Speed is determined by the
actual spindle speed when the orient command is given and is not
controllable by drive tuning. The time required to search for the Orient
Start position can be minimized by increasing the setting of Orient Speed
as much as possible, without causing the drive to be torque limited. The
time for the final deceleration is minimized by increasing the value of Kp
as much as possible. After selecting the Orient Speed, the Orient Start
setting is made as small as possible without forcing the drive into torque
limit, which will result in the highest possible setting of Kp.
On a high inertia spindle, if the Orient Speed is set too high, the orient
position loop gain (Kp), may have to be very low to prevent the drive from
going into torque limit. This low Kp setting may lengthen the deceleration
time, offsetting any reduction in the time spent searching for the Orient
Start position achieved by using the high Orient Speed setting. Quicker
orient performance may be obtained by reducing the Orient Speed setting
and increasing the Kp value.
In a typical system, the orient position loop gain (Kp), should be between 5
and 20 radians/second. Lower gains are usually used with gearboxes and
high inertia systems. Higher gains can be used with direct drive and low
inertia systems.