Rockwell Automation 8510 AC Spindle Drive System Programming Manual - Manual (Page 38)

Drive Tuning
Chapter 3
3-35
Operating the system in its normal metal cutting mode provides a
simplified method to verify stiffness. Load changes occur due to
milling cutter tooth frequency, intermittent cuts, etc. Velocity
disturbance can be observed by monitoring the motor rpm analog
output signal. Proper stiffness can be assumed when the velocity
disturbances are acceptable.
Stiffness can be measured by applying torque to the system when it is
at rest. The shaft will move an amount proportional to the torque.
When torque is removed, the shaft will return to its original position.
Stiffness is the ratio of torque to the angle moved. The greater the
stiffness, the lesser amount a given torque will disturb the shaft
position, and the quicker the system will respond to counteract the
disturbance.
As I Gain is increased, stability will gradually decrease. Frequency of
oscillation will be relatively low and audible noise should be at a
minimum. This suggests another method for adjusting stiffness,
simply increase I Gain until instability is about to occur, then lower
the gain until the system is well damped.
o 4. When operating the 8510 in servo mode in a closed position loop
(such as during C-axis operation or spindle orient), a “stick/slip
hunting” may be observed. Increasing the Droop In Run value can
minimize this.
“Stick/slip hunting” occurs when static friction is significantly higher
than running friction. As the motor approaches the final commanded
position, the position and velocity error decrease until the
commanded torque is less then the running friction. The motion will
then stop. When the system is at zero velocity with a non-zero
position error, the velocity loop integrator will slowly increase the
motor torque until it builds to the point of breaking the static friction
lock and causing the system to move. Once motion starts, friction
immediately drops from the static level to the running level. Since the
torque being generated to overcome static friction is too high for the
lower running friction level, overshoot of the target position may
result. When motion stops in the process of reversing, static friction
again locks the system. The integrator slowly builds up torque in the
opposite direction until it breaks the friction lock and overshoots in
the opposite direction. Hunting now becomes a never ending process,
usually in the range of 0.5 to 2 Hz.
The integral gain compensator has infinite DC gain, meaning that the
smallest position error (velocity command) can be integrated up, to
eventually generate maximum torque. Droop places a limit on DC
gain and creates a threshold below which small position error cannot
generate enough torque to break the static friction lock. The never
ending cycle of stick/slip hunting is broken. Increasing the Droop In
Run parameter setting will increase the level of this threshold.
o 5. Repeat steps 1 through 4 for each gear range, motor winding, and
mode if applicable.