User`s manual

Startup and Tuning
GFK-1742A Appendix D Tuning GE Fanuc Digital and Analog Servo Systems D-13
D
•
Rejects torque disturbances from mechanics or outside influences improving system
accuracy
•
Can expose machine resonance, which occur at frequencies near or below the
bandwidth
The response of a proportional only system, which is what we set up by setting
Integrator Mode
to
“OFF”, is an exponential rise. A time constant for an exponential curve represents 68% of the
remaining rise. For instance, starting at zero velocity, the response of the position loop to a
change in command will require one time constant to reach 68% of the commanded velocity. The
second time constant will reduce 68% of the remaining command. Subsequent time constants will
reduce 68% of remaining command. For example 100% - 68% (one time constant) = 32%,
32%(68%)=21.8%, 68% (first time constant) + 21.8% (second time constant) = 89.8%. We see
that two time constants eliminate 89.8% of the command necessary. Three time constants will
account for 96.7% of the rise in command. Four time constants account for 98.9% of the rise.
Typically three time constants are sufficient for most motion applications.
We can use our knowledge of time constants to predict the required system response. For instance
if we know that the fastest acceleration required in our motion profiles must occur within 200
mSec. The 200 mSec response to the change in command will be 98.9% complete in three time
constants. Simply dividing the 200 mSec by 3 tells us that a time constant will be about 67 mSec.
The
Pos Loop TC
configuration field represents one time constant in mSec.
In the example
above one time constant is 67msec.
Position Loop Proportional Gain Method 2
Similar to the Velocity loop tuning method above. Use an oscilloscope and gradually lower the
Pos Loop TC value (increasing gain). Monitor the
Motor Velocity
analog output for performance
characteristics are appropriate.