User Manual
Appendix D
GFK-1742F
Jan 2020
Tuning Digital and Analog Servo Systems
430
High Bandwidth
•
Allows the servo to more accurately reproduce the desired motion
•
Allows accurate following of sharp corners in motion paths and high machine cycle
rates
•
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 set up by setting Integrator Mode to
“OFF”, is an exponential rise. A time constant for an exponential curve repr
esents 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%. 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.
You can use your knowledge of time constants to predict the required system response. For
instance, if the fastest acceleration required in the 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. Dividing the 200 mSec by 3 results in a time constant of 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.