MTS Series 793 - Manual (Page 28)

MTS Series 793 Tuning and Calibration
About Tuning
Introduction 28
Do you need derivative
gain?
There is a good chance you do not. It is used primarily in systems performing
dynamic tests. Consider this scenario: You have a specimen that is quite springy
(such as fiberglass). The test is calling for rapid changes in direction (say, for
example, more than 5 times a second) and high velocities. Proportional gain
needs to be set quite high to get this kind of response. Because things are
changing so rapidly, the system is electrically noisy. If the system is making a
rumbling sound, you could use some derivative gain.
On the other hand, it is unlikely for you to need derivative gain for soft materials
such as elastomers.
Another quick (and incomplete) rule-of-thumb is to write down the ratings of
your actuator and servovalve in kips and gallons per minute. If kip ÷ gpm>1, then
derivative gain probably has little effect on the loop.
More about derivative
gain
Derivative gain is used with dynamic test programs. It introduces a “derivative of
the feedback signal.” This means it anticipates the rate of change of the feedback
and slows the system response at high rates of change.
Derivative gain:
• Reduces ringing.
• Provides stability and reduces noise at higher proportional gain settings.
• Tends to amplify noise from sensors.
• Tends to decrease system response when set too high.
Keep in mind:
• Too much derivative gain can create instability at high frequencies, and too
much proportional gain may cause a ringing or screeching sound.
• Too little derivative gain can make a rumbling sound. The correct amount of
derivative gain results in the system running quietly.
Note Series 256 and 257 Servovalves may require derivative gain applied to
both the inner control loop and outer control loop.
More than
Optimal Rate
Optimum Rate Excessive Rate
Overshoot
Ringing
Needs Rate