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Servo Loop Rates


dholden
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I would like to get some information on how one determines the best servo loop update rate for the specific application they are working on. From what i understand slower, heavier, and higher inertia axes benefit more from slower servo loop update rates of ~2kHz and faster more dynamic systems benefit from higher rates.

I would like to know specifically how one determines when to use what rate. Are there general rules of thumb that seem to work well or are there more specific ways of finding the best update rate to use?

 

Thanks in advance for you recommendations.

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Your general sense is pretty good. There really are not specific formulas for determining optimal servo update rates.

 

In general, higher servo update rates can provide higher performance, but you will hit a point of diminishing returns. (Note, however, that if you have low position resolution or low-frequency resonances, higher servo update rates can actually harm performance.) The point of diminishing returns hits the "negligible improvement" point sooner for your "heavier" (i.e. low bandwidth) systems than it does for higher bandwidth systems. Very accurate systems generally require higher bandwidth to correct errors more quickly, and therefore can justify higher servo update rates.

 

One of my rules of thumb is that "zero friction" systems (as with air bearings) require about twice the servo update rate of similarly sized systems with mechanical friction, because it is so easy for the system to coast past the set point on a small servo correction.

 

Ultimately, you need to get a feel for the effect on real systems of interest. Tune a system as tightly as you can at 2 kHz, then repeat the operation at 4 kHz, and 8kHz (etc.) and see how much improvement you get.

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