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Unread 27-01-2009, 07:10
Jared Russell's Avatar
Jared Russell Jared Russell is offline
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Re: maximum rpm for encoder.

We know that the digital inputs on the cRIO are sampled at 153KHz. From the Shannon-Nyquist theorem we know that we need to sample at twice the maximum frequency we are expecting in order to fully reconstruct the waveform - so the maximum frequency of each encoder channel is 153KHz/2 = 76.5KHz.

Note that there may be (and there probably is) some filtering component on the input, so the actual limit could be lower. Still, I think that "tens of kilohertz" is probably the right answer.

What this means for encoder shaft RPM is a product of the gearing of your system and the CPR of the encoder.

For example, the kit encoders are 250CPR (1000PPR because of the 4x decode). So you probably want to limit these to a maximum of 10-20 rotations per second, or 600-1200 RPM. You can certainly buy lower resolution encoders (or you can make your own using reflective tape strips and an optical sensor - this is what we did in 2006 for our flywheel).

Last edited by Jared Russell : 27-01-2009 at 07:14.
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