maximum rpm for encoder.

[windell747]

Has anyone found the maximum rpm that the encoder can track a wheel before some of the pulses are missed because the pulse train is too fast? The reason why I ask is that I am considering using the encoder to track the velocity of a cell shoot wheel. any thoughts?

[Jared341]

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).

[windell747]

great stuff! Thank you very much! I have some 100PPR encoders that might be able to do the job, otherwise, i might take you up on your make your own encoder wheel idea.

[Ranana_Prussy]

So its been suggested to me that for ppvs I increase the rp maximum to 5. I thought Id see what the board thinks of that idea.