pic: Encoder Noise

[notmattlythgoe]

Originally Posted by Ether

Your S4 is less than half the CPR of the OP's and has ball bearings. So the two are not comparable. What works for you won't necessarily work for the OP.

At 7K RPM you are exceeding the speed the FPGA could handle if your S4 were at max tolerance for quadrature phase error:

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>
> format(5);
>
> CPR = 120 # cycles per rev (single channel)
      120
>
> EPRS = 2*CPR # edges per rev (single channel)
      240
>
> EPRQ = 2*EPRS # edges per rev quadrature (both channels)
      480
>
> RPEQ = 1/EPRQ # angular distance (revs) between cross-channel edges
0.0020833
>
> per = 261/40e6 # FPGA sampling period, seconds
6.525e-006
>
> RPS = RPEQ/per # maximum revs per sec
   319.28
>
> RPM = RPS*60 # maximum RPM (revs per minute)
    19157
>
> # angular distance (revs) between cross-channel edges
> # with "typical" tolerance:
>
> RPEQ_TYP = (90-10)/90*RPEQ
0.0018519
>
> RPM_TYP = RPEQ_TYP/per*60
    17029
>
>
> # angular distance (revs) between cross-channel edges with "max" tolerance:
>
> RPEQ_MAX = (90-60)/90*RPEQ
0.00069444
>
> RPM_MAX = RPEQ_MAX/per*60
   6385.7
>
>
> # RPM with no tolerance, typical tolerance, and max tolerance:
> RPM
    19157
> RPM_TYP
    17029
> RPM_MAX
   6385.7
>
>

After thinking about it more we weren't at 7k rpm the encoder was reporting a rate of 7k.

[Ether]

Originally Posted by notmattlythgoe

After thinking about it more we weren't at 7k rpm the encoder was reporting a rate of 7k.

7k what?

[notmattlythgoe]

Originally Posted by Ether

7k what?

Pulses per second. Which I believe should put it around 875 RPM.

(7000 pules/s / 480 EPRQ) * 60 s = 875 RPM

[jhersh]

Originally Posted by Ether

@jhersh: (if you are following this thread) would there be any advantage, tolerance-wise, to change the sample size to an even multiple of 4 (say 12 or 8) so that the averaging would always take place over an integer number of cycles?

It depends on the decoding that you have selected. If you select 1x, you have effectively chosen to always average a full cycle, thus averaging out phase noise, If choosing 2x, then multiples of 2 is advised, and if choosing 4x, multiples of 4 is advised (at least if phase noise is something you would like to eliminate, but its a trade-off). I can't imagine a scenario where selecting 4x and 63 samples averaged is better than 64 averaged. I'd love to hear it if you have thought of one.

[Ether]

Originally Posted by notmattlythgoe

...we were decoding in 4x with 10 samples being averaged.

@notmattlythgoe:

if you are still following this thread, you might want to try 8 or 12 samples instead of 10 (see jhersh's post).

[notmattlythgoe]

Originally Posted by Ether

@notmattlythgoe:

if you are still following this thread, you might want to try 8 or 12 samples instead of 10 (see jhersh's post).

I saw that, thanks. We'll have to give it a try.