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Unread 24-04-2010, 14:35
vamfun vamfun is offline
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Re: Unexpected results from Encoder::GetRate()

Quote:
Originally Posted by Alan Anderson View Post
I don't believe that is true. Yours has the ambiguity; a true quadrature decoder does not.
Don't follow you here. All encoders have a resolution ambiguity. The 4x quadrature decoder is 1/4 of 1x but there still exists the ambiguity within the resolution deadband.



Good to hear from you Alan. I am primarily addressing the 1x decoding of a quadrature encoder. So we have two channels to make the best 1x estimate. We are using 1x to minimize the rate errors associated with extra edge phase noise found with 2x and 4x decoding. So please evaluate my comments in this regard. The 1 deg resolution is identical to the kit 360 count encoders.

The comparison is between Joe's 1x and my 1x. The example doesn't favor either but is designed to illustrate the differences in the two approaches, so it focuses on the edge reversal. If there isn't an edge reversal, then the two schemes are identical. You can start anywhere on the cycle... at the exact point where the edge reverses, Joe's error will have a max 1 count error (ambiguity) and mine will be zero. As the reversal continues toward the next edge, Joe's estimate gets better and mine gets worse. If you look at the example carefully, you will notice that the max distance error is the same for both and will not exceed the resolution.

If the Kevin W. algorithm was implemented as I discussed earlier in the thread , I believe it would produce the same output as my 1x case. So I thought I was arguing in your favor.

Quote:
You also left out all of the samples that would reveal that yours gives different values for the same position depending on where it came from. For measuring position, I do not believe hysteresis is a positive attribute.
In my example, I am 0 going up to 1.1 and 1 returning so this is exactly what is illustrated. But , I claim this is the best estimate you can make when a reversal on the same edge is detected. If at the reversal, I ask you where you are and you tell me that you are 1 count further away than just before the reversal... then I know you can do better than that. My algorithm lets you say that you are in the same position that created your last count, hence it gives a more accurate guess at that point. Continuing in the reverse direction , if you happen to move close to the next edge... I will be reporting a 1 count error while Joe would be spot on. Users of either scheme cannot claim any better accuracy than 1 count after an event has been registered. But, in the neighborhood of the reversal event I can claim better accuracy hence I report better rate performance in the presence of your oscillating wheel phenomenon which triggers that event. This is a very subtle point and that's why I'm belaboring it.

Let's nix that "hysteresis" word, since it only applies to same edge reversals not all reversals and may be confusing.

Last edited by vamfun : 24-04-2010 at 16:04.
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