On Tue, 7 Nov 2017, Jon Elson wrote:
Date: Tue, 07 Nov 2017 22:07:36 -0600
From: Jon Elson <el...@pico-systems.com>
Reply-To: "Enhanced Machine Controller (EMC)"
<emc-users@lists.sourceforge.net>
To: "Enhanced Machine Controller (EMC)" <emc-users@lists.sourceforge.net>
Subject: Re: [Emc-users] PID refresher for Gene?
On 11/07/2017 08:58 PM, Gene Heskett wrote:
Hmmm, your raw encoder signals seem to have a LOT of width variation. Since
your time scale is 20 ms, there are 20 samples between screen divisions, so
the sampling rate is not likely to be the cause. Generating encoder velocity
from timing the edges suffers if there is poor accuracy of the state
transitions, and would be expected to produce a jumpy velocity just like what
you showed in a previous Halscope trace. A very poor eyeball guess at the
A-B trace looks like you might get 10 - 20% velocity error easily with those
duty cycle variations. This ought to show up on an actual scope, to verify
it isn't in the Mesa -> HAL part of the system.
Jon
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Just counting graticle dots I see some places where theres almost a 2-1
difference in time from edge to edge (take a look at the violet edges before
and after green falling edge roughly 1 graticle line left of center) The
velocity trace seems to be correctly reflect this timing difference.
You can also see that some of the error is straight quadrature error
and some looks like code wheel variations.
With less than 2 counts per sample, the peak noise is proportional 1/T_err.
With 2 or more counts per sample the peak noise is proportional to 1/T_errN
where N is the number of counts per sample interval.
So higher quadrature accuracy will always improve things and higher resolution
encoders will improve things if they have 2 or more counts per sample period
at normal machining speeds.
Peter Wallace
Mesa Electronics
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