On Fri, 30 Jun 2017, Gene Heskett wrote:
Date: Fri, 30 Jun 2017 12:22:24 -0400
From: Gene Heskett <[email protected]>
Reply-To: "Enhanced Machine Controller (EMC)"
<[email protected]>
To: [email protected]
Subject: Re: [Emc-users] Rough motion with servos
On Friday 30 June 2017 09:27:29 Peter C. Wallace wrote:
On Fri, 30 Jun 2017, Les Newell wrote:
Date: Fri, 30 Jun 2017 12:11:09 +0100
From: Les Newell <[email protected]>
Reply-To: "Enhanced Machine Controller (EMC)"
<[email protected]>
To: [email protected]
Subject: Re: [Emc-users] Rough motion with servos
I think it is time to put a scope on the encoder. it sounds like
there may be a big duty cycle or phase angle error in that encoder.
In other words, at constant speed, the 4 quadrature transitions are
not evenly spaced, but at least one of them is out of time.
I can see the phase angle errors in halscope by looking at the
measured velocity at low speed. There is a regular 4 step cycle. I
can see the same effect on the other encoders, though it is not as
pronounced. All encoders have some phase angle error but I have to
admit this one is worse than I would like. The problem is compounded
by it being a fairly low resolution encoder. However this does not
explain the occasional big spikes, especially the example where I
had a spike of zero velocity for one count. The machine was moving
at low speed so I could see individual counts in Halscope. To read
zero velocity that count should have been MUCH wider than the others
and it was not. There has to be something wrong with the the way the
velocity is calculated.
Les
I dont think so, the velocity calculation in the driver has not
changed for many years, and electrical noise can simulate a reversal
which will cause a 0 velocity reading...
This has been a very consistent error for years Peter. And I've been
sitting here, with halscope running remotely on this machine for the
last hour watching it, and just had a thought. This consistency could be
explained if there was a difference in the gain as applied to the
velocity averager of about 2/1 between the a and b inputs. The timing,
and the resultant error I am seeing right now, with the spindle turning
around 95 rpms, based on hall effect devices on the 60 tooth bull gear,
is telling me the velocity is stepping over a 2 division (400mv p-p)
range centered on nominally 1.2 volts. The shape of this 4 step pattern
can be explained by a gain difference in the a vs b swings. Is this
something we've been ignoring for a decade or more? I can bend the
opto's a tad on TLM, getting the phasing as perfect as I can, and
adjusting the leds for a 50% duty cycle. But the velocity output on TLM,
triggered on the index, looks very similar regardless of which of the 3
machines I can look at. The velocity is consistently low while the b
input is high, and consistently high while the b input is low.
And of course if I crank it up to 1000 rpms, quantization noise is the
dominant noise in the a/b pattern, but the general shape of the velocity
error only changes because of the rise time of the halscope.
It may be Peter, that my encoders are junk, but why are they so
consistently junk in the same repeatable direction pattern? Optical or
hall effect?
Not sure, but the velocity average is nearly perfect with perfect quadrature
and the errors are proportional to the quadrature error as expected.
I have measured this by using a stepgen running in table mode fed back into
an encoder in table mode and making a 16 step long table with a 1 step
quadrature error (3/5 steps from edge to edge instead of 4/4), in this case
you get the expected 1/3 --> 1/5 velocity steps
This does show that a relatively small quadrature error causes a large
velocity modulation is the relative time between edge changes so much
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Peter Wallace
Mesa Electronics
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Cheers, Gene Heskett
--
"There are four boxes to be used in defense of liberty:
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-Ed Howdershelt (Author)
Genes Web page <http://geneslinuxbox.net:6309/gene>
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Mesa Electronics
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