On Wed, 8 Nov 2017, Jon Elson wrote:
Date: Wed, 08 Nov 2017 10:51:37 -0600
From: Jon Elson <[email protected]>
Reply-To: "Enhanced Machine Controller (EMC)"
<[email protected]>
To: "Enhanced Machine Controller (EMC)" <[email protected]>
Subject: Re: [Emc-users] PID refresher for Gene?
On 11/07/2017 10:54 PM, Gene Heskett wrote:
They are visible on my 100 mhz hitachi dual trace, but not to the extent
shown by the pix I just sent, Jon. Quantization errors keep adding up.
Well, this may actually be informative. What are the voltage levels coming
out of the encoder?
Are they a good match for the logic thresholds of the Mesa board? Maybe a
pull-up or terminating resistor might help clean up what the Mesa sees. If
the A and B HAL signals are AFTER the digital filter, then these width
variations may be caused by the Mesa board rejecting noise on the inputs.
I've seen a lot of problems with encoders where cable capacitance causes both
signals to transition at the same time, and this illegal transition is
rejected by the digital filter. The transition is not recognized until the
crosstalk-caused transition corrects itself. With weak pull-up, that can
take some time.
I'm looking at the Omron 2500 line encoder for a touch over $40, and at
least a month to get across the big pond.
But while it looks to have ball bearings, they are probably vxd's. Shitly
stuff thats not really fit for roller skate wheels.
No clue how hard it might be to put decent bearings in it.
Many encoders are not meant to be repaired. The glass disks are plenty
fragile, and getting the read head off and the shaft pressed out without
breaking them is quite tricky. Even the ones with etched steel discs are
fragile.
Can you pretend to advise?
There are also "kit encoders" where you get a disk and read head, and maybe
an enclosure, and fit it to an existing shaft. You could make up a bearing
nest and shaft and fit that to it. Avago, US Digital, Renco and others make
these, and they are not terribly expensive. You can get a wide variety of
the Avago (formerly HP HEDS-series) from Digi-Key and Avnet. You should have
no problem getting one of these kits for $40, but of course that does NOT
have the bearings in it. I think US Digital has complete encoders close to
that price, with the etched steel disk. I have used the Avago HEDS-type
encoders to mount on motors with rear shafts. Quite easy to do. So, if you
can manage to fit a rear shaft, that would be the best way to go.
I'm not sure a higher resolution encoder will really solve your problem. As
you raise encoder resolution, the timing information you get from the edges
gets smaller and smaller.
Noise could be a problem, and can be checked by plotting the unfiltered
inputs (by determining the GPIO pins that correspond to quadrature A,B
inputs and plotting them)
But if you look at the velocity and A/B plots you can see a very distinct
4 edge period to the velocity noise, this really looks like quadrature error
rather then electrical noise.
Note that increasing the encoder resolution will reduce the velocity noise in
inverse proportion to the encoder resolution, as long as there are more than 2
counts per servo period at the speed of interest.
Peter Wallace
Mesa Electronics
(\__/)
(='.'=) This is Bunny. Copy and paste bunny into your
(")_(") signature to help him gain world domination.
------------------------------------------------------------------------------
Check out the vibrant tech community on one of the world's most
engaging tech sites, Slashdot.org! http://sdm.link/slashdot
_______________________________________________
Emc-users mailing list
[email protected]
https://lists.sourceforge.net/lists/listinfo/emc-users
