On Monday 11 October 2021 17:32:31 Peter Hodgson wrote:
Thanks all for your continued support.
I’ve now separated all the earth grounding to individual cables going
to one bolt in the steel control enclosure which is then connected
directly to AC earth wire from the 240v outlet. I’ve also made a
polycarbonate mounting bracket for the encoder housing so it’s
insulated from the machine frame and terminated it’s screen to the
same earth star bolt at the control panel end.
I’m happy to have this now as best practice but it wasn’t the solution
for the ghost pulses.
Interestingly, I noticed that the earth cable coming from the spindle
VFD screen made the encoder signal extremely noisy when it was in
close proximity to the encoder cable.
That smells like a ground loop. If disconnected at the star bolt, it
should be an open circuit, to ground or anything else.
Also, just for your info, the
stepper motors or drivers create a lot of ‘white noise’ on the shop
radio when they are holding or running so I guess they are chucking
out a lot of high frequency noise.
They do.
I generally run my motor cables in shielded cabling. The stepper drivers
control the motor current by turning themselves on and off at an
ultrasonic frequeny we don't hear. If you can find "starquad" cabing in
a gage heavy enough it doesn't run warm at the motors current. It is
actualy the gold standard microphone cable, a top quality microphone
cable available in several gages, all VERY flexible, get the lowest gage
number Suzan has. 22 gage IIRC. Ground the shielding drain wire at the
star bolt, trim and insulate it at the motor end.
It seems I have three options from here.
1) Change to the HPCL2631 opto isolators.
2) Change to the 74HC14 buffer. I think I will need resistance
dividers with this as the max input in the datasheet suggests 6v so I
will need to drop the 12v encoder signal to <6v (?)
3) Try the existing 74HC4050 buffer with resistance dividers.
I’ve got some components on order so I guess whichever turns up first
will be the first I’ll try.
Seems like I also need to find myself some sort of oscilloscope on
eBay!
Digital storage will show you stuff that cannot be seen on an analogue
scope. Definitely worth the extra sheckles. Some of this stuff is at 100
or more megahertz. And tends to be very dim on a analog scope. I
actually have 3, a 30 yo Hitachi 100 mhz dual trace analog, much better
than a tek of the same vintage, a 5 yo digital with the same specs you
can get for $300 or so today, and I just bought Siglents best, a 4
trace, 350 mhz digital sampler. It also costs a down payment on a new
small car. Inheritances are handy.
I’ll keep you posted.
Please.
Cheers,
Pete
On 11 Oct 2021, at 21:11, Gene Heskett <ghesk...@shentel.net> wrote:
On Monday 11 October 2021 15:49:43 Chris Albertson wrote:
You could be correct. High impedance is a recipe for noise.
I had suggested a resistive divider just because it is simpler.
But you are right about providing a ground path. A divider
certainly would do that. If an opto is really needed then use a
high-value resistor to ground to keep the line from floating and
bleed off static.
I also don't like the idea of grounding the shield on the encoder
end as it makes it impossible to know the path from encoder housing
back to true Earth ground. It is "unanalyzable" (if such a word
exists) Running the shield to star ground point makes it easy to
verify it is correct.
+100 Chris. Run a separate ground to the encoder from the star bolt,
and connect the cables overall shield ONLY to that bolt. If that
encoder uses the shield as its ground connection, toss it in the out
bin, and get one that does have a separate ground wie going into it
which is isolated from the metalic case. I would also verify that
the encoder has a good ground to its metalic housing. Painted
brackets are a recipe for failure. As are metallic shaft couplers.
The elastomeric coupler that came with my omron, failed a year ago,
and the coupling is now a couple layers of heat shrink with the
inside layer of thermal glue. If it fails, replace it with a fresh
copy. 50 cents maybe.
Cheers, Gene Heskett.
--
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author, 1940)
If we desire respect for the law, we must first make the law
respectable. - Louis D. Brandeis
Genes Web page <http://geneslinuxbox.net:6309/gene>
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Cheers, Gene Heskett.
--
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author, 1940)
If we desire respect for the law, we must first make the law respectable.
- Louis D. Brandeis
Genes Web page <http://geneslinuxbox.net:6309/gene>
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