On 13/10/2021 15:43, Gene Heskett wrote:
On Wednesday 13 October 2021 06:06:06 Peter Hodgson wrote:
I just tested the VFD shield and supply line shield and both are open
circuit when not connected to the start bolt.
There are three AC motors: Spindle, Oil Lubrication and Coolant Pump
that are all grounded to the machine frame via their mountings but I
have also ran individual earth cables to the star bolt. Is that good
practice??
Pete
I wouldn't think so Pete. Only the house static ground, the third pin of
the power plug, the bare wire in romex cable, s/b connected there. Your
connection to an earth ground is paralleling the static ground to earth,
normally made in the meter head box or just below it. Thats the one and
only place where its even legal to connect static ground and neutral
together here in the US.
That earth connection is potentially a huge ground loop. The machines
themselves are, I assume sitting on concrete, a relatively poor
conductor, and their frames should also be connected to that bolt unless
the bolt itself is on the machine frame which will accomplish, barring
paint, the same thing.
I think I might be creating confusion here with my terminology. When I
say 'grounded' I mean the motor chassis has continuity to the machine
frame via it's mounting bolts so therefore is 'earthed' BUT I have also
run an individual earth cable from the motor chassis to the star bolt.
No AC Neutral or dc 0v are taken to the machine frame or earth (as far
as I know). My question is ......Is it good or bad practice to have the
motor chassis with mechanical continuity to the machine frame AND
continuity to the star bolt with an earth cable i.e. two routes to earth??
I didn't build this machine control. It was an eBay purchase where the
LCNC computer was lost and seems like it had been passed around a bit
whilst various people tried to get it running again. It's running quite
well now after I bought it and got my head around LCNC but I'm now
trying to iron out these last few wrinkles!!
Pete
On 12/10/2021 02:50, Gene Heskett wrote:
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.
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Cheers, Gene Heskett.
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