Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: JohnCNC on February 28, 2018, 09:23:16 PM
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Hi all -
I built a gatton cnc. All good. My Z home, ++ and -- were working fine, then I kept getting resets. I figured it was the z-- and z++ because when I shut them off all good. I check the wiring no issues and when I toggle the limit switches I do get the yellow flash on the diag screen with the home switch. I have the switches in series with active low X, so its hot 5v. I must have toggled something else I am thinking for this to happen, anyone have any ideas?
My x and Y are fine.
Thanks
John
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Hi,
quite possible its electrical noise. Try putting a smallish capacitor (1-10 nf) across the Z axis limit signal line and 0V.
Craig
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Using shielded cable, but i am running along side the motor wire, I will try to separate them.
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Hi,
using shielded cable certainly helps but if you think it will defeat electrical noise you're dreaming. Sorry to be blunt but this myth about shielding has to be busted.
Imagine a signal to a stepper driver, say a 5V pulse working into an impedance of 1kOhm, a fairly typical set of numbers. The energy in a 1 millisecond pulse is
(5)squared/1k *.001= 25uJ, yes only 25 microjoule. An electric motor cable lying next to our signal cable may have a pulse of 100V and 5A for the same 1 millisecond.
Its pulse energy is 100*5*.001=0.5 joule, that is 20,000 times the energy in our signal cable.
If we wish to retain the signal as relatively 'unpolluted' by its electrically noisy neighbor the shielding will have to reduce the radiated energy by something like 50,000
times, just about impossible.
Worse electronically controlled motors tend to have high frequency switching currents and all those jagged current pulses radiate very strongly. Physical separation even
of only an inch or so will markedly improve the situation, separation AND shielding now starts to look like a real solution.
Another area that is overlooked is working impedance levels. You noted that I used a reasonably low impedance (1kOhm) to model the input of the driver but
manufacturers can and do use impedances of 10K, 20K and 50K. The current required to signal the device is markedly reduced, a good good thing, but the pulse
energy is now so low that it will be very very VERY easily swamped by any electrical noise.
The upshot is that currents of around 5-20mA are appropriate for signaling stepper/servo drivers in amateur service, they will often work at very much lower currents
but become increasingly susceptible to noise.
Craig
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So I think I have some sort of short or something somewhere, maybe in the wire itself. Mach3 diag page doesn't show anything when it trips, but one of the ground pins flashes sometimes. Not all the time. I'll keep trying.
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No, it's noise, as Craig said.
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Hi,
it sounds like you may also have an earthing fault. If you attempt to connect a nominally 0V wire to a ground and you
get a flash then your 0V wire is not zero volts at all!
Do you have a sketch of your wiring diagram. If you have two 'earths fighting' strange behaviour results if not outright
damage.
Craig