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Sherlock Ohms
CNC Control in Electrically Noisy Environments
8/14/2012

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naperlou
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Shorter signal path
naperlou   8/14/2012 9:02:24 AM
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I was wondering, as I was reading the article, why you did not look at shortening the signal path.  I can think of reasons, but I am wondering what the situation is in this case.

GlennA
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ground loop ?
GlennA   8/14/2012 11:07:31 AM
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You mention using shielded cables.  Did you ground only one end of the cable shield ?  I have seen installations with both ends of the cable shield connected to ground, which makes ground loops a concern.

kf2qd
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WHy not proper shielding?
kf2qd   8/14/2012 11:30:30 AM
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Having built and serviced CNC plasma cutting machines, proper shielding of cables does wonders. The comercial products we used all had shielding built into the harness to keep the noise out. And the shields were always grounded at 1 end only., and in some cases where the design was older and more sensitive they actually had 2 shields, insulated from each other and each one grounded at opposite ends of the cable.

Also - effort should be made to seperate the motor and encoder cables. Often just having them shielded and a few inches apart is sufficient.

ttemple
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Re: WHy not proper shielding?
ttemple   8/14/2012 1:34:23 PM
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While it doesn't explicitly state it in the article, it appears that the encoders in question have single ended outputs.  It appears that the extenders convert the signals to differential mode, which is much more noise tolerant.  Shielding is a good idea, but with single ended signals I doubt it would have been enough if there is serious high frequency switching noise all around.

Jon Titus
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Consider a current loop?
Jon Titus   8/14/2012 7:32:22 PM
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Just wondered if you considered using a 4-20-mA current loop in place of the voltage signals.  Current loops are not susceptible to electrical noise that can scramble voltage signals. Avago Technologies has 8-pin DIP optically coupled 20-mA transmitters and receivers that simplify current-loop circuits and provide isolation, too.  Texas Instruments has at least one 20-mA transmitter.

I have driven a teletypewriter (circa 1978) with a 20-mA current loop over almost a quarter of a mile of wire.  Granted, at 110 baud.  But in short-distance wires, you can get a high data rate.

TJ McDermott
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Re: WHy not proper shielding?
TJ McDermott   8/15/2012 12:52:19 AM
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Signal cable shielding should be grounded at one end only, but shouldn't motor cable shielding be grounded at both ends?

kf2qd
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Re: WHy not proper shielding?
kf2qd   8/15/2012 11:51:43 AM
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Single ended encoders will work just fine. Plasma machines with 5V single ended encoders and proper shielding and machine grounding will work just fine. Because of the arc involved in Plasma cutting poor machine grounding can induce pulses in the various electrical circuits and cause damage. And grounding is at times a black art... Machines that run fine in one location with minimal attention to grounding practice won't function in another area without special grounding practice.

If you connect a shield at both ends it becomes a ground wire and can cause even more problems if and when there are gound currents. The purpose of a shield is to block and blead off any external electromagnetic garbage, not to act as a possible conductor. There should be a dedicated ground wire in the motor harness.

Another important thing to consider is GROUND LOOPS. If you have multiple ground paths on a machine you have the potential for different currents to flow in the various ground wires/paths. Then your grounds are no longer at the same reference and wierd and wonderful things can happen like solid state components turning on and off seemingly at random.

TJ McDermott
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Re: WHy not proper shielding?
TJ McDermott   8/15/2012 11:59:36 AM
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I should have been more specific - I was talking about the power cable for a motor.  Quoting from one drive manufacturer's installation manual:

The Shield terminal provides a grounding point for the motor cable
shield. It must be connected to an earth ground by a separate continuous lead.
The motor cable shield should be connected to this terminal on the drive (drive
end) and the motor frame (motor end). Use a shield terminating or EMI clamp
to connect shield to this terminal.

Motor power cables are the only time I would do this; I would be curious if this might have eliminated the positioning motor and the spindle motor radiated noise.

 

kf2qd
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Platinum
Re: WHy not proper shielding?
kf2qd   8/15/2012 12:03:57 PM
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Not knowing the particular construction of the motor you refer to - I would assume that the shield connection at the motor end is just continuing the shield connection through to parts on the motor not directly connected to electrical ground. This would keep the noise (pwm is really noisy..) from the motor cables from getting to the encoder/resolver wires in a parallel cable.

TJ McDermott
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Re: WHy not proper shielding?
TJ McDermott   8/15/2012 12:06:17 PM
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The shield is connected to motor frame.  Usually, motor frame and ground have continuity.

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