Electrical Noise Causing Axis Jumps

14 Mar 2023 22:00 #266708 by rmu

Why and how do you get random E-STOPs? 

Generally through noise on a limit switch input. 

I generally debounce the limit switches, as nuisance tripping can be expensive, whereas a 10ms delay in tripping the limit switches probably doesn't matter that much. 

But that is not a general "E-Stop", linuxcnc complains "axis limit exceeded" or something like that.

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16 Mar 2023 03:50 - 16 Mar 2023 06:55 #266801 by jlennox
Andy, I managed to find a RFI filter available local to aus, made by Fuji but has the exact same part number a the Rasmi filters, going to pick it up and install.

Tommylight, I have attached a photo of my new control rack (note this photo doesnt show the encoder wires to the mesacard as in the first post photos). The encoder wiring goes as such; everything is square brackets is inside of my new control rack.
Encoder -> Existing resolver wires (shielded twisted pairs for A/A', B/B' and 5v/Gnd) -> [ MR20 Breakout Board -> UTP cable (using 3 pairs)  -> Mesa 7i77 ]
Do you believe it's possible I am getting noise on the utp cables? They only run 400mm inside the new control rack. I have also orderded shielded twisted pair cable for the 0-10v signals.
You also make a good point on building grounding, my machine lives at my work place and guys running previous machines (robotics and automation company)  have said they have routinly encountered noise issues directly related to grounding, something i'll need to investigate.

RMU, apologies, they a random machine enable dropouts, not full e-stop. As the majority of sensors go through classic ladder fist to replicate a lot of the original machine plc code, it is hard to determine what is exactly tripping it, I am assuming limit switches or something lower level as all the critical e-stops have a gmoccapy message tied to them so would show. I could set a timer up in classic ladder to be able to catch these as they happy too fast normally.

I am sadly out for a week or so (family destruction expansion) but will install an EMI filter when i'm at the machine next and test each 24v PSU (two in machine, one for control rack once for sensors & switches) with a scope and see what the noise looks like before the emi filter goes in, would really like to know the cuplrit rather than 'covreing' it with the emi filter.

Cheers for the help so far guys.
Last edit: 16 Mar 2023 06:55 by jlennox.
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16 Mar 2023 09:53 #266817 by rodw
In .AU the neutral wire is bonded to Ground in the building switchboard.
When I went on a noise busting crusade with an oscilloscope, I found that a lot of the noise was entering via the mains ground and a distinctive 50 Hz AC ripple showed up. Fitting an EMI filter on the mains input basically solved all problems.
The other thing I did is follow a guide about plasma cutters from Hypertherm. This involved setting a star ground point on the frame and running a seperate wire from every motor frame back to the star ground point. This guarantees a low impedence path for noise to dissipate. Without this, the noise is exascerbated as it use high impedence paths though your linear bearings that are not firmly connected.
Some higher end steppers have a grounding point on them. I always use them regardless of the machine...

So I think adding the EMI filter is just part of a quality built machine, and definitely not an admission of defeat!
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25 Apr 2023 04:05 #269804 by jlennox
So I finally acquired some parts and got round to doing some more testing on the machine. I installed an EMI filter on the input of the Spindle Drive and a toroid ring on the output as well. All analog and encoder signal wires are now shielded which are correctly grounded.
I started up the spindle and the nuissance tripping and axis jumping continued. Still thinking the position of the head of the machine had an impact on this, I ran a new cable externally from the spindle drive to the motor to see if this was the issue and the problem still persisted. While I was doing this test, the guys at my work turned on either the induction heater or laser ablation system (automation solution we are developing for a customer) and as soon as it turned on the machine threw a fit and tripped stright into an e-stop. Now knowing this occured, the other day when I was running the machine and it was behaving normally was actually when this automation system we are deveopling was all disconnected as we were reolcating it. This is why the issue was present before and after my sccessful machining job and is worse now than it was at the start as more equpiment has been connected to the automation system in the factory.

With all this in mind now, I am not sure what should be next. I know the power supply that supplis the MESA cards and servo tacho generators has a line filter inbuilt but questionable to exactly what it is. I am thinking of getting a 2-stage line filter and putting it on this power supply and one for the 24v pow supply that supplies all sensors on the machine and see if this will help.
I believe the spinde drive is just a small part of this bigger issue as when it turns on the incoming noise must just be reaching a level to cause issues which is amplified as more equipment is powered on within the factory.

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25 Apr 2023 08:22 #269825 by rmu
I find it quite remarkable that radiated interference has enough energy to trigger opto-inputs. Maybe your induction heater is very "leaky"?

Getting rid of electromagnetic interference is something of a black art. Line filters won't really help with radiated emissions. You can try to minimize the area your circuits span (current always flows in loops, the larger the area those loops span the better antenna they are). Try to avoid "open" inputs, so either use SPST switches or use low-enough pull up/down as appropriate. caveat emptor. IANAEE.
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25 Apr 2023 08:38 #269827 by jlennox
RMU, it my have been the induction heater or the laser ablation system, I think it was the latter but will do a test with the guys this week to try to repeat this. Both these systems are on the other side of the factory ~30m away an on a separate sub-DB to my machine.

My thought process at the moment is when other equipment is on in the factory it is producing noise on the 3PH AC lines and when I start the spindle drive it pulls more current creating more noise which tips it over the edge (I am going to have the induction heater/laser ablation turned on with machine on and spindle drive off to see if it trips again or only with spindle drive on). The machine ran fine milling robot risers flat the other day with no issues or tripping. Once the automation system was reconnected this issue returned. Do note this automation system has 2 robots, conveyor system, laser ablation, induction heting and welding, max amp input is 240A peak.
I need to somehow protect the power supplies for control and sensors from this noise which causes the nuissance trips and axis jumps.

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