Motor noise disrupting LCD character display when connected to FPC overlay

Discussion in 'The Projects Forum' started by ebeowulf17, Jul 1, 2015.

  1. ebeowulf17

    Thread Starter Active Member

    Aug 12, 2014
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    Apologies in advance for the lengthy post - the situation here is complicated.

    There's a 4x20 LCD character display (Newhaven NHD-0420H1Z-FSW-GBW-33V3) in an aluminum housing, connected to the main electronics box of our machine through a 20 pin ribbon cable. All of its power, grounding, data, etc. comes through this cable with very small gauge wires. There's also a flexible printed circuit with integrated buttons that sticks to the outside of the housing and connects to the PCB inside through a ZIF tail. The button traces pass through the PCB from ZIF tail to ribbon cable as directly as possible, essentially running directly to the MCU inputs inside the main body of the machine (they do also get 3.3V through pullup resistors on the PCB in the display housing.) The shared ground trace for the FPC is the same ground used for all display related functions. In the first picture below, you're seeing a clear prototype of the FPC, so the dark lines are the actual traces (except for the S logo!)

    display-housing2.jpg display-housing3.jpg display-housing1.jpg display_05B.png

    The issue is that if the display housing is touching the machine's frame, or even resting on a steel countertop that the machine is sitting on, some sort of noise related to the motors switching on causes bad characters to appear on the display. Interestingly, the character is always a repeat of whatever character was last printed, and it appears in the next position over. Based on that, I suspect that this noise is only being picked up by the display enable pin, and not necessarily affecting anything else.

    The noise seems to get in through the FPC, but it takes the combination of FPC and aluminum housing to make it significant. With the FPC disconnected from the PCB, there are no glitches at all. With an FPC connected, but floating free (not glued onto the housing,) there are very, very few glitches picked up. But when the FPC is stuck to the housing as it's meant to be, the display glitches on almost every motor start.

    I believe the housing is picking up whatever the noise is and that it's getting into the FPC through capactive coupling on the ground trace. The other FPC traces are pretty well isolated from the LCD itself, but the ground is shared, and it's got a pretty crummy path back to real ground through the small-gauge ribbon cable, so I could see how it would get noisy. I tried measuring resistance between the FPC traces and any part of the housing and found no evidence of any shorting. Measuring capacitance between traces and housing, I found a range of values that increased relative to the length of the trace, with the ground trace having the highest capacitance at .109nF. Seems like a very small amount of capacitance to be picking up such destructive noise, but I don't see how else it could be getting in!

    The way I see it, there are a lot of ways one might try to solve this problem:
    1) Eliminate motor noise at source.
    2) Improve grounding overall, and especially in display housing.
    3) Reduce coupling between FPC and display housing.
    4) Make display circuit more robust against noise issues.
    5) Somehow filter, protect, smooth LCD Enable pin.

    We're doing the best we can at sorting out 1 and 2, but I don't feel like we can count on those right now. Reducing coupling (#3) seems like it would be a huge improvement, but I have no idea how much more space between traces and housing it would take to make it reliable. I fear it would be way too much thickness to be practical. So, even though I think these are the less ideal approaches, I'm here humbly asking for help with numbers 4 and 5.

    I've made a rough attempt at simulating a simplified version of the circuit to look at noise filtering approaches. I have no idea how to characterize the noise, so V3 is just a wild guess I created in order to inject a visible spike into the signal through capacitive coupling. Below is my best-guess simulation of how things work now, followed by a version where I added some decoupling caps.

    display-noise_1.png display-noise_2.png
    I'm used to using decoupling caps to stabilize a power level, but I've never tried to couple a stable(ish) power line to stabilize a bad ground. Does this seem like a plausible idea? It seems to have a positive effect in the simulation, but that's based on a LOT of my own assumptions.

    Another zany idea I had, which I suspect is futile, is to do something (I know not what) to the Enable pin signal to make it less sensitive to noise. My fear is that anything that slows down noise spikes would also slow down legitimate signals and that the timing of the display would go wonky and ruin everything!

    Do either of my wacky ideas sound promising? Better yet, do you all have other suggestions for smarter ways to deal with these issues? I'm pretty close to my wit's end!
     
  2. ebeowulf17

    Thread Starter Active Member

    Aug 12, 2014
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    P.S. The aluminum housing definitely plays a large role. We did extensive prototyping with a 3D printed plastic model and never had a single problem, but have had tons of very repeatable problems ever since getting cast aluminum pieces in.
     
  3. RamaD

    Active Member

    Dec 4, 2009
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    I am no expert on EMI/EMC issues.
    Can you try a shield on the back side of the pcb or case, connected to the ground and insulated from the case. Copper foil tape with adhesive backed (3M) could be tried. "Adhesives are non-conductive, even if the manufacturer says it is" I am quoting this from a Silabs Application Note, unfortunately I am unable to locate. This statement I remember because of the way it has been written! I am leaving out the front due to the complexity involved due to display window. And I am assuming that this metal enclosure cannot be grounded as it would come in contact with other structures.
    A cable ferrite on the cable.
    Shielding the cable.
     
    Last edited: Jul 1, 2015
  4. ebeowulf17

    Thread Starter Active Member

    Aug 12, 2014
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    Thanks for the input. I tried a few experiments this morning with little luck.

    First was running an additional ground wire between the main pcb in the machine's main electrical box and the little display pcb in the display housing. That made little, if any, difference.

    Next was adding 10uF and 0.1uF capacitors across 3.3V and display ground. No help there either.

    Next was trying a ferrite bead from some old ribbon cables and putting it around the ZIF tail (between FPC switches and display PCB.) No luck there either.

    Finally, I temporarily replaced the motor switching relay with an SSR, which completely solved the problem. Unfortunately, for several different reasons, we can't switch to SSRs, at least not in the near future. So this may illuminate the situation a little, but doesn't actually solve our problem yet.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If you haven't already I would set up a star ground where the main GND conductor also terminates, also try ground bonding the shield of the ribbon cable at both ends.
    Is all DC supplies grounded at their common?
    Max.
     
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  6. ebeowulf17

    Thread Starter Active Member

    Aug 12, 2014
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    Sorry for misusing ground terminology again. All DC commons are tied to actual ground at just one point where the chassis and the ground wire of the power cord come together. I believe we've got star grounding properly implemented here.

    However, we may have an inadequate ground and/or DC common connection to the display. The display PCB gets its DC common through a single 28 gauge ribbon cable wire, and that DC common is also extended to just outside of the display enclosure on the FPC, via the ZIF tail. The aluminum enclosure itself is not grounded in any way; it's just floating. Unfortunately our ribbon cable is not shielded, so we don't have the shield available to make such a ground connection.

    Given that we only have this problem when the display is touching something grounded, I wonder if we have a ground loop acting as an antenna? There definitely is no conductive connection between the display housing and any machine ground or DC common, but there is the capacitive coupling (.109nF according to my DMM.) Can the antenna effect happen if the loop is only completed through capacitive coupling?
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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    IMO it sounds suspiciously like the reason is that the ribbon is not shielded, (I use a Special shielded ribbon cable for this), and LCD case ungrounded.
    Can you not at least ground bond the LCD case with a proper ground conductor?
    Without proper (equi-potential) bonding there can be loops created inductively or capacitively.
    Industrial method.
    http://www.automation.siemens.com/doconweb/pdf/840C_1101_E/emv_r.pdf?p=1
    Max.
     
  8. ebeowulf17

    Thread Starter Active Member

    Aug 12, 2014
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    All right, sounds like we might be on to something! Thanks, as always, for your help. I don't know how we managed to build a machine for 5 years without ever thinking about the fact that the display enclosure was totally un-grounded! (We're in the testing phase of a new enclosure now, but for the previous 5 years we've been building a different metal enclosure for a similar display - it too had these problems sometimes, but no solution was found.) Changing parts, cables, suppliers, etc. may not happen as quickly as I'd like, but I can try an experiment right away with just running a separate wire to properly ground the display enclosure and see if performance improves. I'll give it a go and report back when I can. Thank you very much!
     
  9. ebeowulf17

    Thread Starter Active Member

    Aug 12, 2014
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    Well, adding a ground connection from display housing to machine chassis ground actually made the noise pickup stronger and more regular. Now, instead of only picking up noise when the housing is touching the machine or touching a grounded countertop, it picks up noise all the time. Also, the glitches are bigger; instead of just repeating whatever character was last written, it now spits out from 3 to 6 random characters every time a motor starts!

    I'm starting to feel like the capacitive coupling between display housing and DC common (within the FPC sticker) is completing a ground loop antenna. It would seem to explain the increase in noise pickup I just experienced, right? By making a permanent, more solid connection to the chassis ground, which works in conjunction with the DC common to complete the loop, I've made a better antenna?
     
  10. RamaD

    Active Member

    Dec 4, 2009
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    I was looking at radiation. An additional ground wire could hardly protect. And Signals & supply wire are still exposed. Shielded wire gives higher protection than beads.
    Is there any length where the Ribbon cable is running alongside the motor cable? Just in case, the usual keep them away and cross them perpendicularly if they need to cross.
    I had a similar problem quite some time back, with a larger NHD LCD. The display would go berserk occasionally, when a phase selector switch to the voltmeter was operated. No switching of any load, just the voltmeter! Maybe, once in 20 times. The cables ran parallel for about 60cm!
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Incidentally what is the nature of the motors, and the method of switching?
    Max.
     
  12. signalflow

    Member

    Mar 12, 2014
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    I am having a very similar problem now. I have a mainboard and a separate LCD board connected via a ribbon cable. I spaced the LCD board off of a metal plate with 0.5" tall nylon spacers. I sit the plate with LCD board on the equipment chassis (earth gnd) and get tons of noise glitches on LCD when motors or contactors turn on/off. If I lift the LCD board and plate off of the equipment chassis, then I get no noise glitches. Even if I hold the LCD in my hand and my hand touches the equipment chassis, then I get noise glitches on the LCD. I am still experimenting myself trying to get this solved. Right now my digital PCB ground is not connected to earth gnd. I've tried doing both ways, but it doesn't help.
     
  13. ronv

    AAC Fanatic!

    Nov 12, 2008
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    I think I understand your hook up - not sure, but I'll wing it.
    Here is what I think is going on:
    While you think the circuit in the box is grounded it is really hanging out there on the inductance of the cable. So what is happening at one end is not really happening on the other end of the cable. Here is what I would try. Since you care about the signal at the micro end of the cable move the pull up to that end. Remove the .1 ufd across the switch in the box or move it to the other end as well. Make the pull up as small as the switch can handle safely. Big difference in the signal the micro sees and on the box end both ground and 3.3 will move together.

    PS. This assumes the noise is picked up on the ground of the flex.
     
    Last edited: Jul 2, 2015
  14. ebeowulf17

    Thread Starter Active Member

    Aug 12, 2014
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    Thanks! I'll play around with this sim a little and see if I can wrap my head around it. Moments like this just make it obvious how far I'm in over my head! I have no doubt that you're right about the "ground" in the display housing being quite a bit different than the ideal ground I'd want it to be. I was just thinking in terms of resistance before, not inductance too. Should've known it would be more sinister than that!
     
  15. ebeowulf17

    Thread Starter Active Member

    Aug 12, 2014
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    Best of luck! If I find any real solutions, whether through this forum or elsewhere, I'll be sure to share the results here.

    For what it's worth, depending on how robust and foolproof of a solution you need, we've found that simply putting rubber feet (even some very thin ones, like 1 or 1.5mm) on the bottom of our display housing provides enough isolation from the grounded counter to eliminate glitches.

    We can't trust the rubber feet to last, plus we're afraid of customers putting our display in contact with other grounded parts of the machine, so for us it's not a very satisfying solution, but it is quite effective as long as the rubber feet are the only points of contact!
     
  16. ebeowulf17

    Thread Starter Active Member

    Aug 12, 2014
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    Let me start by saying I'm pretty ignorant when it comes to the various motor types. To the best of my knowledge, these are centrifugal switch start motors. They're 1/3HP, 208-240VAC single or split phase. 20150625_113751_HDR-1.jpg
    They're being switched by a DPDT relay, which is itself powered by 240VAC, driven by what seems to be a pretty quiet zero cruising opto isolated triac circuit. The motors have snubbers (100ohm, 0.1uF) wired in parallel across the line inputs, which have eliminated the vast majority of the switching noise we used to see... In fact we really thought we were done worrying about motor noise until this display issue came up.
     
  17. ebeowulf17

    Thread Starter Active Member

    Aug 12, 2014
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    [​IMG]
    This is the relay, in case it matters.

    We also sometimes use SSRs instead, and we don't have any of our various motor related noise problems when using SSRs. I wish we could just use them all the time, but we can't, at least not anytime soon.
     
  18. ebeowulf17

    Thread Starter Active Member

    Aug 12, 2014
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    No, the ribbon cable is never anywhere near the motor lines. At their closest, they are maybe ten inches away, near opposite corners of the electronics enclosure. From there, they exit in opposite directions. But you're right about them being unshielded and potentially vulnerable.
     
  19. ebeowulf17

    Thread Starter Active Member

    Aug 12, 2014
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    So I played with the sim a bit finally and I'm not sure I see the improvement I was hoping for. We don't have any [known] issues currently with the switch signals to the micro, so I'm not specifically aiming to make improvements there (although I can hardly argue against making the noise situation better there if there's an easy way!)

    The actual problems we experience are on the 4x20 character LCD. The glitches seem to always be repeats of whatever character was last printed, which makes me think the noise is triggering the Enable pin on the LCD. Since there's no path I can think of for noise that's only picked up with the FPC in place (so presumably noise being induced into the FPC, which would be switch wires or the "ground"/common) to get to the Enable line, I've been guessing that the "ground"/common is moving far enough relative to the Enable line to make it look to the LCD like the Enable pin cycled. I could be wrong on this, but it's my best guess so far.

    As far as I can tell, the rearranged pull up resistors and capacitors have no effect on the ability of the "ground"/common in the display housing to dissipate or reject noise, so it seems like I would still have the same problem. Please correct me if I'm misunderstanding all this (wouldn't surprise me at all.) If not, is there any way to make a virtual ground in the display housing that is less susceptible to noise?
     
  20. ronv

    AAC Fanatic!

    Nov 12, 2008
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    I had trouble reading the schematic, but assumed it was the micro that enabled the LCD based on some signal it got from the switches. Maybe that's not the case. I would think the grounds within the case would be close to the same (short leads), but maybe not. Might need to drag out the scope.:(
     
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