Odd PIC behaviour near a VFD - very subtle


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  1. #1
    Member Peter Truman's Avatar
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    Default Odd PIC behaviour near a VFD - very subtle

    Hi all

    I have a controler based on a 16F876a mounted in a metal enclosure about 2 meters from a 110Kw variable frequency drive.

    Everything on the digital sdie looks ok (not 100% sure of that yet) but I am finding some subttle problems on the analogue output side.

    Im using a TC1320 DAC - then an LM358 opamp with a gain of 4 to give me a 0-10v output. This output forms the referance for the PID in the drive.

    I have the whole unit wired up with good quality shielded cable, grounded at the drive end, although the metal enclosure is actually mounted in a secondary plasic box, the PCB is not electrically connected to the box. Just for completeness the PCB is electrically connected to the metal shed via the braid of an antenna (the system is a remote control for an irrigator).

    The problem is, everything runs fine on the bench, where have a small 3 Kw drive as a test setup. In the field the system mostly works but shows some subtle anomalies, for example, when I try to drive the analogue voltage to zero it will occationally 'stick' at the previous setting. also it will occationally miss some of the incoming radio signals. The rest of the time it all seems to work fine.

    I'm fairly sure this is an EMI problem and I've read the posts from shanantu regarding his problems. I'm just not sure what to do next. I've re run the cables so nothing is in parralelle (although I note the cables on the secondary side of the drive are not shielded) - I'm happy to take my scope down there (it's a 3 hour drive) but I want to be as prepared as I can be when I get there.

    I'm planning to run the connecting cables via 2 turns round a ferrite sleeve, and swap the hardware out for a new set, just in case.

    What would be the correct way to look for ground loops? does one just put an ameter in series with the ground connection?

    Also - what is the normal practice with a PCB in a metal enclosure - should it be electrically connected to the enclosure or should it be left floating? Bearing in mind that the antenna braid is connected to the shed roof.

    Any experiance or ideas would be most welcome.

    Thanks in advance
    Peter
    Peter Truman

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  3. #2
    wastrix
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    Default Re: Odd PIC behaviour near a VFD - very subtle

    If you drive for three hours in Tas surely you end up in the ocean?

    I think it would be important to ascertain here whether the PIC is failing internally (due to interference entering via ground or something), or whether EMI is interfering with signals between the PIC and the DAC, the receiver and the PIC. If you have a good quality MSO which can record what's going on during one of the failures then you can work out what's happening

    I'm certainly no RF/EMI expert but I would say the case should be grounded. (Like a shield)

    Even if there is an earth loop I wouldn't expect this to cause intermittent subtle failures. Probably some hum though is entering through the ground line depending on how it is lain out. I would ensure you've got a good, short, common, solid ground line. I have looked at other peoples' vaguely similar problems to this in much smaller setups which was caused by EMI entering an overly long ground line.

    What I would do is write a simple test program for the PIC, that maybe just outputs a clock pulse, and see if you can see any abnormalities with this pulse occasionally. This would give a more obvious indicator of when the problem is occurring and maybe it can help you can link cause to effect.

    If it turns out to be an internal problem with the PIC and you cannot remove the interference easily you will want to think about coding practices that will reduce the risk of lockups, etc. maybe if you haven't already tried, use a watchdog timer to periodically reset things.
    Last edited by wastrix; 22nd October 2010 at 11:51.

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  5. #3
    Prolific Poster shantanu@india's Avatar
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    Default Re: Odd PIC behaviour near a VFD - very subtle

    Hi Peter,
    Some points are worth considering in cases like this:

    1. For a VFD as big as 110kW you should always use a 3-phase line reactor to eliminate harmonics flowing back into the supply system.I am sure you are aware of this.

    2.You have not mentioned the type of power supply you are using for the PIC , linear or SMPS.Better to use a transformer isolated linear power supply.

    3.In VFD based systems we always use a LC pi-filter for all the instrument power supplies for additional precaution.

    4.If you are sure that there is no problem with the digital IO lines then you can configure two digital inputs of the VFD for "increment" & "decrement" functions(if possible) & try without analog.

    5. Are you sure that the problem lies with the PIC/DAC etc. & not with the RF communication part? What sort of RF module are you using ? Maybe your RF-read routine needs more safeguards to prevent misses.

    6.If it is really EMI which is screwing up the system then I am afraid ferrites etc are not going to help you much.Post your findings with the oscilloscope so that we can think more about it.
    Shantanu

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  7. #4
    Member Peter Truman's Avatar
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    Default Re: Odd PIC behaviour near a VFD - very subtle

    Hi Wastrix

    I'm on the North West Coast and the job is in Bothwell - only about 1/2 way down the state - it must be the dodgy old goat track we call a highway! Beutiful scenary though!

    I've taken on board what you say and much of it I can fairly easily do. The DAC is right next to the PIC on the PCB so I'd hope there was nothing going on there - in fact there is an RTC on the same spi bus and that doesn't appear to be playing up. I like the idea of writting a quick routine that will ramp up and down ove 5 or 10 minutes or so might well tell the story. I've been doing some googling and had a good read around using ferite sleeves and this seems like a good idea. Actually, what I found was that the code calls for the analogue output to go to zero when the system shuts down the drive - the last fault was when the drive shut dowm as it should but the analogue output got 'stuck' at about 2v. By that time the drive had stopped - the radio part still seems to work but I couldn't get the analogue output to function properly until the power was cycled.

    Sadly I've only got a cheap old 100MHz DSO but I do have a Labjack which might do the job?

    Thanks for your help - I'll post my results as they come to hand.
    Peter Truman

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  9. #5
    Kapitan
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    Default Re: Odd PIC behaviour near a VFD - very subtle

    Hi Peter, ground board to chassis with 0.1uF caps for an RF ground.
    Use feed through caps for connections into box.
    Depending on the actual frequency of the VFD, you could use a low pass filter too.

    110kW is a hell of a lot of potential noise if vfd and depending on frequency. also make sure all your circuit power supplies are protected !.
    a good way to do a quick check is run in on batteries to isolate supply problems.. then look at supply if ok.
    J.

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  11. #6
    Member Peter Truman's Avatar
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    Default Re: Odd PIC behaviour near a VFD - very subtle

    Ok - so the normal thing would be to connect the PCB to ground via a 100n cap? What is the story with the antenna braid, which is connected to the PCB directly and to the local earth via the metal shed to which it is attached?

    Just to fill in some of the questions from Shantanu. I'll have to check with the electrical contractor regarding the Line Reactor - There is a choke apparently but I imagine this would be on the DC bus? Outside my experience so far I'm afarid (which is probably more the cause of the trouble :-)

    The power supply is a small (25w) switchmode power supply and I have another small switcher on my PCB - this combination has never given any trouble so far - We mostly use either star delta or small soft starters in our installations, aggriculture is well behind with this stuff - this job is a toe in the water for more complex control systems (any electrical engineeers out there fancy a job?) on the basis of 'bite off more than you can chew - then chew like mad'!

    The digital side of things seems to run fine so the PIC itself seems ok (I can't be 100% sure) so that leaves me with the DAC or, as you say - the radio. The radio is a VHF narrow band module from Radiometrix and I have many hundreds of them working all over the country (Australia) without problems, I use an 8 bit CRC and multiple packets for trapping errors - the great thing about aggriculture is that there is no rush! so bandwidth is not really an issue - whereas range is.

    I'll look up an LC Pi filter as I'm not familier with them as yet.

    I've grounded the shield of the control cable at the drive end and left it floating at the controler end - not sure if that is the best way to do it, I understand tha principle of only grounding one end, in order to prevent any ground current but I'm not clear on which end is best - seemes that convienience plays a role here!

    It's these kind of finer points of electrical installations that I'm falling down on - the control logic is easy - everything works fine on the test rig but subtle anomalies are really hard to fathom.

    I really appreciate and value the experience of those on the list - I know you've all been through these problems before and as they say "what one man can do - another man can do" (I think that was Anthiny Hopkins in the movie where his mate got eaten by a bear?)

    Thanks again
    Peter
    Peter Truman

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  13. #7
    Kapitan
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    Default Re: Odd PIC behaviour near a VFD - very subtle

    Quote Originally Posted by Peter Truman View Post
    Ok - so the normal thing would be to connect the PCB to ground via a 100n cap? What is the story with the antenna braid, which is connected to the PCB directly and to the local earth via the metal shed to which it is attached?

    Just to fill in some of the questions from Shantanu. I'll have to check with the electrical contractor regarding the Line Reactor - There is a choke apparently but I imagine this would be on the DC bus? Outside my experience so far I'm afarid (which is probably more the cause of the trouble :-)

    The power supply is a small (25w) switchmode power supply and I have another small switcher on my PCB - this combination has never given any trouble so far - We mostly use either star delta or small soft starters in our installations, aggriculture is well behind with this stuff - this job is a toe in the water for more complex control systems (any electrical engineeers out there fancy a job?) on the basis of 'bite off more than you can chew - then chew like mad'!

    The digital side of things seems to run fine so the PIC itself seems ok (I can't be 100% sure) so that leaves me with the DAC or, as you say - the radio. The radio is a VHF narrow band module from Radiometrix and I have many hundreds of them working all over the country (Australia) without problems, I use an 8 bit CRC and multiple packets for trapping errors - the great thing about aggriculture is that there is no rush! so bandwidth is not really an issue - whereas range is.

    I'll look up an LC Pi filter as I'm not familier with them as yet.

    I've grounded the shield of the control cable at the drive end and left it floating at the controler end - not sure if that is the best way to do it, I understand tha principle of only grounding one end, in order to prevent any ground current but I'm not clear on which end is best - seemes that convienience plays a role here!

    It's these kind of finer points of electrical installations that I'm falling down on - the control logic is easy - everything works fine on the test rig but subtle anomalies are really hard to fathom.

    I really appreciate and value the experience of those on the list - I know you've all been through these problems before and as they say "what one man can do - another man can do" (I think that was Anthiny Hopkins in the movie where his mate got eaten by a bear?)

    Thanks again
    Peter
    Hi Peter
    try, where possible to keep rf and dc ground seperate..

    theat's why the gorund from and rf connection shield is led to chassis / case ground via 0.1 caps. low imp rf high imp dc.
    all leads into where noise could be apparant via ft caps.
    again, remove psu, use batteries, basic fault finding. if it works, then it's your psu.
    go from there.
    Joe.

    ps, WOOF, the bear just took a bite darn bugger ! lol

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  15. #8
    Prolific Poster John Drew's Avatar
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    Default Re: Odd PIC behaviour near a VFD - very subtle

    G'day Peter,
    Earthing the coax on the shed roof doesn't sound all that good. If you are operating on VHF what kind of antenna are you using that requires grounding to the shed roof?
    I'd be ensuring that your incoming signals were as strong as possible - a nice long yagi perhaps?
    If you are using the shed as a gound plane then dicky connections between the sheets of the iron roof may result in diode action generating RF interference from any strong fields. The only time I'd use a shed roof as a ground plane is for an HF vertcal and even then I'd bond the sheets if it was practical.
    There are plenty of antenna types that will avoid grounding your coax to a roof.
    Grounding to a tower can be a good thing for lightning reasons in which case there are designs that do this but you need to keep anything that influences incoming signals away from unpredictable surfaces.
    Regarding shielding - I have built many amateur radio repeater systems and found that two shields are better than one, use single point earthing and use cap bypass filters on all input/ouptut lines.

    I agree with Joe's comment re running it from batteries as a test.
    John
    VK5DJ

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  17. #9
    Member Peter Truman's Avatar
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    Default Re: Odd PIC behaviour near a VFD - very subtle

    Hi John

    The antenna is a mopole, ground independant 3db gain antenna from RFI - the braid is internally connected to the boss where the locking nut is situated - so, at that end I have no choice (other than to mount on an electrically isolated pole) but to have the braid connected to the shed. I'm not deliberatly using the shed roof as a ground plane (as it's supposed to be ground independant).

    The radiometrix module in my reciever has the RF gnd pins internally connected to the 0v connection so, again, I have no choice but to have the antenna braid connected to 0v.

    I've had a look at the scope and I can see stacks of noise on the analogue line - see note below (only when the test drive is running [0.75kw])- from my op amp to the drive's analogue input. I made up a humungous low pass filter with a 22uh choke and a 100uf electrolytic cap - my output runs through the choke and I have the cap on my side of the connection - now I can still see the noise on the drive side of the connection (doesn't seem to trouble the drive) but my side is now almost complelety clean - not sure if this noise could couple through the op amp into my DAC and cause problems, but it seems like a fair bet?

    One thing I did notice was that when I laid out my board I ommited any pullups on the i2c connections to the DAC (TC1320) - I'm using port B pins with the pullups turned on and I included "Declare SCL_Bus on" anyway - thinking about it now I'm not convinced I should have left off the pullup on the SDA line (any comments anyone?) as I don't know how the internal structure of the command works.

    I'm heading south again on Tuesday and I plan to add the low pass filter to the analogue output - and the power supply if necessary, I will take a plain old transformer power supply and some batteries.

    The drive has an internal 24v power supply but this only provides 100ma which is insufficent to run my system - not sure how to go here but, the 0v output from their power supply is NOT tied to earth, whereas mine is - there is also no connection between my 24v+ and their 24v+

    I'm tempted to tie both + lines together, both 0v lines together and earth the 0v to a 6ft long copper earth stake, right next to the shed. Can anyone offer advice as to whether this is the right thing to do or not?

    It's all very frustrating - the test rig - which is running right behind me as I type, won't fault, whatever I do to it - a similar installation (on a different make of drive) worked well ever since I tied my 0v to earth (similar problems prior) - and every time I visit the problem site it all works fine when I leave. usually get a phone call when I'm about 1/2 km from home!!!

    Just a comment on the caps on the I/O - the output is a relay switched supply - should I still use a cap given that it is only connected through the relay to the input voltage anyway - no route through to the board (or am I being naieve? :-(

    Once again, many thanks for all your interest and help, I appreciate it.

    Regards
    Peter

    PS - Scope notes - interferance on the analogue line - before adding my low pass filter.

    vpp=3.52v, Freq= varies around 700khz

    After adding my low pass filter, measured on the opamp side
    vpp=160mv - the scope shows **** for freq.

    It's all very analogue!!!!!
    Peter Truman

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  19. #10
    Kapitan
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    Default Re: Odd PIC behaviour near a VFD - very subtle

    Hi Peter,
    I still would suggest one step at a time, isolate IF it is a psu problem with batteries, if so, then you know exactly where to look.
    Beware of any antenna that quotes db gain without an i or d suffix, as in dbi or dbd, db gain reference an isotropic source, or dbd reference a dipole. they can and do tend to be fairly good pick up units for the wrong things and unless qualified are often a basic dummy load lol.. but, it is probably psu related, try that first..
    Joe

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  21. #11
    Member Peter Truman's Avatar
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    Default Re: Odd PIC behaviour near a VFD - very subtle

    Thanks Jo - will do. The temptation to try multiple fixes is very strong - the job is very far away and the user getting very grumpy. If the damn thing would fault when I was actually there!!!

    LOL - nature of the game I guess.....

    Where is ruddles in the world at present - are you cruising?
    Peter
    Peter Truman

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    Prolific Poster John Drew's Avatar
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    Default Re: Odd PIC behaviour near a VFD - very subtle

    Peter- fine on the antenna.
    As an adjunct to Joe's suggestion re one thing at a time - agreed but one of the things you can try (if you can get it to misbehave) is to replace the antenna with a dummy load plugged into the antenna socket. If that fixes it then it is coming in the antenna - I suspect it is power supply though and your filter is likely on track.
    Joe, I suspect it is 3dBi antenna (possibly a coax dipole in disguise) system. When no suffix I've come to expect dB means dBi - antenna manufacturers often exaggerate.
    I have had noise problems on I2C in the past - can be a bit susceptible. I overcame mine with improved filtering.
    Cheers
    John

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  25. #13
    Kapitan
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    Default Re: Odd PIC behaviour near a VFD - very subtle

    Hi John, all good points !, if the device is rx only, a standard 1/4 watt 50 ohm resistor will do for a dummy load, if its an rx/tx, then if very low power, it will also do for a quick test.

    I trust antenna manufacturers as far as i can throw them haha...

    can't beat the bamboo special lol, a piece of bamboo, strip the inner and outer coax to lnegth = in meters = .95 *((300/freq Mhz)) / 4), so each leg is that long - the mult by .95 for physical length as opposed to electrical length, tape to bamboo to make dipole, put heat shrink tubing over all and hot glue on centre, instant dipole haha.. with apx 3dbi gain, of course,, 0 dbd !!!. and a slight mismatch of 300 ohms imp at ant, but hey, no worries.. works great !.

    Joe.

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    Prolific Poster John Drew's Avatar
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    Default Re: Odd PIC behaviour near a VFD - very subtle

    Yep, marvellous stuff heat shrink - covers a multitude of sins!
    If I want a quick antenna I strip a piece of 72ohm coax (or 50 for a near enough job) to leave the centre conductor about a quarter wave long (*0.95 probably better) and a short length of braid sticking out. I then solder three or four pieces of 16 gauge copper wire (each a quarter wavelength long) to the braid and lay it alongside the coax to form a rough coax sleeve. Fasten the whole thing to a piece of dowelling or whatever is handy with insulation tape and "Bob's my uncle". Near enough for a quick and dirty although the 70 ohm impedance isn't a perfect match for the 50 ohm of most rigs it's OK. In future I'll use heat shrink and sell the thing for $50 when I'm finished - Hi
    John

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  29. #15
    Member Peter Truman's Avatar
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    Default Re: Odd PIC behaviour near a VFD - very subtle

    Hi Chaps

    Well it seems I found a solution to my problem. I had a look at a variety of points with my scope. The noise was huge, so I connected up my low pass filters - nothing went in or out without going via a filter. I also grounded the 0v DC to a common earth point - the drive earth was connected to the same earth spike via a 12mm cable! Having fired it up I had it working well for about an hour. Thought I had it licked until, for no obvious reason the whole thing locked up. I could get it to reset by cycling the power but obviously that was not going to be good enough for a comercial installation. The board that this system was based on used a rather outdated LM2575 switcher which I think was not upto the job. I replaced the system with a newr design which uses a Recom 78-05-0.5. The system has been running now for about 2 full days with no reported faults. I did leave the low pass filters in place - because I can't be sure if they are necessary without risking another trip down there.

    Obviously, I can't be 100% sure the power supply was the issue - it may have been the different layout of the board but hey, I'm not going to question it too m uch further.

    Thanks for your help with this process, much appreciated.
    Peter Truman

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