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Scary near disaster today

Discussion in 'Cinestar 8' started by Steve Maller, Apr 28, 2014.

  1. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Steve: Given that the motors "like" 22 degrees of timing, have you devised a way of testing that this is correct -- short of the test flight and assess the vertical velocity of the landing? :rolleyes:

    Can you, for example, ground test that you've got things right without needing to get airborne? This motor timing aspect seems to expose a dimension to BL-Ctrls/ESC and motors we've not really had to worry about before.

    Andy
     
  2. Steve Maller

    Steve Maller UAV Grief Counselor

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    I know. This is all getting a bit confusing. I think for now I will reduce the number of variables in my equation and just check for loose connections. On my copter, too. :confused:
     
  3. Gary McCready

    Gary McCready Active Member

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    One of the motor restart errors is before the landing:
     

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  4. Steve Maller

    Steve Maller UAV Grief Counselor

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    Yes, I definitely have an issue with one of my motors. I think the problem is with the motor or the wiring, because I ran the BL with another motor and didn’t see any issues. I strongly suspect my problem is a dicey bullet connector. There are six of them (3 from the motor and 3 from the BL pigtails) so there’s a lot of margin for error. I did check the BL pads today and resoldered them, so that’s not the issue. I even caught a burp today on video, although I think this is a high-current shut-off. But I saw it burp like this another time and it wasn’t at 20A when it happened...

     
  5. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Good catch -- that might be an overcurrent or decommutation at high RPM, Steve. The fact that it was not at 20A, and the motor was running no-load seems to lean more towards decommutation (assuming it's not a dodgy connector as you suggest).

    Let us know what you find.
    Andy.
     
  6. Jeff Scholl

    Jeff Scholl Distributor

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    In the past I've had a motor stop in flight as a result of resin that had built up on the male bullets.
    I was soldering the bullet upright in a jig and the resin flowed/dripped around the springy/clasp thingy that secures the connection when connected into the female bullet.

    Honestly the kopter flew pretty good :) Didn't even notice the failure when looking at the MoVI M10 footage.

    Best,
    Jeff
     
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  7. Nick Kolias

    Nick Kolias Moderator
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    Jeff, I love when you talk technical. ;)

    nick
     
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  8. MIke Magee

    MIke Magee Active Member

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    Steve, it's great you caught that. Assuming this IS a single motor (for whatever reason you determine), why do you think it had such a catastrophic effect on flight? I've seen clips of single motor failures that resulted in "OK" recovery and landing. Is this an inherent weakness in an X8 configuration, or is it possible that this issue somehow cascaded into the other electronics and effected all the motors briefly somehow? All opinions welcome. I'm hanging on everything you are doing here for obvious reasons.
     
  9. Dave King

    Dave King Well-Known Member

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    Andy can you explain what decommutation is in regards to a multirotor and why it occurs?
     
  10. Steve Maller

    Steve Maller UAV Grief Counselor

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    Actually, I was running it with a prop on at nearly full throttle (~200 or so in MK Tools motor test). Needless to say, the copter was WELL anchored down (see below). :rolleyes: Did a nice job of sweeping the floor of my studio, too. I can happily report that my Cinestar frame is fully capable of supporting 60 pounds of sandbags. So is my ghetto PVC transporter rig!

    IMG_9762.jpg
     
  11. Steve Maller

    Steve Maller UAV Grief Counselor

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    The first failure resulted in a wobbly recovery. The second one resulted in a hard landing, as I was only 4-5 feet off the ground, and I also had 10lbs of weight at the bottom of my landing skids, resulting in a pendulum-like effect, and by the time I reacted to the loss of thrust, the copter had nearly touched down by itself.
    Both of these cases were exacerbated by PH, I believe, and I think my compass is a bit out of whack. I have the new external compass but haven’t as yet installed it.
     
  12. Steve Maller

    Steve Maller UAV Grief Counselor

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    After exchanging another set of emails with Holger, I am of the opinion that the problem of my motor cutting out in flight is a motor or wiring issue. I resoldered the pigtails of a few of the motors that looked like they could use it, but I haven’t looked at the bullet connectors yet (all enshrouded in heat shrink, of course). I could also have a dodgy motor. I have a couple of spare motors, so I can test that, too.

    Will be shooting for the next few days, but hope to get back into the hunt on Friday.
     
  13. Steve Maller

    Steve Maller UAV Grief Counselor

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    So far the “resin on the springy/clasp thingy” theory sounds pretty solid. I have had that problem and had to discard a couple of bullets because of it. I’ll take a look under the magnifier. Thank you, Jeff!
     
  14. Jeff Scholl

    Jeff Scholl Distributor

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    bulletconnector.JPG

    This part was completely gummed up with resin and would not even rotate which was the tip off. We have also seen this on a build along. Now I always make sure they are clean and rotate freely, or use the solid pole version.


    Nick - I love your potty mouth :)

    Best,
    Jeff
     
  15. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Hey, Nick: C'mon, you know that the connectors have clippy thingies, right?
    Andy.
     
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  16. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Sure..

    Let me define a couple of things:
    1. The rotor. On the motors we use, the outer bell of the motor spins. It has neodymium magnets epoxied around the inside.
    2. The stator. A series of coils arranged around the inside of the motor -- they're actually wired up into three groups (hence three motor wires, which I've typically heard as group A, B, and C. You'll see references to Phase A, B, and C if you look at a schematic for a speed controller).

    The goal is to energize one of the three phases, A, B, or C in rapid succession, basically trying to lead the rotor (like a donkey with a carrot dangling on a stick in front of it) to rotate to the position where the energized coils are closest to a set of magnets -- however, before the rotor gets there, the sneaky little ESC/BL-Ctrl de-energizes that group of coils, and energizes the next group of coils, so that the rotor is now tempted to rotate even further. This is akin to moving the carrot just slightly ahead of the mouth of the donkey....

    This process is repeated very rapidly, causing the bell of the motor (which, for us, is conveniently attached to a prop) to spin.

    Commutation is the process of switching current in the phases to cause motion. See http://blog.aerotechmotioncontrol.com/2012/08/08/commutation-and-linear-motors/
    Don't be fooled, brushless motors are just linear motors that chase their own tails. :)

    Decommutation is what happens when things go cattywumpus (sorry about the technical term, see http://www.urbandictionary.com/define.php?term=cattywumpus) )....that is, the phases are energized in the wrong sequence -- in effect actually slowing or, worst case, stopping the motor. It's a bit like the donkey suddenly finding the carrot is behind it.

    Decommutation happens when, for example, the ESC/BL-Ctrl is not set up correctly for the physical geometry of the motor. This is what is represented by the number of degrees that Holger has us set the BL-Ctrl to.

    I must confess that the actual meaning of the number of degrees is something I can't remember. I learned this back in the Dim Times (which means anything that happened more than two weeks ago). I think it's the phase angle difference between the electrical currents (but I could well be wrong and it might be the physical angle of the phases (coils) on the motor). Hopefully someone who remembers This Stuff better than I do, can step in and correct my understanding.

    Either way, there is a specific phase angle for a specific motor. You need to match that in the ESC/BL-Ctrl otherwise there is the risk, especially at high RPM (where the timing gets more critical) of the dreaded Things Going Cattywumpus (aka decommutation). :)

    Hope this helps. Remember: when I write this kind of stuff it's likely to be worth what you're paying for it! :)
    Andy.
     
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  17. Dave King

    Dave King Well-Known Member

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    Thanks Andy for the excellent explaination. I have an email into Holger to ask about a lot of the BL settings so we can avoid cattywumpus failure. When I hear back I'll share my info.
     
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  18. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    My explanation got me thinking: I wonder where the word "cattywumpus" comes from?
    See below. Who knew, eh?

    Andy

    catawampous, catawamptious, cattywampus (there are several other spellings on offer) was coined in the US during the 1840s and refers to an imaginary monster or a hobgoblin. The first 'cat' part may be linked to catamount, meaning a panther and its self from 'cat-a-mountain'. The 'wampus' is probably a joke word, although their may be some subconscious link to another slightly ludicrous animal (if only to the ear) the 'grampus' (itself from the Medieval Latin crassus piscem: fat fish). Its earliest (and still main) use was in the South Midland and Southern states. An extension, 'catwampus-cat' means a domineering, aggressive woman, a virago. Used as an adjective it means fierce, unsparing, and destructive. A secondary meaning is parallel to that of 'catty-cornered' or 'cater-cornered' and like them means diagonal or askew. The verb 'catawampus-corner means to force into a difficult position. The adv. catawampously or catawamptiously means utterly or completely and the phr. 'to be catawamptiously chawed up' is cited in Bartlett's Dictionary of Americanisms as meaning to be completely demolished, utterly defeated.​
     
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  19. Gary McCready

    Gary McCready Active Member

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    Andy: thanks for the explanation and the levity! I was in dire need, after having some trouble with my MK. Last week I couldn't get to screen sumo 12 on my MX-20 (unplug the telemetry cable on the Rx..) Today I spent too much time messing with the external compass but it turns out it was just the connecting cable, after replacing the navi and the compass. I was just feeling all cattywumpusafied till I read your post.
     
  20. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Yes, being cattywumpussed is a terrible affliction. We should start a 501(c)(3) non-profit for the Elimination of Cattywumpusification. On the side we could perhaps do some wine tasting. On days we don't do that we could hurl heavier-than-air aircraft into the air that can only stay up because power > weight....

    Andy.
     
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