Possible strategy for finding motor causing vibration

Discussion in 'Cinestar 8' started by Andy Johnson-Laird, Oct 7, 2012.

  1. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    I'll try that, William. One thing I'm currently doing is getting my backup frameset ready for flight (adding an FPV camera to it etc.)
    I plan to fly it just with landing gear and using the same FC settings as the primary frameset to get a baseline for the amount of vibration, and then fly it with the gimbal to see how it behaves.

    If, as I suspect, there is much less vibration, then I'm going to start systematically swapping out motors on the primary frameset to weed out the "bad" ones. I'll try doing some flights with a 3-axis accelerometer on board that can get a decent sample rate so that I can see what frequencies are in the vibration. I keep thinking that knowing that information will help me figure out where the vibration is coming from.

    I've still not ruled out the Yaw PID controller -- but I'm confront with the apparent contradiction that I don't remember seeing this level of vibration two months ago, just recently -- so something, somewhere has changed.....

    Andy.
     
  2. Tabb Firchau

    Tabb Firchau Administrator
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    Hi Andy,

    Did you ever post pics of your FPV camera mounting setup? This vibration does not look like a motor/prop vibe to me. Would be curious to see how you have it all mounted.

    Thanks

    Tabb
     
  3. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    It's just mounted on one bolt of a frame/boom clamp using the U-bracket that comes with it. The clamp has kinda munged up the reflective tape -- must get around to replacing that some day... The PVC tape is just to stop screws/lens from vibrating loose.

    I'd appreciate any thoughts on what you think the vibration source might be. I'm prepping my backup C8 frameset and, if it stops raining I'll test fly it and record the FPV video I get from that.

    Andy


    FPV mounting.png
     
  4. Tabb Firchau

    Tabb Firchau Administrator
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    Hi Andy, Hard mounting anything to a multi rotor and getting good footage is a tall task. First setup I would try is very rigidly mount the FPV camera to the boom and see what you get. As in ditch the clamp, FPV camera frame, etc and double stick tape it to the boom with a zip tie to really cinch it down.

    This will give us an idea of the actual frame vibration versus the vibration translated through the flimsy FPV camera mount etc.

    Curious to see the result of this test. Vibration is a tough one...especially with 8 motors all creating unique signatures, harmonics, etc

    Tabb
     
  5. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Understood -- the real worry is that when I first got this C8 flying, with this same mounting mechanism, I was not getting that lateral wobbling (it's almost below the frequency where I would call it "vibration." I did check, by the way, and the camera mount is pretty darn solid - at least in the pan axis. The camera can tilt, of course -- there only two teensy lamey-ass screws holding it.

    Using your recommended method of mounting (do you have an image, please?), how do you tilt the camera down? I must be having a bad day today 'cos I'm not seeing how to do that. Good job it isn't Saturday...no...wait....

    Andy.
     
  6. Josh Lambeth

    Josh Lambeth Well-Known Member

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    Figured I would continue the discussion on the bad motors from the other thread in here:

    When I mentioned bad motors I meant that mine are making a high-pitch whining noise while running. I have narrowed it down to motors number 1 and 6. Both of which have extra "Play" or movement in them side to side and up and down. This I suspect are the barrings going out on them. I am very frustrated with this since I just replaced 2 other motors about a month ago. I honestly think it is due to running my copter in the excessive heat here in Phoenix. Flights in 110* weather can't be good at all for it. :/

    Josh
     
  7. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Sorry...I thought I'd posted to the wrong thread....so deleted the message and then discovered it was the correct thread. Sigh.

    Anyway, here's a test FPV video with the frameset I build from a kit -- no gimbal, no OSD. The FPV camera is hard mounted on Boom #1 with the supplied U-bracket and a boom clamp.

    Note how smooth it is in comparison to the original video from the RTF frameset that had arboreal adventures. The kit frameset even sounds sooo much smoother, no high frequency motor noises, so I'm pretty sure it's the motors that are causing the vibration. I suppose it could be some damage to the FC or the PDB, but the motors sound really good on the smooth flying frameset.

    Andy.

    Here's the smooth video from the kit frameset:


    Here's the shaky video from the RTF frameset with the bad motors:
     
  8. Tabb Firchau

    Tabb Firchau Administrator
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    Andy, do both of the frames look the same in flight? The one that is shaking almost looks like it is the flight control giving spurious inputs. Can you confirm it is bolted down solid and nothing is bumping into the flight control inducing movement in the frame via vibration?

    Thanks

    Tabb
     
  9. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Hi Tabb:
    Yes, to the degree that eyeballing the two frame is accurate, they both look pretty much the same -- however, they really sound different. The shaky one sounds like a small swarm of agitated banshees, the smooth one sounds much less discordant. Could that still be the FC? Or just the fact that I shock-stopped the motors and they're bent out of shape (literally and/or metaphorically)?

    One thing that is interesting is that hard mounting the FPV camera reveals how smooth the C8 with "good" motors/FC whatever really can be. I was, well, quite impressed (and that's British understatement). Damn, it's smooth....and that's how I remember the FPV from the primary frameset -- I call it the RTF, but I don't want to imply that there's anything wrong with QC's build -- it was that smooth until I adjusted it by means of a tree. :confused:

    We're in the rains now -- I guess it started earlier today for you, so no more test flying for the time being until Tuesday/Wednesday.

    Andy.
     
  10. William Johnston

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    If you don't mind, I would like to see your GPX file for that filght

    Well, I'm assuming you twisted a few motor mounts around the boom a little bit so that they are just a tad off when you landed in the tree. That would impart a slight spin to the CS8 that would have to be constantly corrected for by the flight controller. That would account for before/after difference. The fix would be to straighten up the motors.
     
  11. William Johnston

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    Maybe the gyro sensor is damaged. You might want to start by swapping out the FC board. That's a lot easier than swapping out the motors.
     
  12. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    The GPX file for the smooth flight with the kit C8 frameset is attached. Let me know what you think -- you can certainly tell from the motor current draw and BL temps that there's no gimbal/camera on board.

    For the RTF C8 frameset, I've double checked the motor alignment and prop balance -- all are good. I was thinking I would swap out the FC board as a quick test, but the difference in motor noise between the two framesets is really quite marked -- with the smooth flying frameset having the smooth sounding motors. But, as you say, swapping the FC board is a lot quicker than swapping six motors!

    Thanks
    Andy
     

    Attached Files:

  13. William Johnston

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    Nice! 25 amps, right inline with what Tab measured.

    I am always weirded out by the outliers. Did you see that your motor 8 was only drawing 1.8 amps while all the others are between 2.9 and 3.8. It's even weirder when you break it apart the clockwise and counter clockwise motors.
    clockwise:
    2.9 2.8 2.9 3.4
    clockwise:
    3.6 3.7 3.8 1.8

    Now it's even more of an outlier.

    I'm really hoping it's not the motors. If all it takes to wreck a motor is one good wack on the propeller then I am in trouble. I broke 7 of the 8 propellers when I landed the CS8 in a tree and I broke a few more tipping it as it tried to take off. With everything else that has gone wrong, this I could do without.
     
  14. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Important things first: What! You're also a member of Copter Arboreal Club? I never knew. It's a small, but exclusive, club. Of course, in common with any other organization, our first act needs to be to figure out who not to let in..... <evil grin>

    I did notice that outlier reading. I wondered whether it was a sensor aberration. It really doesn't make sense that motor 8 would be only pulling 1.8 amps does it? It really is an outlier. I also noticed that the temperature for the BL-Ctrl for Motor 8 is not an outlier -- it seems to be in line with the other BL-Ctrls -- which is what made with think it might be the current sensor out of whack.

    As for the motors issue -- I'm not quite sure what might damage a motor. I don't think shock-stopping them can be the best thing for them -- obviously doing that must also cause some kind of lateral shear force across the propeller mount. I've not found a good way to mount a micrometer to the boom to get a good reading on any run-out in the prop shaft. It'd be nice to test for run-out while the motor was still mounted on the boom. I'll give that some thought.

    Thanks
    Andy.
     
  15. Nick Wolcott

    Nick Wolcott Member

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    Hi Andy,
    I have a similar problem to what you're describing. On my CS6 motor 4 is causing the rear landing gear to vibrate, kinda like yours. It'll be fine for a second then vibrate, then stop, and so on. The other landing gears don't do this. They're fairly new motors, maybe have a couple hours on them, I replaced all 6 after a year of hard use just for peace of mind. I only noticed the problem after putting on new motors. They've never met a tree or been shock stopped. There isn't any difference in noise between the motors on my 6 or 8. I checked the balance on the prop - it's fine. It doesn't seem to be effecting/affecting (I never know which one to use) performance at all. Let me know if I can help troubleshoot. I haven't looked at any gpx data yet (my navi is currently on my 8) Here's a quick clip of the issue:
    [​IMG]
     
  16. Adam Paugh

    Adam Paugh Distributor

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    Nick,

    We also notice this behavior on several different multi-rotor variants with a single landing leg of the "6 o'clock" boom. We see the start and stop of the vibration which is the most pronounced at the tip (bottom). Seems like some vibration harmonic that exists within the chasis, as it exhibits a wave period, perhaps.
    ...(Andy's gonna have a field day with this one Lol)
    Greetings,
    Adam
     
  17. Tabb Firchau

    Tabb Firchau Administrator
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    Guys, one important point to remember in all of this....

    if it flies, it vibrates ;)
     
  18. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Hi Nick:
    Thanks for the offer of assistance.
    I'm afraid the link to your video is mangled. Did you insert it using the little Projector Screen icon in the formatting line above the Reply editing window? All I see is a blue square with a white question mark.

    The vibration problem is puzzling isn't it. I've got a shoot practice tomorrow afternoon (Pacific time) and then the real shoot on Thursday morning, so I'm "heads down" getting my backup C8 ready to fly. I hope it's not raining/misting -- I'm itching to move the "shaky" FC/PDB over to the "smooth" frameset and see if it suddenly gets shaky. If it does, then the culprit is the boards. If it doesn't then it's the motors.

    Before I do that board move, though I want to capture some vibration data using one of these instruments so I can at least have data before I go changing things. Vernier makes them for high school physics students but they make really good quality instruments -- not lab grade, of course, but very good for general purpose use.

    There's got to be a way we can bench test motors before we put them on a frameset and know whether they're going to be good or bad.

    Andy.
     
  19. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Nah, Adam, not a field day....just some test flights. "We" may need to find a mass balancer that will absorb the vibrations...OK, over to you Tabb! :)

    Andy.
     
  20. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    True. But the question is how much? Is it ViBrAtEs or ViiBBrAAtEEs? :confused:
     

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