MK Position Hold question and compass errors.

Discussion in 'Electronics' started by Charlie Cushing, Jun 20, 2015.

  1. Charlie Cushing

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    I frequently fly in GPS Position Hold, helps a lot with windy conditions, I've adjusted my GPS Angle in MK Tool to make it fly much better.

    The last several times I've flown my kopter, after doing several compass calibrations, I will take off and get a compass error via my Graupner telemetry. I called QC and found out that I should never be taking off or landing in GPS Position Hold. Why is this a requirement for MK?

    After learning this I started taking off in Manual mode and then engaging the GPS, this seemed to help, but was still getting some compass errors. After that I started getting a new command from the Graupner which was GPS FIX, what does that mean? I've attached the last couple GPX files to this post.

    To anyone willing to take a look, thank you in advance.
     

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  2. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    I've never heard of a requirement not to take off and land in GPS position hold -- Altitude Hold, sure, it's unwise to do that as you're kinda fighting the flight control system.

    I normally make an entire flight in GPS Position Hold -- it results in much smoother flying when I fly the GPS "setpoint" than if I turn PH off. Mind you, you need to see the way I fly. :rolleyes:

    The compass errors would appear to be completely unrelated to whether or not you're using GPS hold. I would look for high current cables passing near the flight control board -- do you have the two LiPo wires tightly bound to each other so that the magnetic field created by the heavy current passing through them is neutralized?

    I've not had a chance to look at the GPX files -- I suspect some images of your cabling might be helpful, Charlie.

    There's also the possibility that the compass might be going bad -- are you using the one built into the Flight Controller board or an external compass?

    Andy.
     
  3. Charlie Cushing

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    QC told me on Friday that is what the manufacturer recommends, first time I had heard it too. I never use Altitude Hold, just position hold.

    It is a very real possibility the compass is bad, if you remember I had a bad fire mid flight and had to replace all of my electronics with the exception of the GPS antenna and flight control board. http://forum.freeflysystems.com/index.php?threads/mikrokopter-motor-failure-and-fire.5606/

    Does this picture show you enough, I can take some better pics with the canopy off if necessary? Thanks!
     

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  4. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Oh....if you have that fire, I suspect the FC board might have taken some damage (eg.. high temperature damaging the compass sensor) -- do you happen to have a spare one you could swap in and see if the problem with compass changes -- or take the plunge and go with the external compass upgrade?

    From the image it looks like the LiPo cables are twisted....you'd be better off to use some cable ties and bind them together every inch so that they're strictly parallel. That may not cure the compass problem though -- I'm just sayin' strictly parallel is better than twisted, IMO.

    I'm still not on board with the "avoid PH on takeoff" philosophy. I could see that if the GPS was giving weird data that it might cause a lateral movement as soon as the copter lifts off, but I've not experienced that problem. The GPS and the compass are two entirely separate systems in the sense that GPS is radio frequency based and the compass is magnetic.

    Do you recall with whom you spoke at QC? (Please send the info to me via a Conversation -- my goal is not to cause embarrassment, but I'll give the person a call to better understand the technical reason for the recommendation).

    Andy.
     
  5. Charlie Cushing

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    Correction, the only two boards that remain on my kopter post fire is the Navi Control Board and the GPS Antenna.
     
  6. Steve Maller

    Steve Maller UAV Grief Counselor

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    I took a quick look at a couple of your GPX files. A couple things stand out. One is that your magnetic compass values are sub-standard, indicating the real possibility that you've got an issue with the compass. That's what the exclamation point is indicating.
    Magnet Field: 66 80 93 % (!)
    It should look something like this (from one of my last flights with my MK rig with External Compass before I sold it):
    Magnet Field: 106 110 115 % (ok)
    It's a tricky problem to diagnose, but the MK External Compass can fix it in a flash in my experience. From your GPX file it appears you have the 2.0 board, which requires a small mod. Unfortunately it's a twitchy thing to install, so if you're not pretty skilled (maybe a 7 or 8 out of 10 in soldering skills) you should send your Nav board to QC to have them do it.

    The other thing I see is your motor currents are seriously out of balance. This can be a result of the copter itself being out of balance (center of gravity problem) and/or the motors being out of vertical alignment. Or even worse, a problem with one of the props being installed incorrectly.
    Motor1: 0.0 11.8 16.5 A Temp: 34 58 69 °C
    Motor2: 0.1 3.8 5.9 A Temp: 32 49 58 °C
    Motor3: 0.0 9.5 12.4 A Temp: 33 56 67 °C
    Motor4: 0.0 3.4 5.4 A Temp: 32 50 59 °C
    Motor5: 0.0 9.9 14.2 A Temp: 34 59 71 °C
    Motor6: 0.0 2.8 4.2 A Temp: 31 49 59 °C
    Motor7: 0.0 8.9 12.3 A Temp: 34 60 72 °C
    Motor8: 0.2 4.0 6.3 A Temp: 32 49 59 °C
    Here's what it should look like (again, from one of my last flights with my MK rig before I sold it). Of note is the #7 motor, whose BL was not properly calibrated...you can ignore that one). You want the motor currents to be 10% or so from the mean...not more.
    Motor1: 0.0 7.8 11.7 A Temp: 13 29 36 °C
    Motor2: 0.0 7.1 10.9 A Temp: 13 30 38 °C
    Motor3: 0.0 9.1 14.3 A Temp: 14 32 41 °C
    Motor4: 0.0 7.8 12.5 A Temp: 14 32 40 °C
    Motor5: 0.0 9.3 15.0 A Temp: 13 31 39 °C
    Motor6: 0.0 8.7 14.2 A Temp: 12 33 41 °C
    Motor7: 0.0 5.5 9.8 A Temp: 14 30 37 °C
    Motor8: 0.0 7.8 13.0 A Temp: 13 30 39 °C
     
  7. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Nice bit of forensics, Mr. Maller!
    Andy.
     
  8. Charlie Cushing

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    I know my props are installed correctly, why are my temps running so high? It's possible the MOVI is slightly out of balance, but should not be extremely out, I've checked it several times. This is what caused my fire a few months ago with different electronics? Do I need the external compass or just a new compass? Thanks again for taking a look into this! -Charlie
     
  9. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    What is most telling, Charlie, is that odd numbered motors are pulling way more current than the even numbered.
    Given that odd numbered motors rotate clockwise (viewed from above), that means they are having to generate more torque to counteract the rotation torque created by the even numbered motors.

    The most likely cause for this is that one of the even numbered motors is not truly vertical and is imparting a horizontal component of rotation to the copter. The easiest way to check for this is power the aircraft up, take off, and hover at eye-level about eight feet away from you -- you can then see across the copter and see whether all of the prop "disks" (they'll look like disks because they're spinning so fast) are all in the same horizontal plane or whether one or more are canted over.

    I think the answer to your question is probably more: you need to replace the compass -- it's got to be your choice whether you get QC to do the modification for the external compass or buy a new flight controller. It's really a dollar question, I'm afraid.

    Hope this helps,

    Andy.
     
  10. Jason Smoker

    Jason Smoker Active Member

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    Sometimes to get all the right motor currents the same and compass problems is a good compass calibration. Even moving the lipo cables slightly can fix compass woes
     
  11. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    That's interesting, Jason. It seems counter intuitive that it would help -- the compass is certainly used for yaw stabilization -- I wonder whether that's why it might work?

    Andy.
     
  12. Jason Smoker

    Jason Smoker Active Member

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    Yeah my cinestar six had compass problems when I first built it, but I just reroute the lipo wires from underneath to new position. It fixed it !weird but sometimes it works! Also buzzers can cause it move that thing far away as possible
     
  13. Charlie Cushing

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    I will definitely try all of the suggestions above. The elevated temperatures really concern me, that's how the fire was caused a few months ago. The compass that is currently in the copter is brand new as of December 2014 with less than 50 flights on it, no crashes. I'm waiting to hear back from QC to see if I should purchase the external compass or not? Thanks again for the help! -Charlie
     
  14. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Charlie: To see if the compass is causing the motor current asymmetry, use MK TOOL to disable the compass (it's the "Compass Effect" under the Misc tab) and see if the currents come back into balance.

    If you disable the compass and the asymmetry still exists then the issue is most likely motor alignment.

    Andy.
     
  15. Steve Maller

    Steve Maller UAV Grief Counselor

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    Another possibility is one or more bad motor bearings or some other mechanical problem. If one motor is "dragging", the others may be forced to compensate. I've seen this happen.
     
  16. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    When I've seen a bad motor (for whatever reason), that usually shows up as both general asymmetry between clockwise and counter clockwise motors and also one of the motors standing out as an outlier in the data. In Charlie's copter, motor #1 seems like it might be such an outlier...what do you think, Steve.

    Andy.
     
  17. Dave King

    Dave King Well-Known Member

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    Let me throw my 2 cents in. Lots of stuff to cover.

    First off, I would never trust the electronics that were from that fire unless you can put it on a test rig without a gimbal and get at least a good 15 flights of it without any issues.

    Your issue with the odd motors pulling much lower current is most likely a function of a motor being slightly out of alignment. This can cause the copter to not fly very smooth as 4 motors are overworking and 4 motors are not working as hard. When this happens it can make yawing difficult or not as smooth because its fighting to keep yaw stable. I have had the same exact problem you did with one motor simple off 2 degrees of alignment.

    I have thousands of flights with MK under my belt and I can tell you that I have experienced many GPS issues that were a result of many different types of issues. I strongly recommend that you always take off in manual mode and wait till you get the copter up in the air at least 15 feet or so for the first 5 to 10 seconds just to make sure every is stable. Then switch to Position Hold. I have had at least 5 incidents where the GPS would freak out and go crazy. Why does this happen? Often because it lacks a GPS fix or because the GPS system has a glitch in it.
     
  18. Charlie Cushing

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    Ok, I did a flight today with a new compass calibration, the GPX file is attached. I didn't get any compass errors, it was a quick flight in my front yard, no payload, just the copter. My next step is to try it with the compass disabled. I also looked at all the motors and they all look aligned. Does anything look out of the ordinary to you guys? Thanks.
     

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  19. Steve Maller

    Steve Maller UAV Grief Counselor

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    Definitely better looking with respect to the compass, but not the motors. Clearly you were flying with very little payload, as your power draw was minimal. But the motor currents are still out of whack, and I suspect that it'd be even worse if your copter was similarly loaded. And if your compass problems were exacerbated by the high current draws from before, then they could return when the copter's at its true flying weight.

    So the verdict? No verdict IMHO. :(
     
  20. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    When you did the test flight did you hover at eye-level and check the alignment of the "disks" formed by looking at the props edge on, Charlie?

    Andy.
     

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