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URGENT: Tighten your props with wrench

Discussion in 'Phantom 2 Vision + Discussion' started by yorlik, Mar 24, 2015.

  1. yorlik

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    Very Strong suggestion: Tighten your props with wrench before your next flight.
    Strong suggestion: When rising, try not to use 100% throttle.

    Until proven wrong, it appears the NEW 2.1 ESCs may have traded popping FETS for throwing props and shutting down smart batteries mid-flight.

    I have document into dji SDK team asking for comment on this potential issue, but in the meantime, there is no reason not to play it a tad safer with the above 2 suggestions.
     
    Redphantom33 likes this.
  2. msinger

    Approved Vendor

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    Are you suggesting that throwing a prop is shutting the smart battery mid-flight? The two incidents seem to be unrelated.
     
  3. tcope

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    I don't think it's even possible to use a wrench on a Vision 2 Plus.

    Can you explain what this means?
     
  4. MapMaker53

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    Can you offer us the scientific explanation behind your theory?
     
  5. yorlik

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    Possible gain issue causing props to fly off in flight & bat

    I have found currentCurrent, as reported by SDK, often acts like this during full throttle ascend:

    velZ currentCurrent
    -------------------------
    0.0 -18697
    0.0 -18759
    0.0 -18759
    0.0 -18618
    -18.0 -18618
    -49.0 -20780
    -63.0 -20780
    -54.0 32767
    -60.0 32767
    -58.0 1349
    -59.0 1349
    -24.0 -15610
    0.0 -15610
    0.0 -15839
    0.0 -15839

    or

    0.0 -17758
    0.0 -18002
    0.0 -18002
    0.0 -17868
    -3.0 -17868
    -21.0 -24009
    -42.0 -24009
    -47.0 32767
    -44.0 32767
    -47.0 32767
    -41.0 32767
    -39.0 -29970
    -38.0 -29970
    -15.0 -21647
    0.0 -21647
    2.0 -12538
    2.0 -12538
    2.0 -16591
    1.0 -16591

    Since I only have data every 1 sec, I cannot tell exactly what is happening, but from looking at various csv files, it appears currentCurrent is going unstable and oscillating to full opposite direction in a saturation to 32676 at times - when ascending current is usually above about -20amps.

    I know it is very difficult to 'tune' a simple velocity loop ESC controller without a proper current loop inside. I suspect that what has happened is that someone tuned this a touch too hot, trying to get the performance required, and it breaks into oscillation occasionally - usually during very high speed command.

    No one has reported the speed command PWM going unstable, so I assume it is in the ESC U1 chip implementation.

    My gut guess (from 36 yrs as a servo engineer often called in to fix other's tuning problems) is if you have an integral gain term, it is too high. If not, then of course the guess goes to the proportional term.

    Without 1msec updates, all I can do is guess looking at this potentially aliased data.

    The other possibility is of course that your SDK is being affected by the high current PWM noise and this data is not actually happening.

    But all outside indicators seem to tie this into props flying off for "no reason" during flight, and yet, if the pv2 still operates afterwards, seeing no indication of any motor issues. It seems to tie the facts together with this data.

    To dji: Please investigate this; if it is indeed so, you may want to offer an immediate firmware update to 3.13 and notice to all owners to upgrade before further flights.

    This also could account for some if not all of the batteries shutting off for "no reason" during flight and dropping the pv2 - instant plug reversal @ 64amps surely would droop the battery voltage to point its software might pull the plug!

    Further note on meaning of this: if a motor has -20 amps going into it spinning, and you IMMEDIATELY change the current to +32amps, this 52 amps makes LOTS of torque - in the opposite direction to normal rotation.
     
  6. yorlik

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    I used my supplied wrench on my v2p about 6 times when I was trying to land via CSC before learning that is a dumb way to shut down. Yes, the wrench fits well (just come down over the top of ridge, not straight into it from the side as it is triangle shaped).

    Better than explaining would be to point to the detailed work Paul did showing how the pre 2.1 units were electronic fuses:

    http://forum.dji.com/forum.php?mod=view ... ic&lang=en

    In recent discussion with Paul, he confirms the 2.1, although same hardware, do not blow FETS like that anymore (firmware update fixed it).
     
  7. yorlik

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    no, just possibly the same cause.
     
  8. Spiritskeeper

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    Yorlik thank you for the explanation. I do believe you are onto the prop loss problem. This would also explain the sudden voltage drop of batteries after only a fraction of the expected flight time.

    Good advice. I will be watching to see the outcome.
     
  9. GSSP

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    I grab the motor, and hand tighten the props, do not over tighten and strip them
     
  10. yorlik

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    It is appearing like this data is probably accurate and not just noise related... doing math on the reported voltage and currents, the voltage droop on my battery seems to correlate with the reported current changes. at about 2amp draw I am seeing only 100mv droop in batt voltage and at the higher 30 amp level almost 1000mv drop. MOre calcs on the data seem to show my battery has somewhere around 25-40mohm internal resistance, which also jives with this data being real.

    Seems to me those threads were about 1/4-20 size; I doubt anyone could strip them by overtightening, although they could no doubt break the captive nut out of the plastic.
     
  11. edmcmeans

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    "is if you have an integral gain term, it is too high. If not, then of course the guess goes to the proportional term."
    That's just what I was thinking! :)
     
    Brent10 likes this.
  12. UKFlyer

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    If this is a real problem then why not does it happen every time? I along with many others i'm sure have gone full throttle many times with no ill effects with our V3's, just today i ascended from around 10ft to 350ft with the stick firmly all the way up, also fly across the sky full throttle never a problem. I only ever hand tighten the props.
     
  13. flyNfrank

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    What app and flight logger are you using to gather your info?

    I spend more time reading my data then I do flying. Based on your answer to my question will help explain things better.
     
  14. yrusoslowh0e

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    This is exactly what I do as well, haven't had any issues so far! Also I take my props on and off for every flight because I use store it in a backpack that isn't big enough to keep the props on!
     
  15. StevenQX

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    Same here...
     
  16. Apparition

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    Same
     
  17. yorlik

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    Me too lately... Ken argo's Ultimate flight.

    BTW, you asked if you could put Paul (na5n) 7 page pdf up here with sticky and appropriate credit to him; he said ok. You can see his written approval here:
    http://forum.dji.com/forum.php?mod=view ... D1&lang=en
    I have no clue how to do that so leave it to you if you still want to. thanks.
     
  18. Meta4

    Meta4 Moderator
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    Re: Possible gain issue causing props to fly off in flight &

    Have you communicated this to DJI?
    Don't count on them finding it here.
     
  19. herkam

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    There esc can't reverse the motor (or?) and I doubt that they apply break on the motor to reduce prop speed. The wind resistance of the prop and the low inertia of the prop/motor system compared to the winds resistance should make it stop fast enough without applying break (or?) which means that you never get a direction of the torque between the prop shaft and the prop that will unscrew the prop.ยด

    A lot of assumptions.....

    The only way to unscrew a prop in flight is a bug in the esc which applies the wrong fets and stops the motor fast, that could unscrew a prop and also fry some fets!

    /hakan
     
  20. yorlik

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    good points herkam. makes me rethink my conclusions. Paul showed only single direction command is given on pwm sig, so like I said, no reversal command can be sent. One down. But we do not have details on the esc velocity loop controller, so we cannot know if it is going unstable and telling the fets to reverse direction or just as bad, command fets in way to effectively try to stop motor with full current. so esc itself can still be doing something not so nice. No control system should allow ANY variable to go to saturation, and we can see currentCurrent does, so there is still some issue.

    but more thinking and i have to remember the currentCurrent variable is current draw from/to(?) battery; since it is hi values like -17amp to idle in sky, -24 to rise, -12 to go down, all values that seem to make sense, I assumed when it is reported as large + value it meant the motor or some motor reversed direction suddenly. dumb assumption since the current is still coming from the battery, not going back into it. When a pwm esc like this, capable of reversing direction, suddenly reverses or commands speed to drop much slower than it is going, the motor becomes a generator and does reverse the current flow - that is what I am used to in the industrial world. it could still happen here, but the problem you made me consider is that the associated battery voltage is going down, not up at these reported current reversals - means the motor is not regening into the battery if that data is accurate... so it may mean their math is off and the 32676, only 1 bit away from the 2^15 probably digital word being used to scale the current, is wrong. How would this effect the smart battery? I do not know, but it sounds dangerous. since it is typical to have signed 2^16 bit word give +/-32767 values, it may be a math issue and really mean n o reversal of current but rather they mean -32.675amps. this would make sense, especially since the big reversed number seems to stay 2-3-4 seconds at a time. A large speed change would likely happen in milliseconds instead. but it may give dji pause to consider if this is a math mistake only, how it may affect smart battery response. The instability issue in the esc is still potentially real and can still potentially explain the quick speed drop which obviously is the cause of props flying off. and I have not tried to compile prop off reports, but doesn't it seem they became much more prevelant after going to 2.1 esc? I will add this comment to dji SDK bug report I entered. Without more data, I will continue to wrench tighten my props so no amount of motor sudden speed drop will unscrew them. I bet none of the prop unscrew cases ever happened to anyone who wrench tightened them. why take the chance?