#Mobula 7 hot motors, noticeable wobble while in hover

54 messages · Page 1 of 1 (latest)

feral lichen
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Hello everybody.

I have two Mobulas 7: one brand new (which I almost never fly, bought it when second one was broken down and I wasn't sure whether I could repair it) and a "custom" one. I bought "custom" one used, I am not sure what version FC is, definitely not 3rd one. I changed a lot on it: frame (this one is from BetaFPV I believe), VTX and camera (from another, probably newer Mobula 7), SPI reciever died so I added ELRS one, also I was playing with GPS reciever on it, and it even flew with it 😄 . Motors and props are stock ones.

It flew always OK, but lately I noticed degradation: heavier that usual jello, motors are pretty hot after a flight, the drone noticebly wobbles while in hover (while watching it LOS). If I fly the new one back-to-back, I immediately notice that the new one flies like butter, as compared to the "custom" one.

Recently I started digging into PID tuning, so my first idea was that the frame has different physical and aerodynamic characteristics, and so another PID tune is required. I decided to check this by swapping frames. It actually worked: wobble almost gone, but not completely.

But then why it degraded only recently? I haven't changed PID tune, and the frame looks intact (it is beaten up, but not broken anywhere).

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lavish dove
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If nothing else swap the frame with a brand new one, they're pretty cheap.

feral lichen
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yep, but then, why?

lavish dove
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As for the rest... wobble/degredation is pretty in line with physical damage. There might be more points since it's used, like bent motor shaft, etc

feral lichen
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and the wobble did disappear completely

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didn't *

lavish dove
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Are they both on the same software version?

feral lichen
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I am not sure, but I think no
furthermore, custom one runs on my own build

lavish dove
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Hot motors are often a case of the ESC pushing the motors really hard (e.g. trying to turn them faster than they are capable, rapid changes in speed) which is also in line with damage or bad gyro data

feral lichen
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can gyro go bad over time?

lavish dove
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Not that I know of.

feral lichen
lavish dove
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I don't believe you have a blackbox.

feral lichen
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I didn't check for blackbox on Mobula yet

lavish dove
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I have a Mobula 7 HD

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On the CrazybeeF4FS

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If there's still a wobble with new frame, swap new motors too

feral lichen
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Okay, I'll try that. Thanks!

lavish dove
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It shouldn't be your FC

feral lichen
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So I've changed the frame, but the behaviour haven't changed.
Also I noticed that batteries drain very fast. Earlier I thought that they were getting older, but now I noticed that even brand new batteries do not last long.

lavish dove
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Huh.

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Now it's a mystery. A wobble that appears out of nowhere. Are all motors equally hot?

feral lichen
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Hm.. I will check this.

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Can I apply some filtering to gyro output? Can this solve the issue?

night iris
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You could

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but we don't know what sort of oscillations we are dealing with

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without a black box log

feral lichen
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is there a way to investigate this without black box?..

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or is there something I could try just to test some theories?

feral lichen
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okay, I might have a theory

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As I explained, SPI reciever had died on it, so I've added external ELRS one, also recently I added a beeper. Originally I just "bundled" them together under the canopy, and I was seeing like they flapped around under the canopy during a hover. So I used some double-sided tape to glue them to the FC.

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But can those wires still cause additional gyro noise?

lavish dove
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You could try moving them.

feral lichen
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May be this is the reason why it flew better in another frame. May be I just re-arranged the wires.

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How can I identify the gyro on the FC?

lavish dove
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That will have to be via the internet.

feral lichen
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Ok. So I put a piece of sponge between FC and wires/RX/beeper. I didn't notice any difference.
I added more filtering by moving both sliders (gyro and D term filtering) two positions to the left. I didn't notice any difference.

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I did notice that one particular motor is VERY HOT (you can't keep a finger on its screws) - motor number 2 (front right). Other motors are hot, but not scorching hot.

lavish dove
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Ooh. That's bad.

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Check if the screws are touching the motor winding.

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If not, replace the motor.

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Or swap the position of 2 motors and see if the old motor is hot or the new one.

feral lichen
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I swapped two front motors (I don't have spare one at hand). Now both two front motors are scorching hot, rear two not too far behind.
Of course I am not sure how accurate my measurements are. These motors are pretty small, so they cool down pretty fast.
I tried lowering D terms for pitch and roll from around 35 to around 20. No change.

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I also tried filming the wobble in hover, but it turns out nearly impossible. This is the best shot I could get.
On video wobble is almost non-noticeable, while with my eyes I can clearly see the wobble.

feral lichen
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Also, I probably changed 3 motor on it during last month. How are they dying? Overheating and bearings sieze?

lavish dove
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I mostly fly my whoop outdoors, so I don't know how common hot motors are.

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You say you swap motors on it often?

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Do they make any bad noise? Can't tell much from video

feral lichen
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I am not sure how "bad" the noises are. But when I fly stock brand new Mobula 7, it definitely makes more smooth motor noise, as well as flies way better.

agile needle
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I have the same problem with my mobula 7. Scorching hot motors that led to crash. So hot they melted through my 3d printed pla frame. So i printed one in petg, still really hot motors.