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Inspire 1 + X5 Harmonic Oscillation Problem (Not from T-Joints) [Must read]

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Hi

I've been lurking here for some time now. Got my Inspire 1 pro for aerial photo- and videowork in 10/2016. First problem was after total of 2hrs of flight I noticed one motor was making a clicking sound when rotating. While running without props on a table it vibrated audibly and the table amplified the sound. Bearings of this motor were going bad. I also checked all the other motors and one more felt loose.

Now here comes the real problem: My Inspire also experienced the harmonic vibration other users have posted here. It only vibrated when raising the landing gear. I originally thought it was because the T-joints are loose as in other cases. They were sufficiently tight. I was even worried about the servo motor because the joints felt so tight when removing the smaller arm and rotating the motor arms.

I sent the inspire for maintenance to DJI through our retailer in Finland so they could address the vibration and bearing fault. It took 3 weeks and DJI had replaced 2 motors though the other motor they replaced wasn't the additional 'loose' one but another. I went for a test flight and the vibration was still there when raising the landing gear. The drone was dead still when landing gear down.

Now it gets interesting. I thought that it must be something else than the T-joints and went for the propellers. I have the quick lock-type props. As I don't have a balancing device I measured all the propellers from the center point to the tip (they were equal to 0.5mm). Then I measured the vertical distance from table top to the both tips of all propellers and looked for balance(so that one tip is not lower or higher than another in one prop). I have in total 12 props and only 4 of these were < 0.5mm. Others were 2-3mm off. Then I collected these 4 good props and measured again. They were off by 1-2mm!? Turns out that if I install prop 1 to motor 1, it is good when installing one way and bad when installing the other way around(keep the motor still and rotate prop 180deg before installing again). This was the case with all the props thus rendering my previous test useless.

I read the post about propellers in which the op said that it's crucial that you don't scratch or bend them in any way. I have also a long background in RC flying and agree with this. I have been extremely careful with the expensive Inspire and the props. Anyway. I set, measured and installed the props so that everything was in balance and the vibration was still there. I still didn't believe that the props would be the cause of this because me and my business partner have bent, scratched and sat on so many props with our previous self-built rc-planes, zagis, quads etc. in 20 years with no problems at all.

Here is a video of the continuing problem:
The video is slow-motion video from LG 5X, around 100fps -> 24fps


I continued to look for the cause of the oscillation and realized that the Inspire + gimbal is a spring-mass-system. I looked into harmonic vibrations theory and it seems that the inspire's motors seed an oscillation to the frame(propably normal for drones) which is amplified in a resonant frequency when the gimbal(X5) is attached. I came to a conclusion that DJI has had bad luck with these masses and the rubber dampers in between them... Maybe not enough testing but the whole combo seems to be too near to the resonant frequency straight out of the package.

I went to prove this and changed the resonant frequency by squeezing the damping rubbers a little bit with zipties. -> No oscillation at all. I mean dead still.

Next thing I did was contact the retailer again with my testing results and he suggested that I should test without the gimbal and check that the plate into which the gimbal connects is securely attached to the Inspire. I did and everything was fine and the bolts tight. I took a test flight with no gimbal (1st time without) and no vibration. Then I measured the weight of the gimbal (534g) and cut a small block of wood about the same weight (539g). Strapped the wooden block with zipties under the gimbal plate being careful not to squeeze the gimbal's wiring on top of the plate and also making sure that the gimbal plate can move freely from the rubber dampers. This time with no zipties on the dampers and a similar weight hanging from the plate -> vibrating again.

This has (at least to me) proven that there is "nothing wrong" with the inspire but it's just poor choise of masses and wrong stiffness in the rubbers between. I still don't rule out that the Inspire wouldn't be in specs and is faulty in some way that I can't measure. They had one situation like this with a phantom 3 and the whole combo had to be replaced before the vibration went away.

So all in all:

- Harmonic oscillation problem when landing gear up -

-> faulty bearing in motor was not the cause of the problem
-> looseness in T-joints was not the cause of the problem
-> propellers being off balance were not the cause of the problem (ALTOUGH may seed the vibration, but still 12 props tested, should brand new props work..?)

Problem is presumably: mass(inspire) + spring(rubber dampers) + mass(gimbal X5) being too close to the resonant frequency of the system into which the motor+prop-combination feeds small vibration. Little reservation still for something else being faulty in the combination.

(Effective mass (spring–mass system) - Wikipedia)

I will now contact DJI directly and try to solve this by presenting all evidence from these tests and hoping that they have a solution for this oscillation. If they have, I think it's new, stiffer rubber dampers for the gimbal plate because you really can't alter the weights of these parts. But if they don't and this is not a common problem (retailer knew about only one other case) I think they have to exchange the drone and X5 for new ones. In the meantime we're losing customers(again), yay...

Any ideas, comments and experiences about these vibrations and your own tests are appreciated. I may have missed something. Also if you have this vibration problem, I can take photos of the DIY solution with the zipties and/or the wooden block test. Going to send an e-mail to dji now->
 
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I'm not sure it isn't something to do with the motors / props setting up the source of the vibration as I've not heard an Inspire sounding like that before - it's more of a growl than a buzz.

Maybe it's just the way it comes across on the video but they usually sound like the one in the clip below (go to 10:30 mark).

 
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Sorry I forgot to mention that the video is slow-motion from my phone. It really does sound weird in slo-mo. In flight my Inspire sounds just like the one in your video.
 
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Sorry I forgot to mention that the video is slow-motion from my phone. It really does sound weird in slo-mo. In flight my Inspire sounds just like the one in your video.
Ah that explains it.

I must say that I don't get any vibrations with my X5 and it's not a hot topic on the forum which suggests this vibration originates somewhere within your aircraft. In the video the Inspire is only a few feet off the ground which suggests it could be to do with the dynamics of the prop wash which changes when the legs are raised, but I'm guessing you've ruled this out and it still vibrates at 5m+ altitude.
What about the top joints/winding gear - have you checked there is no play when the landing gear is up? I've never done this but I'd try starting the motors without props, lift the aircraft by hand, raise the gear then switch off the motors. Alternatively hand catch the aircraft when airborne with gear raised and then kill the motors (obviously a two person job).
 
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Yes, I've tested flying higher also. Today I removed the props and motors running raised the landing gear. Checked for play and found none. All in all the drone seems to be in very good condition (well it's new) and all nuts and bolts tight. I'm really puzzled and the only hypothesis for the vibration must be something related to the "normal" vibration of the aircraft. Of course it might be that it's excessive and not within DJI's limits(?) but they replaced 2 motors of which one was faulty and it seemed at that time that the motor caused the vibration.

Maybe I have to investigate more while waiting for the reply to the e-mail
 
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A real puzzle - last suggestion, is there any play in the prop mounts? I gather the original type can crack around the mounting screws (which is why they shouldn't be too tight) but if yours is a v2 Inspire you've got the later type mounts. Even so just check each one by wiggling the tip of each prop up and down little (to simulate the load in flight) and see if you can feel anything on any of them.
 
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I tested for play in the prop mounts like you suggested. Every mount feels even; there is little play and I think it comes from the motor axles and shouldn't be a problem. I've actually tested 6 motors in total now because I fiddled with them before the first maintentance. When pulling on the bell and wiggling it it feels very solid (simulating flight). Of course the motors/props propably are the culprit of this and the play just may be too excessive(?), can't think of anything else. I'm not sure if we'll ever know the source of the problem if I have to send it back to maintenance/ to be replaced but at least I've done everything I can in home circumstances. Thank you Simon for your help, I'll keep this posted when DJI returns to my mail.
 
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Hi

I've been lurking here for some time now. Got my Inspire 1 pro for aerial photo- and videowork in 10/2016. First problem was after total of 2hrs of flight I noticed one motor was making a clicking sound when rotating. While running without props on a table it vibrated audibly and the table amplified the sound. Bearings of this motor were going bad. I also checked all the other motors and one more felt loose.
...

Any ideas, comments and experiences about these vibrations and your own tests are appreciated. I may have missed something. Also if you have this vibration problem, I can take photos of the DIY solution with the zipties and/or the wooden block test. Going to send an e-mail to dji now->

Thanks a lot for this almost forensic investigation! As DJI does not test their stuff, somebody else must do it. Great job. Curious how this story ends!
 
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My Pro went for repair a couple of months ago after a brush with a shed. The vibration plate was replaced and the dampers fitted were slightly different to the originals in that they had long 'tails' at the bottom. I don't know if the rubber is of a different grade and whether this would make any difference to your.problem.
 
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I have never seen this oscillation in mine. And of course DJI test their stuff, but you can't expect them to test fly each system shipped!
It seems something is not right in yours, but I don't know what. Have you tried changing the rubber dampers? Maybe one is faulty.
 
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My Pro went for repair a couple of months ago after a brush with a shed. The vibration plate was replaced and the dampers fitted were slightly different to the originals in that they had long 'tails' at the bottom. I don't know if the rubber is of a different grade and whether this would make any difference to your.problem.

I found my spare rubber dampers yesterday and they are a also little bit different than the original ones. I actually tested again just before sending the Inspire back to DJI and the vibration was inconsistent even if squeezing the dampers with zipties. Seems that the vibration is sometimes stronger and changing the resonant frequency doesn't always help. I think there must be something else going on in this case.

Anyway I received the inspire from UPS yesterday and got to test it today. DJI had confirmed the problem and for free replaced the flight controller. The vibration is still present when raising the landing gear. Here is a new video in which the problem can bee seen clearly after raising the LG: https://youtu.be/5z0XXvU4xEw

I have contacted the retailer here in Finland and waiting for his reply. DJI didn't pay for the DHL delivery from Finland to central europe (75 eur) so I asked the retailer first what we should do about this.

The story continues...
 
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Alright. Couple of days back I got the inspire back from the retailer and from the 3rd maintenance service. They have now replaced the gimbal mounting plate with a new one and new rubber dampers. These new dampers are a little longer on the underside(you can clearly see that they are different) and definitely stiffer compared to the original ones. DJI also replaced the X5 camera for some reason. I think that they replaced it because it's still cheaper to replace a 1600 eur camera than having to replace the whole inspire when it comes to maintenance once again.

Anyway I got to test the inspire and thought that everything was fine until I saw the footage and images from the new X5. The drone doesn't oscillate anymore against the gimbal. The rubbers have sorted that one out just as I thought. Also the original conclusion was right that the drone suffered from harmonic oscillation and DJI has manufactured stiffer rubbers to counter that. However new rubbers don't affect the real problem here which is the vibration from the motors. I have to state here that the propellers are still in balance and I have tried multiple combinations of propellers and installed them also the other way around (which affects the difference in the height of the tip in my case). The drone vibrates still too much and creates the feared "jello" or "jelly"-effect on shot footage. Also still images are affected. There are two youtube-clips and two images attached here. In the images you can see that part of the image is in focus and other part is blurred.

I also noticed that the jello-effect doesn't always be there. About half of my images from yesterday were sharp. I'm writing an e-mail to the retailer now and asking for a new inspire and also compensation for the lost income (5 months of service and 0 hours of flights on behalf of our business). I'll get back here when I have more news.

half sharp2.jpg half sharp1.jpg
 
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So. I got the Inspire back from the 4th maintenance today and this time DJI exchanged the whole package. New Inspire 1 pro. I really hope that there aren't any problems present in this one. It seems that it took them hit and miss 4 times to actually mend a factory defect. Still they said that the Inspire had had test flight and all after every maintenance and presumably no faults found(?) in the after service check.

I'll get back again if something comes up. Altough this isn't anymore very interesting as we may never know the real reason for the vibration problem.
 
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We got some post spring snow-storms here in Finland but I managed to get couple of hours flying with the new i1. The Drone works perfectly. It's transmitter on the other hand is faulty :D. The video feed goes black and the software freezes on my Nexus 5X. First I thought that the Android on the Nexus was buggy because it had done it before and had to restart my phone. Turns out that the transmitter was faulty. Also the i1 went green light for about 2 seconds when powered on and connected to the transmitter. Then yellow continuous flashing. In the end the transmitter went dead with the video feed altogether. Tested with 2 nexus 5's, ipad pro 2007 and an ipad 2007. Transmitter master/slave and all possible combinations of transmitter+pad.

Luckily I still have the other transmitter which works fine with forementioned devices. Our retailer took the faulty transmitter and it is now being repaired by DJI. Would be nice if DJI had better quality control o_O
 

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