Now that we have version 2.9 and inertial primary control for the Z axis and soon to have it for X and Y axes as well it is necessary to take vibration dampening and isolation of the flight control board much more seriously.

Primary improvements can certainly be made by balancing the props and motors.

So far it seems that the more rigid the frame the better because frame flex introduces undesirable mechanical delay (hysteresis) in translating motor induced actions to the centrally located flight control board. (Do NOT shock mount the motor Arms).

It may be reasonable to somewhat vibration damp the motor mounts themselves because they are on one end of the mechanism.

However, primary damping gains will be made by vibration isolating and or dampening the flight control board itself.

So far we have undertaken this process simply by trial and error sticking on of Foam or Gel pads or using O-ring suspension of the board to outboard standoffs.

This has achieved (barely) acceptable results, but is certainly by no means optimum.

The crucial fact that we have not properly addressed is that the amount and type of dampening medium needs to be matched to the weight (mass) of the item we are trying to isolate.

In fact we are trying to isolate a flight control board that weighs under an ounce or less than 2 ounces in its case which is a very small mass.

Our current "solutions" are actually designed for much larger masses and are not nearly as effective for the light mass of our flight control board as they ought to be.

I have done some on line research which did fully verify this inadequacy.

Virtually all off the shelf solutions (either pad or stud type) basically require a suspended mass that would weigh at a minimum 5 to 10 times what an APM or PX4 / IO board(s) weigh or more for optimal effectiveness.

This includes all pre-made Sorbothane, Alpha gel, memory foam or other silicone or urethane gel or foam mounts including Lord Micro mounts.

However, Alpha Gel or 30 durometer Sorbothane or Kyosho Zeal Gel double sided tape do appear to be the best possible solutions at this time so long as you use small enough pieces of them.

Simply putting a double sided pad under the entire board as we normally do now is entirely inappropriate for maximum vibration isolation and it is amazing it works at all.

Optimally you would use pads of them smaller than 1/2" square (possibly even 1/4" square) on each corner of the board or APM enclosure box. (smaller for the bare board than the board in the box obviously).

You could also improve isolation somewhat by sandwiching the board / enclosure between pads on both sides in slight compression.

So far we have done a dismal job of approaching this like engineers, but the reality is that with the massive excess quantities of vibration absorbing materials that we are using versus the mass of the APM (or PX4) has produced better results than not using them, but no where near what could be achieved by using the proper weight and size of dampening / isolation material.

The basic solution is to reduce the actual isolation medium to the 4 smallest pads you can get by with on each corner and using the softest commercially available dampening materials you can find.

A further gain can be made by placing the item to be damped in 10 to 20 percent compression between 2 pieces of the dampening material.

Thickness of the dampening material does improve dampening and isolation but is not nearly as important as selecting the right material and the right size of the supports made from it.

I believe that Kyosho Zeal tape is 2/10 of an inch thick and that is probably plenty for our use and the frequency range we are trying to damp.

I would very much like to see 3D Robotics produce a APM (and PX4 / IO board for that matter) case with proper internal shock mounting of the board(s) with dampening data for it.

I actually suspected this result from the start of my investigation and a little thoughtful research has completely confirmed it.

Another significant gain in vibration isolation can be had by using a high flex wire and strain relief approach to all wires connected to the Flight control board (and using the minimum number of wires necessary as well.)

I have used the concept of vibration isolation and dampening somewhat interchangeably in this discussion.

Isolation is simple undamped (spring or rubber band support) which allows the movement of the isolated object largely separate from the containing object.

Dampening is the conversion of vibration into heat energy by a shock absorbing medium (car shock absorber for instance.)

Our ultimate goal here is to provide the most high and medium frequency reduction while still allowing low frequency actual board movement to take place with a minimum of delay.

So realistically our methods embody both Dampening and Isolation.

I have covered a lot of ground here, but this is at least a good start for designing some real world vibration solutions that are bound to work better for us than what we have done so far.

Please try your own experiments and kick in your own thoughts here, that's how we get better and this is just a launching point.

Here is an excellent link to some definitive research and testing that will help:

http://fpvlab.com/forums/showthread.php?4251-Vibration-Dampening-amp-Isolation-Solutions-Guide

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Replies

          • MR60

            Great ... the more folks we have testing this the better incase it is prop or other dependent.

            If you can, do the tests indoors in as controlled of a comparison as possible.

  • Frantz, Thanks for  the awesome tool! Would you mind updating the spreadsheet to process the new scale of thrust used by arducopter 3.4 beta?  Here is the analysis of my tarot 680 hexa, using the 3d printed bed which 45degrees dampening balls.

    What do you think?

    3702249404?profile=original

    • MR60

      Hey Thorsten - See ... I was serious!

      Fernando -

      What is your FCU? Pixhawk or one of its smaller brothers?

      These results are interesting in that z starts off really good and then it's like something happened (nut loosened?) and z became tolerable, but not great. Any ideas?

      I'm not familiar with the 3.4 beta new thrust scale. What is it?

      • T3

        Yes! :-)

        Seems there are more and more dudes!

        I'll perform some more tests and tuning next week. 

        • MR60

          So far they are all using FCUs that are hybrids of the Pixhawk, versus the actual Pixhawk. You definitely have discovered something that the community did not know. Now we just need to figure out what. Your FCU is on the way so maybe I'll have data in a few weeks.

      • It is a pixhawk from HK. 

        however I'm trying to build a small tricopter ( quanum trifecta ) with minipx4 and it is been a huge challenge...

        With 3.4 beta, something happened with alitiude control behavior that was really crazy, it was bumping 1-2m when I was around 5 m high. I have posted on http://discuss.ardupilot.org/t/3-4-althold-bumping/9391  forum, and I believe that is something to do with my sonar and the fact that I was flying over grass. However  I'm still not sure yet. I'll try to fly with sonar disbled  today night to see if there is something to do with it.

        It seems that the thrust output is ranging from 0 to 1 now. I have asked also for an explanation on arducopter forum because I couldn't  find anything mentioning that change on the changelog. Will try to search on git to see if I find something. As soon as I fix wherever is letting my altitude controller nuts, I'll redo the hover test to see how Z behaves.

        • MR60

          The HKPilot32 Autonomous Vehicle 32Bit Control Set?

        • MR60

          I ran into something similar with an earlier version of the Pixhawk. The z values would be relatively stable and then jump. Once the z values started to clip, the ship would think that it's dropping (altitude is mostly controlled by the accelerometers) and then do a fly-away. There was a bad batch of IMUs.  I got a new Pixhawk.

          • My pixhawk is pretty solid. It is a year old device. It started happening since the upgrade to 3.4. I have flown more than fifty packs with 3.3 without any issue and I havent upgraded anything except the arducopter code. I could't test today because it was raining. Will try again tomorrow.

            • Hi Fernando,

              It seems you must have been flying with Sonar for some time, so this is likely not going to help, but ordinarily I would say SONAR is a big problem (especially in Z axis of course).

              Every kind of noise imaginable affects Sonar, vibration, sound, EMF and even RF.

              If you look in the index of the wiki I think you can still find the page I wrote regarding implementing SONAR.

              The fact is 9 out of 10 people who try SONAR never get it to work satisfactorily due to all the paths for noise (which translates to Z excursions) to sneak in.

              I got it to work OK, but eventually abandoned it because it was just too fussy.

              A cheap LIDAR (like Lidar lite from Pulsedlight3d) will generally work much better.

              Aside from that I think there were a few early issues with 3.4 beta, you may want to reload now.

This reply was deleted.

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