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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      • I mean the two things, the ship becomes nervous and if I read vibs, are higher. A friend visit me past weekend with a cuad, he had a horrible mount with two velcros, I said, that impossible to fly good but it was possible, It flies good, I try with one velcro and nothing better, the ship goes up and down nervously in windy conditions, I changed props for his APC plastic and nothing better, at the end i fix a weight upside the APM and changes a lot , then I take away and the copter beguins to jump again, then I put two coins upside, then another two and stop when the ship becomes to fly better, that was the same weight (50 grs) than the first time, ( I share a photo in few minutes); now I'm triyng to make something more elegance than the coins and try again; each ship have it's own paranoias UFF!!, the other cuad flies well with hardmounting APM :O

  • I undertook two further flights today with the Pixhawk hard mounted via double side tape. Conditions - quite breezy

    3701903713?profile=originalVibrations are lower than my earlier flight posted a few days ago where the foam padding that came with the Pixhawk was used.

    Not sure how accurate these results are as the program truncated to the last 30,000 and could not find a hover. Not sure how to match up what records were used to the bin file in Mission Planner so that I can graph the specific sample used for comparison.

    • MR60

      Wow!  your vibes look excellent!

      you might try a 2 minute flight indoors.  it can tell if you are trying to hover in poor conditions.

      or ...

      set the hover yourself.  In the grey cells marked "Hover Start & End Rows" input a series that creates about 700 observations (from looking at your chart about 12500 to 13200 maybe).  Then compare statistics.  try to find a spot where roll and pitch are small (not fighting the wind).

      it's not unusual to find that vibrations were less using no pads.  in fact i have yet to see one that didn't improve from not using pads.

      the reason to try it indoors is that the roll and pitch should be better in a no-wind situation (dots mostly in the green).  if lots of dots are outside the green then you can try tightening up the PIDs (increase P & D) until it doesn't fly well in wind.  i've gotten the dots to + 0.3, but it was like a tight violin that quivered in a light breeze.  a bit scary.

  • Jon,: what I do is a screen capture to post as Jpeg

    • Thanks Cala

  • Using Forest Frantzs' Hover analysis tool on my newly built H frame I had been getting an 'OUCH'. Today I managed to get a 'Good' in the 'Roll & Pitch Stability' box. I used the standard foam mountings that come with the Pixhawk and had it mounted on my homemade Nomex sandwich EP. The carbon tubes are 10mm OD 0.5mm thick, multistar 2213 motors with 10 inch props.

    I tried keeping the copter as steady as possible for some of the flight but it had a bit of a starboard drift which I had to control (Probably badly positioned battery). The Hover tool couldn't find a hover in my data so I don't know how accurate the analysis outcome is. I had previously triggered autotune in a previously flight but not sure whether it had completed. I did howerver save the new settings. I now notice in loiter that it twitches a bit so I will have to read up and adjust some of the settings.

    PID settings for the flight are as follows: Stab role & pitch P 11.25. Rate roll P 0.125 I 0.125 D .0070. Rate pitch P 0.22 I 0.22 D 0.01. After I have refined the tuning I will run the Hover tool again and see whether there is any improvement.

    3701900116?profile=original

    I don't know how to save part of an excel files as a jpeg so I copied and pasted into word and saved as a pdf file. I'm hoping I have attached it properly.

    Hover Analysis Log 90 29 Dec 14.pdf

  • Hi Forrest,
    Many thanks for making your spreadsheet available, I am eager to use it but don't have access to the latest MS Excel. I have tried both LibreOffice Calc and Excel '97 with compatibilty pack but Libre's Basic macros aren't compatible & the old Excel doesn't handle macros.

    Can you suggest any compatible alternative to Excel at all?
    Thanks again.

    Philip
    • MR60

      so all of you $$$ goes into UAVs ... good man

      you can use Excel Frantz.  it's free.  following directions stated by Cala, post your log file (in MP download the bin, create the log, then post it).  I'll take a look and post it back.

      We learn a lot by looking at other folks logs.  What's normal.  What's not.  What stats help.  Etc.

      • Yes, you can get rid of $$$ much faster with UAVs than just tearing them up and chucking them in the air!

        Thanks for the offer, my existing logs are a bit on the large side so I'll log a shorter flight and post that. My X8 hovers in Alt Hold rock steady but in fast forward descends at about 15 degrees down until slowed and then rises back up.

        Regards

        Philip
        • MR60

          In fast forward, if it sinks in Alt Hold, then look at two possibilities.

          1) air pressure around the APM bottom side might be decreasing during fast forward flight. The copter thinks it is getting higher (lower air pressure) when it's not. This can happen when the ship is tilted into the wind and a flat surface acting like a wing causes high and low pressure volumes around the sensor chip. On the APM, the ship is on top near the front.  On the Pixhawk it is on the bottom on the leading edge.  This is fixable by using foam to spread out the source of air or change what is causing the wing effect (causing high and low pressure volumes).

          2) the copter is tilting too steep to stay in the air.  you can limit the pitch angle in the extended parameters.

          If you said that the ship becomes unstable during fast flight, then vibration or PID tuning might be a cause.

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