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

          • Thorsten,

            Nice test and data.  Will definitely look to their solutions when I build my next larger auto-drone, I see the Drotek stuff is made in France.  Have never ordered from either before, both look to be in EU.  Found a quick $43 solution with onboard mag for now, which I can use to test the old APM and then re-pin it for the PixRacer therefore taking one variable out.

        • MR60

          I got my M8 at www.drotek.fr (xxl version, excellent product: did a one mile long mission with a landing within one meter of planned waypoint. Maybe some luck too)

          • Thanks Hugues, 

            Going to try a RCT solution for now since I had to order other parts, and am on the west coast in the US.  

            Looks like a really nice board though, especially for auto-pilot missions on a large extended run copter.

  • T3

    I think there can be a stability problem with the soft "balls" when you fly in acro mode at high speeds with flips or rolls. In most other cases they seems to be a good solution.

  • T3

    What would also be interesting is the effect on vibrations when mounting the motors on the arms or below the arms. Normally, I mount them below as shown in the image. This should reduce at least some additional vibrations - apart from a small increase in efficiency.

    Coter_V4-1w1.png

     

    • T3

      What about the distance between the props and the arms? (just joking ;-) )

      I think this will get pretty complicated if you want to collect and analyze data - not to mention the math behind it...

      So in essence: if there is a chance to mount the motors under the arms one should do it - it won't hurt.

      • MR60

        My first ship was under the armpit for that reason ... just a question of if you are comfy being under a flying lawn mower ... but then again, how many times have I leaned over the ship to turn it off and my fat belly pushed the throttle to full ;-{

        The reason why over might be better is that under requires landing gear that adds weight. So like all things, there is a trade that we need to measure to get it right.

    • MR60

      Nice landing gear to be able to use large props under. Do you have more details about these? 

      • T3

        Hugues, it's all custom made:

        Coter_V4-2w1-1.png

        • Hi Thorsten,

          bit off topic - Do you mind sharing the still-pic camera model that you use on your rig?

            I assume it is a Canon and you use CHDK scripting etc.

          Thx,

          Gerhard Laubscher

This reply was deleted.

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