Vibration Isolation and Dampening of APM / PX4 for version 2.9

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-am...

Tags: 2.9, APM, Controller, Dampening, Isolation, Mounting, PX4, Vibration

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Thanks Forrest. Log is attached.

 you in Hood River, OR?

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Looks good.  If you are happy with the way it flies, great.  The signal to noise proxy is greater than 4 so the APM knows what is going on.  Well done.

If you want to improve:

o Check prop balance to see if that helps decrease x/y acceleration.  Z looks real good so prop tracing is probably great.

o See if your THR_MID is set to about 402 so the copter will hover at mid throttle.

o You might be able to tune the PIDs tighter if you want more stability. 

o Tighten up all bolts.  If the APM is suspended on something, try without and see if it improves.  If not, go back to what you are using.

The excel file I sent was looking for RAW records, not IMU so it even gave me an error (a simple change in the File Config Data tab).

Forrest, Thanks for running the log.

I think the additional vibration in this graph compared to my previous is because I took 21+g mass off each motor mount end. My quad is a flame wheel and had 200mm leg extensions, dowels and hardware at the ends of the arms. Now, the landing gear are lighter and mounted under the center section. Also, the arm screws were loose. LOL.

Some questions on your points.

> o Check prop balance to see if that helps decrease x/y acceleration.  Z looks real good so prop tracing is probably great.

I looked up "prop tracing". How does it apply? I looked up "prop tracking" and --- Ha, ha... my cheap plastic props are nicked, tweaked and in several different directions.

>o See if your THR_MID is set to about 402 so the copter will hover at mid throttle.

What? What has this got to do with the topic? Mine is set at 500 because this is related to my ESCs, Motors, Props and how the particular items work. My quad does hover at just a hair under half throttle.

>o You might be able to tune the PIDs tighter if you want more stability.

Stability? It is very stable. I could get it more responsive/twitchy but, I can fly it in a 6' square area with ease. The graph even shows stable flying around for over 5 minutes. Oh, you might try David C's tuning style. I used his to tune the PIDs on my quad to get it stable.

>o Tighten up all bolts.  If the APM is suspended on something, try without and see if it improves.  If not, go back to what you are using.

How'd you know all my screws were loose? ;-)  And here you go on about hard mounting again. Not sure how applicable since the graph is good.  :-)  You just will not give up on hard mounting everything.... Weird! You're trying to put us back into the hard mounted horse drawn wagon era. Even they used springs in the end.

Thanks again for running the log. I am rewiring, reconfiguring and remounting this weekend so, I may ask you to run another one next week. This will be with balanced clipped props, less wire, and firmer mount. Not solid!

Brian.

Good for you Brian.  And you are welcome.  Look forward to seeing more results.  And yes, my screws have been loose for a long time.

The 400 has nothing to do with stability or vibration control.  But the system tracks your throttle setting at hover.  It's at about 400 (or slightly below center).  If you want it right on center ...

I'm actually not an advocate of hard mounting.  I'm only an advocate of testing the limits and learning.  In this case, the two limits are hard mount and suspended.  For example, when you took the mass off of each motor mount, it would have been really wonderful to have tested before and immediately after making that single change and reporting back to the community.  Now I'm curious :-)  What are all of the impacts of reducing mass at the motors?  It's a complex question because by necessity, if nothing else changes the the PIDs need to change (the P becomes smaller if all else stays the same; not a clue what happens to I and D).

Always glad to help and learn.  Look forward to your next fight and log.

Forrest,

If you notice - This is the process of a Pre & Post reporting of changes. Recently I posted my quad results - see above in forum for the Pre condition - and here my removing the mass is the Post report for the actions. That is why I commented about the higher values are likely from the removal of the mass.

Unrelated discussion of THR. 400 is about 1/3 for me - No good! We should not advertise variable settings as constants when they are determined by individual vehicle configurations. Not relevant to our vibration discussion.

Mass at end of arms - I too am now curious.
After my quad goes back together with little changes I'll do a new baseline. Then, I will add simple mass back onto the motor mounts to see the effects. I will publish Pre & Post, as above, when done.

This is the current Hover Analysis Worksheet

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Interesting idea with the dampers Vince. Have you also considered using standard Graupner or Aeronaut folders?

It's interesting, but the pivot axis is in the wrong direction. As Kevin states, you need to use something more like classic folders.

Another option would be to create a teeter-totter for a standard propeller.

I have an Aeronaut 10x6 on my glider, an APM2.5 waiting for install, spare motors, esc's, and a powerful leaf blower to experiment with. I've had some ideas for crude flapping experiments, but I figured I would at least need pitot data for anyone to take me seriously (at least, I think just varying distance with the blower at full throttle won't convince some of you). The results won't be as absolute as using a windtunnel, but I think with a careful setup and comparisons, we could validate if folders are worth further investigation.

What do you guys think? I am not going to set this up unless you guys think it will be useful. I know it will work, but I don't need to convince myself (I don't have the money to buy a bunch of folders anyways, but if the results are promising that may motivate others to flight test folders LOL).

That quick design was just to make use of some broken props for a bit of testing

But copters do have a lead-lag hinge as well as a flapping/teeter one?

Kevin, yes folders were my first thought but they tend to be pitched much higher than copter props, at least thats what I thought until DaveC brought his 90mph copters around here.

I think we need a manufacturer to bring out a plastic prop with a thin, very flexible root, perhaps with Kevlar molded into it.

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