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...

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+1 on the scavenged optical drive parts, I found some particularly soft green isolators -- you can see them in my gimbal mount

The key is to use a shouldered fastener so that you can fully tighten the connection without overly compressing the material.

Aluminum binding posts work well and are lightweight --

Prop choice is an interesting subject. I have some wooden props that I'm excited to try out, but I need to make some longer prop mounts to account for the thicker hubs. It would be interesting to do a comprehensive study on the vibe effects with all the different prop choices out there.

Hi Joshua, any idea what those green Isolators are, they look excellent for this use.

The binding posts are a great idea too.

Propellers are a whole giant topic of which I am only certain that at this point they are entirely under engineered for multicopter use in all ways: efficiency, appropriateness for static thrust and vibration.

We are simply inheriting modifications of designs from planes in particular slow electric 3D flyers.

I have no doubt some are better than others and i am sure Graupner would contest this.

Gary.

A few weeks ago I had my APM mounted at the corners on some very soft ‘sponge’ type foam.

I spent days trying to eliminate a horrible motor/prop vibration until I held up the running quad and put my finger on the APM. Vibration instantly stopped. There was some sort of feedback going on. Motors were being ‘pulsed’ by the APM. It was much better, but way from perfect, on a solid fixing. That was on my old Chinese X frame.

On my new frame its on a 60g block of ali on a 5mm ‘anti vibration foam’ – dont know what sort, like pink skin. The ali base is fully attached. Might get better results with just 4 columns at the corners – or might not.

Gary,

I looked around online trying to find some comparable isolators to get specs off of, but no luck. I went through a laser diode harvesting frenzy a few years back, and I'm positive these isolators came out of a DVD burner. There were a couple of different types and these were the softest.

 I'll keep looking around and report back -- Same on the wood props.

Speaking of prop design: I'd be open to the idea of getting involved with open source component design and manufacture. There are some interesting ways to make things nowadays.

 Props, prop attachment schemes, gimbal components, ... Whatever is lacking in our world.

I have a fantasy about carbon motor cans! Probably not worth the effort, but it just seems super cool, and I would like to optimize the cooling vanes for the reverse rotating motors, because I'm insane.

I believe the problem you are mentioning Vince is probably: "sympathetic resonance".

If you stuck your light foam onto the air frame without the controller on top of it and turned on the motors, you would expect the whole block of foam to pretty much vibrate at the same frequency as the rest of the airframe because no mass was acting against it.

Unfortunately, the flight control board is often light enough to count as no mass.

Furthermore, depending on it's characteristics, a very soft (open pore) sponge like material might provide relatively good isolation characteristics but little or no dampening.

This is like suspending it on a bunch of springs and with this characteristic, the suspended object can easily experience an amplified displacement rather than a dampened one because the vibrations can merge into a higher amplitude lower frequency oscillation.

This sort of mechanism generally also has one or more "natural frequencies" which it will tend to vibrate at when exposed to various input frequencies or harmonics.

Actually, Audio, Electrical and Mechanical mechanisms of this sort are in common use on purpose and are also often common problems.

The characteristics of the best media for our use generally tends towards dampening more than isolation or at least a balanced combination of the two. 

The antivibration foam you are now using probably has a great deal of built in dampening capability whereas the other foam did not.

Dampening converts the motion of vibration into heat (not very much heat), but it actually removes energy from the system, thus "dampens" it.

Isolation just seeks to isolate the object from the vibration, but since it is still connected at various frequencies it can often actually confer a sympathetic vibration at potentially an amplitude even higher than the input amplitude.

Unfortunately there are no perfect isolators, dampeners or isolator / dampeners and they all have optimal dampening frequency ranges and resonant frequency ranges where they are less effective or worse might actually increase displacement or oscillation.

That having been said, one of the greatest mistakes being made now is to use too much dampening medium on our little lightweight flight control boards.

Isolators / dampeners are optimized most importantly for a specific mass (weight) of object that they are supposed to dampen and on our little 1/2 to 2 ounce flight controllers even the lightest most flexible (dampening) materials such as 30 durometer Sorbothane, Alpha Gel, Kyosho Zeal tape and EAR in 1/2" squares at each corner would be as much or more than is necessary for it to work as well as possible.

These materials are all actually specified for how much to use for how big of a mass.

Interestingly most of these materials all work noticibly better in 10 to 25% compression, so sandwiching them (and the flight control board between 2 hard mounted points is definitely be better than just letting the Controller "float" on top of one layer. This also goes a long way towards dampening potential sympathetic vibrations.

For the moment, sub 1/2" square pads on the four corners of the flight control board(s) / case optimally sandwiched on both sides as suggested of one of the materials mentioned or the smallest sized "EAR" shear grommet is very likely to provide the best primary dampening that is currently commonly commercially available. (and even some of these are hard to actually get (EAR Shear grommet)

I am using an APM 2 board suspended between 4 standoffs with folded over 1/16" thick O-rings which Isolates and dampens adequately, but it is by no means ideal or the best that can be done.

Heavier EAR ribbed grommets can also be used to dampen motor mounts and greatly reduce vibration transmitted to the frame in the first place.

Best is to Dampen at both the motors and the Flight controller.

Thanks for info.

I have said it a few times. More use could be made of the battery mass than is currently the practice. They mostly seem to be suspended on some sort of strap adding nothing to the frame mass. If the battery and APM were isolated together that would probably help a lot. I have clamped my battery above the APM with a lid and plastic wing-nut. They are both isolated from the motors (as you have seen) but then the APM is isolated again. Seems OK but one always wonders if it could be better.

 

Do you think there is a danger, if you have very low vibration and manage to get very high tuning without problems (is that how it works?) that any internal shift/change that raises vibration could make it unflyable?

On my first frame (with the bad foam) it was not possible to have ANY rate D else the throttle would become an on/off switch.

 

 

Using the battery to increase the mass of the APM is actually a great idea and certainly can allow more appropriate sized dampeners to be used more effectively.

But there is one problem which may not be major, but is real nonetheless. 

The bigger mass also serves to more significantly decouple the Frame mass from the Flight Controller and while vigbration reduction is very important, the decoupling of them as 2 major masses also results in inducing a more significant lag between the frame "doing something" and the sensors on the flight controller actually seeing it.

It's kind of a Catch 22 really.

You want the flight controller board to be vibration free, yet immediately responsive to anything the Air frame actually does either on purpose or in response to outside forces such as gusts.

So, in reality you end up describing either an arbitrary or calculated frequency domain and a response domain. You want as much dampening as possible in the frequency domain and no dampening at all in the response domain. Obviously, probably not actually achievable.

One of the advantages you do get by keeping the mass of the flight controller small in comparison with the mass of the air frame is that it is easier to separate the vibration frequency domain from the flight response domain.

The most problematic example I have seen of this is where the frame arms on an H copter are connected by a suspended O-ring mount which does a quite good job of dampening the vibrations in the air frame, but at the expense of considerable lag between what the motors and arms are doing and the information getting back to the air frame -> flight controller board.

On the surface it seems like a great idea, but the lag between the motors / arms doing something and the flight controller board actually seeing it induces considerable lag in the system, which in manual mode may be workable, but which at the very least in automatic modes will require considerable tuning to result in not actually having the sensor response loop fighting the mechanical delay.

Even in stabilize mode I would expect that it might be likely to set up a self amplifying oscillation that could result in complete de-stabilization.

I think it is reasonable to dampen either or both ends of the system, but not in the middle.

Dampen the motors and the flight controller, but not the stuff in between.

As soon as you get to 2 large masses you end up introducing low frequency delays which can result in oscillation amplification under various conditions.

The fact of the matter is that once motor direction rotations were reversed to reinforce yaw rather than fight it on the above mentioned quad copter at least initial results were smooth and vibration free, but that was before auto-feedback flight was attempted. Still awaiting the result of that one as I said I have my doubts.

I've done some searching but can't seem to find the way to get this data in a log file. The only thing i can seem to find is people referring to enabling "raw" mode in CLI. I can't seem to find anything that describes how to do it.

If anyone has a link to this process it would be much appreciated.

Thanks

I knew it. As soon as I posted it I found it.

For anyone else looking all you have to do is go to command line. Type "logs" then "enable RAW"

Thanks

Hi Gary.

Its been windy every day since I finished the H frame quad but yesterday I decided to risk it outside.

Stabilise was fine and so was acro,  (since 2.9 I cant see any difference). Loiter was spectacular. The wind was blowing leaves everywhere but the quad just stayed put. I even tried dragging it away and pushing it up and down. It was very determined to keep its place.

I didn’t try an auto mission.

Im not sure if you consider Loiter as fully automatic.

Perhaps I have misunderstood the control system. I thought the  ‘inner loops’ kept it right side up and did all the fast processing, and were then acted upon by either the pilot or a guided mode. I thought any nasty lag/feedback/vibration would show up in the lower modes just as much?

 

Anyway winds dropping, are those batteries charged?

 

 

I have added O-ring suspended standoffs to my FlameWheel F350 based on some of the information I have found in my research on dampening and isolation lately and I think I am actually making some progress.

These standoffs are placed directly outboard of each board corner with approximately 1/10" clearance and are suspended between folded over 1/16" O-rings of just sufficient diameter to suspend the board without movement.

Results were considerably improved over the F450.

I was surprised about the quality of these results but was applying what I had learned so far to the current design and from the surface it seems to have worked out really well.

Since our board is so small and light one of the most important tricks is to have the "solution" be short coupled: Nearer the actual frequency and total vibration displacement zone.

My F450 has the stand offs spaced considerably further from the board and even though it has longer O-rings it is under considerably more tension and although it dampens adequately (+ & -  5) its no where near as good as this and it has more total board movement (way outside the vibration zone) which can induce actual (albeit small) delay.

Restricting total board movement to 1/10" or less and having very progressive dampening (and not much initial tension) has produced this result.

I am pretty sure these principals can be used to produce a practical board mounting method that will result in similar performance.

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