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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I'm having trouble visualizing your new mounts, will you post a pic? 

Done! look back 1.

Oh, sorry. My browser just hung up on the page load the first time I looked : )

Nice solution!

Gary, have you tried silicone O rings? I have ‘acquired’ some and they feel much better than the standard ones. More stretchy but slow to ‘twang’ back. The ones I have are not the right size for my arms but I have suspended the gopro from them.

Mine are orange, I think that’s the normal colour of them.

 

Hi Vince, Silicone has much better dampening characteristics than normal Nitrile (Buna N) O-rings and is often the preferred material used in commercially available dampening solutions.

However, the small diameter cross section necessary to stuff through our APM's circuit board holes I think it would might well be cut on the printed circuit board hole edges and fail more easily than the fairly tough Buna N ones.

Still, definitely worth a try.

I'm not actually advocating this kind of suspension mount as a long term solution in any case, too many clunky problems associated with implementing it.

But it has verified and clarified what sort of characteristics are actually important for our needs in stabilizing the flight control board.

Short coupled (small total permitted movement) starting fairly loose but quickly increasing in snubbing capability  as deflection increases and specifically optimized for the mass of the flight control board.

When dampening material has been used so far almost everybody has used way too much and the flight control board just floats on the surface of it and the whole thing vibrates at the frame frequency.

In all fairness, it does provide some dampening, but no where near what can be accomplished with a properly engineered just right approach.

I am going to continue to pursue this and actually try to develop some more practical dampening and isolation mounts and will keep this thread posted as to my results.

You might be surprised at the silicone o rings, I was. Much harder to snap than the same size nitrile ones.

I discovered another advantage of o ring mounted motor arms. They break in a crash and limit the damage.

Somehow a tiny solder fleck had fallen between the GND and +5v pins on the input rail. Dropped like a brick from 10m. Only damage was 8 snapped o rings.

It's also important to keep in mind that we only need to design the dampening mount to be optimized for the vibration frequencies and amplitudes that are being produced by the motors and props while the copter or plane is in flight.

Basically high frequencies of low physical amplitude.

Occasionally, especially on multicopters because of operationally non synchronized multiple propellers you can get the odd lower frequency higher amplitude vibration resonance, but designing in a little extra snubbing and dampening should cancel out the worst effect of those as well.

Hi Vince, I will try Silicone O-rings.

The ones I see available only go down to 70 durometer that is probably why they are tougher.

Ideally they would work better in the 30 to 40 durometer range for dampening, but with proper design 70's should probably be workable.

I am sure the Buna N ones are at least 70 and they definitely do not have as good dampening characteristics.

Good point on the O-ring suspension mount, I have had numerous less than ideal ground contacts with my 450 Flamewheel and quite likely the board still functions because of the mount.

Gary,

Have you considered a cam tensioner arrangement for your testing (or maybe even as a final) mount?

It would allow you to play around with the pre-tension very easily. You could eliminate the variable induced by the fold over threading through the mount hole.

Gary,

I like your solution.

I would image that placing the APM between the upper and lower board and with the o-rings around the connecting studs of the arms maybe  would produce even better results. I'm reasoning that the place where the studs are is the stiffest part of the F450 frame, so movement would probably the least at that part of the frame. 

I have a F550 clone frame on order and that's what I'm thinking of for that one. 

Gary, is your ACC graph based on 42hz filtering, or 20hz?

Based on the time scale and xy lines shown in your ACC graph, it appears you were testing at relatively low velocity, correct? I've seen logs similar to yours turn to absolute mush once the quad goes over 30mph (prop flapping). Yours may not, but I wanted to mention the effects of speed so nobody kicks themselves for having fuzzy lines at 40mph. For example, my brass plate/foam mount gets thin lines when I flip the quad around my backyard, but shows some fuzzy lines when I put the pedal to the metal at the park.

BTW, I recently removed my rubberbands and glued the foam instead (with UHU POR). I was getting annoyed by APM sliding around after hard landings and plugging in wires. Now it's locked on until I peel it off. No doubt the rubberbands were bridging vibes from the frame, and preloading the foam (making it stiffer). Without the bands it still feels solid enough to not have lag problems, but softer on the z axis. I haven't test flown it yet, but I'm expecting better results.

Hi Geritt,

As long as you have a remote GPS you can place outboard and up on top I'm sure you could place the flight controller in the middle.

On a standard Flamewheel the arms also act as central board spacers so you would still have to add standoffs / spacers of the appropriate length and the problem with that is you would have to drill holes and put machine screws through the bottom plate which is also the printed circuit board on a Flamewheel. I would use nylon machine screws and washers at the least through the bottom if I were you.

The top board is plenty stiff to support the spacers and on my APM 2 the GPS is built in so the control board needs to be on top anyway.

Anyway, I like to stick the battery in the middle (and bought the max sized battery that would fit) as that keeps the center of mass nearest the center of the Air frame and promotes slightly better roll and pitch behavior.

It would require some modifications to the design to be able to get the O-rings on and off, easy on top, not so much in the middle.

The main trick is to just have the support standoffs clear each corner by about a tenth of an inch and then just use 1/16" O-rings that are just long enough to support the control board without any detectable slop but that immediately and progressively dampens any attempt to move or tilt it.

Not much tension but no free play. (Silicone O-rings might work even better.)

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