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:
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.
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.
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.
+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 --
my next try of damping the vibration. the damper are from a old Notebook DVD drive.
Really good read. I would like to point out that many people (myself included) have had success using motor isolators in conjunction with a second layer of isolation - when it comes to dealing with rolling shutter or "jello" for cameras like the gopro. Obviously, balancing props and motors is the main area of concern - getting rid of vibrations at the source - but dual levels of isolation sometimes is the other "trick" to get this things vibration free.
I recommend AGL Hobbies motor isolators. Good success with those
I'll throw in the suggestion of mounting motors and props under the arms, from an engineering point of view it just makes so much sense to me. I can hear the difference.
By the way all of the above is true for camera mounting systems as well.