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:
By the way all of the above is true for camera mounting systems as well.
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.
Yes, Joshua I totally agree.
It definitely does, you not only induce assorted broad frequency vibrations, you are in the way of your own prop wash which results in a direct loss of power and efficiency.
The difficulties are props potentially more exposed to ground contact, and your center of thrust is lower in relation to your CG so potentially less stable.
Reality is that these are small problems given current controllers and easily accounted for.
I will probably be going that way in the future.
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
There are design considerations for sure, but, like you say, they are not huge. On my current craft I have been able to get the APM almost perfectly coplanar with the motor mounts and the majority of the weight on board within 40mm either side of that plane.
I break props no matter where the motors are mounted ; )
Thanks for that link in the thread starter, looking through it now.
Actually a centre of thrust lower than the CG makes for a more stable craft in this situation.
Problem of ground strikes remains though. Probably something to try once you have the bugs worked out of the system and crashes are rare.
I've worked really hard on keeping center of thrust and CG on the same plane, don't you think that is the ideal? Lowest polar moment.
The new gear I'm building passively retract into a prop protection role once the the craft is off the ground
Sorry to deviate from the vibration topic, Gary. I'll stop here : [
I've not seen any definitive conclusion WRT COG being higher vs on the same plane, howerver it is clear that both of these configurations is better than the COG being below the centre of thrust. Unfortunately that's how most copters are built because it is usually easier to sling the battery underneath.
Interesting about the prop protection gear - you should share that on another blog post once it's working well, lots of people would be interested I think.
That looks excellent!
So far + and - 5 seems to be very acceptable and you have + and - 1, so for inertial response and control at least, it seems to me you are covered.
Even those bushings are probably designed for a weight greater than the flight control board, but obviously they are more than adequate to the task.
The way you have them installed would make them work better for a lighter device than the one that was actually installed in their center grooves.
Innovative re-engineering of the dampeners to change their range of operation, definitely a good direction for all of us to think about.
Hi Jonathan, those are truly great motor mounts, definitely going to look into them for my own use.
I would certainly believe that you could get rid of most of the motor and prop induced vibration with those and definitely make the requirements for flight control board dampening less critical.
Clearly a combination of both motor and board isolation along with motor and prop balancing can pretty much eliminate undesirable vibration.