Im in two minds about mounting on rubber rings and particularly if it allows the board to wobble even ever so slightly . Any movement at all of the board relative to the crafts frame I suspect will cause oscillation and probably jitters during quick maneuvers. . I'm more in favor of adding vibration dampening pads to the arms and frame rather than suspending the AMP board. Take for example if the motors are causing a horizontal circular vibration from the motors and this circular rotation is say clockwise, then this is without doubt is going to affect the yaw if the board is moving differently to the frames yaw. So if anything I feel the oring idea is in some cases more counter productive than what its trying to achieve. The APM board in my view must be rock solid fixed to the frame to be able to quickly and accurately read what the aircraft is doing. Any compromise even if only very minute will without doubt affect the stability of the aircraft. We have to realise that if the board can be so easily affected from such small amounts of vibration and it is so sensitive, then the idea of APM board suspension mounts possibly is not a good idea
I feel some of the people having trouble with the PID tuning could be because their boards are moving and are not firmly fixed the the frames.
One method I have used to dampen vibration is to wrap a metal rod in some soft rubber and then slid this into the arm tube just under the motor. The rod floating in the rubber wrap takes quite a bit of the high frequency vibration out of the arms.
Just my thought on this.
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I use 12mm round booms (Multiwiicopter Scarab) I use a short piece of arrow shaft, about 3 inches long with 2 inches glued into the 12mm boom with aquarium silicone at the motor end. I epoxy a another short piece of 12mm boom to the arrow shaft to attach to the motor mount. Nothing scientific just trial and error, I see improvement especially with a camera mounted. The video quality is much improved and the amount of audible noise picked up by the camera is less.
@ Robert & Sami. I've completely forgot about the resonant frequency of the frame, as well as each component that would have dampening in between would have its own resonating frequency, which would be lot to keep track of if you did add many dampeners. I also believe though that the more dampeners you would have, you would essentially lower the rigidity of your frame and lose response time as well.
Though I have little experience, but I am a fan of just very thin foam rings (2/3 mm) that is in between metal contacts, and then tightening with screws. Thats how I have it right now on my quad between board and mount, but personally I plan on switching to a little foam on each engine mount, looking similar to Marc's great diagram. I am biased that I am not a fan of rubber grommets because I worry about the amount of play, though testing would be the best. I feel like a small amount of foam would take care of some frequencies, but it may be little to none.
Theres no way you can tell that some approach for vibration dampening will work without measuring and testing. Yes you can apply some basic technic's that are obvious for vib reduction like prop balance, dynamic motor bell balance etc, other approaches will require specific testing in order to get the best result for the specific frame, and when i mention that is because the vibration will vary in each frame based on materials, installation and hardware. Yes we can design a process to measure the vib in different sections of the frame in order to apply the best suitable vib reduction technic, in this case that section being the nearest point where the APM is mounted. And as I mentioned continuous measuring and testing will provide the correct path for getting the lowest freq.
@John, I actually sort of abandoned my mounting system, at least temporarily. I started it when the "leans" issue was really bad, and I thought damping was our only cure. Then when the fixed the DCM to eliminate "leans", the need for the system sort of fell away. The whole thing was fairly large, and heavy. So it's it's not needed, why bother? I am now going to go back to a simpler system using the vibration absorbing grommets. See how that works.
@Ryan, I wouldn't suggest to start adding rubber between every single mechanical connection all willy-nilly. I don't think that will help, and will probably hurt. You'll be creating a nightmarish collection of natural frequencies and interactions I think it is much better to have the airframe as rigid as possible, and then damp either the source of the vibrations, or damp the APM.
@Marc, I like that design. Nice and simple. I just recently bought some grommets very similar to those which I will use to mount the APM. I wouldn't worry too much causing problems with the motors. A while back I posted some info from a guy who made vibration damping mounts for his quad using those rubber-puck style dampers. They worked well, and the effect was actually much better than anybody would have predicted. It appears that there was a feedback loop created. Vibration from one motor causes vibration in another motor, which causes more vibration in the other motor... With the rubber mounts, the quad is very quiet, and the typical "buzzing" you normally hear is gone.
Thanks John, I take note.
Marc, I would be very cautious about having motors suspended with grommets where they can move even if its minutely. There is a lot of torsional forces when the motors are trying to control the copter and I suspect any movement at all on the arms might give strange flight behavior. For example on my X8 coaxial quad, the motor alignment from the top and bottom motors was very slightly out of vertical and this made the quad constantly yaw. The amount it was out was hardly noticeable, but enough to case problems. No issue to go ahead and try it out, but if there are strange flight behaviors, this is where I would look first.
I am more inline with what Sami stated and try to fine tune the motors and balance the props to start with. But my view is the motors like the APM board need to be fixed firmly to the frame as possible. Its the position of the motors/arms/frame and APM board that the sensors and gyros are working together to control. I feel any movement differential between these elements only leads to problems and hours of PID tuning that get no where. I think there are less problematic way tor remove vibration.
@Sami thanks for the response!
The grommets are made of neoprene rubber. I have a Chinese 3040 CNC so I'll cut the pieces (polycarbonate and carbon fiber sheet).
@Marc:
Just a little something I picked up while working with vibratory feeders: It's hard to tell how the dampers work before testing the full setup with all the parts and weight in it as they all affect the fundamental frequency of the said system. The problem is that if your motor rpm range happens to hit a natural frequency of the mount plate+motor+propeller "system" you'll end up worse off than with any dampers at all. You'll probably have to test the system, with different thicknesses/types of those rubber(?) grommets, to find the optimum setup. In other words you'll want the damping effect to be adjustable to avoid vibration by experimenting (this principle is basically used in reverse with vibratory feeders via an adjustable spring load to fine tune the fundamental frequency of the system to get what ever is being moved to move as efficiently as possible).
As a general rule, in the case of damping the vibration at your motor mounts, you should have the fundamental frequency of the mount as low as possible -- i.e. the mount should be as "loose" as possible -- but still above any frequencies that might be caused by the movement of the frame (which is especially the case with damping the autopilot board). One thing also worth mentioning is the use of multiple stages of dampers that have different fundamental frequencies. That being said, the balancing of the motors/props is probably the easiest way to eliminate the worst of vibrations.
Anyway: nice looking motor mounts! Are you planning to 3D print them, or?
Edit: of course use of materials with good shock absorbing properties also helps.
I'm designing a hexa Y6,
I think this will get rid off most of vibrations:
I'll use this grommet:
http://www.richco-inc.com/products/motion-controlbumpersfeet/100-mo... (VG-1)
What do you think about it? Has someone used something like this?
Yes I would agree if that was the case. I apologize but when I meant between the arm and board mounts I meant between the arm and the body, as that difficulty would vary more depending on your frame. Though I put very thin very dense foam that is tightened between the body board on my quadcopter, and thats all I have currently, the way I see all these things are low pass filters. If you have too much 'cushion' you will not allow the low frequencies to pass, i.e. the actual movement of the quadcopter to register. Another method to essentially produce a low pass filter is to increase mass, which is what I believe you would be doing with the design you have. Mechanical Engineers would have the most experience in this subject and I could ask a few what they would suggest. I personally believe that the quickest way to remove large portion of high frequency vibrations would be to add dampening material between the bolt and plate, plate motor. Theoretically as you increase dampening (increase mass too) you will continue to lose response time, so there is a balance. I just believe that an oring/rubber/foam(I like foam, thin and very well tightened screw to just remove the very high freqs) is most likely the most weight efficient. I would put locktite on the screws too and other necessary precautions. What do you think?
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