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.
Comments
Monroe I agree on that and it was also one idea I have on my list to try out. As it is I use double sided tap on my batteries to stop them sliding around in the battery box. Ive taken most of the stickiness off them so they don't stick, but enough still there to prevent them moving around. This two sided tap foam material looks like it would act as a vib dampener and with the battery working like the shock absorber because of its weight.
Ron, Your simple analogy of the car spring idea is pretty much how I came to this conclusion about the rubber oring idea people are using would be worse than having nothing at all and hence the start of this blog. As I mentioned , any movement at all of the APM relative to the frame position or attitude will be disastrous.
Monroe, On my copter control board because it is so small and light (about 1/8 that of the APM board) I made a sandwich of two types of rubber material and a metal plate which works as the "air anchor" fitted just under the CC board which is pretty much the idea you mentioned as well. .
Robert another reason that big cars ride more smoothly is because of the longer wheel base and that the distance between the wheels to where the driver sits is further than in a small car. Car manufacturers design the seating deliberately so the drivers back is sitting exactly on the centre between the wheels so the ride is smoother. I suspect then that multi rotor copters with wider arms spans would also not be as nervous to fly because of the same?
Wim. although your idea removes the vibrationr really well, my concern again is the possibility that because the motors as suspended then the motors can move slightly independently relative to the APM during flight. There is a lot of torsion when the motors are required to yaw or do quick maneuvers and this may cause them to move out of line relative the the center plate alignment which again will give the APM false readings and poor flight control. The only way your really going to be able to test this is during flight.
Oh, and a key point that has been missed in the discussion thus far: Much of the vibration present in multi-copters doesn't come from the motors, but in fact comes from the "vortex shedding" of the prop-wash on the arms. So even if you rubber mount the motors, there will still be vibrations on the airframe.
Monroe, your description of the Cadillac is not entirely accurate. In many cases, the ratio of sprung to unsprung weight is really not any different than any other car. The real reason they ride so smooth is simply that the spring and damping rates are very soft compared to other vehicles.
As for the O-ring hammock style mount, they can work, and work well. But it depends on the design. Yes, if the orings are too weak, it can create control oscillation. Get it right, and it's fine. Simple as that. And this is much easier to do on a multi than a helicopter, since the control response on a multi is so much softer than a heli.
The reason that the FBL controller manufacturers use foam tape, is not because it is the be-all end-all. It's because it works well enough, it's light and compact, and it's cheap. But that doesn't mean that you can't improve on the performance of the foam tape. Being light, compact, adequate performance and cheap are all good qualities. There's nothing wrong with that. It's a viable option.
But, a really good system can allow a FC to perform even in vibration environments that would ground a foam taped mounted FC. It's easy to make the argument "Well, make sure your machine doesn't vibrate that badly in the first place." And that's fine, but it's not always practical. There are plenty of stories where FC's have failed *mid flight* due to some imperceptible bearing problem that occurred while in the air. I've read so many stories of people blindly replacing every bearing in their machine in search of the one bearing causing problems, even though it feels fine by hand.
Another great example would be that of a mutli-copter losing blades on a motor mid-air. It happens. And it creates massive vibrations. This will shoot down other FC's mounted on foam tape. But a rubber suspended APM will still be able to get safely back on the ground.
Urethane for high harmonics (motor rpm vibrations)
Rubber for low harmonics (Prop to strut ratio, bent axles)
Foam rubber for very low harmonics (not stiff frames)
Delrin for engine mounts, very high kv (>1500).
Identify what vibration you have... Then use the appropriate material. If you're a 3DR frame for example, with 100% perfectly balanced motors and props, urethane is the way to go.
Where you want the dampeners placed depends on you frame design and the 'type' of APM mounting plate. Stiff frames and a thick mounting plate like the 3DR kit and dampeners closer to the vibration source work (you typically want to dampen at the source)... A more flexible frame or thinner mounting plates, it's ok to dampen right at the APM mounting holes.
Ed this is great. and nice to know it does make a difference. Well done and great idea
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.
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.
Ryan, A hard drive doesn't rely on its movement to help compute the hardware position so its not an issue to suspend it. The main issue is if the anti vibration system such as suspending the APM board in any way that allows it to move differently to the frame, then the on board gyros are getting incorrect input feed relative to where the position and attitude of the frame is. For example if you descend quickly and then stop, the APM might continue down slightly more than the frame causing the board to bounce , which then transmits this to the gyros and then the aircraft bounces around in response in an attempt to correct it, and this can increase and cause an ongoing oscillation even to the extent the aircraft would be virtually unflyable. I am looking more into what you suggested in using mechanisms to eliminate vibration in the frame it self rather than isolate the APM board.
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