Rubber suspended mounting of the APM board not a good idea

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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Comment by John Campen on April 19, 2012 at 9:42pm

Ed this is great. and nice to know it does make a difference. Well done and great idea

Comment by Ed Kirk on April 19, 2012 at 10:05pm

I started using my "soft mount" on my camera gymbal and now use it to soft mount my motors.  It is somewhat difficult to make, alignment is critical, I made an assembly tool for alignment and the acquarium silicone has to be applied in two stages so that is will set up (like old school contact cement).  If it doesn't get enough exposure to the air it takes a week to dry.  When done correctly is seems soft and plyable but quite strong, has survived a number of crashes.  The arrow shaft is quite strong, better then I expected, the layer of silicone is pretty thin, doesn't seem to take much.

Comment by Ron Curry on April 19, 2012 at 10:36pm

There actually is a lot of prior art here. In the traditional helicopter comment we've been dealing with this for several years now especially when it comes to Flybarless controllers and especially the more advanced ones that have both gyros and accelerometers (for self balancing and unusual attitude recovery). There has been all kinds of crazy experimentation going on and much of it has just been trial and error without any research. Vibration isolation and dampening is an old field with much research behind it. One source that I've found to be an excellent source of basic theory is this website: http://www.farrat.com/vibration_theory.html . One of the things I've found and you'll find in any professional discussion on vibration isolation is proper dampening is essential . Think about it, if you took the shocks off your car and just had springs it would bounce many times upon encountering a bump.  The shock absorbers  provide a degree of resistance such that the springs don't just oscillate. A while back someone made a post diagraming their idea of using large o-rings to suspend the APM. Creative idea but absolutely would be worse than nothing because the o-rings will oscillate like a guitar string and at some resonant frequency would actually tend to vibrate even more.

My experience with Flybarless controllers has shown that one needs to think simple here. Fancy vibration isolation and absorbing mechanism will rapidly become unwieldy and impractical. We can take some lessons from the Flybarless manufacturers. Everyone of them has done a lot of work to get their controllers to work well in a very high vibration environment which also has a broad range of frequencies to deal with (tail rotor, motor, main blades all at different frequencies). They've realized that an isolation system is essentially like a low-pass filter - we want to let low frequencies pass (the movement of the ship) but isolate and filter out the high frequencies from the motors and props. At the same time the APM must not be so loose that it flops around.

So, the best know method has been to use varying densities of foam mounting tape and, in some applications, metal plates to add mass (in order to reduce the resonant frequency). On my FC's ranging from APM1.0, APM2.0, HoverFly, XA, CopterControl etc. I've had excellent results using the foam mounting pads that come with the Skookum SK720 (available here: http://helidirect.com/skookum-anti-vibration-mounting-kit-p-22097.hdx?) and recently. Recently, due to the weight of the APM along with all the wires connecting to it, I've begun to use the pads that come with the MicroBeast which are a bit stiffer (available here: http://helidirect.com/beastx-special-foam-tape-p-22951.hdx). Remember, it's essentially a tuned circuit - weight of the APM vs springiness of the foam  and the stiffness/resistence of the foam provides dampenings of oscillations. I use one one of these pads and cut it in half. One half goes on each end of the bottom of the APM (thinking lengthwise here).

I've tried the approach of mounting the APM directly to the airframe using the mounting holes and even with rubber vibration mounts on each hole I've not been satisfied with tuning and performance. Mounting use foam mounting pads as I've described above is a proven mechanism and I've found that using that meting I can tune PID's using the "hand" method and, unless I've done something wrong it flies well and doesn't require precise PID tuning. In most cases the default PID values have flow pretty well right out of the box on airframes very different from 3DR's.

One more thing, many guys have tried to "strap down" their flybarless controllers for security or thinking that it will "prevent vibration" but that completely defeats the function of the foam tape. Think about if you put steel cable around the springs of your car and "strapped" them down. It's pretty clear it won't work very well.

Lastly, you can't just use any pads. Some have no springiness and not enough compression "travel" (i.e. how much it can be compress). That would be most "servo tape", so called "gyro tape", any sort of gel, etc. It needs to be mounting pads specifically designed for mounting flybarless controllers. While it's very likely you can buy similar stuff it all seems to not be quite right so I've stuck with the Skookum and MicroBeast stuff. Feel free to experiment if you have the time.

I should also point out that my results on Flybarless stuff was facilitated by the fact that the SK720 has built-in vibration analysis - spectrum analysis which can be monitored on each axis. This proved VERY useful in checking the results of various mounting methods. Would be great to have the APM log the vibrations during flight and then have the Mission Planner chart out the spectrum analysis. This is essentially what Skookum and Mikado do - invaluable. You'd be surprised at how much of your tuning difficulties are related to vibration.

I'm getting ready to leave on a multi-week trip but if photos would be helpful let me know and I'll try to post some.

Cheers,

Ron

Comment by Cliff-E on April 19, 2012 at 10:54pm
From my experience:
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.
Comment by Ryan Alexander Bahr on April 20, 2012 at 12:21am

It's cool to read all the experience here with different noise dampening systems, aquatic silicon is something I would've never thought of myself, even though I have some extra sitting around my house. Saving me a lot of time when I finally get around to doing something worth while with my quad. 

@ Monroe- I believe the camera you were talking about was a Casio EX-ZR100. Not a great camera but has some high speed capabilities for a budget. The resolution as you go in higher speeds can decrease dramatically... Sure you can get an astounding 1000 FPS, but at 224x64 resolution. For use of measuring frequencies of vibrations though, it may be just the tool for the job.

Comment by Rob_Lefebvre on April 20, 2012 at 7:12am

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.

Comment by Wipo on April 20, 2012 at 11:25am

Hi, I first want to tell that I'm no pro at this stuff. I'm currently working on my first quadcopter project and wanted to share my experience. My project isn't finished and my quad doens't fly yet. But I think I may have got some interesting data to share in this topic.

From my experience the best thing you can do to reduce noise/vibrations on the imu is to isolate the motors from the frame by rubbers. Using rubbers/foam/... to isolatie the imu from the frame is less efficient or can make it even worse.

Because I had a lot of trouble to get my quad stable on my testbench, I created a litle vibration testtool and tested every type of motor and imu mount that I was able to test. At the end there was only one option that really gave good result. And the good part is that this option was really easy to make (diy) and it's ultra mega cheap.

You can see my blog for details on my test data and motor mount: quadcopter

Comment by Rob_Lefebvre on April 20, 2012 at 11:36am

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.

Comment by John Campen on April 20, 2012 at 7:02pm

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.

Comment by John Campen on April 20, 2012 at 7:08pm

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.

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