Very good anti-vibration results with following APM2.5 mount, quad/ customized 3D robotics frame

Hi,

I have measured very good Zaccel vibrations with this mounting system. It consists of a suspended acrylic base plate (3mm thick) on which a silicon pad (6mm thick) is placed. The APM case is placed on it with a little pressure from a velcro band.

The O-rings go through holes on each corner of the acrylic plate and are attached to the black nylon spacers. In order to avoid that the o-rings would move up/down the nylon spacers, I used flat metal nuts to block the o-ring (it is screwed between the first 10mm spacer and the bigger 30mm spacer).

The acrylic plate is suspended about 2mm above the fiberglass mounting plate. In order to shield the EMI interferences that could come from the PDB/ESCs, I added special EMI tape on this bottom fiberglass plate.

I show below the obtained results with motors 100% throttle:

This AccelZ measurement in the logs show an average of maximum 0.5g deviation, which is I guess quite good!

Other pictures:

 

Hugues

Views: 19672

Tags: 3D, APM, APM2.5, Arducopter, Robotics, vibration

Comment by Daniel Allen on February 19, 2013 at 10:16am

I think one of the biggest issues with the vibration is due to the cables. Think about it the ESC, Receiver, and sensor cables are very stiff. It can surely induce vibration even if you manage to dampen the controller. I'm going to experiment with using 26-30 gauge wiring to help isolate it further.

Good work, I may use some of these techniques to dampen my quad/hexa

Comment by Gary McCray on February 19, 2013 at 11:13am

Hi Daniel,

I do briefly address the wire issue in the Vibration Control section of the Wiki and yes it is an issue.

Reducing wire gauge can help, but I would recommend some other methods that yield good results before resorting to that.

Use high flex wire which has more strands of copper and a more flexible insulator.

Use less wires, getting rid of unnecessary power and ground wires can reduce the number of wires by half.

Provide a unrestrained strain relief loop for all wires entering the flight control board.

If you want to go all out, you can go to a PPM-Sum receiver or mount the receiver on the same plate as the flight control board and then vibration isolate the whole thing.

These techniques can virtually eliminate interconnecting wires as a vibration entry path.

Comment by F1P on February 19, 2013 at 11:25am

http://fpvlab.com/forums/showthread.php?4251-Vibration-Dampening-am...

get attention to the dampers on the tip after engines =)

Comment by Daniel Allen on February 19, 2013 at 11:35am

Thanks Gary!

I like the idea of dampening the entire Controller + Receiver, especially with rectangular boards like the APM. Combine that with high flex cables would be amazing.

Another question: should the signal wire from the controller to ESC be shielded with a ground wire or other method? Or will it be enough by itself?


Developer
Comment by Rob_Lefebvre on February 19, 2013 at 11:43am

Gary, I agree with mounting the Rx right on the APM.  I assume you've seen my All-In-One blog post from June?

I'm now working on doing the same thing with a full-size FrSky D8R-SP Rx.  Decased, and mounted to a daughterboard.  

Too bad 3DR never went ahead with making an official daughterboard for this purpose.  It would be really nice.

Comment by Hugues on February 19, 2013 at 12:39pm

Update on my experiments, some more improvements versus my first setup, following Gary's advices:

I then get the following Zaccel graph:

Cu,

Hugues

Comment by Hugues on February 19, 2013 at 12:41pm

And I must add that the whole graph corresponds in the middle at Throttle 100% !

Comment by Jim Stricker on February 20, 2013 at 10:05am

Perhaps full throttle is not a good test.  Is this accel Z graph a download from actual flying confditions or bench test?  My quad hovers at 50% throttle and there is a lot of motor speed interaction happening and harmonic frequencies that you probably don't get at 100% throttle.

Comment by Gary McCray on February 20, 2013 at 12:24pm

Hugues, clearly it does seem to indicate even a further improvement with your full throttle vibration now at approximately 0.02G.

Your beginning and end transitions, although still completely excellent at about 0.05 G  are indicative of a less tractable aspect of vibration control in multicopters: Resonance.

Multicopters are inherently non-synchronized (because the motors need to move at different speeds for normal flight maneuvering).

And of course as you are accelerating and decelerating the motors and props are speeding up and slowing down at slightly different speeds, whereas once you get to full throttle they are in rough synchronization.

There are likely always going to be areas of higher vibration during maneuvering and normal flight due to this effect and with your system you have finally gotten the overall vibration down to the point where this effect is clearly visible.

Nonetheless, superb results and I would very much like to see the results in a normal hover just trying to hold it as stable as you can, I suspect you will see results very much like your full throttle experience, which is great. 

Basically, I would presume that if we can come up with a viable reproducible system that can achieve the results you have achieved we have pretty much solved the problem.

I realy like the 2 zone isolation system with the dampened board on the dampened plate using differing dampening techniques.

Even though I am using the O-ring suspension system myself, however I don't really like it very much because it is cumbersome to install properly and O-rings aren't really designed for this use.

I am, however going to convert one of my O-ring suspension mounts to a system like yours with a separate plate and some various pad type secondary supports for the APM.

I am also going to build a system where the intermediate plate is supported by vibration absorbing grommets rather than O-ring suspension mount and give that a try as well.

Your results are the "grail" at this point and I will report my results.

Comment by Hugues on February 20, 2013 at 2:27pm

Yes I agree with you that the O-ring suspension is cumbersome and probable not the best mechanical option. I have another issue with my setup here : as I have cut the silicon pad in four little cubes, I have less friction between the already slippery silicon pads and the also slippery acrylic plate =>I risk having my APM case move in case of shocks. I tried different glues and/or double sided tapes but nothing seems to stick on that silicon. I moulded this silicon pad myself mixing pure liquid silicon with a catalyser (purchased on a crafting/hobby web site : www.artificina.com). I would be very interested to replace this by fixed absorbing grommets.

I also wonder if I could not isolate/dampen now my motors by adding some o-rings on the screws ?

@Jim : yes this is a bench test I did, not a flight. The props are balanced , not the motors.

I will later balance also the motors with a laser test I saw on a forum : motor by motor you attach a small mirror on the active copter arm. You then project a laser (presentation pointer for example) at this mirror to have a reflexion of the laser dot on a distant enough surface (for example the wall of the room). Then you start the engine of that arm and check if the laser dot stays a dot or if it becomes rather a segment. If it remains a dot, the motor is balanced. Otherwise you stick some tape (random I guess) on one side of the motor and check the dot again. As long as the laser projected segment does not reduce its size to a dot, you move the piece of tape around the motor (I would say by 90° each time). You repeat the procedure for all other motors individually.

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