Vibration Isolation and Dampening of APM / PX4 for version 2.9

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:

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

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I have an Aeronaut 10x6 on my glider, an APM2.5 waiting for install, spare motors, esc's, and a powerful leaf blower to experiment with. I've had some ideas for crude flapping experiments, but I figured I would at least need pitot data for anyone to take me seriously (at least, I think just varying distance with the blower at full throttle won't convince some of you). The results won't be as absolute as using a windtunnel, but I think with a careful setup and comparisons, we could validate if folders are worth further investigation.

What do you guys think? I am not going to set this up unless you guys think it will be useful. I know it will work, but I don't need to convince myself (I don't have the money to buy a bunch of folders anyways, but if the results are promising that may motivate others to flight test folders LOL).

That quick design was just to make use of some broken props for a bit of testing

But copters do have a lead-lag hinge as well as a flapping/teeter one?

Kevin, yes folders were my first thought but they tend to be pitched much higher than copter props, at least thats what I thought until DaveC brought his 90mph copters around here.

I think we need a manufacturer to bring out a plastic prop with a thin, very flexible root, perhaps with Kevlar molded into it.

I try vibrations in windy day and notice higth vibs in AccY, I change different pads but the problem persist, motors and props are balanced, What it means vibs in Y axis? any suggestion to solve?

This my last test log

Attachments:

Hi Cala,

Unusually high vibration in Y axis is not normal, usually X and y show similar vibrations.

Because of gravity and throttle effects, unusual vibration excursions are more normally seen in the Z axis.

If you could please describe your copter and its specifications and how you have vibration isolation set up for your flight controller it might be possible to have some better idea what we are dealing with.

I use Kyosho Zeal Gel or the stock as supplied special foam pads under the Pixhawk or alternatively use an O-ring suspension mount which can also work quite well.

As long as you supply a bit of a relief loop in the wires connected to the flight controller and it doesn't bump into anything that is normally sufficient to get most multicopters into the reasonable sub + or - .3 G (3 on the Acc scale) vibration range.

If it is worse than that their may be some other more intrinsic problem with the copter.

On my best copter, a Hoverthings FLIP FPV Pro G Quadcopter, I am using the suspended O-ring mount of the Pixhawk inside the already rubber isolated live frame section.

Best Regards,

Gary

Thank's Gary for your fast response, this cuad flies acceptable in calm days but It' doesn't like windy ones, it's a cheap chinesse frame with cheap motors (perhaps the problem) motors are Turnigy 2836 950 kv with10*4.7 CF props, APM 2.5, Lea GPS, and 3.2 rc 12 firm

FPV isn't connected for test

My last isolation was two layer of this tape http://solutions.3m.com/wps/portal/3M/en_US/Adhesives/Tapes/Brands/... only in corners

 Another test were with this foamand the combination, tape--foam---tape

and always the same results; I'm thinking if changing props with more flexible plastic APC, give me better performance with this low quality setup? Thank's

Photos were great and explains the x versus y vibration on a regular quad.

My recommendations would be to try the following:

1) remove all rubber & foam and direct bolt (or bond) the APM to the frame. the foam is causing huge amplification of the z vibrations.  the frame vibrates at a mix of frequencies. the rubber and foam vibrates at another set.  the two combine for an erratic mess at times amplifying each other and causes latency that makes the APM react incorrectly at the wrong time.  the APM is trying to keep the motor mast stable.  how can it do this when foam and rubber disconnects the APM from what its trying to control?

2) shorten the spacers to the APM platform (keep it less than 10% of the distance between the spacers; in your case a max spacer height of 1/4").  Better yet, mount the APM directly to the electronics platform (EP) closest to the motor masts.  the long spacers are causing huge amplifications of y vibrations (and less so in x) because of the aspect ratio of their spacing.  consider from an engineering point of view of what happens to tall building during an earthquake.  you have the APM mounted on the top of a tall building telling it to figure out what is happening at the motor mast.  can't be done.  the APM needs to know what the motor mast is doing to maintain stable flight.  The APM can be placed just about anywhere on the EP that is directly bolted to the motor masts.  It doesn't have to be in the center.  If needed, move other stuff that doesn't control the ship to the top where the APM is currently located.

3) isolate the FPV, not the APM.

4) make sure that the bolts to the motor masts are really tight and you are getting enough stiffness (if not, start over).

The following is a photo of how to properly design ships.  Note that:

1) the APM (in this case a Pixhawk) is directly bolted to the motor masts on short spacers.

2) the only foam/rubber is between the frame (where the APM is hard mounted) and EP.

3) the FPV is mounted to an isolated EP that contains the rest of the mass (the battery, GPS, and FPV electronics).  [note:  the thick and stiff EP is actually aerospace material and weighs less than 20 grams)]

As Gary pointed out in his response, what isn't optimal about the design pictured is evidence that the sizing, quantity, and material properties of the isolators relative to the weight of the loaded EP has been optimized.  Each of us does what we can.  Isolation of small masses is still new science.

So move your APM as close to the frame as you can.  Use an upper isolated level for the FPV.  And then experiment as Gary indicated to isolate your FPV.  Once you have established that baseline of performance, then it's easy to add shock control to the APM if it is needed and beneficial.  Also, double sided foam is an easy way to adhere the APM to the frame and usually is too stiff to do any noticeable harm.

Great explanation Forrest, I read the post about no isolate the APM but I'm not take care fo my tall building, that's true, I was little afraid to put the APM near the "control board" (a mess of wires uff) but I'm going to try and do the test as You suggest, Thank's!!!!

Thank's Guy, Gary and Forrest, all disagrees are welcome too, I'm going to try the different recomendations up to obtain better performance, in the way I'm learning a lot; thank's

Once you have the firmware fixed, you can get a good log file with IMU, CTUN, ATTITUDE-MED, GPS, and CURRENT enabled.

This is what you should get from the log file.  The analysis of data requires that the ship achieves a good 15 seconds hover.  This takes a poor pilot like myself nearly 5 minutes to achieve when flying inside in a confined space (note the pitch and roll jerks as the ship gets too close to chairs and desks).

This ship is a quad with the APM hard mounted to the motor masts.  As you can see:

- vibrations at the APM are low (averaging +0.07g)

- pitch and roll are stable (averaging +0.04 degrees; the ship is in control) 

- the signal proxy is 3.7 times noise

- Throttle Mid Position should be set to 552

- The ship hovers at about 123 watts

Your data looks like nonsense because it is.  The IMU data in your files is not all IMU, but some sort of strange mix of IMU and other stuff.  So reload firmware in case come 1s and 0s got sideways.

Thank's Forrest, now I'm mounting the APM to the motor mast, i'm going to load the last rc14 (3.2) and try to hover, perhaps I have to redo compass? because more  near wires and battery

Guauu, nice vibs

I hesitate to disagree, because I have great respect for your opinion and achievements, but I think the problem can be distilled down to the following trade-off:

Isolated flight controller: Decrease in feedback resolution, resulting in controls that are not as precise, and potential oscillation/stability issues.

Direct mounted flight controller: Greater accuracy required in balancing motors and propellers, harmonic vibrations in the MEMs accelerometers at certain throttle settings.

Given that you're always going to have a certain amount of vibration, good flight safety protocol requires adherence to the precautionary principle.  It's possible for the DCM algorithm used on the APM to become unstable under excess vibration, with outcomes in extreme cases of a fly-away, so the safest decision is to isolate.  

Also, in practice, the decrease in feedback resolution does not create significant stability or oscillation issues.  For mainstream users, and I would argue all commercial operators, isolation is generally the best practice.  I don't think the foam and gel-tape combo is very suitable, for the same reasons you've outlined above.  For my aircraft, I designed a 3D printed isolation mount that uses silicon dampeners with approximately the right level of stiffness:

http://www.thingiverse.com/thing:160655

It's not a perfect solution.  If the dampeners were 10-20% stiffer, the cutoff frequency would be slightly more ideal, and there was a tradeoff between having the dampeners symmetrically placed to give equal response in each axis, or having them as close to the connectors as possible, to mitigate vibration transferred through the cables.  But I'm coming up to 30 hours of flight time on my multi rotor, and 40 hours on my fixed wing using these mounts, with flawless operation at appropriate PID settings.

Hi Guy,

It is certainly possible to provide too much spring (isolation) and too much or too little damping.

The key is in providing the right amount of each so that it effectively damps normal mechanical chassis vibrations while not providing so much slop as to allow the flight controller to lag or wallow around itself.

Fortunately, most vibrations are relatively high frequency and generally fairly low amplitude and there is a fairly wide gap between what isolation is required to remove vibration and too much which would permit significant (perceivable) lag or wallow.

Generally if you try to isolate the flight controller itself, a thin 1/8" to 1/4" or so appropriate foam or gel damping medium does a good job on most vibrations while not providing even detectable lag or free motion in the controller.

And you can tailor damping versus excess free motion by adjusting the size of the pads you use under each corner of the flight controller.

I use this method to achieve generally sub 1/10G accels in all directions although some of my copters use corner O-ring suspension instead to achieve the same results.

It should also be noted that Z vibrations can be worse and more problematic to fully solve than X and Y vibrations because they directly respond to throttle application and can even set up a self augmenting resonance with a "stabilizing" flight controller.

You occasionally see self growing vibration in the Z direction and I believe this is often the cause.

Of course isolation and damping should simply be applied to augment other good design practices aimed at minimizing vibration in the first place (prop and motor balancing for instance).

On my good Hoverthings FPV Pro G extended arm quadcopter (and my friends nearly identical one) we have an O-ring (Silicone is better than Buna-N) suspension mount of the Pixhawk on a live (rubber mounted) subframe which also contains the large mass of the battery and camera and gimbal.

It produces very low sub 1/10G vibrations.

And by the addition of a couple of 5/8" high hemispheres of 50 durometer Sorbothane between the live and main frames acting as dampers we have achieved vanishingly small vibrations both at the flight controller and at the camera gimbal which are now producing professional grade HD video results.

Spending some time and effort tuning a proper isolation and damping system can produce really excellent results.

Best Regards,

Gary

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