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...

Views: 52222

Reply to This

Replies to This Discussion

Depends on how you contextualise hard data.  There's no contention that the MEMS accelerometers have resonant frequencies, and that these can cause issues when the flight controller is hard mounted and the props and motors are not sufficiently balanced.

As mentioned in the previous post, there is a direct trade off between isolation and feedback resolution.  However, in practice, this typically does not effect practical stability.  If you were to compare two aircraft using either method of vibration management, I very much doubt you be able to pick the performance of one or the other in a blind test.

The practical difference is in the failure conditions.  A motor bearing failure, or clipped/unbalanced propeller, is a much more serious issue for the hard mounted unit, than it is for the isolated one.

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

Hi Forrest. I am working on vibration isolation on my quad. The graphs on your spreadsheet look really helpful. Is that something that is available for download? thanks.

You can find here http://copter.ardupilot.com/wiki/vibration-damping/ are really helpfull

I' made some modifications, APM goes downfloor with a little hard foam, It's look more fim and glued

The GPS goes down too, only for descart movements, then it going to return first floor, I chek near the house with extremly windy and the second log inside the garage (the car survive ;) )

 

Attachments:

I couldn't run in your program Forrest, It doesn't a good log to run or something I'm doing wrong? the program looks very usefull for this thank's for share :)

I try without props and this is the log; I'm not know way y vibs are bigger than x¿?, (not props problem, no batt problem not gps bracket problem)

Attachments:

Good news in the fog of all the bad.  The log file was good.  We can now see what is happening.

First, before we solve the wind issue, let's focus on the indoor problems first.

1) X-Accel is on the margin of bad.

2) Y&Z Accel are now far less, but still 10X too high

3) The PIDs need to be tuned (roll & pitch are marginal) but we can't do that until vibrations are less.

If we solve the three issues above, the wind issue will  most likely also be solved.

First, make the ship stiffer.  From the photos, here are some obvious issues causing the large Z.

- the top EP plate appears to have been cut just behind the forward-left boom bolt.  So the forward-left boom is no longer sandwiched very well.  Can that plate be replaced?

- the motors have been extended onto "diving boards".  Can the motors be moved inward so they directly into the aluminum mast?  Or, can you use longer Aluminum masts?  Or, can you thicken and lengthen the "diving boards" so there is 2x the overlap on the mast as there is overhang?

- The motor masts will be stiffer if they are continuous.  Can the fore-left and aft-right booms be replaced with one continuous boom (this would also solve the weak top EP plate problem)?

- If the motor mast to EP bolts do not tighten well, bond the masts to the EP. 

- Anything else you see that causes the booms to feel flexible or loose.

You might look at this list and conclude, better to start from scratch.  But if you decide to continue, I'll work with you.

the last set of logs is good.  if it fails, then it might be international settings in Excel.

It might be that you have twice as much foam under the APM in one direction as the other.  But let's work on ship stiffness first.

If you want the latest version that incorporates CURR, friend me with a note to remind me what you want and I'll send it to you.

Reply to Discussion

RSS

© 2016   Created by Chris Anderson.   Powered by

Badges  |  Report an Issue  |  Terms of Service