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-amp-Isolation-Solutions-Guide

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  • Liquid damped APM tray?

     

    I was considering making a tray that the APM clipped into that was mounted on the usual foam at the corners. Having the use of a 3D printer I thought why not print some holes in it so that its weight could be altered by pushing in some .22 airgun pellets. This then led on to thinking why not make the entire base hollow and change its weight by filling with water.

     

    It occurred to me that having a viscous fluid in the APM tray could be quite effective at damping out vibrations. Being more practical, I doubt a 3D printed part could be entirely leak proof but I could incorporate spaces for 10mm plastic pipes that could be sealed. Of course other liquids like some oils may be more suitable than water. This system may be more effective on foam that would normally be considered too flexable.

     

    Perhaps some of you theoretical chaps could point out the errors in my thinking.

  • Those who thought I may need lateral damping have a beer on me.

    The frame is now complete and with a 5Kg weight on the platform it osculates forever (laterally) if twanged by a calibrated finger,

     I have added vertical pegs and flat rings and it seems ok now. Time to get some electronics fitted.

    3692815103?profile=original3692814944?profile=original

  • Forrest,

    I tried your Excel worksheet but got "Error 6 - overflow".  Do I need to limit the flight time to reduce total data?

    Anyhow my setup with 4 x 3/8" Zeal squares ~ 1/4 of the way in from the long edges of the APM case in order to miss the holes on the F450's top plate, along with a small 3M velcro cable strap in the middle results in this.

    This is a period of Alt-Hold, post black case mod and blackout foam treatment near the LED's, XY values where the same in Stab-hover minus some Z flux getting my throttle mid dialed with the new GWS 3-blade props I ran today.

    3692808372?profile=original

  • Kyosho Zeal Gel Tape is now back in stock at Amazon, EBay, and A-Main hobbies and others.

    This stuff is the best there is.

    It will do everything you need and it does not turn into a puddle in the heat like Moon Gel.

    Use four 3/4" square pieces of Zeal under your flight controller and secondarily secure it with a Velcro strap lightly tensioned with soft foam between the strap and the flight controller and you will get approximate 1/10 G accels in all directions.

    And ignore the rest of the nonsense on this thread. 

  • Ive been working on my new ‘vibration isolating’ 8 motor design for a couple of weeks now and kept changing between x8 and standard quad. Ive decided the efficiency and loss of total lift was not worth the size advantage of the X8.

    The octa will have folding arms which will help with transport and maintenance. Also the centre frame with all the electronics will be easily removable. Id like to keep commonality of parts with my quad so will probably use MT4008-12 with 14x4.7 props.

    Im estimating the weight with 4 x 5Ah and DSLR to be about 5KG and hover time about 22 mins. Ecalc suggests it will lift another 4kg. Frame alone is about 800g

    There is much scope for adjustment if the o ring suspension by adding or removing, changing length, thickness and material and adjusting pre-tension.

    Comments welcome.

    3692796699?profile=original3689544554?profile=original3692796575?profile=original

  • Your analysis is very good, so I am surprised that the solution isn't obvious: add mass to the APM board.

    I started by mounting the APM to the frame on 4 x 0.5in^3 pads of DuBro foam. There was some improvement, but the vibration levels were still marginal. Then I sticky-taped a stack of 4 US quarters to the bottom of the APM board and bingo, vibration problem solved.

    I understand the concern about mass causing a decoupling of the motion of the frame and APM, but I doubt this is significant at the masses we are dealing with until much higher accelerations than our quads encounter under normal flight conditions.

  • Developer

    The problem is that you are measuring your data using a sample method that is way below the required sample rate to have meaningful data. You need to be able to sample data at 400Hz to see if the APM IMU is not experiencing high frequency interference that can potentially create a problematic situation. Fly away has reported by Rob. (you ask for Devs comments, he is a developer!)

     
    You have also just replied that a commercial company that has invested huge amounts of money in damping is wrong "The air-core damping ball within the dampers will absorb the high frequency vibration. It effectively reduces the high frequency vibration influence to the IMU and gimbal, and makes the S800 flight more stable." Lots about the word 'High Frequency'
    They are saying that the high frequency vibrations are the issue, and from your own admission, with using expensive vibration sensing equipment, that you cannot use it, due to vibrations happening in almost all frequencies. see Permalink These beasts are awash with interference (just like radio wave reception)
     
    Now take another commercial and successful product. The AeroUAV YS-X4. A long time contributor John Anrne Birkeland has done a teardown of this product.  see http://diydrones.com/profiles/blogs/zerouav-ys-x4-teardown
    As you can see "in particular is very impressive with sensors built inside a solid piece of metal to ensure rigidity, and then mounted using internal vibration dampening. A black piece of foam on the processing board, is there so that it presses down on the IMU unit when the boards are stacked together"
    The only real difference in the APM and the DJI and ZeroUAV platforms is that they have added extra max to reduce the high frequency oscillations. And the damping systems use a stiffer foam core (sorbathane is pretty soft and useless) and usually of two densities to target multiple high frequencies.
    USing MP to measure accelerometer data is only indicative towards a solutions. It helps many in the DIYDrones community that are more interested in flying than engineering, make sure there setup complies. We know that high frequency vibrations are happening. We don't have a good way to measure it, but we do know how to deal with it for even the least tuned copter. We want to be able help people bolt something together, and not have to spend hours balancing props and motors etc… and this can only happen by having sound and simple tips top isolate the IMU from these high frequency vibrations.
    And the only craft that has shown the best outcome has two separate masses. ie. the ship and the APM in an extra weighted frame, decoupled by damping material.
    Maybe you could post your result with a replica of how Rob has setup his system and prove that it is no better than a hard mount?
    I don't see, with actually proven techniques from Gary, why you would ask anybody to risk their investment. Make a cast iron case before rubbishing other good work. Buy an off the shelf quad, and hard strap the APM with no prop balancing or other steps and prove we are all wrong! since this is what the majority of customers are doing. They want to fly, not be mechanics.
  • MR60

    Attached is the promised Vibration Analysis Tool for the new system.

    Don't be disappointed if your first try at vibration control looks like mine.  I had read all the BG (before Gary) advice on vibration control and implemented them all.  So I didn't start with the recommended baseline [no isolators, no dampeners, just use double sided tape and stick the APM on tight].  My baseline was motor mount sorbothane (the really light type), spar sorbothane, sorbothane under the APM.  The results were:

    3692790568?profile=original

    It may not look horrible, but the statistics told the truth.  x = .49g, y = 1.34g, and z = 1.24g

    So I slowly replaced sorbothane for earplugs and foams, and ... one at a time and compared ... no improvement.

    So then I started removing one at a time each dampener and isolator and compared each step.  The numbers started to fall, but I had a long ways to go.  At the end, thanks to Gary, I now have this.

    3692790507?profile=original

    With the statistics showing this:

    3692790589?profile=original

    My 99.5% numbers are now .05, .07, .05 for an average of 0.06 g.  Why 99.5%?  The APM only logs at 200Hz, so it's likely not to see the tops of your vibrations.  It's not a law.  It's just a number that you don't change so you can measure progress.

    Now this part is critical.  Do not compare my numbers to yours.  This copter has eight rotating 11" blades run by 750KV motors on a ship that was flying its normal payload of 2.36kg (it weighs more if camera system is added) flying over 30 minutes using 12000mAh of batteries.  If I add a camera, the z goes down.  If I remove batteries, the x, y, z are in the .12 to .18 range.  Less than .15 is good enough.  If you are not satisfied with the camera results, try for lower.

    The Vibration Analysis Tool is attached.

  • It is interesting to me that my little blog on vibration control has managed to stir up so much dissent.

    It's primary purpose was to allow people to achieve sufficient vibration control on a wide variety of traditional and multicopters to be able to control them reasonably when we transitioned to the inertial control paradigm of 3.0.

    In fact there are many means of accomplishing vibration reduction including prop and motor balancing (always good) and even by careful frame design and mass placement.

    However, although motor and prop balancing are generally achievable to good benefit, frame design and mass placement is often not a reasonable expectancy and is also a great deal harder to typify than remedial techniques.

    So while the exercise going on here is certainly interesting from an academic or possibly even top down design standpoint, the simple fact of the matter is that the easiest, least costly and least problematic method of achieving satisfactory for our purposes vibration characteristics is by simple remediation performed by isolation mounting of the flight control system.

    Put simply, four 3/4" square pieces of Zeal, Moon or Alpha gel under each corner of the APM in or out of it's box or a PX4 lightly retained with some foam under velcro will provide very satisfactory approximate 1/10 G vibrations in all 3 axes in the vast majority of applications.

    And it will not permit adequate low frequency (movement) to cause a significant reduction in control because of out of phase flight controller movement versus the airframe.

    The method you have been discussing requires either a very tuned initial design or considerable effort tuning and adjusting an existing design.

    The tuned frame and mass balanced approach has merit, especially if you were designing a specific multicopter for sale, but for universal adaptivity and practical application, Kyosho Zeal works really well and is really easy to do.

  • MR60

    Vibration Analysis Worksheet - A few people have asked me for an update of this analysis tool that goes after your logs and generates a chart and statistics for more precise measurement.

    One pilot sent me three log files and encoded in the log files was a clear message that he wants to do the analysis himself.

    So I'm making the following improvements to make this more fool proof and less work for pilots:

    o Update the s/w to go after IMU versus RAW records per the latest change in log files

    o Update to s/w to the new recording rate

    o Add logic that finds stable hover in a flight log containing motor init, takeoff, hover, and land.

    o Within hover, find the best 20 seconds of hover (the least x, y, z drift).

    o Add logic to allow manual override of that period.

    o Update the statistics to analyze that period.

    This will take a few hours.  I'll post it when done.

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