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:
interesting. but to be truly interesting, you should compare that to attaching the FMU directly to the ship using double sided tape. a test report showing x, y, z for none, under, side placement of the gel would be quite interesting.
I will do as you suggest later ( its set to rain for the next few days in U.K.) I an a little concerned as I once used an APM with inertial guidance
and a rigidly fixed imu resulted in zoom climb when switched to loiter!!
yes. extreme caution there. if the craft is small (300ish) then this is a real possibility as the natural frequency of such a craft is about what it takes to fool the FMU. it can happen in any mode that requires altitude control. it is my belief that this is what happens (someone more knowledgable please chime in here):
- errant or harmonic z-vibrations cause the FCU to think the ship is dropping (it's not)
- the FCU applies power increasing z-vibrations
- this magnifies the issue
- a fly-away as the ship thinks it's dropping
So i'd recommend the following:
- turn on the following parameters in the log
- hover the ship for 3 minutes manually in stabilize mode as best you can
- land, turn off the FMU (saves the log file)
- make the change to the isolation of the FMU from side supported to fixed
- repeat the flight
- send me both logs so i can look at the behavior of your ship
during these tests, stay in stabilize mode.
if your ship is stiffer than the one that flew away, then it might not be impacted by a rigid fixing and fly better because of it.
A photo of your ship showing it's structure might also help.
Good luck. Only try this if you are interested in supporting science.
I have today tested vibrations with rigid mount vs vertical gell(x and z in shear)
mean vibs in hover rigid= x=6 y=11 z=30
mean vibs in hover vertical jel x=4 y=14 z=13
mean vibs full aggressive flight
with vertical gel x=4 y=16 z=18
(attached are logs and jpegs of the beast)
all up weight 3Kg with verrigid frame motors are bedded on a thin (fully compressed) foam tape . frame size=motors on a 22 inch pitch circle diameter Y6.
odd that the y axis seems worse ?? may be over compression with my quick and dirty rearrangement of the gel pads?
I am considering building some 45 deg brackets pointing to the center point so that all axis are equalized regrades shear /compression ( imagine the faces of an inverted pyramid suporting the corners of the pixhawk) ???
Tim - Thanks for doing that test. Clearly, the side mount is doing better. Can you post a photo showing how you mounted the FCU? A photo will show size and location of the pads. The difference in Y is probably just noise.
Also if you want, can you post the logs if you still have them?
please refresh screen I was still editing reply sorry about the bold text????
first log 2015-12-18 14-10-54.bin, is rigid mount red band tight down no pads
the second 2015-12-18 14-13-43.bin is a longer flight with vertical jell red band is released. first minute is stable hover
Hope all this is of interest to you.
Prepping up to deliver wine i see :-)
Excellent attachment method for the FCU. You've done what we call hard mounding as you have basically bonded the FCU to the posts, which are somewhat the strongest part of the electronics platform (EP). Normally we put posts directly into the motor masts and literally bolt the FCU to that, but what you have done is basically the same. Clever. Well done.
Ran the logs through the hover program. you numbers are quite good (.06, .11, .11). you should be pleased.
Don't know if this will help, but you might try going into MP, bring up the flight parameters and set them to the following (these comes from the Hover Analysis program):
- THR_MID = 401
- AHRS_TRIM_X = .0322 (only if the FCU hasn't changed position since the log)
- AHRS_TRIM_Y = .0710 (only if the FCU hasn't changed position since the log)
Yes the "wine" is ballast as have not yet plucked up courage to mount my nex-5 and gimbal yet!!
trim errors are probably from "slippage" due to rough and ready mounting method.
I need small "L" brackets with more surface to stick to . and use a "keeper plate"
to support the FCU when not in flight
will change throttle mid as you suggest.
Thanks for your help