I finally did the test of hard fixing the APM case to the frame, thus not having two separate masses with vibration dampening materials in between, as before (i.e. : frame/props on one side and APM on the other side).
This is explain by Forrest Frantz in a post I made earlier about moongel use here:
and in more details here:
I have to say that I felt on my bottom when I saw the results : TWICE less vibrations with this method WITHOUT any moongel nor any other vibration dampening techniques. JUST ONE SINGLE mass (APM fixed on frame directly).
Look at this graph that shows ZACC within plus or minus 0,1g which is twice less than the best +-0,2g I had with moongel combined with rubber bobbins and/or silicon gel. It is measured on the exact same aircraft (except moongel was removed and rubber bobbins were replaced by hard pylons). APM is stcked to the plate with NON foam double sided tape (hard double sided tape, as I did not want to drill screw holes in the APM plastic case).
Not only have I less vibration but I will also get better control on the craft since I get rid of the lag effect of dampening materials.
I confirm thus the theory of Forrest Frantz. It works for my frame.
It would be nice if others could confirm this also on their frame.
I publish this UPDATE after the information about 200hz dangerous vibrations came out thanks to the debate initiated of this post: the APM and its integrated sensor's hardware have a limit (due to low processing power limiting sampling frequency - that should be solved with the more powerful PX4 hardware) in such a manner that all frequencies of 200hz (and multiple of) will impact the APM control (accel) and will go through the 20hz software filter, undetected (thus noise id processed instead of valid signal, possibly creating catastrophies). As there is no way to analyze in a log AccelZ graph if you get these 200 Hz frequencies on your arducopter, it is necessary as a precaution to use dampening technique(s) to eliminate these high frequencies (even if the AccelZ log seems to show low vibrations by direct fixing APM on the frame).
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Forrest
Forrest said: "On the 50Hz (it actually varies, but that's a knit). The wiki recommends 30 seconds of stable hover because it is a sample (if you are curious as to why sampling works, I can point you to many science journals on this topic). I recommend 40 seconds."
I don't wish to get into an argument about this but it is clear you do not understand what is going on and so we wish you would stop preaching flawed gospel that contradicts the truth we are trying to convey for everyone's safety/enjoyment. Nyquist theorem is BASIC knowledge in this topic, stating that one must sample at twice the frequency of the signal - hence if we're sampling at 50Hz then we can only observe up to 25Hz of frequency. So, in answer to your response, yes there is plenty of information to tell us that sampling does not work in this case because our sampling rate isn't high enough (a trade off - the IMU can sample at 200Hz but that would use significant amounts of flash for the logs - if we can even get the throughput - I don't have the information on that to hand).
To the gent who asked why the vibrations are so catastrophic - this is why. We use measurements from the accelerometer, if the accelerometer says acceleration of 1g for 0.5 second, then we integrate that into velocity, and then integrate again to get position. Noise disrupts this hugely - keep in mind, we're integrating twice the original values, meaning noise is amplified significantly.
We're saying, reduce the vibrations over all - this is likely to reduce vibrations across the spectrum. Bolting to the frame is not advised at all and very strongly discouraged - it easily transmits vibrations across the frequency spectrum.
This is an interesting discussion to me.
When I build my drone a year ago I was talking wtih the local hobby shop guy about how to mount APM.
His suggestion was .... you guessed it.... directly to the frame to avoid any difference in the inertial reference between frame and APM.
This guy built his *real* plane that he fly's by hand.
Not saying what is right or wrong (will test for myself)...... but I think there needs to be more thought and organization in this thread/experiment.... (Because that is what this is an experiment at this point).
Many arguments on both sides are logical... and there are many ways to solve problems.
Example from just this thread.
1. Dampen APM
2. Hard mount APM fix your mechanical imbalances
3. Create a mass that will dampen the main frame which APM rests on
Logically anything that causes the sensors to possibly read differently on movement from the frame will cause some sluggishness in response.... (Maybe even desirable from people who are doing camera work.)
Likely a bad analogy but...
Think about cars.
A sports car is sprung as rigidly as possible to enhance cornering (Control).
A luxury car wont have the handling that a sport car has but is more conformable on bumpy roads.(Camera work)
It would seem that by adding dampening to fix mechanical imbalances you are then causing a less exact response from APM because APM is getting less exact info on its position relative to the frame. In other words your band-aiding a problem.
Back to the car analogy.... it's like putting on bigger tires that are less inflated to your car to make the ride better.
It will work but the problem is the suspension setup not the tires.
I'm looking forward to experimenting myself soon. =)
Cheers everyone and keep the discussion reasonable, gentlemanly, and productive.
this is my first frame from 2010, (arducopter alpha code), 0 vibrations :) that what DJI is doing now, it will make the copter to fly smoother-softer and it is good for video setups, removes small twitches.
But after flying for 3 years... #1 problem is unbalanced Props, after you have perfectly balanced props (the hub !!! good video here ), you'll be surprised that you can actually fly with hard mounted APM on any frame :)
@Oliver,
Like Sgt Ric correctly pointed out, on top of all that is being discussed in this blog, application of little damping on the tone of language will take this discussion even further. There is no point in name calling/foul language by any one as it would drown a good technical discussion even though the content may not be accurate. Cheers
PS: Moon gel or not. these toys are getting serious business and should be handled with due respect and there is no by-pass for safety and discipline while playing with these stuff. So I guess author can put a caution in the end of the blog. " Please note: these are my observation and mileage may vary for others. So pay attention to all the safety concerns and do the bench testing well before attempting a actual flight. I am not responsible for any crash and injury"
Most if not all scientific revelations are not repeatable immediately after even by its own author. It takes time to verify and again result may vary for different set of conditions as in this case. So guys pls tone down, it doesn't help any one and certainly not any one who reads this blog to learn & share. So pls talk like you fly a 3k$ heli. cheers
Here's another weigh in.
3DR Quad
850kv motors
20A ESC
10x4.7 APC props
APM 2.5 - external mag
Battery mounted below frame (4000mAh Traxxas)
Quad seemed noticeably more stable when all vibration dampening was removed.
First here is the vibrations with the stock setup from 3DR. Board mounted on bottom velcroed to plate.
Now here are the vibrations with the APM on a set of M3 Rubber Dampers and Moongel. Flight behavior was erratic and twitchy, only flew for one flight in this setup because stability was null.
And here's the vibrations with the APM hard mounted using nylon screws and spacers. Very stable flight.
EDIT: TURN DOWN YOUR SPEAKERS BEFORE YOU PLAY THIS!! ow my ears.
Attached is log of flight. A bit of loiter in the beginning and then stabilize mode while I chased around a RC sailplane. At one point in the middle the quad lands after I breached my geofence, then it lands itself at the end when the battery goes into failsafe.
All in all I'm impressed with the hard mount solution. None of my props are balanced (some of them even have a little damage). Not very windy while flying either.
2013-08-01 15-49 1.log
Vibrations can be dispersed as heat (this is what dampening actually does). But it is also dispersed as mouvement. So if we can somehow guide the waves to a part of the frame that is purposelu made ro move, would not this reduce vibrations on APM?
I thought about that by seeing my hand made aluminium landing gear, based on openbeams, vibrating a lot during flight but not affecting at all my video nor the zacc measurements.
Am i totally off path here or is it worth drilling deeper ?
Hi,
I am in the process of upgrading my APM 2.5 to the newer APM 2.6. So I thought I would take on board some of the information that I have picked up on vibration, particularly fitting the board and GPS/Compass onto a solid mass.
I machined up an alloy flat bar ( total weight 130g) and fixed that to the frame on 4 small foam blocks 5x5 and 4 thick (mm)
Doubled sized tape for the APM (case) and external GPS and Compass.
I have attached a small log of the results. I am using factory balanced motors and props
flight log.jpg
of course moongel does not work if you only glu APM to it,you have to make sandwich construction and press it.....also can you explain how do you plan to get rid of electric noise comming from LiPos when APM is attached???
@ Forrest,
Your comments:
What I don't know, but should find out. The accelerometers need good signal to noise, electrically and mechanically. So does moving the accelerometer chip diagonally away from CG help or hurt the processing of the signal? Accelerometers measure distance per second per second in x, y, and z. Take the case where the ship tilts forward. Accelerometers located dead center would see nothing (no change in x, y, or z (the gyros would). But if the chip was located forward to the right of CG, z would show a drop, x would show backward movement (the chip via rotation is moving under the ship toward the CG, and y no change. So is that good or bad? I'd guess that would be a false signal so put the chip as close to CG as possible.
WRT the bold part of your statement - This is not true. Accelerometers do not only measure acceleration due to their own movement, or rather rate of movement, but also measure gravity, obviously. So rotating an accelerometer around its own axis will measure a sinusoidal rate of change in the gravitational field it is immersed in. In ALL cases, it is better to have ALL inertial sensors as close to the CG as possible, that way attitudes are more easily computed, since the best signal to noise measurements are obtained this way,; rates are the lowest for such installations.
Joe
The Nampilot
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