Comment by Phil Cole on March 25, 2011 at 8:29pm

I didn't see any mention of balancing the propellers in your tests. This is probably worthwhile, at least for the accelerometers.

Comment by dan on March 26, 2011 at 1:23am

You are right, I have not balanced them.

Everybody "out there" says it's an important part of the vibration reduction effort, but it seems to me that the vast majority of people manage to fly quads without bothering... 

So while it can only help, I don't have the necessary stand (and patience :) ) so for now I'm sill struggling to understand what the problem is in my case... 

I'll try to post today the measured vibrations with an updated frame...

Comment by Phil Cole on March 26, 2011 at 9:29am

Dan,

I was impressed with the work and methodology you've put into this.

As was suggested in your second entry, isolation mounting for the APM will also help. Search for "CSM gyro tape", or the tape used to mount flybarless gyros on model helicopters - this is the same application and they get the units to work on helicopters with piston engines.

If the motors are not producing constant torque you may benefit from isolation around the axis of rotation. Something like a Hyde Mount may help. http://www.ofremmi.info/howto/Hyde-mount/HydeMount.htm

Comment by dan on March 26, 2011 at 10:07am

Phil, thanks a lot for your comments and suggestions !

I'll definitely look into the "gyro tape", even though I've already tried simple 2millimeters thick foam tape, with no noticeable effect...

As for the Hyde Mount, wow... looks quite interesting, but hard to put together, especially 4 times ! lol :)

 

dan

Comment by Luis Mauro on March 26, 2011 at 4:34pm

Hello Dan, I had a lot of experience with vibration I have some clues for you:
- APC propeller are the best, even with no balancing job, they have good balancing,
- Use the original propeller holes, don´t drill it for a larger hole,
- Use motors up to 1000rpm/Volt because the vibration increases with the square of the rpm,
- The stiffness of the quad structure must be tunned to "filter" the vibration like a "low-pass filter (it´s pretty difficult to explain in a few lines). The quad structure must attenuate the vibration that comes from the engines/propellers!
- CSM gyro tape can be used to attenuate even more the vibration,
- Finally, the code (software) that you´re using to read the accelerometers and gyros signals must have a high-pass filter feature

 

/edit by moderator: it needs to be a low-pass filter

Comment by I.S. on March 26, 2011 at 7:19pm

@Luis

- So super stiff motor-motormount-frame bad idea? better to put some dampening between motor and frame to do lowpass? Could you give some hints for this?

- where can I find gyro tape?

- Does de APM code do high-pass filtering for sensors? gyros may need a high pass to avoid drift, but what about the accels? do they need high pass filtering too?

Comment by dan on March 27, 2011 at 5:29am

@Luis
- I am using EMP propellers (do you know anything about their quality?), but it's good to know that not all of them are made equal and APC seem to be better quality
- now with the holes, I had indeed some trouble as the adapters don't fit exactly my motors' shaft
- I use KDA 20-22L 1024kV so it should be ok (but if I were to do it again, I would indeed choose even lower rpms
- I THINK I understand what you mean by the frame "filtering" the vibrations : this means that it should be not too stiff, but stiff enough, to cut out more or less exactly the kind of vibrations that your motors+propellers combination is generating. From what I can see, the stiffer a frame is, the lower the frequency of the amortized vibrations (ie the higher its resonant frequency) and vice versa.
- I'm using some standard 2mm thick foam tape, to no visible effect

For the software I'm not sure:
I'm using some custom code (inspired by Jose Julio's initial code) running on an Arduino.
The sensors are:
- Wii Motion Plus IDG-650 & ISZ-650 for gyroscopes
- ADXL345 for the 3 axis accelerometer (from Sparkfun's 9DOF IMU, that I burnt the MCU of, and now connect to its accelerometers directly through the I2C bus...lol :))

So the code does have filtering for Accelerometers data ONLY, like this :
// Low pass filter on accelerometer data (to filter vibrations)
accelVector[0] = accelVector[0] * 0.4 + _accelX * 0.6; // acc x
But the initial comment said it's a "LOW pass" and to be honest I don't know the difference between low and high pass...

@I.S.
- I don't know what APM code does as I'm not using it...

Comment by I.S. on March 27, 2011 at 5:52am

That's a typical simple LP filter, some kind of weighted moving average.

LPF: LOW freqs pass, HIGH freqs do not or are attenuated

HPF; HIGH freqs pass, LOW freqs do not or are attenuated

Comment by Sebastian Gralla on March 27, 2011 at 5:55am

APM code does filtering

Comment by Luis Mauro on March 27, 2011 at 8:29am

Hello guys
I had a mistake on the follow comment:"- Finally, the code (software) that you´re using to read the accelerometers and gyros signals must have a high-pass filter feature" please read low-pass filter instead of high-pass filter.
- The theory about the quad stiffness is based on its natural frequency. Look at http://en.wikipedia.org/wiki/Mechanical_resonance
- This graph http://en.wikipedia.org/wiki/File:Resonance.PNG shows a typcal response (attenuation or amplification) of a structure like of the quad. The Y axis shows the relation betwwen the intensity of the vibration measured where the sensors are placed and the intensity measured where one of the motors are placed. Note that when this relation is near 1 the motor vibration is been amplified (not desired) and when this relation is above 1.5 the motors vibration is been attenuated (desired). The X axis shows he relation between the motors frequency (WA) and the natural frequency of the quad structure (W0). Note that when WA/W0 equals to 1 there is a big amplification of the motors vibration. This point depends on the quad structure stiffnes (k). If you decrease the stiffnes the W0 decreases too, beyond 1.5 there is a attenuation of the vibration that comes from motors (desired effect). Another parameter that determinate the W0 frequency is the mass (m) on the center of the quad, higher mass lower W0. See the formula f = ..., f in Hz.
- How to find ideal W0? W0 value can be equals to the motors rpm (divide by 60 to transform in Hz) when the quad is a almost taking-off. Above this you need attenuation effect because the quad is flying and bellow this frequency, doesn´t matter because the quad is on the ground ...
- Another important issue is the dampening of the structure: steel and aluminum have low dampening coefficient while carbon-fiber and wood have higher dampening coefficient.
- The structure stiffness is the first barrier against the motors/propellers vibration and few people consider this ...
- I like these IMU units: FreeIMU v.02 see at http://www.varesano.net/projects/hardware/FreeIMU
or
http://www.sparkfun.com/products/10321
they are complete small, light-weight, easy to connect (4 wires) and large range. I have the FreeIMU v.02 that is cheaper and you can connect directly on 5V Arduinos (I2C) with no 3.3V regulator or logic levels converter ...
- Related to the propellers: I had experience only with GWS and APC propellers and I can say that GWS needs a very good balance work before you can use while APC you can use with no balance work ...
- 1024RPM/volt is good

I hope this comment be usefull, See you soon
LMF

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