Review and tests of anti-vibration systems

To optimize videophotography results, vibrations need to be reduced as much as possible. The objective is to disturb as little as possible a brushless gimbal holding a camera. (we all know too well how hard it is to get a brushless gimbal well tuned).

As most people, brushless gimbal tuning is done on the bench using accelerometers & gyros of the brushless gimbal's IMU. And like most, we do not get as good results when flying. Why?: frame & air vibrations. Frame vibrations are created by motors, props and wind. Direct air vibrations (external wind) also impact the brushless gimbal.

This is a blog post showing anti-vibrations objective measurements of different isolation systems, obtained on my Mr. Grey X8 workhorse shown on the introductory picture. All vibrations measurements are made during periods of min 30 sec of hover in stabilize mode. Take-off and landing periods are excluded of measurements.

Configuration of the test craft:

-Pixhawk controlled -Stabilize mode

-T-Motors MT3515-400Kv

-15x5 XOAR props

-6S batteries

-2 axis brushless gimbal - Alexmos

-Sony NEX5 camera

-Weight of the gimbal+camera+fpv+3S 1ah battery : about 1250 g

-AUW : about 6.8Kg

Vibrations measurement setup:

-All tests were made with the craft configured fully ready-to-film : except the gimbal which remained off not to measure its own generated vibrations. The camera was thus simply screwed on the gimbal with no power to the brushless motors.

-X-Cam vibration measurement tester

X-Cam is installed on the different areas of the craft to be tested. Its mesaurements are triggered remotely by a PWM pulse (on/off switch allocated on Taranis). It is powered directly through the PWM servo cable connected on the receiver.

Data are stored on board its flash memory. It is made for vibrations measurements in the 0-200Hz range and can store up to 3 min of continuous measurements.

Through the X-Cam GUI , data is downloaded from the flash memory to produce a time graph (basically useless as it shows the X-Y-Z vibrations measurements along the time axis), and a frequency graph showing X,Y,Z vibrations amplitudes (in number of g) versus frequency (in Hz). Raw data or weighted averaged data can be calculated.

Two anti-vibration systems were measured,

First a silicon ring system as shown below,

Four of these were used in a X configuration between Mr Grey's two plates (see introductory picture). Each plate is made of plywood; square shaped with 20 cm long sides.

Silicon rings come in two stiffnesses: red for softter silicon, black for stiffer silicon. Here, only the red softer ones were tested. Each Secraft system costs about 9 euros each (www.secraft.net) , which is relatively cheap.

Second tested system is a "Bell" shaped  precompressed silicon system. Each "Bell" is made for a 250g payload. Each one of them costs about 39 euros (www.altigator.com) which is quite expensive. It is mostly used in high end professional aerial photography rigs :

Four of those were used, in each corner in between Mr Grey's test plates as shown below:

Reference measurement:

In order to compare results, a reference measurement is made on the frame itself, with the X-Cam placed at the center of Mr Grey's top plate, under its protective "flower pot".

Frequency domain picture shows two frequencies were vibrations are significant : 85Hz and 175Hz; The measurements show about 0,3g of vibrations on all three axis at 85Hz; and 0,4-0,5 g vibrations at 175Hz.

First test result with Silicon rings system:

X-Cam is now placed on the second bottom plywood plate on which the gimbal is fixed,

The results show that frame vibrations are continuing to pass through to the gimbal at the same frequencies (85Hz and 175 Hz) but amplitudes are divided by a factor of 10 at 85 Hz and almost by a factor of 20 at 175Hz, versus non damped frame vibrations.

Second test with the "Bell" system:

X-Cam is placed at the exact same location as for the first test,

The results show that frame vibrations are continuing to pass through to the gimbal at the same frequencies (85Hz and 175 Hz) but amplitudes are divided by a factor of 2 at 85 Hz and by a factor of about 2 at 175Hz, versus non damped frame vibrations.

If we show the silicon ring's system on a graph at the same scale as Bell's system:

The silicon ring anti-vibes system literally erase vibrations in comparison with the "Bell" anti-vibe system....

Comparison of both anti-vibrations systems:

It appears clearly that the "silicon ring system" is much more efficient at reducing vibrations versus the "Bell" system. Further, this silicon ring system is much cheaper than the Bell system.

For once, more expensive, supposedly professionnal parts were not up to the task verus cheaper diy parts.

Cheers,

Hugues

Views: 7982

Comment by Gil Rosenthal on October 21, 2014 at 9:38am

Hi Hugues,

Very pleased with your research and results, as I have just finished a heavy lifter

using a similar O-ring system from FamousHobby.

I am saying this as I hope to get you to do another set of tests, namely:

A) Testing the O-ring layout used by FamousHobby CS036 dampeners.

      These are about double the price of the Secraft suspension, and are supposed to be

      compatible with Cinestar frames. however, they do allow for a nice and compact installation. http://www.aliexpress.com/store/product/CS036-Vibration-Dampener-fo...

B) O-rings actually come in many flex grades and materials, and I am sure that ample testing

     would help tune out the resonant peaks. Take a look at http://www.oringsusa.com/ and

     the O-Ring catalog from www.parker.com:

     https://www.parker.com/literature/ORD%205700%20Parker_O-Ring_Handbo...


MR60
Comment by Hugues on October 21, 2014 at 10:39am

Hi Gil, these you bought are indeed very similar to the tested ones. However you have there a system with a single orientation of the o-rings, while the tested system uses two perpendicular sets of rings which improves the mechanical stability.

Thx for posting these various o-rings possibilities, I will take a look at it.

Comment by Gil Rosenthal on October 21, 2014 at 11:16am

Hi Hugues,

Yes - you are right, they also utilize only 6 rings rather than 8.

However, they are implemented as a rectangular "polar" array -

4 units per gimbal - so this does cancel out a certain amount

of side-to-side (XY plane) movement on the assembly as a whole.

I had to skip using the Secraft solution as they did not fit my frame in terms of dimensions.


MR60
Comment by Hugues on October 21, 2014 at 11:38am

very clean build. How many of these dampers do you use , 4 or 6 ? What are the dimension of these blue dampers ?

The secraft dimensions are :

Comment by Phillip Schmidt on October 22, 2014 at 1:10pm

  Good job with the testing!  It's nice to see a well executed experiment accompanied by a solid write-up.

 I would like to see this type of testing where both acceleration and angular rate are recorded simultaneously.  I think a lot of people get fixated on acceleration, while in my opinion angular rate is what really causes the most significant image distortion in optical devices.  Acceleration might be a good indicator of angular rate, but given how easy it is to gather angular rate, I'm interested to see how they compare.

  Side note: The scale of the "amp (g)" axis on the final graph ("Silicone rings damper (same scale)") is too large by an order of magnitude.  The maximum axis value shown is 3,2 while the maximum value of the Bell damper graph is 0,3.  The silicone ring damper is definitely better, but not quite a significant as that graph makes it look.


MR60
Comment by Hugues on October 25, 2014 at 1:04pm

Thx Philip.

Actually we should be able to use the IMUs we have for gimbals (i have a few laying around) to make the gyro measurements you are talking about, with a piece of arduino code. Any dev volunteering to develop this ?

Comment by Vince Hogg on November 2, 2014 at 6:40am

Hi Hughes, and Gary.

As you know i have been a fan of silicone rings for a while now and blogged the development of my quads and octa. I have been using them successfully now for almost 2 years.

Here are a couple of pics of the way im using them.

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