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: 7778

Comment by Gary McCray on October 19, 2014 at 1:16pm

Hi Hugues, excellent work on vibration analysis comparing the 2 methods.

Recently I have been flying a Hoverthings FLIP FPV Pro G Dead Cat style quadcopter which features a "live" intermediate frame suspended in compression on 4 Lord style rubber mounts.

the rubber mounts are quite flexible so the entire battery, autopilot, camera mount structure tends to move around fairly easily and to not dampen low frequency high amplitude vibrations very quickly, (if you give it a push it moves back and forth several times.)

I wanted to try my Sony AS100V (internally stabilized) camera hard mounted to the floating frame not on a gimbal to see how it would work with this copter.

Results were not as good as I hoped, although there were many "clean" periods, every 5 to 10 seconds a little bit of shutter jello would show up for a bit and then go away.

I tried adding align Gel mounting to the camera mount itself but that only made things worse (it permitted even more camera movement).

So I had an idea, clearly, the flexible Lord style mounts were doing an acceptable job of damping high frequency vibrations (my Pixhawk showed sub .1 G vibrations).

But long period oscillations were not getting damped because the mounts were acting more like a spring than a shock absorber (elastic versus damped).

I bought 8 little 1 1/4" Sorbothane Hemispheres from Amazon which fit perfectly held in a small amount of compression by the "Lord" mounts between the live and main frames of my copter.

Sorbothane is a really good non-elastic damper and I reasoned the hemisphere shape would give progressive resistance and damping.

I gave 4 of them to my friend Oliver who has the same frame to try on his gimbaled copter as well.

Oliver has already tried them and is very happy wit the result.

It has completely eliminated his "jello" problem and he is getting rock solid HD video.

It is about $18.00 for 8 of these on Amazon and if you can incorporate them in a solution they seem to provide nearly ideal damping.

The Sorbothane Hemispheres are available in various sizes.

I will be trying my copter soon as well and will let you know my "results" too.

Best Regards,

Gary


MR60
Comment by Hugues on October 19, 2014 at 2:39pm

Thx Gary. I'd like pictures of your setup if you have any?

Do you also have pictures of these sprbothane hemispheres and those lord rubber things?

Comment by Gary McCray on October 19, 2014 at 3:24pm

This is the Amazon link to the Sorbothane 1 1/4" hemispheres I purchsed:

http://www.amazon.com/Sorbothane-Hemisphere-Non-Skid-Adhesive-Durom...

And here is a picture of My Flip FPV Pro with the Sony AS100V camera:

And a close up showing the frame flaoating on the Lord (like) mounts at each end and one of the hemispheres in the middle.

I have one hemisphere under each end and one under each side.

They are held in place with what seems appropriate tension by the "lord" mounts.

The hemispheres have adhesive on the bottom, but Sorbothane has very high sticktion so I don't think you need to actually use the adhesive for this making them easily removable.

Just stretch the rubber lord mounts a bit to insert or remove the hemispheres.

It looks like these are going to work very well, the Sorbothane is a really good non-springy damper.

Best Regards,

Gary

Comment by Gary McCray on October 19, 2014 at 5:59pm

The above pictures are very high resolution and you can click - zoom in on the right side pair of Lord mounts and the front, rear and right side Sorbothane hemispheres to get a good idea of the fit.

Olivers and my Flip FPV Pros have factory available extended arms, Oliver is using 13" props and I am testing 14" props on the above copter.

My 14" are working fine but as a matter of practicality I would recommend 13" props, tip to tip clearance is a bit close with 14" props as you can see and there is some flex in those CNC G10 fiberglass frame arms.

Both Oliver and I are using KDE 2814 515KV motors and a Quattro 4 in 1 ESC and an O-ring suspended Pixhawk.

Predictably a 1 1/4" hemisphere is about 5/8" high and the frames are held about 1/2" apart by the Lord mounts, so about 1/8" interference to "squish" the hemispheres with.

Oliver is only using one hemisphere in the front and another in the back.

This definitely reduces vibration, particularly Z vibration and also pretty much eliminates that periodic self multiplying sympathetic Z vibration / oscillation you sometimes see.

Although Sorbothane is available in various durometers these hemispheres are 50 durometer and are quite soft, probably just about perfect for this application.

I believe stiffer durometers would not dampen as effectively for our normal uses.

My Pixhawk is O-ring suspended from the front vertical live frame stand offs and is forward of the CG.

This offset (necessitated by positionable battery for C/G determination) has not seemed to have caused any flight handling problems.

Here is a link to Hoverthings "anti vibration bobbins" aka Lord mounts as used in the above airframe:

http://www.hoverthings.com/parts/flipfpv/antivibrationmount.html

We are still experimenting.


MR60
Comment by Hugues on October 19, 2014 at 10:30pm

Thx for posting your details. This is a quite remarkable anti vibration combination. In particular the o-ring suspended pixhawk. Can you please post a better detailed view how the o-rings are fixed to the pixhawk case?

Comment by mP1 on October 20, 2014 at 3:17am

What about Kyosho zeal green gel ? It seems to work well for me, highly worth a shot.

Comment by Gary McCray on October 20, 2014 at 11:48am

Hi Hugues,

All I did was use longer small machine screws in each corner with a nut threaded down to actually secure the top of the case and leaving about a 1/8" gap between the top of the nut and the bottom of the machine screw head.

This is not quite ideal because the threads on the machine screw can cut in to the Oring and eventually cause it to fail. (Oliver has had one fail in this way already).

A 1/8" piece of small heat shrink on the machine screw under the head should love this problem.

On mine, 2 different size orings are used because the front frame standoffs are further apart than the 2 standoffs nearer the middle of the floating frame.

On mine I have also put in a tightly cinched ziptie on each of the standoffs which I position just under each )-ring to keep it from vibrating down the standoff.

And mP1, Kyosho Zeal is great stuff and well damped, it really wouldn't work for damping the entire floating frame in this case because the dimensions are wrong, but using a 1/2" to 3/4" square of it under each  corner of the flight controller is how all my  other PX4's and APM's are supported.

I actually wrote the Wiki page on vibration where I strongly extol the virtues of Kyosho Zeal.

http://copter.ardupilot.com/wiki/vibration-damping/#Gel_pads

(Thats my PX4 Flamewheel in the pictures with the 4 Zeal pads).

This high mass (battery) suspended intermediate frame is a different thing though and it demanded a different solution.

Sorbothane is cool stuff too.

Best Regards,

Gary

Comment by Gary McCray on October 20, 2014 at 11:51am

Hi Hugues,

All I did was use longer small machine screws in each corner with a nut threaded down to actually secure the top of the case and leaving about a 1/8" gap between the top of the nut and the bottom of the machine screw head.

This is not quite ideal because the threads on the machine screw can cut in to the Oring and eventually cause it to fail. (Oliver has had one fail in this way already).

A 1/8" piece of small heat shrink on the machine screw under the head should solve this problem.

On mine, 2 different size O-rings are used because the front frame standoffs are further apart than the 2 standoffs nearer the middle of the floating frame.

On mine I have also put in a tightly cinched ziptie on each of the standoffs which I position just under each )-ring to keep it from vibrating down the standoff.

And mP1, Kyosho Zeal is great stuff and well damped, it really wouldn't work for damping the entire floating frame in this case because the dimensions are wrong, but using a 1/2" to 3/4" square of it under each  corner of the flight controller is how all my  other PX4's and APM's are supported.

I actually wrote the Wiki page on vibration where I strongly extol the virtues of Kyosho Zeal.

http://copter.ardupilot.com/wiki/vibration-damping/#Gel_pads

(Thats my PX4 Flamewheel in the pictures with the 4 Zeal pads).

This high mass (battery) suspended intermediate frame is a different thing though and it demanded a different solution.

Sorbothane is cool stuff too.

Best Regards,

Gary


MR60
Comment by Hugues on October 20, 2014 at 12:04pm

Clever way to mount the o-ring , Gary.

Why did you choose for this method instead of simply putting he pixhawk case down onto the bottom plate ? (with Kyosho zeal for ex or a suspended mount I like very luch such as this one : http://diydrones.com/profiles/blogs/3d-printed-anti-vibration-mount)

I like this FPV pro frame. What would be the maximum payload you could fly with , considering 10" props and 3S batteries on this frame ?

Comment by Gary McCray on October 20, 2014 at 2:31pm

Hi Hugues,

I developed the suspended O-ring platform before I knew about Zeal and have always found it to yield good results, and in fact, the first results we ever had that consistantly got our vibrations down into the "satisfactory" sub 0.3G range.

In fact I have made dual platform autopilot supports using both O-rings and Zeal which had a small improvement over either one alone but they both work fine.

On the Flip FPV Pro, the presence of the vertical standoffs in the right ;location on the "floating" frame made it too tempting to ignore the possibility of easily using O-rings.

My honest estimate is that Zeal would work about the same, so it is a simple choice.

The O-ring supports do make it a lot easier to remove the Pixhawk from the frame than Zeal would though, you can just pop the O-rings off the top of the extended case screws.

Oliver and I have optimized ours for larger props and high efficiency KDE motors, but the standard arms, are in fact designed for 10" props and it flies very well with those.

http://www.hoverthings.com/frame-sets/flip-fpv-pro-g-black-gimbal-v...

Even with 10" props you could carry quite a bit more weight, with even average motors, it hovers at less than 50% throttle with a GoPro and gimbal.

If you really want to carry big payloads or get longer flight times you might be better going with the extended arms and bigger props, they really do make a considerable difference in efficiency and flight time especially (heavy).

I am very fond of this frame and consider it one of the most practical "kit" frames you can get, it is extremely rugged and well laid out for what you need it to do.

It flies very well and because of it's small side profile is very stable even in gusty winds.

I do like the 3D printed vibration isolation frame which, interestingly is a variation on one I saw a couple years ago that used the same O-ring support method I now often use and which I borrowed from them.

The O-ring support method does require a bit of tuning playing with O-ring diameters and the diameter of the oring tubing itself.

Also, Silicone O-rings work a bit better because they have better intrinsic damping then Buna-N.

Our vibration control is always a trade off between elastic (spring) and true damped (shock absorber) methods.

Generally the materials we use incorporate both characteristics, but currently our quantitative appreciation of these is sorely lacking, your Blog here is a great start in the right direction.

Best Regards,

Gary

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