Vibration Isolation and Dampening of APM / PX4 for version 2.9

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:

http://fpvlab.com/forums/showthread.php?4251-Vibration-Dampening-am...

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Whatever you are doing Vince ... keep it up.  Well done.  The z (really good but higher than x/y) would be consistent with heavy lift when keeping your stability tight.  Excellent.

Looking forward to seeing what you've done once you've protected the design.

I have previously posted my Hquad vibration results using Forest's Hover Analysis tool.

I decided that I wanted to know a bit more about vibrations, the different frequencies, impact of an unbalanced system and use of various dampening materials. Gained a good insight from info received and reading various posts  by Forrest,  Hugues, and Gary McCray who started this topic.

I thought that before testing any dampening materials I needed to start with the basics - vibrations created by motor/prop combination with no dampening applied and the additional vibrations caused by an unbalanced system. This post details the results of these tests.

I have used a X-CAM sensor and software to record the frequencies and amplitude. The X-CAM was hot glued to the wooden board just outside the prop perimeter.

The motor used for the tests is mounted horizontally on a piece of wood 15" x 6 1/2" x 3/4", which is bolted to wood 23" x 38" x 3/4". This is screwed to a wooden stand with legs 4" x 3". So a pretty solid structure

I used a RCTimer carbon 16x5.5 prop for all the tests. An elogger was used for recording the rpm. The rpm fluctuated a bit, but all tests were run at approx 2500 rpm. I have only detailed the results for the Z axis.

The table below show the results for the Z axis of the prop as received and after balancing. The prop was balanced with a small piece of plastic insulation tape about 1/8" wide applied approx one third from the prop hub.

My understanding is that the 43Hz relates to the rpm of the motor - 43Hz x 60 seconds (I did say the rpm was approx)

 

MN3508 2500rpm Z Axis

Prop balanced

0.095g (43Hz)

0.091g (86Hz)

0.138g (172Hz)

MN3508 2500rpm Z Axis

Prop unbalanced

0.1g (43Hz)

0.148g (85Hz)

0.15g (172Hz)

 

 

Unbalanced Motor & Prop Vibration Test

 

Motor Weight

Added 0.5g hot glue to outside of motor.

Result - Adding weight to the motor to unbalance it more than doubled the amplitude at 43Hz, also increased amplitude at 86Hz & 130Hz, but decreased amplitude at 172Hz.

 Prop Weight

Removed weight from motor, added 0.3g weight (insulation tape) to one prop tip.  

Result - Significantly increased amplitude at 43Hz. Reduced amplitude at 86Hz, and significant reduction at 172Hz. More than doubled the amplitude at 129Hz

 MN3508 2500rpm Z Axis Prop balanced

0.095g (43Hz)

0.091g (86Hz)

0.083g (128Hz)

0.138g (172Hz)

MN3508 2500rpm Z Axis 0.5g weight added to motor

0.262g (43Hz)

0.138g (86Hz)

0.132g (130Hz)

0.119g (172Hz)

MN3508 2500rpm Z Axis 0.3g weight added to one prop tip

The next stage will be to test vibrations using different diameter of carbon tubing.

0.64g (43Hz)

0.071g (86Hz)

0.182g (129Hz)

0.055g (172Hz)

Excellent and interesting data.

- photo of the test setup (you can use the 2nd button at the top or bottom of the edit box to embed versus attach)?

- frequency/amplitude plot?

- the data would have suggested to me a rpm of 3840 or 7680, so quite interesting.

Very interesting Jon

The above photo was taken on a previous vibration test. For the test detailed above I removed all the electronics/battery except X-CAM off the wooden board.

The images below are the charts for the three tests at 2500rpm:- 1. Prop balanced, 2. Motor Unbalanced, 3. Prop Unbalanced.

I had problems getting the charts into the right format for posting. I've sort of sorted it now. The files once I managed to get them into jpeg format were 5Mb each so I've reduced the size down to approx 200k. They are clear when I view the actual files on my computer but look a little hazy when I upload them here.

Forrest it's interesting what you are saying about the rpm. I tried to incorporate a credibility check by attaching an elogger for rpm, amps, volts & watts plus a GT power meter to compare the amps, volts & watts. In addition to the above tests I did the same tests at 2000rpm. The elogger and GT Power meter were in close agreement for the volts, amps & watts. At 2000 rpm the amps were approx 1.7amps which I thought looked reasonable. At 2500rpm the amps were approx 3.6 to 3.7 amps which seems quite a big jump to me. For the rpm the sensor is attached to the elogger and the two wires go to two of the three motor connectors. I haven't got two rpm sensors to compare to each other. The rpm jumps around a bit when looking at it live on elogger so the readings are roughly 2000rpm and 2500rpm. If I understand it right the Hz for a motor spinning at 2500rpm is 42Hz (2500rpm/60 seconds) On the charts above (unbalanced, motor & prop charts) the larger amplitude spikes are at 43Hz.

At 2000rpm I calculated the Hz to be 33Hz. The chart below details the prop unbalanced test at 2000rpm. The larger amplitude spike is at 34Hz.

I must admit I would really like my test to be wrong on the 2500rpm at 3.7amps and for the rpm to be something like 3840rpm. I'm on a big learning curve here so please let me know if I've got it wrong.

so do you think the results will change if you eliminate potential prop wash feedback?

I'm guessing that spikes will be seen at 43 & 86Hz (2500rpm) but maybe not at the other Hz. I don't know whether the amps will change though. I will need to test. Do youthink the amps look about right at 2000 & 2500rpm?

my database on rotor combinations has been focused on thrust versus watts.  so i rarely recorded rpm. you are exploring new ground.  on rpm measurement, use the system you are using to estimate it but verify your estimation system once using a meter that reads a reflective dot on one blade.

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