G-force Reduction for Electronics in Crashes.


If you consider yourself to be one of the following, then read on:

1) Those with extremely fast airframes, such as jets or hotliners or the like.

2) Those that have more than the average dollar invested in their electronic packages.

3) Those that are just nerdy enough to just want to know if this could work.

4) Those that believe they have a vibration problem and are slightly or totally desperate.

No one likes to have to order another UAV part because something went wrong and now its a pile of little and big pieces in some unwanted location.  It is even worse still to lose your electronics on top of this.  I have two ideas for a system to reduce G-Force loading on the APM2 during a crash and indeed it could be carried over to other electronics as well.  So far as I know this has not been done before.

It occurred to me that nature has two very good examples of mechanical dampening meant to protect a valuable cargo.  I mean to use that as a guide.  Our cargo here being the APM2 board. 

Mother nature did it first in your skull.  Your brain has fluid surrounding it that has multiple purposes including mechanical isolation from forceful blows to the head that might cause brain damage.  The second example would be a developing fetus in the womb.  The amniotic fluid provides a very nice ride for the developing and delicate fetus to float around in.  I'm thinking replace that fetus with the APM2 in a box.  Oh jeez, that just sounds bad, sorry.

It should be easy....RIGHT....

Lots of things to mitigate here.  The first being what liquid could you use?  If you search the internet you can find really neat electronic submersions in a non conductive liquid such as Mineral Oil.  See it here.

Various properties need to be considered should you want to choose another liquid, but here is my take on mineral oil.

1) It would need to be thermally conductive to help dissipate the small amount of heat the APM2 would generate.  If you were to perhaps want to send an APM2 into orbit (but no one wants to do that:) you could deliberately choose a liquid with low thermal conductivity to act as a perfect blanket that would not freeze until at about -22F.

2) It would need to be non conductive right from the get go.  I think this goes without saying.  There are a lot of products out there that fit this bill but Mineral Oil is both transparent and non conductive so you could still see your APM2.

3) It would need to allow the GPS to pass an un-attinuated signal, big fancy word to mean that the liquid must not change the frequency or power of the incoming satellite signals. I've got no clue on this and I could not measure it even if I did for I lack the equipment.

4) It would also need to not interfere with the magnetometer.  Since mineral oil's origin is that it is a derivative of petroleum I doubt it will have anything ferrous and would probably fit this bill.

5) Not be heavy, typical mineral oil is about 0.8 g/cm3  so I think we are safe here.

6) Since this liquid could be released into the environment it needs to be non toxic.  Mineral Oil fits that bill too.

7) Since we are usually on a tight budget it should be CHEAP and low and behold, it really is around 10 bucks a gallon.

8) It's also a mild laxative...just thought I'd throw in that icing on the cake since we are in awe of mineral oil at the moment.

Ok, so maybe now you buy in just a little to the mineral oil idea.  Now what?

Two ways to use this, one is very simple and clean and will most likely do the job well.  To picture this think of those water filled barrels you see on some dangerous exits on the highway.  When a car (or APM2 board) hits them the dispersion of the water being thrown into the air acts as a siphon of energy in the collision.  It is quite effective and saving lives because the force felt by the passengers (or components on the APM2 board) is otherwise reduced to a safe level.

What this safe level is I don't know.  That is going to be the crux of this experiment.  So lemme get to the mechanics of it.

The first thing you could do is use some sort of plastic bag that is filled with this mineral oil.  You take a custom made box and line the entire inside with these bags.  Place your APM2 board inside this box and you have the basic idea.  The box does NOT need to be stout, it just needs to hold everything in place to let the mineral oil do its job.  In fact, if the box itself was to explode on impact that too would dissipate energy.  To provide much better protection you would need bags under and over the APM2 as well.  It seems that in theory, you could run the cables out of the top of the box without serious degradation to impact resistances.  That is idea one at the most simple implementation.

The second idea is a bit harder to pull off but could have better results.  You take a box, put the APM2 inside of it, loosely mount it, as in the mounting mechanisms would "easily" break upon impact.  Fill the entire box with mineral oil and put a lid on it.  The lid must be water tight.  All electrical connections would need a water tight exit as well.  The US Navy has a lot of methods to accomplish these connections so while it may not be practical for our purposes it is at least a possibility.  This works because once the mounts are broken the APM2 board must "swim" to the wall of your box in a thick viscous fluid which takes a lot of energy to get there. 

I can foresee testing this second idea by giving the completed box (minus the external wires) to a minor league baseball pitcher and letting him have at it with a brick wall...or you can just drop it from your roof whichever tickles your fancy.  

This is a weak outlining for a solution to crash-proofing valuable electronics.  I have no idea how many would even be interested in such an item or if anyone is willing to give this a try.  I personally don't have APM2's lying around to throw at brick walls, but some of you might!

As a side note, I can also see how this MIGHT reduce vibration issues.  Since vibration in the typical aircraft/copter/whatever for the RC world is due to mechanical oscillation from various sources (motors and airframes for example) then it stands to reason that those forces would propagate to this housing.  These boxes could have plenty of room for custom rubber mounts that you may not have room for with traditional mounting methods (whatever those are for you).  The cool thing is, if you want to reduce vibration overall then you need to have dissimilar materials used in construction.  The mineral oil will act as a very different type of material for the mechanical energy passing into the box to be mitigated before it reaches the IMU on your APM2 board.  I don't know by what degree, but it seems logical that it will indeed have an effect.

I will install SolidWorks on my main desktop at a later date and design some example boxes that are in my head to help those that can do this to get started.  Could be fun!

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  • sorry for multiple digitation errors, I am not comfortable with this kbd and sent the message as mistake

  • most liquids react as slmost solids when sudden speed object try to force it.

    try to shot a bullet on water to see it (don't do it, bullet will bounce out)

    anyway when crashing g forces are so great that apm will not break only for the omogeneous pressure distribution. this reason make me think that a simple silicone layer guled to the apm will distribute force equally and is not damaged until the board underneat borke itself for the crash. a sturdy steel board will solve this.

    I usually have electroncis inside foam and I use foam gliders as uav so never experienced a crash, but maybe with quad is different and bulky to use foam.

  • I like your idea but I think in solution 2 the liquid will only act as a damper, not a spring.  I would think you would want both. You give the human brain as an example, but the human brain is very susceptible to long duration shocks because it is mostly damper (fluid) and little spring (the facia between the brain and the skull, whatever that is called)

    I would think an open cell foam would treat you better, gives both spring and damper, but choosing the right spring rate will be the key and to do that you need to know how many Gs you experience in a crash and the duration (have fun with that!).

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