Altitude/Accelerometer

[JeffD 0]

I am deciding on a senior design project and am wondering if I could possibly sell this device to fellow skydivers.

It would basically be an audible altimeter, which logs alitutde, vertical speed, and the acceleration in 2 axis (so the jumper could have a reference on how hard/soft an opening was)

I know this is similar to a Pro-track but with accelerometers.

My main curiosity is, how many people would want/need/use the accelerometer data?

[FrogNog 1]

Which two axes? How would you guarantee orientation so you knew it was measuring all the acceleration? Would the opening acceleration be given as the vector sum of the measured acceleration axes (of which I would hope there were three...)? Would it be susceptible to reading unimportant acceleration by being mounted on extremities? (I.e. my feet probably experience much more acceleration, or at least a wildly different acceleration profile, than my head.)

How much would it cost, how big would it be, how hard would it be to operate, maintain, and extract data from, and what would be its mounting requirements? These are all things people think about when they decide to aquire additional skydiving equipment.

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Pull.

[billvon 2,435]

You'd need a 3 axis accelerometer and you'd need to position it at the center of mass, which in a belly-flying skydiver is near the small of their backs. I suspect it wouldn't give you much in the way of new info (over and above what a protrack gives you) but the exercise might be interesting.

[masher 1]

Quote

I know this is similar to a Pro-track but with accelerometers.

Doesn't the protrack have accelerometers? or does it just use atmospheric pressure?

Anyway, couldn't you get your acceleration just by (double) differentiating your velocity curve from the protrack?

--
Arching is overrated - Marlies

[DancingFlame 0]

It measures pressure and does some calculations (based on the pressure changes, just like a Cypres, but not that precisely)

[thelem 0]

Wouldn't a protrack which measured the speed more accuratly be just as good?

eg, if you take 1000 altitude measurements over the skydive, you should be able to get a fairly good indication of open speed.

[darkwing 4]

I suspect the accuracy limitations for the ProTrack (and similar devices) are related to the pressure fluctuations right next to the falling object, not due to limitations of the device itself. That is why pitot tubes on aircraft extend into clean air to make their measurements.

-- Jeff
My Skydiving History

[JeffD 0]

Yes but using an accelerometer would not be affected by air pressure changes.

Plus a piezo electric accelerometer is on the order of $15-$20 so its not too expensive (and the size of a quarter)

[billvon 2,435]

>Wouldn't a protrack which measured the speed more accuratly be just as good?

Almost but not quite. As an example, take an AAD. If you integrated an accelerometer into an AAD, it could detect opening shock and integrate that into the speed model, resulting in a detection of decreasing speed. If you measure just air pressure, you can see an _increasing_ speed as the jumper rotates into a head-up position, and this could cause a misfire. This feature could prevent many AAD misfirings that occur when the jumper deploys around 1500 feet.

[darkwing 4]

I'm not up on the state of the art for $20 accelerometers, but there is a butt-load of stuff you have to do with the accelerometer data before it tells you how high you are. You have to integrate a noisy vector function twice, which is even worse than what the air pressure based instruments have to do. I really, really doubt it would work any better, or cheaper. Butg hey, lots of people have been told something wouldn't work, so go ahead and give it a try. At the very least, you would learn a lot, and that is valuable.

-- Jeff
My Skydiving History

[AggieDave 6]

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This feature could prevent many AAD misfirings that occur when the jumper deploys around 1500 feet.

And I don't believe that it is truely a "misfire." It fired due to the lack of awareness on the jumper's part, it did exactly what it is supposed to do.

--"When I die, may I be surrounded by scattered chrome and burning gasoline."

[ToeUp 0]

I assume you're still using a pressure sensor to determine altitude and vertical speed? Doing that with just a 2-axis accelerometer would be pretty much impossible.
But I've considered some very similar projects and have done a few projects with accelerometers. Mostly I have used the ADXL311 from Analog Devices, which is cheap ($6.38 each), but only measures up to 2g. They also have some cheap 10,50 and 100g devices though.

I think that 2 axes would be sufficient to register opening shock if mounted correctly, since you generally wouldn't experience much sideways force during deployment. But you would probably have to mount it somewhere on the harness because the flexibility and motion of the human body will dampen the opening shock. To measure peak opening shock you should be able to use just a single axis sensor mounted on the riser, since most of the decelerating force will be parallel to the riser.

Anyway, I don't think there would be a very big market for the added information, especially when you consider that retail price will normally be at least 4 or 5 times part cost.

What parts are you planning to use? I'm working on a gadget right now and am shopping for a good pressure sensor and maybe a good ADC. I've used the Motorola MPX5100 before, but I'm looking for something smaller.

[riddler 0]

I doubt I would get much use out of it. My openings are categorized as "normal", "twisty" and "hard" - occasionally "very hard". I don't have much need to know how fast the deceleration was - it would probably only make me grumpy

If you could also get horizontal speed and distance out of it, it would be nice to know, for instance, how far and fast I track, or how much I backslide when freeflying. But integrating those values without good starting criteria (for instance, how fast is the plane traveling? How do the upper winds affect the initial acceleration?) would be challenging - you would have to make some assumptions that would give you less than accurate results.

Trapped on the surface of a sphere. XKCD