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Altimeter accelerometer anomalies?

  • tlainhart
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9 months 1 week ago - 9 months 1 week ago #11812 by tlainhart
Altimeter accelerometer anomalies? was created by tlainhart
A question for the panel:

A couple of Acton launches ago, I put up a BT-60 based rocket on a D12. It had onboard a FlightSketch mini altimeter , and (probably, not sure) a Jolly Logic chute release. The charts are here:  flightsketch.com/flights/4121/ . The graphical display navigation is wonky - you'll need to place your mouse pointer within the data rectangle and click/drag to navigate.

While the altitude and velocity look reasonable, the acceleration data does not, with large spikes between 11 and 21 seconds. I'm wondering if this is a bug, or a side-effect of the altimeter banging around freely during descent as it's attached to the shock cord. Or something else. The accelerometer is described as "3-axis". Perhaps this is reporting of the x and z axis?

Theories?

 
Last edit: 9 months 1 week ago by tlainhart.

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9 months 1 week ago - 9 months 4 days ago #11813 by DeltaVee
Replied by DeltaVee on topic Altimeter accelerometer anomalies?
My expectation is that the acceleration curve is the total acceleration.  I would expect large spikes at ejection, chute release and landing, but during descent a somewhat noisy looking acceleration since the rocket is swaying and being buffeted... 
Your altitude vs. time plot suggests there was no chute release being used... the slope of the descent curve should have a discontinuity in it.

My JL altimeter three usually looks like this:

 

Acceleration is the red curve...   I cant explain the big spikes you see from time to time on your acceleration plot though.

Added:  after thinking about it for a while, I'm wagering it's the shock cord attachment... 
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Last edit: 9 months 4 days ago by DeltaVee.

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9 months 5 days ago #11819 by tlainhart
Replied by tlainhart on topic Altimeter accelerometer anomalies?
Thanks John. Good catch on the no-chute release observation, according to the data. (Delayed response here because still not getting forum update notifications)

Does the vertical velocity show a steady negative at 6 secs because the bird is under steady descent? I figure a rocket scientist would know :-)

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9 months 4 days ago #11820 by DeltaVee
Replied by DeltaVee on topic Altimeter accelerometer anomalies?

Thanks John. Good catch on the no-chute release observation, according to the data. (Delayed response here because still not getting forum update notifications)

Does the vertical velocity show a steady negative at 6 secs because the bird is under steady descent? I figure a rocket scientist would know :-)

That's exactly right...you can see the curve as it crosses the x axis toward terminal negative (downward) velocity.
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9 months 8 hours ago - 9 months 8 hours ago #11825 by hermanjc
Replied by hermanjc on topic Altimeter accelerometer anomalies?
At work when we encounter significant noise in accelerometer data we will typically apply smoothing by taking a rolling average of acceleration data. I applied this to your data (averaging over 10 samples), and the magnitude of the spikes you see at 11 and 21 seconds seem to be at least in the ballpark with the rest of your data. It's possible that just the FS Mini whipping around a little or a breeze making it shift caused this noise.
 
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8 months 4 weeks ago #11831 by tlainhart
Replied by tlainhart on topic Altimeter accelerometer anomalies?
That's awesome Jeremy - thanks!

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