How do I figure out the orientation of a accellerometer?

Question

Hi,

I am trying to make my own "TrakDot" luggage tracking device, and for that I am gonna make the feature that disables all the antennas when the airplane leaves the ground (accelerates) and when it lands again (decelerates).

But since I don't have a lot of experience with accellerometers, I can't really figure out how to approach the following:

How can I figure out what orientation the accellerometer is in? I mean, if I put it in my suitcase, it could be pointing north, south, east, west, 45* angle or be upside down after the baggage-guys have thrown it around a few times.. How do I know/register that the airplane accelerates when I don't know if it's accelerating on X or Y or somewhere in between?

I am using the MPU-6050MPU-6050 module which also has an integrated gyroscope maybe somehow that can help me determine on what axis I should measure the acceleration

Thanks!

-Kenneth

michaelkellett · Accepted Answer

It's no at all easy to work out when a plane leaves the ground or lands just by acceleration. You could probably detect the take off by looking at acceleration along the ground but it would be very easy to get false triggers for landing. You would need to record lots of flight data and then devise a way of processing the data to correctly identify what was happening. Adding a pressure sensor might help (if it's in the hold). It is easy to work out the orientation of a DC coupled accelerator if it isn't being accelerated (it might be hard to know this) - but you can vector sum the readings from the XYZ axes and this (when the accelerometer is at constant velocity) will give you the angle of the earth's 1g gravity relative to the accelerometer. The accelerometers and gyros in MPU-6050 MPU-6050 are not good enough to do classical inertial guidance but they can work in sensor fusion type applications to give useful information InvenSense may have some applications data that will help otherwise expect to do a lot of Googling and reading MK

dougw · Answer

Interesting problem.

The only constant acceleration is gravity, all other accelerations are transient, so when the acceleration doesn't change for long periods of time it means the bag is stationary or at constant velocity.

After takeoff the plane climbs for many minutes at an angle of 10-15 degrees. This is much longer than any conveyor belt trip, so it is fairly easy to detect. But it means the steady state (cruising) will take at least 45 minutes to stabilize. Sitting on the tarmac before takeoff will also be much longer than any baggage handling accelerations, so you will know which way id down before takeoff.

On landing aircraft decelerate at 3-6 knots each second (about 1/4 g) for 10 - 20 seconds, which is longer than any turbulence, so it should be detectable. It is also in the opposite direction from the takeoff acceleration.

An alternative is just to set a timer for the ETA to turn the radio on.

How do I figure out the orientation of a accellerometer?

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