Accelerometer data interpretation

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1. Jun 22, 2017

Cota

• Member advised to use the homework template for posts in the homework sections of PF.
Hello, this is my first experience in this forum. :)

I was wondering if someone can help me understanding my accelerometer data installed in a project consisting in an object that is dropped in the air from an altitude of approximately 1000 meters. The descent is controlled by a parachute.

Well, while at the ground level I am getting the normal values witch are 0m/s2 for X and Y axis and 9,8m/s2 for Z axis. So when the object is dropped I get readings in the Z axis from 11 to 14 m/s2 (average 12.2m/s2). This value seem to be not consistent with my idea that a falling object with a parachute cannot have an acceleration grater then gravity acceleration.

Can anyone help me with this?

D Cot@

PS-I am sorry about my English.

2. Jun 22, 2017

FactChecker

two things to check:
1) When the parachute drop has reached a steady decent rate, the acceleration should be the same as if not dropping at all since there is no longer downward acceleration.
2) Is the higher reading when the parachute is slowing the downward decent? That would be a greater acceleration -- not just holding steady but decreasing a downward velocity.

3. Jun 22, 2017

Cota

Well, I am sorry but I didn't understand your first point.

Regarding the second point, yes, the higher readings are when during the descending with the parachute opened.

My accelerometer is a Adafruit 9 DOF L3DG20H gyroscope + LSM303DLHC accelerometer/compass sensors:

The mass of the object is 316 grams and the average descending velocity is 9.42 m/s.

Thank you very much.

4. Jun 22, 2017

FactChecker

Once the accelerometer and parachute has reached a steady decent rate, R, there is no more acceleration in the downward direction -- just a constant downward velocity. When that has happened, the accelerometer reading should be the same as it was before it was dropped.

If the accelerometer is dropped without the parachute fully open, it might get a higher downward velocity than the steady parachute decent rate R. When the parachute opens and begins to slow the decent, the acceleration numbers would be higher than before it was dropped.

5. Jun 22, 2017

Cota

You wrote:
"Once the accelerometer and parachute has reached a steady decent rate, R, there is no more acceleration in the downward direction -- just a constant downward velocity. When that has happened, the accelerometer reading should be the same as it was before it was dropped."

Yes, they are almost the same. There is a slight diference (about 0.15m/s2) maybe because of airplane vibration/movement.

Then, you wrote:
"If the accelerometer is dropped without the parachute fully open, it might get a higher downward velocity than the steady parachute decent rate R. When the parachute opens and begins to slow the decent, the acceleration numbers would be higher than before it was dropped."

Considering the underlined part, can you tell me why?

Meanwhile I did another thinking and calculations, considering the 3 axis values not the system acceleration:

During the descending I have this average values: X=0,46; Y=-1,45; Z=9,66m/s2. This seams to be ok.

Then considering the three axis and calculating the system acceleration: (ax^2+ay^2+az^2)^0,5 = 9,94m/s2

For the flight I have similar axis values, but I have X with 9,51m/s2 because the sensor is 90 degrees rotated in the drop tube.

Does this values seem consistent to you?

Thank you.

D Cot@

6. Jun 22, 2017

FactChecker

If the object is at a constant height, not dropped, the accelerometer will detect the acceleration of gravity. If the object is dropping at a velocity V1 but being slowed by the parachute to a lower velocity V2 < V1, that adds acceleration in the upward direction compared with the non-dropped object. So the accelerometer should read a larger number.
I don't understand what you are saying.

7. Jun 23, 2017

Cota

Thank you very much again.

I think I am starting to understand. So when the object hits the ground I should expect a much lower value then 9,8m/s2, correct? Because of the high force on the upward direction. So if the accelerometer gives, for example, 2m/s2, can I conclude that the acceleration of the system when he hits the ground is 7,8m/s2 (9,8-2)?

To simplify I am just considering the z axis (vertical).

Cot@

8. Jun 23, 2017

FactChecker

When the object hits the ground, you should expect a large acceleration number at the instant of impact. After that, on the ground you should get the 9.8 m/s2.