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08-31-2018 06:08 AM

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Hey, as I am working with a free fall detection code with Accelerometer with I2C connection with Arduino, I searching for the tutorial which is used to help me monitor the output with these kinds of sensors. Any useful suggestion for these kinds of work will be very helpful.

08-31-2018 07:58 AM

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I supervised some BME Graduate Students who were working with a little triaxial accelerometer to measure muscle twitches. The accelerometer put out three voltages for accelerations along X, Y, and Z axes. The axes were (by chip design) reasonably orthogonal and linear, but the offsets and gains of the three channels were specified ±10%! Not so accurate! The concept of __calibration__ just didn't occur to them (and these were __graduate__ students!).

So, have you calibrated your accelerometer? How many axes does it have? If you have a linear triaxial accelerometer __that has been calibrated__, then your task is basically complete -- the accelerometer is in free-fall when the net acceleration (measured as a vector) is 0. You can calibrate a linear triaxial accelerometer by taking six measurements under easily-realized experimental conditions (requiring no additional equipment except your hand and a benchtop) and a little thought (and some math).

Bob Schor

09-03-2018 08:57 AM

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Hello Bob

Thanks for the reply, according to the datasheet it can work as 3D accelerometer and 3D gyroscope and I want to test the gravitation force by using free-fall detection but I am confused how it will be possible to use it by testing free fall detection

09-03-2018 06:07 PM

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@Robiejo32 wrote:

Hello Bob

Thanks for the reply, according to the datasheet it can work as 3D accelerometer and 3D gyroscope and I want to test the gravitation force by using free-fall detection but I am confused how it will be possible to use it by testing free fall detection

I don't understand the concept of "free fall detection". Ah, having just written that sentence, maybe you mean "detect if the accelerometer is falling", i.e. if the __net acceleration__ it is experiencing is less than 1g. Is that what you mean?

Is your question really "Given a 3-axis accelerometer, one that gives me the acceleration along three mutually orthogonal axes, how do I get the __magnitude__ of the Acceleration Vector given by the components along these three axes"? If you know the X, Y, and Z components of a vector (e.g. the acceleration along the accelerometer's X, Y, and Z axes), do you know how to compute the magnitude of this vector?

I believe I said earlier that if you have a __calibrated__ Accelerometer (meaning all three Axes give you readings that are +1 when oriented (call this "up"), -1 when anti-parallel (call this "down") and 0 when perpendicular to gravity (call this "horizontal"), then you've basically got your answer. But you need to know how to combine the X, Y, and Z components of the gravity vector to get the net acceleration, which should be 1g if the accelerometer is "at rest", and <1g if it is falling.

If I've missed your point, feel free to write a few paragraphs explaining what you are trying to do, and why I "don't get it".

Bob Schor