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View Full Version : L3G4200D Gyroscope _ Measuring Angle!?



Jigar4Electronics
03-17-2013, 06:07 AM
Hello All,

I am making an IMU for 6DOF which has L3G4200D 3-Axis Digital Gyroscope.

I've checked so many Blogs, Forums, Application notes and have programmed accordingly to get the proper reading/information from my gyro.

Though I am very near to solve it - a single problem has made me mad since last two nights!
I've 16 bit digit - 1 signed bit + 15 bit value

Suppose I want to measure an angle with respect to X-axis at 2000 dps, for that I am doing the following:

X = X*70/1000; // X is of type int & having the 16bit value in it
Print(X); // to print X on UART

I am getting some values most probably <800 when I move gyro in counter clock direction with respect to X-axis and getting >4000 for the opposite direction!!

I want to have positive angle values while rotating it on counter clockwise direction and negative angle values for the clock wise direction... For that what I should add on my calculations?? [Angles should be in degrees 1 to 180 & (-1) to (-180)]

// in code 0.007 is in mdps which is the value of each LSB of gyro's data

What I should add in could and why I am having >4000 & <800 values right now?

Could anyone please help me with this?

Thank you so much for your valuable time.

Jigar4Electronics
03-17-2013, 08:37 AM
I've observed(on internet) that everyone is getting +/- readings while I am not getting it.
so I've tried it with different data type...
and now I am getting +/- readings at both the side of the axis..

but still my main question is unsloved!

Can you tell me what should I do to getting the angles now???
should I take x1 at time t1 and x2 at time t2 and then :
X_angle = (x2-x1)/(t2-t1)

Am I right???

jwatte
03-17-2013, 03:47 PM
You are probably treating the 16-bit value as an unsigned short, instead of a signed short.
Are you sure the value is sign bit plus mantissa? More normal would be two's complement, where the hex value 0xffff means "-1" and 0xfffe means "-2" and so on.

shimniok
08-22-2013, 12:28 AM
Agree on the signed/unsigned thing.

Also, what's the reading of the gyro when it's sitting perfectly still? The bias, that is. In my rover project, I did this to read L3G4200D values and convert to heading rate (on mbed, but it's just C/C++)

void Sensors::Read_Gyro()
{
_gyro.read(g); // this just does the I2C stuff to get the int16_t (short) values back
gTemp = (int) _gyro.readTemp();

gyro[_z_] = (g_sign[_z_] * (g[_z_] - g_offset[_z_])) / g_scale[_z_];


return;
}

Hope this helps.