Hi, all!
Place the MMA7361 on the desk and tighten it, keep it actionless.
I measured the X, Y, Z voltage. They are 1.63V, 1.71V, 2.25V
VDD is 3.3V
So, Ax(acc of X) = (1.63-1.65)/0.8 = -0.025
Ay(acc of Y) = 0.075
Az = 0.75
But (-0.025)^2 + 0.075^2 + 0.75^2 <> 1
Why?
Hello Gary Wang,
The magnitude of a resulting vector is the square root of the sum of its three vectors (X,Y and Z). After obtaining the square root of your sum, you'd get 0.75:
SQRT((-0.025)^2 + 0.075^2 + 0.75^2) = 0.75
Remember that you’re using the MMA7361LC with a +/-1.5g range, so, you’d need to multiply the magnitude of the resulting vector by the selected range:
0.75*1.5g= 1.13g
A non accurate voltage measurement or the specific device offset could cause the extra 0.13g, I'd recommend you to use at least a 10bit ADC.
For optimal 0g offset performance, you could implement “Auto-Zero”, a software technique to calibrate accelerometers:
Implementing Auto-Zero Calibration Technique for Accelerometers
http://cache.freescale.com/files/sensors/doc/app_note/AN3447.pdf
Hope it helps!
Josh