android - alpha calculation in complementary filter -

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i using complementary filter sensor fusion. equation complementary filter using is

angle=(1-alpha)*(angle+gyro*dt)+(alpha)*(accel_mag_angle)

i have confusion calculation of alpha. read somewhere alpha calculated using formula 

alpha= da/(da+dt)

where da rate @ want values change (which refers cutoff frequency of filter) , dt sampling time. using sensor_delay_ui. sampling rate of ui around 15hz. used alpha=0.2(by trial , error method) , app worked fine. later realised according equation cannot use alpha=0.2(theoretically) since give me cut off frequency of 60hz , sampling frequency 15hz. cut off frequency calculation wrong ? or understanding of equation wrong ?

i wouldn't sweat it. if find alpha=0.2 satisfactory trial , error, use it. filters have tuned in practice anyway. theory helps start (usually) cannot avoid tuning.

i don't see wrong 60 hz cut off frequency. means motions above frequency cut off. , thing, since noise anyway.


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