[Python] Magnetometer Heading Calculation?

I am using MPU9250 from adafruit (https://learn.adafruit.com/adafruit-tdk-invensense-icm-20948-9-dof-imu)

This sensor is outputting raw data of magnetometer.

I was able to calibrate data from



to



where I get calibration number by

x offset = (max_x+min_x)/2
y offset = (max_y+min_y)/2
z offset = (max_z+min_z)/2


and I subtract offset to raw data.

However, I used calculation from (https://cdn-shop.adafruit.com/datasheets/AN203_Compass_Heading_Using_Magnetometers.pdf)

Direction (y>0) = 90 - [arcTAN(x/y)]*180/PI
Direction (y<0) = 270 - [arcTAN(x/y)]*180/PI
Direction (y=0, x<0) = 180.0
Direction (y=0, x>0) = 0.0


However, direction is alway off and incorrect.

for example, when I amd heaind 0 degree (directly to the north)

x 15.749999999999998
y -0.9749999999999996
z 47.4


and calculation returns 40.44 degrees.

When I am heading 90 degree (East)

x 4.049999999999999
y -14.475
z 46.949999999999996


heading is 84.906

When I am heading 270 degree (West)

x 1.3499999999999996
y 7.125
z 45.0


Heading is 75.774 Degree

When I am heading 180 degree (to South)

x -11.55
y -2.9250000000000007
z 46.5


and calculation result is 128.99 degrees.

Does my methodology to calculate heading is wrong? Or does my calibration need more steps?

[Edit]

Magnetometer calibration:

def calibrationDataCollection():
i2c = board.I2C() # uses board.SCL and board.SDA
icm = adafruit_icm20x.ICM20948(i2c)

lx = kbhit.lxTerm()
lx.start()
x=[]
y=[]
z=[]
while True:
#print(icm.magnetic)

if lx.kbhit():
c = lx.getch()
c_ord = ord(c)
if c_ord == 32: # Spacebar
print("\nStop")
break
x.append(icm.magnetic[0])
y.append(icm.magnetic[1])
z.append(icm.magnetic[2])

print((max(x)+min(x))/2)
print((max(y)+min(y))/2)
print((max(z)+min(z))/2)
print()
time.sleep(0.1)

comp_df = pd.DataFrame({"x": x, "y": y, "z": z})
comp_df["offset_x"] = (comp_df.x.max()+comp_df.x.min())/2
comp_df["offset_y"] = (comp_df.y.max()+comp_df.y.min())/2
comp_df["offset_z"] = (comp_df.z.max()+comp_df.z.min())/2
toCSV(analysisPath, "Calibration_2.csv", comp_df)


I calibrate the sensor by moving 8 shape in 3D (like infinite loop shape)

Then, whenever offset values become steady, I stop calibration and convert it to csv file.

Then, use offset value, I am getting calibrated value

def Compass_2():
i2c = board.I2C() # uses board.SCL and board.SDA
icm = adafruit_icm20x.ICM20948(i2c)


while True:
x = icm.magnetic[0]- 12.15 #(these are my calibration offsets)
y = -icm.magnetic[1]+ 6.225
z = icm.magnetic[2]+ 14.4
print("x ", x)
print("y ", y)
print("z ", z)

heading = magToHeading2([x,y,z])

print("heading: ", heading)
time.sleep(0.5)


The heading(magToHeading2) is calculated by

def magToHeading2(magnetic):
heading = -1

if magnetic[1] > 0:
heading = 90-math.atan(magnetic[0]/magnetic[1]) * 180 / math.pi
elif magnetic[1] < 0:
heading = 270-math.atan(magnetic[0]/magnetic[1]) * 180 / math.pi
elif magnetic[1] == 0:
if magnetic[0] < 0:
heading = 180.0
else:
heading = 0.0

return round(heading,3)

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