Magnetometer Heading Calculation?

Question

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

enter image description here

to

enter image description here

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)

• jasonharper: Please show the exact code you're using. I cannot replicate your results using only those formulas you posted.

Answer 1

The problem was calibration. Eventhough after calibration data are all well centered, whenever I rotate the sensor with horizonal surface, the data were not well cenetered. So, instead of shaking around the sensor for calibraiton, I just put sensor on the table and rotate around (since I am using only xy for heading).

• st2000: To make a compass you also need to normalize the magnitude of the 3 magnetometer sensors. I don't think I saw that in your code. Also you can use atan2 instead of that last bit of code you posted. Atan2 will return -180 to 180 (or -Pi to +Pi). This gets you 90 to 99 percent there. The last bit can be thought of as the correction of an imperfectly aligned magnetometer. The result is called a 3 by 3 soft iron correction matrix. And the explanation will not fit into the space of a comment. Lastly, if you want to create a usable compass you need to add an accelerometer and a bit of sensor fusion.