[CMPS2 compass module with Arduino - Heading steps accuracy]

Thanks @JColvinfor the quick reply! I've been trying to modify the code in various ways for while now but still don't quite get it. Here's what I'm thinking/wondering so far:

On 4/6/2021 at 11:26 PM, JColvin said:

The CMPS2_init() sets the global maximum variable to -32768, and the global minimum variable to 32767. This mismatch causes problems later. Global midpoint is set to 0.

Could this be fixed by just setting the globlal minimum value to 32768? Or does it even matter considering the lines where it's actually used look like this?

if (measured_data[0] < Min[0]) { //x min
    Min[0] = measured_data[0];
  }

 

On 4/6/2021 at 11:26 PM, JColvin said:

In the CMPS2_getHeading() a Reset function is performed (to get an opposite phase shift I guess) after bullet point 2 and then CMPS_read_XYZ is performed again. However, because the global max and min values have not been reset, the new measurement data is getting compared to the exact same data. I guess in theory if you continually repeated this process as the module was rotated around, you would get the max and min values for the X and Y axis, but this isn't done in the code. So the max or minimum may have changed slightly, but not both, and the global midpoint is the bifurcation of this potential small change. I might be misunderstanding the code though.

I'm wondering if it would be possible at all to get an accurate heading without rotating it 360 degrees first for calibration? I don't see how that would work since it won't have any values to compare with unless it gets rotated.

 

On 4/6/2021 at 11:26 PM, JColvin said:

In the CMPS2_getHeading() after bullet point 3, it says offset is eliminated between the two measurements, but all it does is assign the calculated offset to a temporary variable, component[i].

Looking at the guide for calibration here https://reference.digilentinc.com/reference/pmod/pmodcmps2/reference-manual (and assuming it's correct) where it says

Output1 = H + Offset

 

Output2 = −H + Offset

I think the math in the code makes sense since the line

components[0] = (components[0] - X) / 2.0; // H = (Output1 - Output2) / 2.0

should give just H ( H = (Output1 - Output2) / 2.0 ), or am I missing something? (where H is the desired measured value to use in the trigonometry calculations)

 

 

Another thing I've noticed is the line 

measured_data[0] = 1.0 * (int)(tmp[1] << 8 | tmp[0]); //x

which supposedly reconstructs the raw data. Doesn't interpreting the 16 bit binary number as an int cause overflows if the MSB from the sensor is unsigned? Or is the raw data supposed to be signed? Is the leftmost bit of tmp[1] part of the value or just indicating the sign?

When plotting the reconstructed raw data I'm seeing these overflow jumps (?) every now and then when rotating the compass around the z-axis.

 

As before, all help is greatly appreciated!

[CMPS2 compass module with Arduino - Heading steps accuracy]

Hi!

I'm using the CMPS2 compass module with an Arduino Uno and the example code from https://reference.digilentinc.com/reference/pmod/pmodcmps2/start#example_projects to get a heading from the compass module, but it seems to be only jumping between specific values and never taking the values in between. For example, running the program while keeping the module horizontal and slowly spinnng it 360° around the z-axis gives an output like this:

Heading = 231.18°    
Heading = 231.19°    
Heading = 186.09°    
Heading = 141.09°    
Heading = 186.12°    
Heading = 231.10°    
Heading = 231.09°    
Heading = 231.08°    
Heading = 231.09°    
Heading = 231.03°    
Heading = 230.96°    
Heading = 321.25°    
Heading = 321.15°    
Heading = 321.04°    
Heading = 51.20°    
Heading = 51.06°    
Heading = 51.06°    
Heading = 50.96°    
Heading = 50.91°    
Heading = 96.12°    
Heading = 96.10°    
Heading = 51.00°    
Heading = 141.17°    
Heading = 141.11°    
Heading = 141.12°    
Heading = 141.04°    
Heading = 141.04°    
Heading = 186.11°    
Heading = 141.01°    
Heading = 231.16°    
Heading = 231.16°    
Heading = 231.16°    

Basically, plotting the heading as a function of time doesn't give a continuos plot no matter how slowly the module rotates.

I have tried to check for disturbances from metal objects or electronic devices using another compass, so I'm pretty sure that's not the problem.

I don't need to get down to the 1° accuracy promised by the maker (5° is probably enough for my project), but these giant leaps from 320° to 50° won't work.

All help is appreciated!