# Madgwick sensor fusion algorithm on iOS

I am trying to run Madgwick's sensor fusion algorithm on iOS. Since the code is open source, I have already included it in my project and call methods with the provided sensor values.

But it looks like the algorithm is expecting the sensor to measure in a different coordinate system. On the right side is the Apple CoreMotion sensor system, on the left is Madgewick. Here is a picture of the different coordinate systems . Both systems follow the right-hand rule. To me it is like rotating 90 degrees around the z-axis. But it didn't work.

I also tried flipping the x and y axes (and inverting the z) as suggested by this stackoverflow question for WP, but that didn't work either. So do you have a hint? It would be ideal if the output of Alogithm Madgwick could be on the same system as the output of CoreMotion (CMAttitudeReferenceFrameXMagneticNorthZVertical).

Also, I'm looking for a good working value for **betaDef** on an iPhone. **betaDef** is the proportional gain and is currently set to 0.1f.

Any help in achieving the goal will be appreciated.

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I'm not sure how to write this in lens c, but this is how I accomplished coordinate transformations in vanilla c. I also wanted to rotate the orientation so that + y is north. This translation is also reflected below.

This method expects a 4-element quaternion as wxyz and returns the translated quaternion in the same format:

```
void madgeq_to_openglq(float *fMadgQ, float *fRetQ) {
float fTmpQ[4];
// Rotate around Z-axis, 90 degres:
float fXYRotationQ[4] = { sqrt(0.5), 0, 0, -1.0*sqrt(0.5) };
// Inverse the rotation vectors to accomodate handedness-issues:
fTmpQ[0] = fMadgQ[0];
fTmpQ[1] = fMadgQ[1] * -1.0f;
fTmpQ[2] = fMadgQ[2];
fTmpQ[3] = fMadgQ[3] * -1.0f;
// And then store the translated Rotation into ret:
quatMult((float *) &fTmpQ, (float *) &fXYRotationQ, fRetQ);
}
// Quaternion Multiplication operator. Expects its 4-element arrays in wxyz order
void quatMult(float *a, float *b, float *ret) {
ret[0] = (b[0] * a[0]) - (b[1] * a[1]) - (b[2] * a[2]) - (b[3] * a[3]);
ret[1] = (b[0] * a[1]) + (b[1] * a[0]) + (b[2] * a[3]) - (b[3] * a[2]);
ret[2] = (b[0] * a[2]) + (b[2] * a[0]) + (b[3] * a[1]) - (b[1] * a[3]);
ret[3] = (b[0] * a[3]) + (b[3] * a[0]) + (b[1] * a[2]) - (b[2] * a[1]);
return;
}
```

Hope it helps!

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