Compass: implement Bills new offset nulling algorithm

this seems to work much better than the old algorithm, converging
faster and more accurately. Even better, it has no linkage to DCM, so
no possibility of nasty feedback effects

libraries/AP_Compass/Compass.cpp

@@ -182,38 +182,62 @@ Compass::calculate(const Matrix3f &dcm_matrix)
 #endif
 }
 
+
+/*
+  this offset nulling algorithm is based on a paper from
+   Bill Premerlani
+
+  http://gentlenav.googlecode.com/files/MagnetometerOffsetNullingRevisited.pdf
+ */
 void
-Compass::null_offsets(const Matrix3f &dcm_matrix)
+Compass::null_offsets(void)
 {
     if (_null_enable == false || _learn == 0) {
         // auto-calibration is disabled
         return;
     }
 
-    // Update our estimate of the offsets in the magnetometer
-    Vector3f    calc;
-    Matrix3f    dcm_new_from_last;
-    float       weight;
-
     Vector3f mag_body_new = Vector3f(mag_x,mag_y,mag_z);
 
-    if(_null_init_done) {
+    // this gain is set so we converge on the offsets in about 5
-        dcm_new_from_last = dcm_matrix.transposed() * _last_dcm_matrix;      // Note 11/20/2010: transpose() is not working, transposed() is.
+    // minutes with a 10Hz compass
+    const float gain = 0.5;
+    const float max_change = 2.0;
 
-        weight = 3.0 - fabs(dcm_new_from_last.a.x) - fabs(dcm_new_from_last.b.y) - fabs(dcm_new_from_last.c.z);
+    if (!_null_init_done) {
-        if (weight > .001) {
+        // first time through
-            calc = mag_body_new + _mag_body_last;                // Eq 11 from Bill P's paper
-            calc -= dcm_new_from_last * _mag_body_last;
-            calc -= dcm_new_from_last.transposed() * mag_body_new;
-            if(weight > 0.5) weight = 0.5;
-            calc = calc * (weight);
-            _offset.set(_offset.get() - calc);
-        }
-    } else {
         _null_init_done = true;
+        _mag_body_last = mag_body_new;
+        return;
     }
-    _mag_body_last = mag_body_new - calc;
+
-    _last_dcm_matrix = dcm_matrix;
+    Vector3f delta, diff;
+    float diff_length, delta_length;
+
+    diff = mag_body_new - _mag_body_last;
+    diff_length = diff.length();
+    if (diff_length == 0.0) {
+        // the mag vector hasn't changed - we don't get any
+        // information from this
+        return;
+    }
+
+    // equation 6 of Bills paper
+    delta = diff * (mag_body_new.length() - _mag_body_last.length()) / diff_length;
+
+    // limit the change from any one reading. This is to prevent
+    // single crazy readings from throwing off the offsets for a long
+    // time
+    delta_length = delta.length();
+    if (delta_length > max_change) {
+        delta *= max_change / delta_length;
+    }
+
+    // set the new offsets
+    _offset.set(_offset.get() - (delta * gain));
+
+    // remember the last mag vector
+    _mag_body_last = mag_body_new;
 }
 
 

libraries/AP_Compass/Compass.h

@@ -97,11 +97,9 @@ public:
 	///
 	void set_offsets(int x, int y, int z) { set_offsets(Vector3f(x, y, z)); }
 
-	/// Perform automatic offset updates using the results of the DCM matrix.
+	/// Perform automatic offset updates
 	///
-	/// @param  dcm_matrix          The DCM result matrix.
+	void null_offsets(void);
-	///
-	void null_offsets(const Matrix3f &dcm_matrix);
 
 
 	/// Enable/Start automatic offset updates 
@@ -134,7 +132,6 @@ protected:
 
 	bool                _null_enable;        	///< enabled flag for offset nulling
 	bool                _null_init_done;        ///< first-time-around flag used by offset nulling
-	Matrix3f            _last_dcm_matrix;       ///< previous DCM matrix used by offset nulling
+	Vector3f            _mag_body_last;         ///< used by offset correction
-	Vector3f            _mag_body_last;         ///< ?? used by offset nulling
 };
 #endif