AP_AHRS: add get_accel_ef method to return earth frame accelerometer values for use in ArduCopter's inertial nav and accel based throttle

libraries/AP_AHRS/AP_AHRS.h

@@ -62,6 +62,9 @@ public:
 	    return _ins;
     }
 
+    // accelerometer values in the earth frame in m/s/s
+    Vector3f        get_accel_ef(void) { return _accel_ef; }
+
     // Methods
     virtual void update(void) = 0;
 
@@ -192,6 +195,9 @@ protected:
     // radians/s/s
     float _gyro_drift_limit;
 
+    // accelerometer values in the earth frame in m/s/s
+    Vector3f        _accel_ef;
+
     // acceleration due to gravity in m/s/s
     static const float _gravity = 9.80665;
 

libraries/AP_AHRS/AP_AHRS_DCM.cpp

@@ -396,8 +396,11 @@ AP_AHRS_DCM::drift_correction(float deltat)
     // apply trim
     temp_dcm.rotate(_trim);
 
+    // rotate accelerometer values into the earth frame
+    _accel_ef = temp_dcm * _accel_vector;
+
     // integrate the accel vector in the earth frame between GPS readings
-    _ra_sum += temp_dcm * (_accel_vector * deltat);
+    _ra_sum += _accel_ef * deltat;
 
     // keep a sum of the deltat values, so we know how much time
     // we have integrated over

libraries/AP_AHRS/AP_AHRS_MPU6000.cpp

@@ -70,6 +70,9 @@ AP_AHRS_MPU6000::update(void)
 
     // Calculate pitch, roll, yaw for stabilization and navigation
     euler_angles();
+
+    // prepare earth frame accelerometer values for ArduCopter Inertial Navigation and accel-based throttle
+    _accel_ef = _dcm_matrix * _ins->get_accel();
 }
 
 // wrap_PI - ensure an angle (expressed in radians) is between -PI and PI