AP_AHRS: internalise application of correct_centrifugal

This is a DCM-only option but that's not clear with the interface.
Internalise the variable and base it off build type.

libraries/AP_AHRS/AP_AHRS_Backend.h

@@ -38,10 +38,7 @@ class AP_AHRS_Backend
 public:
 
     // Constructor
-    AP_AHRS_Backend() {
+    AP_AHRS_Backend() {}
-        // enable centrifugal correction by default
-        _flags.correct_centrifugal = true;
-    }
 
     // empty virtual destructor
     virtual ~AP_AHRS_Backend() {}
@@ -281,17 +278,6 @@ public:
         return _flags.have_initial_yaw;
     }
 
-    // set the correct centrifugal flag
-    // allows arducopter to disable corrections when disarmed
-    void set_correct_centrifugal(bool setting) {
-        _flags.correct_centrifugal = setting;
-    }
-
-    // get the correct centrifugal flag
-    bool get_correct_centrifugal(void) const {
-        return _flags.correct_centrifugal;
-    }
-
     // helper trig value accessors
     float cos_roll() const  {
         return _cos_roll;
@@ -494,7 +480,6 @@ protected:
     // flags structure
     struct ahrs_flags {
         uint8_t have_initial_yaw        : 1;    // whether the yaw value has been intialised with a reference
-        uint8_t correct_centrifugal     : 1;    // 1 if we should correct for centrifugal forces (allows arducopter to turn this off when motors are disarmed)
         uint8_t wind_estimation         : 1;    // 1 if we should do wind estimation
     } _flags;
 

libraries/AP_AHRS/AP_AHRS_DCM.cpp

@@ -609,6 +609,20 @@ Vector3f AP_AHRS_DCM::ra_delayed(uint8_t instance, const Vector3f &ra)
     return ret;
 }
 
+/* returns true if attitude should be corrected from GPS-derived
+ * velocity-deltas.  We turn this off for Copter and other similar
+ * vehicles while the vehicle is disarmed to avoid the HUD bobbing
+ * around while the vehicle is disarmed.
+ */
+bool AP_AHRS_DCM::should_correct_centrifugal() const
+{
+#if APM_BUILD_TYPE(APM_BUILD_ArduCopter) || APM_BUILD_TYPE(APM_BUILD_ArduSub) || APM_BUILD_TYPE(APM_BUILD_Blimp)
+    return hal.util->get_soft_armed();
+#endif
+
+    return true;
+}
+
 // perform drift correction. This function aims to update _omega_P and
 // _omega_I with our best estimate of the short term and long term
 // gyro error. The _omega_P value is what pulls our attitude solution
@@ -762,7 +776,7 @@ AP_AHRS_DCM::drift_correction(float deltat)
     bool using_gps_corrections = false;
     float ra_scale = 1.0f/(_ra_deltat*GRAVITY_MSS);
 
-    if (_flags.correct_centrifugal && (_have_gps_lock || fly_forward)) {
+    if (should_correct_centrifugal() && (_have_gps_lock || fly_forward)) {
         const float v_scale = gps_gain.get() * ra_scale;
         const Vector3f vdelta = (velocity - _last_velocity) * v_scale;
         GA_e += vdelta;

libraries/AP_AHRS/AP_AHRS_DCM.h

@@ -156,6 +156,13 @@ private:
     // P term yaw gain based on rate of change of horiz velocity
     float           _yaw_gain(void) const;
 
+    /* returns true if attitude should be corrected from GPS-derived
+     * velocity-deltas.  We turn this off for Copter and other similar
+     * vehicles while the vehicle is disarmed to avoid the HUD bobbing
+     * around while the vehicle is disarmed.
+     */
+    bool should_correct_centrifugal() const;
+
     // state to support status reporting
     float _renorm_val_sum;
     uint16_t _renorm_val_count;