Copter: smooth waypoint transitions

Speed of intermediate point is initialised and constrained based on
current speed vector vs direction to the next waypoint.
This means the copter does not slow if the previous segment and next
segment are in line and also the intermediate point is not advanced
towards the next waypoint if the copter is moving quickly in the
opposite direction.

libraries/AC_WPNav/AC_WPNav.cpp

@@ -311,8 +311,10 @@ void AC_WPNav::set_origin_and_destination(const Vector3f& origin, const Vector3f
     _target = origin;
     _flags.reached_destination = false;
 
-    // reset limited speed to zero to slow initial acceleration away from wp
-    _limited_speed_xy_cms = 0;
+    // initialise the limited speed to current speed along the track
+    Vector3f curr_vel = _inav->get_velocity();
+    float speed_along_track = curr_vel.x * _pos_delta_unit.x + curr_vel.y * _pos_delta_unit.y + curr_vel.z * _pos_delta_unit.z;
+    _limited_speed_xy_cms = constrain_float(speed_along_track,0,_wp_speed_cms);
 
     // default waypoint back to slow
     _flags.fast_waypoint = false;
@@ -338,12 +340,35 @@ void AC_WPNav::advance_target_along_track(float dt)
     curr_delta.z = curr_delta.z * _vert_track_scale;
     curr_delta_length = curr_delta.length();
 
-    // increase intermediate target point's velocity if not yet at target speed
-    if(dt > 0 && _limited_speed_xy_cms < _wp_speed_cms) {
-        _limited_speed_xy_cms += WPNAV_WP_ACCELERATION * dt;
+    // get current velocity
+    Vector3f curr_vel = _inav->get_velocity();
+    // get speed along track
+    float speed_along_track = curr_vel.x * _pos_delta_unit.x + curr_vel.y * _pos_delta_unit.y + curr_vel.z * _pos_delta_unit.z;
+
+    // calculate point at which velocity switches from linear to sqrt
+    float linear_velocity = _wp_speed_cms;
+    float kP = _pid_pos_lat->kP();
+    if (kP >= 0.0f) {   // avoid divide by zero
+        linear_velocity = MAX_LOITER_POS_ACCEL/kP;
     }
-    if(_limited_speed_xy_cms > _wp_speed_cms) {
-        _limited_speed_xy_cms = _wp_speed_cms;
+
+    // let the limited_speed_xy_cms be some range above or below current velocity along track
+    if (speed_along_track < -linear_velocity) {
+        // we are travelling fast in the opposite direction of travel to the waypoint so do not move the intermediate point
+        _limited_speed_xy_cms = 0;
+    }else{
+        // increase intermediate target point's velocity if not yet at target speed (we will limit it below)
+        if(dt > 0 && _limited_speed_xy_cms < _wp_speed_cms) {
+            _limited_speed_xy_cms += WPNAV_WP_ACCELERATION * dt;
+        }
+        // do not go over top speed
+        if(_limited_speed_xy_cms > _wp_speed_cms) {
+            _limited_speed_xy_cms = _wp_speed_cms;
+        }
+        // if our current velocity is within the linear velocity range limit the intermediate point's velocity to be no more than the linear_velocity above or below our current velocity
+        if (fabsf(speed_along_track) < linear_velocity) {
+            _limited_speed_xy_cms = constrain_float(_limited_speed_xy_cms,speed_along_track-linear_velocity,speed_along_track+linear_velocity);
+        }
     }
 
     // calculate how far along the track we are
@@ -558,9 +583,6 @@ void AC_WPNav::reset_I()
     // reset target velocity - only used by loiter controller's interpretation of pilot input
     _target_vel.x = 0;
     _target_vel.y = 0;
-
-    // reset limited speed to zero to slow initial acceleration
-    _limited_speed_xy_cms = 0;
 }
 
 /// calculate_wp_leash_length - calculates horizontal and vertical leash lengths for waypoint controller