AC_WPNav: check acceleration is non zero

libraries/AC_WPNav/AC_WPNav.cpp

@@ -56,7 +56,7 @@ const AP_Param::GroupInfo AC_WPNav::var_info[] PROGMEM = {
     // @DisplayName: Waypoint Acceleration 
     // @Description: Defines the horizontal acceleration in cm/s/s used during missions
     // @Units: cm/s/s
-    // @Range: 0 980
+    // @Range: 50 500
     // @Increment: 10
     // @User: Standard
     AP_GROUPINFO("ACCEL",       5, AC_WPNav, _wp_accel_cms, WPNAV_ACCELERATION),
@@ -281,6 +281,11 @@ void AC_WPNav::set_wp_origin_and_destination(const Vector3f& origin, const Vecto
         _pos_delta_unit = pos_delta/_track_length;
     }
 
+    // check _wp_accel_cms is reasonable
+    if (_wp_accel_cms <= 0) {
+        _wp_accel_cms.set_and_save(WPNAV_ACCELERATION);
+    }
+
     // initialise position controller speed and acceleration
     _pos_control.set_speed_xy(_wp_speed_cms);
     _pos_control.set_accel_xy(_wp_accel_cms);
@@ -526,6 +531,11 @@ void AC_WPNav::set_spline_origin_and_destination(const Vector3f& origin, const V
     // mission is "active" if wpnav has been called recently and vehicle reached the previous waypoint
     bool prev_segment_exists = (_flags.reached_destination && ((hal.scheduler->millis() - _wp_last_update) < 1000));
 
+    // check _wp_accel_cms is reasonable to avoid divide by zero
+    if (_wp_accel_cms <= 0) {
+        _wp_accel_cms.set_and_save(WPNAV_ACCELERATION);
+    }
+
     // segment start types
     // stop - vehicle is not moving at origin
     // straight-fast - vehicle is moving, previous segment is straight.  vehicle will fly straight through the waypoint before beginning it's spline path to the next wp