Plane: added Q_ASSIST_ALT parameter

this allows for quadplane assistance in fixed wing modes when the
aircraft drops below a given altitude. This could help save an
aircraft that is flying badly in fixed wing mode

ArduPlane/quadplane.cpp

@@ -461,6 +461,15 @@ const AP_Param::GroupInfo QuadPlane::var_info2[] = {
     // @User: Advanced
     AP_GROUPINFO("TKOFF_ARSP_LIM", 15, QuadPlane, maximum_takeoff_airspeed, 0),
 
+    // @Param: ASSIST_ALT
+    // @DisplayName: Quadplane assistance altitude
+    // @Description: This is the altitude below which quadplane assistance will be triggered. This acts the same way as Q_ASSIST_ANGLE and Q_ASSIST_SPEED, but triggers if the aircraft drops below the given altitude while the VTOL motors are not running. A value of zero disables this feature. The altutude is calculated as being above ground level. The height above ground is given from a Lidar used if available and RNGFND_LANDING=1. Otherwise it comes from terrain data if TERRAIN_FOLLOW=1 and comes from height above home otherwise.
+    // @Units: m
+    // @Range: 0 120
+    // @Increment: 1
+    // @User: Standard
+    AP_GROUPINFO("ASSIST_ALT", 16, QuadPlane, assist_alt, 0),
+
     AP_GROUPEND
 };
 
@@ -1377,6 +1386,7 @@ bool QuadPlane::assistance_needed(float aspeed)
         angle_error_start_ms = 0;
         return false;
     }
+
     if (aspeed < assist_speed) {
         // assistance due to Q_ASSIST_SPEED
         in_angle_assist = false;
@@ -1384,6 +1394,31 @@ bool QuadPlane::assistance_needed(float aspeed)
         return true;
     }
 
+    const uint32_t now = AP_HAL::millis();
+
+    /*
+      optional assistance when altitude is too close to the ground
+     */
+    if (assist_alt > 0) {
+        float height_above_ground = plane.relative_ground_altitude(plane.g.rangefinder_landing);
+        if (height_above_ground < assist_alt) {
+            if (alt_error_start_ms == 0) {
+                alt_error_start_ms = now;
+            }
+            if (now - alt_error_start_ms > 500) {
+                // we've been below assistant alt for 0.5s
+                if (!in_alt_assist) {
+                    in_alt_assist = true;
+                    gcs().send_text(MAV_SEVERITY_INFO, "Alt assist %.1fm", height_above_ground);
+                }
+                return true;
+            }
+        } else {
+            in_alt_assist = false;
+            alt_error_start_ms = 0;
+        }
+    }
+
     if (assist_angle <= 0) {
         in_angle_assist = false;
         angle_error_start_ms = 0;
@@ -1412,7 +1447,7 @@ bool QuadPlane::assistance_needed(float aspeed)
         in_angle_assist = false;
         return false;
     }
-    const uint32_t now = AP_HAL::millis();
+
     if (angle_error_start_ms == 0) {
         angle_error_start_ms = now;
     }

ArduPlane/quadplane.h

@@ -280,6 +280,11 @@ private:
     AP_Int8 assist_angle;
     uint32_t angle_error_start_ms;
 
+    // altitude to trigger assistance
+    AP_Int16 assist_alt;
+    uint32_t alt_error_start_ms;
+    bool in_alt_assist;
+
     // maximum yaw rate in degrees/second
     AP_Float yaw_rate_max;