Copter: add collective stick low flag to catch bad setups for land detector

ArduCopter/Copter.h

@@ -553,6 +553,7 @@ private:
         uint8_t dynamic_flight          : 1;    // 0   // true if we are moving at a significant speed (used to turn on/off leaky I terms)
         uint8_t inverted_flight         : 1;    // 1   // true for inverted flight mode
         uint8_t in_autorotation         : 1;    // 2   // true when heli is in autorotation
+        bool coll_stk_low                  ;    // 3   // true when collective stick is on lower limit
     } heli_flags_t;
     heli_flags_t heli_flags;
 
@@ -761,6 +762,7 @@ private:
 #if MODE_AUTOROTATE_ENABLED == ENABLED
     void heli_set_autorotation(bool autotrotation);
 #endif
+    void update_collective_low_flag(int16_t throttle_control);
     // inertia.cpp
     void read_inertia();
 

ArduCopter/heli.cpp

@@ -136,6 +136,7 @@ bool Copter::should_use_landing_swash() const
 void Copter::heli_update_landing_swash()
 {
     motors->set_collective_for_landing(should_use_landing_swash());
+    update_collective_low_flag(channel_throttle->get_control_in());
 }
 
 // convert motor interlock switch's position to desired rotor speed expressed as a value from 0 to 1
@@ -237,4 +238,19 @@ void Copter::heli_set_autorotation(bool autorotation)
     motors->set_in_autorotation(autorotation);
 }
 #endif
+
+// update collective low flag.  Use a debounce time of 400 milliseconds.
+void Copter::update_collective_low_flag(int16_t throttle_control)
+{
+    static uint32_t last_nonzero_collective_ms = 0;
+    uint32_t tnow_ms = millis();
+
+    if (throttle_control > 0) {
+        last_nonzero_collective_ms = tnow_ms;
+        heli_flags.coll_stk_low = false;
+    } else if (tnow_ms - last_nonzero_collective_ms > 400) {
+        heli_flags.coll_stk_low = true;
+    }
+}
+
 #endif  // FRAME_CONFIG == HELI_FRAME

ArduCopter/land_detector.cpp

@@ -61,11 +61,12 @@ void Copter::update_land_detector()
     } else {
 
 #if FRAME_CONFIG == HELI_FRAME
-        // check for both manual collective modes and modes that use altitude hold. For manual collective (called throttle 
-        // because multi's use throttle), check that collective pitch is below mid collective (zero thrust) position.  For modes 
+        // check for both manual collective modes and modes that use altitude hold. For manual collective (called throttle
+        // because multi's use throttle), check that collective pitch is below land min collective position or throttle stick is zero.
+        // Including the throttle zero check will ensure the conditions where stabilize stick zero position was not below collective min. For modes
         // that use altitude hold, check that the pilot is commanding a descent and collective is at min allowed for altitude hold modes.
-        bool motor_at_lower_limit = ((flightmode->has_manual_throttle() && motors->get_below_land_min_coll() && fabsf(ahrs.get_roll()) < M_PI/2.0f) 
-                                    || (motors->limit.throttle_lower && pos_control->get_vel_desired_cms().z < 0.0f));
+        bool motor_at_lower_limit = ((flightmode->has_manual_throttle() && (motors->get_below_land_min_coll() || heli_flags.coll_stk_low) &&
+                                    fabsf(ahrs.get_roll()) < M_PI/2.0f) || (motors->limit.throttle_lower && pos_control->get_vel_desired_cms().z < 0.0f));
 #else
         // check that the average throttle output is near minimum (less than 12.5% hover throttle)
         bool motor_at_lower_limit = motors->limit.throttle_lower && attitude_control->is_throttle_mix_min();