mc_att_control: keep integral enabled based on land detector

Previously the rate controller disabled updating the integral below 20%
throttle. This is not ideal for several reasons:
- some racers already hover with 20% throttle.
- for acro it is important to always keep the integral enabled, it has a
  noticeable effect on flight performance.

src/modules/mc_att_control/mc_att_control.hpp

@@ -55,6 +55,7 @@
 #include <uORB/topics/vehicle_control_mode.h>
 #include <uORB/topics/vehicle_rates_setpoint.h>
 #include <uORB/topics/vehicle_status.h>
+#include <uORB/topics/vehicle_land_detected.h>
 
 /**
  * Multicopter attitude control app start / stop handling function
@@ -99,6 +100,7 @@ private:
 	void		battery_status_poll();
 	void		parameter_update_poll();
 	void		sensor_bias_poll();
+	void		vehicle_land_detected_poll();
 	void		sensor_correction_poll();
 	void		vehicle_attitude_poll();
 	void		vehicle_attitude_setpoint_poll();
@@ -136,6 +138,7 @@ private:
 	int		_sensor_gyro_sub[MAX_GYRO_COUNT];	/**< gyro data subscription */
 	int		_sensor_correction_sub{-1};	/**< sensor thermal correction subscription */
 	int		_sensor_bias_sub{-1};		/**< sensor in-run bias correction subscription */
+	int		_vehicle_land_detected_sub{-1};	/**< vehicle land detected subscription */
 
 	unsigned _gyro_count{1};
 	int _selected_gyro{0};
@@ -160,6 +163,7 @@ private:
 	struct sensor_gyro_s			_sensor_gyro {};	/**< gyro data before thermal correctons and ekf bias estimates are applied */
 	struct sensor_correction_s		_sensor_correction {};	/**< sensor thermal corrections */
 	struct sensor_bias_s			_sensor_bias {};	/**< sensor in-run bias corrections */
+	struct vehicle_land_detected_s		_vehicle_land_detected {};
 
 	MultirotorMixer::saturation_status _saturation_status{};
 

src/modules/mc_att_control/mc_att_control_main.cpp

@@ -52,7 +52,6 @@
 #include <mathlib/math/Limits.hpp>
 #include <mathlib/math/Functions.hpp>
 
-#define MIN_TAKEOFF_THRUST    0.2f
 #define TPA_RATE_LOWER_LIMIT 0.05f
 
 #define AXIS_INDEX_ROLL 0
@@ -358,6 +357,19 @@ MulticopterAttitudeControl::sensor_bias_poll()
 
 }
 
+void
+MulticopterAttitudeControl::vehicle_land_detected_poll()
+{
+	/* check if there is a new message */
+	bool updated;
+	orb_check(_vehicle_land_detected_sub, &updated);
+
+	if (updated) {
+		orb_copy(ORB_ID(vehicle_land_detected), _vehicle_land_detected_sub, &_vehicle_land_detected);
+	}
+
+}
+
 /**
  * Attitude controller.
  * Input: 'vehicle_attitude_setpoint' topics (depending on mode)
@@ -541,8 +553,8 @@ MulticopterAttitudeControl::control_attitude_rates(float dt)
 	_rates_prev = rates;
 	_rates_prev_filtered = rates_filtered;
 
-	/* update integral only if motors are providing enough thrust to be effective */
+	/* update integral only if we are not landed */
-	if (_thrust_sp > MIN_TAKEOFF_THRUST) {
+	if (!_vehicle_land_detected.maybe_landed && !_vehicle_land_detected.landed) {
 		for (int i = AXIS_INDEX_ROLL; i < AXIS_COUNT; i++) {
 			// Check for positive control saturation
 			bool positive_saturation =
@@ -614,6 +626,7 @@ MulticopterAttitudeControl::run()
 
 	_sensor_correction_sub = orb_subscribe(ORB_ID(sensor_correction));
 	_sensor_bias_sub = orb_subscribe(ORB_ID(sensor_bias));
+	_vehicle_land_detected_sub = orb_subscribe(ORB_ID(vehicle_land_detected));
 
 	/* wakeup source: gyro data from sensor selected by the sensor app */
 	px4_pollfd_struct_t poll_fds = {};
@@ -673,6 +686,7 @@ MulticopterAttitudeControl::run()
 			vehicle_attitude_poll();
 			sensor_correction_poll();
 			sensor_bias_poll();
+			vehicle_land_detected_poll();
 
 			/* Check if we are in rattitude mode and the pilot is above the threshold on pitch
 			 * or roll (yaw can rotate 360 in normal att control).  If both are true don't
@@ -846,6 +860,7 @@ MulticopterAttitudeControl::run()
 
 	orb_unsubscribe(_sensor_correction_sub);
 	orb_unsubscribe(_sensor_bias_sub);
+	orb_unsubscribe(_vehicle_land_detected_sub);
 }
 
 int MulticopterAttitudeControl::task_spawn(int argc, char *argv[])