mc_att_control_main: zero minimal thrust when landed

but this time compared to
2fbb70d9ca
the minimum thrust is ramping to not produce a thrust jump.

The better long term solution will be to always have airmode but
with the effect limited to a magnitude of MPC_MANTHR_MIN.

src/lib/slew_rate/SlewRate.hpp

@@ -49,6 +49,7 @@ class SlewRate
 {
 public:
 	SlewRate() = default;
+	SlewRate(Type initial_value) { setForcedValue(initial_value); }
 	~SlewRate() = default;
 
 	/**

src/modules/mc_att_control/mc_att_control.hpp

@@ -50,10 +50,12 @@
 #include <uORB/topics/vehicle_attitude.h>
 #include <uORB/topics/vehicle_attitude_setpoint.h>
 #include <uORB/topics/vehicle_control_mode.h>
+#include <uORB/topics/vehicle_land_detected.h>
 #include <uORB/topics/vehicle_local_position.h>
 #include <uORB/topics/vehicle_rates_setpoint.h>
 #include <uORB/topics/vehicle_status.h>
 #include <lib/mathlib/math/filter/AlphaFilter.hpp>
+#include <lib/slew_rate/SlewRate.hpp>
 
 #include <AttitudeControl.hpp>
 
@@ -100,6 +102,7 @@ private:
 	uORB::Subscription _manual_control_setpoint_sub{ORB_ID(manual_control_setpoint)};
 	uORB::Subscription _vehicle_attitude_setpoint_sub{ORB_ID(vehicle_attitude_setpoint)};
 	uORB::Subscription _vehicle_control_mode_sub{ORB_ID(vehicle_control_mode)};
+	uORB::Subscription _vehicle_land_detected_sub{ORB_ID(vehicle_land_detected)};
 	uORB::Subscription _vehicle_local_position_sub{ORB_ID(vehicle_local_position)};
 	uORB::Subscription _vehicle_status_sub{ORB_ID(vehicle_status)};
 
@@ -118,12 +121,14 @@ private:
 	float _man_yaw_sp{0.f};                 /**< current yaw setpoint in manual mode */
 	float _man_tilt_max;                    /**< maximum tilt allowed for manual flight [rad] */
 
+	SlewRate<float> _manual_throttle_minimum{0.f}; ///< 0 when landed and ramped to MPC_MANTHR_MIN in air
 	AlphaFilter<float> _man_roll_input_filter;
 	AlphaFilter<float> _man_pitch_input_filter;
 
 	hrt_abstime _last_run{0};
 	hrt_abstime _last_attitude_setpoint{0};
 
+	bool _landed{true};
 	bool _reset_yaw_sp{true};
 	bool _heading_good_for_control{true}; ///< initialized true to have heading lock when local position never published
 	bool _vehicle_type_rotary_wing{true};

src/modules/mc_att_control/mc_att_control_main.cpp

@@ -60,8 +60,9 @@ MulticopterAttitudeControl::MulticopterAttitudeControl(bool vtol) :
 	_loop_perf(perf_alloc(PC_ELAPSED, MODULE_NAME": cycle")),
 	_vtol(vtol)
 {
-
 	parameters_updated();
+	// Rate of change 5% per second -> 1.6 seconds to ramp to default 8% MPC_MANTHR_MIN
+	_manual_throttle_minimum.setSlewRate(0.05f);
 }
 
 MulticopterAttitudeControl::~MulticopterAttitudeControl()
@@ -101,10 +102,10 @@ MulticopterAttitudeControl::throttle_curve(float throttle_stick_input)
 	// throttle_stick_input is in range [0, 1]
 	switch (_param_mpc_thr_curve.get()) {
 	case 1: // no rescaling to hover throttle
-		return math::interpolate(throttle_stick_input, 0.f, 1.f, _param_mpc_manthr_min.get(), _param_mpc_thr_max.get());
+		return math::interpolate(throttle_stick_input, 0.f, 1.f, _manual_throttle_minimum.getState(), _param_mpc_thr_max.get());
 
 	default: // 0 or other: rescale to hover throttle at 0.5 stick
-		return math::interpolateN(throttle_stick_input, {_param_mpc_manthr_min.get(), _param_mpc_thr_hover.get(), _param_mpc_thr_max.get()});
+		return math::interpolateN(throttle_stick_input, {_manual_throttle_minimum.getState(), _param_mpc_thr_hover.get(), _param_mpc_thr_max.get()});
 	}
 }
 
@@ -265,6 +266,14 @@ MulticopterAttitudeControl::Run()
 			}
 		}
 
+		if (_vehicle_land_detected_sub.updated()) {
+			vehicle_land_detected_s vehicle_land_detected;
+
+			if (_vehicle_land_detected_sub.copy(&vehicle_land_detected)) {
+				_landed = vehicle_land_detected.landed;
+			}
+		}
+
 		if (_vehicle_local_position_sub.updated()) {
 			vehicle_local_position_s vehicle_local_position;
 
@@ -324,6 +333,13 @@ MulticopterAttitudeControl::Run()
 			_vehicle_rates_setpoint_pub.publish(rates_setpoint);
 		}
 
+		if (_landed) {
+			_manual_throttle_minimum.update(0.f, dt);
+
+		} else {
+			_manual_throttle_minimum.update(_param_mpc_manthr_min.get(), dt);
+		}
+
 		// reset yaw setpoint during transitions, tailsitter.cpp generates
 		// attitude setpoint for the transition
 		_reset_yaw_sp = !attitude_setpoint_generated || !_heading_good_for_control || (_vtol && _vtol_in_transition_mode);