fixed-wing landing: add a touchdown time and subsequent ramp down to rwto pitch setpoint after the flare to keep wheels on ground

src/modules/fw_pos_control_l1/FixedwingPositionControl.cpp

@@ -1782,10 +1782,21 @@ FixedwingPositionControl::control_auto_landing(const hrt_abstime &now, const flo
 		const float height_rate_setpoint = flare_ramp_interpolator_sqrt * (-_param_fw_lnd_fl_sink.get()) +
 						   (1.0f - flare_ramp_interpolator_sqrt) * _flare_states.initial_height_rate_setpoint;
 
-		const float pitch_min_rad = flare_ramp_interpolator_sqrt * radians(_param_fw_lnd_fl_pmin.get()) +
+		float pitch_min_rad = flare_ramp_interpolator_sqrt * radians(_param_fw_lnd_fl_pmin.get()) +
-					    (1.0f - flare_ramp_interpolator_sqrt) * radians(_param_fw_p_lim_min.get());
+				      (1.0f - flare_ramp_interpolator_sqrt) * radians(_param_fw_p_lim_min.get());
-		const float pitch_max_rad = flare_ramp_interpolator_sqrt * radians(_param_fw_lnd_fl_pmax.get()) +
+		float pitch_max_rad = flare_ramp_interpolator_sqrt * radians(_param_fw_lnd_fl_pmax.get()) +
-					    (1.0f - flare_ramp_interpolator_sqrt) * radians(_param_fw_p_lim_max.get());
+				      (1.0f - flare_ramp_interpolator_sqrt) * radians(_param_fw_p_lim_max.get());
+
+		if (_param_fw_lnd_td_time.get() > FLT_EPSILON) {
+			const float touchdown_time = math::max(_param_fw_lnd_td_time.get(), _param_fw_lnd_fl_time.get());
+
+			const float touchdown_interpolator = math::constrain((seconds_since_flare_start - touchdown_time) /
+							     POST_TOUCHDOWN_CLAMP_TIME, 0.0f,
+							     1.0f);
+
+			pitch_max_rad = touchdown_interpolator * _param_rwto_psp.get() + (1.0f - touchdown_interpolator) * pitch_max_rad;
+			pitch_min_rad = touchdown_interpolator * _param_rwto_psp.get() + (1.0f - touchdown_interpolator) * pitch_min_rad;
+		}
 
 		// idle throttle may be >0 for internal combustion engines
 		// normally set to zero for electric motors

src/modules/fw_pos_control_l1/FixedwingPositionControl.hpp

@@ -166,6 +166,9 @@ static constexpr float MAX_TOUCHDOWN_POSITION_NUDGE_RATE = 4.0f;
 // [.] normalized deadzone threshold for manual nudging input
 static constexpr float MANUAL_TOUCHDOWN_NUDGE_INPUT_DEADZONE = 0.15f;
 
+// [s] time interval after touchdown for ramping in runway clamping constraints (touchdown is assumed at FW_LND_TD_TIME after start of flare)
+static constexpr float POST_TOUCHDOWN_CLAMP_TIME = 0.5f;
+
 class FixedwingPositionControl final : public ModuleBase<FixedwingPositionControl>, public ModuleParams,
 	public px4::WorkItem
 {
@@ -867,13 +870,16 @@ private:
 
 		(ParamFloat<px4::params::FW_LND_FL_TIME>) _param_fw_lnd_fl_time,
 		(ParamFloat<px4::params::FW_LND_FL_SINK>) _param_fw_lnd_fl_sink,
+		(ParamFloat<px4::params::FW_LND_TD_TIME>) _param_fw_lnd_td_time,
 		(ParamFloat<px4::params::FW_LND_TD_OFF>) _param_fw_lnd_td_off,
 		(ParamInt<px4::params::FW_LND_NUDGE>) _param_fw_lnd_nudge,
 		(ParamInt<px4::params::FW_LND_ABORT>) _param_fw_lnd_abort,
 
 		(ParamFloat<px4::params::FW_WIND_ARSP_SC>) _param_fw_wind_arsp_sc,
 
-		(ParamFloat<px4::params::FW_TKO_AIRSPD>) _param_fw_tko_airspd
+		(ParamFloat<px4::params::FW_TKO_AIRSPD>) _param_fw_tko_airspd,
+
+		(ParamFloat<px4::params::RWTO_PSP>) _param_rwto_psp
 	)
 
 };

src/modules/fw_pos_control_l1/fw_pos_control_l1_params.c

@@ -1034,6 +1034,26 @@ PARAM_DEFINE_FLOAT(FW_WING_HEIGHT, 0.5);
  */
 PARAM_DEFINE_FLOAT(FW_LND_FL_TIME, 1.0f);
 
+/**
+ * Landing touchdown time (since flare start)
+ *
+ * This is the time after the start of flaring that we expect the vehicle to touch the runway.
+ * At this time, a 0.5s clamp down ramp will engage, constraining the pitch setpoint to RWTO_PSP.
+ * If enabled, ensure that RWTO_PSP is configured appropriately for full gear contact on ground roll.
+ *
+ * Set to -1.0 to disable touchdown clamping. E.g. it may not be desirable to clamp on belly landings.
+ *
+ * The touchdown time will be constrained to be greater than or equal to the flare time (FW_LND_FL_TIME).
+ *
+ * @unit s
+ * @min -1.0
+ * @max 5.0
+ * @decimal 1
+ * @increment 0.1
+ * @group FW Auto Landing
+ */
+PARAM_DEFINE_FLOAT(FW_LND_TD_TIME, -1.0f);
+
 /**
  * Landing flare sink rate
  *