PX4-Autopilot/src/modules/land_detector/MulticopterLandDetector.cpp

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/**
 * @file MulticopterLandDetector.cpp
 *
 *The MC land-detector goes through 3 states before it will detect landed:
 *
 *State 1 (=ground_contact):
 *ground_contact is detected once the vehicle is not moving along the NED-z direction and has
 *a thrust value below 0.3 of the thrust_range (thrust_hover - thrust_min). The condition has to be true
 *for GROUND_CONTACT_TRIGGER_TIME_US in order to detect ground_contact
 *
 *State 2 (=maybe_landed):
 *maybe_landed can only occur if the internal ground_contact hysteresis state is true. maybe_landed criteria requires to have no motion in x and y,
 *no rotation and a thrust below 0.1 of the thrust_range (thrust_hover - thrust_min). In addition, the mc_pos_control turns off the thrust_sp in
 *body frame along x and y which helps to detect maybe_landed. The criteria for maybe_landed needs to be true for MAYBE_LAND_DETECTOR_TRIGGER_TIME_US.
 *
 *State 3 (=landed)
 *landed can only be detected if maybe_landed is true for LAND_DETECTOR_TRIGGER_TIME_US. No farther criteria is tested, but the mc_pos_control goes into
 *idle (thrust_sp = 0) which helps to detect landed. By doing this the thrust-criteria of State 2 will always be met, however the remaining criteria of no rotation and no motion still
 *have to be valid.

 *It is to note that if one criteria is not met, then vehicle exits the state directly without blocking.
 *
 *If the land-detector does not detect ground_contact, then the vehicle is either flying or falling, where free fall detection heavily relies
 *on the acceleration. TODO: verify that free fall is reliable
 *
 * @author Johan Jansen <jnsn.johan@gmail.com>
 * @author Morten Lysgaard <morten@lysgaard.no>
 * @author Julian Oes <julian@oes.ch>
 */

#include <cmath>
#include <mathlib/mathlib.h>

#include "MulticopterLandDetector.h"


namespace land_detector
{

MulticopterLandDetector::MulticopterLandDetector()
{
	_paramHandle.maxRotation = param_find("LNDMC_ROT_MAX");
	_paramHandle.maxVelocity = param_find("LNDMC_XY_VEL_MAX");
	_paramHandle.maxClimbRate = param_find("LNDMC_Z_VEL_MAX");
	_paramHandle.minThrottle = param_find("MPC_THR_MIN");
	_paramHandle.hoverThrottle = param_find("MPC_THR_HOVER");
	_paramHandle.minManThrottle = param_find("MPC_MANTHR_MIN");
	_paramHandle.freefall_acc_threshold = param_find("LNDMC_FFALL_THR");
	_paramHandle.freefall_trigger_time = param_find("LNDMC_FFALL_TTRI");
	_paramHandle.altitude_max = param_find("LNDMC_ALT_MAX");
	_paramHandle.landSpeed = param_find("MPC_LAND_SPEED");

	// Use Trigger time when transitioning from in-air (false) to landed (true) / ground contact (true).
	_landed_hysteresis.set_hysteresis_time_from(false, LAND_DETECTOR_TRIGGER_TIME_US);
	_maybe_landed_hysteresis.set_hysteresis_time_from(false, MAYBE_LAND_DETECTOR_TRIGGER_TIME_US);
	_ground_contact_hysteresis.set_hysteresis_time_from(false, GROUND_CONTACT_TRIGGER_TIME_US);
}

void MulticopterLandDetector::_initialize_topics()
{
	// subscribe to position, attitude and velocity changes
	_vehicleLocalPositionSub = orb_subscribe(ORB_ID(vehicle_local_position));
	_vehicleLocalPositionSetpointSub = orb_subscribe(ORB_ID(vehicle_local_position_setpoint));
	_attitudeSub = orb_subscribe(ORB_ID(vehicle_attitude));
	_actuatorsSub = orb_subscribe(ORB_ID(actuator_controls_0));
	_sensor_bias_sub = orb_subscribe(ORB_ID(sensor_bias));
	_vehicle_control_mode_sub = orb_subscribe(ORB_ID(vehicle_control_mode));
	_battery_sub = orb_subscribe(ORB_ID(battery_status));
}

void MulticopterLandDetector::_update_topics()
{
	_orb_update(ORB_ID(vehicle_local_position), _vehicleLocalPositionSub, &_vehicleLocalPosition);
	_orb_update(ORB_ID(vehicle_local_position_setpoint), _vehicleLocalPositionSetpointSub, &_vehicleLocalPositionSetpoint);
	_orb_update(ORB_ID(vehicle_attitude), _attitudeSub, &_vehicleAttitude);
	_orb_update(ORB_ID(actuator_controls_0), _actuatorsSub, &_actuators);
	_orb_update(ORB_ID(sensor_bias), _sensor_bias_sub, &_sensors);
	_orb_update(ORB_ID(vehicle_control_mode), _vehicle_control_mode_sub, &_control_mode);
	_orb_update(ORB_ID(battery_status), _battery_sub, &_battery);
}

void MulticopterLandDetector::_update_params()
{
	param_get(_paramHandle.maxClimbRate, &_params.maxClimbRate);
	param_get(_paramHandle.maxVelocity, &_params.maxVelocity);
	param_get(_paramHandle.maxRotation, &_params.maxRotation_rad_s);
	_params.maxRotation_rad_s = math::radians(_params.maxRotation_rad_s);
	param_get(_paramHandle.minThrottle, &_params.minThrottle);
	param_get(_paramHandle.hoverThrottle, &_params.hoverThrottle);
	param_get(_paramHandle.minManThrottle, &_params.minManThrottle);
	param_get(_paramHandle.freefall_acc_threshold, &_params.freefall_acc_threshold);
	param_get(_paramHandle.freefall_trigger_time, &_params.freefall_trigger_time);
	_freefall_hysteresis.set_hysteresis_time_from(false, (hrt_abstime)(1e6f * _params.freefall_trigger_time));
	param_get(_paramHandle.altitude_max, &_params.altitude_max);
	param_get(_paramHandle.landSpeed, &_params.landSpeed);
}


bool MulticopterLandDetector::_get_freefall_state()
{
	if (_params.freefall_acc_threshold < 0.1f
	    || _params.freefall_acc_threshold > 10.0f) {	//if parameter is set to zero or invalid, disable free-fall detection.
		return false;
	}

	if (_sensors.timestamp == 0) {
		// _sensors is not valid yet, we have to assume we're not falling.
		return false;
	}

	float acc_norm = _sensors.accel_x * _sensors.accel_x
			 + _sensors.accel_y * _sensors.accel_y
			 + _sensors.accel_z * _sensors.accel_z;
	acc_norm = sqrtf(acc_norm);	//norm of specific force. Should be close to 9.8 m/s^2 when landed.

	return (acc_norm < _params.freefall_acc_threshold);	//true if we are currently falling
}

bool MulticopterLandDetector::_get_ground_contact_state()
{
	// When not armed, consider to have ground-contact
	if (!_arming.armed) {
		return true;
	}

	// land speed threshold
	float land_speed_threshold = 0.9f * math::max(_params.landSpeed, 0.1f);

	// Check if we are moving vertically - this might see a spike after arming due to
	// throttle-up vibration. If accelerating fast the throttle thresholds will still give
	// an accurate in-air indication.
	bool verticalMovement;

	if (hrt_elapsed_time(&_landed_time) < LAND_DETECTOR_LAND_PHASE_TIME_US) {

		// Widen acceptance thresholds for landed state right after arming
		// so that motor spool-up and other effects do not trigger false negatives.
		verticalMovement = fabsf(_vehicleLocalPosition.vz) > _params.maxClimbRate  * 2.5f;

	} else {

		// Adjust maxClimbRate if land_speed is lower than 2x maxClimbrate
		float maxClimbRate = ((land_speed_threshold * 0.5f) < _params.maxClimbRate) ? (0.5f * land_speed_threshold) :
				     _params.maxClimbRate;
		verticalMovement = fabsf(_vehicleLocalPosition.vz) > maxClimbRate;
	}

	// Check if we are moving horizontally.
	bool horizontalMovement = sqrtf(_vehicleLocalPosition.vx * _vehicleLocalPosition.vx
					+ _vehicleLocalPosition.vy * _vehicleLocalPosition.vy) > _params.maxVelocity;

	// if we have a valid velocity setpoint and the vehicle is demanded to go down but no vertical movement present,
	// we then can assume that the vehicle hit ground
	bool in_descend = _is_climb_rate_enabled()
			  && (_vehicleLocalPositionSetpoint.vz >= land_speed_threshold);
	bool hit_ground = in_descend && !verticalMovement;

	// TODO: we need an accelerometer based check for vertical movement for flying without GPS
	if ((_has_low_thrust() || hit_ground) && (!horizontalMovement || !_has_position_lock())
	    && (!verticalMovement || !_has_altitude_lock())) {
		return true;
	}

	return false;
}

bool MulticopterLandDetector::_get_maybe_landed_state()
{
	// Time base for this function
	const hrt_abstime now = hrt_absolute_time();

	// When not armed, consider to be maybe-landed
	if (!_arming.armed) {
		return true;
	}

	if (_has_minimal_thrust()) {
		if (_min_trust_start == 0) {
			_min_trust_start = now;
		}

	} else {
		_min_trust_start = 0;
	}

	float landThresholdFactor = 1.0f;

	// Widen acceptance thresholds for landed state right after landed
	if (hrt_elapsed_time(&_landed_time) < LAND_DETECTOR_LAND_PHASE_TIME_US) {
		landThresholdFactor = 2.5f;
	}

	// Next look if all rotation angles are not moving.
	float maxRotationScaled = _params.maxRotation_rad_s * landThresholdFactor;

	bool rotating = (fabsf(_vehicleAttitude.rollspeed)  > maxRotationScaled) ||
			(fabsf(_vehicleAttitude.pitchspeed) > maxRotationScaled) ||
			(fabsf(_vehicleAttitude.yawspeed) > maxRotationScaled);

	// Return status based on armed state and throttle if no position lock is available.
	if (!_has_altitude_lock() && !rotating) {
		// The system has minimum trust set (manual or in failsafe)
		// if this persists for 8 seconds AND the drone is not
		// falling consider it to be landed. This should even sustain
		// quite acrobatic flight.
		return (_min_trust_start > 0) && (hrt_elapsed_time(&_min_trust_start) > 8_s);
	}

	if (_ground_contact_hysteresis.get_state() && _has_minimal_thrust() && !rotating) {
		// Ground contact, no thrust and no movement -> landed
		return true;
	}

	return false;
}

bool MulticopterLandDetector::_get_landed_state()
{
	// When not armed, consider to be landed
	if (!_arming.armed) {
		return true;
	}

	// reset the landed_time
	if (!_maybe_landed_hysteresis.get_state()) {

		_landed_time = 0;

	} else if (_landed_time == 0) {

		_landed_time = hrt_absolute_time();

	}

	// if we have maybe_landed, the mc_pos_control goes into idle (thrust_sp = 0.0)
	// therefore check if all other condition of the landed state remain true
	return _maybe_landed_hysteresis.get_state();

}

float MulticopterLandDetector::_get_max_altitude()
{
	/* ToDo: add a meaningful altitude */
	float valid_altitude_max = _params.altitude_max;

	if (_battery.warning == battery_status_s::BATTERY_WARNING_LOW) {
		valid_altitude_max = _params.altitude_max * 0.75f;
	}

	if (_battery.warning == battery_status_s::BATTERY_WARNING_CRITICAL) {
		valid_altitude_max = _params.altitude_max * 0.5f;
	}

	if (_battery.warning == battery_status_s::BATTERY_WARNING_EMERGENCY) {
		valid_altitude_max = _params.altitude_max * 0.25f;
	}

	return valid_altitude_max;
}

bool MulticopterLandDetector::_has_altitude_lock()
{
	return _vehicleLocalPosition.timestamp != 0 &&
	       hrt_elapsed_time(&_vehicleLocalPosition.timestamp) < 500_ms &&
	       _vehicleLocalPosition.z_valid;
}

bool MulticopterLandDetector::_has_position_lock()
{
	return _has_altitude_lock() && _vehicleLocalPosition.xy_valid;
}

bool MulticopterLandDetector::_is_climb_rate_enabled()
{
	bool has_updated = (_vehicleLocalPositionSetpoint.timestamp != 0)
			   && (hrt_elapsed_time(&_vehicleLocalPositionSetpoint.timestamp) < 500_ms);

	return (_control_mode.flag_control_climb_rate_enabled && has_updated && PX4_ISFINITE(_vehicleLocalPositionSetpoint.vz));
}

bool MulticopterLandDetector::_has_low_thrust()
{
	// 30% of throttle range between min and hover
	float sys_min_throttle = _params.minThrottle + (_params.hoverThrottle - _params.minThrottle) * 0.3f;

	// Check if thrust output is less than the minimum auto throttle param.
	return _actuators.control[actuator_controls_s::INDEX_THROTTLE] <= sys_min_throttle;
}

bool MulticopterLandDetector::_has_minimal_thrust()
{
	// 10% of throttle range between min and hover once we entered ground contact
	float sys_min_throttle = _params.minThrottle + (_params.hoverThrottle - _params.minThrottle) * 0.1f;

	// Determine the system min throttle based on flight mode
	if (!_control_mode.flag_control_climb_rate_enabled) {
		sys_min_throttle = (_params.minManThrottle + 0.01f);
	}

	// Check if thrust output is less than the minimum auto throttle param.
	return _actuators.control[actuator_controls_s::INDEX_THROTTLE] <= sys_min_throttle;
}

} // namespace land_detector