PX4-Autopilot/src/modules/mc_pos_control/PositionControl.cpp

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/**
 * @file PositionControl.cpp
 */

#include "PositionControl.hpp"
#include <float.h>
#include <mathlib/mathlib.h>
#include "Utility/ControlMath.hpp"
#include <px4_defines.h>

using namespace matrix;

PositionControl::PositionControl(ModuleParams *parent) :
	ModuleParams(parent)
{}

void PositionControl::updateState(const PositionControlStates &states)
{
	_pos = states.position;
	_vel = states.velocity;
	_yaw = states.yaw;
	_vel_dot = states.acceleration;
}

void PositionControl::_setCtrlFlag(bool value)
{
	for (int i = 0; i <= 2; i++) {
		_ctrl_pos[i] = _ctrl_vel[i] = value;
	}
}

bool PositionControl::updateSetpoint(const vehicle_local_position_setpoint_s &setpoint)
{
	// by default we use the entire position-velocity control-loop pipeline (flag only for logging purpose)
	_setCtrlFlag(true);

	_pos_sp = Vector3f(setpoint.x, setpoint.y, setpoint.z);
	_vel_sp = Vector3f(setpoint.vx, setpoint.vy, setpoint.vz);
	_acc_sp = Vector3f(setpoint.acc_x, setpoint.acc_y, setpoint.acc_z);
	_thr_sp = Vector3f(setpoint.thrust);
	_yaw_sp = setpoint.yaw;
	_yawspeed_sp = setpoint.yawspeed;
	bool mapping_succeeded = _interfaceMapping();

	// If full manual is required (thrust already generated), don't run position/velocity
	// controller and just return thrust.
	_skip_controller = PX4_ISFINITE(_thr_sp(0)) && PX4_ISFINITE(_thr_sp(1))
			   && PX4_ISFINITE(_thr_sp(2));

	return mapping_succeeded;
}

void PositionControl::generateThrustYawSetpoint(const float dt)
{
	if (_skip_controller) {

		// Already received a valid thrust set-point.
		// Limit the thrust vector.
		float thr_mag = _thr_sp.length();

		if (thr_mag > _param_mpc_thr_max.get()) {
			_thr_sp = _thr_sp.normalized() * _param_mpc_thr_max.get();

		} else if (thr_mag < _param_mpc_manthr_min.get() && thr_mag > FLT_EPSILON) {
			_thr_sp = _thr_sp.normalized() * _param_mpc_manthr_min.get();
		}

		// Just set the set-points equal to the current vehicle state.
		_pos_sp = _pos;
		_vel_sp = _vel;
		_acc_sp = _acc;

	} else {
		_positionController();
		_velocityController(dt);
	}
}

bool PositionControl::_interfaceMapping()
{
	// if nothing is valid, then apply failsafe landing
	bool failsafe = false;

	// Respects FlightTask interface, where NAN-set-points are of no interest
	// and do not require control. A valid position and velocity setpoint will
	// be mapped to a desired position setpoint with a feed-forward term.
	// States and setpoints which are integrals of the reference setpoint are set to 0.
	// For instance: reference is velocity-setpoint -> position and position-setpoint = 0
	//               reference is thrust-setpoint -> position, velocity, position-/velocity-setpoint = 0
	for (int i = 0; i <= 2; i++) {

		if (PX4_ISFINITE(_pos_sp(i))) {
			// Position control is required

			if (!PX4_ISFINITE(_vel_sp(i))) {
				// Velocity is not used as feedforward term.
				_vel_sp(i) = 0.0f;
			}

			// thrust setpoint is not supported in position control
			_thr_sp(i) = NAN;

			// to run position control, we require valid position and velocity
			if (!PX4_ISFINITE(_pos(i)) || !PX4_ISFINITE(_vel(i))) {
				failsafe = true;
			}

		} else if (PX4_ISFINITE(_vel_sp(i))) {

			// Velocity controller is active without position control.
			// Set integral states and setpoints to 0
			_pos_sp(i) = _pos(i) = 0.0f;
			_ctrl_pos[i] = false; // position control-loop is not used

			// thrust setpoint is not supported in velocity control
			_thr_sp(i) = NAN;

			// to run velocity control, we require valid velocity
			if (!PX4_ISFINITE(_vel(i))) {
				failsafe = true;
			}

		} else if (PX4_ISFINITE(_thr_sp(i))) {

			// Thrust setpoint was generated from sticks directly.
			// Set all integral states and setpoints to 0
			_pos_sp(i) = _pos(i) = 0.0f;
			_vel_sp(i) = _vel(i) = 0.0f;
			_ctrl_pos[i] = _ctrl_vel[i] = false; // position/velocity control loop is not used

			// Reset the Integral term.
			_thr_int(i) = 0.0f;
			// Don't require velocity derivative.
			_vel_dot(i) = 0.0f;

		} else {
			// nothing is valid. do failsafe
			failsafe = true;
		}
	}

	// ensure that vel_dot is finite, otherwise set to 0
	if (!PX4_ISFINITE(_vel_dot(0)) || !PX4_ISFINITE(_vel_dot(1))) {
		_vel_dot(0) = _vel_dot(1) = 0.0f;
	}

	if (!PX4_ISFINITE(_vel_dot(2))) {
		_vel_dot(2) = 0.0f;
	}

	if (!PX4_ISFINITE(_yawspeed_sp)) {
		// Set the yawspeed to 0 since not used.
		_yawspeed_sp = 0.0f;
	}

	if (!PX4_ISFINITE(_yaw_sp)) {
		// Set the yaw-sp equal the current yaw.
		// That is the best we can do and it also
		// agrees with FlightTask-interface definition.
		if (PX4_ISFINITE(_yaw)) {
			_yaw_sp = _yaw;

		} else {
			failsafe = true;
		}
	}

	// check failsafe
	if (failsafe) {
		// point the thrust upwards
		_thr_sp(0) = _thr_sp(1) = 0.0f;
		// throttle down such that vehicle goes down with
		// 70% of throttle range between min and hover
		_thr_sp(2) = -(_param_mpc_thr_min.get() + (_param_mpc_thr_hover.get() - _param_mpc_thr_min.get()) * 0.7f);
		// position and velocity control-loop is currently unused (flag only for logging purpose)
		_setCtrlFlag(false);
	}

	return !(failsafe);
}

void PositionControl::_positionController()
{
	// P-position controller
	const Vector3f vel_sp_position = (_pos_sp - _pos).emult(Vector3f(_param_mpc_xy_p.get(), _param_mpc_xy_p.get(),
					 _param_mpc_z_p.get()));
	_vel_sp = vel_sp_position + _vel_sp;

	// Constrain horizontal velocity by prioritizing the velocity component along the
	// the desired position setpoint over the feed-forward term.
	const Vector2f vel_sp_xy = ControlMath::constrainXY(Vector2f(vel_sp_position),
				   Vector2f(_vel_sp - vel_sp_position), _param_mpc_xy_vel_max.get());
	_vel_sp(0) = vel_sp_xy(0);
	_vel_sp(1) = vel_sp_xy(1);
	// Constrain velocity in z-direction.
	_vel_sp(2) = math::constrain(_vel_sp(2), -_constraints.speed_up, _constraints.speed_down);
}

void PositionControl::_velocityController(const float &dt)
{
	// Generate desired thrust setpoint.
	// PID
	// u_des = P(vel_err) + D(vel_err_dot) + I(vel_integral)
	// Umin <= u_des <= Umax
	//
	// Anti-Windup:
	// u_des = _thr_sp; r = _vel_sp; y = _vel
	// u_des >= Umax and r - y >= 0 => Saturation = true
	// u_des >= Umax and r - y <= 0 => Saturation = false
	// u_des <= Umin and r - y <= 0 => Saturation = true
	// u_des <= Umin and r - y >= 0 => Saturation = false
	//
	// 	Notes:
	// - PID implementation is in NED-frame
	// - control output in D-direction has priority over NE-direction
	// - the equilibrium point for the PID is at hover-thrust
	// - the maximum tilt cannot exceed 90 degrees. This means that it is
	// 	 not possible to have a desired thrust direction pointing in the positive
	// 	 D-direction (= downward)
	// - the desired thrust in D-direction is limited by the thrust limits
	// - the desired thrust in NE-direction is limited by the thrust excess after
	// 	 consideration of the desired thrust in D-direction. In addition, the thrust in
	// 	 NE-direction is also limited by the maximum tilt.

	const Vector3f vel_err = _vel_sp - _vel;

	// Consider thrust in D-direction.
	float thrust_desired_D = _param_mpc_z_vel_p.get() * vel_err(2) +  _param_mpc_z_vel_d.get() * _vel_dot(2) + _thr_int(
					 2) - _param_mpc_thr_hover.get();

	// The Thrust limits are negated and swapped due to NED-frame.
	float uMax = -_param_mpc_thr_min.get();
	float uMin = -_param_mpc_thr_max.get();

	// make sure there's always enough thrust vector length to infer the attitude
	uMax = math::min(uMax, -10e-4f);

	// Apply Anti-Windup in D-direction.
	bool stop_integral_D = (thrust_desired_D >= uMax && vel_err(2) >= 0.0f) ||
			       (thrust_desired_D <= uMin && vel_err(2) <= 0.0f);

	if (!stop_integral_D) {
		_thr_int(2) += vel_err(2) * _param_mpc_z_vel_i.get() * dt;

		// limit thrust integral
		_thr_int(2) = math::min(fabsf(_thr_int(2)), _param_mpc_thr_max.get()) * math::sign(_thr_int(2));
	}

	// Saturate thrust setpoint in D-direction.
	_thr_sp(2) = math::constrain(thrust_desired_D, uMin, uMax);

	if (PX4_ISFINITE(_thr_sp(0)) && PX4_ISFINITE(_thr_sp(1))) {
		// Thrust set-point in NE-direction is already provided. Only
		// scaling by the maximum tilt is required.
		float thr_xy_max = fabsf(_thr_sp(2)) * tanf(_constraints.tilt);
		_thr_sp(0) *= thr_xy_max;
		_thr_sp(1) *= thr_xy_max;

	} else {
		// PID-velocity controller for NE-direction.
		Vector2f thrust_desired_NE;
		thrust_desired_NE(0) = _param_mpc_xy_vel_p.get() * vel_err(0) + _param_mpc_xy_vel_d.get() * _vel_dot(0) + _thr_int(0);
		thrust_desired_NE(1) = _param_mpc_xy_vel_p.get() * vel_err(1) + _param_mpc_xy_vel_d.get() * _vel_dot(1) + _thr_int(1);

		// Get maximum allowed thrust in NE based on tilt and excess thrust.
		float thrust_max_NE_tilt = fabsf(_thr_sp(2)) * tanf(_constraints.tilt);
		float thrust_max_NE = sqrtf(_param_mpc_thr_max.get() * _param_mpc_thr_max.get() - _thr_sp(2) * _thr_sp(2));
		thrust_max_NE = math::min(thrust_max_NE_tilt, thrust_max_NE);

		// Saturate thrust in NE-direction.
		_thr_sp(0) = thrust_desired_NE(0);
		_thr_sp(1) = thrust_desired_NE(1);

		if (thrust_desired_NE * thrust_desired_NE > thrust_max_NE * thrust_max_NE) {
			float mag = thrust_desired_NE.length();
			_thr_sp(0) = thrust_desired_NE(0) / mag * thrust_max_NE;
			_thr_sp(1) = thrust_desired_NE(1) / mag * thrust_max_NE;
		}

		// Use tracking Anti-Windup for NE-direction: during saturation, the integrator is used to unsaturate the output
		// see Anti-Reset Windup for PID controllers, L.Rundqwist, 1990
		float arw_gain = 2.f / _param_mpc_xy_vel_p.get();

		Vector2f vel_err_lim;
		vel_err_lim(0) = vel_err(0) - (thrust_desired_NE(0) - _thr_sp(0)) * arw_gain;
		vel_err_lim(1) = vel_err(1) - (thrust_desired_NE(1) - _thr_sp(1)) * arw_gain;

		// Update integral
		_thr_int(0) += _param_mpc_xy_vel_i.get() * vel_err_lim(0) * dt;
		_thr_int(1) += _param_mpc_xy_vel_i.get() * vel_err_lim(1) * dt;
	}
}

void PositionControl::updateConstraints(const vehicle_constraints_s &constraints)
{
	_constraints = constraints;

	// For safety check if adjustable constraints are below global constraints. If they are not stricter than global
	// constraints, then just use global constraints for the limits.

	const float tilt_max_radians = math::radians(math::max(_param_mpc_tiltmax_air.get(), _param_mpc_man_tilt_max.get()));

	if (!PX4_ISFINITE(constraints.tilt)
	    || !(constraints.tilt < tilt_max_radians)) {
		_constraints.tilt = tilt_max_radians;
	}

	if (!PX4_ISFINITE(constraints.speed_up) || !(constraints.speed_up < _param_mpc_z_vel_max_up.get())) {
		_constraints.speed_up = _param_mpc_z_vel_max_up.get();
	}

	if (!PX4_ISFINITE(constraints.speed_down) || !(constraints.speed_down < _param_mpc_z_vel_max_dn.get())) {
		_constraints.speed_down = _param_mpc_z_vel_max_dn.get();
	}

	if (!PX4_ISFINITE(constraints.speed_xy) || !(constraints.speed_xy < _param_mpc_xy_vel_max.get())) {
		_constraints.speed_xy = _param_mpc_xy_vel_max.get();
	}
}

void PositionControl::updateParams()
{
	ModuleParams::updateParams();
}