// update L1 control for waypoint navigation
// this function costs about 3.5 milliseconds on a AVR2560
void AP_L1_Control::update_waypoint(const struct Location &prev_WP, const struct Location &next_WP)
{
struct Location _current_loc;
float Nu;
float xtrackVel;
float ltrackVel;
// Calculate L1 gain required for specified damping
float K_L1 = 4.0f * _L1_damping * _L1_damping;
// Get current position and velocity
_ahrs.get_position(_current_loc);
Vector2f _groundspeed_vector = _ahrs.groundspeed_vector();
// update _target_bearing_cd
_target_bearing_cd = get_bearing_cd(_current_loc, next_WP);
//Calculate groundspeed
float groundSpeed = _groundspeed_vector.length();
if (groundSpeed < 0.1f) {
// use a small ground speed vector in the right direction,
// allowing us to use the compass heading at zero GPS velocity
groundSpeed = 0.1f;
_groundspeed_vector = Vector2f(cosf(_ahrs.yaw), sinf(_ahrs.yaw)) * groundSpeed;
}
// Calculate time varying control parameters
// Calculate the L1 length required for specified period
// 0.3183099 = 1/1/pipi
_L1_dist = 0.3183099f * _L1_damping * _L1_period * groundSpeed;
// Calculate the NE position of WP B relative to WP A
Vector2f AB = location_diff(prev_WP, next_WP);
// Check for AB zero length and track directly to the destination
// if too small
if (AB.length() < 1.0e-6f) {
AB = location_diff(_current_loc, next_WP);
}
AB.normalize();
// Calculate the NE position of the aircraft relative to WP A
Vector2f A_air = location_diff(prev_WP, _current_loc);
// calculate distance to target track, for reporting
_crosstrack_error = AB % A_air;
//Determine if the aircraft is behind a +-135 degree degree arc centred on WP A
//and further than L1 distance from WP A. Then use WP A as the L1 reference point
//Otherwise do normal L1 guidance
float WP_A_dist = A_air.length();
float alongTrackDist = A_air * AB;
if (WP_A_dist > _L1_dist && alongTrackDist/max(WP_A_dist, 1.0f) < -0.7071f)
{
//Calc Nu to fly To WP A
Vector2f A_air_unit = (A_air).normalized(); // Unit vector from WP A to aircraft
xtrackVel = _groundspeed_vector % (-A_air_unit); // Velocity across line
ltrackVel = _groundspeed_vector * (-A_air_unit); // Velocity along line
Nu = atan2f(xtrackVel,ltrackVel);
_prevent_indecision(Nu);
_nav_bearing = atan2f(-A_air_unit.y , -A_air_unit.x); // bearing (radians) from AC to L1 point
} else { //Calc Nu to fly along AB line
//Calculate Nu2 angle (angle of velocity vector relative to line connecting waypoints)
xtrackVel = _groundspeed_vector % AB; // Velocity cross track
ltrackVel = _groundspeed_vector * AB; // Velocity along track
float Nu2 = atan2f(xtrackVel,ltrackVel);
//Calculate Nu1 angle (Angle to L1 reference point)
float xtrackErr = A_air % AB;
float sine_Nu1 = xtrackErr/max(_L1_dist, 0.1f);
//Limit sine of Nu1 to provide a controlled track capture angle of 45 deg
sine_Nu1 = constrain_float(sine_Nu1, -0.7071f, 0.7071f);
float Nu1 = asinf(sine_Nu1);
Nu = Nu1 + Nu2;
_nav_bearing = atan2f(AB.y, AB.x) + Nu1; // bearing (radians) from AC to L1 point
}
_last_Nu = Nu;
//Limit Nu to +-pi
Nu = constrain_float(Nu, -1.5708f, +1.5708f);
_latAccDem = K_L1 * groundSpeed * groundSpeed / _L1_dist * sinf(Nu);
// Waypoint capture status is always false during waypoint following
_WPcircle = false;
_bearing_error = Nu; // bearing error angle (radians), +ve to left of track
}
// update L1 control for loitering
void AP_L1_Control::update_loiter(const struct Location ¢er_WP, float radius, int8_t loiter_direction)
{
struct Location _current_loc;
// scale loiter radius with square of EAS2TAS to allow us to stay
// stable at high altitude
radius *= sq(_ahrs.get_EAS2TAS());
// Calculate guidance gains used by PD loop (used during circle tracking)
float omega = (6.2832f / _L1_period);
float Kx = omega * omega;
float Kv = 2.0f * _L1_damping * omega;
// Calculate L1 gain required for specified damping (used during waypoint capture)
float K_L1 = 4.0f * _L1_damping * _L1_damping;
//Get current position and velocity
_ahrs.get_position(_current_loc);
Vector2f _groundspeed_vector = _ahrs.groundspeed_vector();
//Calculate groundspeed
float groundSpeed = max(_groundspeed_vector.length() , 1.0f);
// update _target_bearing_cd
_target_bearing_cd = get_bearing_cd(_current_loc, center_WP);
// Calculate time varying control parameters
// Calculate the L1 length required for specified period
// 0.3183099 = 1/pi
_L1_dist = 0.3183099f * _L1_damping * _L1_period * groundSpeed;
//Calculate the NE position of the aircraft relative to WP A
Vector2f A_air = location_diff(center_WP, _current_loc);
//Calculate the unit vector from WP A to aircraft
Vector2f A_air_unit = A_air.normalized();
//Calculate Nu to capture center_WP
float xtrackVelCap = A_air_unit % _groundspeed_vector; // Velocity across line - perpendicular to radial inbound to WP
float ltrackVelCap = - (_groundspeed_vector * A_air_unit); // Velocity along line - radial inbound to WP
float Nu = atan2f(xtrackVelCap,ltrackVelCap);
_prevent_indecision(Nu);
_last_Nu = Nu;
Nu = constrain_float(Nu, -M_PI_2, M_PI_2); //Limit Nu to +- Pi/2
//Calculate lat accln demand to capture center_WP (use L1 guidance law)
float latAccDemCap = K_L1 * groundSpeed * groundSpeed / _L1_dist * sinf(Nu);
//Calculate radial position and velocity errors
float xtrackVelCirc = -ltrackVelCap; // Radial outbound velocity - reuse previous radial inbound velocity
float xtrackErrCirc = A_air.length() - radius; // Radial distance from the loiter circle
// keep crosstrack error for reporting
_crosstrack_error = xtrackErrCirc;
//Calculate PD control correction to circle waypoint_ahrs.roll
float latAccDemCircPD = (xtrackErrCirc * Kx + xtrackVelCirc * Kv);
//Calculate tangential velocity
float velTangent = xtrackVelCap * float(loiter_direction);
//Prevent PD demand from turning the wrong way by limiting the command when flying the wrong way
if (ltrackVelCap < 0.0f && velTangent < 0.0f) {
latAccDemCircPD = max(latAccDemCircPD, 0.0f);
}
// Calculate centripetal acceleration demand
float latAccDemCircCtr = velTangent * velTangent / max((0.5f * radius), (radius + xtrackErrCirc));
//Sum PD control and centripetal acceleration to calculate lateral manoeuvre demand
float latAccDemCirc = loiter_direction * (latAccDemCircPD + latAccDemCircCtr);
// Perform switchover between 'capture' and 'circle' modes at the
// point where the commands cross over to achieve a seamless transfer
// Only fly 'capture' mode if outside the circle
if (xtrackErrCirc > 0.0f && loiter_direction * latAccDemCap < loiter_direction * latAccDemCirc) {
_latAccDem = latAccDemCap;
_WPcircle = false;
_bearing_error = Nu; // angle between demanded and achieved velocity vector, +ve to left of track
_nav_bearing = atan2f(-A_air_unit.y , -A_air_unit.x); // bearing (radians) from AC to L1 point
} else {
_latAccDem = latAccDemCirc;
_WPcircle = true;
_bearing_error = 0.0f; // bearing error (radians), +ve to left of track
_nav_bearing = atan2f(-A_air_unit.y , -A_air_unit.x); // bearing (radians)from AC to L1 point
}
}
// update L1 control for waypoint navigation
void AP_L1_Control::update_waypoint(const struct Location &prev_WP, const struct Location &next_WP)
{
struct Location _current_loc;
float Nu;
float xtrackVel;
float ltrackVel;
uint32_t now = AP_HAL::micros();
float dt = (now - _last_update_waypoint_us) * 1.0e-6f;
if (dt > 0.1) {
dt = 0.1;
}
_last_update_waypoint_us = now;
// Calculate L1 gain required for specified damping
float K_L1 = 4.0f * _L1_damping * _L1_damping;
// Get current position and velocity
_ahrs.get_position(_current_loc);
Vector2f _groundspeed_vector = _ahrs.groundspeed_vector();
// update _target_bearing_cd
_target_bearing_cd = get_bearing_cd(_current_loc, next_WP);
//Calculate groundspeed
float groundSpeed = _groundspeed_vector.length();
if (groundSpeed < 0.1f) {
// use a small ground speed vector in the right direction,
// allowing us to use the compass heading at zero GPS velocity
groundSpeed = 0.1f;
_groundspeed_vector = Vector2f(cosf(_ahrs.yaw), sinf(_ahrs.yaw)) * groundSpeed;
}
// Calculate time varying control parameters
// Calculate the L1 length required for specified period
// 0.3183099 = 1/1/pipi
_L1_dist = 0.3183099f * _L1_damping * _L1_period * groundSpeed;
// Calculate the NE position of WP B relative to WP A
Vector2f AB = location_diff(prev_WP, next_WP);
// Check for AB zero length and track directly to the destination
// if too small
if (AB.length() < 1.0e-6f) {
AB = location_diff(_current_loc, next_WP);
if (AB.length() < 1.0e-6f) {
AB = Vector2f(cosf(_ahrs.yaw), sinf(_ahrs.yaw));
}
}
AB.normalize();
// Calculate the NE position of the aircraft relative to WP A
Vector2f A_air = location_diff(prev_WP, _current_loc);
// calculate distance to target track, for reporting
_crosstrack_error = A_air % AB;
//Determine if the aircraft is behind a +-135 degree degree arc centred on WP A
//and further than L1 distance from WP A. Then use WP A as the L1 reference point
//Otherwise do normal L1 guidance
float WP_A_dist = A_air.length();
float alongTrackDist = A_air * AB;
if (WP_A_dist > _L1_dist && alongTrackDist/MAX(WP_A_dist, 1.0f) < -0.7071f)
{
//Calc Nu to fly To WP A
Vector2f A_air_unit = (A_air).normalized(); // Unit vector from WP A to aircraft
xtrackVel = _groundspeed_vector % (-A_air_unit); // Velocity across line
ltrackVel = _groundspeed_vector * (-A_air_unit); // Velocity along line
Nu = atan2f(xtrackVel,ltrackVel);
_nav_bearing = atan2f(-A_air_unit.y , -A_air_unit.x); // bearing (radians) from AC to L1 point
} else { //Calc Nu to fly along AB line
//Calculate Nu2 angle (angle of velocity vector relative to line connecting waypoints)
xtrackVel = _groundspeed_vector % AB; // Velocity cross track
ltrackVel = _groundspeed_vector * AB; // Velocity along track
float Nu2 = atan2f(xtrackVel,ltrackVel);
//Calculate Nu1 angle (Angle to L1 reference point)
float sine_Nu1 = _crosstrack_error/MAX(_L1_dist, 0.1f);
//Limit sine of Nu1 to provide a controlled track capture angle of 45 deg
sine_Nu1 = constrain_float(sine_Nu1, -0.7071f, 0.7071f);
float Nu1 = asinf(sine_Nu1);
// compute integral error component to converge to a crosstrack of zero when traveling
// straight but reset it when disabled or if it changes. That allows for much easier
// tuning by having it re-converge each time it changes.
if (_L1_xtrack_i_gain <= 0 || !is_equal(_L1_xtrack_i_gain, _L1_xtrack_i_gain_prev)) {
_L1_xtrack_i = 0;
_L1_xtrack_i_gain_prev = _L1_xtrack_i_gain;
} else if (fabsf(Nu1) < radians(5)) {
_L1_xtrack_i += Nu1 * _L1_xtrack_i_gain * dt;
// an AHRS_TRIM_X=0.1 will drift to about 0.08 so 0.1 is a good worst-case to clip at
_L1_xtrack_i = constrain_float(_L1_xtrack_i, -0.1f, 0.1f);
}
// to converge to zero we must push Nu1 harder
Nu1 += _L1_xtrack_i;
Nu = Nu1 + Nu2;
_nav_bearing = atan2f(AB.y, AB.x) + Nu1; // bearing (radians) from AC to L1 point
}
_prevent_indecision(Nu);
_last_Nu = Nu;
//Limit Nu to +-pi
Nu = constrain_float(Nu, -1.5708f, +1.5708f);
_latAccDem = K_L1 * groundSpeed * groundSpeed / _L1_dist * sinf(Nu);
// Waypoint capture status is always false during waypoint following
_WPcircle = false;
_bearing_error = Nu; // bearing error angle (radians), +ve to left of track
}
