ardupilot/APMrover2/mode_follow.cpp

#include "mode.h"
#include "Rover.h"

// initialize follow mode
bool ModeFollow::_enter()
{
    if (!g2.follow.enabled()) {
        return false;
    }

    // initialise waypoint speed
    set_desired_speed_to_default();

    // initialise heading to current heading
    _desired_yaw_cd = ahrs.yaw_sensor;
    _yaw_error_cd = 0.0f;

    return true;
}

void ModeFollow::update()
{
    // stop vehicle if no speed estimate
    float speed;
    if (!attitude_control.get_forward_speed(speed)) {
        // no valid speed so stop
        g2.motors.set_throttle(0.0f);
        g2.motors.set_steering(0.0f);
        return;
    }

    Vector3f dist_vec;  // vector to lead vehicle
    Vector3f dist_vec_offs; // vector to lead vehicle + offset
    Vector3f vel_of_target; // velocity of lead vehicle

    // if no target simply stop the vehicle
    if (!g2.follow.get_target_dist_and_vel_ned(dist_vec, dist_vec_offs, vel_of_target)) {
        _reached_destination = true;
        stop_vehicle();
        return;
    }

    // calculate desired velocity vector
    Vector2f desired_velocity_ne;
    const float kp = g2.follow.get_pos_p().kP();
    desired_velocity_ne.x = vel_of_target.x + (dist_vec_offs.x * kp);
    desired_velocity_ne.y = vel_of_target.y + (dist_vec_offs.y * kp);

    // if desired velocity is zero stop vehicle
    if (is_zero(desired_velocity_ne.x) && is_zero(desired_velocity_ne.y)) {
        _reached_destination = true;
        stop_vehicle();
        return;
    }

    // we have not reached the target
    _reached_destination = false;

    // scale desired velocity to stay within horizontal speed limit
    float desired_speed = safe_sqrt(sq(desired_velocity_ne.x) + sq(desired_velocity_ne.y));
    if (!is_zero(desired_speed) && (desired_speed > _desired_speed)) {
        const float scalar_xy = _desired_speed / desired_speed;
        desired_velocity_ne *= scalar_xy;
        desired_speed = _desired_speed;
    }

    // calculate vehicle heading
    _desired_yaw_cd = wrap_180_cd(atan2f(desired_velocity_ne.y, desired_velocity_ne.x) * DEGX100);

    // run steering and throttle controllers
    calc_steering_to_heading(_desired_yaw_cd, desired_speed < 0);
    calc_throttle(calc_reduced_speed_for_turn_or_distance(desired_speed), false, true);
}