L1 control——ArduPilot——更新航点update_waypoint（二）

本文详细讲解了ArduPilot中AP_L1_control库中的update_waypoint函数的具体实现！！！

下面是源代码：

// 更新L1 control的航点
void AP_L1_Control::update_waypoint(const struct Location &prev_WP, const struct Location &next_WP, float dist_min)
{

    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;
        _L1_xtrack_i = 0.0f;
    }
    _last_update_waypoint_us = now;

    // 用特定的阻尼来计算L1的增益
    float K_L1 = 4.0f * _L1_damping * _L1_damping;

    // 获取当前飞机的位置
    if (_ahrs.get_position(_current_loc) == false) {
        // if no GPS loc available, maintain last nav/target_bearing
        _data_is_stale = true;
        return;
    }

    Vector2f _groundspeed_vector = _ahrs.groundspeed_vector();

    // 更新目标方位角
    _target_bearing_cd = get_bearing_cd(_current_loc, next_WP);

    //计算地速
    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(get_yaw()), sinf(get_yaw())) * groundSpeed;
    }

    // 计算时变控制参数
    // 根据_L1_period计算特定L1长度
    // 0.3183099 = 1/pi
    _L1_dist = MAX(0.3183099f * _L1_damping * _L1_period * groundSpeed, dist_min);

    // 计算航点A与B的北东位置长度
    Vector2f AB = location_diff(prev_WP, next_WP);
    float AB_length = AB.length();

    //如果AB距离很短，那么就直接将B点记作当前目标点，径直朝向B
    if (AB.length() < 1.0e-6f) {
        AB = location_diff(_current_loc, next_WP);
        if (AB.length() < 1.0e-6f) {
            AB = Vector2f(cosf(get_yaw()), sinf(get_yaw()));
        }
    }
    AB.normalize();

    // 计算航点A到飞机的北东距离长度信息
    Vector2f A_air = location_diff(prev_WP, _current_loc);

    // 计算航迹跟踪误差
    _crosstrack_error = A_air % AB;


    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)
    {
        //飞机航向与A_air航向反方向夹角超过135度，且L1_dist < WP_A_dist,则指向A
        //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 if (alongTrackDist > AB_length + groundSpeed*3) {
        // 飞机超过了航点B3秒的时间，那么指向B
        // Calc Nu to fly To WP B
        Vector2f B_air = location_diff(next_WP, _current_loc);
        Vector2f B_air_unit = (B_air).normalized(); // Unit vector from WP B to aircraft
        xtrackVel = _groundspeed_vector % (-B_air_unit); // Velocity across line
        ltrackVel = _groundspeed_vector * (-B_air_unit); // Velocity along line
        Nu = atan2f(xtrackVel,ltrackVel);
        _nav_bearing = atan2f(-B_air_unit.y , -B_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);

        // 计算积分项，减少稳态误差
        if (_L1_xtrack_i_gain <= 0 || !is_equal(_L1_xtrack_i_gain.get(), _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/2)
    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

    _data_is_stale = false; // status are correctly updated with current waypoint data
}

单纯看代码可能略显枯燥，可以结合ArduPilot——AP_L1_control——更新航点update_waypoint（一）与下图一起参考借鉴。

../_images/L1_WP_following.png

注意：update_waypoint<——>waypoint following。上图给出的就是飞机在追踪航点的一个较为直观，且提供了代码对应的数学公式，以及一些跟踪逻辑，这更加有助于理解L1 guidance。

_latAccDem最终会被用来计算飞机的目标roll—nav_roll_cd，见下面代码。从而将nav_roll_cd作为Roll controller的输入值，进一步通过PID控制器来控制roll角。

int32_t AP_L1_Control::nav_roll_cd(void) const
{
    float ret;
    ret = cosf(_ahrs.pitch)*degrees(atanf(_latAccDem * 0.101972f) * 100.0f); // 0.101972 = 1/9.81
    ret = constrain_float(ret, -9000, 9000);
    return ret;
}

画圈跟踪可以参看L1 control——ArduPilot——更新圆圈update_Loiter

L1 control——ArduPilot——更新航点update_waypoint（二）

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