ardupilot/libraries/AC_Avoidance/AC_Avoid.cpp

#include "AC_Avoid.h"

const AP_Param::GroupInfo AC_Avoid::var_info[] = {

    // @Param: ENABLE
    // @DisplayName: Avoidance control enable/disable
    // @Description: Enabled/disable stopping at fence
    // @Values: 0:None,1:StopAtFence
    // @User: Standard
    AP_GROUPINFO("ENABLE", 1,  AC_Avoid, _enabled, AC_AVOID_STOP_AT_FENCE),

    AP_GROUPEND
};

/// Constructor
AC_Avoid::AC_Avoid(const AP_AHRS& ahrs, const AP_InertialNav& inav, const AC_Fence& fence)
    : _ahrs(ahrs),
      _inav(inav),
      _fence(fence)
{
    AP_Param::setup_object_defaults(this, var_info);
}

void AC_Avoid::adjust_velocity(const float kP, const float accel_cmss, Vector2f &desired_vel)
{
    // exit immediately if disabled
    if (_enabled == AC_AVOID_DISABLED) {
        return;
    }

    // limit acceleration
    float accel_cmss_limited = MIN(accel_cmss, AC_AVOID_ACCEL_CMSS_MAX);

    if (_enabled == AC_AVOID_STOP_AT_FENCE) {
        adjust_velocity_circle(kP, accel_cmss_limited, desired_vel);
        adjust_velocity_poly(kP, accel_cmss_limited, desired_vel);
    }
}

/*
 * Adjusts the desired velocity for the circular fence.
 */
void AC_Avoid::adjust_velocity_circle(const float kP, const float accel_cmss, Vector2f &desired_vel)
{
    // exit if circular fence is not enabled
    if ((_fence.get_enabled_fences() & AC_FENCE_TYPE_CIRCLE) == 0) {
        return;
    }

    // exit if the circular fence has already been breached
    if ((_fence.get_breaches() & AC_FENCE_TYPE_CIRCLE) != 0) {
        return;
    }

    // get position as a 2D offset in cm from ahrs home
    const Vector2f position_xy = get_position();

    float speed = desired_vel.length();
    // get the fence radius in cm
    const float fence_radius = _fence.get_radius() * 100.0f;
    // get the margin to the fence in cm
    const float margin = get_margin();

    if (!is_zero(speed) && position_xy.length() <= fence_radius) {
        // Currently inside circular fence
        Vector2f stopping_point = position_xy + desired_vel*(get_stopping_distance(kP, accel_cmss, speed)/speed);
        float stopping_point_length = stopping_point.length();
        if (stopping_point_length > fence_radius - margin) {
            // Unsafe desired velocity - will not be able to stop before fence breach
            // Project stopping point radially onto fence boundary
            // Adjusted velocity will point towards this projected point at a safe speed
            Vector2f target = stopping_point * ((fence_radius - margin) / stopping_point_length);
            Vector2f target_direction = target - position_xy;
            float distance_to_target = target_direction.length();
            float max_speed = get_max_speed(kP, accel_cmss, distance_to_target);
            desired_vel = target_direction * (MIN(speed,max_speed) / distance_to_target);
        }
    }
}

/*
 * Adjusts the desired velocity for the polygon fence.
 */

void AC_Avoid::adjust_velocity_poly(const float kP, const float accel_cmss, Vector2f &desired_vel)
{
    // exit if the polygon fence is not enabled
    if ((_fence.get_enabled_fences() & AC_FENCE_TYPE_POLYGON) == 0) {
        return;
    }

    // exit if the polygon fence has already been breached
    if ((_fence.get_breaches() & AC_FENCE_TYPE_POLYGON) != 0) {
        return;
    }

    // get polygon boundary
    // Note: first point in list is the return-point (which copter does not use)
    uint16_t num_points;
    Vector2f* boundary = _fence.get_polygon_points(num_points);

    // exit if there are no points
    if (boundary == NULL || num_points == 0) {
        return;
    }

    // do not adjust velocity if vehicle is outside the polygon fence
    const Vector3f& position = _inav.get_position();
    Vector2f position_xy(position.x, position.y);
    if (_fence.boundary_breached(position_xy, num_points, boundary)) {
        return;
    }

    // Safe_vel will be adjusted to remain within fence.
    // We need a separate vector in case adjustment fails,
    // e.g. if we are exactly on the boundary.
    Vector2f safe_vel(desired_vel);

    uint16_t i, j;
    for (i = 1, j = num_points-1; i < num_points; j = i++) {
        // end points of current edge
        Vector2f start = boundary[j];
        Vector2f end = boundary[i];
        // vector from current position to closest point on current edge
        Vector2f limit_direction = closest_point(position_xy, start, end) - position_xy;
        // distance to closest point
        const float limit_distance = limit_direction.length();
        if (!is_zero(limit_distance)) {
            // We are strictly inside the given edge.
            // Adjust velocity to not violate this edge.
            limit_direction /= limit_distance;
            limit_velocity(kP, accel_cmss, safe_vel, limit_direction, limit_distance);
        } else {
            // We are exactly on the edge - treat this as a fence breach.
            // i.e. do not adjust velocity.
            return;
        }
    }

    desired_vel = safe_vel;
}

/*
 * Limits the component of desired_vel in the direction of the unit vector
 * limit_direction to be at most the maximum speed permitted by the limit_distance.
 *
 * Uses velocity adjustment idea from Randy's second email on this thread:
 * https://groups.google.com/forum/#!searchin/drones-discuss/obstacle/drones-discuss/QwUXz__WuqY/qo3G8iTLSJAJ
 */
void AC_Avoid::limit_velocity(const float kP, const float accel_cmss, Vector2f &desired_vel, const Vector2f limit_direction, const float limit_distance) const
{
    const float max_speed = get_max_speed(kP, accel_cmss, limit_distance - get_margin());
    // project onto limit direction
    const float speed = desired_vel * limit_direction;
    if (speed > max_speed) {
        // subtract difference between desired speed and maximum acceptable speed
        desired_vel += limit_direction*(max_speed - speed);
    }
}

/*
 * Gets the current xy-position, relative to home (not relative to EKF origin)
 */
Vector2f AC_Avoid::get_position()
{
    const Vector3f position_xyz = _inav.get_position();
    const Vector2f position_xy(position_xyz.x,position_xyz.y);
    const Vector2f diff = location_diff(_inav.get_origin(),_ahrs.get_home()) * 100.0f;
    return position_xy - diff;
}

/*
 * Computes the speed such that the stopping distance
 * of the vehicle will be exactly the input distance.
 */
float AC_Avoid::get_max_speed(const float kP, const float accel_cmss, const float distance) const
{
    return AC_AttitudeControl::sqrt_controller(distance, kP, accel_cmss);
}

/*
 * Computes distance required to stop, given current speed.
 *
 * Implementation copied from AC_PosControl.
 */
float AC_Avoid::get_stopping_distance(const float kP, const float accel_cmss, const float speed) const
{
    // avoid divide by zero by using current position if the velocity is below 10cm/s, kP is very low or acceleration is zero
    if (kP <= 0.0f || accel_cmss <= 0.0f || is_zero(speed)) {
        return 0.0f;
    }

    // calculate point at which velocity switches from linear to sqrt
    float linear_speed = accel_cmss/kP;

    // calculate distance within which we can stop
    if (speed < linear_speed) {
        return speed/kP;
    } else {
        float linear_distance = accel_cmss/(2.0f*kP*kP);
        return linear_distance + (speed*speed)/(2.0f*accel_cmss);
    }
}

/*
 * Returns the point closest to p on the line segment (v,w).
 *
 * Comments and implementation taken from
 * http://stackoverflow.com/questions/849211/shortest-distance-between-a-point-and-a-line-segment
 */
Vector2f AC_Avoid::closest_point(Vector2f p, Vector2f v, Vector2f w) const
{
    // length squared of line segment
    const float l2 = (v - w).length_squared();
    if (is_zero(l2)) {
        // v == w case
        return v;
    }
    // Consider the line extending the segment, parameterized as v + t (w - v).
    // We find projection of point p onto the line.
    // It falls where t = [(p-v) . (w-v)] / |w-v|^2
    // We clamp t from [0,1] to handle points outside the segment vw.
    const float t = ((p - v) * (w - v)) / l2;
    if (t <= 0) {
        return v;
    } else if (t >= 1) {
        return w;
    } else {
        return v + (w - v)*t;
    }
}