1. Centered Corridors

We define centered corridors for our drones through our Ground Control Station. The drones should decide how to move in these corridors without collision in a decentralized manner. These centered corridors have two specifications, their center points and directions. In what follows they are described.

1.1. Points of Corridors

We define a list of ordered points as center of a corridor in which each drone should fly without collision based on their decentralized swarming rules. It is noteworthy that the points are not time-based, but they are ordered which means that when a drone is flying based on its current point of the list, tries to pass the next point. When it has passed the next point, the point is considered as the drone’s current one. The list containing the points of each drone’s corridor is sent to each drone thorough our Ground Control Station. For instance, the equation of center points of a corridor as an ellipse with origin point of (x0, y0, z0) and width of 2a and length of 2b is: (x-x0)2/a2 +(y-y0)2/b2 = 1, z=z0. The list containing coordination of the points is created through the following piece of code in Python:

for i in range(points_number):
    Points.append(numpy.array([a*math.sin(i*6.28/points_number) + x_0, b*math.cos(i*6.28/points_number)+y_0, z_0]))

If we consider x0 as 25, y0 as 50, z0 as 20, a (width) as 25 and b (length) as 50, we have an ellipse as shown below:

1.2. Directions of Corridors

A direction vector at a point of a corridor, is a vector that a drone should fly and move forward along with in the corridor. Therefore, we create a list of directions, in which nth element (nth direction) is a normalized vector from point nth to (n+1)th. The list containing vector of directions is obtained through the following piece of code in Python:

for i in range(len(Points)):
    if i==len(Points)-1: # for the last point
        Directions.append((Points[0]-Points[i])/numpy.linalg.norm(Points[0]-Points[i])) #normalized direction vector
        Directions.append((Points[i+1]-Points[i])/numpy.linalg.norm(Points[i+1]-Points[i])) #normalized direction vector

We use these direction vectors as the direction of the migration velocity, and to obtain lane cohesion error vector described in what follows.

2. Velocities & Swarming Rules During a Mission

There are four target velocities to make drones move forward and rotate at the right distance from the direction vectors without collision in corridors. These velocities are described in what follows.

2.1. Lane Cohesion Velocity

We want each drone to rotate around the line from its current point to the next point at a radius called lane radius. So we should have a velocity/force to make the drone at the right distance (lane radius) from the line. Therefore, we call this velocity/force as lane cohesion velocity/force that provides centripetal force to keep the drone at the right distance from the line. To obtain the magnitude and direction of this velocity, at first, we define lane cohesion error as shown in the following figure.

As it seems from the previous figure, lane cohesion error is the shortest vector from the line to the drone. So, magnitude of lane cohesion velocity should be proportional to magnitude of lane cohesion error minus lane radius, and its direction should be along with opposite direction of the normalized lane cohesion error vector. Therefore, the code to obtain it in Python is:


if np.linalg.norm(lane_cohesion_position_error) != 0:
    v_lane_cohesion = -1*(lane_cohesion_error_magnitude -lane_radius)*lane_cohesion_error/np.linalg.norm(lane_cohesion_error)

We will use lane cohesion error to obtain velocity of rotation in the following sections.

2.2. Migration Velocity

Migration velocity/force is along with the direction of the current point (a normalized vector from the previous point of current point to the current point) to make drones move forwad. Keep in mind that this velocity is perpendicular to lane cohesion error and velocity. The magnitude of this velocity is 1 that we can adjust it by gains assigned by a user.

v_migration = Directions[current_point]

2.3. Rotation Velocity

Each drone should rotate around its current direction vector. The direction of this rotation is perpendicular to both the direction vector and lane cohesion error vector. So the direction of this velocity should be the cross product of them. We define the magnitude of rotation velocity as the magnitude of lane cohesion error over lane radius if lane cohesion error is less than lane radius, and define it as lane radius over the magnitude of lane cohesion error if lane cohesion error is equal or greater than lane radius. Therefore, the magnitude is always less than or equal to 1, and it decreases as a drone gets farther from the lane radius around the direction vector of its current point.

if lane_cohesion_error_magnitude < lane_radius:
    v_rotation_magnitude = (lane_cohesion_error_magnitude /lane_radius)
    v_rotation_magnitude = (lane_radius / lane_cohesion_error_magnitude)
cross_prod=numpy.cross(lane_cohesion_error, directions[current_point]
if (np.linalg.norm(cross_prod)) != 0):
    v_rotation = v_rotation_magnitude*cross_prod/numpy.linalg.norm(cross_prod))
    v_rotation = numpy.array([0, 0, 0])

2.4. Separation Velocity

To prevent collision of drones in swarming, we define a velocity/force called separation velocity to separate them. Each drone has a feedback of other drones’ velocities, so each drone can calculate its own separation velocity. The whole separation velocity for a drone, is the sum of separation velocities between the drone and other drones. The direction of each separation velocity is along with a vector from another drone to the drone (to push the drone away), and its magnitude is based on their distance. The figure of the magnitude of separation velocity between two drones are shown below:

And the equation of separation velocity between two drone based on the above figure is:

In the above equation, rconf, rcoll and vsep refer to radius of conflict area, radius of collision area and separation velocity, respectively. The Python code to calculate total separation velocity is:

v_separation = np.array([0, 0, 0])
for drone in swarm:
    if drone == Drone:
    x = Drone.position - numpy.array(drone.position)
    d = np.linalg.norm(x)
    if d <= r_conflict and d > r_collision and d != 0:
        v_separation = v_separation + ((x / d) * (r_conflict - d / r_conflict - r_collision))
    if d <= r_collision and d != 0:
        v_separation = v_separation + 1 * (x / d)

Note that object Drone in the above code refers to the drone currently running the code, and swarm contains objects of all the drones.

2.5. Passing Points

We should define a mechanism to check if the drone has passed the points next to its current point or not. If we just consider closest distance to determine the drone’s current point, we will face a problem as shown in the photo below:

As we can see, when the drone was in the middle of its way from point 0 to 1, it changed its current point to 1, and changed its path and direction to point 1 to 2 without completing its path from point 0 to 1. This problem is more sensible when the number of the points are not high. To solve this problem and make the movement of the drones smoother, we define a mechanism based on dot product (not closest distance) to determine and find out when a drone has passed a point next to its current point. The mechanism explains that a drone has passed point m if:

  1. It has passed all points from the current point to m-1.

  2. The dot product of the vector from point m to drone and direction vector from point m-1 to m is equal or greater than zero. The dot product is shown as follows:

Then if the drone has passed point m, point m will be the current point. We continue to find the passed points as long as the dot product for points after the current point is zero or positive. When the dot product is negative, the loop to find the current point stops until the next time we run the loop to find the current point. And the result of the mechanism for the previous example is:

3. Preparations after and before a Mission