Safety Node

The Safety Node implements Automatic Emergency Breaking (AEB) through Time to Collision (TTC) calculations. This the time it would take for the car to collide with an obstacle if it maintained its current heading and velocity.

We approximate the time to collision using Instantaneous Time to Collision (iTTC), which is the ratio of instantaneous range to range rate calculated from current range measurements and velocity measurements of the vehicle:

where $r_i$ is the instantaneous range measurements and $\dot{r}$ is the current range rates. The operator $[x]_+$ is defined as $\max(x, 0)$.

We obtain:

• $r_i$ by using the current measurements from the LaserScan message
• $\dot{r}_i$ by mapping the vehicle’s current longitudinal velocity onto each scan beam’s angle by using $v_x\cos(\theta_i)$, where $v_x$ is the forward speed in the vehicle’s frame of reference (obtained from Odometry message), and $\theta_i$ is the beam angle obtained from LaserScan messages.

The code has been implemented in both Python3 (under scripts/wall_follow_node.py) and C++ (under src/safety_node.cpp). Both have the same functionalities.

Starting Up the Node

• To start up the Python node, run ros2 run safety_node safety_node.py
• To start up the C++ node, run ros2 run safety_node safety_node