# My F1TENTH Journey — Lab 2, Automatic Emergency Braking

This series of blogs marks the journey of my F1/10 Autonomous Racing Cars.

All my source codes can be accessed here.

Previous post:

## Overview

This lab focuses on implementing an AEB (Automatic Emergency Braking) in ROS node for F1/10 racing cars. AEB is widely used in autonomous vehicles as a basic safety guarantee to avoid collisions with objects.

The lab materials can be accessed here. A PDF version is also attached below.

The lab was built on the F1Tenth Simulator, which can be accessed here.

## Time-To-Collision

TTC (Time-To-Collision) is the time it would take for the vehicle to collide with an obstacle given its current heading and velocity. TTC can be calculated with the following format:

$TTC = \frac{r}{[-\dot{r}]_+}$

where $r$ is the distance between vehicle and obstacle, $\dot{r}$ is its 1st derivative with respect to time, and the symbol $[x]_+$ denotes $\max(0,x)$.

In practice, we use LiDAR results to calculate TTC for each beam. Specifically, we project the current vehicle velocity onto the direction of each beam as $\dot{r}$, namely $\dot{r}=v\cos(\theta)$.

sensor_msgs/LaserScan provides us with LiDAR beams in all directions. In each LaserScan message, we have angle_min, angle_max and angle_increment, which corresponds to the starting angle, the ending angle, and the step angle between two adjacent laser beams. All the angles are given in the local coordinate frame (positive $x$ direction is the vehicle’s heading) and positive $x$ direction is marked as angle value $0$.

We also have float32[] ranges in each LaserScan message, which gives us the distance to obstacle in the corresponding direction angle. Namely, ranges[i] <--> angle_min + i * angle_increment.

Given the above information, it is not difficult for us to write our AEB ROS node.

F1/10 Autonomous Racing Lecture recordings: My F1TENTH Journey — Lab 3, PID-Controlled Wall Follower My F1TENTH Journey — Lab 1, Introduction to ROS