An I/O board such as Arduino can be connected to multiple sensors and actuators. In this section, we will program Arduino to control HC-SR04, an ultrasonic range sensor to be used in ROS. HC-SR04 sensors are low-cost and are commonly employed to measure distances between robots and obstacles.

This device is a Printed Circuit Board (PCB) with two piezoelectric transducers that are similar to microphones and piezoelectric headphones, but for the fact that they work in the ultrasonic range. One of the transducers is used as an emitter and the other as a receiver. The principle for measuring distance with this sensor is based on the echolocation system of animals such as bats. The sensor will emit ultrasound. If the ultrasound bounces against an obstacle, it will return to the sensor. Knowing the speed of sound in the air, we will measure the time between the emission and the reception of the sound to calculate the distance to the obstacle.

The HC...