The HC-SR04 uses ultrasound to navigate the robot. The picture show two HC-SR04s, front on the top and back on the bottom. The piezoelectric ceramic transducers transmit (T) and receive (R) ultrasound.
The robot will mimic bats by using echolocation to navigate. Bats use a variety of ultrasonic ranging (echolocation) techniques frequencies beyond 100 kHz, possibly up to 200 kHz. The human can hear sound frequencies from 20 to 20,000 Hz. The HC-SR04 ultrasound device contains piezoelectric ultrasound sensors that emit and resonant to ultrasound at 42 kHz that we cannot hear.
The backside of the HC-SR04 unit shows three chips, left is an amplifier, center is microcontroller, and right is for serial communication. This project contains a microcontroller to control a microcontroller. This HC-SRO4 is triggered to send out a burst of ultrasound, measure the time it takes the echo to return, and then send the time report to the Arduino as serial communication. The center chip is another microcontroller and its ID is being kept proprietary.