Ultrasonic Sensor

Overview of ultrasonic sensors

Ultrasonic sensors are widely used in mobile robots to detect obstacles. They are also used in functions such as automatic braking at low speeds and obstacle warning in automobiles. Compared to millimeter-wave radar, LiDAR, and stereo cameras, which are expected to be powerful sensors for obstacle detection to realize automatic driving functions, ultrasonic sensors are not as accurate in estimating the position of objects. Nevertheless, because they are compact and inexpensive, ultrasonic sensors are actively used in autonomous driving robots such as self-driving cars to detect obstacles in a short distance at low speeds. It is one of the most important sensors for engineers to understand.

Ultrasonic sensors are sensors that use a type of sound wave called ultrasound to detect obstacles. Sound waves are what we call “sound”. Sound (sound waves) are waves that travel through the air. It propagates in the form of a repetition of low and high density areas of air. This is called a “coarse wave” and is classified as a “longitudinal wave. As shown in the figure below (left), a graph with the air density on the vertical axis and the position in space on the horizontal axis shows a sine wave, indicating that it is a wave. Also, as shown in the figure below (right), if time is taken on the horizontal axis, it becomes a sine wave.

Coarse and dense waves
Coarse and dense waves

Ultrasound refers to sound waves with a frequency higher than the human audible range. The human audible range is 20 kHz or less, so ultrasonic waves are in the range of 20 kHz or higher. Ultrasonic sensors use sound waves in this range to detect the presence or absence of obstacles. However, there is no exact definition of the frequency of ultrasonic waves; in some cases, it is considered to be 40 kHz or higher. In principle, it is possible to create a sensor that uses sound waves in the audible range, but this would be annoying because the sound would be heard every time the sensing action is performed, so the ultrasonic range is used to reduce the impact on humans. Since some animals, such as dogs, have a wide audible range and can hear sounds of 20 kHz or higher, consideration is sometimes given to using sounds of 40 kHz or higher or 100 kHz or higher with a margin.

(To be continued.)

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