Sensoria Obscura: Ultrasonic Sensors

Ultrasonic sensors have been around for decades. If you’ve ever been inside of a car that beeps when it’s close to something, you’ve certainly experienced them in action. They’re also commonly found in factories, hospitals, warehouses, forklifts, robots, autonomous vehicles, and in every Tesla model until now (given their recent announcement to leave them behind).

In this chapter of our Sensoria Communia, Sensoria Obscura series, we’ll take a look at how ultrasonic sensors work, their pros and cons, and how they are used in perception arrays for robotic and autonomous vehicle systems. We’ll name a few noteworthy sensors and discuss the technical and commercial reasons why they may or may not remain a staple choice for developers of autonomous platforms.

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How do ultrasonic sensors work?

Much like bats use echolocation, ultrasonic sensors emit high-frequency sound waves (typically between 20-40 kHz) to measure the amount of time it takes for the sound to bounce back in order to detect objects, determine distance, and navigate environments.

Since ultrasonic sensors measure with sound, they can be used under any lighting conditions. And unlike LiDAR and depth cameras, they are not thrown off by the translucency or transparency of objects nor materials that warp or bend light like water or glass. The precision of their measurements are instead impacted by the acoustic transparency of objects (soft materials that absorb sound like foam or wool) and environmental conditions like temperature and humidity.

As (near or) far as range goes, most ultrasonic sensors can detect as near as a two centimeters up to around ten meters. Given that they operate at the speed of sound, their measurements are quick (multiple range measurements per second), with refresh rates between 1 to 50 Hz depending on the application. While many ultrasonic sensors boast ± 1% accuracy, it’s safe to assume that specification was determined in near perfect conditions.

Let’s recap their pros and cons:

Pros

• Can work in any lighting scenario
• Not affected by color or transparency
• Low-cost
• Some are self-cleaning
• Reliable, well understood technology

Cons

• Not great at detecting poles, small objects, or multiple objects at high speeds
• Can’t operate without air
• Precision affected by soft materials
• Limited range
  

How Ultrasonic Sensors are Used in Autonomy

Since the 1990s, ultrasonic sensors embedded in vehicles have provided human drivers with parking assistance, and with blind spot detection in more recent years — but their unique sensing abilities support several important perception capabilities for autonomy. In autonomous vehicles and ADAS systems, ultrasonic sensors serve as proximity sensors in parking technology and anti-collision safety systems that perform reliably through fog, rain, dust, and snow. In mobile robots, they are mainly used for object detection and avoidance.

By emitting sound waves and measuring the time it takes for them to bounce back, ultrasonic sensors can detect changes in velocity and the distance of the objects in their field of view. This allows autonomous systems to respond to changes in their surroundings, like the appearance of new obstacles or the movement of other objects.

Ultimately, ultrasonic sensors help autonomous vehicles and robots navigate their environments, react to their surroundings in real-time, and avoid collisions — all of which are critical for safety.

Noteworthy Sensors

HC-SR04 Ultrasonic Sensor

The HC-SR04 Ultrasonic Sensor is an inexpensive choice that’s easy to use and is one of the most popular ultrasonic sensors used in robotics and automation projects by companies and hobbyists alike. You can find its specs here in our datasheet library.

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Bosch

The 6th generation of Bosch’s ultrasonic sensor is known for its sensitivity and is available in three mechanically compatible sensor variants:

1. Detection range: max. 2.5 m, min. 15 cm; object presence 6 cm
2. Detection range: max. 4.5 m, min. 15 cm; object presence 6 cm
3. Detection range: max. 5.5 m, min. 15 cm; object presence 3 cm (Variant for both passenger car and off-highway systems)

You can also find all of its specifications in our datasheet library here.

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Toposens’ ECHO ONE®

In July of 2022, Munich-based startup Toposens released the 3D Ultrasonic Echolocation Sensor ECHO ONE® which provides a point cloud-like output. Their approach provides greater levels of accuracy and object information as a result. It boasts a range between 10 mm - 3000 mm and a field-of-view of up to 160° horizontal by 80° vertical. As for price, you’ll need to go through their sales team to get one.

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Will ultrasonic sensors remain a staple in autonomy?

For most developers, the price point, accuracy, and reliable performance in challenging environments is tough to beat. With decades of use in a wide range of applications, ultrasonic sensor technology is mature, reliable, well understood, and a safe choice yielding consistent results. And while ultrasonic sensors provide many useful perception functionalities for autonomous systems, we have yet to fully realize how the collective ultrasonic data from connected vehicles can be used on a larger scale for vehicle-to-vehicle (V2V), vehicle-to-infrastructure (V2I), and vehicle-to-pedestrian (V2P) communications, collectively known as V2X (Vehicle-to-Everything Communications). That’s exciting stuff!

If you’re looking into adding an ultrasonic sensor to your own array and have questions about integration, calibration, or fusion, we’re always happy to chat! And if you enjoyed this post, we welcome you to subscribe to our newsletter to get our next post delivered straight to your inbox.