Researchers at the Georgia Institute of Technology have created a passive motion/contact sensor that detects motion using only sound. Without electricity, electronics, or batteries. The sensors are small, uniquely shaped metal tags that each emit very specific ultrasonic sounds upon contact. They are smaller than a penny and cost only a few cents each.
Most smart home sensors require some form of power to function. Either they are permanently plugged in or require a battery. Sensors that require direct power are highly limited in where they can be positioned. Battery-powered sensors, on the other hand, need to be regularly recharged or have their batteries replaced.
The researchers found a much simpler way to build sensing technology. Their invention is a small, flat, lightweight metal tag that’s basically a hollowed-out ring. Kind of like a washer but with various cuts on its outer circumference.
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The physics behind these tags is simple but ingenious. Most objects make a sound when you strike them, with their shape and material determining how they sound. Metals are particularly sonorous, as anyone who has been around toddlers (and pots and pans) can testify.
The researchers took ordinary metal rings and precisely shaped them into specific patterns designed to produce distinct ultrasonic frequencies when struck. To create these patterns, they developed a modeling and simulation tool that generated ring geometries matched to specific frequencies. Using the tool, the team identified more than a thousand unique designs, each capable of producing its own ultrasonic tone.
They then fabricated some of these designs, creating 15 distinct metal tags, each with its own unique ultrasonic “fingerprint.” When a tag is struck, a wearable mic, such as a smartwatch or phone, picks up the sound, and accompanying software identifies exactly which tag was struck based on its unique signature.
“Those unique fingerprints can be used for smart home sensing, or what we call ‘activity recognition,’” said Yibo Fu, PhD student and leader of the research team.
An interesting feature of the tags is that they retain a high signal-to-noise ratio. This means that even in noisy environments, the signals remain clearly perceptible to the mics.
The full sensor is a three-part device consisting of a metal tag, a plastic base on which the tag sits, and a small, separate plastic tab that strikes the tag. The sensor is designed for areas where there is natural recurring contact between two surfaces or parts of an object. For example, a door, a window, or a toilet seat.
Here’s how it works, using the door example: the plastic base with the mounted metal tag is attached to the door frame. The plastic tab is attached to the moving door in a position that aligns it closely enough to contact the metal tag. Whenever the door is opened or closed, the tab strikes the tag, producing a distinct ultrasonic sound that is picked up by a nearby microphone. Because the sound is unique, a user can tell exactly which door was opened.
The same setup can be applied to faucets, toilet seats, drawer doors, and so on. They would also work great in the gym for counting reps.
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In a recent Instagram video, Fu presented the tags to the public, detailing how they worked and potential use cases. The video went viral, garnering almost two million views, an indication that the researchers had an interesting product on their hands. Among the hundreds of comments, people proposed various other interesting use cases for the tags.
“One mentioned using the tags in archiving systems where you have huge shelves and libraries of boxes. When you remove a box or store a new box, there’s a rapid motion, and you would trigger the tags and know exactly what thing you just opened, closed, or archived. Someone else mentioned tracking locations for thousands of garbage and recycling bins in waste management systems,” says Fu.
While they are not high-tech or feature-rich, the tags are highly functional and practical for many use cases. Plus, ultra-cheap, maintenance-free sensing that could quietly operate for years without ever needing a battery replacement or wired connection is definitely a yes.
These tiny ultrasonic tags could one day enable massive networks of low-cost sensors embedded into homes, buildings, warehouses, and public infrastructure, all without adding to the growing burden of battery waste and power consumption.
A paper on the research was recently published in the journal Proceedings of the ACM on Interactive, Mobile, Wearable and Ubiquitous Technologies.
Source: Georgia Tech
