Deploying thousands of sensors strategically throughout a city supports informed decision making in areas such as infrastructure management, public health, and resource management. When considering the benefits, it seems like a no-brainer to deploy sensors broadly and make our cities smarter. But how many city-scale sensor deployments can you name? Likely very few, because they are rare cases. The reality is that each weather-hardened sensor, which requires power and some form of connectivity, can cost thousands of dollars just to install, not to mention future maintenance costs.
Carnegie Mellon University researchers are working to bring these costs down with a scheme that requires only a small number of centralized sensing devices. They call their method Vibrosight++, and it focuses on one particular type of sensor called a laser vibrometer. Laser vibrometry has applications in sensing traffic flow, bus arrival times, wind conditions, and infrastructure health. This type of sensor is generally only used at a range of a few meters. However, the team showed that through the use of reflective surfaces, that range can be extended over one kilometer. And as they point out, reflective surfaces are already ubiquitous in cities on street signs, construction barriers, road studs, and license plates, for example.
The Vibrosight++ system consists of a pan-tilt motor platform that allows for precise aiming, and a sensor bundle. Among the sensors are a photodetector and range finder, as well as a 200mW 650nm laser. An optical band-pass filter was included to remove ambient light (e.g., sunlight, reflections, etc.). The total cost of the prototype was approximately $600.
The system is connected to a laptop via USB for data processing. The processing pipeline initially scans an area of interest with the rangefinder and photodetector. This allows a map of all appropriate, reflective surfaces to be created. Vibrometry data is then captured by pointing the laser at an object of interest. Vibrations in the object cause oscillations in the reflected pattern of light, which are sensed by the photodetector as a rise and fall of received light intensity.
Vibrosight++ was compared with an accelerometer, which is considered a gold standard sensor, to determine the system’s level of accuracy. Both the accelerometer and reflective tape were placed on a large diaphragm speaker and measurements were collected from a distance of 128 meters. The signal from Vibrosight++ was found to be considerably more coarse and noisy, yet the fidelity was found to be more than sufficient for many use cases.
The system was next evaluated in a real-world city environment. The team found Vibrosight++ to work well in daylight conditions, where it was able to cover a circular area of one kilometer in diameter.
It should be noted that the system requires line-of-sight for all sensing applications, which may complicate the task of placing a small number of devices around the city. Moreover, since Vibrosight++ only periodically looks at each object of interest, transient events may be missed while other objects are being investigated. The high-power laser used also has the potential to cause vision damage if inadvertently pointed at the eyes of any bystanders. They have worked to mitigate this risk by limiting the laser pulses to 50 microseconds, which is considered a safe amount of exposure.