To the naked eye, buildings and bridges appear fixed in place, unmoved by forces like wind and rain. But in fact, these large structures do experience imperceptibly small vibrations that, depending on their frequency, may indicate instability or structural damage.
MIT researchers have developed a technique to “see” the vibrations invisible to the naked eye, combining high-speed video with computer vision techniques called motion magnification. “This technique provides a faster, cheaper, and noninvasive alternative to existing monitoring techniques. This could be a noncontact sensor technology that can be used for economic and speedy applications”,Oral Buyukozturk, an MIT professor says.
Normally, high-speed video wouldn’t pick up such subtle vibrations from a building. But, the researchers employed a computer vision technique to break down high-speed frames into certain frequencies, essentially exaggerating tiny, subpixel motions.
In laboratory experiments, the researchers were able to detect tiny vibrations in a steel beam and a PVC pipe. The vibrations measured by the technique matched those picked up by accelerometers and laser vibrometry — precise but expensive techniques commonly used in infrastructure monitoring.
Today, engineers typically monitor infrastructure using multiple accelerometers — sensors that measure acceleration, which can then be used to calculate velocity and, ultimately, motion. Accelerometers are very precise, but expensive, costing more than $1,000 each, and a single accelerometer only measures a single point along a structure. Even with an array of sensors, Buyukozturk notes, accelerometers “can’t achieve high density of spatial measurements.”
As an alternative, laser vibrometry is a noncontact technique that exposes a structure to a laser beam and an acoustic wave, the velocities of which can be translated to calculate a structure’s displacement, or motion. This method also is incredibly precise — but, like accelerometers, laser vibrometry is time-consuming, measuring only a single point at a time.
Instead, the researchers speculated that a high-speed camera might quickly and easily track vibrations across an entire structure, without making physical contact.
To test the theory, Buyukozturk worked with Durand and Freeman, the original developers of the motion magnification algorithms.
In 2012, the pair presented a software that effectively boosts certain frequencies in video frames, making it possible to “see” tiny motions, like a person’s pulse, or a vibrating violin string.
Durand and Freeman worked with Buyukozturk, Chen, Wadhwa, and Cha to adapt their code to monitor infrastructure. The code essentially filters a video image into amplitude and phase signals, which can then be combined to reconstruct the video image in which the apparent motions of certain objects are magnified at certain frequencies.
The team carried out experiments using a Phantom v10 high-speed camera. The researchers set up an experiment to compare the technique with standard accelerometers and laser vibrometers. With each technique, the researchers measured the vibrations from a cantilever beam and a PVC pipe after striking them with a hammer. The subsequent measurements by the motion magnification technique compared well with those of the other sensors.
The researchers observed that, without implementing the algorithms, the high-speed videos showed both the beam and pipe remaining apparently immobile. Once they ran the algorithm on the video data, however, they observed a range of shape deformations in each structure as they vibrated. For instance, the beam appears to wobble back and forth, while the pipe’s circumference changes from a circle to an oval, and back again.
This technique may be useful in remotely monitoring buildings and bridges, and may be especially useful in surveying pipelines.
The group plans to carry out video-monitoring experiments of MIT’s Green Building (Building 54), as well as Boston’s John Hancock Tower, Prudential Tower, and Zakim Bridge. Buyukozturk points out that detecting vibrations in a building or bridge doesn’t necessarily mean there’s something wrong; every structure has a “fundamental frequency” at which it vibrates. Knowing that frequency, he says, may give engineers an idea of how a structure may respond to forces like wind, or even earthquakes.
