According to the James Consulting, researchers at the National Institute of Standards and Technology (NIST) used the green laser pointer radar (LiDAR) system to perform 3D imaging of objects that melt in flames. An accurate, safe, and easy way to measure structural changes in an object that collapses in a flame.
LiDAR systems based on optical distance measurement have been widely used in manufacturing, surveying, and autonomous driving, or can help solve the practical challenges of structural fire resistance design. In this case, the structural temperature is too high to be installed on the building. The traditional electromechanical sensor is measured.
The study has been published in the Optica journal. According to the article, NIST uses a commercial LiDAR system to measure distances in objects in a smog-like flame. The 3D surface accuracy measured by the experiment at a distance of 2 m has reached more than 30 um. According to the article, this measurement accuracy can meet the requirements of most structural fire research applications.
“We need materials that melt too fast and not too slow, but at the same time let us see the results,” explains project leader Esther Baumann. “And, I like chocolate!”
With 3D imaging through the flame, LiDAR has several advantages. The technology is very sensitive and can image objects even when faced with a small amount of soot in the flame. The method can also work in a safe area at a certain distance from the flame to avoid the equipment being affected by high temperatures. In addition, LiDAR measuring instruments can be made compact and portable with fiber optics and simple photodetectors.
“When we tried to communicate with the 'optical experts' and the 'burning experts', the project was born by chance,” said NIST structural engineer Matthew Hoehler. “The collaboration between the two research areas is not only productive but also very interesting. ”
In the 3D mapping system, the green laser pointer performs a continuous scan over a range of frequencies. The initial laser output is combined with the reflected light from the target to produce a "beat frequency" signal, which is then detected by digital signal processing to obtain time-dependent delay data. (The frequency difference between the initial signal and the signal received from the target is increased as the distance increases)
Even in turbulent flame environments with strong signal scattering and distortion, researchers can successfully apply LiDAR measurements and map 3D point clouds (point clouds are "voxels" that make up 3D imaging). In contrast, the team also produced a video of the melting of the chocolate in the flame, as well as a melted image of the more complex plastic skeleton.
NIST researchers have shown that LiDAR can generate a continuous point cloud map of a piece of chocolate that melts behind the flame. The chocolate melts and deforms for 6 minutes and the video is accelerated. In the video, the depth is represented by pseudo color, blue is the closest, and red is the farthest.
For melted chocolate, each frame of LiDAR image consists of 7,500 point clouds sufficient to capture the chocolate deformation process. The melting of the plastic skeleton is hidden in the flame in the traditional video, almost invisible, but the 3D point cloud image can reveal complex shape changes and clearly show the details of the chest and hips.
The researchers determined through experiments that the speed of the LiDAR system is sufficient to overcome the signal distortion problem, and the green laser pointer beam deflection caused by the flame can be adjusted by the time-averaged signal to maintain high precision.
The initial experiment was performed on a laboratory burner at the University of Colorado Boulder with a 50 mm wide flame. Preliminary results indicate that LiDAR technology can be applied to larger objects and combustion sites. The NIST team is now planning to expand the scope of the experiment by first projecting a 3D image of the object through a flame of about 1 m wide, and if possible, quantitative observations of larger structural combustion.