Tyler O'Neal, Staff Editor VISUALIZATION

KICT's solution for monitoring massive infrastructures

A trailblazer for developing a new paradigm for structural monitoring

The Korea Institute of Civil Engineering and Building Technology (KICT) has announced the development of an effective structural monitoring technique to monitor massive infrastructures, such as long-span bridges. The method provides accurate and precise responses over the whole structural system densely by fusing the advantages of multi-fidelity data.

Rapid advances in sensing and information technologies have led to condition-based monitoring in civil and mechanical structural systems. The structural monitoring system plays a key role in condition-based monitoring to evaluate structural safety from responses measured by sensors. In other words, the following method allows examining the health of existing structures, such as a long-span bridge. The structural monitoring system can enable early detection for an unsafe condition and enable proactive maintenance. As a result, it greatly reduces the inspection burden as well as maintenance costs. The prerequisite for successful condition-based monitoring is to obtain accurate responses through the whole structural system. Especially in civil-infrastructures, high cost, and technical difficulty are some challenging issues. 

Complementary data-fusion framework using multi-fidelity data

To solve this problem, a research team in KICT, led by Dr. Seung-Seop Jin, has developed an effective as well as an efficient data-fusion method for condition-based monitoring. With the following method, the complementary data-fusion for the point and distributed strain sensor is performed to combine their advantages to obtain the accurate strain distribution over whole infrastructures; thereby, responses can be estimated with high accuracy densely over whole infrastructures.

For structural response, the multi-fidelity data consists of point and distributed sensors, which have different fidelities. The point sensor provides highly accurate and reproducible responses at discrete measurement positions (High-fidelity data, HF-data), while a distributed sensor utilizes the scattering-based or scanning sensing technique to obtain very dense responses using the quasi-continuous sensing (Low-fidelity data, LF-data). The LF data is relatively easy to acquire, so it is possible to produce large amounts of relatively inaccurate data for response trends over the whole infrastructure. On the contrary, the HF data provides high accuracy; however, it is limited to acquire in terms of both time and technical limitations. Therefore, a limited amount of data is available and this can significantly impair the ability to diagnose structural conditions over whole infrastructures. Although they can be complemented each other, their complementary data-fusion has not been studied yet for the structural monitoring system. KICT firstly recognizes their potentials and developed the complementary data-fusion framework by exploiting multi-fidelity modeling in Computational statistics and Geo-statistics. The basic concept of the developed method is to transfer knowledge of the abundant but potentially inaccurate LF-data (response trend) to enhance their accuracies by fusing the information from the HF-data (accuracy at some points).

The newly developed method was verified by several numerical tests with other existing multi-fidelity data-fusion methods. To consider possible situations in real applications, the developed method was extensively evaluated through Monte Carlo simulations by varying the number and locations of multi-fidelity data with the noise. The results show that the prediction performance of the developed method is consistently superior to other existing methods. In both experiments, the relative percentages of accuracy (maximum absolute error) are improved up to 171.3% and 192 % to the existing methods.

The method is very versatile for structural monitoring, especially for massive infrastructures. The developed method is currently improved to make it more robust and efficient. For better-generalized capability, the developed method requires flexible learning capability for extracting the information from both the HF-data and LF-data and fusing them. Such improvements include the following.

Dr. Jin said, "The rationale behind this improvement is similar to our decision-making in real life. We seek different options and combine them to make the best decision. Similar to our decision-making, we do not have high confidence in the unknown damages. In the current method, we have to utilize specific models and some parameters of these models. It should be noted that the best model and its parameters are case-dependent. Therefore, we pursue several options for flexible modeling. To deal with various conditions in infrastructures, we can adopt a flexible and self-learning framework such as optimal kernel learning for better data fusion. This idea takes a step towards autonomous and efficient monitoring system for massive infrastructures."

A new design of ultra-small silicon chip that manages terahertz waves leads to the next generation of communications

Tyler O'Neal, Staff Editor VISUALIZATION

Researchers from Osaka University, Japan, and the University of Adelaide, Australia have worked together to produce the new multiplexer made from pure silicon for terahertz-range communications in the 300-GHz band.

"In order to control the great spectral bandwidth of terahertz waves, a multiplexer, which is used to split and join signals, is critical for dividing the information into manageable chunks that can be more easily processed and so can be transmitted faster from one device to another," said Associate Professor Withawat Withayachumnankul from the University of Adelaide's School of Electrical and Electronic Engineering.

"Up until now compact and practical multiplexers have not been developed for the terahertz range. The new terahertz multiplexers, which are economical to manufacture, will be extremely useful for ultra-broadband wireless communications.

"The shape of the chips we have developed is the key to combining and splitting channels so that more data can be processed more rapidly. Simplicity is its beauty." Schematic of the integrated multiplexer, showing broadband terahertz wave being split into four different frequencies, where each is capable of carrying digital information.

People around the world are increasingly using mobile devices to access the internet and the number of connected devices is multiplying exponentially. Soon machines will be communicating with each other in the Internet of Things which will require even more powerful wireless networks able to transfer large volumes of data fast.

Terahertz waves are a portion of the electromagnetic spectrum that has a raw spectral bandwidth that is far broader than that of conventional wireless communications, which are based upon microwaves. The team has developed ultra-compact and efficient terahertz multiplexers, thanks to a novel optical tunneling process.

"A typical four-channel optical multiplexer might span more than 2000 wavelengths. This would be about two meters in length in the 300-GHz band," said Dr. Daniel Headland from Osaka University who is the lead author of the study.

"Our device is merely 25 wavelengths across, which offers dramatic size reduction by a factor of 6000."

The new multiplexer covers a spectral bandwidth that is over 30 times the total spectrum that is allocated in Japan for 4G/LTE, the fastest mobile technology currently available and 5G which is the next generation, combined. As bandwidth is related to data rate, ultra-high-speed digital transmission is possible with the new multiplexer.

"Our four-channel multiplexer can potentially support an aggregate data rate of 48 gigabits per second (Gbit/s), equivalent to that of uncompressed 8K ultrahigh definition video being streamed in real-time," said Associate Professor Masayuki Fujita, the team's leader from Osaka University.

"To make the entire system portable, we plan to integrate this multiplexer with resonant tunneling diodes to provide compact, multi-channel terahertz transceivers."

The modulation scheme employed in the team's study was quite basic; terahertz power was simply switched on and off to transmit binary data. More advanced techniques are available that can squeeze even higher data rates towards 1 Terabit/s into a given bandwidth allocation.

"The new multiplexer can be mass-produced, just like computer chips, but much simpler. So large-scale market penetration is possible," said Professor Tadao Nagatsuma from Osaka University.

"This would enable applications in 6G and beyond, as well as the Internet of Things, and low-probability-of-intercept communications between compact aircraft such as autonomous drones."

UCL investigators' simulations of black hole-neutron star collisions may help settle the debate over Universe's expansion

Tyler O'Neal, Staff Editor VISUALIZATION

Studying the violent collisions of black holes and neutron stars may soon provide a new measurement of the Universe's expansion rate, helping to resolve a long-standing dispute, suggests a new supercomputer simulation study led by researchers at UCL (University College London).

Our two current best ways of estimating the Universe's rate of expansion - measuring the brightness and speed of pulsating and exploding stars, and looking at fluctuations in radiation from the early Universe - give very different answers, suggesting our theory of the Universe may be wrong.

A third type of measurement, looking at the explosions of light and ripples in the fabric of space caused by black hole-neutron star collisions, should help to resolve this disagreement and clarify whether our theory of the Universe needs rewriting.

The new study, published in Physical Review Letters, simulated 25,000 scenarios of black holes and neutron stars colliding, aiming to see how many would likely be detected by instruments on Earth in the mid-to-late-2020s.

The researchers found that, by 2030, instruments on Earth could sense ripples in space-time caused by up to 3,000 such collisions, and that for around 100 of these events, telescopes would also see accompanying explosions of light. 

They concluded that this would be enough data to provide a new, completely independent measurement of the Universe's rate of expansion, precise and reliable enough to confirm or deny the need for new physics.

Lead author Dr. Stephen Feeney (UCL Physics & Astronomy) said: "A neutron star is a dead star, created when a very large star explodes and then collapses, and it is incredibly dense - typically 10 miles across but with a mass up to twice that of our Sun. Its collision with a black hole is a cataclysmic event, causing ripples of space-time, known as gravitational waves, that we can now detect on Earth with observatories like LIGO and Virgo.

"We have not yet detected light from these collisions. But advances in the sensitivity of equipment detecting gravitational waves, together with new detectors in India and Japan, will lead to a huge leap forward in terms of how many of these types of events we can detect. It is incredibly exciting and should open up a new era for astrophysics."

To calculate the Universe's rate of expansion, known as the Hubble constant, astrophysicists need to know the distance of astronomical objects from Earth as well as the speed at which they are moving away. Analyzing gravitational waves tells us how far away a collision is, leaving only the speed to be determined.

To tell how fast the galaxy hosting a collision is moving away, we look at the "redshift" of light - that is, how the wavelength of light produced by a source has been stretched by its motion. Explosions of light that may accompany these collisions would help us pinpoint the galaxy where the collision happened, allowing researchers to combine measurements of distance and measurements of redshift in that galaxy.

Dr. Feeney said: "Computer models of these cataclysmic events are incomplete and this study should provide extra motivation to improve them. If our assumptions are correct, many of these collisions will not produce explosions that we can detect - the black hole will swallow the star without leaving a trace. But in some cases a smaller black hole may first rip apart a neutron star before swallowing it, potentially leaving matter outside the hole that emits electromagnetic radiation." 

A still from a NASA animation of a black hole devouring a neutron star. CREDIT Dana Berry/NASA

Co-author Professor Hiranya Peiris (UCL Physics & Astronomy and Stockholm University) said: "The disagreement over the Hubble constant is one of the biggest mysteries in cosmology. In addition to helping us unravel this puzzle, the spacetime ripples from these cataclysmic events open a new window on the universe. We can anticipate many exciting discoveries in the coming decade."

Gravitational waves are detected at two observatories in the United States (the LIGO Labs), one in Italy (Virgo), and one in Japan (KAGRA). A fifth observatory, LIGO-India, is now under construction.

Our two best current estimates of the Universe's expansion are 67 kilometers per second per megaparsec (3.26 million light-years) and 74 kilometers per second per megaparsec. The first is derived from analyzing the cosmic microwave background, the radiation left over from the Big Bang, while the second comes from comparing stars at different distances from Earth - specifically Cepheids, which have variable brightness, and exploding stars called type Ia supernovae.

Dr. Feeney explained: "As the microwave background measurement needs a complete theory of the Universe to be made but the stellar method does not, the disagreement offers tantalizing evidence of new physics beyond our current understanding. Before we can make such claims, however, we need confirmation of the disagreement from completely independent observations - we believe these can be provided through black hole-neutron star collisions."