Terahertz imaging on the cheap

New theory could reduce number of sensors required for high-resolution imaging systems.

05/15/2014


Illustration courtesy: Jose-Luis Olivares, MITTerahertz imaging, which is already familiar from airport security checkpoints, has a number of other promising applications—from explosives detection to collision avoidance in cars. Like sonar or radar, terahertz imaging produces an image by comparing measurements across an array of sensors. Those arrays have to be very dense, since the distance between sensors is proportional to wavelength.

In the latest issue of IEEE Transactions on Antennas and Propagation, researchers in MIT's Research Laboratory for Electronics describe a new technique that could reduce the number of sensors required for terahertz or millimeter-wave imaging by a factor of 10, or even 100, making them more practical. The technique could also have implications for the design of new, high-resolution radar and sonar systems.

In a digital camera, the lens focuses the incoming light so that light reflected by a small patch of the visual scene strikes a correspondingly small patch of the sensor array. In lower-frequency imaging systems, by contrast, an incoming wave—whether electromagnetic or, in the case of sonar, acoustic—strikes all of the sensors in the array. The system determines the origin and intensity of the wave by comparing its phase—the alignment of its troughs and crests—when it arrives at each of the sensors.

As long as the distance between sensors is no more than half the wavelength of the incoming wave, that calculation is fairly straightforward, a matter of inverting the sensors' measurements. But if the sensors are spaced farther than half a wavelength apart, the inversion will yield more than one possible solution. Those solutions will be spaced at regular angles around the sensor array, a phenomenon known as "spatial aliasing."

Narrowing the field

In most applications of lower-frequency imaging, however, any given circumference around the detector is usually sparsely populated. That's the phenomenon that the new system exploits.

"Think about a range around you, like five feet," said Gregory Wornell, the Sumitomo Electric Industries Professor in Engineering in MIT's Department of Electrical Engineering and Computer Science and a co-author on the new paper. "There's actually not that much at five feet around you. Or at 10 feet. Different parts of the scene are occupied at those different ranges, but at any given range, it's pretty sparse. Roughly speaking, the theory goes like this: If, say, 10 percent of the scene at a given range is occupied with objects, then you need only 10 percent of the full array to still be able to achieve full resolution."

The trick is to determine which 10 percent of the array to keep. Keeping every tenth sensor won't work: It's the regularity of the distances between sensors that leads to aliasing. Arbitrarily varying the distances between sensors would solve that problem, but it would also make inverting the sensors' measurements—calculating the wave's source and intensity—prohibitively complicated.

Regular irregularity

So Wornell and his co-authors—James Krieger, a former student of Wornell's who is now at MIT's Lincoln Laboratory, and Yuval Kochman, a former postdoc who is now an assistant professor at the Hebrew University of Jerusalem—instead prescribe a detector along which the sensors are distributed in pairs. The regular spacing between pairs of sensors ensures that the scene reconstruction can be calculated efficiently, but the distance from each sensor to the next remains irregular.

The researchers also developed an algorithm that determines the optimal pattern for the sensors' distribution. In essence, the algorithm maximizes the number of different distances between arbitrary pairs of sensors.

With his new colleagues at Lincoln Lab, Krieger has performed experiments at radar frequencies using a one-dimensional array of sensors deployed in a parking lot, which verified the predictions of the theory. Moreover, Wornell's description of the sparsity assumptions of the theory—10 percent occupation at a given distance means one-tenth the sensors—applies to one-dimensional arrays. Many applications—such as submarines' sonar systems—instead use two-dimensional arrays, and in that case, the savings compound: One-tenth the sensors in each of two dimensions translates to one-hundredth the sensors in the complete array.

Massachusetts Institute of Technology (MIT)

www.mit.edu 

- Edited by CFE Media. See more Control Engineering sensor and vision stories.



No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
World-class maintenance: The three keys to success - Deploy people, process and technology; 2016 Lubrication Guide; Why hydraulic systems get hot
Your leaks start here: Take a disciplined approach with your hydraulic system; U.S. presence at Hannover Messe a rousing success
Hannover Messe 2016: Taking hold of the future - Partner Country status spotlights U.S. manufacturing; Honoring manufacturing excellence: The 2015 Product of the Year Winners
The digital oilfield: Utilizing Big Data can yield big savings; Virtualization a real solution; Tracking SIS performance
Getting to the bottom of subsea repairs: Older pipelines need more attention, and operators need a repair strategy; OTC preview; Offshore production difficult - and crucial
Digital oilfields: Integrated HMI/SCADA systems enable smarter data acquisition; Real-world impact of simulation; Electric actuator technology prospers in production fields
Improving flowmeter calibration; Selecting flowmeters for natural gas; Case study: Streamlining assembly systems using PC-based control; CLPM: Improving process efficiency, throughput
Putting COPS into context; Designing medium-voltage electrical systems; Planning and designing resilient, efficient data centers; The nine steps of designing generator fuel systems
Warehouse winter comfort: The HTHV solution; Cooling with natural gas; Plastics industry booming

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
This article collection contains several articles on the vital role that compressed air plays in manufacturing plants.
This article collection contains several articles on the Industrial Internet of Things (IIoT) and how it is transforming manufacturing.
This article collection contains several articles on strategic maintenance and understanding all the parts of your plant.
click me