Now, a group of University of Michigan (U-M) engineering undergraduate students have developed a new way to detect them.
The students invented portable, palm-sized metal detectors that could be hidden in trash cans, under tables or in flower pots, for example.
The detectors are designed to be part of a wireless sensor network that conveys to a base station where suspicious objects are located and who might be carrying them.
Compared with existing technology, the sensors are cheaper, lower-power and longer-range. Each of the sensors weighs about 2 pounds.
"Their invention outperforms everything that exists in the market today," said Nilton Renno, a professor in the U-M Department of Atmospheric, Oceanic and Space Sciences.
The students undertook this project in Renno's Engineering 450 senior level design class.
"They clearly have an excellent understanding of the problem. They also thought strategically and designed and optimized their solution. The combination of a movable command center with a wireless sensor network can be easily deployed in the field and adapted to different situations," said Renno.
The core technology is based on a magnetometer, or metal detector, explained Ashwin Lalendran, an engineering student who worked on the project and graduated in May.
"We built it entirely in-house - the hardware and the software," Lalendran said.
"Our sensors are small, flexible to deploy, inexpensive and scalable. It's extremely novel technology," he added.