Researchers Develop a Smart Fence

Nov. 2 -- The first line of defense against intruders at airports, prisons, military bases, and nuclear power plants is often nothing more than a chain-link fence.

At some high-risk areas, such as nuclear weapons storage facilities, complex sensors that detect an intruder's movement, body heat, or footsteps can augment perimeter security.

But adding these detection capabilities also adds as much as $165 per foot of perimeter fence, making them prohibitively expensive for large areas such as an airport or Navy base, which may have miles of fencing to monitor.

However, a team of researchers at Pennsylvania State University's Applied Research Laboratory (ARL) has developed an extremely simple — and inexpensive — solution to make almost any fence smarter at detecting, locating and classifying trespassers.

Secured by a String of Wire

The experimental fence monitoring system uses a single high-tension metal wire that can be embedded within the fence's chain links or strung along the top of fence structures. Any disturbance to the fence — a person attempting to climb it or a piece of trash brushing up against it — will cause the tightly wound wire to vibrate like a guitar string.

Geophones — electronic sensors commonly used to monitor earthquake activity — or other sensors strung along the wire translate these vibrations into signals that are sent back to a central computer. Software created by ARL's researchers then compares the strength and pattern of the signals to determine the type and location of the "threat" against the fence.

Sorting the Bad Vibes From Good

A tree branch that is blown against the fence, for example, produces only a momentary pattern of vibrations. But a person cutting the fence and the sensor wire creates a very brief pattern of intense signals. Even careful climbing by an intruder would stress the fence and create a heavy, sustained vibration pattern indicating a human invader was present rather than a squirrel.