Sandia LabNews

Tiny vibrations in structures power embedded sensors

Vibration-powered sensor harvests structural shakes, stores data for later readout

Civil engineers assessing the health of a structure following an earthquake, storm, bomb blast, or other insult need to know how severely structural elements have been stressed. A beam or buttress strained beyond its tolerance limits might be dangerous.

In a project initiated by Sandia’s Architectural Surety program, Labs researchers have demonstrated the key components of a self-contained microsensor system that powers itself by converting mechanical energy from the subtle vibrations of structures into electrical power that drives the system.

It uses this vibe-power to take simple sensor measurements, then stores the data in a memory device that can be read from outside the structure — through concrete, steel, and other building materials — with a commercial radiofrequency (RF) tag reader used by trucking and warehousing operations to track tagged inventories.

No batteries, no wires

Because the sensor system requires no hookups to batteries or wires, it could be embedded into a structure during construction and forgotten until a need arises to assess the health of the structure.

"If you bend a beam to a certain point, the next time you bend it, it is going to break," says Kent Pfeifer (1744), who conceived the device and demonstrated its feasibility along with colleagues Sarah Leming, Art Rumpf (both 1744), and Robert Waldschmidt (former contractor). "This technique could lead to self-powered sensor packs that can take a variety of measurements over a long period of time and store them until needed."

The system’s power plant is a swath of piezoelectric material attached to a structural element, such as a beam. (A piezoelectric material produces electricity when subjected to stress or strain. The strain changes the electromagnetic alignments of vibrating crystals in the material — the same natural vibrations that keep time in a quartz crystal watch — to produce a net electrical charge.)

Each time the beam bends from a load on the structure (for instance, when a tall building sways in the wind or a truck traverses a bridge), the piezoelectric ceramic generates a tiny parcel of charge — about 100 microcoulombs is all — which is stored temporarily in the system’s capacitance bank.

This stored charge is sufficient to power the microsensor system for a fraction of a second, long enough to take a simple reading.

Later, if strains on the piezoelectric exceed a predetermined threshold — as the result of a significant insult on the structure, for instance — the system’s low-power microprocessor could turn on, command the sensor to take a measurement, commit the reading to the RF tag’s flash memory, and quickly shut down into its power-conserving sleep mode.

To retrieve the stored data, a structural engineer can point a commercial hand-held tag reader at the structure near the embedded microsensor system. The embedded tag’s resonant circuit harvests RF energy from the hand-held reader through changes in the circuit’s impedance, making it possible for the hand reader to power the tag’s response remotely.

Uses not yet explored

So far the Sandia team has demonstrated a system that powers a microprocessor generated from vibration only and illustrated an approach for storing and retrieving sensor data taken by such a system, says Kent.

"There’s still more to be done," he says, including integrating and testing a complete self-powered microsensor system. "But we’ve shown that this is a viable approach to monitoring critical infrastructures."

"A finished device could provide thermal measurements, stress measurements, deflection and strain measurements, or other information that could be stored over time in a database or captured immediately after an event to help plan an evacuation of a tall building, for instance," says Rudy Matalucci, Architectural Surety program manager. "It would be a great way to monitor performance and health of high rises, bridges, dams, tunnels, and other infrastructures."

Wireless microsystems might initially be employed in structures to predict fatigue or failure of key structural elements, keep track of the number of stress cycles a bridge endures as heavy trucks cross it, or determine the integrity of key structures such as hospitals or emergency command centers following a disaster.

The technique might also be used in any application where a self-contained, no-maintenance monitoring capability is needed over a long period of time, says Kent.

Stacks of piezoelectric generators might make higher-power applications possible, perhaps continuously powering embedded clocks.

"I’m sure we haven’t thought of all the possible applications," adds Kent. "This has never been done before."

The system’s development was funded through the Laboratory Directed Research and Development program and Sandia’s MESA Institute.