By Steven W. Arms, Microstrain Inc.
Recent developments in combining sensors, microprocessors, and radio frequency (RF) communications hold the potential to revolutionize the way we monitor and maintain critical systems. In the future, literally billions of wireless sensors may become deeply embedded within machines, structures, and the environment. Sensed information will be automatically collected, compressed, and forwarded for condition-based maintenance.
But who will change billions of dead batteries?
MicroStrain (www.microstrain.com) believes the answer is to harvest strain, vibration, light, and motion energy from the environment and store it. Combined with strict power management, smart wireless sensing networks can operate indefinitely, without the need for battery maintenance.
 MicroStrain’s energy-harvesting wireless strain/load sensing node enabled load tracking on the Bell model 412 helicopter. |
MicroStrain’s electronics feature smart comparators-switches that consume only nanoampere levels of current-to control operation of a wireless sensing node. The system waits until there is sufficient energy to perform a programmed task, and only when the stored energy is high enough will the nanoamp comparator switch allow current draw. This is critical for applications where the ambient energy levels may be low or intermittent; without this switch, the system may never successfully start up, because stored energy levels may always remain insufficient for the task at hand.
MicroStrain’s miniaturized energy harvesting sensing nodes feature a precision timekeeper, non-volatile memory for on-board data logging, and frequency agile IEEE 802.15.4 transceiver. Sampling rates, sample durations, sensor offsets, sensor gains, and on-board shunt calibration are all wirelessly programmable.
Tracking helicopter component loads
The US Navy, through its SBIR program, has supported MicroStrain’s development of wireless sensor nodes that use piezoelectric materials to convert cyclic strain and vibration into power. One compelling application is in monitoring the critical rotating structures of helicopters. Direct measurement of loads on these structures allows enhanced maintenance, and thus greatly reduces operational costs.
Rather than replacing parts on a fixed schedule of flight hours, parts could be replaced based on their actual usage severity. Better loads tracking also has the potential to save lives through improved performance and increased safety.
 Solar-powered wireless G-Link seismic sensor on the Corinth Bridge in Greece. |
One critical rotating helicopter component is the “pitch link.” Responsible for controlling the rotor blade’s angle of attack as the rotor rotates through the air, pitch link loads vary strongly with aircraft flight regimes, reaching much higher loads (6X) during maneuvers as compared to straight and level flight. Therefore the pitch link is an excellent indicator of vehicle usage severity and can provide critical data for improved condition-based maintenance.
Working in collaboration with Bell Helicopter/Textron, the first successful flight test of our energy harvesting wireless sensor node was performed in March 2007. We instrumented the pitch link of a Bell Model 412 helicopter with our energy harvesting sensor node, along with piezoelectric materials and a full strain gauge bridge, which canceled thermal and bending influences while amplifying tension and compression loads. The smart wireless sensor nodes flown on the Bell M412 are capable of adapting their operating modes in accordance with the amount of energy available.
The flight test showed that our energy harvesting strain and load sensor will operate continually, without batteries, even under low energy generation conditions of straight and level flight.
Monitoring large bridge spans
These techniques can be used for other applications, too, such as monitoring large civil structures. Three years ago, the Federal Highway Administration rated ~20% of the U.S. interstate bridges (nearly 12,000 bridges) as deficient. Wireless sensor networks have the potential to enable cost efficient, scalable monitoring systems that could be tailored for each particular bridge’s requirements. Eliminating long runs of wiring from each sensor location greatly simplifies system installation and allows rapid deployment of large arrays of sensor nodes.
We recently supported two major wireless installations that are actively monitoring the structural strains and seismic activity of major spans. Leveraging energy harvesting technology supported by the U.S. Navy, these wireless sensor networks are powered by the sun, and therefore do not require battery maintenance.
 Multiple solar-powered nodes monitor strain and vibration at key locations on the Goldstar Bridge over the Thames River in New London, Conn. |
MicroStrain has previously described battery-powered wireless strain sensors for structural health monitoring. One example is the Ben Franklin Bridge, which spans the Delaware River from Philadelphia, Penn., to Camden, N.J. The system was accessed remotely over commercial cellular telephone networks, and sensor data was provided to the customer via secure access to a web-based server. The nodes measured structural strains in the cantilever beams as passenger trains traversed the span. Measurements taken over several months’ time, under contract from the Delaware River Port Authority (DRPA), were used to document the bridge’s cyclic structural strains-and find that fatigue was not a problem.
MicroStrain’s first solar-powered bridge installation was recently made in Corinth, Greece. This system uses arrays of wireless tri-axial accelerometer nodes to monitor the span’s background vibration levels at all times. Each node and solar panel is packaged within a watertight enclosure for outdoor use. In the event that seismic activity is detected at any node, the entire wireless network of nodes is alerted, and data are collected simultaneously from the entire network.
Steven W. Arms is president of MicroStrain. He can be contacted at (802) 862-6629 or at [email protected].