Harvester takes flight

An energy harvesting wireless strain sensing module has taken a flight test aboard a Bell helicopter to help track any damage to rotating parts.

The module, from Microstrain, will operate indefinitely without the need for batteries, by converting the component’s cyclic strains into DC power using piezoelectric materials.

MicroStrain’s modules, called ESG-LINK, feature a precision time keeper, non-volatile memory for on-board data logging, and a frequency agile IEEE 802.15.4 transceiver. Sampling rates, sample durations, sensor offsets, sensor gains and on-board shunt calibration are all wirelessly programmable.

Recent flight tests on the Bell helicopter showed that MicroStrain’s nodes could operate continually, without batteries, even under low energy generation conditions of straight and level helicopter flight.

By continuously monitoring the strains on rotating components, the nodes recorded operational loads, computed metal fatigue, and estimated remaining component life.

The critical component instrumented on the Bell M412 was the pitch link. The pitch link controls 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 pull ups and gunnery turns as compared to straight and level flight.

For that reason, the pitch link is an excellent indicator of how severely the vehicle is being used, and can provide critical data for maintenance engineers.

In operation. strain gauges bonded to the pitch link were arranged to cancel thermal and bending effects and to amplify tension/compression loads. Bench calibrations allowed the strain gauge bridge to provide load data during flight.

While spinning at 5 revolutions per second, data were logged within the wireless node’s non-volatile memory and also periodically transmitted to a small mobile base station located in the helicopter’s cabin. Flight test data from MicroStrain’s wireless nodes were compared to data collected by from hard wired strain gauges (using slip rings) with close agreement.

MicroStrain’s latest adaptive energy harvesting wireless sensors can sample pitch link static and dynamic loads at a rate of 32 samples/sec, while consuming only 250 microwatts. Compared to conventional Wheatstone bridge signal conditioning electronics (which draw 72 milliwatts), MicroStrain’s e-harvesting wireless sensor node delivers an improvement of 288 fold.

'Our first successful flight test, performed in concert with Bell Helicopter, has demonstrated that our technology works. The potential of energy harvesting combined with wireless sensing will now begin to be fully realised, not only on rotating helicopter components, but on a wide range of machines, structures, and systems', said Steven Arms, MicroStrain’s President.

This work was supported by a Navy Phase II SBIR award. MicroStrain is currently at the one year point of a two year SBIR contract with Navy/NAVAIR to transition this technology for health monitoring and structural health monitoring of the Navy’s helicopter fleet.