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Development of technology building blocks For structural health monitoring sensing devices in aeronautics

Periodic Report Summary - TRIADE (Development of technology building blocks for structural health monitoring sensing devices in aeronautics)

The industrial specifications of the TRIADE project would require health and usage monitoring system (HUMS) performing structural health monitoring (SHM) functions during flights that should exhibit a very high level of autonomy. The power consumption would be minimised to achieve optimal use of the energy obtained from the battery and energy harvesting units subjected to very stringent space limits. During the project the following applications were considered:

1. to explore the 'out of flight domain' conditions and to monitor the integrity and health of parts and structures which were repaired in the field
2. to monitor critical parts made out of composites in helicopters
3. to increase the efficiency of aircraft maintenance.

The inspection of completed or ongoing European Community's projects showed that these goals could not be currently achieved in aeronautics due to the the non-intrusiveness, autonomy, size and cost requirements. However, the following technical outputs differentiated TRIADE from other projects, and in particular from the ADVICE project:

1. the proposed energy harvesting used seismic mass and electromagnetic conversion
2. the energy storage used rechargeable devices
3.the fully depleted silicon on insulator (SOI) complementary metal-oxide semiconductor (CMOS) technology targeted for ultra low power (ULP) functions, sensors and interfaces
4. breakthrough solutions were also developed to bring embedded neural network intelligence.

The overall TRIADE objective was to contribute towards solving application issues by providing technology building blocks and fully integrated prototypes to achieve power generation, power conservation, embedded powerful intelligence-data processing/storage and energy management for structural health monitoring sensing devices in aeronautical applications.

The objectives of TRIADE were assessed in comparison to the following measurable goals:

1. to implement an architecture, where energy management would bridge the gap that existed between a need of 250 mAh in most modern comparable products, to less than 30 mAh available power in most modern harvesting and storage solutions
2. to include a neural tool and data processing in the prototype, which would fulfil the power consumption requirements
3. to validate the robustness of the solution to the aeronautic environment.

The technical development resulted in a disposable smart tag that included a battery, an antenna, an radio frequency (RF) inductive coupling link, a memory, an energy harvesting part, a power management circuit and a microprocessor. Remote sensors were connected to the tag, i.e. a humidity sensor, one or two XY strain gauges, an acceleration sensor and ultra low power (ULP) temperature and pressure sensors. This tag was stuck in the last layer of the composite, whose lifetime was at least 10 years, or on the structure, whose lifetime ranged from six months to one year.

The project was divided into six technical work packages (WPs). WP1 was essentially concerned with HUMS environmental and functional requirements, overall architecture and interfaces with low power in mind. WP2 dealt with peripheral components. In particular, this WP was concerned with the adaptation of peripheral components to the aeronautics requirements, i.e. with energy harvesting sources and batteries. Several interfaces were implemented, namely RF link, microprocessor, power interface between energy source and batteries and remote sensors implemented with low power solutions. WP3 aimed at upgrading and establishing the SOI CMOS microelectromechanical systems (CMOS/MEMS) platform for embedded electronics and sensors. Selected critical functions were developed using ULP concepts. WP4 focussed on the study of the embodiment of the electronics, how it adapted to processes and process temperature, environmental and service life. Simulations for structural integrity assessment were performed and transferred for use in WP6. WP5 consisted of neural network computation. It focussed on developing a software computing tool, compatible with previous requirement and choices. WP6 was concerned with the development of a prototype to be put on a small technological specimen containing fasteners. Stuck on the specimen, the prototype was tested with an environmental cycle that was defined by the end users.

The work performed since the beginning of the TRIADE project allowed to finalise the end users' requirements that led to common needs. The different building blocks were developed in parallel and the trade-offs between harvesters, battery, electronics and overall size of the smart tag were made to favour an optimal integration versus autonomy. Hence, the total dimensions of the tag did not exceed 85 times 55 times 4.5 mm3.

The different building blocks, i.e. vibration and electromagnetic harvesters, SOI parts for sensor interfaces and neural tool, were delivered. A first lithium iron phosphate (LFP) packaged battery, which was 1.5 mm thick, was manufactured and other operating batteries were very soon expected to finalise the smart tag integration. Concerning the Li-ion battery, an enhanced electrolyte allowing an operating range from -40 °C to 60 °C, or even better, was under development.

Preliminary tests were conducted to evaluate the behaviour of the electronics, showing very good results with respect to temperature, humidity, pressure, vibrations and strain. A first non-functional smart tag was also successfully integrated.

The major deliverable of the project would be the HUMS smart tag device that could be stuck on the structure or in the last layer of the composite of an aircraft in order to record the external parameters like temperature, pressure, moisture and vibrations. The smart tag would respect the compatibility with the manufacturing processes and service life. It targeted the credit card size to be easily used in the aeronautics domain and further allowed for other monitoring applications. From this smart tag several other technological results with breakthrough building blocks were issued, namely:

1. a battery optimised for aeronautics embedded devices
2. harvester devices using vibration and electromagnetic coupling
3. a neural network for smart record triggering and damage assessment
4. SOI based ultra low power components.

The expected impact of TRIADE was the contribution to the reduction of the aircraft operating costs by 50 % through reduction in maintenance, inspection and other direct operating costs by 2020. Before TRIADE, smart maintenance systems were not embeddable onboard aircrafts; nevertheless after TRIADE they would be embeddable. Before TRIADE, smart maintenance systems consumed 250 mAh and lasted a few hours when continuously powered; however after TRIADE they would consume 30 mAh and might be used intermittently for 10 years.

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