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Autonomous Damage Detection and Vibration Control Systems

Final Report Summary - ADVICE (Autonomous Damage Detection and Vibration Control Systems)

The ADVICE project was a multidisciplinary research project that aimed at the development of state-of-the-art technologies for structural health monitoring and vibration damping in aeronautical structures. Bringing together different research activities in one common project is expected to drive new synergies that can lead to new possibilities for aircraft design, maintenance and cabin environment concepts.

Maintenance and service life evaluation has always been a concern in the aeronautical industry. Over the past century, the approach taken by manufacturers has evolved from safe-life static designs to fail-safe, then damage tolerant, allowing an overall reduction of the weight of the structure, increasing its performances and better predicting the possible failure mechanisms. Maintenance takes an important part when certifying the airworthiness of an aircraft. Regular overhaul and various inspection techniques range from visual inspection to eddy current and fluorescent penetrant inspection. They are required to follow the evolution of the integrity of the structure.

The industry is now turning its attention to a new approach to increase the reliability of an aircraft and reduce the time it must spend in maintenance: structural health monitoring (SHM) aims at continuously tracking the state of the structure to record any changes in behaviour and give out a warning when a situation is identified as potentially threatening (using threshold values, neural networks, user interfaces, etc). This domain of research has been described as a promising upcoming technology, but still requires the development of new integrated approaches to be available for use on aircrafts.

Focus is also set on vibration damping in aircrafts. Reducing vibration levels in the structure can have an impact on the service life and maintenance requirements in structures. Fatigue is often the source of unexpected failure or crack propagation in parts and vibration damping can have an impact on the amplitude of vibration at a local level, which can increase, in an optimistic case, the average time allowed, and at least increase the safety factor between maintenance checks in some parts.

In direct link with both previously cited research areas is energy scavenging, that is a growing area of research due to the increase in demand for microsensors. The devices that can be developed for health monitoring and vibration damping sometimes need to be placed in remote or hidden areas and renewable energy sources must be sought out to provide the necessary power for their proper use. Mechanical vibrations are one of the possible sources of energy. Thanks to the creation of ultra low power devices and new energy storage and energy management techniques, new solutions can be proposed to create autonomous sensors forming a distributed network, harvesting vibration energy available in the structure.

These three research areas are at the basis of the developments foreseen in the ADVICE project.

WP1: Specifications and requirements
The content of Work Package 1 dedicated to the elaboration of specifications and requirements was completed over the first two years of the project. It regrouped all the disciplines as well as all the partners of the project.

WP2: System design and virtual integration
The objectives of the second work package of the ADVICE project are to design and to virtually integrate the whole VDC system with regards to the requirements and characteristics of the VDCu, of the network, of the electrical systems and of the receiver and of the central station which were all defined in the previous work package.

The work performed comprises:
- the validation of the strategy feasibility using the one-dimensional demonstrator;
- the SOI integration of the energy harvesting SSHI (power management module) and SSDI (damping module) circuitry;
- the design of the RF communication module (in particular, the development of a unique low power RF module featuring both emission (TX) and reception(RX) modes);
- the measurement of the consumption the digital signal processing (DSP) module of a platform based on system-on-chip (SoC from Texas Instrument) for the estimation of the damage index;
- the investigation of bonding issues for the integration of piezo-patches to the host structure;
- the completion of the system design.
The achievements of WP2 were the design of the system, based on the VDC strategy selected in WP1 and in accordance with the identified constraints and requirements and the design of its components: VDC unit, network, 'data treatment & analysis' station (base station).

WP3: System development and manufacture: Preliminary tests
WP3 objectives were the development and manufacture of VCDus, of the central station and of the network according to the system design and following the visa for development both delivered in WP2. Development and test of these main features were done in parallel, each of them being managed by a leading partner responsible for checking dependencies, identifying blocking points, and monitoring the global progress of the manufacture and tests. Individual tests of the different modules precede joint tests that require the assembly of the test rig and the test structures.

To fulfil its ambitious requirements, the VDCu should comprise a microgenerator, a Lamb wave transmitter, a radio frequency unit, a core processor and a power management module. Assembling all these features in one single energetically autonomous device was a great challenge.

It has proved to be feasible thanks to the joint efforts and the good coordination of the different developers involved in WP3 combining a wide range of expertises such as:
- wireless network communication
- piezoelectric materials
- low power technology
- energy harvesting
- vibrations analysis
- neural networks and embedded electronics development and programming.

The VDCu was initially planned to be assembled in one single package with limited surface and weight, to minimise the effect of the presence of the device on the vibration of the structure. Due to some delays in the integration of the different features of the VDCu, it has not been possible to built the appropriate packaging, but the final assembly has been done with compactness and lightweight in mind, the electronics being limited to a surface equal to the total surface occupied by the different piezoelectric patches to be bonded on the host structure.

The whole system integration, including VDCus, end-node, gateway and base station was only made possible by following important integration steps such as the permanent compatibility check through design phase by integrator, the virtual integration to evaluate interaction between components and the final compatibility check and recommendations for actual integration.

WP4: Integration and validation - safety and reliability assessment
The system to validate consists in a set of test structures fitted with autonomous VDCu and an end node. A gateway and a central station complete the system for data collection and processing. The system ensures the functions of health diagnostic of the test structures. It is in part autonomous as some energy is harvested from the mechanical strains generated under structural vibrations. This energy is scavenged by the VDCu electronics and used for sending an ultrasonic wave along the test structure skin. This wave (Lamb wave) propagation is affected by any damage on the structure. An analysis of the received wave by the end node electronics allows damage detection capability.

During the integration, the following functions had to be checked:
- energy harvesting
- power management
- Lamb wave generation and reception
- damage computation
- wireless network
- data storage, consolidation and display.
The tests objectives were to submit the system to vibration spectra representative of aircraft structures during flight and then evaluate the damage detection capability under these conditions.

The testing campaign has allowed characterising the quality of the system developed and tested within the ADVICE project, presenting the various achievements made in a wide range of technical domains which have required numerous expertises and great coordination within the consortium to commonly converge towards an ambitious goal. Combining energy harvesting, structural damping, RF network, health diagnostic algorithm has proved to be feasible. The system showed encouraging results and paves the way for further activities to increase the maturity of the technology.

WP5: Dissemination and exploitation of the results
The ADVICE project led to the successful development of an autonomous wireless system that can be used for the detection of damage in structures, but also for the damping of structural vibrations. Several important milestones and technical improvement were achieved first at component level (each individual function of the system) and also on a global level (the system operating as a whole). The major technological developments and research included:
- low power vibration harvesting through direct coupling on the structure and non linear processing of the electric signal;
- low power energy management solutions developed with SOI technology and optimised for a given environment;
- communication and polling strategies based on predictions of available energy;
- automated structural analysis of composite structures through Lamb waves;
- development of a centralised network and data management tool with associated APIs and man-machine interface;
- representative numerical and experimental analysis of a composite structure installed on a electromagnetic shaker.

Most of the initial objectives of the project were achieved, although some difficulties were encountered and some changes in strategy or objectives were made during the project. A nine-month extension of the project was requested due to problems in the developments and testing of electronic boards as well as delays in the different testing phases due to compatibility issues during integration. Actions were taken on problems that were identified, but a few issues still remained at the time of the demonstration mostly in terms of robustness and communication errors between the gateway and the end-nodes.

The project led to interesting results in the characterisation of a direct coupling harvesting solution installed on a composite panel. An important amount of additional effort was put in the identification and the proper exploitation of vibration harvesting tests on the shaker. Damage detection algorithms such as neural networks were investigated throughout the project and led also to interesting outputs related to the parameters that can influence the healthy / damaged signal as well as the possible solutions and the requirements to be able to identify damage and eventually locate it. These elements show a high potential of damage detection through Lamb waves, but require further research work that can based on the outcome and conclusions of this project.

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