Shenfang Yuan

A time reversal focusing based impact imaging method and its evaluation on complex composite structures

The growing use of composite structures in aerospace structures has attracted much interest in structural health monitoring (SHM) for the localization of impact positions due to their poor impact resistance properties. The propagation mechanism and the frequency dispersion features of signals on complex composite structures are more complicated than those on simple composite plates. In this paper, a time reversal focusing based impact imaging method for impact localization of complex composite structures is proposed. A complex Shannon wavelet transform is adopted to extract frequency narrow-band signals of impact response signals of a PZT sensors array at a special time–frequency scale and to measure the phase velocity of the signals. The frequency narrow-band signals are synthesized using software, depending on the time reversal focusing principle, to generate an impact image to estimate the impact position. A demonstration system is built on a composite panel with many bolt holes and stiffeners on an aircraft wing box to validate this method. The validating results show that the method can estimate the position of impact efficiently.

Distributed structural health monitoring system based on smart wireless sensor and multi-agent technology

This paper presents a new parallel distributed structural health monitoring technology based on the wireless sensor network and multi-agent system for large scale engineering structures. The basic idea of this new technology is that of adopting the smart wireless sensor with on-board microprocessor to form the monitoring sensor network and the multi-agent technology to manage the whole health monitoring system. Using this technology, the health monitoring system becomes a distributing parallel system instead of a serial system with all processing work done by the central computer. The functions, the reliability, the flexibility and the speed of the whole system will be greatly improved. In addition, with wireless communication links instead of wires, the system weight and complexity will be lowered. In this paper, the distributed smart wireless sensor network is designed first based on the Berkeley Mote Mica wireless sensor platform. Two kinds of sensor have been adopted: piezoelectric sensors and electric resistance wires. They are connected to a Mica MPR board though a designed charge amplifier circuit or bridge circuit and MTS101 board. Seven kinds of agents are defined for the structural health monitoring system. A distributed health monitoring architecture based on the defined agents is proposed. Finally, a composite structural health monitoring system based on a Mica wireless platform and multi-agent technology is developed to evaluate the efficacy of the new technology. The developed system can successfully monitor the concentrated load position or a loose bolt position.

Multi-agent system design and evaluation for collaborative wireless sensor network in large structure health monitoring

Much attention has been focused on the research of structural health monitoring (SHM), since it could increase the safety and reduce the maintenance costs of engineering structures. In recent years, wireless sensor network (WSN) has been explored for adoption to improve the centralized cable-based SHM system performances. This paper presents a multi-agent design method and system evaluation for wireless sensor network based structural health monitoring to validate the efficiency of the multi-agent technology. Through the cooperation of six different agents for SHM applications, the distributed wireless sensor network can automatically allocate SHM tasks, self-organize the sensor network and aggregate different sensor information. In the evaluation work, the strain gauge and PZT sensors are used to monitor strain distribution change and joint failure of an experimental aluminum plate structure. A dedicated sensor network platform including the wireless strain node, wireless PZT node and wireless USB station is designed for the evaluation system. Based on the hardware platform, the multi-agents software architecture is defined. The multi-agent monitoring principle and implementation in the validation work for two typical kinds of structure states are presented. This paper shows the efficiency of the multi-agent technology for WSN based the SHM applications on the large aircraft structures.

High spatial resolution imaging for structural health monitoring based on virtual time reversal

Lamb waves are widely used in structural health monitoring (SHM) of plate-like structures. Due to the dispersion effect, Lamb wavepackets will be elongated and the resolution for damage identification will be strongly affected. This effect can be automatically compensated by the time reversal process (TRP). However, the time information of the compensated waves is also removed at the same time. To improve the spatial resolution of Lamb wave detection, virtual time reversal (VTR) is presented in this paper. In VTR, a changing-element excitation and reception mechanism (CERM) rather than the traditional fixed excitation and reception mechanism (FERM) is adopted for time information conservation. Furthermore, the complicated TRP procedure is replaced by simple signal operations which can make savings in the hardware cost for recording and generating the time-reversed Lamb waves. After the effects of VTR for dispersive damage scattered signals are theoretically analyzed, the realization of VTR involving the acquisition of the transfer functions of damage detecting paths under step pulse excitation is discussed. Then, a VTR-based imaging method is developed to improve the spatial resolution of the delay-and-sum imaging with a sparse piezoelectric (PZT) wafer array. Experimental validation indicates that the damage scattered wavepackets of A0 mode in an aluminum plate are partly recompressed and focalized with their time information preserved by VTR. Both the single damage and the dual adjacent damages in the plate can be clearly displayed with high spatial resolution by the proposed VTR-based imaging method.

Designing strategy for multi-agent system based large structural health monitoring

Density and different kinds of sensors have to be adopted to have a reliable monitoring of large scale engineering structures. To deal with the diverse, heterogeneous and distributed information obtained by these sensors at different sites on the structure, multi-agent system (MAS) based structural health monitoring (SHM) technology is researched in detail in this paper. A general framework to design the multi-agent system for large scale structural health monitoring is proposed. Compared to past work, this framework is typical and flexible, which is easy to be referenced by other researchers to develop their own multi-agent based SHM system. The design strategy of the MAS based SHM is discussed in detail, including function design, ontology design, individual agent design, facilitator design, society behavior and learning behavior design. A MAS based composite SHM system is researched as a case study to evaluate the efficiency of the proposed MAS based SHM system design strategy and the advantages of the MAS based SHM system. Very detail introduction of the implementation of this system is given in the paper.

Recent Progress on Distributed Structural Health Monitoring Research at NUAA

This study introduces recent research advances on the structural health monitoring (SHM) at Nanjing University of Aeronautics and Astronautics (NUAA). Distributed SHM technology for large-scale engineering structures is discussed. First, a hybrid wing box health monitoring evaluation system is presented. This evaluation system is developed to verify the piezoelectric sensor array-based active Lamb wave diagnostic method and the fiber Bragg grating sensor-based structural strain monitoring method. Second, distributed health monitoring technology based on the multi-agent system (MAS) technology is discussed. This research includes the individual agent design, the MAS-based SHM system architecture design, and three important aspects in designing the MAS-based SHM system. An evaluation case is presented in detail to show the advantages of the MAS-based SHM system. Finally, the study illustrates the motivation of the agents realized by a wireless sensor network. The developed wireless sensor nodes are also des...This study introduces recent research advances on the structural health monitoring (SHM) at Nanjing University of Aeronautics and Astronautics (NUAA). Distributed SHM technology for large-scale engineering structures is discussed. First, a hybrid wing box health monitoring evaluation system is presented. This evaluation system is developed to verify the piezoelectric sensor array-based active Lamb wave diagnostic method and the fiber Bragg grating sensor-based structural strain monitoring method. Second, distributed health monitoring technology based on the multi-agent system (MAS) technology is discussed. This research includes the individual agent design, the MAS-based SHM system architecture design, and three important aspects in designing the MAS-based SHM system. An evaluation case is presented in detail to show the advantages of the MAS-based SHM system. Finally, the study illustrates the motivation of the agents realized by a wireless sensor network. The developed wireless sensor nodes are also described.

A wireless sensor network node designed for exploring a structural health monitoring application

This paper presents a wireless sensor network node designed for building a structural health monitoring (SHM) application. To develop a low-cost, low-power, dedicated wireless sensor node for a composite SHM system, a modular approach is taken in the design of the wireless sensor node. Three functional modules are adopted, including a sensor input unit, processing core and wireless communication. Different from existing wireless sensor nodes, the signal conditioning circuit is designed on this developed node for two typical SHM sensors, the piezoelectric sensor and the strain gauge. The developed wireless sensor nodes can be expediently used to deploy the dedicated wireless sensor network for SHM application. A two-tier wireless sensor network is deployed adopting the designed wireless sensor nodes to verify the efficacy of developing SHM systems. An embedding pattern matching method and a directed diffusion routing algorithm are developed to monitor the strain distribution or the bolt loosening position successfully.

Design of piezoelectric transducer layer with electromagnetic shielding and high connection reliability

Piezoelectric transducer (PZT) and Lamb wave based structural health monitoring (SHM) method have been widely studied for on-line SHM of high-performance structures. To monitor large-scale structures, a dense PZTs array is required. In order to improve the placement efficiency and reduce the wire burden of the PZTs array, the concept of the piezoelectric transducers layer (PSL) was proposed. The PSL consists of PZTs, a flexible interlayer with printed wires and signal input/output interface. For on-line SHM on real aircraft structures, there are two main issues on electromagnetic interference and connection reliability of the PSL. To address the issues, an electromagnetic shielding design method of the PSL to reduce spatial electromagnetic noise and crosstalk is proposed and a combined welding?cementation process based connection reliability design method is proposed to enhance the connection reliability between the PZTs and the flexible interlayer. Two experiments on electromagnetic interference suppression are performed to validate the shielding design of the PSL. The experimental results show that the amplitudes of the spatial electromagnetic noise and crosstalk output from the shielded PSL developed by this paper are ???15?dB and ???25?dB lower than those of the ordinary PSL, respectively. Other two experiments on temperature durability (???55??C?80??C ) and strength durability (160?1600??, one million load cycles) are applied to the PSL to validate the connection reliability. The low repeatability errors (less than 3% and less than 5%, respectively) indicate that the developed PSL is of high connection reliability and long fatigue life.

Design and Evaluation of a Wireless Sensor Network Based Aircraft Strength Testing System

The verification of aerospace structures, including full-scale fatigue and static test programs, is essential for structure strength design and evaluation. However, the current overall ground strength testing systems employ a large number of wires for communication among sensors and data acquisition facilities. The centralized data processing makes test programs lack efficiency and intelligence. Wireless sensor network (WSN) technology might be expected to address the limitations of cable-based aeronautical ground testing systems. This paper presents a wireless sensor network based aircraft strength testing (AST) system design and its evaluation on a real aircraft specimen. In this paper, a miniature, high-precision, and shock-proof wireless sensor node is designed for multi-channel strain gauge signal conditioning and monitoring. A cluster-star network topology protocol and application layer interface are designed in detail. To verify the functionality of the designed wireless sensor network for strength testing capability, a multi-point WSN based AST system is developed for static testing of a real aircraft undercarriage. Based on the designed wireless sensor nodes, the wireless sensor network is deployed to gather, process, and transmit strain gauge signals and monitor results under different static test loads. This paper shows the efficiency of the wireless sensor network based AST system, compared to a conventional AST system.

An evaluation on the multi-agent system based structural health monitoring for large scale structures

Different kinds of sensors have to be adopted to have a reliable monitoring of large scale engineering structures. Multi-agent system (MAS) based structural health monitoring (SHM) technology is researched to deal with the diverse, heterogeneous and distributed information. This paper introduces an evaluation on a multi-agent system based structural health monitoring to validate the efficiency of the multi-agent technology. Through the cooperation of different agents and different subsystems, the whole system can fuse three kinds of sensor information including strain gauge sensor, fiber optic sensor and piezoelectric sensor, can automatically choose sensing object, can self-organize the sensor network and discard useless sensor data, and can recognize three typical kinds of structure states which may indicate structural damages including impact load, joint failure, strain distribution change on every substructure and the edge area between two adjacent substructures. In the evaluation work besides adopting conventional monitoring instruments, the strain signal of the strain gauge is also demonstrated to be monitored by smart wireless sensor with on-board microprocessor developed in the lab. In this paper the evaluated experimental aluminium plate and the sensor distribution are firstly introduced. The corresponding agents are defined based of the designed multi-agent system frame presented by the authors. The monitoring strategies in the validation work including monitoring principle and monitoring equipments for three typical kinds of structure states are proposed. After the process of developing the impact load diagnostic agent as the basis work of the whole system is introduced in detail, the implementation of the whole multi-agent system based structural health monitoring for the plate is presented. This paper shows the efficiency of the multi-agent technology for the development of the SHM system on the large practical structures.