The development of multi-functional sensors together with advancement of electronics and miniaturization in the past two decades, has resulted in novel sensor technologies which can be tailored to the SHM application in operational conditions. Our research group has been developing novel SHM technologies addressing key factors which have driven our work:

• Survivability of the on-board components through life-time of the structure
• Reliability of the acquired data under operational condition
• Redundancy of the sensors
• Repairability of the SHM units
• Self Sufficient - Energy Harvesting
• Cost benefit analysis
• Additional weight and disturbance to the host structure

We have developed a SHM technology cluster with innovations in both sensor technology and data acquisition system. In practical applications, when a large coverage area is required, or a large number of sensors are distributed remotely, data transmission, sensor placement and wiring costs can become an issue for wired sensing systems. The combination of SHM and wireless sensing networks (WSN) results in a fundamentally different cyber-physical approach which involves both the considerations of SHM systems (the physical component) and the newly-introduced WSN system (the cyber component). Our team has developed compact energy-aware high-performance cyber-physical systems for both impact detection and identification and damage detection and characterization. The SHM systems combined with our diagnostic film, offers a reliable, compact diagnostic package which includes both technological innovation and software advances which enables the full system to be applicable to aircraft structure under operational conditions.

Wireless Paive Sensing Module (WiPas) :

For a passive system to detect and characterize impact events on an aircraft in operation, the coordinator and routers are required to be active all the time in order to receive any data from end devices. These units are connected to the main power line in the aircraft. Therefore, power consumption is not a major issue for these devices. However, for end devices, they are normally powered by batteries or energy harvesters, which have a limited power supply capability. The probable impacts which can occur on an aircraft are rare, random and transitory. In order to capture these events, it is ideal for the sensing system to operate continuously. In order to achieve long-term operation and to reduce the cost for maintenance, our team has developed a wireless sensing unit with low-power consumption. For aircraft in operation, vibrations in a wide frequency bandwidth are abundant. These vibrations also affect the output of piezoelectric sensors. False trigger events may be generated due to local noises. In order to adapt the sensing system at run-time on board, filters and an appropriate triggering threshold have been included in our WiPas module to avoid recording false impacts or impacts of very low energy which are not alarming for the structure. The local processing module is required to be responsive to impact events, capable of processing multiple impact inputs and energy-efficient when no impact occurs. The wireless module (ZigBee) transmits the processed impact data to the host station for impact detection and evaluation.

Selected publication:

• Fu, H., Z. S. Khodaei, et al. (2018). "An Event-Triggered Energy-Efficient Wireless Structural Health Monitoring System for Impact Detection in Composite Airframes." IEEE Internet of Things Journal. DOI: 10.1109/JIOT.2018.2867722

Wireless Active Sensing Module:

Our team has developed a compact energy-aware high-performance cyber-physical systems, for monitoring aircraft composite structures. The Wireless active sensing module includes a network coordinator and a certain number of sensor nodes as routers or end devices. Wireless communication methods, is used for data transmission between sensor nodes and the coordinator. The coordinator is attached to a central station that is responsible for detection control, sensing data repository and post-signal processing for damage detection and evaluation. For sensor nodes, they have the functions of sensing data acquisition, local processing and wireless communication with the coordinator. The innovative features of our design are:

1. Compactness:  the system is designed to be implemented on a printed circuit board with small dimensions and light weight:
2. Multiple sensing channels: All components of the system is designed to allow interrogation of up to 24 multiple sensors (e.g. piezoelectric sensors) which can be connected to the system.
3. Wireless communication:  a wireless module is designed in the system for the communication between end devices and the host station (coordinator).
4. Energy-Efficient:  The system is designed to operate in a low power mode when no actuating requests are received from the host station and operate with a high performance when it is woken up by the actuating request from the host.
5. Low cross-talk: To allow compactness as well as multiple sensing channels, a switching module is designed with high crosstalk attenuation. The influence of the actuating signal on sensing channels due to crosstalk is significantly reduced.
6. Energy harvesting powered: The wireless sensor nodes are powered by an energy harvesting module. This module converts ambient energy sources (e.g. vibration, heat, or airflow) into electricity for sensing application.

Diagnostic Film:

Through the use of inkjet printing for the development of a highly innovative advanced lightweight structural health monitoring (SHM) system has been developed by our group. The developed SMH system consisted of a printed network of conductive wires on the surface of a dielectric thin-film and an array of distributed piezoelectric (PZT) sensors. In comparison to wired systems the film reduced the weight of almost 70% . Its reliability under operational conditions of an aircraft has been verified through electrical, mechanical and thermal loading tests. The diagnostic film can be either surface mounted or embedded within a composite interlayer.

The Diagnostic Film consists of an array of PZT sensors and an inkjet-printed conductive network. It can be either attached on the surface of a structure or embedded within a composite interlayer. Due to the versatile character of the inkjet printing technology, the developed conductive network can be purposely designed to allow for accurate sensor placement and better meet application demands. The reliability of the developed SHM system has been verified through extensive tests that simulated the operational environment of an aircraft.

Selected publication:

• Bekas, D. G., Sharif-Khodaei, Z., & Aliabadi, M. H. F. (2018). An Innovative Diagnostic Film for Structural Health Monitoring of Metallic and Composite Structures. Sensors (Basel), 18(7). doi:10.3390/s18072084