doi: 10.1111/j.1460-2695.2008.01248.x Structural health monitoring using electro-mechanical impedance sensors S. PARK 1 , C.-B. YUN 1 and D. J. INMAN 2 1 Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, Daejeon, 305-701, Korea, 2 Center for Intelligent Material Systems and Structures, Virginia Polytechnic Institute and State University, Blacksburg, VA, 24061, USA Received in final form 4 June 2008 ABSTRACT This paper reports recent achievements of novel structural health monitoring (SHM) techniques for damage diagnosis for critical members of civil, mechanical and aerospace structures using electro-mechanical impedance sensors. The basic concept of this tech- nique is to use simultaneously both high-frequency structural excitations and responses employing piezoelectric sensors to monitor the local area of a structure for changes in structural impedance that would indicate imminent damage. In this paper, several princi- pal software and hardware issues on these topics are described. A new impedance model is proposed that incorporates the effects of sensor and bonding defects for sensor self- diagnosis. Temperature effects-free impedance-based damage detection algorithm using effective frequency shifts based on cross-correlation coefficients is presented. In a sense of tailoring wireless sensing technology to the impedance methods, an active sensor node incorporating a miniaturized impedance sensing device, an on-board microcontroller, and a radio frequency (RF) telemetry is introduced. A data compression algorithm is embedded into the on-board chip of the active sensor node to enhance its local data processing-capability. Finally, this paper concludes with a discussion of further studies and future applications. Keywords electro-mechanical impedance; piezoelectric sensors; sensor diagnosis; sen- sor nodes; structural health monitoring; temperature effects. INTRODUCTION Damage in civil, mechanical and aerospace structures may come from fatigues or excessive external loads such as strong winds, earthquakes, explosions and vehicle im- pacts. Early detection of the damage or structural degra- dation prior to local failure can prevent catastrophic col- lapse of those structures. In particular, the large physi- cal size of host structures may require innovative sens- ing technologies utilizing appropriate software and hard- ware systems for data acquisition/reduction for rational structural health monitoring (SHM) applications. At its simplest application, a risk alarm can be provided when the continuously measured responses from some sensors at the specific locations of those structures exceed the pre-set threshold level. In this sense, electro-mechanical impedance-based SHM techniques have been developed Correspondence: S. Park. E-mail: shparkpc@kaist.ac.kr by utilizing the electro-mechanical coupling property of piezoelectric materials and utilized as a powerful and in- novative tool for local damage detection for a variety of structures. 1–9 The basic concept of these techniques is to use simultaneously both high-frequency structural ex- citations and responses employing piezoelectric sensors to monitor the local area of a structure for changes in structural impedance that would indicate imminent dam- age. In this paper, several principal software and hard- ware issues on this topic are reviewed. A new impedance model is described that incorporates the effects of sen- sor and bonding defects for sensor self-diagnosis. Tem- perature effects-free impedance-based damage detection algorithm using effective frequency shifts based on cross- correlation coefficients is presented. In a sense of tailor- ing wireless sensing technology to the impedance meth- ods, an active sensor node incorporating a miniaturized impedance sensing device, an on-board microcontroller, and a radio frequency (RF) telemetry is introduced. A data compression algorithm is embedded into the on-board chip of the active sensor node to enhance its local data 714 c  2008 Blackwell Publishing Ltd. Fatigue Fract Engng Mater Struct 31, 714–724 Fatigue & Fracture of Engineering Materials & Structures Fatigue & Fracture of Engineering Materials & Structures