Improved Algorithm for Efficient Measurement of Electromechanical (e/m) Impedance for Structural Health Monitoring

Electro-mechanical impedance method is emerging as an important and powerful technique for structural health monitoring. The E/M impedance method utilizes as its main apparatus an impedance analyzer that reads the in-situ E/M impedance of the PWAS attached to the monitored structure. Present day impedance analyzer equipments (e.g. HP4194) are bulky, heavy and expensive laboratory equipment that cannot be carried into the field for on-site structural health monitoring. To address this issue, several investigators have explored means of miniaturizing the impedance analyzer making the impedance analyzer more compact and field-portable. In this paper we present an improved algorithm for efficient measurement of the E/M impedance using PWAS transducers. Instead of using a sinusoidal as the input to the PWAS and slowly changing its frequency, our method utilizes a chirp signal which is abundant in frequency components. By applying Fast Fourier Transform (FFT) to both the input and response signals, the impedance spectrum of the PWAS is acquired. The hardware system architecture consisting of reference signal generation, voltage and current measurement and digital signal acquisition have been explored. The size and the implementation of the overall system using either a laptop or a digital signal processor (DSP) are also discussed. Finally, practical results are presented and comparatively examined. INTRODUCTION Structural health monitoring (SHM) is a method of determining the health of a structure from the readings of an array of permanently-attached sensors that are embedded into the structure and monitored over time. SHM can be performed in basically two ways, passive and active. Passive SHM consists of monitoring a number of parameters (loading stress, environment action, performance indicators, acoustic emission from cracks, etc.) and inferring the state of structural health from a structural model. In contrast, active SHM performs proactive interrogation of the structure, detects damage, and determines the state of structural health from the evaluation of damage extent and intensity. Both approaches aim at performing a diagnosis of the structural safety and health, to be followed by a prognosis of the remaining life. The difference is, passive SHM uses passive sensors which only “listen” but do not interact with the structure. Therefore, it does not provide direct measurement of the damage presence and intensity. On the contrary, active SHM uses active sensors that can interact with the structure and thus determine the presence or absence of the damage. The methods used for active SHM resemble the methods of nondestructive evaluation (NDE), e.g., ultrasonics, eddy currents, etc., only that they use the embedded sensors. Hence, the active SHM could be seen as “embedded NDE”. One widely used active SHM method employs piezoelectric wafer active sensors (PWAS), which send and receive Lamb waves and determine the presence of cracks, delaminations, disbonds, and corrosion. Due to its similarities to NDE ultrasonics, this approach is also known as “embedded ultrasonics”.