Analog demodulation circuit for structural health monitoring based on nonlinear wave modulation

Abstract. The purpose of this study is the development of analog demodulation system for a structural health monitoring system based on nonlinear wave modulation. When the structure with contact-type failure, such as tips of fatigue cracks, delamination of composite materials and bolt loosening is vibrating at low-frequency, the amplitude and phase modulation of ultrasonic waves or vibrations occurs and this phenomenon is called nonlinear wave modulation or vibro-acoustic modulation. The demodulation process is necessary to evaluate the failure development and localize the failure location with a simple and inexpensive signal processing system. The authors proposed a demodulation system using a complex analog filter. This complex analog circuit, which operates as Hilbert filter, has the characteristic of widening the operating frequency range as Hilbert filter by increasing the number of filter stages; however, the gain, which is ratio of output voltage to input voltage decreases with the increase of the number of filter stages. In this paper, numerical investigation for design of this complex analog circuit was performed. First, the basic performance of this complex analog circuit was introduced. Next, the design result of this complex analog circuit for specific condition was shown. Moreover, evaluation terms for the design of this complex analog circuit using performance characteristics was proposed.