In this paper, a self-diagnosis technique is introduced for structural health monitoring based on the timefrequency analysis of electromechanical impedance (EMI) signatures. It is first shown that the EMI signature is essentially a pulse-echo signal represented in the frequency domain, and it can be converted into time domain using the convolution theorem. The time-frequency plot is then generated from the recovered time domain signals to cover a wide range of excitation frequencies and provide a more comprehensive damage detection capability. Presenting the EMI signal in the time and the time-frequency domains provides the physical insights that explain how different factors influence the EMI signature. As such, the time domain signal acquired from the EMI is divided into “resonant phase” and “echo phase”. The resonant phase includes the immediate response of the sensor to the excitation and is used to monitor the sensor bonding layer condition, while the echo phase only includes wave reflections from structural damages and boundaries, and is implemented for the structural damage detection. Finally, the proposed method is implemented on a beam structure to detect and localize structural damage in the presence of a damage in the sensor bonding layer.
Proc. SPIE. 9803, Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems 2016
KEYWORDS: Actuators, Digital signal processing, Modulation, Waveguides, Sensors, Receivers, Demodulation, Tomography, Transducers, Structural health monitoring, Antennas, Ultrasound tomography, Frequency conversion
In this paper, an unpowered wireless ultrasound tomography system is presented. The system consists of two subsystems; the wireless interrogation unit (WIU) and three wireless nodes installed on the structure. Each node is designed to work in generation and sensing modes, but operates at a specific microwave frequency. Wireless transmission of the ultrasound signals between the WIU and the wireless nodes is achieved by converting ultrasound signals to microwave signals and vice versa, using a microwave carrier signal. In the generation mode, both a carrier signal and an ultrasound modulated microwave signal are transmitted to the sensor nodes. Only the node whose operating frequency matches the carrier signal will receive these signals and demodulate them to recover the original ultrasound signal. In the sensing mode, a microwave carrier signal with two different frequency components matching the operating frequencies of the sensor nodes is broadcasted by the WIU. The sensor nodes, in turn, receive the corresponding carrier signals, modulate it with the ultrasound sensing signal, and wirelessly transmit the modulated signal back to the WIU. The demodulation of the sensing signals is performed in the WIU using a digital signal processing. Implementing a software receiver significantly reduces the complexity and the cost of the WIU. A wireless ultrasound tomography system is realized by interchanging the carrier frequencies so that the wireless transducers can take turn to serve as the actuator and sensors.
This paper presents a compact, batteryless wireless ultrasound pitch-catch system that wirelessly transmits the excitation signals to the actuator installed on the structure, and acquires the ultrasound sensing signal from the wireless sensor. The principle of frequency conversion is used to transform the ultrasound signals to microwave signals so that it can be wirelessly transmitted without digitization. As such, the power hungry digital-to-analog data conversion at the wireless actuator is eliminated. The wireless sensor node is equipped with a low power amplifier, which can be powered continuously by a microwave energy harvester. In addition, compact microstrip patch antennas are implemented for wireless transmissions, which help to achieve a compact interrogation unit.
Battery-less wireless transmission of acoustic emission (AE) signal acquired using a PWAS is demonstrated in this paper. The wireless AE sensor is equipped with a passive wireless transponder that receives a microwave carrier signal and up-converts the AE signal to microwave frequencies for wireless transmission. A low voltage ultrasound amplifier was designed, fabricated, and tested to amplify the AE signal and to provide a better impedance matching between the PWAS and the 50 Ω wireless transponder. A light-based energy harvester was adopted to drive the low-power voltage amplifier so that no battery is needed at the wireless sensor node. The energy harvesting devices and the amplifier were characterized using ultrasound pitch-catch and pencil lead break experiments. The design, implementation, and characterization of the wireless AE sensing system are described.