The aim of this study is to develop a simple sensor node with a sensor and actuator system which interacts with its host structure in an autonomous and self-sufficient manner. Such a sensor and actuator system embedded in a structure may perform like a biological system surrounded by an environment that eventually earns a recognition of its internal and external state through the embodied interaction with the host as well as other nodes in the neighborhood. To this end, this paper presents a structurally embedded piezoelectric ultrasonic active sensor that intermittently oscillates using energy harvested by itself from the operational vibration of the host structure. The presented sensor node consists of three parts: a piezoelectric element (PE) that performs as an energy harvester, sensor, and actuator; an oscillation circuit which induces an ultrasonic burst of the PE at one of the natural frequencies of the host structure, and a charge controller that switches the PE between an energy storage and the oscillation circuit. Since the oscillation frequency of the ultrasonic burst and the burstto-burst period respectively reflect the structural dynamics in high-frequencies and the magnitude of the low-frequency vibration at the sensor location, both of which can be used to understand the structural integrity and operating conditions. In this paper, an energy harvesting performance measure for the PEs directly attached on the structural surface is first presented. A design of the oscillation circuit with negative resistance is then presented, and experimental verifications are made to show the basic validation and future possibility of the proposed concept.
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