Theoretical and simulation studies on designing a phase-reversal-based broadband CMUT with flat passband and improved noise rejections for SHM

In the past two decades, capacitive micromachined ultrasonic transducers (CMUTs) have been greatly explored for applications in structural health monitoring (SHM); however, relevant theories about their broadband sense have not been investigated systematically. Therefore, broadband CMUTs have been specifically developed from the aspects of theory and simulation in this work. Based on these theoretical developments, we propose a new design of phase-reversal-based CMUT, which has a flat passband for broadband sensing and two stopbands at both sides for improved noise rejections. First, the expressions for the evaluation of the total output current and the sensitivity of a CMUT constituted of multiple cells are deduced from the theoretical spring–mass–damping model. Then, theoretical and simulation analysis on a CMUT combined with two different cells have revealed that reversing the current phase of one of these two cells can produce significant stopbands for rejecting the low- and high-frequency noises, which are useful not only for a CMUT in coarse vacuum (low pressure) but also a CMUT in the air (atmospheric pressure). Especially, for a CMUT in a coarse vacuum, this design can effectively build a passband among the resonant frequencies of each cell instead of compensating each other to zero. Finally, the genetic algorithm is adopted to design a broadband CMUT with a given passband in air, the results of which are verified by the frequency- and time-domain simulations concurrently. Our research work may produce a theoretical way for the design of broadband CMUTs with noise rejections.