| In order to adapt to the complex underwater environment and meet the changeful mission requirements,advanced sonar equipment should have a wider operation band.An ultra-wide operation band can provide more frequency points for applications,obtain more underwater information,and further enhance the anti-interference ability and stability of the system,showing great significance to improve the comprehensive performance of the sonar equipment.A large number of underwater sonars operate in the middle-and high-frequency band above 10 k Hz and below 1MHz.The acoustic signal in this frequency range is mainly generated by longitudinal transducer and 1-3 piezoelectric composite transducer.In this paper,we investigate the drive method that can achieve the ultra-wide operation band(width greater than two octaves)for the two transducers mentioned above.To form an ultra-wide operation band,the transducer should have the ability of activating multiple resonances.The longitudinal transducer looks like a rod and mainly works in longitudinal vibration mode.The 1-3 composite is driven by piezoelectric rods inside it,whose longitudinal vibration produces the macroscopic thickness vibration of the composite.Therefore,the resonance characteristic of the 1-3 composite is also determined by the longitudinal resonance of the rods,which is the same as the longitudinal transducer.Longitudinal resonances are the inherent vibration modes of a rod.Using high-order longitudinal resonances to expand the bandwidth can remain the structure of the transducer simple,with no need to introduce other resonances activated by designing complex structure.However,it is not easy to activate multiple,continuous,couplable longitudinal resonances with similar strength.This thesis will carry out the research in the following aspects.First,a 1D rod transducer is established.Different drive methods can be produced by changing the position,length and applied voltage of the drive sections.The drive method that can activate multiple,continuous resonances by referring to vibration modes is investigated.When drive methods obey vibration modes,the corresponsding resonances can be activated.Even if resonances are activated,response notches may occur between these resonances,resulting in their uncoupling.The analysis results show that the position and length of the primary drive section determine whether the resonances can be coupled.In addition,the response values of high order resonances are much higher than that of the first one.In order to obtain a flat band,the response values of these resonances are required to be close to each other.According to the vibration mode,we introduce extra drive sections to suppress high order resonances,and the peak values can be balanced by controlling the degree of drive against the mode.In this case,the rod transducer contains multiple drive sections distributed along the longitudinal direction.Finally,based on above results,general design principles are proposed for achieving an ultra-wide operation band.A two-layer 1-3 composite high frequency transducer is presented.The total thickness of the two layers is 10 mm.Only the lower-layer is excited,while the upper-layer in contact with the medium is not excited.Finite element method is employed to investigate the influences of the lower-layer thickness and the rod volume fraction on the transmitting voltage response.The influence when the upper-layer is excited is also studied.In addition,using a matching layer can further smooth the band.Finally,the transducer attains the ultra-wide band by coupling five resonances,nearly covering the frequency range from 100 k Hz to 1MHz.Subsequently,the proposed design principle is applied to the longitudinal transducer;the transducer has two drive stacks.First,a 1D theoretical model of the longitudinal transducer is established.The effects of tail mass length and drive stack length on the position of the response deep notch is explored.The effect of the length of the two drive stacks on the three response peaks is analyzed.In addition,the flexural mode of the head mass can be additionally activated to further improve the band fluctuation.Finally,a wide operation band is obtained in the middlefrequency range 15-70 k Hz.In addition,in order to solve the problem of ultra-wide band for middle-frequency transducer,this paper further proposes a three drive stack longitudinal transducer,which can activate and couple first four longitudinal resonances.At this time,the longitudinal structure becomes very complex,alternately distributing multiple drive-stacks and passive sections.Because many variables need to be optimized,traditional methods appear ineffective.This paper uses the simulated annealing optimization algorithm,combining with the 1D equivalent circuit theory,to optimize multiple variables simultaneously,and finally we obtain the drive method that makes all the response peak values consistent.The actual structure of the transducer will affect the frequency band.To obtain accurate design results,finite element re-optimization is required.Based on the results from simulated annealing optimization,the optimal results can be easily obtained by fine-tuning variables.Finally,an ultra-wide band is obtained in the middle-frequency range 15-90 kHz. |
