Central Bolts And Prestressed Piezoelectric Transducer Performance Parameters

Sandwich vertical vibration transducer has the advantages of solid structure, simple manufacturing process, big power-to-weight ratio and high electro-acoustical efficiency. In the field of power ultrasound, it has been widely used. With the development of power ultrasonic technology, the power, measurement range, and measurement accuracy of the transducer are becoming more demanding. As the core component of the ultrasonic vibration system, it is utmost important for improving the performance of ultrasonic transducers.The performances of sandwich piezoelectric transducer are related to the material, the shape and the size of the main components, in addition to the prestress of the bolt. In sandwich piezoelectric transducer, because piezoelectric ceramics tensile strength is rather poor, the prestress is usually imposed through the bolt and the metal pieces so that avoiding the rupture of piezoelectric ceramics in the high-power state. However, due to the impact of the prestress of the bolt, the performance of the transducer will change.In theoretical analysis on sandwich piezoelectric ultrasonic transducer, most were concentrated in a simple structure and the bolt was neglected. In the actual processing and assembly of the transducer, the diameter of the bolt is designed in accordance with the hole of the piezoelectric ceramic and the length is given arbitrarily. Thus the performance of the transducer may not be able to achieve the best condition.To further enhance the performance of the transducer, the effects of the bolt and the prestress on the performance parameters of the transducer were studied. The performance of the transducer can be improved by optimizing the geometric size, the location of the bolt and controlling the prestress in the assembly process. There are two main aspects:(1) The effects of the prestress on the performance parameters of the transducer were studied through experimental tests. The relation between the performance parameters and the prestress is as follows: With increase of the prestress, resonance frequency, anti-resonance frequency, effective electromechanical coupling coefficient, static electric capacity and dynamic electric capacity increase first and tend towards stability when the prestress achieves certain value. Dynamic inductance hardly varies with the prestress and remains constant. With increase of the prestress, the value of the impedance decreases at first. When the prestress reaches about 30 MPa, the impedance is smallest. Thereafter, the impedance begins to increase. When the prestress reaches about 50 MPa, the impedance basically remains constant.(2) The bolt is considered as a four-end network in the equivalent circuit. The resonance frequency equation of the transducer is derived. The effects of the position, the diameter and the length of the bolt on the resonance frequency, the anti-resonance frequency and the effective electromechanical coupling coefficient are studied.When the displacement node is located at the piezoelectric ceramic piles, resonance frequency, anti-resonance frequency and effective electromechanical coupling coefficient of the transducer will change with the length of the bolt. When the length of the bolt is equivalent to about a half of the length of the transducer, resonance frequency and anti-resonance frequency of the transducer achieve the largest value. Resonance frequency, anti-resonance frequency and effective electromechanical coupling coefficient change monotonously with the diameter of the bolt. The greater the diameter of the bolt is, the greater resonance frequency and anti-resonant frequency are, on the contrary, the smaller effective electromechanical coupling coefficient is. When the bolt is located at the centre of the transducer, resonance frequency and anti-resonance frequency achieve the largest value. However, there is a small change of effective electromechanical coupling coefficient with the position of the bolt.When the displacement node is located at the metal pieces, resonance frequency, anti-resonance frequency and effective electromechanical coupling coefficient of the transducer will change with the length, the diameter and the position of the bolt. Resonance frequency, anti-resonance frequency and effective electromechanical coupling coefficient change monotonously with the diameter of the bolt. The greater the diameter of the bolt is, the greater resonance frequency and anti-resonant frequency are, on the contrary, the smaller effective electromechanical coupling coefficient is. If the bolt moves ahead from the back-end, there exists a maximum value of resonance frequency and anti-resonance frequency corresponding to a particular position. However, effective electromechanical coupling coefficient of the transducer becomes smaller.