| Bone conduction auditory device carry the wave signal to inner ear through the skull which gains it a wide application to patients with auditory impairment and normal human since it conquers the masking effect in the noise to ensure clear listening and protects ears from hearing loss by replacing the air conduction earphone.The piezoelectric bone conduction aditory device produces hearing by the vibration of a vibrator driven by electric signal with certain frequencies. The key vibrator of this device has superiorities in small energy loss, light weight, thin surface, none electromagnetic radiation, radio resistance, etc. The frequency band and deformation of common piezoelectric ceramic materials are not large enough for bone conduction auditory device due to its inherent properties. Nevertheless, a piezoelectric ceramic vibrator with the deformation amplified might meet the requirement of bone conduction auditory device.The research in this paper is combined with the Ministry of Education in Colleges and Universities Technology Innovation Program fund major cultivating Project (Research on the new piezoelectric drive structure for universal measurement and control, No.708028) and Scientific and Technological Development Project of Jilin Province (Study on the working theory and key technologies of piezoelectric bone conduction auditory device).1.Basic theory of piezoelectric bone conduction auditory device The study on piezoelectric bone conduction auditory device is the integration of bone conduction auditory theory and the converse piezoelectric effect including the analysis on the physical properties of sound, the function of auditory organ, the transmission pathway of sound and the auditory parameters which provides evidence to the design of the key vibrator. Being the key vocal element of bone conduction auditory device, the piezoelectric materials have a direct impact on the performance of the vibrator via its inherent performance and parameters. Based on the analysis of the performance and parameter index of piezoelectric materials, PZT is selected as the vocal element to meet the strong and static converse piezoelectric effect requirement of the vibration membrane. As an elastomer with piezoelectric effect, the piezoelectric materials has special parameters such as dielectric constant d , elastic constant s , piezoelectric constant d , electromagnetic coupling coefficient K , mechanical quality coefficient Qm , etc. There are four piezoelectric equations considering the different boundary conditions and independent variables, two of which related to the PZT are analyzed in this paper.2.Theoretical analysis on the basic performance of piezoelectric vibrator Piezoelectric bimorphs are selected and the vibration membrane based on the analysis of the performance of piezoelectric vibrator. The power is connected in parallel to increase the displacement and force of the piezoelectric vibrator. The sensitive zone of bone conduction is 800Hz to 2000Hz, where the low frequency signal is usually too small to be perceived. As the key performance parameters of bone conduction auditory device, the fundamental frequency and displacement of piezoelectric vibrator can be determined by configuring the resonance frequency to the low frequency. This configuration may increase the displacement of the vibrator in the low frequency area so as to improve the low frequency response. The hysteresis characteristic of piezoelectric materials is analyzed as it determines the stationarity of the sound loudness. The mechanical model of rectangular and circular piezoelectric bimorph is established. The vibration equation and surface deformation equation are solved to find the formula of stiffness and amplitude of the piezoelectric vibrator. The relationship between the piezoelectric bimorph and the dimensions and materials of the metal substrate is analyzed.3. Analysis on the performance of the piezoelectric vibrator structure With the PZT-5 piezoelectric ceramic is chosen as the bone conduction auditory device vibration membrane material, beryllium bronze is selected as the substrate of piezoelectric vibrator on the finite element simulation analysis of the displacement and fundamental frequency of different substrate. Finite element simulation analysis of the rectangular piezoelectric vibrator fixed on one end and two ends and circular piezoelectric vibrator fixed on the centre and edge gives the displacement and fundamental frequency of them. Experiments on different supporting ways of the two piezoelectric vibrators are conducted to determine the major parameters such as the output force, hysteresis, step response. A conclusion can be made from the experiment that the rectangular piezoelectric vibrator in bone conduction application should be fixed on the two ends and the circular one should be fixed on the edge. The influence of the structure parameters of the piezoelectric vibrator—length, width, and thickness of the substrate and bimorph to the rectangular one and diameter, width, thickness of the substrate and bimorph to the circular one—on the displacement and fundamental frequency is simulating analyzed, which provides evidence to the determination of these parameters. Experiment is conducted on the selected piezoelectric vibrator to prove the feasibility of the vibrator. The experiments involve the test of the relationship between the driven voltage and displacement, the deformation displacement test, the fundamental frequency test, the step response test, the sine signal response test, and the amplitude-frequency test.4.Study on the vibrator structure design of single membrane bone conduction auditory devicePrototypes of the rectangular and circular membrane bone conduction vibrator are designed with a conduction column and a rubber plate to transmit the sound vibration to the skull. Experiments on the relationship between the diameter of the conduction column and the displacement and fundamental frequency of the piezoelectric vibrator and the relationship between the thickness of the rubber plate and the displacement and fundamental frequency of the piezoelectric vibrator are conducted to determine the diameter of the conduction column and the thickness of the rubber plate. The dynamic model and simulation model of the rectangular piezoelectric vibrator are established. The simulation analysis shows the dynamic characteristic of the vibrator. The static test of the two prototypes on the relationship between the displacement and voltage and fundamental frequency shows that the rectangular piezoelectric vibrator has a lower fundamental frequency and consequently a larger displacement in the low frequency response compared with the circular piezoelectric vibrator. The dynamic test of the dynamic response of the circular and rectangular piezoelectric vibrator shows that the rectangular one gives better response in the low frequency area and the circular one excels in other frequency area.5.Study on the vibrator structure design of double membranes bone conduction hearing device A double membrane frequency based piezoelectric bone conduction auditorydevice prototype is design with a rectangular piezoelectric vibrator working in the low frequency area and a circular piezoelectric vibrator working in the medium and high frequency area. Simulation and experiment on the rectangular vibrator with a hole in the centre are conducted to analyze the influence of the hole diameter on the performance of the vibrator. The appropriate hole diameter is given. The dynamic model of the double membrane bone conduction auditory device is established to analyze the coupling characteristics of the two membranes. Rubber damping material is added to the fixed point of the piezoelectric vibrator to decrease the influence of the membrane on the structure and the coupling interference between the two membranes. A comparison test is conducted between the piezoelectric vibrator and the electromagnetic bone conduction earphone. The voltage-displacement test and fundamental frequency test show that the piezoelectric bone conduction vibrator has a lower fundamental frequency and consequently larger amplitude in the low frequency area. The sine wave and step signal test demonstrate that the piezoelectric vibrator has a faster step response and better sine wave response than its counterpart. The voltage-displacement test of both the piezoelectric vibrator and the electromagnetic one under different frequency is conducted to determine the frequency division point. The dynamic performance of the piezoelectric vibrator with the rectangular membrane working in the low frequency area and the circular membrane working in the medium and high frequency area is proved to be better than the electromagnetic one in the whole frequency zone. |
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