| BackgroundFocused ultrasound transducer is the core component of ultrasound treatment equipment,its stable and effective work is the effective protection of safe treatment.The electrical impedance of the transducer was an important basis for the choice of treatment frequency and the design of the drive power source.According to the ideal electrical impedance model of the piezoelectric oscillator,the resonant frequency point can be determined by the resistance impedance curve to determine the optimal operating frequency of the piezoelectric transducer and the electrical impedance matching parameters of the transducer.However,the actual focus ultrasonic transducer was affected by the structure,process and acoustic lens.There was a certain gap between the electrical impedance model and the ideal piezoelectric oscillator.It was difficult to directly determine the optimum operating frequency of the piezoelectric ceramic material from the impedance spectrum.By establishing and studying the electrical impedance model of the transducer,this problem can be effectively solved and the theoretical basis for the design of the transducer and the driving power supply was provided.In addition,the electrical impedance of the transducer during the work of the characteristics of the acoustic load,the working temperature and aging of piezoelectric materials and other factors affect the range of changes,affecting the sound poweroutput and treatment effect,therefore,the transducer resistance real-time measurement of resistance to the safety and effectiveness of treatment is of great significance.The main indicator of the effective output of the transducer was the electro-acoustic conversion efficiency,which changed with the frequency,so the transducer need to select the appropriate frequency point,and the actual situation of the transducer in the dynamic changes,and static conditions of the electro-acoustic conversion characteristics of the deviation,so the poor choice of working parameters will affect the stability of the transducer output,or even damage the transducer,thus affecting the safety of treatment.PurposeThe electrical impedance model of the spherical self-focusing ultrasonic transducer was constructed to analyze its electrical impedance characteristics,and it provided the basis for the stable output of the transducer.At the same time,the automatic test of the electro-acoustic conversion characteristic of the transducer was studied,and the appropriate working frequency point of the transducer was found quickly and accurately to met the requirement of improving the effectiveness and safety of the focused ultrasound system.Methods1.Establish a spherical self-focusing ultrasonic transducer electrical impedance modelThe existing transducer electrical impedance model is based on the plane ultrasonic transducer.In order to analyze the impedance model of the spherical self-focusing ultrasonic transducer,the finite element software was used to construct the electrical impedance model.The simulation results were compared with the actual measured impedance characteristics to verify the effectiveness of the model.2.Measurement of electrical resistance spectra of transducer based on impulse response methodIn order to measure the electrical impedance characteristics of the transducer in real time,based on the electrical impedance model of the transducer obtained above,the impedance spectrum of the transducer was measured by the impulse response method.In order to verify the impulse response method,the simulation results were compared with the experimental measurements,and the excitation signals were optimized and selected.From the point of view of the power spectral density of the excitation signal,the method of optimizing the selection of the excitation signal was energy concentration and the '3dB' decay method.The energy concentration method was divided into two cases: monotonic and non-monotonous,the power spectral density distribution was analyzed to select the appropriate excitation waveform.According to the waveform pulse width will affect the waveform energy distribution in different bands,the latter optimization selection method was proposed.In order to verify the effectiveness of the optimization method,the corresponding measurement platform was constructed.Taking the square wave pulse,the Gaussian pulse and the first order differential Gaussian pulse as an example,the impedance spectra of the transducer under different pulse width and different types of pulse signals were obtained.The electrical impedance of the transducer under the signal was used to verify the reliability of the selection method.3.Automatic testing of electro-acoustic conversion characteristics of transducersIn order to obtain the electro-acoustic conversion characteristic of the focused ultrasonic transducer in real-time,an automatic test system was constructed.The input electric power of the transducer was measured by anelectric power meter based on a directional coupler.The output power of the transducer was measured by using a radiation balance.Graphical development environment Lab VIEW developed automated test software.Based on the developed test system,the electro-acoustic conversion efficiency of the transducer at different frequencies and driving power was tested,the working frequency band of the transducer was analyzed,and the stability of the system was analyzed by using the coefficient of variation and compared with the artificial test method.Result1.The spherical self-focusing ultrasonic transducer was constructed by the simulation software,compared with the actual value,the change of the resonant and anti-resonant frequencies of transducers under the theoretical model were less than 3%,and the change of the two-frequency point electrical impedance value were not more than 15%,which show that the simulation results were in good agreement with the measured values,thus confirmed the effectiveness of the constructed electrical impedance model.2.The simulation results show that the resonant frequency of the transducer was reduced by 0.13 MHz and the anti-resonant frequency was0.22 MHz,and the rate of change was less than 3%,and the impedance value changed by 0.5 ? and 2.09 ?,and the change rate of the two values did not exceed 10%,which indicated that the simulation results were in good agreement with the measured values,which confirmed the effectiveness of the impulse response method to measure the electrical impedance spectrum of the transducer.In the case of pulse width was 100 ns and amplitude was 50 V.For the different excitation signals,the impedance spectra obtained by the square wave pulse excitation were distorted.The impedance spectra of theGaussian pulse signal and the first order differential Gaussian pulse signal were in good agreement with the actual measured values.The rate of change of the resonant and anti-resonant frequencies obtained by these two excitation signals was less than 2%,and the corresponding anti-resonant frequency was different in the electrical impedance value,the former was23%,the latter was only 5%.It was shown that the more exciting energy of the pulse excitation waveform was,the more favorable the measurement result was.In the case of excitation voltage of 50 V,for the different pulse width of the excitation signal,the pulse width of 31.5ns square wave pulse and pulse width of 123 ns first order differential Gaussian pulse excitation transducer,the corresponding impedance spectra were in good agreement with the actual measurements,and the power spectral densities of the two signals were in accordance with the 3 dB attenuation method.The effectiveness of the 3dB decay method for measuring the impedance spectrum of the transducer was described.3.For the measurement of electro-acoustic conversion efficiency,the manual test took about 110 min,the automatic test took only 17 min,the test time of the automatic test system was shortened by more than 5 times.The coefficient of variation of the system was 4.06%,4.31% and 4.65% at the driving power of 10 W,20 W and 30 W respectively.The coefficient of variation of the artificial test was 4.14%,4.69% and 5.83% respectively.Conclusion1.An electrical impedance model based on focused ultrasonic transducer was constructed.Based on the theoretical calculation and experimental verification of the planar ultrasonic transducer,the finite element simulation software was used to simulate the focused ultrasonic transducer,and the results were compared with the actual measurement results.The results show that the two coincides well,so the establishmentof the electrical impedance model can be more realistic simulation of the work of focusing the transducer.2.Optimize the impulse response method for measuring the impedance spectrum of the transducer.The finite element method was used to simulate the impedance spectrum,which provided the basis for measuring the impedance spectrum of the transducer.The optimization of the selection of the pulse excitation signal was carried out.The method of selecting the excitation signal was proposed.The effectiveness of the method was verified by comparing the impedance spectrum of the transducer under different excitation signals.The optimization of the selection method can be quickly and easily selected the optimal excitation waveform,so as to met the relevant areas of resistance impedance measurement needs.3.Set up the transducer electro-acoustic conversion efficiency measurement platform,compared to manual testing,the platform can automatically test,convenient,fast,a single test time reduced by more than5 times,and better stability,the drive power from 10 W to 30 W,the coefficient of variation was only increased by 15%,well below the artificial test coefficient of variation of 41%.Therefore,the establishment of this method to facilitate the rapid and accurate measurement of the electro-acoustic conversion efficiency of the transducer,the selection of the best working frequency of the transducer had important guiding significance,the actual production of the transducer detection,had a good application and promotion value. |
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