Design Of Ultrasonic Ranging System Based On Piezoelectric Micromachined Ultrasonic Transducer

Ultrasonic sensing technology has outstanding application value in smart sensing,industrial manufacturing,smart medical,home automation and other fields.As a kind of "non-contact" distance measurement,ultrasonic ranging has unique advantages such as single directionality,low power consumption,and little external interference,which is extensively used in measurement field.However,with the rapid evolvement of MEMS technology and craft,ultrasonic sensing systems are developing towards miniaturization and intelligence,while traditional large-scale ultrasonic transducers based on piezoelectric ceramic cannot meet the needs of miniaturized distance measuring equipment.The MEMS piezoelectric ultrasonic transducer(PMUT)based on Al N piezoelectric film,which is small in size,simple in drive,compatible with CMOS process,and easy to miniaturize the measurement system,has received extensive attention and research.However,due to the low piezoelectric coefficient and electromechanical coupling coefficient of Al N,the ultrasonic signal intensity generated by this type of PMUT is low,and it attenuates quickly in the air,making it difficult to achieve longdistance measurement.For the purpose of settling the problem,this thesis conducted the theoretical research of PMUT based on Al N piezoelectric film,proposed a structure design of array element PMUT,elaborated its fabrication process,and tested its key performance about mechanical properties and electrical characteristics.According to its performance parameters,a complete back-end matching circuit was designed.Based on temperature compensation and ultrasonic Time-of-Flight(ToF)ranging principle,a high-precision ultrasonic ranging system was realized.The main research contents of this thesis include:(1)The characteristics and the sound velocity of ultrasonic waves are deduced.A distance measurement method based on ToF was elaborated.The piezoelectric effect and piezoelectric equation of Al N were discussed,the basic structure of PMUT based on Al N piezoelectric film was explained,the mechanical and electrical properties and the working principle were analyzed.An equivalent model of PMUT in the electrical domain,mechanical domain and acoustic domain was established.(2)According to the basic structure and working principle of PMUT,an arrayelement PMUT was innovatively proposed to make up for the shortcomings of Al N’s low piezoelectric coefficient and coupling coefficient,and to solve the problem of low ultrasonic energy generated by this type of PMUT.The fabricated process based on MEMS technology was described,and the morphology characterization and related performance test analysis of the fabricated PMUT device were carried out.(3)According to the PMUT performance parameters,a complete back-end matching circuit was designed,the hardware and software schemes of the PMUT-based ultrasonic ranging system were developed,and the related circuit principle design,layout design and debugging were completed.(4)The construction and debugging of the ultrasonic distance measuring system have been completed,and the distance and temperature measurement has been realized.The experimental results show that the measurement range of this system is about4cm～100cm,the maximum error is about 4mm,and the relative error is about 0.4%.