The SBF (short baseline framework) is commonly used in ultrasonic parameter measurement, and ToF (time of flight) of which is essential in sound velocimeter and other applications. Different from general ultrasonic measurement research work mainly on ToF estimation method, this paper studies the criterion and way of establishing the optimum signal. For given transducers, the optimum signal achieves the most accuracy, and the pulsed form signal is stress here. To find the optimum signal, firstly, the transmitting is illustrated in its Fourier series form, which establishes the waveform, and the problem of transmitting wave design is converted into finding the optimum Fourier series. Secondly, as the accuracy is in inverse ratio to the sensitivity of the measured signal, the problem of finding the optimum Fourier series is equivalent to maximizing the sensitivity, resulting into a form of nonlinear optimization problem. For optimum parameter derivation, the sensitivity is expressed in form of vector and matrix, and a novel method based on SCHUR decomposition on the matrix is proposed to solve the nonlinear optimization problem. Simulation with a typical narrowband system testifies that the proposed method can synthesize transmitting waveform efficiently.
The short baseline framework is commonly used in ultrasonic parameter measurement, and ToF (time of flight) of which is essential in sound velocimeter and other applications. Different from conventional steady-state and transient methods, this paper proposed a new method using DSSS (Direct Sequence Spread Spectrum) signal to extract the ToF. DSSS signal possesses excellent auto-correlation features, thus it does not need to adjust path-length or the acoustic frequency, nor avoid the impact of the multiple echoes. The DSSS signal and measurement system are modeled and the detailed theoretical process of ToF estimation is derived to demonstrate the measurement principles, and the principles of echoesresistance are analyzed. Measurement experiments are carried out using distilled water at ambient pressure with temperature ranging from 19°C to 30°C. The test results indicate that the relative precision of ToF is in bounds of 2 ppm and the bias of sound speed derived is within 0.06m/s compared to Del Grosso's equation.
The ultrasonic ranging equipment usually need to measure the “time of flight”, that it takes for acoustic waves to travel the measured distance. Due to the influence of various noises, ultrasonic distance measurement using the pulse signal is difficult to obtain accurate measurement results. Continuous signal phase detected method can achieve the precision measurement of the transmission time, and has a strong noise resistance capability. When single-frequency signals are used, the measurement range is limited to a small extent. This problem can be solved by dual-tone or multi-frequency method. But in short distance ranging applications especial in the restricted space fields, there usually exists strong multiple echoes. Because of the echoes, the ranging accuracy will be significantly decreased. In this paper, a singlefrequency continuous signal modulated by a pseudo-random sequence is used, since the pseudo-random sequence has excellent autocorrelation properties. On the other hand, an improved signal phase estimation algorithm is used to realize the high-precision measurement of TOF. An experimental measurement system was built, and experiments showed that the method can get micron degree precision.