This paper proposes an innovative ultrasonic time of flight (TOF) measurement method with narrow-band transducers. By introducing the received ultrasonic wave peak time sequences (PTSs) of two slightly different frequencies, the relative TOF can be accurately identified with a much better resolution than a wave period. The new PTS TOF measurement is achieved in two steps. First, a PTS is built for receiving the ultrasound signal of each frequency according to the arrival time of the wave peaks by calculating the mean value of the adjacent crossover time. Second, the arrival time of the wave front is rebuilt by estimating the common initiation time of the PTSs for the received waves of slightly different frequencies. A mathematical model is derived to describe the signal reception, from which the TOF estimation algorithm was derived. A simulation model describing the characteristics of the ultrasonic transducer and the ultrasonic wave propagation physics was developed to verify the feasibility of the new algorithm. Finally, an experimental system for measuring the relative TOF over the known distance of 550-1450 mm was implemented to confirm feasibility and to demonstrate that a subwavelength resolution of 0.015 times of the wavelength was indeed achievable in measuring distances over meters.
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