Abstract
Coaxial rotor helicopters have a wide range of civilian and military applications; however, the collision risk of the upper and lower blades that comes with the coaxial rotor system remains. This paper introduces a blade-tip distance measurement method based on coded ultrasonic ranging to tackle this challenge. Coded ultrasonic ranging with phase modulation was adopted to improve the measurement rate. In this paper, seven-bit M-sequences and Gold codes are chosen with four periods of 200 kHz sine wave carriers as the excitation signals, and the received signals are filtered by a bandpass filter and decoded by a matching filter. The coding performance is evaluated by the distinguishability and energy level of the received signals. The experimental results show that the measurement rate can reach 3060 Hz for a distance of one meter. They also give the potential solution for other high-speed measurement problems.