A pulse-to-pulse incoherent flow measurement with frequency-coded signals

This article describes a pulsed method, whereby receiving and transmitting takes place intermittently with the same transducer. Similar to the pulse-to-pulse coherent method, in which the velocity and range unambiguousness is limited by the Nyquist-criterion and the pulse repetition frequency, the pulse-to-pulse incoherent method is also subject to restrictions. The boundary conditions for both methods are demonstrated in this paper. With greater penetration depth, it becomes more and more difficult to ensure the unambiguousness. Therefore, signal-coding is required, even for relatively narrow-band ultrasonic transducers. To overcome this weakness a frequency coding concept for a pulse-to-pulse incoherent method is presented. The features of those encoded signals are given by a nearly orthogonal relation to each other and optimal autocorrelation properties. Beyond that, a filter bank with an envelope detector is demonstrated to separate the respective signal codes from the received signal and to determine the runtime of the echo. This concept has been verified by measuring the velocity distribution of air bubbles from a pipe stream. The measurement of the velocity distribution shows a high correlation to the actual flow rate of the pipe and corresponds to the Doppler-spectrum of a continuous wave (CW) sensor.