Wind speed and direction measurement with narrowband ultrasonic sensors using dual frequencies

Knowing the wind speed and direction in an electrical power generating wind farm is an important factor in its management. A measuring device for this function with no moving parts is desirable to reduce maintenance due to wear of the mechanical parts. Ultrasonic measuring techniques can be employed in such a device to fulfill this requirement by measuring the time it takes for a signal to propagate from an emitter to a receiver sensor. Wind in the direction of the sound wave, or against it will affect the travel time of the sound wave and the wind speed can be extracted from these Time Of Flight (TOF) measurements. Commonly used narrowband ultrasonic transducers have the disadvantage of generating a long oscillating signal where the start of the received signal has a very small amplitude and cannot be directly detected and flagged with a simple threshold circuit. The present paper describes a method where two signal bursts of different frequencies within the bandwidth of the transducers are used to obtain the TOF. The phase difference between the emitted and the received signal is measured at two frequencies and the results are then combined to give the TOF. The wind direction can be obtained with an additional measurement in the orthogonal direction by a second pair of sensors.