Operating principle - WMT700

WMT700 uses the Vaisala WINDCAP ultrasonic sensor technology in the wind measurement. The sensor has an onboard microcontroller that captures and processes data and communicates over serial interfaces.

WMT700 has an array of 3 equally-spaced ultrasonic transducers on a horizontal plane. Wind speed (WS) and wind direction (WD) are determined by measuring the time it takes the ultrasound to travel from each transducer to the other 2.

WMT700 measures the transit time (in both directions) along the 2 paths established by the array of transducers. The transit time depends on the wind speed along the ultrasonic path. For zero wind speed, both the forward and reverse transit times are the same. With wind along the sound path, the upwind direction transit time increases and the downwind transit time decreases.

The following figure shows how the time shift of the ultrasonic signals is measured and how the tail wind and forward wind affect the measurement.

1

Ultrasonic measurement with zero wind

2

Impact of tail wind on ultrasonic measurement

3

Impact of head wind on ultrasonic measurement

The microcontroller calculates the wind speed from the measured transit times using the following formula:

VW=0.5·L·(1/tf-1/tr)

Measuring the 6 transit times allows V_w to be computed for each of the 3 ultrasonic paths. Using V_w values of 2 array paths is enough to compute the wind speed and the wind direction.

The different paths of WMT700 and the vectors provided by the wind sensor:

1–6

Measurement paths 1 to 6 of WMT700

L_a, L_b, L_c

Distance between 2 transducers

The vectors are calculated as follows:

Va=0.5·La·(1/A1-1/A2)
Vb=0.5·Lb·(1/A3-1/A4)
Vc=0.5·Lc·(1/A5-1/A6)

The equation depends on the accurate distance of the measurement path (L). The computed wind speeds are independent of altitude, temperature, and humidity, which are canceled out when the transit times are measured in both directions, although the individual transit times depend on these parameters.