WIND ENERGY SYSTEMS - Cd3wd

Chapter 3—Wind Measurements 3–18
Figure 9: Wind direction vane and transmitter. (Courtesy of Electric Speed Indicator Company,
12234 Triskett Rd., Cleveland, OH 44111.)
north, V B /4 when the wind is from the east, V B /2 when the wind is from the south, etc. The
A/D converter filters noise and converts V d to a digital form for recording. The potentiometer
is usually wire-wound so V d changes in small discrete steps as the wiper arm rotates. The
A/D output always changes in discrete steps even for a smoothly varying input. These two
effects make direction output resolutions of less than 3 degrees rather impractical, but such a
resolution is usually quite adequate.
Figure 10: Potentiometer-type wind direction transmitter.
Each digital number represents a range of wind direction, in a similar manner to the
wind speed ranges discussed earlier. This range can be changed by adjusting the voltage V B .
Suppose that we have a 7 bit A/D converter, which will have 128 different digital numbers if
the input voltage varies from zero all the way to rated voltage. V B can be adjusted to a value
less than the rated voltage of the A/D converter so the A/D output just reaches 120 at the
maximum setting of the potentiometer, which makes each digital number, or bin, represent
a range of 3 degrees. Each of the eight cardinal directions then can easily be determined by
adding 15 adjacent bins to get a total of 45 degrees. This increment size does not work as well
for a 16 direction system since this would require splitting a bin. If 16 directions are required,
an 8 bit A/D converter with 1.5 degree bins may be desirable.
Wind Energy Systems by Dr. Gary L. Johnson November 12, 2001