An Anemometer Telemetry System for Airport Use
Tehachapi Wind FarmBy Kevin Cousineau
Tehachapi, California is one windy place where the Tehachapi Wind Farm includes about 5,000 wind turbines, but the privately-owned Mountain Valley Airport – with its two 5,000 foot runways – doesn't have the government funds other airports have to measure wind speed and wind direction. Best known for its glider operations and maybe the Raven's Nest cafe, wind telemetry is obviously critical to this facility. That's where I was able to help.
Fig. 1 Mountain Valley Airport Wind Sock and Wind Direction Glider
The wind sock and winder direction glider (mounted on its own yaw bearing) are located about 1,000 feet from the airport's main office and tell the arriving pilot something about the direction and strength of the winds. If the sock is straight out, the winds are exceeding 25 nautical miles per hour (knots) and if the sock is halfway down (as it is shown here) the winds are between 10 and 15 knots. To the right of the wind sock are the remote anemometer and its real-time telemetry system for signaling wind speed data to the wind speed meter.
In the picture above and directly blow the digital clock is the wind speed meter for real time display of wind direction and speed. The anemometer telemetry receiver and decoder are located just below the meter. The front panel mounted switch is used to select either the remote telemetry anemometer or a local roof-mounted anemometer. The wind direction sensor is mounted on the roof of the office with this extra anemometer and is not remotely monitored.
The requirements for this system were simple enough: connect the frequency output of a cup anemometer to a FM transmitter, send it about 1,000 feet to a FM receiver and reconstruct that signal to drive a wind speed meter. As with all engineering requirements, however, the problems are in the details. In this case, I needed to not only meet the environmental, electrical and electronics requirements, but I was under great pressure to maintain a reasonable budget as well. To that end, I had to work with everything I had in my archives to achieve the goal. In other words, it was time to raid the junk box and the Jameco catalog as well.
Encoder Circuit DesignThe cup anemometer installed was a Maximum type 41. This is a popular, rugged and inexpensive sensor that outputs a zero crossing sine wave with a frequency proportional to the wind speed. With an output of 59 Hz at 100 mph (86.9 knots) and 0 Hz at 0 wind speed, this signal cannot be directly connected to the microphone input on the FM transmitter and, therefore, some kind of encoder system would be needed.
The encoder simply amplifies and squares the anemometer output and uses that digital signal to switch a 1,000 Hz tone that is then connected to the FM transmitters microphone input terminals. The FM transmitter chosen was a Hamtronics T304-3 UHF exciter. At my request, Hamtronics modified their circuit by reducing the power output from 1 watt down to 100 mW. This would keep the overall current draw on the encoder system less than 175 mA and reduce the size of the expensive photovoltaic power system.
The encoder and FM exciter are powered by a 26 amp hour sealed lead-acid battery, which in turn is charged by a 20 watt photovoltaic panel. I modified one of my glider battery charge controllers to accept the solar panel input. The complete encoder system (including a battery charger, encoder, transmitter and battery) is mounted inside a Hoffman all-weather enclosure and attached to the mast shown in Fig. 1.
The encoder system is designed to operate over an extended temperature range of -20°C to +4°C including long periods of rain, snow and cloudy skies during winter conditions. It is not uncommon to go more than a week without any sunshine, so the system needed the large storage battery to continue working under these conditions.
The completed system is shown below undergoing bench testing. The Hamtronics exciter is located in the upper left-hand side of the enclosure cover with the anemometer encoder in the lower right and the battery charge controller occupying the lower left side of this same cover.
Decoder Circuit DesignOnce encoded, the next task was to design and build the decoder circuitry and mount it along with its associated FM receiver for operation inside the airport office. The decoder consisted of a Hamtronics R-306 UHF FM receiver coupled to the decoder circuit board itself. This circuit employed a simple audio amplifier, rectifier, filter and output amplifier that worked as an envelope detector to remove the 1,000 Hz tone and leave only the original low frequency anemometer signal in its place. The decoder circuit had to be built up using breadboard techniques as I have yet to complete a PCB for this portion of the project. Standard LM-385 op amps were used, one for the envelope detector and one for the output anemometer amplifier.
Finishing UpThe entire project took about a week to complete, including stuffing all of the boards and wiring the two enclosures. With the anemometer mast already in place, field installation required only about three hours to complete. My biggest challenge was to keep the project simple, low budget and reliable during the winter conditions in Tehachapi, located at an elevation of 4,200 feet in the Sierra Nevada foothills. Additionally, the decoder and the wind speed meter itself (KLC electronics Wind Check II) were mounted on the top of a 7 watt VHF transmitter and needed to operate without interference from this powerful source.
It was a fun way to spend my Christmas vacation time and since I am also a pilot, I get to take advantage of its output every time I fly into the Mountain Valley Airport.
Kevin Cousineau is the founder of KLC Electronics located in Lompoc, California and a Jameco customer.
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