A Novel Cat's Paw* Wind Sensor - Self Powered and Wireless
( No Batteries and No Cables )

William Stein

Every once in a while a simple idea for sailing technology comes along and one wonders why they didn't think of that, or in my case why I didn't think of it years ago. I had been working as an engineer both in wind power and instrument design for over thirty years. A couple of years ago I had the opportunity to develop a battery powered weather station which monitored wind speed and direction, barometric pressure, relative humidity and temperature. With a high powered RF transmitter it could transmit its weather readings to a receiver and base station up to 10 miles away.

It occurred to me some time later that a simpler device with a couple of important features would have a more widespread application. I had seen small wireless weather stations, usually requiring batteries or a solar panel to operate. These certainly have their place. But what if a wind sensor could be wholly self-contained, needed no batteries, and wirelessly radio the wind speed data back to a remote receiver? It seemed ideal. And a cup anemometer, in another sense, is the tiniest of wind powered generators. At just a few miles per hour of wind speed, plenty of energy is available in the wind to turn a generator and run a small bit of modern electronics, including a wireless transmitter.

Wind speed sensors, or conventionally, anemometers, are ideally placed at as high a location on a mast or tower as is practical. Once installed, one never wants to revisit the installation again! On the boat mast, on the roof top or up a tower, it's a job requiring removal of rigging or climbing or both. And conventionally, with a wired sensor, the sensor cable must be attached, routed, and secured. Holes need to be drilled, the wires fished through, and then the holes caulked to properly finish the job. It seemed this new concept of a self-powered sensor eliminated all these drawbacks... and one more. Putting any electrical gear at the highest point on a boat (or house) and then running wires from it to the cabin or house you can see where I am going with this: LIGHTNING. Now it is true the sensor is still susceptible to a strike, but the expensive equipment, be it a display, laptop computer, data logger, or control system, is now totally isolated and protected.

It has taken just about one year of development, but the anemometer is now ready for Prime Time. Figure 1 is a photo of the anemometer. The anemometer's innards consist of two highly refined components.

The first is a brushless generator which has zero impact on the free turning requirements of the anemometer's wind cups. This anemometer starts turning in the slightest of breezes, as low as 1 1/2 Knots. The second is the ultra-low power transmitter and signal conditioner. Very little energy is demanded of the generator. The very latest in low-power-consuming computer chips along with careful attention to low-power-circuit design were utilized to cut the power consumption to a minimum.

Two modes of operation are possible. If the average winds at your site are really low, say less than 5 knots (this is almost never the case in coastal areas), then the anemometer will run in ultra low power mode and transmit new data every 20 seconds. With increasing winds, the anemometer is capable of transmitting data more frequently up to once per second.

In Figure 1 you see what looks like an ordinary anemometer, with one exception. Near the bottom right is the anemometer's antenna. The transmitter operates at 900 MHz. This is the band the FCC has set aside for a variety of consumer and industrial products.

The receiver is shown in Figure 2. There are three connections to the receiver: the power cord, the serial cable to the PC and an antenna connector. For shorter range applications, up to 140 feet, a small three-inch stick antenna similar to that on the sensor is suitable. For larger distances a directional antenna may be used. Currently the receiver has two types of outputs. It has a serial port for connection to a computer, with software to display (and data log if desired) the incoming wind data. It also has a voltage output which can be fed to meter or bar graph for a convenient visual display. One other feature: for those interested in measuring the wind's direction, this anemometer is available with a wind vane option for transmitting the direction data as well.

Figure 3 is a screen snapshot of the software available with the instrument which runs on a PC. The software provides an immediate visual display of the current wind conditions. Additionally, it provides a "virtual" strip chart for observing the recent history of the wind conditions. And lastly for the weather buffs and wind energy prospectors, it provides an option for saving the data in a spreadsheet file for further analysis.

This new "Cat's Paw" wind speed sensor would seem to solve many of the problems associated with measuring wind at the top of a mast. Once clamped in place at the top of the mast, no cables need to be routed, and no batteries or solar panels are required. It is sufficiently rugged to withstand bird landings, although birds seldom land on moving objects such as wind cups.

About the Author

William Stein is a Research Fellow at the Renewable Energy Research Laboratory, Mechanical Engineering Department, at the University of Massachusetts in Amherst, MA He can be reached at support@etesian-tech.com He has recently formed Etesian Technologies to market and manufacture his new products. The above described sensor is patent pending.

* For Land Lubbers: Cat's Paw is a wind which ripples the surface of a body of water.