Weather: Sensing Wind- SC3sww

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Sensing and Control for hobbyists and schools: Weather: Wind

Page contents...

    Wind Speed.
    Wind Direction.
    Issues for both.

Wind speed

First catch your anemometer. I.e., you need something that spins around when the wind blows, and creates some kind of electrical signal.

The actual cups are tedious to make... you might want to spend your energies tracking down a commercial manufacturer of wind speed sensors who will sell you just the cups.

Don't skimp on the mechanicals! In even a moderate wind, the anemometer will spin very rapidly. Bad bearings or an unbalanced device will degrade your instrument's reliability. Also, you want it to last.

Once you have a spinning shaft, detecting how fast it is turning has much in common with other "how fast is it spinning" projects. Click through to this page, and look down towards the bottom.

Be sure to consider the Dallas/ AAG unit. It is well made, on the mechanical level. You can use it with "off the shelf" wiring and software, or you can adapt it, if you are more adventurous. You won't make a better job of the basic mechanical elements. ($75, 12/05. If you don't already have one, you'll need an adapter... DS9490 or DS9097U ($20 or $15). The signals from the AAG unit come down from the roof via the Dallas MicroLan, for 1-Wire chips, unless you embark on extensive (and warrantee destroying) modifications... but MicroLan is pretty cool!!)

Alternatively, and perhaps not for the hobbyist, but you might be interested in an alternative way of measuring wind speed: A wire is exposed to the elements. It is heated by passing a current through it. The more wind there is, the harder it is to achieve a particular temperature-above-ambient.

Another advanced approach derives from the wind direction sensor described below which uses ultrasonic transmissions.

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In this sub section: Some unpolished, not-fully-checked/tested information about sources of anemometers, or parts for making them.

Before I start on that: A brief note of encouragement: One day, fed up, I said "I'll make my own". It went surprisingly well! Very crude, but not entirely useless. A little wood, some wood dowels (about 5mm diameter), some coat hanger wire, a reed switch, a magnet, and some plastic balls from a toy store.

I used a small bit of wood to mount three horizontal arms to a vertical shaft. (Arms and shaft: Dowel wood). Stuck in the bottom of the shaft: A vertical pin. It "rides" on a horizontal surface. It is kept from drifting horizontally by a collar of coat hanger wire. (Essentially, it is a home made screw-eye). Near the top of the shaft, there's another screw eye, to keep the shaft upright. This is the weakest link in the design.

The cups are made from hollow semi-rigid plastic balls cut in half. Bits of wire keep them from flying off the ends of the spokes... a problem which I underestimated on my first attempt at this device.

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Moving on to more "serious" answers....

I haven't seen them yet, but the price (at 12/05: $20+p&p) was right, and they looked good online: You can get plastic cup assemblies from "ForceField, in Colorado. If you go this route, and find a good source of shafts and bearings, please get in touch?

A UK retailer, Weather Front, sells what looks like a nice set of cups for slightly more (at 12/05, Ł22 + p&p)

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Assembled anemometers-

Peet Brothers, in Florida, do what looks like a nice combined wind speed / wind direction sensor. Price at 12/05: $90+p&p. It is also available in a heated version, for places where freezing rain is a problem. They have a very clever way of determining the wind direction... clever in that it is achieved with very simple mechanical elements in the sensor head, so the sensor is likely to be reliable. There just two switches. The frequency of pulses on one gives you the wind speed. The relationship between the pulse trains from the two switches tells you the wind direction... if both switches go on at the same moment, the wind is from the north. If one goes on half-way between two "on"s from the other, the wind is from the south. If you have ideas you'd be willing to share here for how you interpret this data easily, or links to the same, please contact me? (Peet Brothers, obviously, will sell you a box which takes care of that... but some of you will share my urge to do things the hard way.) (Peet also offer a tipping bucket type rain gauge for $90 (12/05), and a fancier rain gauge for $190.)(and other things!))

Vortex have a nice looking unit, which is almost certainly of the "switch" type. At 12/05, price was $80+p&p.

Rainbird offer what look like nice anemometers and rain gauges. The anemometer looks like the Vortex unit- see above. I may come back to these devices when I have more time, but I couldn't (in the time I was willing to spend) discover where the company is headquartered... I'd guess somewhere in the US... nor could I be sure that both devices gave outputs equivalent to a momentary switch opening and closing. (Rain gauge seemed to be switch based. Rain collecting opening only 3.8"... for US Weather Service work, I think 8" is the standard.) The "spec sheet" was disappointing... more a marketing glossy than geek heaven. Neither could I find prices! The company seems to have its roots in the world changing (I'm not being snide... I believe this) impact sprinkler that we've all seen, and probably given too little respect. There's an interesting note on the site under "Company History".

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Visit any marina, and you will see an anemometer at the top of many masts. I've yet to discover where you can buy one. I also strongly suspect that the actual rotors are frequently damaged in regions where the boats are pulled from the water for the winter, but, again, I can't discover where to buy them. Yes... I have asked at a marina!

Wind direction

Be sure to consider the Dallas/ AAG unit. It is very well made, on the mechanical level. You can use it with "off the shelf" 1-Wire(tm) (also known as) MicroLan (tm) wiring and software, or you can adapt it, if you are more adventurous. You won't make a better job of the basic mechanical elements.

Some of the comments on my page about shaft encoders are relevant to the question of wind direction sensing.

Perhaps not for the hobbyist, but you might be interested in an alternative way of measuring wind direction. It involves three pairs of ultrasonic transmitters and receivers in a horizontal triangle. The system works from the Doppler shift arising due to the wind across each transmission path. (See also at least one thing in the previous section)

Issues Applying To Both Windspeed and Direction

What do you mean by windspeed? Direction? Are you speaking of the maximum gust for the day? The hour? Do you mean the most frequent speed or direction in the past ten minutes?

How would you display windspeed or direction. Speed is pretty easy... Direction- less so.

Even in displaying speed, the obvious answer may not be the easiest to program. Click here to have a look at the alternative display system. Besides being much easier to program, it has real advantages, once you learn (not too hard) to read it.

Displaying historical wind direction is even harder, especially if you want your display to show the difference between a day with a nice steady wind, and one when the wind changes it's direction capriciously. Some would say that your wind "direction" graph should show that there was essentially no direction when there was essentially no wind.

Two systems seem, to me, to be good. Both are based on circular "graph paper", and both are dynamic. By this I mean they are not like, say, a thermograph plot, with time the x axis, temperature the y axis. Both of the wind direction systems I like present changing "pictures", like the picture on a radar screen is dynamic. For a history of the day's wind, you'd need to take snapshots of the display from time to time. In both cases, they give a good idea of what is happening now, and what has happened recently.

The first draws lines from the center of the circle outward. If the wind has been towards the north most of the time, there will be a long, or fat, or brightly colored line from the center of the circle towards it's top, the traditional direction for "north" on the printed page. This basic idea can have many variations, depending on what you want to show.

The second scheme requires you to declare the wind to be from just one direction at any given moment. The direction it is from now gives rise to a dot on the edge of the circle. Again, if the wind is from the north, the dot is at the "top". The direction the wind was from five minutes ago gives rise to a dot one millimeter in from the edge of the circle. The direction the wind was from ten minutes ago gives rise to a dot two millimeters in from the edge of the circle... and so on. If you did a time-lapse movie of the graph, you would see the line being "sucked" into oblivion at the center of the circle. I don't fancy programming this display, but it would certainly give plenty of information. The dots could be drawn in different colors to indicate the windspeed. If the sampling period was short, you would be able to distinguish days of steady wind and swinging wind. The following shows a time when the wind is blowing towards the north. On the oldest part of the graph (at the center), we see that the wind was blowing to the east. We can see that the change happened quite smoothly and rapidly.

Wind Direction Display

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