Saturday, December 28, 2013

An Anemometer for your model airport

Motivation

As noted before, I fly mostly small - mini and micro - copters of various kinds. A lot of time, this means flying indoors. But flying outdoors is a lot more fun - having a sky to play in instead of a living room provides a lot more room for tricks, mistakes, and recovering from them.
However, the wind outdoors is really important. I can go outside and look, but that doesn't tell me how the wind has been behaving, so I might be looking at either calm or gust and draw the wrong impression. I can check the weather reports on a variety of devices, but that's not local, and a bit of a pain.
I thought about putting up a wind sock, but that's just a cooler version of going outside to look. So I decided I wanted an anemometer of some sort.

Hardware

First step - what kind of off the shelf choices are there?

Handheld devices

There were lots of small handheld devices were common and either cheap or with nifty features, but if I had to go outside to check them, or stay outside so the nifty features could do their thing, I'd already know the wind conditions.
Smartphone apps (that either used the builtin-microphone to check wind sound, or had a sensor that plugged into the audio port) were both cheap and had nifty features, but still required going outside. Not what I wanted.

Complete stations

Then there are home weather stations of various sorts. Getting one that actually had the wind speed information I wanted were very expensive, because they invariable included a nice, complete weather station. And none had a display that really worked for what I wanted to do.
Wind Sensor mounted on fence corner
Wind Sensor

A solution

One of the interesting things that turned up in the was an Inspeed pole mount anemometer. It had great reviews from meteorologists at the local university. The problem with it is that it came with a bicycle speedometer as a display. Not really an improvement over a wind sock, but at least I didn't have to go outside. However, it was available without a display and documentation on what was put on the wire - which could easily be dealt with by an arduino.


Display

Given that I get to design the display, there's now lots to choose from. I eventually settled on a 16x2 RGB display from Adafruit, because they made a nice, simple display case for it.

Back of display, showing LCD control & Arduino
Back of Anemometer showing cirtuitry.

The blue PCB that dominates the image is the LCD control circuit. The bit of electricians tape on it covers the power LED. The Arduino is the green PCB on the right with the power connector coming off of it. It's stuck to the side of the display stand with a bit of double-sided tape.
With a 16x2 display, the top line becomes labels, and the bottom line values, displaying current, average, max and a trending indicator. While this was more readable from across the room than a bicycle speedometer, it was still a bit small.
Anemometer in cyan
Anemometer next to a center speaker with a temperature/humidity readout behind it.


One solution would have been to use a poorly documented double-height character set for the display, which would have lost the labels. Given that I was displaying current, average and max wind speeds, I'd rather keep the labels.
Since I could get an RGB display, I could use the display color to indicate the nature of the wind conditions. The conditions of interest are the recent average wind speed, and the maximum wind speed for that period. A high average speed means flying against the wind will be difficult, if not impossible, so there's no point in trying if the aircraft has too little power. High gust speeds will lead to erratic flight, which means I want an aircraft that's naturally stable. The color of the display is used to indicate how high those are:

Average↓ Gust→LowMediumHigh
LowGreenCyanYellow
MediumNPBlueMagenta
HighNPNPRed
The slots marked NP aren't possible, because the average wind speed can't be higher than the max wind speed. Since the color is pretty much obvious from anywhere I can see the display, I can tell what kind of models I might consider flying at a glance.

Software

A more complete discussion of the software is available on my software blog.

Conclusions

I use this on a regular basis. While I still tend to check the weather stations for wind speed forecasts, I'll check this when grabbing a CP copter to decide if I want to fly indoors or out.
The one downside is that I chose the wrong wind sensor. Oh, it works great, and I don't have any complaints about it's performance or accuracy. However, the local meteorologists who raved about it all worked for the National Severe Storms Lab. They use it for chasing tornadoes and dealing with speeds at and above 100 mph. But it lacks accuracy at the lower wind speeds of interest to me. Since I built it, Inspeed has come out with a more suitable version, and has been receptive to the idea of a variant of this one whose accuracy is between those two. If I can get one that doesn't replacing the underground wiring I'll probably try upgrading, but this one does the job I got it for.