AMF 14H MARANDA SN:1026 Making Sawdust October 8, 2013
Where did September go? I didn’t even get a chance to post last month. Oh, well. I guess I was too busy working on Sawdust and stuff to take time to post.
The right wing and fuselage are in the hanger and the left wing is nearing completion in the garage. Plus I have signed up for an EAA Webinar that is taking place tomorrow night and I am registered for an EAA SportAir Workshop in Dallas on the 19th and 20th.
The most visible project of the past month is my Arduino based electronic altimeter. A couple months ago we took a drive to see our son and grandson in Dallas and while I was there I picked up an Arduino starter kit. For those who don’t know what an Arduino is, it is a very small computer. The Arduino has lots of connections that can be used to connect various parts and it has a program that runs on your computer that you can use to program it. Lots of people are using Arduinos to build controllers for their sprinkler systems, blinking lights with lots of lights, robots and UAVs or drones. There is even a special version of the Arduino called the Ardupilot that is designed as an autopilot for model airplanes.
I started checking around the web to see if anyone had used an Arduino as the base for instrumentation for real airplanes, like homebuilts. Sure enough! There were a couple of sites that came up with altimeters and airspeed indicators. The catch is that these sites have been done mostly by non-fliers. Therefore they have big disclaimers about how the projects presented are not for use in real airplanes. However, homebuilt airplanes can use homebuilt instruments and avionics. With that in mind, I want to make my own disclaimer with regard to my current airplane project. Because this is my first homebuilt airplane and I want to be very careful with it, I am going to install regular, old fashioned, steam gages for the primary gages. My experimental gages will be installed on the right side of the panel until I am comfortable with their accuracy and reliability. Since this is a homebuilt aircraft, I can redo the panel later on and install my experimental instruments later.
The altimeter consists of a barometric pressure and temperature sensor, two two digit seven segment displays, four shift registers, three push button switches and an Arduino Uno R3. There are also a bunch of resistors and some wire. The parts are soldered to a perf-board that I got at Radio Shack.
I started by assembling the display and writing a program to count up from 1 to 9999, then start over. There is a decimal point LED on each display and the right most decimal point is lit at the start of the program. As the counter goes past 9999 and starts back at 1 again the next decimal point is lit and the others are turned off.
Next I put together the sensor and switches on a project board and modified the program to show the altitude, temperature, barometric pressure and rate of climb, well sort of.
So, I took the display, the Arduino, my battery pack and a project board with the pressure sensor on it and mounted it in my “test panel” in my pickup and drove around and watched the numbers change. Here is how it looked:
Here is a closer look at the pressure sensor:
This shows the altitude. Notice that the right most decimal point is lighted.
This is the temperature in degrees Fahrenheit. Notice that this is to the tenth of a degree and the appropriate decimal point is lighted.
The barometric pressure is next. Well, not really. This is the equivalent of the Kohlsman window on a regular altimeter. In this mode I can adjust the reference pressure to pressure at the airport. The program sets a default pressure of 29.72. There is a button to increase the value displayed by .01 and another button that will decrease the value by .01.
The fourth mode is to display the rate of climb. I will need to do a bit more work on this since it is currently setup to sample the altitude once per second. The altitude is saved for that second and is subtracted from the next altitude. This gives the change in altitude per second, but we want the change per minute, so I multiply this value by 60. That gives me the rate of climb in multiples of 60. I need to figure out some way of getting a more granular value here.
Of course, just sitting in the truck doesn’t really have a rate of climb, so it is showing “000” FPM.
Next I wanted to move everything to the display board. So now I have it all together and could theoretically complete a package to house the unit.
Here is a closer look at the display board, so you can see the sensor and the switches. There are three push button switches which are used to set the display mode (top switch), increase the pressure setting (middle switch) and decrease the pressure setting (the bottom switch). In my haste I forgot to add the wires that go from the switches to the Arduino, but I will fix that pretty quickly.
The left wing is coming along nicely. I need to pull some staples and sand the filler blocks between the ribs on the bottom of the wing. Then I can start applying sealer and install bolts and brackets and tighten the internal bracing. I need to order a few more sheets of plywood and more T88, but I want to get all the bolts in to be sure that I order everything that I need to complete the wing in one order. If the weather holds I think I will make my goal of completing the wings before winter.