Compass belt update.

Ack. I'm late for the second week in a row.

Slow news week. Naj and I made a wire harness for the motor wires, but we're havint to redo because I didn't think far enough ahead. It brought all the wires to a single plug in front, which meant you had to shimmy into it. We started moving the circuit from a protoboard to soldered perfboard but only got about halfway.

[Above, Xander helps clear up a common misconception]

What's a compass belt?

Last week I had the circuit mocked up and the motor assemblies constructed. This week I got the motors sewed into some elastic, and give it a spin for the first time.

I added a brief explanation of what a compass belt actually is and does to the MOPTAW wiki: https://bandgap.rsnsoft.com/mediawiki/index.php/Compass_Belt

Here's the Wired article that inspired it: http://www.wired.com/wired/archive/15.04/esp.html

I've got some assembly photos of the compass belt up on my Flickr account:

As I mentioned in this update, I found the correct magnetic field model to be using to compute magnetic declination and magnetic inclination. Right now the program is busy crunching the values at quarter-degree latitude and longitude increments. It's been crunching away for about 30 hours, and is a bit over halfway done.

In order to compute the direction of "true" north from magnetic north, it's necessary to know the magnetic declination, the angle between the two. In general, this changes with location and the passage of time. (The magnetic elevation is also an important number; most places on earth, the magnetic field lines actually point down into the ground, not parallel it. More on that in another post.)

The digital compass module is working, and talking to an LED ring.

Transcript follows in the full post.

I got the compass module working! The key was that in addition to the PORTx registers on the AVR there are PINx registers. Reading from the former returns the value of the output latch for the corresponding GPIO line, whereas reading from the latter returns the actual current value of the pin. Once I switched to reading from PINx things looked up quickly. The other hitch was that I hadn't been correctly waiting for the data ready line on the MicroMag3 (compass) module so I was getting garbage data with the more precise settings, which take longer to measure.

I managed to get serial communication with the AVR board working, so I have more than just LEDs for debug, but I think I'm going to have to borrow a scope or logic analyzer from work for this one. So far as I can tell I'm sending the correct sequence to the digital compass module, but it keeps returning all zeros for every command, which makes me suspicious. It's receiving at least some commands correctly because if I put it into oscillator test mode I can see a voltage across one of the magnetic sensors. This is a bit puzzling.

I have a few plans for projects that'll need some sort of low-end microcontroller, and all the cool kids are using AVRs these days, so I thought I'd give it a shot. I picked the Digilent Nanocon to get started. I have the AVR GNU toolchain working on my Powerbook as well as my windows machine, but so far I'm only able to program the device from a PC. I have a USB Jtag cable from Digilent, but I've only gotten the parallel port programmer to work so far. I kinda dragged my heels, though, so I may be past the warranty period. I may switch to using a serial-based programmer like the STK500 for maximum compatibility.

Anyway, I have the equivalent of "Hello world" working: I'm blinking an LED. Next step is to interface it with the digital compass module.