Saturday, January 9, 2021

Improved Magnetometer Calibration

Much of this new year has been spent working on improving the calibration of AMOS's magnetometers. The calibration jig discussed in the last blog entry has been completed and tested out, and seemed to work pretty well! Next up was an apparatus for performing a simple temperature calibration of the magnetometer sensors. To keep the temperature calibrator as simple as possible, it was decided to just put a small heater and fan into a small box with the device under test, and have everything hooked up to the Raspberry Pi on AMOS. Fortunately I had nearly all of the required parts already on hand: a spare heater cartridge from the 3D printer, an old heatsink and 12 V fan from a decades-ago junked desktop PC (never throw anything out, you might need it someday!!! 😜), an electronics enclosure box that had previously been embedded in the failed foam hull (see https://readerforgmail.blogspot.com/2020/11/breaking-mold.html), a DC-DC converter, and a 4-port relay (with just one fried port from the previous paddle experiments: https://readerforgmail.blogspot.com/2020/08/paddle-test-take-2.html).  I did need to buy a 24 V supply though to power the heater. 

Here's a pic of the interior of the temperature calibration box once it was all put together:


A simple bracket was 3D-printed to hold the PC fan and heat sink securely in place. Calibrating with this box requires chilling it for a while down to some minimum temperature, and then turning it on and letting the Pi collect data from the device under test (i.e. the magnetometer sensors). Fortunately the weather is pretty cold at this time of year, so I just did the temperature calibration outside:


The heater was powerful enough to heat up the interior from about -2 deg C up to about 37 deg C. Good enough I think to get some pretty decent linear calibrations on the magnetometer sensors:



The code on AMOS has been updated to include linear temperature calibrations for the magnetometers, so hopefully tomorrow I'll try it out in the house and then in the backyard to make sure that it's working.