Friday, August 22, 2014

A low-cost marine compass (Part 3 – Calibration)

When connected to a terminal program through the FTDI breakout at 115200 baud, the compass displays the results formatted as NMEA sentences. This is the default operating mode (Mode 4 in the software).

Two calibration helper modes are available:
        Mode 1 : Accelerometer  calibration (type M1<ENTER>)
        Mode 2 : Magnetometer calibration (type M2<ENTER>)

These calibration modes are used to produce the raw data files required by the calibration software Magneto. In these modes, the compass stops displaying results until it receives an ‘x’ character (type x<ENTER> in the terminal program).

In Mode 1 (Accelerometer), it will then displays 64 consecutive (x,y,z) filtered raw data (at the 200 Hz rate), then pauses until it receives another ‘x’ character.

In Mode 2 (Magnetometer), it will displays 16 consecutive (x,y,z) raw data (at the 6.5 Hz rate), then pauses until it receives another ‘x’ character.

By logging the results to a text file in the terminal program, it is then possible to create the Magneto input files, by moving the compass around and pressing the ‘x’ character at each new position.

I recommend moving the compass with roughly equal angle steps around three perpendicular axis. For the accelerometer, you can use any support (metallic or not) to help position the compass, but it is very important to wait until the compass is completely at rest (no vibration) before hitting the ‘x’ character.

For the magnetometer, try to use the lightest non-magnetic support (wood or plastic), and use the ribbon cable to keep away the FTDI brekout and especially the laptop (all this in a magnetic clean environment!).

The 2 raw data files can then be processed by the Magneto software. The Pro Mini source file can then be edited with the new values and recompiled.

Unfortunately, this is not the end of the game if we want to be rigorous. We will have to look for misalignments between three independent reference frames: the compass base, the accelerometer plan and the magnetometer plan, with a final offset correction when compared to a known heading.

But the compass should already be quite usable, as these last corrections will be quite small compared to what has been accomplished.

A low-cost marine compass (Part 1 - Hardware)
A low-cost marine compass (Part 2 - Software)

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