Added magnetometer documentation.

Documentation/Chapters/04_Users Guide.tex

@@ -290,6 +290,7 @@ The application offers calibration tools to determine the exact ambient field wi
 These are both integrated into one application view, depicted in Figure \ref{fig:ambientcalibrationpure}.
 All the calibration tools require access to the complete Helmholtz cage hardware, as well as a magnetometer.
 These must be connected before starting the test.
+It is important that the magnetometer is centered as well as possible to achieve accurate results.
 For these tests, where the magnetometer itself is not the \gls{dut}, it is recommended to use the IRS's FGM3D reference magnetometer.
 Further, an adapter script (\code{fgm3d\_adapter.py}) already exists for this sensor.
 For more details on writing adapter scripts and connecting magnetometers, please refer to Section \ref{sec:tcp_api}.
@@ -332,6 +333,21 @@ To understand the calibration methods in all detail, it is recommended to look a
 \subsubsection*{Magnetometer Calibration}
 \label{sec:magnetometer_calibration}
 
+The helmholtz control software supports calibrating magnetometers.
+In preparation, a calibration of the ambient field using the reference magnetometer (FGM3D) should be conducted beforehand to achieve accurate results.
+Afterwards, the magnetometer must be replaced with the \gls{dut}.
+Since the software only supports one magnetometer at once, the reference magnetometer (meaning: its adapter script) should be disabled beforehand.
+Tip: To mount CubeSat magnetometers a purpose built PC104 holder can be found in accompanyment of the Helmholtz cage.
+
+As the coordinate system of the Helmholtz Cage and the magnetometer may not match, there are two options that available.
+First, the calibration can be conducted as-is, and the calibration results will reflect the unexpected coordinate system by often yielding negative sensitivities and untypical sensor axis angles.
+These values are not invalid, but describe a sensor correction that would transform it into the Helmholtz coordinate system.
+With some work, the desired calibration parameters could be extracted mathematically by changing signs and adding or subtracting \SI{90}{\degree} increments.
+The second option would be to change the sensors orientation in software in the adapter script that is used.
+This yields a more expected set of calibration parameters, but these now do not describe the initial axes anymore.
+In this case, the parameters must be correlated back to their initial axes and angles must be negated if the axis was flipped.
+This common issue should be solved robustly in the control/calibration software in the future.
+
 \begin{figure}[h]
 	\centering
 	\includegraphics[width=\linewidth]{media/magnetometer_calibration_pure}
@@ -339,6 +355,24 @@ To understand the calibration methods in all detail, it is recommended to look a
 	\label{fig:magcalibrationpure}
 \end{figure}
 
+The calibration method used (Zikmund \cite{ref:calibration_procedure_magnetometer_helmholtz_cage}) relies on a simplified error model and the Helmholtz test bench.
+After either measuring the local geomagnetic field or cancelling it, the magnetometer-under-test runs through a sequence of magnetic fields generated by the Helmholtz coils, which supplies sufficient data to solve a system of equations containing the coefficients of interest.
+The non-linear system is constructed with the equation below on a per-axis basis, with one row for every sample.
+
+\begin{equation*}
+    B_{meas} = S \left( B_E \sin{\alpha_E} + B_x \cos{\alpha} \cos{\beta} + B_y \cos{\alpha} \sin{\beta} + B_z \sin{\alpha} \right)
+\end{equation*}
+
+The calibration procedure makes use of nearly equidistantly distributed vectors in all directions as test points.
+The number of these points, as well as the settle time before taking a measurement, can be set in the \gls{ui}.
+Generally, a high number of points, such as larger than 8, is recommended to achieve both an accurate result and also to get useful residual data.
+The meaning of the individual calibration coefficients can be derived from Zikmund \cite{ref:calibration_procedure_magnetometer_helmholtz_cage} or also the thesis accompanying the Helmholtz test bench \cite{ref:leons_test_bench}.
+
+After completing, the results of the calibration procedure will be saved into the data fields on the right hand side of Figure \ref{fig:magcalibrationpure}.
+This also enables the save buttons underneath.
+The ``Copy to clipboard'' will put the results table as shown in the \gls{ui} into the system clipboard, from which it can be pasted into software such as Microsoft Excel and LibreOffice Calc.
+If semi-raw experiment data is required to verify the program functioning or to apply custom algorithms, it can be exported with the ``Export raw CSV'' button.
+
 \subsubsection*{Data Logging Configuration Page}\label{sec:logging_guide}
 The application has the ability to log test bench data to a \gls{csv} file.
 The data is temporarily stored internally and must be saved to an external file by user request.

Documentation/References.bib

@@ -172,4 +172,23 @@
 	author={Stier, Annika and Schweigert, Robin and Galla, Daniel and Lengowski, Michael and Klinkner, Sabine},
 	year={2020},
 	publisher={University of Leicester}
+}
+
+@inproceedings{ref:calibration_procedure_magnetometer_helmholtz_cage,
+    author = {Zikmund, A. and Janosek, Michal},
+    year = {2014},
+    month = {05},
+    pages = {473-476},
+    title = {Calibration procedure for triaxial magnetometers without a compensating system or moving parts},
+    isbn = {9781467363860},
+    journal = {Conference Record - IEEE Instrumentation and Measurement Technology Conference},
+    doi = {10.1109/I2MTC.2014.6860790}
+}
+
+@thesis{ref:leons_test_bench,
+	author = {Leon Teichröb},
+	title = {Mapping and Calibration of a Helmholtz Magnetic Field Cage and Test of the EIVE Attitude Control System},
+	date = {2021},
+	institution = {Institute for Space Systems (IRS), University of Stuttgart},
+	location = {Stuttgart},
 }