Collection of tools and code examples designed to help users improving the accuracy of the MPS MagAlpha magnetic angle sensor by implementing a calibration curve.

This project contains the following features:

• Calibration curve generator
• Angle Interpolation
• Ma-Cal-Generator App

C source code that generate a lookup table with the angle error.

The source code can be found in src/calibration-curve-generator

C source code that compute the corrected angle using the previously generated lookup table.

The source code can be found in src/angle-interpolation

C++ application that embed the two aforementioned source codes inside a executable test program.
The application take a CSV file containing the reference and measured angle as input parameter, generate a lookup table of the angle error and then use this lookup table to correct the measured angle. The application return a CSV file with the both the input parameters and the computed corrected angle. This output file will be placed on the output-files folder.

The source code can be found in src/ma-cal-generator

The Ma-Cal-Generator application use Qt Open Source Framework. You can either directly use the compiled Windows executable or you can build it yourself.

The executable can be found in the bin.zip on the Releases page. The executable is provided along the required Qt DLLs.

Unzip the files and copy the bin folder into the main MagAlpha-Calibration-Curve-Toolbox folder. You should have something like this:

MagAlpha-Calibration-Curve-Toolbox ├───bin ├───docs ├───input-files └───src ├───angle-interpolation ├───calibration-curve-generator └───ma-cal-generator 

Start by downloading and installing the Qt Open Source Framework.

Then open Qt Creator, go to File => Open File or Project... and select src/ma-cal-generator/ma-cal-generator.pro. The last step is to built the project. The executable will be located on the bin folder.

This project was built and tested with Desktop Qt 5.7.0 MinGW 32bit for Windows but it should works as well with the latest Qt version.

These source files doesn't require any installation, simply copy the .c and .h files in you project. They only require the math.h library to works and should therefore be portable to almost any microcontrollers, embedded systems and desktop environments that use C language.

Navigate to the MagAlpha-Calibration-Curve-Toolbox\bin folder and launch one of the following commands:

You can use the default input CSV file located in ..\input-files\calibration_data_input_example.csv.

ma-cal-generator.exe 

Or you can specify the CSV input file you want to use.

ma-cal-generator.exe ..\input-files\calibration_data_input_example_add_75.csv 

In both cases the output file will be located in MagAlpha-Calibration-Curve-Toolbox\output-files\calibration_curve.csv.

The input file must use the following structure. You can use a much row as you want.

Which gives the CSV format below.

Reference Angle [degree],Measured Angle [degree] 258.047,0.703125 260.156,2.10938 262.266,3.51563 263.672,5.625 265.781,7.73438 ..., ... 

The generated output file will use the following structure.

Which gives the CSV format below.

Reference Angle,Measured Angle,Measured Angle with Zero Correction,Angle Error,Angle Error Fitting,H1,H2,H3,H4,Ph1,Ph2,Ph3,Ph4,Number of points,Corrected Angle Cst + Slope,Angle Error after fit Cst + Slope,Corrected Angle Cst + Slope Lin Search,Angle Error after fit Cst + Slope Lin Search,Corrected Angle Fitted,Angle Error after Fit 258.047,0.703125,258.75,0.0596442,-0.461852,0.779883,3.42582,0.26283,1.64956,-1.56693,-1.56693,-1.56693,-1.56693,200,259.219,-0.469254,259.219,-0.469254,259.219,-0.469254 260.156,2.10938,260.156,-0.643115,-0.477579,0.779883,3.42582,0.26283,1.64956,-1.56693,-1.56693,-1.56693,-1.56693,200,260.655,-0.498378,260.655,-0.498378,260.655,-0.498378 262.266,3.51563,261.563,-1.34685,-0.496033,0.779883,3.42582,0.26283,1.64956,-1.56693,-1.56693,-1.56693,-1.56693,200,262.09,-0.527503,262.09,-0.527503,262.09,-0.527503 263.672,5.625,263.672,-0.643481,-0.530978,0.779883,3.42582,0.26283,1.64956,-1.56693,-1.56693,-1.56693,-1.56693,200,264.243,-0.571189,264.243,-0.571189,264.243,-0.571189 265.781,7.73438,265.781,-0.643115,-0.577405,0.779883,3.42582,0.26283,1.64956,-1.56693,-1.56693,-1.56693,-1.56693,200,266.396,-0.614876,266.396,-0.614876,266.396,-0.614876 ...,...,...,...,...,...,...,...,...,...,...,...,...,...,...,...,...,...,...,... 

First you need to extract the harmonics components from the angle measurements.

#include "calibrationcurvegenerator.h" //input parameters const unsigned int sizeAngleArray = 200; //change if needed float referenceAngleInDegree[sizeAngleArray]; //fill with ref angle float measuredAngleInDegree[sizeAngleArray]; //fill with sensor angle //output parameters float angleErrorArrayInDegree[sizeAngleArray]; float h1; float h2; float h3; float h4; float phi1; float phi2; float phi3; float phi4; extractAngleErrorHarmonics( referenceAngleInDegree, measuredAngleInDegree, angleErrorArrayInDegree, sizeAngleArray, &h1, &h2, &h3, &h4, &phi1, &phi2, &phi3, &phi4);

Define the lookup table size and angles to use.

//Generate the lookup table that will be use in the final application const unsigned int lookupTableSize = 32; //Define the lookup table size float angleStep = 360.0/(float)lookupTableSize; unsigned int i; //output parameters float lookupTableInputAngleArray[lookupTableSize]; for(i = 0;i<lookupTableSize;++i) { lookupTableInputAngleArray[i]=(float)i*angleStep; }

Compute the lookup table correction coefficients using the computed harmonics parameters. Their is several methods available to generate the calibration table.

This method use two coefficients to represent the angle error:

• angleErrorConstants represent the angle error offset for a given angle input
• angleErrorSlopes represent the angle error curve slope for a given angle input

This method use more memory to store the lookup tables but is easier to use to interpolate the angle error of any given points.

//output parameters float angleErrorConstants[sizeAngleArray]; float angleErrorSlopes[sizeAngleArray]; generateAngleErrorLookupTableUsingConstantsAndSlopes(lookupTableInputAngleArray, angleErrorConstants, angleErrorSlopes, lookupTableSize, &h1, &h2, &h3, &h4, &phi1, &phi2, &phi3, &phi4);

This method use a single coefficients to represent the angle error:

• angleErrorInDegree represent the angle error value for a given angle input

This method use less memory to store the lookup tables but is slightly more complex to use to interpolate the angle error of any given points.

//output parameters float angleErrorInDegree[lookupTableSize]; generateAngleErrorLookupTableUsingFittedCurve(lookupTableInputAngleArray, angleErrorInDegree, lookupTableSize, &h1, &h2, &h3, &h4, &phi1, &phi2, &phi3, &phi4);

The function used to perform the interpolation depends of the method chosen to generate the lookup table.

//input parameters float measuredAngleInDegree; //measured angle to correct float zeroDegreeOffset = 0.0; //Reference angle when the sensor return 0 degree (use 0.0 by default) //output parameters float interpolatedAngleInDegree; float interpolatedAngleErrorInDegree; interpolatedAngleInDegree=interpolateAngleFromConstantsAndSlopes(measuredAngleInDegree, angleErrorConstants, angleErrorSlopes, lookupTableSize, zeroDegreeOffset, &interpolatedAngleErrorInDegree);

//input parameters float measuredAngleInDegree; //measured angle to correct float zeroDegreeOffset = 0.0; //Reference angle when the sensor return 0 degree (use 0.0 by default) //output parameters float interpolatedAngleInDegree; float interpolatedAngleErrorInDegree; interpolatedAngleInDegree=interpolateAngleFromFittedCurve(measuredAngleInDegree, lookupTableInputAngleArray,angleErrorInDegree, lookupTableSize, zeroDegreeOffset, &interpolatedAngleErrorInDegree);