Software Gimbals for your compass

Software Gimbaled Compass

The data were acquired using the 12 element calibration of a LSM9DS1 IMU

The 12 element calibration was obtained using a collection of raw data points taken at many different orientations, and processed using am Excel spreadsheet and the solver function to fit the data to a spherical surface. This method is discussed further in the following conversation link. (note a long cable was used to minimize the IMU from the magnetic fields of the table and computer)
http://forum.espruino.com/conversations/298287/
The following code was used to apply the calibrations:

//Use 12 element calibration on raw count magnetometer data
function cal8G(S){
 var i;
 var mm=[0,0,0];
 var scale=8;
 var mres=[
     [0.970972836,0.171154738,-0.000344029],
     [-0.095223453,0.956653319,-0.000125418],
     [0.017179224,0.005190024,1],
     ];
 var moff=[1057.176483,120.7727841,549.551664];
 for(i=0;i<3;i++) S.m[i] -=moff[i];
 for(i=0;i<3;i++)
  mm[i]=S.m[0]*mres[i][0]+S.m[1]*mres[i][1]+S.m[2]*mres[i][2];
  for(i=0;i<3;i++)S.m[i]=mm[i]*scale/32768;
}//end cal8G(S)
function readall(W){
  var heading,dip;
  var pirate=180/Math.PI;
if(W.magAvailable()){
  W.readMag();
  W.readAccel();
  cal8G(W);
  W.m[2]=-W.m[2];
heading=pirate*Math.atan2(W.m[0],-W.m[1]);
if(heading<0)heading +=360;
dip=pirate*Math.atan(W.m[2]/Math.sqrt(W.m[0]*W.m[0]+W.m[1]*W.m[1]));
console.log(W.a,',',W.m,',',heading,',',dip);
}//endif
}//end readall

The dataset

The following dataset was obtained using the above calibration with the IMU approximately level starting at 0 and proceeding by approximate 45 degree steps.
(360, 315, 270 ,225 ,180 ,135 ,90 ,45)

Using the Dip Angle Statistic to Determine Axis Signs

The data were then processed using the Rodrigues.js module into datasets where various combinations of the signs of the X, Y , and Z axis were manipulated. The following table shows the statistics of the Dip angle as the signs of the X, Y amd Z data were changed. Compare the Dip Range

Code used to apply the Rodrigues.js module

The follwing code was used to apply the Rodrigues.js module.

function test(){
 FF=require("Rodrigues").connect();
 var i;
 for(i=0;i<Adata.length;i++){
// for(i=0;i<4;i++){

 A=FF.Setup(Adata[i],[0,0,1]);
   Theta=FF.theta;
   Kn=FF.Kn;
 A=FF.An;
// Mdata[i][0]=-Mdata[i][0];
// Positive x, negate y and z to minimize dip range
// z was negated in acquisition program
 Mdata[i][1]=-Mdata[i][1];
 B=FF.RotVector(Mdata[i]);
   An=FF.An;
heading=pirate*Math.atan2(B[1],B[0]);
if(heading<0)heading +=360;
dip=pirate*Math.atan(B[2]/Math.sqrt(B[0]*B[0]+B[1]*B[1]));
console.log(i,',',A,',',Kn,',',An,',',B,',',
 Theta,',',heading,',',dip);
}//next i
}

The Results

The final output using positive X and negative Y and Z follows:
( ais accelerometer, k is rotation axis, m raw data, m1 is processed data; vectors have been Normed to unit vectors)

1 Attachment

• 8gauss.ods