PLUME – Calibration


These last few days I have worked on the calibration of the magnetic system. We want the calibration to be as easy as possible. By easy, I mean we will not have to measure precisely the position of the receiver to calibrate. The solution we have come up with is to fix the receiver on an optical table.We don’t know the position of the table nor its orientation.

Once the receiver is on the table ,we slide it several times by a known amount. s. At each position the measure the voltages in the receiver coilThese voltages are used  to compute : the position of the optical table, its orientation, and the two calibration matrix.

These values are found using a gradient descent in Octave,but the result are not satisfactory yet. Octave stops the algorithm when the gradient is too low and I have not found yet how to prevent it. I get a ~5-10cm precision using the artificially noised measures (the measures are simulated) after calibration. I hope there won’t be too much noise on the real system.

5 comments to PLUME – Calibration

  • Just curious, have you tried measuring the magnetic field of your emitter using a magnetometer ?

  • Virgile

    Yes we measured the magnetic field using a magnetometer to get some values of the field strength during the early prototype phase. I don’t remember the values but they were very low.

  • OK, and anyway, I guess the magnetometer sampling rate is definitely not as fast as (the double of) the frequency of your emitting coil oscillation.
    Do you remember what this frequency is by the way?
    Do you think it could work at 100Hz for example?
    Thanks 😉

  • Virgile

    The frequency of the emitting coil is approximatively 20kHz. We chose to use an audio codec to sample the signal in the coils.
    We aim at 200 position measures per second. To get the magnetic field strength, we will do a FFT using the samples from the codec to get the amplitude of the field.

  • Haha, OK I don’t know any magnetometer sampling that fast…
    Thanks for the details 😉