I measured the open-loop transfer function of the magnetic levitation system.

The schematic block diagram for this measurement is the following:

I injected a signal at a level of 20mV between two preamplifiers, and the corresponding open-loop

transfer function is given by B/A.  I took a picture of the resulting measurement, because

I encountered some difficulties to save the data to the computer via the wireless network.

The bode plots for the transfer function shown on the screen is the following:

I am puzzled with the zero near 10 Hz. I think it should come from the mechanical response function, because there is no zero in the transfer functions

of the preamplifer and the coil itself. I am not sure at the moment.

The corresponding configuration of the levitated magnet is