Building a Fluxgate Magnetometer Part 2

With school starting progress has slowed some, but currently most of the system is constructed.  First off the sense coil had to be finished.  The wire ends were coated in fingernail polish to keep the coil from slowly working undone and the entire setup was placed into a clear acrylic tube to protect it from wear.  The tube was stopped with standard rubber plugs and a computer power cord was soldered on for connection purposes.

With the function generator working it was time to amplify its ~100mV output to something that would induce a larger field via the driver coil.  Finally I decided to go with an operational amplifier (op-amp) design.  This requires both a positive and negative voltage source which is easily accomplished with two 9V batteries.  The signal generator will be run off a third battery because it is crucial that the two op-amp supply batteries remain at equal voltages.  My initial breadboard design (below) clipped the waveform badly (also below).  After some readjustments and gain fiddling a nice waveform was reached.  I built two amplifiers on a perf-board (one to amplify the signal to the driver coil and one to amplify the signal coming back from the sense coil).

It was also time to being thinking about a case/display.  Lexan seemed like the obvious choice so students can see inside.  I bought 2 sheets of lexan and nylon hardware to separate them.  Leaving the sides open allows easy oscilloscope probe access for recalibrating the amplifiers (I left little copper connections on the board for this purpose).  I designed the front control panel (not implemented yet) and drilled all the holes required.  Finally after mounting all the boards down to the lexan I powered up the amplifiers and they worked great (below)!

Next the bandpass filter needs to be nailed down.  I've worked on it some, but cannot get a satisfactory result to build up onto the last perf-board.  The signal that carries the information we are interested in is the 2nd harmonic of the 1kHz driver signal.  It will be weak so it is likely that the amplifier will need a bit of reworking and hopefully I can build some gain into the bandpass design (also op-amp).  The classic catch is increasing the Q of the filter, but killing the amplitude of the signal.  More to come...

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