Barry,

You say that the LVDT core had to be electrically isolated for your
LVDT to work. So I am mystified. The effect in the LVDT is entirely
electromagnetic: (for the benefit of the reader): the AC excitation 
signal of the central winding is coupled transformer-style to the 
windings at both sides of center. When the sensing core cylinder is 
centered, equal voltages are induced in both of the outside windings, 
which are connected in opposite directions so the net output is zero;
a displacement of the core increases the voltage induced in one as 
the other is reduced, resulting in the displacement voltage output.

Grounding or floating the core should make no difference except for
some weak electrostatic forces that may be present. The floating or
grounding condition of the excitation and output windings should make
no difference as far as the core is concerned. As I mentioned before,
the excitation is usually transformer coupled to prevent any DC currents
from magnetically moving the core around.

Maybe there is something else going on with your LVDT. A common problem
is that the moving core can touch and drag on the inside of the winding
forms, possibly making contact with a winding in a formless configuration;
the clearance on commercial LVDTs is really minimal, like less than 0.5 mm,
which is why I don't like to use them. But I assume that yours is not 
dragging or the seismometer wouldn't work. So, as I said, I am mystified.

Regards,
Sean-Thomas.

PS: a good treatment of the LVDT and its' noise can be found in:

Agnew, D.C., "Strainmeters and Tiltmeters",
Reviews of Geophysics, vol 24, No 3, 579-634, 1986

_____________________________________________________________________

Public Seismic Network Mailing List (PSN-L)