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Subject: VRDT vane
From: S-T Morrissey sean@...........
Date: Thu, 20 Jan 2000 21:04:48 -0600 (CST)


Regarding the vane in the VRDT:

It is electrically passive, (as is the core on the LVDT). The sensitivity
of the sensor is due to the fact that the vane changes the inductance
and therefore the reactance of both of the inductances on either side of
it. As it moves between the pole faces, it increases one inductance as 
it decreases the other, and therefore changes the balance of the bridge. 
The reactance change is of the order of 2 ohms per micron with a 2mm gap 
between the inductors at 600hz excitation frequency. (using the TL021
transformers modified as in the drawing).

The relative inductance change depends on the initial distance from the
pole face, so a smaller gap between both is more sensitive. The gross
effect of the inductance change is quite non-linear, but the balanced
effect over a small distance can be very linear, especially if the vane 
dimensions are about 20% larger than the pole faces. I dismantle a larger,
like 1-watt, audio transformer and use the center of the E laminates as
the vane; audio transformers have thinner laminates with higher permitivity
to reduce hysteresis losses compared with AC transformers.

An advantage of the VRDT is that there are minimal forces acting on the
vane, compared to a LVDT, where magnetic forces from the central driving
winding are measurable, and a capacitive sensor, where electrostatic
forces can be appreciable. Speaking of which, the VRDT vane should be
grounded to the frame to prevent static problems. Naturally, it should
be suspended with non-magnetic material, like brass brazing rod or 
small diameter hobby-metals tubing (ACE hdwe) which can be finely slit
from the end on one side and the vane soldered or epoxied in.



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Larry Cochrane <cochrane@..............>