Barry, 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. Regards, Sean-Thomas _____________________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>