In the fluid power industry, one of the ways that LVDTs offer great value is in their ability to separate the core from the coil assembly, with the LVDT's core operating in a pressurized medium and the LVDT's coil installed in an unpressurized environment. A classic example is the use of an LVDT for measuring spool position in a 2-stage hydraulic valve where there is need for proportional position feedback between the second stage main spool and the input to the first stage pilot valve's current driver. The feedback permits the main spool to stay in position, producing a specified flow through the valve at the operating pressure.

In a typical LVDT installation on a servo valve or an electrically controlled proportional valve, a non-magnetic core connecting rod with an external thread attaches to an internal thread in the main spool, often using a jam nut to lock the rod at a specific dimension. The other end of the core connecting rod is also attached to the core by smaller thread. The main spool usually is inserted into the valve through an o-ring sealed port of suitable size. After the spool has been inserted, a port plug with a blind bore non-magnetic tube extending from it that is sufficiently long to permit the maximum motion of the core attached to the main spool is screwed into the o-ring port. The tube, often called an "isolation tube", is made with a thick enough wall to easily withstand the typical pilot stage control pressures of 250 to 2000 psig. The bore of the LVDT coil assembly fits over the isolation tube, with the LVDT thus mounted outside of any pressurized medium. The input/output of the LVDT goes to a standard LVDT signal conditioner with a bipolar DC output that is fed to the valve driver to close the loop.

In some types of valve installation, where the valve is operated by a solenoid instead of a force motor, it is merely enough to indicate that the spool has actually moved to a full flow or zero flow condition. In such cases, the desired output from the LVDT is a state change or switch action similar to that of a proximity sensor. The LVDT serves that purpose when used with the correct type of signal conditioning, which could be field programmable if needed.

There is a similar application using the same technique with poppet-type safety and relief valves. The requirement is to be certain that the poppet has fully reseated and is not leaking. This is especially important on safety valves used on hydraulic operated drawing presses, press brakes, metal shears, and other types of machinery. In fact, this type of sensor is now a legal requirement in Canada for valves on hydraulic operated machinery.

In these installations, the LVDT's core attached to the poppet stem, using a non-magnetic core connecting rod in between. The core sticks up into a non-magnetic isolation tube. The coil and built-in electronics module are concentrically mounted over the tube and sealed from the medium, either with a flange and gasket, or screwed into an o-ring port. What makes this application unusual is that the LVDT is only measuring the position of the poppet when it is nearly or completely seated, nominally over only a millimeter or so. The actual motion of the poppet when the valve is open is many times that distance, typically 10 to 50 mm.

For relief valves or hydraulic control valves used in refineries and process plants, there is often a need for a hazloc listed sensor. Macro Sensors has several lines of Intrinsically Safe and/or hazloc-rated LVDT sensors that can be utilized for these applications, and we are rapidly increasing the breadth of our product lines of position sensors for hazlocs.