LVDT Installation Considerations

Mounting the Body

It is recommended that the body (coil) of the LVDT be mounted using split blocks or collets. The use of these types of mounts applies uniform clamping pressure around the outside diameter (O.D.). For units exceeding about 6 inches in length, it is suggested that two mounting blocks be used to provide proper rigid support to the LVDT.

The use of bored blocks with set screws is specifically not recommended. Set Screws can exert large forces capable of denting or deflecting the outer housing which can cause non-repairable damage to the internal coils of the LVDT.

While most LVDTs are magnetically shielded, it is recommended that ferrous metals should not be located closer than 1/4 inch from the open end of the LVDT body. Closer spacing can cause anomalies in the magnetic properties of the unit which can show up as offsets, non-symmetry or non-linearity in the output signal. The use of ferrous materials for mounting on the outside diameter of the LVDT as well as locating these materials in close proximity to the shielded areas is acceptable.

LVDT Core

The core is made from a nickel-iron alloy which is cut to length and threaded to facilitate attachment to an extension rod that transfers motion or position from the object being measured to the LVDT core. After machining, the core is annealed to relieve stress and optimize its magnetic properties. Do not machine, file, grind, or otherwise alter or modify the core, as this can induce stresses which can cause degradation of rated performance. Special form cores or core assemblies with extension rods can be provided which have been properly heat treated. Consult the factory for availability.

Core extension rods

Core extension rods should be made from non-magnetic stainless steel (300 series). They can also be made from plastics or laminates, provided that these materials offer adequate mechanical strength and are not used over a wide temperature range (as these materials may have high coefficients of thermal expansion and/or contraction).

The use of brass or aluminum for core extension rods is not recommended, as these materials contain trace elements which can cause degradation of performance. Cores are threaded internally to accept #1-72, #4-40, or #6-40 threaded extension rods.

Cores are also available with M2, M3, or M4 metric threads. Refer to the specific data sheet for the unit being used to determine thread type. Continuously threaded, stainless steel rods can be supplied for use as a core extension and is available in the above mentioned threads and in lengths of up to 24 inches. Consult factory for availability.

Core orientation

Some LVDTs and their cores are supplied with orientation marks. On units having these marks, the core should be oriented with the red marked end of the core facing toward the end of the LVDT body marked in red. This will duplicate the orientation used at the factory during calibration. On units having no marks, the core may be oriented either way without affecting LVDT operation.

Electrical connections

Electrical connections are shown on individual data or catalog sheets for the specific unit being used. Care should be taken in routing LVDTwiring. AC-operated units produce low millivolt signals at core positions close to null which could be susceptible to interference from high voltage or high current cables located in close proximity to LVDT wires or cable.

The use of shielded cable in high electrical noise environments is recommended or where long cable runs are required. Ideally, cables should be selected which have the minimum number of conductors needed and which possess low capacitance, such as that found in Category 5 computer cable. Caution: Be sure not to apply voltage to the output connection of DC-operated units or catastrophic failure may occur.

DC-operated units

DC-operated units are typically used in environments which have a moderate temperature range and excursion [typically +32°F to +160°F (0°C to +70°C)] and where units are not subjected to extremely high shock and vibration. DC-operated units having an extended temperature range [-40°F to +257°F (-40°C to +125°C)] are also available. Consult factory for details. Power to operate DC-operated units can be easily obtained from small inexpensive power supplies or often can be powered from PLCs and other signal processing or data gathering instrumentation.

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