Page 457 - Sensors and Control Systems in Manufacturing

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requirements. Although the operating environments vary greatly,
these LVDT designs use similar materials of construction and share
the same physical configurations.
Currently, these LVDT sensors are built entirely with inorganic
materials. The coil form is made of dimensionally stable, fired ceramic
wound with ceramic-insulated high-conductivity magnet wire spe-
cially formulated for the application. Joints between the windings
and lead wires are brazed or welded for mechanical reliability and
electrical continuity. Ceramic cements and fillers are chosen to opti-
mize heat transfer and bonding between windings, coil form, and
housing. The potted assembly is cured at elevated temperatures, fus-
ing the components together into a solidified structure.
Most inorganic insulations tend to be hygroscopic by nature, so
the cured coil assembly is encased in an evacuated stainless steel shell
that is hermetically sealed by electron beam (EB) welding. This evac-
uation and sealing process prevents moisture accumulation and sub-
sequent loss of the insulation’s dielectric strength. It also seals out
surrounding media from the windings, while permitting the core to
move freely.
Electrical connections are made to the windings with nickel con-
ductors mutually insulated from each other by magnesium oxide
filler and sheathed in a length of stainless-steel tubing. This cable
assembly can be terminated by a hermetically sealed header for a
connector when the application requires it.
The preceding description gives a brief insight into the material
and techniques currently used in constructing the sensor for extremely
severe environments. However, the state of the art in materials tech-
nology is being continually advanced. As new materials and methods
of construction are evaluated, tested, and proved to upgrade perfor-
mance, they will be incorporated into these sensors.

8.10.2 Cryogenic Manufacturing Applications
An LVDT sensor connected to the gripper of a robot is designed to
cover a wide range of cryogenic applications ranging from general
scientific research to space vehicle analysis and cryogenic medicine.
A significant feature of the LVDT sensor is its ability to withstand
repeated temperature cycling from room ambient conditions to the
liquefaction temperatures of atmospheric gases such as nitrogen and
oxygen. In order to survive such rigorous temperature changes, the
sensor is constructed of materials selected for compatible coefficients
of expansion while maintaining good electrical and magnetic proper-
ties even at –450°F (–270°C). The evacuated and hermetically sealed
stainless-steel case prevents damage that could otherwise result from
repeated condensation, freezing, and revaporization. Internal mag-
netic and electrostatic shielding renders the sensor insensitive to
external magnetic and electrical influences.