What Is LVIT Technology?
Linear variable inductive transducers (LVITs) have been around for more than 30 years and are becoming very popular due to their relatively low cost and flexibility to be packaged in many different forms. LVITs are contactless position sensing devices that utilize eddy currents developed by an inductor in the surface of a conductive movable element to vary the resonant frequency of an L-C tank circuit.
The most common form of an LVIT uses a small diameter inductive probe surrounded by a conductive tube called a spoiler that is mechanically coupled to the moving object whose position is being measured. Typical LVITs have full ranges from fractions of an inch to 30 or more inches. Modern electronics incorporating microprocessors make outstanding performance possible, achieving linearity errors of less than ±0.15% of full scale, and output and temperature coefficients of 50 ppm/°F along with either analog or digital outputs. The range of housing sizes for LVITs can be seen in figure 1 and a cutaway view of an LVIT is shown in figure 2.
Fig.1: Typical housing sizes for LVITs
Fig.2: Descriptive view of an LVIT
LVIT Applications
LVITs are found in a wide variety of applications that require position information or feedback. Typical LVIT applications include mobile hydraulics, subsea hardware, civil engineering testing, power generation and energy development, and factory automation.
In mobile hydraulics, the LVIT is commonly used to measure hydraulic or pneumatic cylinder position. Usually the sensor has a pressure-sealed head and a probe long enough to insert into a gun-drilled hole in the cylinder's ram. The ID of this hole in the ram then acts as the spoiler. The sensor head can either be port-mounted or embedded into the end cap of the cylinder.
A typical in-cylinder LVIT installation is shown in figure 3. This packaging fulfills many different applications in mobile hydraulics such as bulldozer shovel or snow plow positioning, boom positioning on hydraulic cranes and manlifts, and in a variety of agricultural vehicle accessory position feedback requirements
Fig. 3: Typical in-cylinder LVIT installation
For subsea cylinder applications involving pumps, chokes, blowout preventers, and ROV-based actuators, the LVIT is designed to withstand the internal and/or external pressures of a pressure balanced, oil filled (PBOF) system. Other technologies commonly used to satisfy these applications require additional hardware like a ring magnet to operate, which adds cost to the machining of the cylinder ram and complexity to the installation. Typical subsea LVITs are shown in figure 4.
Fig. 4: Subsea LVITs
Typical civil engineering applications include measuring bridge expansion and contraction due to seasonal heating and cooling, and related shifts in trunnions and roller support mechanisms. This problem of expansion and contraction is compounded with railway bridges and tracks,where the expansion of a mile-long section of rail could be as much as four feet over a change of temperature in some climates of 100°F. This could lead to rail buckling, known in the industry as sun kink, and the derailment of a train.
In this type of application, instead of having a bare probe protruding from the sensor head, the LVIT's probe coil, spoiler, and electronics are packaged inside of a cylindrical housing for heavy duty protection, allowing the sensor to be exposed to its environment. The LVITs are connected to the pier and deck of the bridge to measure the relative position of the two, or directly to the rails to measure rail buckling.
LVIT technology is extremely robust and so can withstand seasonal weather conditions such as heat, cold, rain, and snow. An example of an installation of heavy duty LVITs on a bridge is shown in figure 5.
Fig. 5: An LVIT monitors changes in conditions under a bridge.
LVITs are used in many factory automation applications, including packaging and material handling equipment, die platen position in plastic molding machines, roller position and web tension controls in paper mills or converting facilities, and robotic spray painting systems. Being contactless, the basic measurement mechanism of an LVIT does not wear out over time. LVITs also do not have the higher installed cost often associated with other contactless technologies.
LVITs are being deployed in specific areas of the energy sector. In power generation applications, LVITs are used for valve position feedback, feed-water pump displacement, or generator shell movement. In oil fields, LVITs are used in hydraulic-operated pumps that replace Lufkin-style pump jacks and to measure the poppet position inside check valves.
As just one example, all LVIT products offered by Alliance Sensors Group also feature proprietary SenSet field programmable scaling, which allows a user to adjust for any mechanical variations after installation by pushing a button or grounding a single connection. This SenSet feature reduces setup time and total cost of ownership. As a typical example, consider a 6-inch range LVIT being used to measure the position of a process valve stem that moves 5.5 inches. Using SenSet to scale the output of the sensor after it has been installed, the full 4 mA to 20 mA output can be obtained over the 5.5 inches that the stem actually moves.
About the Author
Edward E. Herceg is currently Vice President and Chief Technology Officer of Alliance Sensors Group, a division of H. G. Schaevitz LLC. Before joining ASG, he held senior technical and marketing positions at Everight Sensors Corp., AST Macro Sensors, Massa Products Corp., and Schaevitz Engineering (now part of Measurement Specialties Inc.), and is well known as the author of the Schaevitz Handbook of Measurement and Control.
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