Image Credits: Cooper Instruments & Systems, Everight Position, Measurement Specialties
Linear Variable Differential Transformer (LVDT) sensors are inductive type devices that measure linear displacement.
How LVDT Linear Position Sensors Work
These sensors consist of three coils-a primary and two 180o out-of-phase secondaries-wound around a hollow tube. A moveable ferromagnetic core (an "armature"), connected to the object being measured, slides along the tube. An AC reference voltage (the "excitation signal") is applied to the primary winding which in turn induces an EMF signal into the two adjacent secondary windings.
Image Credit: Electronics Tutorials
Types of LVDT Linear Position Sensors
One way to distinguish a LVDT is whether it measures the relative currents (AC-in, AC-out; DC-in, DC-out) or resonant frequencies of the coils as a function of core position.
LVDTs can also be categorized according to armature mechanism.
Unguided armatures are attached to the specimen to be measures, fit loosely in the tube, and require that the body of the LVDT be separately supported. This type has no wear and does not restrict the resolution of measured data ("infinite resolution").
Captive armatures are guided and restrained by a low friction assembly. They are better for long working ranges and help prevent misalignment.
A force-extended armature uses an internal spring, pneumatic force, or electrical motor to continuously push the armature to its fullest possible extension. Such LVDTs are good for slow moving applications and require no connection between the armature and the specimen.
Specifications and Features
This video presents a number of considerations when selecting a LVDT sensor.
Video Credit: Stellar Technology Inc.
Sensor measurement range varies (e.g., less than 0.083 to more than 8 inches), but all have a maximum usable range.
Accuracy ranges (e.g., better than 15% to better than 0.02%).
Devices can tolerate certain temperature ranges (e.g., -65 to 300 F).
The low level of friction between the hollow shaft and the core prolong the life of the LVDT and enable very good resolution. In addition, the small mass of the core allows for good sensitivity in dynamic tests. LVDTs find use in modern machine-tools, robotics, avionics, and computerized manufacturing.
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