Introduction to LVDT - Basic Principles
Basics of the LVDT
The word LVDT stands for Linear Variable Differential Transformer, an electromechanical transducer that can linearly convert position of an object to which it is mechanically attached into DC voltage. LVDT transducers can measure movements as small as 0.01 microns (0.4 micro-inch), up to several millimeters, but are also capable of measuring positions up to 200 mm (±4 inch).
A traditional LVDT consists of 3 coils wound in a sporadic manner on a plastic bobbin, which is separated into 3 sections. See drawing.
The figure shows the components of a traditional LVDT. The internal structure consists of a primary winding centered between a pair of identically wound secondary windings, symmetrically spaced about the primary. The coils are wound on a hollow coil form of glass reinforced polymer or plain plastic material, encapsulated against moisture, and then secured in a cylindrical high permeability steel housing. This coil assembly is usually the stationary element of the position sensor.
The moving element of an LVDT, commonly called the core, is a separate cylinder of magnetically permeable material. The core diameter is usually slightly smaller than the internal diameter of coil form, to ensure frictionless movement. The core usually connects to the moving part of the system, and can move axially within the LVDT bore.
Steel coil Form LVDT
When temperature stability is required, the best option is to choose steel made bobbin, due to much lower coefficient of thermal expansion of steel, compared to that of plastic materials.
Layer wound LVDT
To achieve high accuracy over temperature changes and time, it is recommended to use Layer Wound LVDT. In this type of LVDT, there is no separation between secondary coils. The primary coil is neatly wound across the full width of the LVDT. Layer Wound LVDTs usually consist of a steel coil form.
Helpful Resources Online:
- How LVDT Works by Sensorland
- The DC-LVDT Displacement Transducer also by Sensorland
- eFunda: Theory of LVDT
- Engineers Edge: Explanation of LVDT