LVDTs, or Linear Variable Differential Transformers, can amplify your ability to measure and monitor precise movements in equipment and industrial automation systems. Choosing an LVDT for your application can be confusing since so many types of LVDTs exist. LVDTs are offered with different armatures, stroke lengths, with or without built-in electronics, and manufactured for various temperature ranges.
The LVDT Device
The main device consists of a hollow metallic cylinder in which a shaft of smaller diameter moves freely along the cylinder’s axis. Usually, the shaft if physically attached to the moveable object while the coil assembly is attached to a fixed reference point. The moving element is made of magnetically permeable material in order to measure the back and forth movement. Coils are wound on a one-piece hollow form of thermally stable glass reinforced polymer, encapsulated against moisture, wrapped in a high permeability magnetic shield, and then secured in a cylindrical housing. These are arranged as a primary winding between a pair of two identically wound secondary windings.
Different armature types are made to customize LVDTs for specific purposes.
Captive Guided Spring Return
Captive guided spring return is made for measuring multiple targets or for targets that move transverse to the armature. In spring return armatures, an internal spring makes contact with the target’s surface to measure surface displacement. This is particularly suited for applications measuring changes in a structure’s surface.
This armature type is specialized for longer measurement ranges of – ±0.5” to ±18.5”. Here, the armature is attached to the body as well as the structure it is measuring. The armature is threaded to allow free movement across the machined bearings. Additionally, armatures can be free unguided to measure targets that move parallel to the LVDT or need frequent measurements. In this type of unit, the armature is disconnected from the LVDT body. Since the contactless arrangement removes friction and drag, free unguided LVDTs have a practically infinite mechanical life.
The electrical signal the LVDT emits represents the distance that the object has traveled from a reference point, or its displacement. It transfers the electrical energy from one circuit to another by inducing a change in voltage or current. LVDTs may use alternating or direct current.
Alternating current (AC) types have a better shock and vibration resistance and can operate over higher temperature ranges (–200°C to 500°C). It can also work with remotely located electronics.
Direct current LVDTs can function in temperatures as low as –40° F or up to 200°C, and are compatible with internal electronics. Built-in electronics eliminate the volume, weight and cost of external AC excitation equipment. These can send digital outputs directly to computer systems. Although AC LVDTs may rank higher in terms of performance and ability, DC LVDTs are more cost-effective and can work just as well in most environments.
Having a variety of LVDTs expands the device’s portfolio of abilities and applications. Application possibilities range from measuring bill thickness in ATM machines to scanning laser tomography for precise optic positioning. LVDTs can control weight and thickness, measure distance between objects, and monitor fluid levels. With its durability in high temperature and other rugged environments, the LVDT increases quality standards and process performance for distance-measuring technologies.
Ariadna is a junior at Cornell University studying Materials Science and Engineering. She is a staff writer for The Cornell Daily Sun and a member of the Nanoscale Materials for Energy Lab.