What is LVDT?

 

LVDT is an acronym for Linear Variable Differential Transformer and is a common type of electromechanical sensor that can convert the rectilinear motion of an object along an axis into a corresponding electrical signal.

Common applications that use LVDT are vibration, traction, compression, stress measurement

Construction:

• An LVDT consists of a hollow cylinder with a shaft or push rod that moves freely back and forth inside the hollow cylinder. 

• The pushrod is connected to a magnetically conducted core. 

• A primary coil is wrapped around the middle of the hollow cylinder and is energized by constant amplitude AC source, known as the primary excitation 

• While two secondary coils having an equal number of turns are wrapped on either side of the cylinder at equal distances from the primary coil.  

• The two secondary coils are connected in the series generating AC output voltage. 

• A signal conditioning circuit that is housed inside the LVDT structure is required to convert the output to either 5V dc or 4-20 mA’s whichever format is compatible with your data acquisition system.  

Working:

• When the core is at the center, called the null point, no voltage appears at the secondary output.

• But as soon as the coil moves by even the smallest amount, a differential voltage is induced at the secondary output. 

• The phase of the output voltage is determined by the direction of the core’s displacement while the amplitude is determined more or less linearly by the magnitude of the core’s displacement from the center. 

• Hence, the phase shift of the output voltage and the amplitude of the output voltage can be used to determine the direction of the core from the null and amount of displacement of the core from the null point by means of appropriate circuitry respectively. 

LVDT Characteristics:

• The magnitude of the differential output voltage varies with the core position. 

• The value of output voltage at maximum core displacement from null depends upon the amplitude of the primary excitation voltage and the sensitivity factor of the particular LVDT but is typically several volts RMS.

Why use an LVDT?

Friction-Free Operation

One of the most important features of an LVDT is its friction-free operation. In normal use, there is no mechanical contact between the LVDT’s core and coil assembly, so there is no rubbing, dragging, or other sources of friction. This feature is particularly useful in materials testing, vibration displacement measurements, and high resolution dimensional gaging systems.

Infinite Resolution

Since an LVDT operates on electromagnetic coupling principles in a friction-free structure, it can measure infinitesimally small changes in core position. This infinite resolution capability is limited only by the noise in an LVDT signal conditioner and the output display’s resolution. These same factors also give an LVDT its outstanding repeatability.

Unlimited Mechanical Life

Because there is normally no contact between the LVDT’s core and coil structure, no parts can rub together or wear out. This means that an LVDT features unlimited mechanical life.  This factor is especially important in high-reliability applications such as aircraft, satellites, and space vehicles, and nuclear installations. It is also highly desirable in many industrial process control and factory automation systems.

Overtravel Damage Resistant

The internal bore of most LVDTs is open at both ends. In the event of unanticipated overtravel, the core is able to pass completely through the sensor coil assembly without causing damage. This invulnerability to position input overload makes an LVDT a suitable sensor for applications like extensometers that are attached to tensile test samples in destructive materials testing apparatus.

Single Axis Sensitivity

An LVDT responds to the motion of the core along the coil’s axis but is generally insensitive to the cross-axis motion of the core or to its radial position. Thus, an LVDT can usually function without adverse effect in applications involving misaligned or floating moving members, and in cases where the core does not travel in a precisely straight line.

Separable Coil And Core

Because the only interaction between an LVDT’s core and the coil is magnetic coupling, the coil assembly can be isolated from the core by inserting a non-magnetic tube between the core and the bore. By doing so, pressurized fluid can be contained within the tube, in which the core is free to move, while the coil assembly is unpressurized. This feature is often utilized in LVDTs used for spool position feedback in hydraulic proportional and/or servo valves.

Environmentally Robust

The materials and construction techniques used in assembling an LVDT result in a rugged, durable sensor that is robust to a variety of environmental conditions. Bonding of the windings is followed by epoxy encapsulation into the case, resulting in superior moisture and humidity resistance, as well as the capability to take substantial shock loads and high vibration levels in all axes. And the internal high-permeability magnetic shield minimizes the effects of external AC fields. Both the case and core are made of corrosion-resistant metals, with the case also acting as a supplemental magnetic shield. And for those applications where the sensor must withstand exposure to flammable or corrosive vapors and liquids, or operate in pressurized fluid, the case and coil assembly can be hermetically sealed using a variety of welding processes. Ordinary LVDTs can operate over a very wide temperature range, but, if required, they can be produced to operate down to cryogenic temperatures, or, using special materials, operate at the elevated temperatures and radiation levels found in many nuclear reactors. 

Null Point Repeatability

The location of an LVDT’s intrinsic null point is extremely stable and repeatable, even over its very wide operating temperature range. This makes an LVDT perform well as a null position sensor in closed-loop control systems and high-performance servo balance instruments.

Fast Dynamic Response

The absence of friction during ordinary operation permits an LVDT to respond very fast to changes in a core position. The dynamic response of an LVDT sensor itself is limited only by the inertial effects of the core’s slight mass. More often, the response of an LVDT sensing system is determined by the characteristics of the signal conditioner.

Absolute Output

An LVDT is an absolute output device, as opposed to an incremental output device. This means that in the event of loss of power, the position data being sent from the LVDT will not be lost. When the measuring system is restarted, the LVDT’s output value will be the same as it was before the power failure occurred.