LVDT Sensors

Linear Variable Differential Transformers  and Transducers

What is LVDT and how does it work?

LVDT (Linear Variable Differential Transformer aka Linear Variable Displacement Transducer)  is a type of analog electromechanical sensor or transducer to measure displacement of an object. This transducer can convert the rectilinear motion of an object to which it is coupled mechanically into a corresponding electrical signal. LVDT linear position sensors can measure movements as small as a few millions of an inch up to several inches.

The internal structure of LVDTs looks much like a transformer, it consists of a primary winding centered between a pair of two identically wound secondary windings. The 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 stainless-steel housing. This coil assembly is the stationary element of the position sensor and the core is the moving part.

LVDT Components

Above the primary winding is illustrated in the center of the LVDT. Two secondary coils are wound symmetrically on each side of the primary coil. The two secondary windings are typically connected in “series opposing” (Differential).

The moving element of an LVDT is a ferromagnetic tubular armature, the core.

This core moves freely within the coil’s hollow center and in most cases, is mechanically coupled to the object whose position is being measured.

The center of a LVDT usually is large enough to provide substantial radial clearance between the core and bore, making it a non-contact sensing devise.

In operation, the LVDT’s primary winding is energized by alternating current of appropriate amplitude and frequency, known as the primary excitation.

The excitation AC is provided by external signal conditioner or in certain LVDT models already integrated as a feature.

How Does it Work?

The LVDT’s primary winding, P, is energized by a constant amplitude AC source. The resulting magnetic flux is coupled by the core to the adjacent secondary windings, S1 and S2. If the core is positioned midway between S1 and S2, equal flux is coupled to each secondary so the voltages, E1 and E2, induced in windings S1 and S2 are equal.

This point is referred to as null point, since at this reference midway core position the differential voltage output, (E1 – E2), is zero.

Now, if the core is moved closer to S1 than to S2, more flux is coupled to S1 and less to S2, so the induced voltage E1 is increased while E2 is decreased, resulting in the differential voltage (E1 – E2). Conversely, if the core is moved closer to S2, more flux is coupled to S2 and less to S1, so E2 is increased as E1 is decreased, resulting in the differential voltage (E2 – E1).

Through these very fine electrical changes the displacement of the core and hence the object can be measured.

Why use an LVDT Sensor?

Friction-Free Operation –
One of the most important features of an LVDT is its contact-free operation. Under normal operating conditions, there is no mechanical contact between the LVDT’s core and coil assembly, so there is no rubbing, dragging, or other source 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 any 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.

Single Axis Sensitivity –
An LVDT responds to motion of the core along the coil’s axis, but is generally insensitive to 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.

Null Point Repeatability –
The location of an LVDT’s 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 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 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.

Our LVDT Product Line and Features

Standard LVDT Series

Our Standard LVDT series comes in a nickel plated 20 mm diameter steel casing and is available in stroke length from 5mm to 200mm. This series requires external signal conditioning. Available with swivel head, spring return and as unguided armature.

Miniature LVDT Series

Our Miniature LVDT series comes in a nickel-plated steel casing with 4mm and 8mm diameter. This series requires external signal conditioning and is available in stroke length from 2mm to 20mm. Available with spring return and as unguided armature.

LVDT with External Thread for Easy Installation 

This product series comes in a nickel-plated steel as well as blue zinc-plated steel casing with M12 and M18 thread. Available in stroke length from 2mm to 20mm. This series requires external signal conditioning. Available with spring return and as unguided armature.

LVDT with Integrated Signal Processing

This product series comes in a nickel-plated 20mm diameter steel casing and is available with stroke length from 2mm to 200mm. This is series is equipped with internal signal conditioning. Available with swivel head, spring return and as unguided armature.

LVDT with External Thread and Integrated Signal Processing

This product series comes in a blue zinc-plated steel and stainless-steel casing with M18 thread.  Available with stroke length from 2mm to 200mm. This is series is equipped with internal signal conditioning. Available with spring return and as unguided armature.

Economy LVDT Series

Our economy series comes in a nickel-plated 20mm diameter steel casing. This series is equipped with internal signal conditioning. Available in stroke length from 2mm to 50mm. Available as spring return only.

NOTE: All LVDT sensors with internal signal conditioning come in two voltage supply options (15 VDC and 24 VDC) and two output options (4..20mA and 0..10V)

LVDT Accessories

IMA2-LVDT

Designed for top hat rail mounting, fully configured.
Available with the following specifications:
IMA2 LVDT Specifications
Further options available on request

IVM2-LVDT

Designed for PCB mounting, self-configurable, ideal for instrumentation and equipment manufacturers.
Available with the following specifications:
IVM2 LVDT Specifications

Further options available on request

IMK-LVDT (AVAILABLE SOON)

Fully enclosed electronics, integrated in a small housing including connection cable, completely configured.

Features

  • Temperature ranges up to 125°C
  • Protection class up to IP68
  • Linearity tolerances from 0.25%
  • Extremely high measuring accuracy
  • Dynamic measurements
  • External thread

Areas of Application

  • Perfect for use in harsh industrial environments such as high temperature and pressure ranges, and for high acceleration and measuring cycles
  • Systems where high vibration and acceleration occur
  • For oscillating movement with very small strokes and measurements under one millimeter
  • Automation technology
  • Test facilities
  • Mechanical engineering
  • Medical technology Production lines

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