AC LVDT Transducers General Description & Features

Daytronic Linear Displacement Transducers are rugged, sensitive measuring devices that produce an electrical output signal precisely proportional to the mechanical displacement of a sensing probe.

These robust, full ratiometric instruments yield exceptional dimensional stability.  With applications in gaging, automatic inspection, process control, and countless specific research operations, these LVDT's yield repeatable measurements from less than a micron to over three feet.

Models include

  • GENERAL-PURPOSE LVDT'S with various electrical and mechanical configurations, for use in a broad range of industrial and research applications (many units have a threaded shank and locking nut for secure positioning in a simple mounting fixture)
  • Exceptionally reliable PRECISION LVDT's with high-quality linear bearings, for the most sensitive gaging and quality control operations

  • Rugged SUBMERSIBLE LVDT's, hermetically sealed for use in hostile industrial and research environments (corrosive fluids and gases, high temperature and vibrations, etc.)

Based on the linear variable differential transformer (LVDT) principle, the performance of these sensors depends on inductance effects that do not involve flexing wires or sliding electrical contacts.  All coils are magnetically shielded, and are cased in hardened stainless-steel housings.

As a result, these transducers are virtually "noise-free," displaying extreme resistance to the effects of vibration, rotation, and electrical interference, as well as to adverse environmental factors like humidity, ambient temperature variation, and corrosive atmospheric conditions.  Calibration remains stable for years of operation—even in the most unfavorable industrial surroundings.

Daytronic offers both AC-OPERATED and DC-OPERATED LVDT probes.  The major advantages of DC-operated ("DC-to-DC") LVDT's are ease of installation and signal conditioning, the ability to operate from dry cell batteries in remote locations, and lower system cost (especially in multipoint applications).  AC-operated LVDT's are generally smaller in body size and more accurate than DC versions.  They will also normally operate at higher temperatures.

An optional flat contact tip (Model 110) is available for use with all DC-OPERATED LVDT's.  Replacing the standard rounded tip on any standard "DSD" model, this tip provides a precision ground-flat surface, 10 mm in diameter, for contacting rounded objects.

For both AC- and DC-operated transducers, there are

  • SHORT-STROKE LVDT's, which have full-scale linear ranges from ±0.01 inch (±0.25 mm) to ±0.5 inch (±12.7 mm).  A number of high-precision short-stroke models are available.
  • LONG-STROKE LVDT's, which have full-scale linear ranges from ±0.5 inch (±12.7 mm) to ±18.5 inch (±470 mm).  Long-stroke DC-to-DC models offer both ±2-V and ±5-V output.

Three different armature types are available, to meet varying application requirements:

  • Unguided armature
  • Captive (guided) armature
  • Spring-extended armature

Sturdy LVDT mounting blocks are available for securing to a flat surface any LVDT with a body diameter of 0.315" (8 mm), 0.375" (9.5 mm), or 0.813" (20.6 mm).

How They Work

Differential transformers (also known as linear variable differential transformers, or LVDT's) are inductive sensing devices that produce an AC output voltage proportional to the mechanical displacement of a small iron core.  They are simple and rugged, have completely stepless resolution, and can resolve fractions of a microinch, if required.

One primary and two secondary coils are symmetrically arranged to form a hollow cylinder, as shown in the following cross-section diagram of a typical LVDT with spring-extended armature.  A magnetic nickel-iron core, supported by a nonmagnetic push rod, moves axially within the cylinder in response to mechanical displacement of the probe tip.

With excitation of the primary coil, induced voltages will appear in the secondary coils.  Because of the symmetry of magnetic coupling to the primary, these secondary induced voltages are equal when the core is in the central ("null" or "electric zero") position.  When the secondary coils are connected in series opposition, as shown in the figure, the secondary voltages will cancel and (ideally) there will be no net output voltage.

If, however, the core is displaced from "null" position, in either direction, one secondary voltage will increase, while the other decreases.  Since the two voltages no longer cancel, a net output voltage will now result.  If the transducer has been properly designed, this output will be exactly proportional to the magnitude of the displacement, with a phase polarity (as referenced to the primary excitation voltage) corresponding to the direction of displacement.

The actual (as opposed to the "ideal") AC output voltage of an LVDT would be represented by the solid line in the graph.  Notice that there is no ability to distinguish between displacements on either side of null, and that the voltage does not go to zero at null, but retains some finite minimum value.
This "residual null" voltage, which is always present to some degree, is composed partly of extraneous electrical pickup and partly of quadrature voltage components arising from capacitive and other effects.

To achieve useful readout of LVDT-generated measurement data, a signal conditioner must be used that can eliminate the effects of residual null voltages, and also discriminate between positive and negative inputs, thereby producing an output conforming to the "ideal" characteristic represented by the dashed line in the graph.

In the case of AC-excited LVDT's, a conditioner of phase-sensitive carrier amplifier design—as exemplified by all Daytronic LVDT conditioning instruments with a “30” or “3x”  in the last two digits of the part number—provides optimum sensitivity and accuracy.  Responding only to the modulated carrier frequency, such an instrument is insensitive to extraneous DC and AC "noise" voltages at other frequencies.

Unguided Armature

This is the simplest mechanical configuration, where the armature fits loosely in the bore of the LVDT, being attached to the moving point by a male thread.  It can be completely separated from the transducer body without demounting either part.

Proper installation of this type of LVDT requires that the armature and LVDT body be separately supported so as to ensure relative movement along a common axis.  When properly aligned, this noncontact arrangement allows essentially frictionless movement with zero wear, and ensures continued repeatability with infinite resolution.  With no internal springs or bearings, the unit has virtually unlimited fatigue life.

A free unguided armature is most suitable for short-range, high-speed applications (such as mechanical vibration measurements) or applications with a very high number of cycles.  It is also recommended for applications in which the target being measured moves parallel to the transducer body.

Captive (Guided) Armature

This configuration can be used for both static and dynamic applications, including applications where the target being measured moves in a direction transverse to the transducer body.

Here, the armature is both restrained and guided by a low-friction bearing assembly.  This allows all units to be mounted vertically between optional self-aligning bearings, if desired.  Units measuring ranges of ±3 inches (±76 mm) or less can also be mounted horizontally.  Units with a range greater than ±3 inches, however, may require additional support along the transducer body, to prevent flexing.

Captive-armature LVDT's are suitable for applications with a longer working range, where the transducer is to be mounted by its ends only, or where misalignment might occur if the armature were unguided.

Spring-Extended Armature

In this configuration, the armature is both restrained and guided by a low-friction bearing assembly (as with the "captive" armature, above).  In addition, it has an internal spring to continuously push the armature to its fullest possible extension, thereby maintaining light yet reliable contact with the measured object.  This feature is appropriate where the contact surface moves periodically beyond the range of the transducer (as, for example, when multiple separate parts are being gaged on a production line).

Spring-loaded LVDT's only require a fixing point for the transducer body, and are most suited to static or relatively slow-moving applications.  The probe end of the armature is normally tipped with a ball, although optional flat or roller ends are also available (see the Model 106 Contact Tip).


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