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MODELS
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For precise electrical measurement of linear motion, size, strain, position, distortion, expansion, and similar mechanical phenomena
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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
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 operationeven in the most unfavorable industrial surroundings.
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
As explained below, three different armature types are available, to meet varying application requirements:
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 (see the graph in the figure). 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 designas exemplified by all Daytronic LVDT
conditioning instrumentsprovides 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.
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.
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.
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).
Daytronic's LVDT products are mainly classified by
The following table shows how these characteristics are employed in each LVDT's model-number designation. (NOTE: There are a few exceptions. The following spring-extended AC units, for example, have retained their "classic" Daytronic model numbers: DS20B through DS400B; DS1000A through DS6000A; DS200B, DS500, and DS2000.) The Table of LVDT's in the next section shows the general LVDT categories that correspond to the general model-numbering scheme.
For many more LVDT applications, see the Applications pages for specific Daytronic instrument families.
The stated linearity characteristics represent minimum values at the stated excitation level, and refer to the full-scale range over which the LVDT is calibrated. Use of less than nominal range results in the same percentage linearity but in proportionally better absolute linearity. LVDT's (unlike strain gages) cannot be supplied with meaningful calibration data. System sensitivity is a function of excitation frequency, cable loading, amplifier phase characteristics, and other factors. It is a practical necessity to calibrate each LVDT/cable/instrument system after installation, using a known input standard. The sensitivity and repeatability values given are typical minimum values only. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
As
shown in the 
Sturdy
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