LINE A R VA R IA BLE D IFFER E NTIA L
TRANSFORMERS
Reliability and
Ruggedness
are their Hallmarks
Andy Anthony, Managing Director of Monitran, outlines
why the popularity of Linear Variable Differential
Transformers (LVDTs) for displacement and position
measurement remains so strong.
T
Pressurised
special LVDT
he Linear Variable Differential Transformer (LVDT) is a
well-established transducer technology. Whilst there
are other ways of measuring displacement, the LVDT’s
popularity within industry continues unabated. Indeed,
as an OEM, we have seen our year-on-year sales of LVDTs
increasing steadily. So what’s the attraction with this
transducer technology?
As is the case with most transducer types, LVDTs can
be used for analysis or monitoring purposes. For example, they
are frequently used on automated test rigs to take dimensional
readings. Similarly, they are
equally at home in industrial
applications; for example, as
part of a control loop within a
servo-hydraulic system.
As a rule, LVDTs are of
rugged construction - most
being made of stainless
steel – and have standard
operating temperature ranges
of -30 to +85°C. They can
also be sealed to different
levels of Ingress Protection
(IP); such as water jets (65),
heavy seas (66), immersion
(67) and submersion (68). In
addition, vibration resistance will tend to be up to a few tens of
g and shock resistance much higher (several hundred g).
Yet these rugged characteristics are accompanied by high
precision, as and when required. For example, some miniature
LVDTs will give their full output voltage swing in response to a
displacement of less than 1mm. Furthermore, an LVDT’s output
is linear - typically deviating by no more than 0.5% from the
mathematical relationship between the electrical output range
and mechanical displacement range. But possibly the greatest
attraction of the LVDT – compared to other displacement
transducers – is its simple principle of operation.
In essence, an LVDT is a transformer in a stainless
steel cylindrical enclosure – see Figure 1. The primary and
secondary windings (copper wire coated in synthetic enamel)
are wrapped around a multi-section hollow bobbin. The bobbin
is typically made of either polyoxymethylene (a thermoplastic)
or fibreglass (glass reinforced plastic, GRP). A core, commonly
of high permeability Nickel-Iron, is free to move (though
sometimes against a spring’s tension) along the length of the
bobbin’s core.
14 | Sept/Oct 2011 | ME
The primary winding is energised with a constant
amplitude AC supply, typically between 1 and 10 kHz,
and the position of the core affects not only the amplitude
of the signal induced in the secondary windings but also
whether the output is in phase with or the inverse of the
excitation. This latter aspect is down to how the two
halves of the secondary winding are wound in opposite
directions. When the induction in both halves of the
secondary winding is equal (but opposite) the output is
zero – a.k.a ’the null’.
The secondary output signal is then processed by
a phase-sensitive demodulator which is switched at the
same frequency as the excitation signal. Depending on
how the LVDT is to be interfaced with other systems, the
output is further conditioned to produce either a unipolar
or bipolar voltage or current. Example output ranges
include 0 to 5VDC (i.e. unipolar) and ±5VDC (i.e. bipolar).
Also, many LVDTs output industry-standard 24VDC
4-20mA, with 12mA representing the null, for interfacing
with monitoring/control circuitry.
This simple principle of operation (and rugged
construction) results in high reliability; chiefly because of
how the core moves along the bore of the GRP bobbin
means there is no contact with the coils. Note; unless
the LVDT is spring loaded there is no friction either which
is important when using the transducer for materials
analysis, as you don’t want mechanical resistance it to
affect the results.
And whilst few, if any, LVDTs are ATEX-certified (for
use in hazardous areas) they non-energy-storing devices
and can therefore be classified as ‘Simple Apparatus’
under current guidelines.
As well as being invaluable transducers in their own
right, LVDT technology is often embedded within other
sensors. For example, PSM Instrumentation – an OEM of
process measurement and control instrumentation – uses
LVDTs within its Tankstar 260 and Optima 360 series of
hydrostatic pressure sensors (used for level sensing and
transmission).
Within these sensors, an LVDT measures by how
much the centre of a metal diaphragm moves in response
to external pressure; and the displacement over the
full pressure range is only 0.25mm. The LVDT’s output
passes to a remote amplifier module that produces a
24VDC 4-20mA signal corresponding to pressure/depth.
In summary the LVDT is a rugged, reliable
transducer type that boasts high reliability, accuracy and
repeatability. It has served the industry well for several
decades and will undoubtedly continue to do so.
Figure 1: LVDT cross-section. The primary winding (P) is
energised with a constant amplitude AC supply and the position
of the core affects the amplitude of the signals induced in the
secondary windings (S1 and S2).
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