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Five Things You Didn’t Know About
Today’s LVDT Linear Position Sensors
Since its introduction more than six decades ago, the LVDT linear position sensor has gained
prominence as a reliable linear displacement measurement tool for various industrial and
aerospace applications requiring position feedback. Capable of providing linear displacement
measurements from micro inches to 2 feet, LVDT position sensors offer high accuracy and
durability.
While the basic concept of LVDT technology has remained the same over the years (see figure
1), the advent of microelectronics, new manufacturing techniques and construction materials
has evolved the LVDT linear position sensor into a more cost-effective and highly precise linear
position measurement device that can be designed to meet the specifications of a wider range
of applications.
Here are five things you might not know about today’s LVDT technology:
1. New Materials Enable Operation in Different Environments
New materials and construction techniques enable LVDT linear position sensors to be
constructed for use in hostile environments including those with high and low temperature
extremes, radiation exposure and subsea or vacuum pressure conditions. LVDTs can be
hermetically sealed or vented and constructed from a wide variety of materials such as
stainless steel, Monel or Inconel. They can perform under very hostile chemical
conditions (See Figure 2) and ,when suitably housed, LVDT linear position sensors can
withstand many years of use in seawater and corrosive acids and very high pressures
and temperatures in conjunction with the chemical abuse.
For example, when designed from stainless steel and Inconel 718 for pressure and
corrosion resistance, an LVDT assembly can provide reliable operations for many years,
even if the device is fully exposed to seawater.
For shallow and warm waters with high levels of oxygen, Monel 400, a special nickelbased alloy, provides excellent resistance against pitting and attack by micro organisms.
Fig.3 shows a typical sub-sea LVDT linear position sensor with a heavy duty connector.
Titanium and Hastelloy offers resistance to pressure and corrosion when measurements
must be obtained in seawater depths down to 7500 ft. and with an external pressure of
approximately 3800psi. These materials enhance the high reliability required of this
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sensor so that it will continue to operate for at least 10 years.
For sulfidation environments in the presence of high levels of H2-H2S concentrations and
high temperatures up to 425 deg C, an LVDT linear position sensor constructed from
exotic alloys (cobalt, nickel and chromium) with mineral insulation can deliver high
performance while other technologies will not survive.
As a result of its construction options, LVDT position sensors are becoming popular
alternatives to less reliable technology when it comes to position measurement in harsh
and deepwater environments such as offshore applications, downhole drilling, and power
generation.
2. Today’s LVDTs perform better
While linear position sensors are inherently highly reliable devices, new LVDT materials
and manufacturing techniques make it possible for linear position sensors to achieve
even better performance than LVDT technology of ten or twenty years ago. For instance,
microprocessor-enhanced linear position sensors, in which errors caused by the sensor’s
characteristics or environment are corrected, are more accurate than uncorrected LVDTs.
For example, while a standard LVDT linear position sensor may have a linearity of
+/-.25% of full scale output, an LVDT with microprocessing can linearize its output to
+/-.05% of full scale output.
Today’s LVDTs offer high repeatability – the ability to reproduce the same output for
repeated trials of exactly the same input under constant operating and environmental
conditions. A well-made LVDT is so repeatable that overall transducer repeatability is
affected only by the mechanical factors of the physical members or structures to which
the LVDT's core is attached and to which the LVDT's coil is mounted.
For example, in many aerospace and metrology applications the need for unit to unit
consistency is required. Enhanced computer winding machines allow for unit to unit
sensitivities to be specified to within +/-1% instead of the typical +/-5%.
3. Shorter Strokes/Smaller Diameters
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While linear position sensors were once considered too long for applications with limited
space, the development of new winding techniques, such as computerized layer winding,
and improved microprocessing, has considerably reduced the length of the linear position
sensor body compared to its measurable stroke length. In fact, for short stroke actuators
and cylinders (less than 6”) where a magnetostrictive sensor was the preferred method of
measurement, Macro Sensors linear position sensors can now be much shorter.
With the improved stroke to length ratio (now up to 80%), the linear position sensor
becomes a viable position measurement device for machine tool positioning, hydraulic
cylinder positioning, valve position sensing and automatic assembly equipment.
Unique coil winding configuration also supports a compact diameter design, enabling the
LVDT linear position sensor to serve as an integral part of devices with tight space
restrictions. A lightweight low mass core also makes the linear position sensors ideal for
applications having high dynamic response requirements or where weight is a premium
such as on aircraft and satellite.
4. LVDT costs are more attractive
The total cost of ownership of linear position sensors has continued to drop over the
years. Previously, a user would have an AC-operated LVDT linear position sensor and a
separate signal conditioner. The time to calibrate the two components together and the
additional equipment needed to do so accurately doubled or tripled the piece price of the
sensor and conditioner. DC-operated LVDT linear position sensors, those with built in
electronics, have become much more popular, especially with younger engineers.
“Plug and play” are the buzz words the market place has continued to whisper in sensor
manufacturers’ ears. Improved microprocessor technology has allowed sensor
manufacturers to embed more and more control into the housing of the sensors while
keeping the overall package size small. For those applications where the AC sensor and
separate electronics are still required, push button zero and span controls have replaced
digital pots and improved software has reduced the time needed to properly calibrate and
select the needed gain.
Modern micro-controller based electronics also allow complete drive and processing
circuitry for the LVDT that is compact and easy to use. These electronics allow the
customer to select the drive frequency, filtering options for speed and noise. And, with
certain options, one can get the information displayed on a local display in engineering
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units.
5. Elimination of Analog to Digital Cards
High-density microelectronics enable the incorporation of signal conditioning and
processing functions inside the LVDT housing rather than requiring an external box. In
addition, LVDTs can be produced with digital outputs directly compatible with computer–
based systems and the standardized buses now in use. This practice has become
extremely popular in metrology applications. Other industrial customers are beginning to
implement this control in their plants to improve control and reduce cabling.
Figure 1: The LVDT is an electromechanical transducer that can convert the rectilinear motion of
an object to which it is coupled mechanically into a corresponding electrical signal. Shown are the
components of a typical LVDT. The transformer's internal structure consists of a primary winding
centered between a pair of identically wound secondary windings, symmetrically spaced about
the primary. In operation, the LVDT's primary winding is energized by alternating current of
appropriate amplitude and frequency, known as the primary excitation. The LVDT's electrical
output signal is the differential AC voltage between the two secondary windings, which varies with
the axial position of the core within the LVDT coil. Usually this AC output voltage is converted by
suitable electronic circuitry to high level DC voltage or current that is more convenient to use.
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Fig. 2
Sealed construction inside a heavy-duty steel housing lets the LVDT core move freely while
protecting the windings from the environment. It also lets the core withstand temperatures in
excess of 400°F.
Figure 3: Offshore LVDTs are hermetically sealed with a welded subsea connector that is gold
plated and rated up to 7500psi. Dependent upon ocean temperature and depth levels, the LVDT
casing is typically composed of special alloy that supports long-term operation in different
elements
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