“Five Things You didn’t know about LVDT Linear Position ...



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:

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 nickel-based 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 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.

1. 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%.

2. Shorter Strokes/Smaller Diameters

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.

3. 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 units.

4. 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.

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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.

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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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