Macro Sensors LVDT: LVDTs Measure Valve Position for Steam, Water and Lubricants

It is a common type of electromechanical transducer, but what sets it apart from ordinary transducers?

Linear variable differential transformers (LVDTs) are a common type of electromechanical transducer that can convert the rectilinear motion of an object to which it is coupled mechanically into a corresponding electrical signal. LVDT linear position sensors are readily available that can measure movements as small as a few millionths of an inch up to several inches, but are also capable of measuring positions up to ±20 inches (±0.5 meters).

LVDTs have certain significant features and benefits, most of which derive from their fundamental physical principles of operation or from the materials and techniques used in their construction. These commendable features have made LVDTs very useful in a wide variety of applications. Some of these features are unique to the LVDT and are not available in other transducers. Many of the features of an LVDT arise from the basic fact that it is an electrical transfomer with a separable, non-contacting core.

This schematic shows the inner composition of an LVDT.

Friction-Free Operation

One of the most important features of an LVDT is its friction-free operation. In normal use, there is no mechanical contact between the LVDT’s core and coil assembly, so there is no rubbing, dragging or other source of friction. This feature is particularly useful in materials testing, vibration displacement measurements and high-resolution dimensional gaging systems.

Infinite Resolution

Since an LVDT operates on electromagnetic coupling principles in a friction-free structure, it can measure infinitesimally small changes in core position. This infinite resolution capability is limited only by the noise in an LVDT signal conditioner and the output display’s resolution. These same factors also give an LVDT its outstanding repeatability.

Unlimited Mechanical Life

Because there is normally no contact between the LVDT’s core and coil structure, no parts can rub together or wear out. This means that an LVDT features unlimited mechanical life. This factor is especially important in high reliability applications such as aircraft, satellites and space vehicles, and nuclear installations. It is also highly desirable in many industrial process control and factory automation systems.

Overtravel Damage Resistant

The internal bore of most LVDTs is open at both ends. In the event of unanticipated overtravel, the core is able to pass completely through the sensor coil assembly without causing damage. This invulnerability to position input overload makes an LVDT the ideal sensor for applications like extensometers that are attached to tensile test samples in destructive materials testing apparatus.

Single Axis Sensitivity

An LVDT responds to motion of the core along the coil’s axis, but is generally insensitive to cross-axis motion of the core or to its radial position. Thus, an LVDT can usually function without adverse effect in applications involving misaligned or floating moving members, and in cases where the core doesn’t travel in a precisely straight line.

Separable Coil And Core

Because the only interaction between an LVDT’s core and coil is magnetic coupling, the coil assembly can be isolated from the core by inserting a non-magnetic tube between the core and the bore. By doing so, a pressurized fluid can be contained within the tube, in which the core is free to move, while the coil assembly is unpressurized. This feature is often utilized in LVDTs used for spool position feedback in hydraulic proportional and/or servo valves.

Environmentally Robust

The materials and construction techniques used in assembling an LVDT result in a rugged, durable sensor that is robust to a variety of environmental conditions. Bonding of the windings is followed by epoxy encapsulation into the case, resulting in superior moisture and humidity resistance, as well as the capability to take substantial shock loads and high vibration levels in all axes. And the internal high-permeability magnetic shield minimizes the effects of external AC fields.

Both the case and core are made of corrosion resistant metals, with the case also acting as a supplemental magnetic shield. And for those applications where the sensor must withstand exposure to flammable or corrosive vapors and liquids or operate in pressurized fluid, the case and coil assembly can be hermetically sealed using a variety of welding processes.

Ordinary LVDTs can operate over a very wide temperature range, but, if required, they can be produced to operate down to cryogenic temperatures, or, using special materials, operate at the elevated temperatures and radiation levels found in many nuclear reactors.

Null Point Repeatability

The location of an LVDT’s intrinsic null point is extremely stable and repeatable, even over its very wide operating temperature range. This makes an LVDT perform well as a null position sensor in closed-loop control systems and highperformance servo balance instruments.

Fast Dynamic Response

The absence of friction during ordinary operation permits an LVDT to respond very fast to changes in core position. The dynamic response of an LVDT sensor itself is limited only by the inertial effects of the core’s slight mass. More often, the response of an LVDT sensing system is determined by characteristics of the signal conditioner.

Linear variable differential transformers, such as these from Macro Sensors,
can measure movement as small as a few millionths of an inch up to several inches.

Absolute Output

An LVDT is an absolute output device, as opposed to an incremental output device. This means that in the event of loss of power, the position data being sent from the LVDT will not be lost. When the measuring system is restarted, the LVDT’s output value will be the same as it was before the power failure occurred.

About the Author Ed Herceg is director of technology applications development at Macro Sensors, 7300, US Rte. 130 N., Bldg. 22, Pennsauken, NJ 08110. More information is available by calling 856-662-8000 or at www.macrosensors.com.

---------------------------------------------------------------------------
Company Information:
Name: Macro Sensors
Address: 7300 US Route 130 North, Bldg. 22
City: Pennsauken
State: NJ
ZIP: 08110 1334
Country: USA
Phone: 856-662-8000
FAX: 856-317-1005
http://www.macrosensors.com



Press Release: www.pddnet.com	Why Use an LVDT? Product Design & Development, March 2007 by Ed Herceg
Printable PDF of this article [148 KB]	It is a common type of electromechanical transducer, but what sets it apart from ordinary transducers? Linear variable differential transformers (LVDTs) are a common type of electromechanical transducer that can convert the rectilinear motion of an object to which it is coupled mechanically into a corresponding electrical signal. LVDT linear position sensors are readily available that can measure movements as small as a few millionths of an inch up to several inches, but are also capable of measuring positions up to ±20 inches (±0.5 meters). LVDTs have certain significant features and benefits, most of which derive from their fundamental physical principles of operation or from the materials and techniques used in their construction. These commendable features have made LVDTs very useful in a wide variety of applications. Some of these features are unique to the LVDT and are not available in other transducers. Many of the features of an LVDT arise from the basic fact that it is an electrical transfomer with a separable, non-contacting core. This schematic shows the inner composition of an LVDT. Friction-Free Operation One of the most important features of an LVDT is its friction-free operation. In normal use, there is no mechanical contact between the LVDT’s core and coil assembly, so there is no rubbing, dragging or other source of friction. This feature is particularly useful in materials testing, vibration displacement measurements and high-resolution dimensional gaging systems. Infinite Resolution Since an LVDT operates on electromagnetic coupling principles in a friction-free structure, it can measure infinitesimally small changes in core position. This infinite resolution capability is limited only by the noise in an LVDT signal conditioner and the output display’s resolution. These same factors also give an LVDT its outstanding repeatability. Unlimited Mechanical Life Because there is normally no contact between the LVDT’s core and coil structure, no parts can rub together or wear out. This means that an LVDT features unlimited mechanical life. This factor is especially important in high reliability applications such as aircraft, satellites and space vehicles, and nuclear installations. It is also highly desirable in many industrial process control and factory automation systems. Overtravel Damage Resistant The internal bore of most LVDTs is open at both ends. In the event of unanticipated overtravel, the core is able to pass completely through the sensor coil assembly without causing damage. This invulnerability to position input overload makes an LVDT the ideal sensor for applications like extensometers that are attached to tensile test samples in destructive materials testing apparatus. Single Axis Sensitivity An LVDT responds to motion of the core along the coil’s axis, but is generally insensitive to cross-axis motion of the core or to its radial position. Thus, an LVDT can usually function without adverse effect in applications involving misaligned or floating moving members, and in cases where the core doesn’t travel in a precisely straight line. Separable Coil And Core Because the only interaction between an LVDT’s core and coil is magnetic coupling, the coil assembly can be isolated from the core by inserting a non-magnetic tube between the core and the bore. By doing so, a pressurized fluid can be contained within the tube, in which the core is free to move, while the coil assembly is unpressurized. This feature is often utilized in LVDTs used for spool position feedback in hydraulic proportional and/or servo valves. Environmentally Robust The materials and construction techniques used in assembling an LVDT result in a rugged, durable sensor that is robust to a variety of environmental conditions. Bonding of the windings is followed by epoxy encapsulation into the case, resulting in superior moisture and humidity resistance, as well as the capability to take substantial shock loads and high vibration levels in all axes. And the internal high-permeability magnetic shield minimizes the effects of external AC fields. Both the case and core are made of corrosion resistant metals, with the case also acting as a supplemental magnetic shield. And for those applications where the sensor must withstand exposure to flammable or corrosive vapors and liquids or operate in pressurized fluid, the case and coil assembly can be hermetically sealed using a variety of welding processes. Ordinary LVDTs can operate over a very wide temperature range, but, if required, they can be produced to operate down to cryogenic temperatures, or, using special materials, operate at the elevated temperatures and radiation levels found in many nuclear reactors. Null Point Repeatability The location of an LVDT’s intrinsic null point is extremely stable and repeatable, even over its very wide operating temperature range. This makes an LVDT perform well as a null position sensor in closed-loop control systems and highperformance servo balance instruments. Fast Dynamic Response The absence of friction during ordinary operation permits an LVDT to respond very fast to changes in core position. The dynamic response of an LVDT sensor itself is limited only by the inertial effects of the core’s slight mass. More often, the response of an LVDT sensing system is determined by characteristics of the signal conditioner. Linear variable differential transformers, such as these from Macro Sensors, can measure movement as small as a few millionths of an inch up to several inches. Absolute Output An LVDT is an absolute output device, as opposed to an incremental output device. This means that in the event of loss of power, the position data being sent from the LVDT will not be lost. When the measuring system is restarted, the LVDT’s output value will be the same as it was before the power failure occurred. About the Author Ed Herceg is director of technology applications development at Macro Sensors, 7300, US Rte. 130 N., Bldg. 22, Pennsauken, NJ 08110. More information is available by calling 856-662-8000 or at www.macrosensors.com. --------------------------------------------------------------------------- Company Information: Name: Macro Sensors Address: 7300 US Route 130 North, Bldg. 22 City: Pennsauken State: NJ ZIP: 08110 1334 Country: USA Phone: 856-662-8000 FAX: 856-317-1005 http://www.macrosensors.com