Macro Sensors LVDT: LVDTs for spool or poppet postion sensing in hydraulic valves

As LVDTs can separate from their core from the coil assembly, the LVDT’s core can operate in a pressurized medium while its coil can be installed in an unpressurized environment. In a typical LVDT installation on a servo valve or an electrically controlled proportional valve, a non-magnetic core connecting rod with an external thread attaches to an internal thread in the main spool, often using a jam nut to lock the rod at a specific dimension. The other end of the core connecting rod is also attached to the core by smaller thread.

The main spool typically is inserted into the valve through an o-ring sealed port of suitable size. After the spool has been inserted, a port plug configured with a blind bore non-magnetic tube that extends long enough from it to permit the maximum motion of the core attached to the main spool is screwed into the o-ring port. The tube, often called an “isolation tube,” is constructed with a wall thick enough to easily withstand the typical pilot stage control pressures of 250 to 2,000 psig. The bore of the LVDT coil assembly fits over the isolation tube, with the LVDT mounted outside of any pressurized medium. The input/output of the LVDT connects to a standard LVDT signal conditioner with bipolar DC output that feeds to the valve driver to close the loop.

In installations where the valve is operated by a solenoid instead of a force motor, it is sufficient to indicate that the spool has actually moved to a full flow or zero flow condition. In such cases, the desired output from the LVDT is a state change or switch action similar to that of a proximity sensor. The LVDT serves that purpose when used with the correct type of signal conditioning.

This same technique is used with poppet-type safety and relief valves. It is a stringent industry requirement that the poppet has fully reseated and isn’t leaking during operation. This requirement is especially important on safety valves used on hydraulic-operated drawing presses, press brakes, metal shears and other types of machinery. In fact, this type of sensor is now a legal requirement in Canada to monitor valves on hydraulic-operated machinery.

In these installations, the LVDT’s core is attached to the poppet stem using a nonmagnetic core connecting rod in between. The core sticks up into a non-magnetic isolation tube. The coil and a built-in electronics module are concentrically mounted over the tube and sealed from the medium, either with a flange and gasket, or screwed into an o-ring port. What makes this application unusual is that the LVDT only measures the position of the poppet when it is nearly or completed seated, nominally over only a millimeter or so. The actual motion of the poppet, when the valve is open, is many times that distance, typically 10 to 50 mm.

Fig 1: LVDT Cutaway

For relief valves or hydraulic control valves used in refineries and process plants, there is often a need for a hazloc-listed sensor. In these cases, intrinsically safe and/or hazloc-rated LVDT sensors can be utilized for these applications.

Fig 2: Schematic

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.

Figure 3: This figure shows the components of a typical LVDT

This figure shows 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 around the primary. The coils are wound on a one-piece hollow form of thermally stable glass-reinforced polymer, encapsulated against moisture, wrapped in a high permeability magnetic shield, and then secured in a cylindrical stainless steel housing. This coil assembly is usually the stationary element of the position sensor.

This moving element of an LVDT is a separate tubular armature of magnetically permeable material called the core, which is free to move axially within the coil’s hollow bore, and mechanically coupled to the object whose position is being measured. This bore is typically large enough to provide substantial radial clearance between the core and bore with no physical contact between it and the coil.

Macro Sensors offers a variety of AC- and DC-operated Free Core LVDTs for position sensing in various applications. Shown is the new HSE-750 Series of hermetically-sealed HSE-750 LVDTs that operated from 24 V DC and offer a 0-10 V DC output. In addition to a heavy-duty housing, the sensor’s coil windings are sealed against hostile environments to IEC standard IP-68. This allows for free movement of the core while sealing out the surrounding media from the winding. Units are ideal for use in hydraulic cylinder position, valve position sensing, automatic assembly equipment and other industrial applications with hostile conditions.

Figure 4: HSE-750 Series of Hermetically Sealed Free Core LVDTs

About The Author: Ed Herceg has been involved in the development of sensors and related instrumentation for more than four decades. He wrote the Schaevitz Handbook of Measurement and Control, which is regarded by some as the authoritative treatise on LVDT-based sensors. He has written numerous articles and papers covering diverse topics on sensors and physical measurement and was actively involved in development of the IEEE 1451.4 smart sensor interface standard.

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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.ifps.org www.fluidpowerjournal.com Printable PDF of this article [245 KB]	LVDTs for spool or poppet postion sensing in hydraulic valves Tech Directory, 2006 by By Ed Herceg, Director of Technology Applications Development Macro Sensors, Pennsauken, N.J. In the fluid power industry, Linear Variable Differential Transformers (LVDTs) are used to measure spool position in two-stage hydraulic valves where there is a need for proportional position feedback between the second stage main spool and the input to the first stage pilot valve’s current driver. The feedback provided by the LVDT permits the main spool of the valve to stay in position, providing a specified flow through the valve at the operating pressure.
	As LVDTs can separate from their core from the coil assembly, the LVDT’s core can operate in a pressurized medium while its coil can be installed in an unpressurized environment. In a typical LVDT installation on a servo valve or an electrically controlled proportional valve, a non-magnetic core connecting rod with an external thread attaches to an internal thread in the main spool, often using a jam nut to lock the rod at a specific dimension. The other end of the core connecting rod is also attached to the core by smaller thread. The main spool typically is inserted into the valve through an o-ring sealed port of suitable size. After the spool has been inserted, a port plug configured with a blind bore non-magnetic tube that extends long enough from it to permit the maximum motion of the core attached to the main spool is screwed into the o-ring port. The tube, often called an “isolation tube,” is constructed with a wall thick enough to easily withstand the typical pilot stage control pressures of 250 to 2,000 psig. The bore of the LVDT coil assembly fits over the isolation tube, with the LVDT mounted outside of any pressurized medium. The input/output of the LVDT connects to a standard LVDT signal conditioner with bipolar DC output that feeds to the valve driver to close the loop. In installations where the valve is operated by a solenoid instead of a force motor, it is sufficient to indicate that the spool has actually moved to a full flow or zero flow condition. In such cases, the desired output from the LVDT is a state change or switch action similar to that of a proximity sensor. The LVDT serves that purpose when used with the correct type of signal conditioning. This same technique is used with poppet-type safety and relief valves. It is a stringent industry requirement that the poppet has fully reseated and isn’t leaking during operation. This requirement is especially important on safety valves used on hydraulic-operated drawing presses, press brakes, metal shears and other types of machinery. In fact, this type of sensor is now a legal requirement in Canada to monitor valves on hydraulic-operated machinery. In these installations, the LVDT’s core is attached to the poppet stem using a nonmagnetic core connecting rod in between. The core sticks up into a non-magnetic isolation tube. The coil and a built-in electronics module are concentrically mounted over the tube and sealed from the medium, either with a flange and gasket, or screwed into an o-ring port. What makes this application unusual is that the LVDT only measures the position of the poppet when it is nearly or completed seated, nominally over only a millimeter or so. The actual motion of the poppet, when the valve is open, is many times that distance, typically 10 to 50 mm. For relief valves or hydraulic control valves used in refineries and process plants, there is often a need for a hazloc-listed sensor. In these cases, intrinsically safe and/or hazloc-rated LVDT sensors can be utilized for these applications. 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. This figure shows 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 around the primary. The coils are wound on a one-piece hollow form of thermally stable glass-reinforced polymer, encapsulated against moisture, wrapped in a high permeability magnetic shield, and then secured in a cylindrical stainless steel housing. This coil assembly is usually the stationary element of the position sensor. This moving element of an LVDT is a separate tubular armature of magnetically permeable material called the core, which is free to move axially within the coil’s hollow bore, and mechanically coupled to the object whose position is being measured. This bore is typically large enough to provide substantial radial clearance between the core and bore with no physical contact between it and the coil. Macro Sensors offers a variety of AC- and DC-operated Free Core LVDTs for position sensing in various applications. Shown is the new HSE-750 Series of hermetically-sealed HSE-750 LVDTs that operated from 24 V DC and offer a 0-10 V DC output. In addition to a heavy-duty housing, the sensor’s coil windings are sealed against hostile environments to IEC standard IP-68. This allows for free movement of the core while sealing out the surrounding media from the winding. Units are ideal for use in hydraulic cylinder position, valve position sensing, automatic assembly equipment and other industrial applications with hostile conditions. About The Author: Ed Herceg has been involved in the development of sensors and related instrumentation for more than four decades. He wrote the Schaevitz Handbook of Measurement and Control, which is regarded by some as the authoritative treatise on LVDT-based sensors. He has written numerous articles and papers covering diverse topics on sensors and physical measurement and was actively involved in development of the IEEE 1451.4 smart sensor interface standard. --------------------------------------------------------------------------- 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