Difference Between Free-Core vs. Guided-Core (Spring-Loaded) LVDT Linear Position Sensors | Automation.com

Difference Between Free-Core vs. Guided-Core (Spring-Loaded) LVDT Linear Position Sensors

Difference Between Free-Core vs. Guided-Core (Spring-Loaded) LVDT Linear Position Sensors

One of the primary factors in choosing an LVDT is determining the correct mechanical Interface.  Both free-core and guided-core LVDT Linear Position Sensors provide the benefit of infinite resolution, with each type offering models that can operate in harsh environments.  While free-core LVDTs are generally less expensive and available in many ranges, they are more difficult to install than guided-core sensors.  Available in spring and air-loaded cores, guided-core LVDTs are simpler to install but more expensive and not available in as many ranges.  In addition to these differences, some applications are just better suited for a free-core LVDT, while others can benefit from a guided-core LVDT.

Free-Core LVDTs
The moving element of a free-core 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 for frictionless measurement and infinite mechanical life.

Free-core LVDTs are very versatile, non-contacting and robust, offering long-term reliability in harsh or hostile environments when constructed with the right materials. Frictionless operation translates into higher repeatability and resolution.  Based on these properties, LVDTs are typically used in systems where cost of ownership, safety and high reliability are priorities, such as aircraft wing-flap indication, subsea applications, high-temperature steam valves, and power plant installations.   Other ideal applications for free-core LVDTs include:

•    Where the measured object is mechanically coupled to the reference surface or object  (valves, hydraulic cylinders, actuators, strain testing machines)
•    Measuring ranges above 4.00”
•    Frequency response >10 Hz (vibration measurements)
•    Critical measurement of delicate materials or highly elastic materials that allow the measured object to move with little to no mechanical resistance

Guided Core: Spring-Loaded LVDTs
Spring-loaded LVDTs, commonly referred to as dimensional gaging probes, incorporate a non-contacting, inductive position sensor, either an LVDT or half bridge, that includes a spring-loaded movable armature coupled to a shaft supported in a high-precision linear bearing.

Most gaging probes have a maximum gaging range of ±0.50 mm to ±50.0 mm (±0.020 in. to ±2.00 in.), with resolutions of fractions of microns.  When electronics are built into a spring-loaded assembly, there is no need for external electronics, making the mechanical setup of the sensor into automated machinery less complicated and more cost effective.  

Electronic gaging probes are commonly used in the dimensional gaging of manufactured parts, serving as important components of quality assurance systems.  Ideal applications are where the measured object is NOT mechanically coupled to the reference:

•    Gaging in heavy industrial: hermetically-sealed gaging probes can solve many of the problems associated with dimensional gaging in harsh environments
•    High precision gaging:  High precision gaging probes utilize a linear ball bearing assembly precisely fitted to a hardened and ground, non-rotating probe shaft to minimize radial play and the effects of side loading. This results in the probes' exceptional repeatability of 0.000006 inch (0.15 μm)
•    Assembly Line Quality Measurements: Air extend/spring-retract gauging probes are recommended for these applications as they extend to make measurements and then retract so the probe is not damaged as product moves down the line
•    Flatness measurements on plates
•    Potential cross-axial movement of the measured object:  For example, making a roundness measurement on a rotating part
•    Thickness measurements


Free-Core LVDTs Ensure Proper Currency Dispensing in ATM Machines
TE Connectivity’s PR750 Contactless Free Core LVDT serves as a measurement feedback device in the paper bill detector of automated teller machines (ATMs) to ensure proper currency dispensing.   As the bills pass between a pair of rollers, the LVDT output changes according to their thickness.  With a typical bill measuring 0.0003” (0.0075mm) thick, the LVDT will detect any derivation should two or more bills get stuck together.  

As the ATM can be used hundreds of times during the course of a day, the repeatability and reliability of the linear position sensor are of key importance.  Since there is no contact between the LVDT core and coil of the PR Series LVDTs, there is no friction to cause inaccuracy or part of the sensor to wear out during the service life of the ATM.  

DC-operated Spring-Loaded LVDT Linear Position Sensors Ensure Proper Operation of Presses & Dyes
In the press and dye industry, precise mechanical control of machine operations is imperative to prevent improper operation that can lead to broken dyes, downed machines as well as misshapen and out-of-spec parts. Serving as components of a computer-controlled system, spring-loaded LVDT linear position sensors are used to ensure proper machine operation.

A ruggedized linear measurement device, the spring-loaded LVDT can survive the heavy pounding and shocks common on punch presses. TE Connectivity’s GHSER 750 Series Spring-Loaded LVDTs with built-in electronics and radial connectors are installed on presses so that the LVDT plunger is compressed as the punch press comes in contact with the metal being shaped. The LVDT output is fed back into the machine’s control system, providing feedback on how far a press has moved and when to stop.

AC-operated Spring-Loaded LVDTs Replace Failing Units on Automotive Parts Production Line
TE Connectivity’s Spring-Loaded LVDTs Sensors are being used on an automated auto part production line as part of a quality control system that ensures the precise dimensions of hydraulic valve adjusters manufactured for use in different car engines.

Every automobile uses a number of different engine valves mounted on the cylinder head that allows air flow in and out of the cylinder.  A 4-cylinder engine will have 16 valves, while an 8-cylinder engine will have 32 valves. Valve adjusters, very small oil-filled devices, keep valves from knocking when the engine starts by maintaining correct valve pressure in the internal combustion engine.   

During mass production, the ID of each valve adjuster is measured prior to oil fill.  If dimensions are not within specification, the valve adjuster is removed from the manufacturing line.  LVDT gage heads ensure the highly accurate and precise measurement of dimensions as part of the industrial quality control system.

A standard LVDT gage head quickly wore out because the springs could not tolerate the high production volume of 300K units per day.  As a result, the sensors needed to be replaced about three times during a production cycle, which resulted in productivity loss and higher costs associated with sensor inventory.

To solve the problem, TE Connectivity’s GHSAR 750 Series Spring-Loaded AC LVDT Position Sensors were customized with a spring manufactured from high strength steel that is 3x to 4x stronger than the competitive units.  The LVDT gage heads are also specified with a 90° radial connector to provide 2” to 3” more clearance for the sensor to adjoin with the mating connector.  The axial connector of the previous unit would bend and break, requiring the entire sensor to be replaced.

As an AC-operated sensor, electronics of the gage head can be remotely located in a box outside the hostile conditions of the production area. In addition to enabling the gage head to operate where temperature, vibration and other parameters exceed the limits of the electronics, sensors can be more inexpensively replaced if necessary as the entire LVDT would not be required.

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