Attributes and Benefits of Free-Core and Spring-Loaded LVDT Position Sensors

LVDTs can come in both free-core or spring-loaded mechanical packages.  Learn about each one, and how to select the best one for your application…

LVDTs (Linear Variable Differential Transformers) are electromechanical devices used in many industrial applications to measure the displacement or position of an object.  They convert the linear position or motion of a measured object into an electrical output that is displayed on a local readout or input into a programmable logic controller as part of an automated process control system.  LVDTs come in different core configurations: free-core and spring-loaded. (Read How LVDTs Work)

Free-Core

Free-core LVDTs (see Photo 1) consist of a fixed housing containing a single primary winding, two secondary windings, and a moveable core that is mechanically linked to the measured object.   Most LVDTs have a free-floating core unit unattached to any of the coils.  Its position between the coils determines the LVDT’s electrical output.  

LVDTs with free-floating cores feature frictionless operation that enables measurement of very sensitive displacements without any interference due to friction or stiction.   In applications that measure very soft/easily deformed material, frictionless operation is crucial because friction or stiction can lead to erroneous measurements.

Free-core LVDTs are the ideal choice when high accuracy, cost of ownership, and long life are priorities.  

Photo 1:  Free-core LVDTs come in a variety of sizes, constructions, and configurations to address the requirements of different applications.  LVDTs also are available in both AC and DC versions to perform in different environments.  Read about the Attributes and Benefits of AC-and DC-operated LVDT Position Sensors.

Spring-Loaded

Another LVDT configuration includes a core connected to a spring-loaded probe shaft having a removable tip.  Known as a spring-loaded LVDT position sensor, they are often described as gaging probes with LVDT assemblies. 

In construction, an internal spring continuously applies force to the core and extends a probe shaft that contacts a measured object. The probe shaft is guided in a sleeve bearing retained in a case that also encloses the LVDT coil windings.  Only the ball tip maintains physical contact with the part under measurement. These sensors are commonly used for highly accurate and repeatable dimensional gaging of components as part of quality assurance systems.

Spring-loaded LVDTs also are available in air-actuated versions (See Photo 2) that operate with low-pressure air (10 – 40 pi) that extends the sensor shaft.  Measurement is made when the probe is extended.  Probe contact force is regulated by air pressure.  With the removal of air pressure, an internal spring retracts the probe to avoid any damage to the sensor and conveyed parts between inspection readings.

Photo 2: In the configuration of spring-loaded air-actuated LVDTs, the probe spring returns the probe shaft to its fully retracted position between measurements.  During measurement cycles, the shaft is extended by the introduction of a low-pressure, clean, and dry air supply. Probe contact force can be set by adjusting air pressure. Air outflow through the probe’s bearing aids the exclusion of dirt, dust, oil, solvents, and other contaminants from entering the unit.

Benefits of Free-Core LVDTs

The lack of physical contact between the core and its hollow bore during operation makes for frictionless measurement that translates into high repeatability, superior resolution, and infinite mechanical life.  Other attributes include:

 

  • High application versatility
  • Robustness for long life and lower cost of ownership
  • High reliability with the absence of friction during operation
  • Shock- and vibration-tolerance
  • Standard, miniature, hermetically-sealed, high temperature and high-pressure designs
  • AC or DC operation

Free-Core LVDT Applications

Free-core LVDTs are used in a variety of applications in different industries (Photo 3).

Applications include:

  • Factory automation
  • Medical equipment
  • Food process control
  • Materials testing
  • Fluid level measurement
  • Packaging equipment
  • Power generation
  • Ovens and castings
  • Autoclaves
  • Engine control systems
  • Cryogenic storage tanks
  • Subsea choke position and strain measurements

Photo 3: Robust and highly accurate, free-core LVDTs perform in a range of environments (depending on construction) within the industry, aerospace, and military sectors. 
NewTek Real-world Applications

NewTek free-core LVDTs ensure accurate monitoring and control of gas turbine valves that regulate the flow of fuel through different components of a gas turbine.  By knowing the position of micro fuel valve openings, operators can more efficiently operate turbines with minimal wasted energy.

 

Implemented as part of steam power plant rehabilitation projects, these free-core LVDT sensors are used in a control system to monitor the position of modulating governor and throttle valves. 

 

The linear position sensors can significantly increase operating efficiency by enabling operators to meet specific control algorithms.  NewTek LVDT sensors also monitor valve position at petrochemical plants as well as within off-road vehicles, farm equipment, construction equipment, and aircraft.  Check out NewTek Free-Core LVDTs

Benefits of Spring Loaded LVDTs

Benefits of Spring-Loaded LVDTs

As the sensor probe maintains constant contact with the measured object, spring-loaded LVDTs provide highly repeatable and accurate measurements.  With the spring reducing the effect of friction between the core and housing, the sensor has greater accuracy. Other benefits include:

 

  • Easy installation
  • Gaging ranges from ±0.50 mm to ±50.0 mm (±0.020 in. to ±2.00 in.)
  • Resolution to fractions of microns
  • Choice of probe contact tips

Spring-Loaded LVDT Applications

Spring-loaded LVDTs are popular for use in the dimensional gaging of components as part of manufacturing quality assurance systems.  They can provide single-point gaging where the tip touches the measured object at one point or multi-gaging of objects with complex shapes or profiles such as the lenses of a space telescope.   Other applications include:

 

  • Product thickness control
  • TIR measurements
  • Inline inspection
  • Automotive testing
  • Automated production lines (gaging of parts in go/no go inspection)
  • R&D labs for quality control

NewTek Real-world Applications

Spring-loaded LVDTS are used for surface measurement of paper, rubber, paneling, and other materials as part of quality control processes in sawmills, wood processing, wall paneling, and automotive production plants. 

Outfitted on precision wheels, NewTek Spring-Loaded Gage Heads are mounted at different points of the manufacturing line to make multiple contacts of a product surface in precisely measuring its thickness, flatness, and other surface parameters.  Data collected from the gaging probes enables operators to monitor a product profile for abnormalities or an out-of-tolerance spec.

 

DC-operated LVDT gage heads are ideal for operation in less hostile environments as they eliminate the installation and cost of signal conditioning equipment. With electronics built into the assembly, the sensor simplifies mechanical set-up into automated machinery.  

In manufacturing conveyor lines, NewTek Air Extend/Spring Retract Gage Heads are used to conduct tolerance checks as part of quality assurance systems, providing dimensional feedback to verify if products have any variance in size and other physical properties.

Tolerance checks are important to maintain product quality from lot to lot throughout manufacturing cycles. Machine manufacturers and quality control departments use these sensors for “go/no-go” inspection of complex mechanical parts.

These Air Extend/Spring Retract Gage Head Sensors check dimensions when their probes contact the measured object.  In operation, low-pressure air (10 – 40 psi) extends the sensor shaft. Probe contact force is regulated by the air pressure. With the removal of air pressure, an internal spring retracts the probe to allow room for automated parts to move along the conveyor without damaging the probe or the next product on the production line.

Check out NewTek Spring Loaded LVDT Gage Heads.

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