LINEAR VARIABLE DIFFERENTIAL TRANSFORMER VISHAL VASISTHA ASSISTANT PROFESSOR (ME) DRONACHARYA COLLEGE, GURGAON
LVDT WORKING + (50-20K Hz) E1 E2 (E1-E2) -
INTRODUCTION • A transformer with variable secondary coupling dependent upon the core position (linear movement) • position-to-electrical sensor whose output is linearly proportional to the position of a movable magnetic core • Core moves linearly inside a transformer consisting of a primary coil and two secondary coils • Primary winding is excited with an ac voltage source(50-20K Hz)
INTRODUCTION • Voltage is induced by mutual induction principle (same as transformer) in secondary coils that vary with the position of the magnetic core within the assembly • Core is attached to the object whose movement or displacement is being measured • Core is not in contact with primary and secondary coils. • Secondary windings are wound out of phase with each other
LVDT COMPONENTS Ferrous core Epoxy encapsulation Primary coil Secondary coil Bore shaft Magnetic shielding Stainless steel end caps Secondary coil High density glass filled coil forms Signal conditioning circuitry
ADVANTAGES • Frictionless Measurement • Infinite Mechanical Life • Infinite Resolution • Null Position Repeatability • Cheap, easy to use way of measuring deflection
ADVANTAGES • Low risk of damage • Core and Coil Separation • Environmental Compatibility
TYPES OF LVDT’S AC DC Ease of installation Can operate from dry cell batteries Usually lower system cost • Smaller size • More accurate • Can operate within a wide temperature range • Lower unit cost.
DISADVANTAGES • Area of dynamic measurement due to appreciable mass of core itself • Very high displacement is required for generating high voltages • Shielding is required since it is sensitive to magnetic field • The performance of the transducer gets affected by vibrations • Requires alignment to be correct • The range of measurement is limited to 12 in.
APPLICATIONS • Measure displacement of thermostat valve stem for diesel truck engine monitoring system • Blood-testing device measuring the displacement of blood cells as they contract • Measuring displacement of diamond tip to determine material hardness
LVDT ACCELEROMETER • LVDT core acts as a seismic mass • Core is attached with two spring steels with the help of rods • Spring steels are attached to the case firmly • Spring steels provide spring action • As instrument is subjected to vibration, core moves up and down and induces resultant output in secondary coils
TYPES OF LVDT’S • Unguided Armature • Captive (Constrained) Armature • Spring-extended Armature
UNGUIDED ARMATURE Measured Object Armature must be attached to the specimen Armature fits loosely in the bore Body must be separately supported & properly aligned
UNGUIDED ARMATURE • There is no wear on the LVDT because no contact is made between armature and bore. • LVDT does not restrict the resolution of measured data (“infinite resolution”). Measured Motion
UNGUIDED ARMATURE • Well-suited for short-range, high-speed applications (vibration) Measured Motion time displ.
CAPTIVE ARMATURE Measured Object Armature must be attached to the specimen Armature is both guided and restrained by a low friction assembly Body must be separately supported
CAPTIVE ARMATURE Advantages compared to unguided armature: • Better for longer working ranges • Preferred when misalignment may occur
Spring-Extended Armature Measured Object Like the captive armature, it has a low-friction bearing assembly Internal spring to continuously push the armature to its fullest possible extension
Spring-Extended Armature • The spring-extended armature is best suited for slow-moving applications. • Attachment between armature and specimen is not required.