Displacement Transducer / Position Sensor, LVDT ... - Of the Clux

HOME DISPLACEMENT PRESSURE ACCELERATION CALIBRATION TECHSUPPORTINDEX LOAD ELECTRONICS TORQUE ANGLECONTACTUSLinear Variable DifferentialTransformer Principle of OperationL.V.D.T. stands for Linear Variable Differential Transformer.The main advantage of the LVDT principle over other types of displacement transducer istheir high degree of robustness. This is derived from their very principle in which there is nophysical contact across the sensing element and so there is zero wear in the sensing element.This also means that RDP Electronics LVDTs can be made waterproof and in a formatsuitable for the most arduous applications.The LVDT principle of measurement is based on magnetic transfer which also means that theresolution of LVDT transducers is infinite. The smallest fraction of movement can be detectedby suitable signal conditioning electronics.The combination of these two factors plus other factors such as accuracy and repeatablity hasensured that this technology is still at the forefront of displacement measurement after over 90years.An LVDTtransducercomprises acoil former orbobbin ontowhich threecoils arewound. Thefirst coil, the primary is excited with an a.c. current, normally in the region of 1 to 10kHz at0.5 to 10V rms. The other two coils, the secondaries are wound such that when a ferritic coreis in the central linear position, an equal voltage is induced into each coil. However, thesecondaries are connected in opposition so that in the central position the outputs of the twosecondaries cancel each other out.

The excitation is applied tothe primary winding of theposition sensor by theoscillator circuit. Theoscillator is an externalitem, not shown in thisanimation. The excitation isnormally a sinusoidalvoltage signal, of 0.5V to5V amplitude and 1kHz to30kHz frequency.The armature (the movingpart or slider of thedisplacement transducer)assists the induction ofcurrent into the secondarycoils Sec. 1 and Sec. 2. Thearmature is made of aspecial magnetic materialand is often connected to apush rod that is notmagnetic. The push rodconnects the armature tothe outside world.When the armature is in thecentral position there is anequal voltage induced intoboth Sec.1 and Sec. 2.However, as they are wiredin opposition, the sum ofthe position sensorsecondary outputs canceleach other out resulting in azero output.As the armature moves intoSec.1 (and out of Sec. 2)the result is that the sum ofSec.1 and Sec. 2 favoursSec.1 which in thisillustration is in-phase withthe excitation voltage.Conversely, as the armaturemoves into Sec. 2 (and outof Sec.1) the sum favoursSec. 2 (the out-of-phasevoltage).

The output of an is an a.c. waveform and so it does not actually have a polarityas such. The magnitude of the output of the transducer rises regardless of thedirection of movement from the electrical zero position.In order to know in which half of the displacement transducer coil the centre of the armatureis located, one must consider the phase of the output as well as the magnitude. The outputphase of the position sensor is compared with the excitation phase and it can be either in orout of phase with the excitation, depending upon which half of the coil the centre of thearmature is in.The electronics therefore, must combine information on the phase of the output withinformation on the magnitude of the output . This will then allow the user to know exactlywhere the armature is rather than how far from the electrical zero position it is.RDP Electronics manufacture both LVDT transducer units and signal conditioning systemsfor measurement in a wide range of environments.Back to main LVDT page.Due to the RDP Electronics policy of on-going development, specifications may changewithout notice.Any modification may affect the specification of our equipment.Document Reference:- www.rdpe.com/displacement/lvdt/lvdt-principles.htm, updated20020419©RDP Electronics Ltd, Grove Street, Heath Town, Wolverhampton, WV10 0PY, UnitedKingdom.Tel +44 (0)1902 457512, Fax +44 (0)1902 452000, URL www.rdpe.com, e-mail (sales only)sales@rdpe.com, other departments mail@rdpe.com.