DISPLACEMENT MEASUREMENT

LINEAR VARIABLE DIFFERENTIAL TRANSDUCER

  • Diagram of LVDT (1)

  • Diagram of LVDT (2)

  • Block Diagram of a LVDT

  • OUTPUT GRAPH OF LVDT

    • The most widely used inductive transducer to translate the linear motion into electrical signals is the LVDT.The transformer consists of a single primary winding and two secondary windings wound on a cylindrical former. The secondary windings have equal number of turns and are identically placed on either side of the primary winding. An alternating supply is given to the primary winding. A movable soft core is placed inside the former. The displacement to be measured is applied to the arm attached to the soft iron core. This core is made up of high permiability, nickol iron which is hydrogen annealed. The frequency of the a.c suuplied tot he primary winding has to be between 50 Hz and 20 KHz.

    Since the primary winding is excited by an alternating current source, it produces an alternating magnetic field which in turn induces alternating current voltages in the two secondary windings.If the output voltage of secondary windings are E1 and E2, in order to convert the outputs from the sec.windings into a single voltage signal, the two secondaries are connected in series. The output of the transducer is the difference between the two voltages E1 and E2.

    Differential output voltage E0=E1-E2.

    WORKING OF LVDT

    The main advantage of the LVDT transducer over other types of displacement transducer is their high degree of robustness. This is derived from their very principle in which there is no physical contact across the sensing element and so there is zero wear in the sensing element. LVDTs can be made waterproof and in a format suitable for the most arduous applications.

    The LVDT principle of measurement is based on magnetic transfer which also means that the resolution of LVDT transducers is infinite. The smallest fraction of movement can be detected by suitable signal conditioning electronics.

    The combination of these two factors plus other factors such as accuracy and repeatablity has ensured that this technology is still at the forefront of displacement measurement after over 90 years.

    An LVDT comprises a coil former or bobbin onto which three coils are wound. The first coil, the primary is excited with an a.c. current, normally in the region of 1 to 10kHz at 0.5 to 10V rms. The other two coils, the secondaries are wound such that when a ferritic core is in the central linear position, an equal voltage is induced into each coil. However, the secondaries are connected in opposition so that in the central position the outputs of the two secondaries cancel each other out.

    The excitation is applied to the primary winding of the position sensor by the oscillator circuit. The oscillator is an external item, not shown in this animation. The excitation is normally a sinusoidal voltage signal, of 0.5V to 5V amplitude and 1kHz to 30kHz frequency.

    The armature (the moving part or slider of the displacement transducer) assists the induction of current into the secondary coils Sec. 1 and Sec. 2. The armature is made of a special magnetic material and is often connected to a push rod that is not magnetic. The push rod connects the armature to the outside world.

    When the armature is in the central position there is an equal voltage induced into both Sec.1 and Sec. 2. However, as they are wired in opposition, the sum of the position sensor secondary outputs cancel each other out resulting in a zero output.

    As the armature moves into Sec.1 (and out of Sec. 2) the result is that the sum of Sec.1 and Sec. 2 favours Sec.1 which in this illustration is in-phase with the excitation voltage.

    Conversely, as the armature moves into Sec. 2 (and out of Sec.1) the sum favours Sec. 2 (the out-of-phase voltage). The output of an LVDT is an a.c. waveform and so it does not actually have a polarity as such. The magnitude of the output of the transducer rises regardless of the direction of movement from the electrical zero position .

    In order to know in which half of the displacement transducer coil the centre of the armature is located, one must consider the phase of the output as well as the magnitude. The output phase of the position sensor is compared with the excitation phase and it can be either in or out of phase with the excitation, depending upon which half of the coil the centre of the armature is in. The electronics therefore, must combine information on the phase of the output with information on the magnitude of the output . This will then allow the user to know exactly where the armature is rather than how far from the electrical zero position it is.