Principle of Operation A LVDT inductive displacement transducer is constructed using a static transformer (primary winding) and two secondary windings.
The windings are formed on a hollow bobbin through which a magnetic core can travel.
The magnetic core povides a path for linking the coils via the magnetic flux.
When the primary winding is connected to an AC supply, current starts to flow in the secondary coils. A simplified electrical schematic is shown in the figure.
The secondary coils A and B are connected in series opposition so that the two voltages VA and VB have opposite phase and the transducer output is VA-VB.
If the core is in the centre position then voltages of equal magnitude but opposite phase will be induced in each secondary and the net output is zero.
As the core is moved in one direction, the voltage in the corresponding secondary coil increases while the other coil experiences a complementary voltage reduction. The net effect is a transducer voltage output that is proportional to displacement.
Knowledge of the magnitude and the phase of the output with respect to the excitation signal allows one to deduce the position and direction of the core motion from the null position.
The output of an LVDT is a linear function of displacement over its calibrated measurement range. Beyond this range the output becomes increasingly non-linear. Measurement range is defined as ± distance from the transducer null position.
Half Bridge, LVDT and digital transducers
The linear Variable Differential Transformer (LVDT) and Half-Bridge are two alternative approaches to the coil format and are described in this section. LVDT and Half-Bridge transducers convert the movement of a core within the magnetic field produced by an energised coil into a detectable electrical signal.
Conventional Half Bridge The Half Bridge transducer forms half of a Wheatstone bridge circuit, which enables change from null to be readily determined. The other half of the bridge is built into the amplifier. When the core is in a central position, the two signals VAand VB are equal. As the core is displaced, the relative inductance of the two windings changes producing a complimentary change in VA and VB.
Conventional LVDT When the core is in a central position, the coupling from the primary (VEXC) to each secondary is equal, so VA=VB and the output Vout=0. As the core is displaced VA differs from VB, and the output VOUT changes in magnitude and phase in proportion to the movement.
Solartron Half Bridge and LVDT Transducers Solartron Metrology's continuous development of precision bobbin mouldings and multi chambered coil windings ensure excellent linearity and thermal stability throughout the range.
Solartron Orbit digital transducers Solartron Metrology digital transducers are calibrated using a traceable interferometer and are issued with a calibration certificate. All digital transducers are fitted with integrated electronics, which store information such as probe ID, range, calibration error, etc. Digital transducers provide superior performance compared to traditional analogue transducers.
