Inductive technology

6 Principle of operation Half bridge, LVDT and digital transducers 7

An LVDT inductive displacement transducer isconstructed using a static transformer (primarywinding) and two secondary windings. The windings are formed on a hollow bobbin through which a magnetic core can travel. The magnetic core provides a path for linking the coils via the magnetic flux. When the primary winding is connected to anAC 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 inseries opposition so that the two voltages V
V A V B The Linear Variable Differential Transformer (LVDT) and Half-Bridge are two alternativeapproaches to the coil format and aredescribed 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 electricalsignal.Conventional Half Bridge

Conventional half bridge Conventional LVDT Solartron half bridge and LVDT transducers

The Half Bridge transducer forms half of aWheatstone bridge circuit, which enableschange 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 V
V EXC Armature (core) Static transformer A and V
Secondary coil ASecondary coil B Primary coil B have opposite phase and thetransducer output is V
A -V
B .If the core is in the centre position thenvoltages of equal magnitude but oppositephase will be induced in each secondary coil and the net output is zero. As the core is moved in one direction, the voltage in the corresponding secondary coilincreases while the other coil experiences acomplementary voltage reduction. The net effect is a transducer voltage output that is proportional to displacement. Knowledge of the magnitude and the phase ofthe 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 ofdisplacement over its calibrated measurementrange. Beyond this range the output becomesincreasingly non-linear. Measurement range is defined as ±distance from the transducer null position.
A and V
B are equal.As the core is displaced, the relative inductanceof the two windings changes producing acomplimentary change in V
Linearity Linearity Linearity A and V
B .
Displacement Core Conventional LVDT When the core is in a central position, thecoupling from the primary (V
EXC ) to eachsecondary is equal, so V
A =V
B and the outputV
0UT =0. As the core is displaced V
A differsfrom V
V OUT = V A -V B B , and the output V
0UT changes inmagnitude and phase in proportion to themovement.
ACV EXC Solartron Half Bridge and LVDT transducers

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Voltage out Voltage out opposite phase Core at ACore at 0 (null)Core at B Solartron Metrology’s continuous development ofprecision bobbin mouldings and multichambered coil windings ensure excellentlinearity and thermal stability throughout the range. Solartron Orbit digital transducers Solartron Metrology digital transducers arecalibrated using a traceable interferometer andare issued with a calibration certificate. All digitaltransducers are fitted with integrated electronics, which store information such as probe ID, range, calibration error, etc. Digital transducers provide superior performance compared to traditionalanalogue transducers. Performance figuresquoted in this catalogue include all mechanical errors within the probe head together with any errors in the electronics interface modules.

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Measurement rangeAB