LVDT, Half Bridge and Digital Transducer Theory
Principle of Operation
A LVDT Displacement Sensor works by moving the core through the body. The position of the core within the body is detected by coils wound on the bobbin. The coils are supplied with an AC signal and then returned an AC signal. The signal is then processed by conditioning electronics to provide a measure of the core position. The body is normally mounted on the static part of an element and the core attached to the moving part. 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 VA and VB are equal. As the core is displaced, the relative inductance of the two windings changes producing a complimentary change in VA and VB.
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 Metrology's continuous development of precision bobbin mouldings and multi-chambered coil windings ensure excellent linearity and thermal stability throughout the range.
Solartron Orbit® 3 digital sensors
Solartron Metrology digital sensors 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 sensors provide superior performance compared to traditional analogue sensors. Performance figures quoted in his catalouge include all mechanical errors within the prove head together with any errors within the probe head together with any errors in the electronics interface modules.