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Displacement Measurement Using LVDTs

This article describes how an LVDT sensor works, the process measurement applications that it can be used for.

The LVDT Sensor

Linear Variable Differential Transformers are position sensors that are used in industrial applications where reliability is vital. Unlike most position sensors (e.g. potentiometers) LVDTs have no moving electrical contacts which will eventually wear out or corrode. Typical LVDT measurement ranges are ±0.25mm up to ±600mm and can have IP68 protection. This allows their use in very harsh conditions, including submersible and many aerospace applications.

LVDT sensors are transformers that operate on the electromagnetic coupling between a primary coil and two secondary coils, which are wound out of phase with each other. An AC current energizes the primary coil typically at between 1-10KHz and 2-5V amplitude. The LVDT's output signal is a differential AC voltage between the two secondary windings, which varies with the position of the core (armature) within the coils. The phase of the output indicates the core position relative to the null (middle) position of the core.


The Tracker 260 LVDT Signal Conditioner / Indicator

The Tracker 260 accepts AC outputs from LVDT in 2-wire, 3-wire (ratio-metric) and 4-wire (ratio-metric). There is a "Null Position" feature that allows the user to quickly find the electrical mid position of the core. This allows the user to position the LVDT so that the required measurement is taken through the most linear portion of the transducer range.

The Tracker 260 can be quickly calibrated to any engineering units and has user selectable sensor excitation frequency and amplitude, analogue outputs and serial communications, including Modbus RTU, all fitted as standard. Alarm outputs are also available as an option.

The Colour of Light

Combinations of LVDT sensors and Tracker 260s are used in Beamline applications where the angle of a silicon coated monochromatic mirror converts the hugely bright "white light" of synchrotron radiation to the required colour temperature for a particular application or experiment. Small motors move the mirror and its position measured by LVDTs. The LVDTs are connected to the Tracker 260s, which gives a local indication and feedback to the beamline control system via analogue or digital links. Mirror systems can have has up to five mirror position motors (mirror height, pitch, roll, yaw and horizontal adjustment).