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How it Works - 4 to 20mA LVDT

animation of LVDT transducer showing how an LVDT works
Primary excitation
Secondary 1
Secondary 2
Secondary 1 + Secondary 2
An LVDT Displacement Transducer comprises 3 coils; a primary and two secondaries.

The transfer of current between the primary and the secondaries of the LVDT displacement transducer is controlled by the position of a magnetic core called an armature.

On our position measurement LVDTs, the two transducer secondaries are connected in opposition.

At the centre of the position measurement stroke, the two secondary voltages of the displacement transducer are equal but because they are connected in opposition the resulting output from the sensor is zero.

As the LVDTs armature moves away from centre, the result is an increase in one of the position sensor secondaries and a decrease in the other. This results in an output from the measurement sensor.

With LVDTs, the phase of the output (compared with the excitation phase) enables the electronics to know which half of the coil the armature is in.

The strength of the LVDT sensor's principle is that there is no electrical contact across the transducer position sensing element which for the user of the sensor means clean data, infinite resolution and a very long life.

An oscillator/demodulator circuit built into the displacement transducer is powered by and controls the 4-20mA output.
As the transducer contains internal signal conditioning electronics, there is no need for external signal conditioning.