External measuring amplifier LVDT

An LVDT displacement sensor consists of a corrosion-protected, magnetically shielded hollow body, a primary coil (primary winding), two secondary coils connected in series (secondary winding) and a soft iron core (iron-nickel alloy with high magnetic permeability) . The push rod can be moved axially within the hollow body without contact via a coil, using the inductive measuring principle as a "linear variable differential transformer": LVDT.

The primary winding is typically supplied with an AC voltage of constant frequency (excitation voltage or primary voltage) in the range of 1-10 kHz. Depending on the position of the soft iron core, this alternating voltage is induced in the secondary coils of the two secondary windings connected in series in opposite phase.

If the soft iron core is exactly in the middle of the secondary coil, the magnetic field of the secondary coil is cancelled out and the output voltage is 0 V. If the soft iron core is axially displaced, the induced voltage is directional. The voltage on one secondary coil increases steadily while the voltage on the other secondary coil decreases steadily.

An LVDT is a differential transformer that is operated with an alternating voltage (AC) and also produces an alternating voltage (AC) as an output signal. For versatile output signals, inductive displacement transducers are supplied with corresponding evaluation electronics such as IMA2LVDT or integrated measuring amplifier electronics. The latter consists of an oscillator for generating the excitation voltage, a demodulator, a difference former and an output amplifier with filter. Due to the different output voltage ranges that can be realised, displacement transducers can be easily adapted to a wide variety of measuring, control and regulating devices. The voltage ratio is differentially evaluated by the electronics and usually converted into a standardised output signal (0...10 V, 4...20 mA, etc.). LVDT sensors have very good linearity within the specified measuring range. Supply voltage - output voltage: For inductive systems with external amplifiers, an input voltage of 10 VRMS from 500 Hz to 5 kHz is required. DC-DC types are normally fixed at ±15 V or 24 V DC.

However, if, for example, a potentiometric displacement transducer has a DC power supply and further processing electronics for the DC measurement signal, the inductive displacement transducer has DC electronics that can generate and convert AC voltages in the transducer circuit (oscillator). Feedback of the measuring signal to a DC voltage (demodulator) as schematically shown in sketch 2. The displacement transducer DG is supplied with a DC voltage of ±15 V or 24 V, 0..5 V / ±5 V / 0..10 V / ±10 V / 0..20 mA / 4..20 mA . As a measured value at the output.