LVDT- Linear Variable Differential transformer

LVDT- Linear Variable Differential transformer

Position and displacement may be sensed by methods of electromagnetic induction. The most commonly used inductive transducer to convert linear motion into electrical signals is the Linear Variable Differential Transformer or LVDT as they are commonly known.

An LVDT consists of one primary coil and two secondary coils. The primary coil carries ac excitation (Vref) that induces a steady ac voltage in the secondary coils. The induced amplitude depends on flux coupling between the coils.

There are two techniques for changing the coupling.

• One is the movement of a core made of ferromagnetic material within the flux path. This changes the reluctance of the path, which, in turn, alters the coupling between the coils. This is the basis for the operation of a LVDT (linear variable differential transformer), a RVDT (rotary variable differential transformer).
• The other method is to physically move one coil with respect to another.

Generally, the core is made of high permeability, nickel iron which is hydrogen annealed. It gives low harmonics, low null voltage.

The ac excitation in the primary coil produces an alternating magnetic field which induces an alternating current in the secondary coils. The secondary coils are connected in series opposition. Hence the polarity of voltage induces is opposite in the two secondary coils. When the core is positioned in the magnetic center of the transformer, the secondary output signals cancel and there is no output voltage. This is known as “null position”.

Moving the core away from the central position unbalances the induced magnetic flux ratio between the secondaries, developing an output. As the core moves, reluctance of the flux path changes. Hence, the degree of flux coupling depends on the axial position of the core. At a steady state, the amplitude of the induced voltage is proportional to the core displacement. Consequently, voltage may be used as a measure of a displacement. The LVDT provides the direction as well as magnitude of the displacement. The direction is determined by the phase angle between the primary (reference) voltage and the secondary voltage. Excitation voltage is generated by a stable oscillator.

The LVDT is considered a passive transducer because the displacement of the core, which is being measured, itself provides energy for changing the induced voltage in the secondary coil. Even though an external power supply is used to energize the primary coil, which in turn induces a steady voltage at the carrier frequency in the secondary coil, which is not relevant in the definition of a passive transducer.

Note: Because of opposed secondary windings, the LVDT provides the direction as well as the magnitude of displacement. When the output signal is demodulated, its sign gives the direction. If the output signal is not demodulated, the direction is determined by the phase angle between the primary (reference) voltage and the secondary (output) voltage, which includes the carrier signal.

For an LVDT to measure transient motions accurately, the frequency of the reference voltage (the carrier frequency) has to be at least 10 times larger than the largest significant (useful) frequency component in the measured motion, and typically can be as high as 20 kHz. For quasi-dynamic displacements and slow transients of the order of a few hertz, a standard ac supply (at 50/60 Hz line frequency) is adequate. The performance (particularly sensitivity and accuracy) is known to improve with the excitation frequency, however. Since the amplitude of the output signal is proportional to the amplitude of the primary signal, the reference voltage should be regulated to get accurate results. In particular, the power source should have a low output impedance.

Ideally the output at the null position should be equal to zero. However in practice there exists a small voltage at the null position. This may be due to presence of harmonics in the input supply  voltage, stray magnetic fields, temperature effect among other reasons.

Calibration and Compensation

An LVDT may be calibrated in millimeter per volt (mm/V), in its linear range. In addition, and a displacement offset (mm) may be provided. This typically represents the least squares fit of a set of calibration data. Since ambient temperature and other environmental conditions will affect the LVDT output, in addition to the primary and secondary coils, a reference coil may be available for compensation of the LVDT output.

Advantages of the LVDT include the following:

1.       It is essentially a non-contacting device with no frictional resistance. Lightweight core will result in very small resistive forces.

2.       Hysteresis (both magnetic hysteresis and mechanical backlash) is negligible.

3.       It has low output impedance, typically in the order of 100 Ω.

4.       It provides directional measurements (positive/negative).

5.       It is available in miniature sizes as well (e.g., length of 1 or 2 mm, displacement measurements of a fraction of a millimeter, and maximum travel or stroke of 1 mm).

6.       It has a simple and robust construction (inexpensive and durable)

7.       It has fine resolutions (theoretically, infinitesimal resolution; practically, much better than a coil potentiometer)

Disadvantages

1.      They are sensitive to stray magnetic field.

2.      Output signal need amplification due to its small magnitude.

3.      Measurement is affected by vibrations.

4.      Affected by temperature variation.