What are LVDT, RVDT, and their comparison?

Linear variable differential transformer LVDT and rotary variable differential transformer RVDT are two specialized transformers.

A basic LVDT and RVDT comprise one primary coil, two secondary coils, and a movable core. If a first coil across which the AC voltage applied is called the Primary coil and two coils that produce output are called secondary coils.

LVDT and RVDT are position sensors and non-contact type. The suitable mechanical arrangement is done to keep the core in a noncontact position in the coil to keep throughout the stroke.

LVDT:

LVDT is an analog inductive passive transducer used as a primary transducer and also used as a secondary transducer in some applications.

LVDT consists of a primary winding P and secondary windings S1, S2 wound on a hollow cylindrical bobbin.

The secondary has an equal number of turns, connected in series opposition so that emf induced int he coils opposite to each other.

The primary winding is connected to an alternating current (AC) source. A movable straight, linear ferromagnetic iron core slides inside the cylindrical bobbin.

The position of the movable core determines the flux linkage between an excited (AC) primary winding and each of the two secondary windings.

As shown in the above figure, at the null position the output voltage is zero.

When the position of the core moves towards the B-side (right), the voltage increases.

As the position of the core moves towards the A-side (left), negative voltage increases.

RVDT:

RVDT operates on the same principle as LVDT with the exception that a rotary ferromagnetic core is used.

One primary winding P and two secondary windings S1 and S2 connected in series internally and differential output is available. The core is cam-shaped and rotated inside the windings through the shaft.

The prime use of RVDT is for angular displacement.

RVDTs are capable of continuous 360 degrees rotation but linear over the range +/- 40 degrees.

The core is rotated in clockwise and anti-clockwise directions with respect to the null position.

When the core at the null position the output voltage is zero because the voltage induced in S1 and Voltage induced in S2 are in equal and opposite directions. Differential voltage gets developed across the secondary windings at the output when an angular displacement which is to be measured with respect to the null position is applied to the shaft.

When the core is rotated in a clockwise direction, the voltages increase. If the core is rotated in opposite direction, the voltages increase but there is a phase opposition in the voltages developed.

In both directions, the voltages are increasing but there is a phase opposition with respect to the rotating shaft (clockwise and anti-clockwise).

Comparison of LVDT and RVDT

LVDT

  • Converts linear displacements into electrical voltages.
  • Its core is placed in a linear fashion.
  • Used for converting left side and right side linear displacements in electrical voltages.

RVDT

  • Converts angular displacements into electrical signals.
  • Its core is cam-shaped.
  • Used for converting clockwise and anticlockwise angular motions into electrical signals.

Advantages of LVDT:

  • Good linearity. The relationship is linear between input displacement and output voltage.
  • LVDT can withstand a high degree of shock and vibration.
  • Useful for displacement measurements ranging from 1.25 mm 250 mm to electrical signal.

Applications of LVDT:

  • It is used as a secondary transducer in measuring force, pressure, and load.
  • Used in thickness measurement, level indicators.

Advantages of RVDT:

  • It can measure slight angle changes precisely.
  • RVDT can withstand extreme environmental conditions.
  • Produce high voltage changes with slight movement.

Applications of RVDT:

  •  It can be used in Aircraft, Oil drilling, hydraulics.

Author: PSS Bapu Rao

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