LVDT Sensor vs RVDT Sensor-Difference between LVDT and RVDT

This page on LVDT sensor vs RVDT sensor describes difference between LVDT sensor and RVDT sensor types. LVDT stands for Linear Variable Differential Transformer and RVDT stands for Rotary Variable Differential Transformer.

Both LVDT and RVDT are position or displacement sensors. Both LVDT and RVDT have following merits.
•  The sensor is a noncontact device with no or very little friction resistance with small resistive forces
•  hystereses (magnetic and mechanical) are negligible
•  output impedance is very low
•  there is low susceptibility to noise and interferences
•  its construction is solid and robust
•  infinitesimal resolution is possible

LVDT (Linear Variable Differential Transformer) sensor

LVDT sensor circuit diagram

Position and displacement may be sensed by methods of electromagnetic induction. A magnetic flux coupling between two coils may be altered by the movement of an object and subsequently converted into voltage. Variable-inductance sensors that use a nonmagnetized ferromagnetic medium to alter the reluctance (magnetic resistance) of the flux path are known as variable-reluctance transducers.

The basic arrangement of a multi-induction transducer contains two coils: primary and secondary. The primary carries ac excitation that induces a steady ac voltage in the secondary coil. The induced amplitude depends on flux coupling between the coils.

There are two techniques for changing the coupling. One is the movement of an object 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 sensor, RVDT sensor and mutual inductance proximity sensors. The other method is to physically move one coil with respect to another.

The LVDT is a transformer with a mechanically actuated core. The primary coil is driven by a sine wave (excitation signal) having a stabilized amplitude. The sine wave eliminates error-related harmonics in the transformer. An ac signal is induced in the secondary coils. A core made of a ferromagnetic material is inserted coaxially into the cylindrical opening without physically touching the coils. The two secondaries are connected in the opposite phase. When the core is positioned in the magnetic center of the transformer, the secondary output signals cancel and there is no output voltage. Moving the core away from the central position unbalances the induced magnetic flux ratio between the secondaries, developing an output. As the core moves, the 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, in the linear operating region, 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 full form of LVDT is Linear Variable Differential Transformer. For the LVDT sensor to measure transient motions accurately, the frequency of the oscillator must be at least 10 times higher than the highest significant frequency of the movement. For the slow-changing process, stable oscillator may be replaced by coupling to a power line frequency of 60 or 50 Hz.

One useful application for the LVDT sensor is in the so-called gauge heads, which are used in tool inspection and gauging equipment. In that case, the inner core of the LVDT is spring loaded to return the measuring head to a preset reference position.

RVDT (Rotary Variable Differential Transformer) sensor


The full form of RVDT is Rotary Variable Differential Transformer sensor. The RVDT sensor operates on the same principle as LVDT sensor, except that a rotary ferro-magnetic core is used. The prime use for the RVDT sensor is the measurement of angular displacement. The linear range of measurement is about +/-40 degree, with a nonlinearity error of about 1%.

The figure-2 depicts RVDT from Moog Components Group. It is basically an electro-mechanical transducer. It generates variable AC output voltage linearly proportional to angular displacement of input shaft.

When fixed AC source is used then output is linear within certain range over angular displacement.

Following table compares LVDT and RVDT sensors.

Measurement range between +/-100µm to +/-25 cm They are linear over range around +/-40 degrees.
Sensitivity is equal to 2.4 mV/Volt/degree of rotation sensitivity is equal to 2 to 3 mV/Volt/degree of rotation
Input voltages are from 1V to 24V rms (at frequency range from 50 Hz to 0 KHz) Input voltages have range of about 3V(at frequency range from 400Hz to 20 KHz)

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