Xi'an lekong Electromechanical Technology Major explains LVDT & rvdt for you

LVDT (linear variable displacement transformer) is the abbreviation of linear variable differential transformer. It is a common type of electromechanical sensor, which can convert the linear motion of the mechanically coupled object into the corresponding electrical signal. The LVDT linear displacement sensor is plug and play, and can measure all kinds of movements, from small to one millionth of an inch to several inches, and even up to ± 30 inches (± 0.762 meters). Figure 1 shows a typical LVDT component. The internal structure of the transformer consists of a primary winding and a pair of secondary windings wound in the same way. The two secondary windings are symmetrically distributed on both sides of the primary winding. The coil is wound on a heat stable one-piece insulating glass reinforced polymer, and then wrapped in a high permeability magnetic shielding layer, and then fixed in a cylindrical stainless steel sheath. The coil fitting is usually the static element of displacement sensor.

Figure 1: shows the primary winding in the center of the LVDT. The two secondary coils are symmetrically wound on both sides of the primary coil (as shown in the figure for the "short stroke" LVDT) or on the top of the primary coil (for the "long stroke" LVDT). The two secondary windings are usually connected in "reverse series" (differential).

It is a common type of electromechanical sensor, which can convert the linear motion of the mechanically coupled object into the corresponding electrical signal.

The moving element of the LVDT is a separate tubular armature made of permagnetic material. Generally known as the iron core, it can move freely along the axial direction in the hollow hole of the coil, and is mechanically coupled to the object at the position to be measured. The hole is usually large enough to provide a large radial gap between the core and the hole so that there is no physical contact between the core and the coil. During operation, the primary winding of the LVDT is energized by alternating current with appropriate amplitude and frequency. This process is called primary excitation. The electrical output signal of the LVDT is the differential AC voltage between the two secondary windings, which varies with the axial position of the iron core in the LVDT coil. In general, the AC output voltage is converted from an appropriate electronic circuit to a more convenient high-level DC voltage or current.

How does LVDT work?

Figure 2 shows what happens when the cores of the LVDT are in different axial positions. The primary winding P of the LVDT is energized by a constant amplitude AC source. The resulting magnetic flux is coupled from the core to the adjacent secondary windings S1 and S2. If the core is located in the middle of S1 and S2, equal magnetic flux is coupled to each secondary winding, so E1 and E2 contained in windings S1 and S2 are equal. At this reference intermediate core position (called zero), the differential voltage output (E1-E2) is essentially zero. As shown in Fig. 2, if the iron core is moved so that its distance from S1 is less than that from S2, the magnetic flux coupled to S1 will increase, while the magnetic flux coupled to S2 will decrease, so the induced voltage E1 will increase, while E2 will decrease, resulting in differential voltage (E1-E2). On the contrary, if the iron core moves closer to S2, the magnetic flux coupled to S2 will increase, while the magnetic flux coupled to S1 will decrease, so E2 will increase and E1 will decrease, resulting in differential voltage (e2-e1).

Figure 2: shows what happens when the cores of the LVDT are in different axial positions.

Figure 3A shows how the magnitude of the differential output voltage eout varies with the position of the core. The eout value of the maximum core displacement from zero depends on the amplitude of the primary excitation voltage and the sensitivity factor of a particular LVDT, but is usually several volts rms. The phase angle of the AC output voltage eout (with primary excitation voltage as reference) remains constant until the center of the core passes through zero, at which time the phase angle suddenly changes by 180 degrees, as shown in Figure 3B. The 180 degree phase shift can be used to determine the direction of the core leaving the zero point through the corresponding circuit. It is shown in Fig. 3C, where the polarity of the output signal represents the position relationship between the iron core and the zero point. The figure also shows that the output of the LVDT has good linearity over its specified core movement range, but the sensor can be used in a larger range, and the output linearity will be reduced.

Figure 3: the output characteristics of the LVDT vary with the position of the core. The full range output is a large signal (usually one volt or more) and usually does not require amplification. Note that the LVDT will continue to operate over 100% travel, but linearity will decrease.