What is LVDT?
LVDT linear differential transformer displacement sensor detailed introduction
LVDT is the abbreviation of Linear Variable Differential Transformer, a type of electromechanical converter. LVDT can convert the mechanical change of the linear motion of an object into the corresponding electronic signal. The LVDT linear position sensor (LVDT displacement sensor) can measure displacements as small as several millionths of an inch to several inches, but it can also measure displacements of ±20 inches.
figure 1
Figure-1 shows the components of a typical LVDT displacement sensor. The internal structure of the transformer includes a primary coil placed in the middle of a pair of identical secondary coils. The primary coil and the secondary coil have symmetrical inter-rows. These coils are wound on a hollow thermoplastic glass fiber reinforced polymer. In order to prevent moisture, they are covered with a high-permeability separator, and then sealed in a cylindrical stainless steel tube. This coil combination is usually an essential component of the position sensor.
The movable component of the LVDT is a magnetically permeable tubular iron core. This iron core can move freely within the hollow-formed coil and integrate with the object to be measured. The aperture of the coil is so large that when the core is placed in it, it can still provide an effective gap, so that there is no substantial contact between the core and the coil.
In use, we add a certain frequency of alternating current to the primary coil, which is the so-called primary excitation. The output signal of the LVDT is the AC voltage difference between the two sets of secondary coils. This voltage difference will change with the position of the core in the coil. Usually for convenience of use, the AC output voltage is processed by an electronic circuit and converted into a high-level DC voltage or current.
How does the LVDT displacement sensor work?
figure 2
Figure-2 illustrates what happens when the core of the LVDT displacement sensor is in different positions. After the primary coil (P) of the LVDT is excited by the AC power supply of 2.5KHz～5KHz, the generated magnetic flux will be coupled to the adjacent secondary coils S1 and S2 via the iron core. If the core is located between S1 and S2, the same magnetic flux will be coupled to the secondary side. Therefore, the voltages E1 and E2 induced in each coil will also be equal. Therefore, when the core is at the center point, which is the so-called origin, the output voltage difference (E1-E2) should be equal to zero.
image 3
Figure-3 shows that the size of the output voltage difference Eout will change with the position of the iron core. The voltage value of Eout from the origin to the maximum displacement of the iron core depends on the magnitude of the primary excitation voltage and the sensitivity of the LVDT itself, usually several Vrms. The phase angle of the AC output voltage Eout related to the primary excitation voltage is a constant when the core is at the origin. As shown in Figure-4, the phase angle of the origin position changes steeply by 180 degrees.
With the proper circuit, the 180-degree phase displacement can be used to determine the direction of the core displacement from the origin. As shown in Figure-5, the polarity of the output signal represents the relative relationship between the core position and the origin. This figure also shows that when the iron core moves within the rated measurement range of the LVDT, its output signal is very linear. It is also possible to use it beyond the rated measurement range, but the linearity of the output signal will be reduced.
Electronic circuit with LVDT
Although LVDT is a kind of transformer, it needs an AC power supply with a fixed amplitude and frequency to operate normally. This power supply is different from the commonly known AC power supply, usually 3Vrms @2.5kHz ~ 3.0kHz. Supplying the excitation power required by the LVDT is one of the functions generally known as the LVDT signal amplifier. Other functions include converting the low-level AC output voltage of the LVDT into a high-level DC signal, which is more convenient to use. In addition, when the LVDT core passes through the origin, the phase detector can be used to determine the direction of core movement from a phase shift of 180 degrees, and the zero point level adjustment of the output signal. Figure-6 shows the cross-sectional view of the DC LVDT with the amplifier on the right. The module of the amplifier circuit should be protected by sealing compound, but for the sake of illustration, it is not shown.
Why use LVDT displacement sensor?
Due to the basic physical principle of the operation of the LVDT displacement sensor itself or the materials and technology used in its structure, it has certain characteristics and advantages.
Frictionless operation
One of the most important features of LVDT is frictionless operation. Under normal operation, there is no mechanical contact between the core and coil assembly structure of the LVDT, such as friction, drag, or other factors that can cause friction. This feature is particularly useful in material testing, vibration displacement measurement, and high-resolution dimensional measurement systems.
Infinite resolution
Because the LVDT is operated by the electromagnetic coupling principle of the frictionless structure, it can measure extremely small changes in the position of the iron core. This infinite resolution capability is only limited by the resolution of the LVDT signal amplifier and the number of bits of the output display. Therefore, LVDT also has excellent repeatability.
Infinite mechanical life
Generally, there is no contact between the core and the coil assembly structure of the LVDT, so there is no friction or wear between any components, which means that the LVDT has no mechanical life limit. This feature is particularly important in applications requiring high reliability, such as the installation of aircraft, satellites, space vehicles, and nuclear energy equipment. In addition, there is a high demand for this feature in many industrial process control and factory automation control systems.
Will not be damaged by exceeding the measurement range
The inner bore of most LVDTs is not closed at both ends. If the rated measurement stroke is exceeded due to careless operation, the iron core will completely pass through the sensor coil assembly structure without causing any damage. This feature of not being damaged by exceeding the displacement stroke makes LVDT an ideal sensor for material damage testing equipment, such as a tensiometer attached to the test component to test its tension. Refer to the HSTA750 series.
Single axis induction
LVDT only reacts to the movement of the iron core along the axis of the coil, and usually does not respond to the movement of the iron core transversely cross-axis or the radial position of the iron core. Therefore, for non-aligned installation or floating mobile applications, and when the LVDT movement is not very precise in a straight line, the LVDT can still work normally without any adverse effects.
Detachable coil and core
Because there is only the interaction of magnetic coupling between the iron core and the coil assembly of the LVDT, a non-magnetic tube can be inserted between the iron core and the tube wall of the inner bore to isolate the iron core and the coil assembly. Using this method, the pressurized fluid can be enclosed in the isolation tube, so that when the coil assembly is released, the core will move freely. Proportional valves or servo valves in hydraulic systems, as LVDTs for spool position feedback, often use this feature, refer to HSIR750 series.
Good environmental resistance
LVDT is a very rugged and durable sensor because of the materials used and the technology of the assembly structure, which can be used in a variety of different operating environments. After the coil winding is encapsulated with epoxy resin and placed in a stainless steel tube, it can achieve excellent moisture-proof and moisture-proof effects, and has good vibration and impact resistance. In addition, due to the internal high permeability shielding, the influence of external AC magnetic field effects is minimized.
The shell and core are made of corrosion-resistant metal materials, and the shell also has the effect of enhancing the magnetic field isolation. In some applications, when the sensor must be exposed to flammable or corrosive vapors and liquids, or pressurized fluids for operation, various welding processing methods can be used to completely seal the entire housing and coil assembly structure.
Generally, the operating temperature range of LVDT is very wide, but if necessary, it can also be specially customized and used in low temperature environment. Or use special materials, which can be applied to applications requiring high temperature and radiation resistance such as nuclear reactors.
Zero point repeatability
The zero position of the LVDT itself is very stable and has high repeatability, even if it exceeds its applicable operating temperature range. This feature makes LVDT a good zero position sensor for applications in closed-loop control systems and high-performance servo balance instruments.
Fast dynamic response
Under normal operating conditions, this frictionless feature enables the LVDT to quickly reflect the change in the position of the core. The dynamic response of LVDT itself is limited only by the inertial effect of the tiny mass of the iron core. Generally, the response speed of the LVDT sensing system depends on the characteristics of the amplifier.
Absolute value output
LVDT is an absolute value output component, which is different from an incremental output component. This means that even when the power is turned off, the position data sent by the LVDT will not disappear.