LVDT - Linear Variable Differential Transformer Basics

Catalog

Ⅰ Definition and principle

LVDT belongs to the linear displacement sensor. It consists of a primary coil, two secondary coils, iron core, coil frame, shell, and other components. The primary coil and the secondary coil are distributed on the coil frame, and there is a freely movable rod-shaped iron core inside the coil. When the iron core is in the middle position, the induced electromotive force generated by the two secondary coils is equal, so the output voltage is zero; when the iron core moves inside the coil and deviates from the center position, the induced electromotive force generated by the two coils is not equal, and the output voltage depends on the displacement. In order to improve the sensitivity of the sensor, improve the linearity of the sensor, and increase the linear range of the sensor, the two coils are connected in reverse. So the voltage polarity of the two secondary coils is opposite and the voltage output by the LVDT is the difference between the two secondary coils. This output voltage value has a linear relationship with the displacement of the iron core.

Working of LVDT

The working circuit of the LVDT is called a regulating circuit or a signal conditioner. A typical regulation circuit should include a voltage regulator circuit, a sine wave generator, a demodulator, and an amplifier. The sine wave generator should have a constant amplitude and frequency, and not be affected by time and temperature. The sine can be generated by a Wien bridge, square wave, step wave, filtered, or other suitable methods.

The demodulator can be a simple diode structure. When the AC output of the LVDT secondary coil is greater than 1VF.S, a simple diode demodulator is used. If the signal amplitude is lower than this value, due to the difference in the forward voltage of the two diodes, there will be temperature sensitivity problems, but for larger signal voltages, the effect of diode errors is not obvious. A synchronous demodulator can also be used. In the synchronous demodulator, two FETs switch alternately, and the timing is synchronized with the sine wave that supplies the primary power. The amount of phase shift required between the primary and the demodulator switch depends on the LVDT index and the wire length between the LVDT and the signal conditioner.

Sine wave generators, demodulators, and amplifier circuits have been combined into commercial ICs. The use of these devices will greatly simplify the design of LVDT signal conditioners. The most commonly used are NE5521 produced by Philips and AD598/698 produced by ADI. In addition, the appearance of standard analog and digital devices in fine-pitch packages has simplified the circuit design and can be fixed inside the LVDT housing.

Ⅱ Advantages and features

LVDT has many commendable advantages and features, and has a wide range of applications:

(1) Frictionless measurement

There is usually no physical contact between the movable iron core and the coil of the LVDT, which means that the LVDT is a frictionless part. It is used for important measurements that can bear the load of a lightweight iron core but cannot bear the friction load. Two examples are impact deflection or vibration testing of precision materials, or tensile or creep testing of fibers or other highly elastic materials.

(2) Unlimited mechanical life

Since there is no friction and contact between the LVDT's coil and its iron core, there will be no wear. In this way, the mechanical life of the LVDT is theoretically unlimited. This is an extremely important technical requirement in applications such as fatigue testing of materials and structures. In addition, unlimited mechanical life is also important for highly reliable mechanical devices in aircraft, missiles, spacecraft, and important industrial equipment. Therefore, LVDT is widely used in the aero-engine digital control system to accurately measure and control the position of the throttle lever, the position of the oil needle, the position of the guide vane, and the position of the nozzle.

(3) Unlimited resolution

LVDT's frictionless operation and its sensing principle make it possess two notable characteristics. The first feature is a truly infinite resolution. This means that the LVDT can respond to the smallest movement of the core and generate output. The readability of external electronic devices is the only limit to the resolution.

(4) Zero repeatability

The LVDT structure is symmetrical and the zero position can be restored. The electrical zero position of LVDT is highly repeatable and extremely stable. It is used in high profit and loss closed-loop control systems. LVDT is an excellent electrical zero indicator. It is also used in a ratio system where the composite output is proportional to the two independent variables of the zero position.

(5) Axial suppression

LVDT is very sensitive to the axial movement of the iron core, and the radial movement is relatively slow. In this way, the LVDT can be used to measure iron cores that do not move in a precise straight line. For example, the LVDT can be coupled to the end of the Bourdon tube to measure pressure.

(6) Durable

The materials used to make the LVDT and the processes used to join these materials make it a robust sensor. LVDT can continue to function even if it is subjected to strong shocks and large vibrations that are often found in industrial environments. The iron core and the coil are separated from each other, and a non-magnetic spacer is inserted between the iron core and the inner wall of the coil to isolate the pressurized, corrosive or alkaline liquid from the coil assembly. In this way, the coil assembly is hermetically sealed, and it is no longer necessary to dynamically seal the moving components. For the coil assembly in the pressurized system, only static sealing is required.

(7) Environmental adaptability

LVDT is one of the few sensors that can work in a variety of harsh environments. For example, the sealed LVDT uses a stainless steel housing and can be placed in corrosive liquids or gases. Sometimes, LVDTs are required to work in extremely harsh environments. For example, in a low temperature environment similar to liquid nitrogen. For another example, the LVDT working in the main containment of a nuclear reactor has an operating temperature of 550°C, plus 10 Rads of radiation and/or 3X10 NVT of neutron flux. Another example is the LVDT working in 210bar pressure-bearing fluid. LVDT is cleverly designed and can adapt to a variety of harsh environments at the same time. However, it is important to note that although in most cases, LVDTs have unlimited working life (in theory), LVDTs placed in harsh environments have different working life due to different environments.

(8) Input/output isolation

LVDT is considered to be a kind of transformer because its excitation input (primary) and output (secondary) are completely isolated. LVDT does not need a buffer amplifier and can be considered as an effective analog signal calculation component. In an efficient measurement and control loop, its signal wire is separated from the power ground wire.

Ⅲ Conclusion

As mentioned above, LVDT has many outstanding qualities. Its main limitation is that in order to obtain linear performance, the housing of the sensor is longer than the stroke, and the output signal has a certain non-linearity to the input is measured. The use of special adjustment techniques can improve the stroke-to-shell length ratio and non-linear problems. One technique is to add a microcontroller for correction. LVDT has good repeatability, this technique is feasible.

Although LVDT has been around for many years, it is still an effective solution to many position sensing problems. The sturdy structure provides high reliability, and its performance is very suitable for most applications with strokes less than ±100mm.

LVDT can also be made into a rotating device. The working method is similar to the linear model, except that the processed iron core moves along a curved path. This is the RVDT. RVDT is the abbreviation of the rotary variable differential transformer, which belongs to the angular displacement sensor. It uses the same differential transformer principle as LVDT, that is, the rotation of the mechanical parts is transmitted to the axis of the angular displacement sensor, and the spoiler/iron core connected to it is driven to change the induced voltage/inductance in the coil and output Voltage/current signal proportional to the rotation angle.

RVDT non-contact design has the characteristics of unlimited resolution, long service life, and high accuracy. It can realize 360° rotation measurement. It is widely used in the transmission and feedback control of ball valve position, hydraulic pump, forklift, robot, fan, and other equipment.