What Is a Linear Variable Differential Transformer?

LVDT (Linear Variable Differential Transformer) is an abbreviation of Linear Variable Differential Transformer, which belongs to linear displacement sensors. The working principle is simply an iron core movable transformer. It consists of a primary coil, two secondary coils, an iron core, a coil bobbin, and a housing. During the LVDT operation, the movement of the iron core cannot exceed the linear range of the coil, otherwise non-linear values will be generated, so all LVDTs have a linear range.

LVDT

LVDT (Linear Variable Differential Transformer) is an abbreviation of Linear Variable Differential Transformer, which belongs to linear displacement sensors. The working principle is simply an iron core movable transformer. It consists of a primary coil, two secondary coils, an iron core, a coil bobbin, and a housing. The primary coil and the secondary coil are distributed on the coil bobbin, and a freely movable rod-shaped core is arranged inside the coil. When the iron core is in the middle position, the induced electromotive forces generated by the two secondary coils are equal, so that the output voltage is zero; when the iron core moves inside the coil and deviates from the center position, the induced electromotive forces generated by the two coils are different, and there is a voltage Output, its voltage depends on the amount of 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 series in the design, and the voltage polarity of the two secondary coils is opposite. The voltage output by the LVDT is two secondary coils. The difference between the voltage and the output voltage is linearly related to the displacement of the core.

LVDT can be used to measure accurate data such as elongation, vibration frequency, amplitude, thickness and expansion of objects. It can also be used for the positioning of machine tools and hydraulic cylinders, as well as the control of roll gaps and valves. ^[1]

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

(1) Friction-free measurement

There is usually no physical contact between the LVDT's movable iron core and the coil, which means that the LVDT is a friction-free component. It is used for important measurements that can withstand light core loads but not friction loads. 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 core, no wear is caused. In this way, the mechanical life of LVDT is theoretically infinite. This is an extremely important technical requirement in applications such as fatigue testing of materials and structures. In addition, infinite mechanical life is equally important for high-reliability machinery in aircraft, missiles, spacecraft, and important industrial equipment. Therefore, LVDT is widely used in aero engine digital control system to accurately measure and control the displacement of throttle stick position, needle position, guide vane position and nozzle position.

(3) Unlimited resolution

LVDT's frictionless operation and its induction principle make it have two significant characteristics. The first feature is a truly infinite resolution. This means that the LVDT can respond to the smallest movements of the core and generate an output. The readability of external electronic devices is the only limitation on resolution.

(4) Zero position repeatability

The LVDT structure is symmetrical and the zero position can be recovered. LVDT's electrical zero position is highly repeatable and extremely stable. Used in high-loss-loss closed-loop control systems, the 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 cores that do not follow a precise linear movement. For example, the LVDT can be coupled to the end of a Bourdon tube to measure pressure.

(6) Durable

The materials used to make LVDTs and the processes used to join them make it a robust sensor. LVDT can continue to play its role, even when subjected to strong shocks and large vibrations often found in industrial environments. The core and the coil are separated from each other. A non-magnetic spacer is inserted between the core and the inner wall of the coil to isolate the pressurized, corrosive or alkaline liquid from the coil group. In this way, the coil group is hermetically sealed, and there is no longer a need to dynamically seal the moving members. For coil sets in pressurized systems, 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, sealed LVDTs have a stainless steel housing that can be placed in corrosive liquids or gases. Sometimes LVDTs are required to work in extremely harsh environments. For example, in low temperature environments like liquid nitrogen. For another example, an LVDT working in the main containment of a nuclear reactor has an operating temperature as high as 550 ° C, plus 10 Rads of radiation and / or 3X10 NVT neutron flux. Another example is an LVDT that works in 210bar pressure fluid. LVDT is cleverly designed to adapt to a variety of harsh environments at the same time. However, it is important to note that although in most cases LVDTs have an infinite working life (in theory), LVDTs placed in harsh environments will have different working life due to different environments.

(8) Input / output isolation

LVDT is considered a type of transformer because its excitation input (primary) and output (secondary) are completely isolated. LVDT does not require a buffer amplifier and can be considered as an effective analog signal computing element. In an efficient measurement and control loop, its signal line is separated from the power ground.

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 with respect to the input being measured. Special adjustment techniques can be used to improve the stroke-to-case length ratio and non-linearity. One of the techniques is to add a microcontroller for correction. LVDT has good repeatability, and this technique is feasible.

Although LVDT has been around for many years, it is still an effective solution for many position sensing problems. The rugged construction provides high reliability, and its performance is ideal 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 to be introduced next.

RVDT (Rotary Variable Differential Transformer) is an abbreviation of Rotary Variable Differential Transformer.

LVDT displacement sensors cannot be used alone. Generally, supporting display instruments are needed to complete the conversion and transmission of LVDT sensor data. LVDT is generally used in conjunction with digital display instruments to convert the displacement voltage of LVDT into displacement digital quantity for visual display and processing. Or transfer data to PLC or computer through data bus, including RS232, RS485, USB, Ethernet, or WIFI. Then you can use the SPC software to analyze and store the data of the measured object. If the measurement system is applied to the production line, according to the measurement results, a PLC or a computer can issue control instructions to discharge the measured and unqualified workpieces out of the production line for disposal as scrap or repaired products.

The two most commonly applied principles of LVDT and their applications:

The DC rebound type LVDT has excellent performance. It uses a convenient single power supply of 9-28V DC. The measuring contact probe of the product is made of wear-resistant chrome-plated hard tool steel. The 304 stainless steel metal tube can work in harsh environments such as humidity and dust. The output signal is standard 0-5V or 4-20mA output that can be used by computers or PLCs.

Differential transformer type displacement sensors (LVDT) can be widely used in aerospace, machinery, construction, textile, railway, coal, metallurgy, plastics, chemical and scientific research institutions and other national economic industries to measure elongation and vibration High-tech products such as object thickness and expansion.

l Shaft diameter runout detection

l Valve position detection and control

l Roll gap clearance control

l Metal processing inspection

DC drag-type LVDT has excellent performance. It adopts convenient single power supply 9-28V DC for power supply. The electronic circuit is sealed in 304 stainless steel metal tube. It can work in harsh environments such as humidity and dust. 0-5V or 4-20mA output used by PLC.

l Machine tool and tool positioning

l Hydraulic cylinder positioning

l Roll gap clearance control

l Valve position detection and control

LVDT is the position of the steam turbine through the coil