The linear variable differential transformer ( LVDT ) (also called linear variable displacement transformer , linear variable displacement transducer , or only differential transformer ) is a type of electrical transformer used to measure linear displacement (position). The companion of this device used to measure rotary movement is referred to as a rotary variable differential transformer (RVDT).
Video Linear variable differential transformer
Introduction
LVDT is a strong, absolute linear position/displacement transducer; inherently without friction, they have an almost limitless life cycle when used appropriately. Since LVDT-operated air conditioners do not contain any electronics, they can be designed to operate at cryogenic temperatures or up to 1200 ° F (650 ° C), in harsh environments, under high vibration and shock levels. LVDT has been widely used in applications such as electric turbines, hydraulics, automation, aircraft, satellites, nuclear reactors, and many others. This transducer has low hysteresis and excellent repetition.
The LVDT changes the position or linear displacement of a mechanical reference (zero, or zero position) into a proportional electrical signal containing phase (for direction) and amplitude (for distance) information. The LVDT operation does not require electrical contact between the moving parts (probe or core assembly) and the coil assembly, but depends on the electromagnetic coupling.
Maps Linear variable differential transformer
Operation
The linear variable differential transformer has three solenoidal coils placed end to end around the tube. The middle coil is the main one, and the two outer windings are upper and lower secondary. The ferromagnetic core is cylindrical, attached to an object whose position must be measured, sliding along the tube axis. The alternating current moves the primer and causes the voltage induced in each proportional secondary to the length of the nucleus that connects to the secondary. Frequencies are usually in the range of 1 to 10 kHz.
As the core moves, the primary relationship to the two secondary windings changes and causes the induced voltage to change. The coil is connected so that the output voltage is the difference (hence "differential") between the upper secondary voltage and the lower secondary voltage. When the core is in its center position, equidistant between two seconds, the same voltage is induced in two secondary windings, but the two signals cancel, so the output voltage is theoretically zero. In practice, minor variations in the way in which the primers are connected to each secondary means that small voltages are removed when the nucleus is central.
This small residual voltage is caused by phase shift and is often called quadrature error. This is a disruption in a closed loop control system as it may lead to oscillations about the zero point, and may not be acceptable in a simple measurement application as well. This is a consequence of using synchronous demodulation, with a direct reduction of the secondary voltage in the AC. Modern systems, especially those involving safety, require error detection of LVDT, and the normal method is to demodulate each secondary separately, using half-wave precision or full wave rectifier, based on the op-amp, and calculate the difference by reducing the DC signal. Because, for the constant excitation voltage, the sum of two secondary voltages is almost constant throughout the LVDT operation stroke, its value remains in the small window and can be monitored such that any internal failure of LVDT will cause the total voltage to deviate from its limit and quickly detect, causing errors to indicated. There is no quadrature error with this scheme, and the voltage difference depends on the position passing smoothly through zero at the zero point.
Where digital processing in the form of a microprocessor or FPGA is available in the system, it is customary for the processing device to carry out fault detection, and possibly ratiometric processing to improve accuracy, by dividing the difference in secondary voltage by the sum of the secondary voltages, to make independent measurements of the amplitudes the right excitation signal. If sufficient digital processing capacity is available, it is common to use this to produce sinusoid excitation via DAC and may also perform secondary demodulation via multiplex ADC.
When the core is moved to the top, the voltage in the upper secondary coil increases when the voltage at the bottom decreases. The resulting output voltage increases from zero. This voltage is in phase with the primary voltage. When the nucleus is moving in the other direction, the output voltage also increases from zero, but the phase is opposite to the main. The phase of the output voltage determines the direction of displacement (up or down) and the amplitude shows the number of displacements. The sync detector can determine the signed output voltage associated with displacement.
The LVDT is designed with long slender coils to make the output voltage essentially linear over the displacement of up to several inches (several hundred millimeters) in length.
LVDT can be used as an absolute position sensor. Even if the power is off, when restarted, LVDT shows the same measurement, and no position information is lost. The biggest advantage is repetition and reproducibility once configured correctly. Also, regardless of the uni-axial linear movement of the core, any other movement such as the rotation of the nucleus around the axis will not affect its measurement.
Since the sliding core does not touch the inside of the tube, it can move without friction, making LVDT a very reliable tool. The absence of sliding or spinning contacts allows LVDT to be completely closed to the environment.
LVDT is typically used for position feedback in servomechanisms, and for automatic measurements in machine tools and many other industrial and scientific applications.
See also
- dot convention
- The linear encoder
- Rotary encoder
References
- Baumeister, Theodore; Marks, Lionel S., eds. (1967), Standard Handbook for Mechanical Engineering (Seventh ed.), McGraw-Hill, LCCNÃ,16-12915
External links
- How LVDT Works (Interactive)
- How LVDT Works
- Explain Explanation
- LVDT models and apps
- Analog Devices AD598 datasheet
- http://www.meas-spec.com/downloads/LVDT_Selection,_Handling_and_Installation_Guidelines.pdf Selected LVDT, handling and installation guidelines; explains the significant parameters in LVDTs implementation
- http://www.meas-spec.com/downloads/Principles_of_the_LVDT.pdf LVDT: construction and operating principle
- http://www.meas-spec.com/downloads/LVDT_Technology.pdf March 2013; internal construction drawings; rejection of cross axis, shield, corrosion problem
Source of the article : Wikipedia
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