LVDT (Linear Variable Differential Transformer) sensor : A differential transformer-type linear displacement sensor used for liquid level measurement, metering valve opening measurement in engines and gas turbines, and pressure sensors in conjunction with strain gauges. LVDT sensors are the most widely used sensors in hydraulic control systems.
RVDT sensor (Rotary Variable Differential Transformer): A differential transformer-type angular displacement sensor, typically used to collect displacement signals from the control stick, control wheel, rudder pedals, etc., and to display control surface position signals (such as rudder, elevator, aileron, flaps, spoilers, etc.) and convert them into angle values for display.
LVDT and RVDT sensors are recommended for use in applications requiring high reliability and tolerance to harsh environments.
Brief Introduction to the Principles of LVDT and RVDT Sensors
An LVDT sensor mainly consists of a core, frame, excitation winding, output winding, connecting rod, baffle, and housing. The excitation winding and output coils are located on the frame, while the rod-shaped core is located inside the coils and can move freely. When the core is in the center, the two output coils generate the same induced electromotive force, resulting in a zero output voltage. When the core deviates from the center but remains within the coils, the two output coils generate unequal induced electromotive forces, resulting in a non-zero output voltage. The magnitude of the output voltage is determined by the amount of displacement.
Figure 1: LVDT sensor structure diagram
An RVDT sensor is essentially a movable transformer with an iron core, consisting of a primary coil, two secondary coils, an iron core, a coil frame, and a housing. When the iron core is in the middle position of the coil frame, the electromotive forces V1 and V2 output by the two secondary coils are equal, and the output voltage is zero. When the iron core deviates from the middle position, the electromotive forces V1 and V2 of the two secondary coils are not equal, resulting in a certain output voltage.
The left image below shows the structure of a classic 4-pole RVDT, while the right image shows a schematic diagram of a three-phase 6/4 structure switched reluctance motor. The comparison reveals structural similarities between the RVDT and the switched reluctance motor, offering design advantages for both.
Figure 2: Classic 4-pole RVDT structure diagram and schematic diagram of a three-phase 6/4 structure switched reluctance motor
Ignoring the parasitic capacitance of the coil and the losses generated by the iron core, the equivalent schematic diagrams of LVDT and RVDT can be as follows:
Figure 3: Equivalent schematic diagram of LVDT and RVDT
When the primary coil is excited by a voltage of appropriate frequency, an induced electromotive force will be generated in the two secondary coils according to the working principle of a transformer. Assuming that the transformer structure is perfectly symmetrical in terms of manufacturing process, then when the core is in its initial equilibrium position, the mutual inductance coefficients of the two secondary coils will be equal, meaning that the output voltage of the differential transformer is zero at this time. When the core position moves, the output voltage of the two output coils connected in reverse series is linearly related to the core displacement.
The sensor body of LVDT and RVDT consists of an external demodulation circuit. The demodulation circuit is divided into phase-sensitive demodulation circuit and phase-shift-free demodulation circuit. Currently, phase-shift-free demodulation circuit is the main type used.
Advantages of LVDT and RVDT sensors
01. Highly adaptable to various environments, robust and durable.
LVDT and RVDT sensors are non-contact displacement sensors; there is no frictional contact between the coil and the iron core, eliminating any wear. Excluding level conversion and conditioning modules, the sensor body is entirely mechanical, exhibiting extremely high reliability. A certain type of RVDT sensor was deployed in an aircraft airborne environment (24 units per aircraft). From its test flight onwards, over nearly three years, it accompanied six aircraft on a total of 4478 takeoffs and landings/1153 hours and 46 minutes of flight time, without a single one returning from the field.
02. Stable zero position and high precision
The primary and secondary coils are electrically isolated, and the accuracy is almost unaffected by load, common-mode voltage, or input harmonics and noise.
03. High impact resistance and high vibration resistance limit
LVDT and RVDT sensors can be installed on aircraft engines and subjected to shock and vibration tests.
04. Good dynamic performance
Theoretically, it has infinite resolution. It has good dynamic performance and can be used for high-speed online detection.
The table below shows a comparison of the advantages and disadvantages of several displacement sensors; you can read it carefully.
