LVDT is an inductive device. It has a primary coil, two secondary coils, and a linearly movable core. The primary coil is driven by an AC excitation current, which creates a varying magnetic field. This changing field induces an AC voltage or current in the secondary coil (usually two or three windings) that can be measured and sent to an output circuit. It can measure linear displacement, force, acceleration, and pressure.
LVDT Working Principle
The LVDTs operate on changing the mutual inductance between primary and secondary coils. Figure 1 shows the internal design of an LVDT, its circuit diagram, and its typical output. The primary coil driven by an AC excitation current (typically several kHz) induces an AC current in each of the secondary coils (they are connected in the opposed phase). A ferromagnetic core, usually threaded to a moving object, is inserted coaxially into the tube opening without physically touching the coils.
When the core is in the center of the transformer (null position), the outputs from the two secondary coils are canceled out, and the net output voltage is zero. When the core moves away from the central position, a nonzero net output is produced due to unbalances in magnetic field in the secondary coils. There is also a phase change as the core moves. Thus, by measuring both the voltage amplitude and its phase angle, the direction and the displacement of the object motion can be determined.
An LVDT requires a circuit to generate a proper output. Figure 2 shows a simplified industry-standard AD598 LVDT signal conditioning circuit. Its on-chip oscillator can be set to generate a 20 Hz to 20 kHz excitation frequency through a single external capacitor.
The two secondaries’ outputs—V1 and V2, followed by two filters—are used to generate the ratiometric function (V1 − V2)/(V1 + V2). This function is independent of the amplitude of the primary winding’s excitation voltage, assuming the sum of the LVDT output voltage amplitudes remains constant over the operating range.
This is usually the case for most LVDTs except that the manufacturer specifies otherwise on the LVDT data sheet. This method often requires the use of a 5-wire LVDT. A single external resistor sets the AD598 excitation voltage from approximately 1 V (RMS) to 24 V (RMS) with a current drive capability of 30 mA (RMS).
The AD598 can drive an LVDT at the end of 300 ft of cable, since the circuit is not affected by phase shifts or absolute signal magnitudes. The position output range of Vout is ±11 V for a 6 mA load and it can drive up to 1000 ft of cable.
The V1 and V2 can be as low as 100 mV RMS.
Rotary Variable Differential Transformer
Rotary variable differential transformer (RVDT) has a rotating core (Figure 3) and can measure angular position. A RVDT works in the same way that an LVDT does, except that its movable magnetic core rotates instead of translates. Thus, it measures the angular position or speed.
Most RVDTs are composed of a wound, laminated stator and a salient two-pole rotor. The stator contains both the primary and the two secondary windings. Some secondary windings may also be connected together. RVDTs utilize brushless, noncontacting technology to ensure long-life, reliable, repeatable position sensing with infinite resolution under the most extreme operating conditions.
Features of LVDTs and RVDTs
Many LVDTs and RVDTs have virtually no friction, possess excellent null stability, and operate in broad temperature spectrum. They are often used for linear or angular displacement measurement.
LVDTs have a wide measurement range typically from ±100 μm to ±25 cm. Typical excitation voltages range from 1 to 24 V (RMS), with frequencies from 50 Hz to 20 kHz. A well-designed LVDT can provide a linear output within ±0.25% over the range of core motion and with a very fine resolution (limited primarily by the ability to measure voltage changes) such as 1 part in 100,000 within the linear range.
The time response depends on the moving object to which the core is connected. The slope of the transfer function is typically given in millivolts per millimeter (mV ⋅ mm−1 ).
Some LVDT displacement sensors come with integral electronics that internally generate the alternating current and convert the measured signal into a calibrated DC output.
LVDT Pressure Sensor
By attaching the core to a moving object or a diaphragm, an LVDT can measure position, acceleration, and pressure.To use an LVDT sensor to measure pressure, the pressure must be converted to a linear displacement.
Any pressure-sensitive element that changes size or shape as a pressure is applied, such as bellows, Bourdon tubes, or aneroid capsules, can be used. As a pressure is applied, the core is displaced from its null position, causing an output voltage.
Figure 4 shows an LVDT pressure sensor in which a pressure diaphragm drives the core up and down and varies the inductive coupling between the primary and secondary windings. The LVDT is a differential device and can be made to measure absolute, relative, or differential pressures.