Linear Variable Differential Transformer

Linear Variable Differential Transformer is a very precise and frictionless positional converter used to measure linear displacement of an object with an output voltage proportional to the position of its moving core.

A linear variable transformer or LVDT is a short electromechanical position transmitter (sensor) that provides accurate and frictionless positional feedback information about the linear mechanical condition of an external force or object.As the name suggests, the linear variable differential transformer operates on the same principle as the AC transformer, but instead of providing a load current or high voltage, the basic transformer uses the principles of mutual inducing to measure linear motion.

In our tutorial on mutual inducing, we found that when two or more long solenoid coils are wrapped together on the same mold or core, the magnetic battery produced by any of the coils is connected to the others by the magnetic flux produced by the drive coil. against the flux produced by other coils.Thus, any AC current flowing from one coil will induct a voltage to other magnetically connected coils, and this is the basic principle of LVDT.

Linear Variable Differential Transformer

Then the LVDT is a passive inductive converter that requires an external power supply to operate.It uses coils and an alternate magnetic field to produce analog output voltage, making it a variable inductive converter.Thus, the "linear variable differential transformer" measures the distance along a linear axis.

Linear Variable Differential Transformer

LVDT consists of three separate coils, which are wrapped sequentinously around a hollow, non-magnetic insulated tube.One coil of the magnetic wire is classified as the primary coil, and the other coils form two identical secondary coils.

The two secondary coils are connected to each other in a contrasting series configuration, that is, they are electrically out of phase 180 with each other. Therefore, there is the differential part of the name.

The only centrally positioned primary coil of LVDT is energized by a constant source of AC sinusoidal waveform with a frequency ranging from about 1khz to 10kHz. The magnetic flux produced by the primary winding is connected from inside the nucleus to one or both of the two secondary coils placed on both sides.

This arrangement produces a differential output voltage proportional to the displacement of the nucleus, thereby calling it the "space sensor".Then the linear variable differential transformer consists of a primary stimulation coil and two secondary coils connected to the "serial opposite" (Differential).

Linear Variable Differential Transformer

Linear Variable Differential Transformer

The image above shows the generalized principle of LVDT.When the moving soft iron ferromagnetic core is placed in the center of two secondary coils, the "empty position" is exactly the same as the amount of primary magnetic flux induced into each of the two secondary coils.Since the two secondary coils are wrapped out of phase 180 with each other, the two induced emks in the two secondary windings cancel each other out as VSEC1 = VSEC2, so the resulting secondary output voltage is zero (V OUTPUT = 0).Thus, zero volt means that the nucleus is fully centered at zero position.

When the core is slightly shifted from one zero position to one side or another, the magnetic flux induced in one of the secondary coils due to the joining effect of the ferromamanyetic nucleus will be greater than the other.This causes two secondary coils to become unstable, as the voltage induced to the secondary coil further away from the core shrinks, while the voltage induced to the secondary coil closest to the nucleus becomes larger.This magnetic imbalance between the two secondary windings produces an output voltage (V OUT) according to the sinusoidal frequency of the peak voltage applied to the winding of the primary stimulation coil.

Obviously then the difference voltage between the two second outputs is multiplied by the cosine of phase shift in one direction V Sec1 – V SEC2 and V SEC2 – V Sec1 in the other direction, which will be the RMS voltage.Therefore, the larger the displacement of the moving nucleus from the central zero position to one end or another (stroke length), the greater the resulting output voltage.

The polarity and size of the output signal depends on the direction and quantity of displacement of the moving nucleus determined by the movement of the connected object.These displacement results are a differential voltage output that changes linearly with the core position.Therefore, the rms output voltage from this type of position sensor has both an amplitude, which is a linear function of core displacement, as shown, and a polarity indicating the direction of movement.

LVDT Output Voltage

Linear Variable Differential Transformer

From the position-voltage graph above, as the core moves from one end of its range to the other along its center position, we can see that whichever consists of the primary and two secondary coils has a larger magnetic connection.Output voltages vary from maximum to zero in the opposite direction and again to maximum in some way related to how far the core is moving from zero.This allows the LVDT to produce an output AC signal, the magnitude of which represents the amount of movement from the central "empty" position and represents the direction of movement of the phase angle moving core.

Connecting an object to the nucleus allows the linear variable differential transformer converter to provide fairly precise information about the object's position.Since their output is calibrated to produce a certain voltage per millimeter, for example, 20 or 200 mV/mm, the range or stroke can range from several millimeters to hundreds of millimeters.This means that a one millimeter core displacement will produce a voltage output of 200 mV.The phase angle of the output voltage (0 o or 180 o) is compared to the primary coil exctimulation voltage (0 o), it is possible to know which half of the secondary coil the nucleus is located and thus its direction.

A variable differential transformer has many advantages and uses for position measurement compared to resistant ponciometer-based converters.LVDTs have very good linearity, that is, the voltage output to displacement is excellent, it provides frictionless operation due to very good accuracy, good resolution, high precision, as well as the lack of mechanical connectivity between the coils and the core, and there is no part to erode.In addition, the Transformer part of its name means that there is electrical insulation between the primary and secondary windings, which allows for more electrical connections.

Since the only interaction between the primary, secondary windings and core of an LVDT is magnetic coupling, the primary and secondary windings of LVDT are usually sealed in an epoxy encapsulation, and the entire sensor is placed in a metal enclosure, allowing safe use of humid or harsh environmental conditions as well as in a wide range of areas.

LVDT Question Sample 1

A linear variable differential transformer has a stroke length of ±150mm and produces a resolution of 40mV/mm.

Calculate: a) the maximum output voltage of the LVDT,

b) when the core is moved 120 mm from zero position,

c) when the output voltage is 3.75 volts, the position of the core from the center,

d) displace the core from +80mm to -80mm.

a).Maximum output voltage, V OUT

If the 1 mm movement produces 40mV, the 150 mm movement produces:

V OUTPUT = 40mV x 150mm = 0.04 x 150 = ±6 Volts

B).120 mm core movement V OUT

If a 150 mm core displacement produces a 6-volt output, a movement of 120 mm produces:

Linear Variable Differential Transformer

C).V OUT = core position when 3.75 volts

Linear Variable Differential Transformer

NS).Voltage change from +80mm to -80mm displacement

Linear Variable Differential Transformer

Thus, as the core moves from +80 mm to -80 mm respectively, the output voltage changes from +3.2 volts to -3.2 volts.

Displacement transducers come in many lengths and sizes, from a few millimeters to those that can measure strokes longer than a few millimeters.However, while LVDTs can measure linear motion in a straight line, LVDT has a variation called Rotary Variable Differential Transformer, or RVDT, that can measure angular motion.

Rotary Variable Differential Transformer (RVDT)

Ponsiometer-based transducers are easy and simple to use, but resistant posiometers are subject to mechanical wear due to contact between the sliding wiper and the resistant rail, and also the wiper produces electrical noise as it slides and bounces along the resistance path.Rotary variable differential transformers operate on the same basic principle as the previous LVDT, except for the use of a rotating ferromamanyetic core.

Here the transformer core is not flat, but forms part of a circle (as with toroidal transformers), which allows the sensor to measure the angular displacement of the connected object.The ferromamanyetic moving core of the RVDT combines with secondary coils depending on its angular position, thereby allowing the measurement of angular displacement.

The electrical operation of an RVDT is exactly the same as the linear version in that it is based on changing the mutual inducing coupling between the primary and secondary coils.The primary coil is still driven by an AC stimulation current (typically in the kilo-hertz, kHz range), which induces an AC current in each of the opposing secondary coils.

One of the main drawbacks of the rotary variable differential transformer is that it can operate only in a relatively narrow range of angular rotations.Although theoretically they can measure continuous rotation and speed, the output of the typical RVDT is really linear at a range of about ±60 o or less from zero zero position (0 o) mainly due to limitations in magnetic coupling.Beyond that, the output signal becomes nonlinear and less useful.In addition, their sensitivity is much smaller than their linear cousins, which produce about 2 to 5mV per degree of rotation.

Linear Variable Differential Transformer Summary

We saw in this tutorial about the linear variable differential transformer that LVDT is a positional sensor used to measure small linear (straight line) displacements from a few millimeters to hundreds of millimeters.LVDT does not have direct sliding mechanical contact or moving parts to wear, so it makes it almost frictionless, offering greater electrical performance and service life compared to the reactive linear pocinciometer type displacement sensor.

LVDT consists of a transformer with a single primary winding and two secondary windings that are electrically 180 o-phase out of each other.LVDT also consists of a moving core.When the core is in its central position, the voltages induced in the two secondary windings are equal and opposite and signal zero output.As the core moves away from the central position, the voltage induced in one half-secondary winding will be greater than the other, giving a signal whose amplitude is proportional to the amount of linear displacement and whose phase represents the direction of movement.Thus, LVDT produces a linearly changing differential voltage output with the core position where the phase angle of the output voltage changes by 180 o.