POSITION SENSORS
THE LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)

The Linear Variable Differential Transformer (LVDT) is a displacement measuring instrument and is not a strain-based sensor.

The LVDT models closely the ideal Zeroth-order displacement sensor structure at low frequency, where the output is a direct and linear function of the input.

The LVDT is a variable-reluctance device, where a primary center coil establishes a magnetic flux that is coupled through a mobile armature to a symmetrically-wound secondary coil on either side of the primary.

Two components comprise the LVDT: the mobile armature and the outer transformer windings. The secondary coils are series-opposed; wound in series but in opposite directions.

When the moving armature is centered between the two series-opposed secondaries, equal magnetic flux couples into both secondaries and the voltage induced in one half of the secondary winding is balanced and 180 degrees out-of-phase with, the voltage induced in the other half of the secondary winding.

The balanced condition provides total cancellation of secondary voltages and therefore zero voltage output. When the moveable armature is displaced from the balanced condition, more magnetic flux will couple into one half of the secondary than into the other producing an imbalance voltage output at the primary coil excitation frequency. The output voltage of the LVDT is therefore a direct function of the displacement of the mobile magnetic armature. The LVDT is, by definition, a transformer and requires an oscillating primary coil input.

The DC LVDT is provided with onboard oscillator, carrier amplifier, and demodulator circuitry. The AC LVDT requires these components externally. Due to the presence of internal circuitry, the DC LVDT is temperature limited operating from typically -40 C to +120 C.

The AC LVDT is able to tolerate the extreme variations in operating temperature that the internal circuitry of the DC LVDT could not tolerate.

Typically, LVDT’s will be excited by a primary carrier voltage oscillating at between 50 hertz and 25 Kilohertz with 2.5 Kilohertz as a nominal value.

The carrier frequency is generally selected to be at least 10 times greater than the highest expected frequency of the core motion.

The external housing of the LVDT is fabricated of material having a high-magnetic permeability therefore desensitizing the device from the effects of external magnetic fields.

No sensing spring element exists within an LVDT and therefore, the output of the sensor is hysteresis-free. Some LVDT displacement measuring sensors are, however, provided with internal armature return springs to allow profile measurement. When there exists no direct contact with the moving armature is allowed no mechanical wear results. The provision of linear bearings to prevent armature to coil structure contact and to limit wear can greatly extend LVDT operating life expectancies.

The strong relationship between core position and output voltage yields a sensor design that shows excellent resolution, limited more by the associated circuitry than the sensing method.
The internal core of the LVDT is generally constructed of an annealed nickel iron alloy with the high-temperature limitations of the device limited to the curie point of the core and the winding insulations used.

The thermal response characteristics of the LVDT are excellent for static and quasi-static thermal environments due to the physical and electrical symmetry of these devices. The physical symmetry also contributes to excellent zero repeatability over time and temperature.

Most thermal-sensitivity shift errors result from the significant thermal coefficient of resistance (TCR) of the copper transformer windings.

With increasing temperature, the primary coil resistance will increase causing a decrease of the primary current in the constant-voltage-excited case and therefore decreasing the magnetic flux generated and voltage output correspondingly.

The use of constant-current excitation will ensure a constant primary flux regardless of the coil resistance. Since the equivalent circuit of the constant-current source is a voltage source with an infinite series resistance, the use of a low-TCR resistance, in series with the primary, will function in much the same manner as the piezoresistive span-compensation resistor by causing the primary voltage to increase as a function of temperature thus offsetting the TCR-induced losses. The use of the series low-TCR resistor in the primary circuit allows the constant-voltage source to appear to the LVDT as a constant-current source.

Other thermally-active methods may also be used to compensate for the primary winding TCR by causing the primary voltage to increase, with rising temperature, in proportion to the increase in the primary coil resistance. The temperature coefficient of magnetic permeability is another contributor to the thermal-sensitivity shift and is compensated out as a net effect by the means described above. Within approximately 2 seconds of power application the LVDT oscillator and emodulator circuitry will stabilize sufficiently for dynamic measurement.

Due to self-heating of the primary coil, warm-up times for high precision static measurement are comparable to strain gaged sensors and are dependent upon the thermal stability of the measuring environment.

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