Linear Variable Differential Transformer (LVDT) is extensively used transducer in industry and so explained here in detail.
Figure 1 shows this mutual inductance transducer. The primary is excited by an external AC voltage. The secondaries, both of equal turns and identical placement about the primary are so connected that if the core is centred between the identical secondary windings, the voltages induced in each of the secondaries V, and V2 are identical but 180°C out of phase, so there is no net output. If the core is moved off centre, the mutual inductance of the primary with one secondary will be greater than the other.
V, and V2 are then no more equal and a differential voltage will appear across the secondaries in series. For off-centre displacements within the range of operation, this voltage is a linear function of displacement.
Thus LVDT produces an electrical output proportional to the displacement of a separate movable core. Because there is no physical contact between the core and coil, the mechanical components of an LVDT do not wear out or deteriorate. The corresponding absence of friction gives truly infinite resolution and no hysteresis.
The small core mass and the lack of friction enhance response capabilities for dynamic measurements. The LVDT is not affected by mechanical overload, so its reliability is very high. The LVDT and its rotary counterpart, the rotary variable differential transformer (RVDT), are used with many types of sensing elements that translate physical inputs to displacement.
A typical standard core consists of a uniformly dense cylinder of annealed nickel-iron alloy. Usually it is threaded internally prior to annealing to accept nonmagnetic core rods or screws from an external actuating element. The annealing process improves the magnetic permeability of the core and makes it uniform. The process also relieves machining stresses.
The magnetic flux path through the core is essentially a surface phenomenon. In applications where a considerably smaller core mass is required to satisfy extremely fast response requirements, it is possible to use a tubular core of nickel-iron alloy without reducing the effective flux-carrying cross-section.
The stroke of commercial LVDT’s Fig 2.22 varies from ± 100 /ym to ± 25 cm, and the linearity is of the order of 0.25 %. Sensitivity depends on the supply voltage and stroke length, but is usually in the range 100 mV/cm to 40 mv7/vm. Resolutions down to 0.1 mm are obtainable.
The principal advantageLVDT over resistance potentiometers is the absence of any moving contacts, and reasonably constant output impedance with infinite resolution.
SPECIFICATIONS OF LVDT NOMINAL LINEAR RANGE OF THE LVDT
The basic variable in LVDT selection is the maximum displacement of the core from its null position that produces an output of specified linearity as Shown in Fig 2.5b. This distance of core travel is called full-scale displacement. Since the core can be displaced from null toward either end, the linear operating range is twice the full-scale displacement. The linear range of LVDT varies to some degree with frequency, and load across which the differential voltage of the secondaries is fed.
The primary current of and LVDT excited at a fixed voltage decreases as excitation frequency increases. Thus, the input voltage at higher excitation frequencies can be increased, within the limitation-of maximum input power. However, the input voltage must not produce core saturation or exceed the breakdown voltage of the winding insulation. Furthermore, increased excitation may produce undesirable output signal distortion.
INPUT VOLTAGE REGULATION OF THE LVDT
The output voltage of an LVDT is directly proportional to primary voltage. Therefore, unless the LVDT is being used in an application where primary voltage (or current) is one of the variables, the excitation voltage should be closely regulated and monitored. A constant-current power source, rather than a constant-voltage source, is sometimes preferable for accurate LVDT operation.
This is especially true when using -an excitation level that produces a substantial temperature rise in the transformer. A constant-current source eliminates the output variation that would result from the normal variation of primary resistance with ambient temperature. This primary resistance variation is particularly important at low excitation frequencies where the resistance is a significant fraction of the primary impedance, but becomes less significant at high frequencies, where the primary impedance becomes principally inductive.