Linear Variable Differential Transformers (LVDTs) are widely used in industrial and scientific applications to measure displacement accurately. They are contactless transducers and operate on the principle of electromagnetic induction. Here's how LVDTs measure displacement:
Basic Construction: An LVDT consists of a primary coil (excitation coil) and two secondary coils wound on a common cylindrical former. The former is typically made of a non-magnetic material like nickel-iron alloy. The secondary coils are placed symmetrically on either side of the primary coil, and all coils are wound around the former.
Excitation: An alternating current (AC) is applied to the primary coil. The primary coil serves as the input to the system, and the alternating current induces a varying magnetic field in the core.
Displacement Application: When an object whose displacement is to be measured is attached to the LVDT's core, it causes the core to move linearly within the former. This core movement is directly proportional to the displacement being measured.
Principle of Induction: The varying magnetic field generated by the AC current in the primary coil induces EMF (electromotive force) in the two secondary coils. The amount of EMF induced in the secondary coils is dependent on the position of the core within the former.
Differential Output: The LVDT is designed in such a way that the EMF induced in the secondary coils has a phase difference. When the core is at its central or null position, the EMF in the two secondary coils is equal and opposite, resulting in a net output voltage of zero. This null position is the reference point for displacement measurement.
Measurement: As the core moves due to the applied displacement, the magnitude of the EMF induced in the secondary coils changes. This change causes a differential voltage output proportional to the core's position relative to the null position. This differential voltage represents the magnitude and direction of the displacement.
Signal Processing: The differential voltage output is then conditioned and amplified using signal processing electronics to produce a usable output signal, such as a voltage or current output. This output can be further converted and calibrated to display the actual displacement in the desired units (e.g., millimeters or inches).
LVDTs are known for their high accuracy, linearity, and reliability in measuring displacement. They are commonly used in various industrial applications, including position feedback in control systems, automation, and quality control processes.