Feedback Device (e.g., LVDT)

A feedback device, such as a Linear Variable Differential Transformer (LVDT), is a critical component within a servo valve that continuously monitors and reports the precise position of the valve's spool or actuator. This feedback signal is essential for the closed-loop control system to compare the actual position with the commanded position, enabling high-precision control of fluid flow, pressure, or force in hydraulic or pneumatic systems.

Typically, an LVDT-based feedback device consists of a movable ferromagnetic core connected to the valve spool, which moves inside a stationary transformer assembly (primary coil flanked by two secondary coils). An AC excitation signal applied to the primary coil induces voltages in the secondary coils. The core's position changes the magnetic coupling, creating a differential voltage output from the secondary coils. This output's magnitude and phase are linearly proportional to the core's (and thus the spool's) displacement from its null position, providing an accurate electrical signal representing spool position.