A feedback sensor, such as a Linear Variable Differential Transformer (LVDT), is a critical component within a servo control valve. It continuously measures the precise linear displacement of the valve's spool or actuator. This position data is fed back to the valve's electronic controller, enabling closed-loop control to achieve accurate flow regulation, pressure control, or force application based on the input command signal. The sensor is typically mounted on the valve body, with its core mechanically linked to the spool. The output signal, often an AC voltage, is conditioned and converted to a digital or analog format for the controller. The measuring range (stroke) is a key specification, typically specified in millimeters, and must be matched to the valve's spool travel. The sensor's materials include a ferromagnetic core (e.g., nickel-iron alloy), copper windings, epoxy potting compound, and a stainless steel housing, which provide durability and environmental protection. For selection, engineers must verify the stroke length, electrical interface (e.g., voltage or current output), and environmental ratings (e.g., temperature, vibration) against the specific valve model and application. Maintenance signals include erratic output, drift, or failure to return to zero, which may indicate core wear, coil damage, or loose connections. Verification questions should address calibration, installation alignment, and compatibility with the controller's input requirements. The sensor is not a standalone product; it is an integral part of the servo valve assembly, and its performance directly affects the valve's response and accuracy. Always confirm model-specific values and standards with the legal manufacturer or supplier.
An LVDT-type feedback sensor operates on the principle of electromagnetic induction. It consists of a primary coil energized with an AC signal and two secondary coils wound symmetrically on a cylindrical former. A movable ferromagnetic core, attached to the valve spool, moves within the coil assembly. The core's position changes the mutual inductance between the primary and secondary coils, producing differential output voltages from the secondaries. The phase and amplitude of this differential AC voltage are proportional to the direction and magnitude of the core's displacement, providing a precise, contactless measurement of spool position.
