The D633-509B is a high‑flow, single‑stage electrohydraulic direct drive servo valve (DDV) in the MOOG D633 series, with integrated closed‑loop electronics and LVDT spool position feedback. It uses a permanent magnet linear force motor to drive the spool directly—no pilot oil required. Designed for high‑dynamic, high‑force applications (e.g., hydraulic presses, fatigue test rigs, aerospace simulators), it delivers ultra‑low hysteresis, excellent repeatability, and high contamination tolerance.