This paper considers flow, in particular, leakage flow, between a servovalve spool and valve body. Although often ignored, experimentation and analysis indicate that at small spool displacements this flow is much larger than orifice flow. In precision positioning applications, where the servovalve operates within the null region, leakage flow may severely degrade the performance of conventional servohydraulic designs based on ideal orifice relation. Hence, servovalve models for such applications should include leakage flow. We have developed a model for leakage flow in a servovalve. This has been achieved by reviewing experimental leakage flow data and identifying a simple mathematical form that (1) makes physical sense and (2) can replicate experimental data. The resulting model accurately captures leakage behavior, which is modeled as turbulent flow with a flow area inversely proportional to the overlap between the spool land and valve orifices. When combined with orifice relations, the model extends the accuracy and region of applicability of existing servovalve models. Furthermore, the leakage model is easily parameterized, an important feature for controller design and adaptive methods.

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