The pressure drop needs to be kept constant in the flow rate/input signal performance test of proportional directional control (PDC) valve. In general, the control of valve pressure drop is implemented by regulating the relief valve or flow control valve that located between port A and port B of the PDC valve. But in this study, the load of the test valve is fixed and the stable pressure drop is obtained by changing the proportional relief valve which is placed in the inlet of the PDC valve. Then the mathematical model of the test rig and several controllers are established based on this idea. To be specific, proportional-integral (P-I) controller, proportional-integral-double-integral (P-I-II) controller, and fuzzy proportional-integral-double-integral (FP-I-II) controller are all applied to stabilize the pressure drop in this study. And the FP-I-II controller with compensation (FP-I-II-WC) is proved to be the best for this work both in the simulation and the actual experiment.

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