Identification and Modeling of a Servo Pump-Controlled Hydraulic System

Compared with the valve-controlled hydraulic system, the servo pump-controlled hydraulic system (SPCHS) utilizes the servomotor to drive and control the pump, and therefore, owns the superiorities of high energy efficiency, compact structure, and surpassing control accuracy, exhibiting great prospects. However, to achieve the highest performance, the SPCHS must be appropriately modeled, identified, and controlled. In this context, this article develops the theoretical model, meanwhile the simulation model, of an SPCHS, which consists of a servomotor, a planetary gearbox, an external gear pump, an oil pipeline, and an orifice at the end; especially, taking the compressibility, the inertia, and the damping of the oil into account, an analogous two-mass-spring system is proposed to model the oil pipeline; also, the pump flow leakage is theoretically modeled, which is a function of the pump outlet pressure. For exact modeling of the SPCHS, the actual pump flow leakage is measured and modeled: by utilizing an angular sensor to evaluate the theoretical flow rate of the pump, a flowmeter to measure the actual flow rate of the pump, and a relief valve to adjust the outlet pressure of the pump, the flow leakage under different pressures are determined; by employing the least squares fitting, various flow leakage models are derived, which suggests that the cubic polynomial achieves satisfactory fitting goodness. Also, an approach is proposed for measuring the effective bulk modulus: based on the simulation system, the Bode plots of the hydraulic system under different modulus are derived; the actual Bode plots are experimentally measured; by comparing the measured Bode plots with the simulated ones, the effective bulk modulus is determined. Experiments are performed to verify the constructed model.