Frequency domain modeling, analysis and verification of electro-hydraulic servo steering system for heavy vehicles

The electro-hydraulic servo steering system is one of the core components of a heavy vehicle, and frequency response characteristics of this system are essential to guarantee the vehicle flexibility. However, it is difficult to establish frequency domain model directly for the frequency response characteristics analysis due to the strong nonlinearity of steering trapezoidal mechanism and hydraulic power system in electro-hydraulic servo steering system. This paper proposes a simplified linearization analysis method for the electro-hydraulic servo steering system. By variable substitution defining the load flow and load pressure, and linear fit between double tire angles and cylinder displacement, the original model is simplified to a frequency domain model. Based on this model, the essential frequency response characteristics and the effects of key parameters to electro-hydraulic servo steering system can be obtained. Through the sweep frequency response analysis, the linearized frequency domain model is compared with the nonlinear time domain model and the actual test system, respectively. As shown in Bode plots, the amplitude-frequency phase-frequency characteristic curves of models match well, which verifies linearization analysis method and linear frequency domain model. The key parameters affecting the system frequency domain characteristics are the valve flow gain, the area of cylinder rodless and rod chamber, and the linearization coefficient between the left and right tire angles and so on. The electro-hydraulic servo steering system bandwidth is only 7.38 rad/s (1.17 Hz). This research is helpful for the design and optimization of heavy vehicle dynamic steering system.