# Analysis and Suppression of Self-Excited Oscillations in Pressure Servo Valve System
## 1 Introduction  
A pressure servo valve is a special jet pipe hydraulic servo valve, which can realize the feedback control of output pressure through the internal flow channel. Therefore, a control system using the pressure servo valve can realize the accurate control of output pressure in open-loop condition (without pressure sensor). Pressure servo valves are widely used in aircraft wheel braking systems with the advantage of easy control and high reliability. As a key component of large aircraft wheel brake servo control system, the pressure servo valve plays a very significant role in bearing impulse load and providing braking force [1, 2]. However, the feedback structure inside the pressure servo valve can also cause positive feedback, which can lead to the nonlinear self-excited oscillation of aircraft wheel braking system and cause serious safety accidents [3]. At present, much research on self-excited oscillation of hydraulic valves and valve-controlled servo systems has been carried out. To solve the self-excited oscillation of the check valve, [PERSON] et al. have carried out a lot of theoretical analysis and experimental research, and finally determined that the inducement of check valve self-excited oscillation is the fluid-solid coupling between spool and fluid during the rotation of the ball valve [4]. [PERSON] et al. established the nonlinear dynamic model of the pilot pressure control valve system, and the effect of valve inlet pressure on the spool self-excited oscillation had been investigated [5]. A nonlinear dynamic model of the pneumatic cartridge valve considering gas compressibility was established by [PERSON] et al., and the pneumatic cartridge valve self-excited oscillation is attributed to the water hammer effect in the upstream/downstream pipeline of the valve [6]. [PERSON] et al. established a nonlinear dynamic model of pilot electromagnetic pneumatic valve considering fluid-solid coupling, and clarified the influence of working parameters on valve self-excited oscillation [7]. [PERSON] et al. established a model to describe the nonlinear dynamic behavior of the check valve system, and the method of increasing damping to suppress the self-excited oscillation had been put forward [8]. [PERSON] et al. established the pneumatic check valve self-excited oscillation dynamic model,and solved the characteristic roots of linear equations based on the principle of small disturbance, and the stable working range of the check valve system without self-excited oscillation has been obtained [9]. [PERSON] et al. established a finite element model of a pressure servo valve torque motor considering the thermal field, and concluded that a high temperature is more likely to cause resonance [10]. [PERSON] et al. established a nonlinear dynamic model of relief valve, and the influence of system parameters on the relief valve nonlinear dynamic behavior had been obtained [11]. Through mathematical modeling and experimental research, the self-excited oscillation in many hydraulic systems has been analyzed. However, for the self-excited oscillation mechanism of the pressure servo valve system (PSVS), there is a lack of relevant research work. To address this situation, the mechanism and influencing factors of PSVS self-excited oscillations has been analyzed in depth, starting from the establishment of PSVS accurate dynamic model. The structure of this paper is as follows:  
In the second part, the PSVS dynamic model is established, and the accuracy of the model is verified by experiments. Meanwhile, the PSVS nonlinear self-excited oscillation phenomenon is analyzed. In the third part, the nonlinear self-excited oscillation mechanism of the PSVS is obtained, and the influencing factors of the oscillation are also evaluated qualitatively in this part. In the fourth part, based on the above study, the pressure servo valve structure optimization method is proposed, and PSVS nonlinear self-excited oscillation is eliminated effectively.