註釋 Certain types of Load-Sensing (LS) pumps utilize a hydro-mechanical control system designed to regulate the pressure difference, or margin pressure, between the inlet and outlet of a flow control valve. With a constant margin pressure, predictable flow control and improved efficiency can be achieved by controlling the orifice area of the flow control valve. Instability due to limit cycles (sustained oscillations) that stem from nonlinearities within the system is a common issue related to hydraulic LS systems. In this work, the stability of the pressure control system was investigated using describing function analysis. Describing function analysis is a method used to approximate a nonlinearity within a nonlinear system and was conducted to predict the existence and stability of limit cycles that occur due to saturation nonlinearities within the mechanical components of the LS system. A combination of linear and nonlinear analysis and modeling was employed to assess the stability of a particular LS pump system. Among many nonlinearities present in the hydro-mechanical LS system, of particular interest was the saturation inherent in the actuator that is used to displace the pump swash plate and the saturation within the 3-way spool valve that permits flow to reach the actuator. This saturation nonlinearity was believed to be a problematic source for limit cycles that tend to appear in LS systems. A comprehensive nonlinear model was developed as the foundation for this research as it was used for validation in direct comparison to experimentally acquired data. The nonlinear model proved to be precise and accurate in matching to the experimental test bed response based on the data that was gathered. The acquired data was compared to the NL model simulation through a root mean squared error evaluation and frequency response analysis. The nonlinear model was then used to generate a linearized model necessary for stability analysis. The saturation nonlinearities for two separate mechanical systems were isolated from the rest of the linearized system dynamics to provide a system structure necessary for conducting describing function analysis. Analysis of model characteristics and dynamics was used to make conclusions about the stability of the system. The results of the describing function analysis demonstrated an accurate diagnosis of the stability of the system through three separate scenarios. Each scenario indicated that describing function analysis can be very useful and certainly is appropriate for predicting the stability for a high-order LS system model such as the one presented in this research. Investigation into the results also revealed the critical importance of following the fundamental assumption of describing function analysis. The linear dynamics used for the hydraulic system must satisfy the filtering hypothesis required for describing functions. A scenario in which the filtering hypothesis was violated was encountered and an adjustment of the model by removing higher frequency dynamics could allow for even more improved results in the future. 