Abstract: Aiming at the problem that passive hydro-pneumatic suspension could not effectively reduce the vertical vibration of vehicle body, the dynamic model of elbow in semi-active suspension was established. A method 
to adjust the size of the throttle hole by using multi-objective output feedback control to increase ride comfort was proposed. In the framework of linear matrix inequality (LMI), the multi-objective output feedback was used to adjust the size of adjustable orifice, the suspension was satisfied with the the hard constraints and the gain of disturbance channel was reduced. The application of LMI region pole placement in suspension performance optimization was studied. The results show that comparing with passive hydro pneumatic suspension, the multi-objective output feedback control based on LMI method can effectively reduce the vertical acceleration caused by the external road disturbance, and the stiffness characteristics of the system can be optimized within a certain range of disturbance. Meanwhile, the dynamic travel, dynamic to static load ratio and the output force of damping damper in time domain do not exceed the constraints. The grounding performance of the vehicle is ensured and the pole distribution of the system meets the design requirements.

Key words: elbow-type hydro-pneumatic suspension; dynamic model; linear matrix inequality(LMI)method; multi-objective output feedback