A Self-Contained Electro-Hydraulic Cylinder: Modeling, Control Design, Simulation and Experimental Validation

Abstract

In the constant effort to improve the efficiency of hydraulic systems, the pressure drop over hydraulic valves has been one of the major obstacles. In the Self-Contained Electro-Hydraulic Cylinder, the directional valve and the hydraulic power unit have been replaced by a bi-directional axial piston pump and a Permanent Magnet Synchronous Motor. Actuation of the cylinder is performed by controlling the direction of rotation and the velocity of the motor. Originally the purpose of this project was to: "Design a proper control algorithm for an existing novel selfcontained electro-hydraulic cylinder, implement it on the main boom of a loader crane, and troubleshoot it". Due to a time delay in having the test bed ready for experiments, more effort was put into modelling and control design. The system is divided into three major parts: The hydraulic system, the electric drive and the external load in the form of a vertical boom with a payload. The main parts of the hydraulic system are the asymmetric cylinder, the fixed displacement axial pump and the load holding system. The load holding part consists of two pilot operated check valves, and a system for controlling the pilot pressure. The electric drive is divided into the inverter and the PMSM. The kinematics of the vertical boom was derived using classical static mechanics. In addition, a dynamic model is available, that was derived from a previous PhD research program, which was tested and compared to the static model. When all three models were combined into one, the overall simulation time was beyond what was considered useful, and it was decided to simplify the electric drive and use the static mechanic model. Then the control design phase started, a linearised model of the system is derived and compared with the the high-fidelity Simulink model. A frequency response test is performed, both model based and on the test bed. Different control architectures are tested; such as P and PI controllers. A feedforward term is also implemented in the structure. The overall performance is then tested on several motion profiles. Search words: Self-Contained Electro Hydraulic Cylinder, Fluid Power, Asymmetric Actuator, Axial Piston Pump, Passive Load Holding, Electric Drives, Permanent Magnet Synchronous Machine, Modelling, Control Design, Experiments.