Development and control of a single rod electro hydrostatic actuator

Çalışkan, Hakan
In this thesis a variable speed pump controlled electro hydrostatic drive system for a single rod actuator is developed. The problem of the single rod actuator in hydrostatic applications is the differential flow rate which is formed due to the asymmetric structure of the piston rod assembly. The hydraulic circuit solutions proposed in the literature commonly suffer from undesired pressure oscillations, which are named as system internal instability. This thesis, addresses the stability problem of the pump controlled asymmetric hydraulic actuators, proposes a physical to solution to this problem and introduces a novel single rod electro hydrostatic actuator (EHA) which demonstrates a high dynamic performance and efficiency. The system under consideration utilizes a shuttle valve to compensate for the differential flow rate. For the stability analysis a simple linear model of the system, which constitutes of load pressure and actuator velocity states, is derived. It is shown that there exists a critical load pressure region in which any equilibrium point requiring a partially opened spool position is unstable during the retraction of the actuator. It is proposed that an underlapped shuttle valve provides a stable operation region up to certain retraction speed. Theoretical findings are validated by both numerical simulations and experimental tests. The deficiency of the underlapped shuttle valve solution, which is the circulating leakage flow over the spool pre-openings, is investigated. An inverse kinematic model of the system is developed to compensate the effects of flow losses and verified with numerical simulations. The circulating leakage losses are physically eliminated by a novel shuttle valve spool structure, which provides asymmetric orifice pre-opening. The theoretical stability analysis is extended and a numerical program is developed, which is suitable to investigate all possible spool structures. The effects of several valve parameters on system stability are investigated and second novel valve solution is proposed, which provides symmetric pre-opening, removes the spring pre-loading and increases its stiffness. The first valve solution, with asymmetric orifice pre-opening, is manufactured and implemented on the electro hydrostatic actuator (EHA). A combined feedback and feedforward position control system is designed, based on a linearized model that includes the servo motor and hydraulic actuator dynamics. The developed single rod EHA is tested experimentally on a previously developed load simulator test set up and its performance is evaluated in terms of positioning, set point tracking and disturbance rejection.
Citation Formats
H. Çalışkan, “Development and control of a single rod electro hydrostatic actuator,” Ph.D. - Doctoral Program, Middle East Technical University, 2015.