【正文】 just required. Especially, in cases that a real experimental system is directly used for evaluating candidates, ESs are more suitable than other evolution algorithms due to its own characteristic called selfadaptation. In this study, a time delay controller (TDC) is designed as a controller for position control of an electrohydraulic servo system. The controller designed to have 2nd order error dynamics has three controller gains implicitly. By using an ES as optimization algorithms, the optimal controller gain set in the speci?ed gain spaces is determined through online experiments. For the veri?cation of the obtained results, the ?tness function topologies in the gain spaces experimentally are made out, and analyzed. Finally, the experimental results searched through ESs are shown to coincide with the optimal peak point that has best ?tness value on the speci?ed gain spaces. 2. Hydraulic servo system . Electrohydraulic position control system is the schematic diagram of the electrohydraulic position control system used in this study. This system is a test rig for the rollgap control of hydraulic mill stand, which was made for the improvement of thickness control performance in plate hotrolling processes (Gizburg, 1984。 Watton, 1989): the pressure?ow characteristics of valve, the saturation of valve and cylinder, the leakage ?ow characteristics of valve and cylinder with variation of supply pressure, the friction characteristic in cylinder, the variation of viscosity and pressibility of working ?uid with the temperature, the ?ow characteristic due to the shape of pipeline, and most importantly, the variation of the system gains with the supply pressure and the load pressure. Therefore, when these ?uid power systems are controlled, the controller gains are adjusted on the foundation of expert’s intuitive knowledge about the system and the tuning experience of the controller gains in general. It needs very excessive experiments through trial and error. But though some controller gains are obtained, it is hard to say that the results are the best gain set at a given situation. For the automatic adjustment of the controller gains in ?uid power systems, the research to application of a fuzzy gain adapter (FGA) has been performed (Jeon, 1997。 Hyun amp。 Purshouse, 2020。 Evolution strategies。 Time delay control。 Klein, 1992。 Lee, 1998。 Klein, 1992). In this case, the knowledge base is needed for transplantation of the expert knowledge to the systems, and some general rules to variation of the system response due to variation of the controller gains are demanded for the construction of this knowledge base. Therefore, much expert’s experiences and many experiments are necessary for the implementation of this algorithm. In this study, evolution strategies (ESs) is proposed as a method of the automatic optimization of the controller gains in a electrohydraulic system. ES is one of the evolutionary algorithms based on the natural geics and the survival of the ?ttest (Rechenberg, 1973。 Lee, Lim, amp。 Gao, 1990), which is based on the estimation of unknown effects due to system uncertainties using the time delay technique, that is, sampling technique of the discrete control. The time delayed information (the control input and the derivatives of state variables at the previous sampling time) plays a role to cancel various effects which result from variations of system parameters, unpredictable external disturbances, and unmodeled nonlinear system dynamics. Since the current control input can be obtained by using just the control input and the system responses observed at the previous sampling time, the controller is little affected by a system modeling, so that can be effectively applied whenever there is the large variation of system paramenters and a disturbance. paring with the Proportional plus integral plus derivative (PID) controller, the state feedback controller, or adaptive controller, the structure of this controller is as simple as the PID controller, because it does not require a realtime estimation of state variables or system parameters using observers, nor does it a putation of inverse system dynamics. Recently, a research has been performed on a ?uid power system using a TDC by Chin, Lee, and Chang (1994). The application results show easiness of the real implementation of this controller and robustness on various uncertainties of the plant. For t