**Abstract**

The paper presents the results of the researches performed in order to find out the static and dynamic performances of some recent types of hybrid electrohydraulic servomechanisms. The new concept was generated by the needs of the modern aerospace technology called "flight by wire", directed to the use of compact actuators composed mainly by a brushless motor driving a hydrostatic pump connected in a close loop with a hydraulic cylinder. The numerical simulations performed with Simcenter Amesim language pointed out the possibility of saving a lot of energy with this new concept, already used on civil airplanes and fighters. The new wave of autonomous driving generated a lot of combinations of electric and hydraulic components, according the peculiarities of applications. The authors studied the accuracy of the hybrid steering system of an articulated tractor based on a digital industrial servo valve combined with an ORBITROL standard unit. The structure of the last generation of the trucks hybrid steering systems was also studied, taking into account the real performances of each component. Finally, the effect of the strong penetration of the hybrid servo systems in the automotive manufacturing systems is assessed.

**Keywords:** energy saving, hybrid electrohydraulic servo systems, modeling, simulation, experimental validation

### **1. Introduction**

The classical electrohydraulic servomechanisms, which include different types of high-speed two or three-stages servo valves (with flapper - nozzles, jet - pipes or piezo ceramic actuators) are still widely used in high performance control systems due to their very high dynamics reaching even 1000 Hz. However, the complex and expensive maintenance program, and the low overall energy efficiency due to the lamination of the fluid are strong drawbacks. The maximum efficiency of a classical hydraulic position servo system is lower than 65%. Consequently, all the advanced fluid control systems include oil coolers, and use special synthetic fluids for wide temperature ranges. In spite of these peculiarities, the modern servo valves offer minor time constants, which allow a high positioning accuracy. In the recent evolution of the electrohydraulic servo systems two important structural improvements can be pointed out:

a. The replacement of the classical servo valves by proportional directional flow control valves in different industrial applications (Rexroth, from 1989).
