Servoactuators, Hydraulic or Mechanical, in the Era of Electric (Aero)Space and Automotive Vehicles

Dear Colleagues,

Recently, the use of electrohydraulic actuators (EHAs) has expanded significantly, becoming the best choice (until the recent rise of a strong competitor in the Electric Aircraft and Automotive Vehicle, namely the Electromechanical Actuators, EMAs). EHAs found widespread application across various fields including civil engineering, machine tools, mobile equipment and robots, radar antenna, land vehicles, naval and aerospace systems, etc. In flight controls systems, EHA or EMA, are in fact position tracking systems, but EHA or EMA as stabilizing systems can be viewed as a special case of the tracking system. An eloquent example of practical interest for EHA or EMA as a stabilizing system is the well-known problem of altitude-hold autopilot synthesis, involving an EHA or EMA, where the target is the maintenance of the desired altitude of the aircraft, enabling the pilot to focus on higher-priority tasks.

This Special Issue is dedicated to the coverage of various advanced approaches in control techniques of EHAs and EMAs in helicopters, fixed wing aircraft, electric aircraft and automotive vehicle.

Electrohydraulic Servoactuators (EHAs) not only enable the generation of large forces, but, thanks to advancements in control technology and sensors, are also capable of performing critical control tasks as, for example, highly precise positioning of heavy loads. However, controlling an EHA is far from being straightforward due to several factors, including nonlinear hydraulic dynamics, significant parameter uncertainties, and the switching and discontinuous nonlinearities due to control input saturation and directional change of valve ports opening. The basic, classical EHA is servovalve controlled and consists of an electrohydraulic servovalve (EHSV) and a Hydrocylinder. Another variant of EHA, known as Electrohydrostatic Servoactuators, is pump controlled and represents an emerging aerospace technology in aerospace and automotive vehicles. It is designed to replace centralised hydraulic system with a self-contained and localised direct drive servoactuator system. This type of EHA has become an important part of modern flight control systems due to the increased efficiency, reduced leakages and lower overall weight compared to conventional hydraulic systems. We can therefore distinguish the two variants by the acronyms EHS-SC and EHS-PC.

Electromechanical Servoactuators (EMAs) are emerging as strong candidates for next-generation electric aircraft, offering notable advantages in portability, reliability, and efficiency. EMAs eliminate the hydraulic pump and oil pipelines, replacing them with electric motors and flexible cables. This shift results in reduced weight, improved portability, and enhanced reliability due to the removal of potential leaks and the simplification of the system. Furthermore, EMAs provide faster and more accurate actuation, contributing to improved actuation efficiency and enabling greater levels of digitalization and full electrification in aircraft systems.
Yet, only practice and the future will determine the place and importance of these servoactuator variants in aircraft, aerospace and automotive vehicles control in the coming era. For example, to date, EHA-SCs seem unbeatable in terms of weight-to-power ratio.

(Note that the prefix "servo-" signifies that all these actuators - EHS-SC, EHS-PC and EMA - are automatic systems, so they have feedback in their constitution; we also note that "flight controls" refers to the primary flight surfaces).

Dr. Ioan Ursu
Dr. Liviu Dinca
Dr. Daniela Enciu
Guest Editors

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• PID control
• LQG control
• backstepping, sliding mode and geometric control
• adaptive control synthesis
• intelligent control synthesis: neural, and fuzzy control
• systems with delay
• switching systems
• critical stability
• absolute stability
• robustness theory