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Electro-hydraulic actuator
Electro-Hydrostatic actuators (EHAs), replace hydraulic systems with self-contained actuators operated solely by electrical power. EHAs eliminate the need for separate hydraulic pumps and tubing, simplifying system architectures and improving safety and reliability. This technology originally was developed for the aerospace industry but has since expanded into many other industries where hydraulic power is commonly used.
==Conventional designs==
Aircraft were originally controlled by small aerodynamic surfaces operated by cables, attached to levers that magnified the pilot's mechanical advantage. As aircraft grew in size and performance, the aerodynamic forces on these surfaces grew to the point where it was no longer possible for the pilot to manually control them across a wide range of speeds - controls with enough advantage to control the aircraft at high speed left the aircraft with significant overcontrol at lower speeds when the aerodynamic forces were reduced. Numerous aircraft in the early stages of World War II suffered from these problems, notably the Mitsubishi Zero and P-38 Lightning.
Starting in the 1940s, hydraulics were introduced to address these problems. In their early incarnations, hydraulic pumps attached to the engines would feed high-pressure oil through tubes to the various control surfaces. Here, small valves were attached to the original control cables, controlling the flow of oil into an associated actuator connected to the control surface. One of the earliest fittings of a hydraulic boost system was to ailerons on late-war models of the P-38L, removing the need for great human strength in order to achieve a higher rate of roll.〔"Lockheed P-38 Lightning Pilot's Flight Manual", pg. 5〕
Over time the systems evolved to replace the mechanical linkages to the valves with electrical controls, producing the "fly-by-wire" design,〔Frischemeir, pg. 2〕 and more recently, optical networking systems in what is known as "fly-by-light". All of these systems require three separate components, the hydraulic supply system, the valves and associated control network, and the actuators. Since any one of these systems could fail and render the aircraft inoperable, redundancies are needed that greatly increase the complexity of the system. Additionally, keeping the hydraulic oil pressurized is a constant power drain.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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