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.^[1]

Over time the systems evolved to replace the mechanical linkages to the valves with electrical controls, producing the "fly-by-wire" design,^[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.

EHAs

The primary development that lead to the possibility of EHAs was the precision feedback controlled conventional motor, or high-power stepper motor.^[3] Stepper motors are designed to move a fixed angle with every application of energy, and do so repeatedly in an extremely precise fashion. Both types of motor drives have been in use for years, powering the controls on motion control rigs and numeric control machine tools for instance.

With an EHA, high-power versions of these motors are used to drive a reversible pump, which is tied to a hydraulic cylinder. The pump pressurizes a working fluid, typically hydraulic oil, directly raising the pressure in the cylinder, and causing it to move. The entire system, consisting of the pump, the cylinder and a reservoir of hydraulic fluid, is packaged into a single self-contained unit.

Instead of the energy needed to move the controls being supplied by an external hydraulic supply, it is supplied over normal electrical wiring, albeit larger wiring than what would be found in a fly-by-wire system. The speed of the motion is controlled through the use of pulse-code modulation. The result is a "power-by-wire" system, where both the control and energy are sent through a single set of wires.^[4]

Redundancy can thus be provided by using two such units per surface, and two sets of electrical wires. This is far simpler than the corresponding systems using an external hydraulic supply. Additionally, the EHA has the advantage that it only draws power when it is being moved, the pressure is maintained internally when the motor stops. This can reduce power use on the aircraft by eliminating the constant draw of the hydraulic pumps. EHAs also reduce weight, allow better streamlining due to reduced internal routing of piping, and lower overall weight of the control system.^[5]

References

Notes

↑ "Lockheed P-38 Lightning Pilot's Flight Manual", pg. 5
↑ Frischemeir, pg. 2
↑ Frischemeir, pg. 3
↑ C. W. Helsley, Jr., "Power By Wire for Aircraft - the All-Electric Airplane", SAE, #771006, February 1977
↑ Adams

Bibliography

Robert Navarro, "Performance of an Electro-Hydrostatic Actuator on the F-18 Systems Research Aircraft", NANASA/TM-97-206224, October 1997
Stefan Frischemeir, "Electrohydrostatic Actuators for Aircraft Primary Flight Control", Technical University Hamburg-Harburg
Charlotte Adams, "A380: 'More Electric' Aircraft", Avionics, 1 October 2001

External links

Electrohydrostatic actuation, Young & Franklin
Electrohydrostatic actuation, Parker

Aircraft components and systems

Airframe structure	Aft pressure bulkhead Cabane strut Canopy Cruciform tail Dope Empennage Fabric covering Fairing Flying wires Former Fuselage Hardpoint Interplane strut Jury strut Leading edge Lift strut Longeron Nacelle Rib Ring tail Spar Stabilizer Stressed skin Strut T-tail Tailplane Trailing edge Triple tail Twin tail Vertical stabilizer V-tail Y-tail Wing root Wing tip Wingbox

Flight controls	Aileron Airbrake Artificial feel Autopilot Canard Centre stick Deceleron Dive brake Electro-hydraulic actuator Elevator Elevon Flaperon Flight control modes Fly-by-wire Gust lock Rudder Servo tab Side-stick Spoiler Spoileron Stabilator Stick pusher Stick shaker Trim tab Wing warping Yaw damper Yoke

Aerodynamic and high-lift devices	Active Aeroelastic Wing Adaptive compliant wing Blown flap Channel wing Dog-tooth Droop Flap Gouge flap Gurney flap Krueger flap Leading edge cuff LEX Slats Slot Stall strips Strake Variable-sweep wing Vortex generator Vortilon Wing fence Winglet

Avionic and flight instrument systems	ACAS Air data computer Airspeed indicator Altimeter Annunciator panel Attitude indicator Compass Course deviation indicator EFIS EICAS Flight data recorder Flight management system Glass cockpit GPS Heading indicator Horizontal situation indicator INS Pitot-static system Radar altimeter TCAS Transponder Turn and slip indicator Vertical Speed Indicator Yaw string

Propulsion controls, devices and fuel systems	Autothrottle Drop tank FADEC Fuel tank Gascolator Inlet cone Intake ramp NACA cowling Self-sealing fuel tank Splitter plate Throttle Thrust lever Thrust reversal Townend ring Wet wing

Landing and arresting gear	Arrestor hook Autobrake Conventional landing gear Drogue parachute Landing gear Landing gear extender Oleo strut Tricycle landing gear Tundra tire

Escape systems	Ejection seat Escape crew capsule

Other systems	Aircraft lavatory Auxiliary power unit Bleed air system Deicing boot Emergency oxygen system Entertainment system Environmental control system Hydraulic system Ice protection system Landing lights Navigation light Passenger service unit Ram air turbine Weeping wing

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