Electro-hydrostatic actuator
Encyclopedia
Electro-hydrostatic actuators (EHAs), are an emerging aerospace
technology that aims at replacing hydraulic systems
with self-contained actuators operated solely by electrical power. EHAs would eliminate the need for separate hydraulic pumps and tubing, simplifying aircraft layout and improving safety and reliability.
. 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 aileron
s late-war models of the P-38L, curing the need for great human strength in order to achieve a higher rate of roll.
Over time, the systems evolved to replace the mechanical linkages to the valves with electrical controls, producing the "fly-by-wire
" design, 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.
s. 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.>
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.
Aerospace
Aerospace comprises the atmosphere of Earth and surrounding space. Typically the term is used to refer to the industry that researches, designs, manufactures, operates, and maintains vehicles moving through air and space...
technology that aims at replacing hydraulic systems
Hydraulic cylinder
A Hydraulic cylinder is a mechanical actuator that is used to give a unidirectional force through a unidirectional stroke. It has many applications, notably in engineering vehicles.- Operation :...
with self-contained actuators operated solely by electrical power. EHAs would eliminate the need for separate hydraulic pumps and tubing, simplifying aircraft layout and improving safety and reliability.
Conventional designs
Aircraft were originally controlled by small aerodynamic surfaces operated by cables, attached to levers that magnified the pilot's mechanical advantageMechanical advantage
Mechanical advantage is a measure of the force amplification achieved by using a tool, mechanical device or machine system. Ideally, the device preserves the input power and simply trades off forces against movement to obtain a desired amplification in the output force...
. 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
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
suffered from these problems, notably the Mitsubishi Zero and P-38 Lightning
P-38 Lightning
The Lockheed P-38 Lightning was a World War II American fighter aircraft built by Lockheed. Developed to a United States Army Air Corps requirement, the P-38 had distinctive twin booms and a single, central nacelle containing the cockpit and armament...
.
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 aileron
Aileron
Ailerons are hinged flight control surfaces attached to the trailing edge of the wing of a fixed-wing aircraft. The ailerons are used to control the aircraft in roll, which results in a change in heading due to the tilting of the lift vector...
s late-war models of the P-38L, curing the need for great human strength in order to achieve a higher rate of roll.
Over time, the systems evolved to replace the mechanical linkages to the valves with electrical controls, producing the "fly-by-wire
Fly-by-wire
Fly-by-wire is a system that replaces the conventional manual flight controls of an aircraft with an electronic interface. The movements of flight controls are converted to electronic signals transmitted by wires , and flight control computers determine how to move the actuators at each control...
" design, 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 leads to the possibility of EHAs are precision feedback controlled conventional motors, or high-power stepper motorStepper motor
A stepper motor is a brushless, electric motor that can divide a full rotation into a large number of steps. The motor's position can be controlled precisely without any feedback mechanism , as long as the motor is carefully sized to the application...
s. 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
Motion control
Motion control is a sub-field of automation, in which the position or velocity of machines are controlled using some type of device such as a hydraulic pump, linear actuator, or an electric motor, generally a servo...
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
Pulse-code modulation
Pulse-code modulation is a method used to digitally represent sampled analog signals. It is the standard form for digital audio in computers and various Blu-ray, Compact Disc and DVD formats, as well as other uses such as digital telephone systems...
. The result is a "power-by-wire" system, where both the control and energy are sent through a single set of wires.>
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.
External links
- Electrohydrostatic actuation, Young & Franklin
- Electrohydrostatic actuation, Parker