Airbus A350 - General Discussion (2013)
May 5, 2013 9:33:29 GMT 1
FabienA380, tomparis1, and 2 more like this
Post by philidor on May 5, 2013 9:33:29 GMT 1
Baroque's post made me try to learn more about electro-hydrostatic actuators (EHAs). Here are some quotations from Wikipedia :
About conventional designs :
"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."
About EHAs :
"The primary development that lead to the possibility of EHAs was the precision feedback controlled conventional motor, or high-power stepper motor. Stepper motors are designed to move a fixed angle with every application of energy, and do so repeatedly in an extremely precise fashion....
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.
About conventional designs :
"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."
About EHAs :
"The primary development that lead to the possibility of EHAs was the precision feedback controlled conventional motor, or high-power stepper motor. Stepper motors are designed to move a fixed angle with every application of energy, and do so repeatedly in an extremely precise fashion....
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.