Electro Hydrostatic Actuators

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.

The primary development that lead to the possibility of EHAs was the precision feedback controlled conventional motor, or high-power stepper motor^{[citation needed]}.^[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]