Actuators on aircraft perform a number of important functions such as adjusting flight control surfaces like the elevator, rudder, ailerons, flaps, slats and spoilers, extending and retracting landing gear, positioning engine inlet guide vanes and thrust reversers, and opening and closing cargo or weapon bay doors.

Commercial aircraft actuation applications. Source: Qiao et al. A review of electromechanical actuators for More/All Electric aircraft systems. Image adapted from Jean-Charles Maré: Aerospace Actuators 1, Wiley 2016. (Click image to enlarge)Commercial aircraft actuation applications. Source: Qiao et al. A review of electromechanical actuators for More/All Electric aircraft systems. Image adapted from Jean-Charles Maré: Aerospace Actuators 1, Wiley 2016. (Click image to enlarge)

Over the past several decades, the source control signals as well as power for actuators onboard aircraft has evolved. Starting with manual sources like cables and rods, actuation technology gradually advanced to hydraulically- and electrically-driven solutions. The transition away from manual power sources began with hydromechanical systems in which the movement of a control column or lever was transmitted mechanically to operate control valves in the hydraulic circuit, filling and emptying cylinders to produce actuator movement.

Later, fly-by-wire systems replaced mechanical linkages with electrical cables. In a fly-by-wire system, the pilot’s control column movements are interpreted by a flight computer that sends electrical signals to actuator control electronics. The control electronics instruct the operation of either hydraulic control valves to set in motion hydraulic actuators or electric motors to move electromechanical actuators. Fly-by-wire systems enabled aircraft manufacturers to integrate more electrically powered actuators in airplane systems such as electrohydrostatic actuators and electromechanical actuators. These actuators are powered by electricity produced by engine-driven generators delivered over power-by-wire systems.

Elements of a fly-by-wire flight control system. Source: Garg et al. Application of fiber optics in aircraft control system & its development. 2014 International Conference on Electronics and Communication Systems (ICECS). (Click image to enlarge)Elements of a fly-by-wire flight control system. Source: Garg et al. Application of fiber optics in aircraft control system & its development. 2014 International Conference on Electronics and Communication Systems (ICECS). (Click image to enlarge)The shift comes as the aerospace industry moves toward more-electric aircraft with a long-term ambition of all-electric aircraft. The motivation to convert mechanical, pneumatic and hydraulic systems to electric systems is driven by a desire to optimize aircraft performance, reduce maintenance and operating costs, increase fuel efficiency and reduce emissions.

When selecting an actuation technology, a number of considerations are taken into account. Application requirements like force and velocity specifications as well as the size and weight of the actuator are critical. The total cost of ownership factors like energy efficiency, reliability and safety are also important when evaluating total operating costs over the life of the system. Every additional ounce of weight and watt of power required to operate an actuator results in additional fuel burn, making it more expensive to fly the aircraft.

This article discusses three actuation technologies – traditional electrohydraulic actuators, electrohydrostatic actuators and electromechanical actuators – examining their operating principles, advantages and disadvantages as well as focusing on the linear varieties of these actuators in which hydraulic or electric power is converted to linear motion. Hydraulic rotary actuators and electric rotary actuators are also available, in which power is converted to rotational motion.

Simplified schematics for a traditional electrohydraulic actuator (left), an electromechanical actuator ( middle), and an electrohydrostatic actuator (right). Source: Lee et al. Achieving High-Performance Electrified Actuation System with Integrated Motor Drive and Wide Bandgap Power Electronics. (Click image to enlarge)Simplified schematics for a traditional electrohydraulic actuator (left), an electromechanical actuator ( middle), and an electrohydrostatic actuator (right). Source: Lee et al. Achieving High-Performance Electrified Actuation System with Integrated Motor Drive and Wide Bandgap Power Electronics. (Click image to enlarge)

Electrohydraulic actuators

Traditional electrohydraulic actuator systems require a central hydraulic power supply with hydraulic lines leading to each actuator. Electric command signals control a servo valve to vary the amount of hydraulic fluid delivered to the actuator from the main hydraulic fluid supply.

Aircraft control-surface control system powered by an electrohydraulic servovalve. Source: NASA. Edwards, J. W. (1972). Analysis of an electrohydraulic aircraft control surface servo and comparison with test results. (Click image to enlarge)Aircraft control-surface control system powered by an electrohydraulic servovalve. Source: NASA. Edwards, J. W. (1972). Analysis of an electrohydraulic aircraft control surface servo and comparison with test results. (Click image to enlarge)

This type of electrohydraulic servo actuator system, consisting of an electrohydraulic servo valve and hydraulic actuator, is capable of producing very high forces with no backlash.

Electrohydraulic servoactuator for the A380 fly-by-wire system used to actuate spoiler surface position 4. Source: Liebherr (Click image to enlarge)Electrohydraulic servoactuator for the A380 fly-by-wire system used to actuate spoiler surface position 4. Source: Liebherr (Click image to enlarge)The system, however, requires a centralized hydraulic network which is kept at constant pressure (common aircraft hydraulic system operating pressures are 3,000 psi or 5,000 psi) by hydraulic pumps continuously bleeding energy from the engines. The continuous energy expenditure results in heating of the hydraulic fluid, requiring a cooling system to maintain acceptable hydraulic fluid temperature.

The central hydraulic network also requires a system of pipes to deliver pressurized hydraulic fluid to actuators distributed throughout the aircraft, adding additional weight and taking up space. The large hydraulic network increases the risk of leakages and requires a large volume of hydraulic fluid.

Conventional electrohydraulic actuators have excellent power density (kW/kg) at the equipment level but poor power density at the power distribution network level.

Electrohydrostatic actuators

Electrohydrostatic actuators, by contrast, are self-contained hydraulic units that eliminate the need for a central source of hydraulic power and associated hydraulic plumbing. Electrohydrostatic actuators exploit high equipment-level power density and eliminate inefficiencies associated with a central hydraulic power distribution network.

Electrohydrostatic actuation systems convert electrical energy to hydraulic energy to mechanical energy locally at the actuation site. The self-contained unit consists of an electric motor which drives a hydraulic pump to pressurize fluid for a hydraulic actuator.

To control the power output of an electrohydrostatic actuator (for example, to move loads of various magnitudes at the same speed), the pump output flow must be controlled. This can be accomplished with either a variable speed electric motor driving a fixed displacement hydraulic pump, or a fixed speed electric motor driving a variable displacement hydraulic pump.

Electronic command signals are delivered to the electrohydrostatic actuator over electrical cables. In the common variable speed electric motor-driven electrohydrostatic actuator, the signals control the speed of the electric motor to provide rotational power to the hydraulic pump which generates pressurized hydraulic fluid to move the hydraulic cylinder locally at the actuator.

Diagram illustrating the main components involved in converting electric power to actuator movement in an electrohydrostatic actuator. Source: Qiao et al. A review of electromechanical actuators for More/All Electric aircraft systems. (Click image to enlarge)Diagram illustrating the main components involved in converting electric power to actuator movement in an electrohydrostatic actuator. Source: Qiao et al. A review of electromechanical actuators for More/All Electric aircraft systems. (Click image to enlarge)

Compared to traditional electrohydraulic actuators, electrohydrostatic actuators operate with higher energy efficiency. Instead of continuously bleeding power from the engines to maintain a large hydraulic network at constant pressure, electrohydrostatic actuators only consume power (in the form of electricity produced by engine-driven generators) when moving the load. Lower energy expenditure produces less heat in the hydraulic fluid, eliminating the need for a cooling system. With fewer points of failure compared to a central hydraulic network with extensive plumbing, the potential for leaks is reduced and maintenance demands are diminished.

The fly-by-wire flight control system of the F-35 multirole combat aircraft. F-35s have a full power-by-wire system with no centralized hydraulic network. Source: Moog (Click image to enlarge)The fly-by-wire flight control system of the F-35 multirole combat aircraft. F-35s have a full power-by-wire system with no centralized hydraulic network. Source: Moog (Click image to enlarge)

Electrohydrostatic actuators also have advantages over electromechanical actuators. Higher power density allows them to produce higher forces in a more compact package. They also have no backlash issues, enabling precise positioning with no error caused by gaps between mechanical components. In addition, electrohydrostatic actuators do not suffer from the risks of jamming that arise due to interference between gear teeth or screw threads in electromechanical actuators.

A drawback of electrohydrostatic actuators is that they require hydraulic fluid for operation. Although fluid volume is much reduced compared to conventional electrohydraulic actuation systems, the presence of the fluid negates the potential for 100% leak-free operation.

Actuation components of the primary flight control system of the F-35 multirole combat aircraft. The aircraft’s rudder, flaperon and horizontal tail are powered by electrohydrostatic actuators. Source: Moog (Click image to enlarge)Actuation components of the primary flight control system of the F-35 multirole combat aircraft. The aircraft’s rudder, flaperon and horizontal tail are powered by electrohydrostatic actuators. Source: Moog (Click image to enlarge)

Electromechanical actuators

Electromechanical actuators do not use hydraulic fluid, eliminating the presence of the toxic and flammable liquid and its associated piping, power sources and potential for leaks.

Electromechanical actuators convert electrical energy to mechanical energy. An electric motor drives a linear actuator. The rotary motion of the servo motor is coupled mechanically through a gearbox to an acme leadscrew, ball screw or planetary roller screw for conversion to linear motion. Direct-drive versions are also possible in which the motor is directly coupled to the screw mechanism without a gearbox.

[Discover AC servomotors and DC servomotors on Engineering360.]

Diagram illustrating the main components involved in converting electric power to actuator movement in an electromechanical actuator. Source: Qiao et al. A review of electromechanical actuators for More/All Electric aircraft systems. (Click image to enlarge)Diagram illustrating the main components involved in converting electric power to actuator movement in an electromechanical actuator. Source: Qiao et al. A review of electromechanical actuators for More/All Electric aircraft systems. (Click image to enlarge)

A primary benefit of electromechanical actuators compared to conventional electrohydraulic actuators is their elimination of hydraulic fluid. The absence of this liquid and the pipes needed to carry it results in increased safety, reduced weight, saved space, higher energy efficiency and lower environmental impact. Easier maintenance is facilitated by no risk of leaks and a lack of fluid conditioning tasks like filling, charging, purging and filtering.

Electromechanical actuators may appear to be an optimal solution for more-electric and all-electric aircraft, since they completely eliminate the need for hydraulic fluid. Current electromechanical actuator technology, however, runs into limitations for applications requiring high output forces. For example, a large commercial airliner’s main landing gear retraction actuator requires a load capability exceeding 100,000 pounds of force. In such cases, hydraulic actuators have a power density advantage, able to generate huge forces in a small space in harsh conditions without requiring a liquid cooling system for the motor.

Planetary roller screw mechanisms convert rotary motion to linear motion. Pictured are (a) inverted and (b) standard configurations. Planetary roller screw mechanisms can handle higher loads than ball screw mechanisms because rollers have more contact points with the screw compared to balls, distributing the load and reducing maximum contact stresses. Source: Qiao et al. Friction Torque Modelling and Efficiency Analysis of the Preloaded Inverted Planetary Roller Screw Mechanism. (Click image to enlarge)Planetary roller screw mechanisms convert rotary motion to linear motion. Pictured are (a) inverted and (b) standard configurations. Planetary roller screw mechanisms can handle higher loads than ball screw mechanisms because rollers have more contact points with the screw compared to balls, distributing the load and reducing maximum contact stresses. Source: Qiao et al. Friction Torque Modelling and Efficiency Analysis of the Preloaded Inverted Planetary Roller Screw Mechanism. (Click image to enlarge)

The downsides of electromechanical actuators include backlash, jamming and thermal management issues. Backlash can occur due to gaps between interlocking gear teeth or screw threads and results in positional inaccuracies. Backlash can increase as repeated wear cycles cause surface degradation. Jamming is a risk for electromechanical actuators due to potential failures involving screw components interfering or seizing up, preventing motion of the actuator. This can occur as a result of:

  • Mechanical wear of the gear and screw assembly including fatigue due to external loads causing high contact stresses on the raceway
  • Reduced lubricant viscosity and thickness due to high temperatures
  • Catastrophic failure of components

Thermal management is also an issue for electromechanical actuators in high-load applications. In electromechanical actuators, heat is generated in the electric motor due to electrical resistance in the copper stator windings and iron stator core, as well as friction in the gearbox and screw mechanism. Heat dissipation in conventional hydraulic systems is more easily achieved by hydraulic fluid circulation and heat exchange in the main tank (e.g. using cooler fuel to absorb heat from the hydraulic fluid). Electromechanical actuators, by contrast, must deal with heat dissipation locally. Potential solutions include heat sinks, heat pipes, liquid cooling and phase-change materials.

As a relatively new actuation technology for aircraft, the body of knowledge and data around the safety and reliability of electromechanical actuators in operation on aircraft is lacking. The reliability concerns associated with electromechanical actuators point to a need for solutions to improve their operational safety. These solutions could include designs incorporating fault tolerance and redundancy as well as systems for health monitoring and predictive maintenance. Health monitoring systems could incorporate sensors to measure position, backlash, load, torque, vibration and temperature with data generated used in models to predict ideal maintenance intervals to prevent failures before they occur.

Electromechanical actuators are not yet considered mature enough as actuation solutions for primary flight controls that continuously perform safety-critical aircraft flight trajectory corrections (e.g. the rudder adjusts yaw, the ailerons control roll and the elevator changes pitch). Electromechanical actuators, however, are entering service in less critical roles onboard aircraft. They are used on the Boeing 787 for trimming the horizontal stabilizer, actuating the mid-board spoilers and activating the landing gear brakes.

A selection of hydromechanical, electrohydraulic, electrohydrostatic and electromechanical actuators used on commercial and military aircraft and the European Vega rocket launch system illustrating the progression of aerospace actuation technologies toward all-electric actuation. Source: Maré & Fu. Review on signal-by-wire and power-by-wire actuation for more electric aircraft. (Click image to enlarge)A selection of hydromechanical, electrohydraulic, electrohydrostatic and electromechanical actuators used on commercial and military aircraft and the European Vega rocket launch system illustrating the progression of aerospace actuation technologies toward all-electric actuation. Source: Maré & Fu. Review on signal-by-wire and power-by-wire actuation for more electric aircraft. (Click image to enlarge)

Despite the challenges, electromechanical actuator technology continues to advance as the industry seeks improvements in reliability, thermal efficiency and package sizes. A future may be within sight in which hydraulic actuation is no longer needed onboard aircraft. To bridge the gap, self-contained electrohydrostatic actuators are reducing the need for central hydraulic systems in the transition toward more-electric aircraft with reduced fuel burn and lower maintenance costs.

Resources

Jean-Charles, Maré, & Jian, Fu. (2017). name="_Hlk22651717">Review on signal-by-wire and power-by-wire actuation for more electric aircraft. Chinese Journal of Aeronautics, 30(3), 857-870.

Qiao, G., Liu, G., Shi, Z., Wang, Y., Ma, S., & Lim, T. C. (2018). A review of electromechanical actuators for More/All Electric aircraft systems. Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science, 232(22), 4128–4151.

Lee, W., Li, S., Han, D., Sarlioglu, B., Minav, T. A., & Pietola, M. (2017, September). Achieving High-Performance Electrified Actuation System with Integrated Motor Drive and Wide Bandgap Power Electronics. In 2017 19th European Conference on Power Electronics and Applications (EPE'17 ECCE Europe) (pp. P-1). IEEE.

Helbig, Achim. Electro-hydrostatic Actuation. Moog [PDF]

Maré, J. C. (2016). Aerospace Actuators 1: Needs, Reliability and Hydraulic Power Solutions. John Wiley & Sons.

Maré, J. C. (2017). Aerospace Actuators 2: Signal-by-wire and Power-by-wire. John Wiley & Sons.