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Electromechanical Actuators Advance But Hydraulic Still Reigns

Electromechanical actuation technology in aviation is advancing but will require a lot more work for its practical applications of high output force.

Printed headling: Actuation, Actually

Aircraft actuators—those devices that can move something as large as a landing gear or as small as a seat-recline system—are trending increasingly toward electromechanical technology.

“As the need for safer, more reliable, more fuel-efficient aircraft has evolved, the aerospace actuation industry has progressively migrated its power sources over several decades from manual—pulley and cable—to pneumatic and hydraulic, and ultimately to electrical power,” says Brian Berry, senior director for motion control solutions at Honeywell. “During this evolution to more electric actuation, the industry has had to address technical challenges in thermal management—Arctic cold and the extreme heat of sustained operation,” he adds.

In addition, engineers are developing sophisticated electronic controls, incorporating technology that promotes enhanced aircraft safety not only in commercial aircraft applications but also for the up-and-coming urban air mobility market.

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The move toward the more electric aircraft and electric actuators addresses a couple of key issues, according to Stan Griffin, director of engineering for Triumph Group’s fluid power and actuation business. “One is that hydraulic actuators leak. Despite all the subtle improvements in sealing technology and surface finishing, seals need to have a small amount of leakage to properly work,” he says. “And since most of the hydraulic actuator’s operation is in challenging environments, the failure of the occasional seal is inevitable. Secondly, utilizing electric actuators precludes the need for hydraulic lines, which reduces weight, space and installation costs.”

Griffin reports that as aircraft systems become increasingly electric, many hydraulic actuators are being replaced by electromechanical actuators. “However, we also see centralized hydraulic systems moving toward smaller, electrically powered distributed systems, which power hydraulic actuators that are not easily replaced by electromechanical actuators,” he notes.

He cites landing gear actuation as an example. “The power density of hydraulic actuators,” he explains, “gives them the ability to create a tremendous amount of force, in a relatively small package, compared to an electromechanical actuator.” A main landing-gear-retract actuator for a commercial widebody platform can have an output force capability well in excess of 100,000 lb. “An electromechanical actuator with the same output force and stroke requires a huge amount of electrical power, generates a tremendous amount of heat, and probably requires liquid cooling to keep the motor cool,” he says.

Griffin stresses that a “lot of work and advancements” in electromechanical actuation technology will be required for its practical application to high-output force, jam-proof or jam-resistant operation under rough environmental conditions. “That is why hydraulic actuators still reign in these kinds of applications,” he says.

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Nonetheless, he adds that electromechanical actuators are becoming more compact and thermally efficient every year and in the future will rival the power densities of a hydraulic actuator. “In the meantime, we are looking at mounting a smaller, noncentralized hydraulic system in the landing gear bay to power hydraulic landing gear actuators,” Griffin says. “This saves the cost and weight of a centralized hydraulic system with lots of fluid and lines running all over the airplane.”

Heavy Weight, High Power

Griffin says that weight reduction has also been a trend with actuation, ever since 5,000-psi hydraulic pressure systems were incorporated initially on the Airbus A380 and Boeing 787. “While a 5,000-psi actuator with the same rod size and cylinder bore size as a standard 3,000-psi actuator will be heavier, in order to handle the increased psi, the benefit is less hydraulic fluid,” he explains. “That results in an overall lighter hydraulic system since it requires a smaller reservoir and smaller hydraulic lines.”

Werner Buecheler, director of flight controls and actuation systems at Liebherr-Aerospace Lindenberg in Germany, says that while hydraulic actuator systems still play an important role, the market is demanding more electrically activated actuators. “The requirements are also pointing to actuators with more integrated electronics and the capability to be controlled by digital busses,” he explains. “Examples are our rotary actuators, where we are going to replace the conventional mechanical torque limiter with a magnetostrictive sensor to provide more flexibility and higher accuracy with fewer mechanical parts.”

He says that for more electric aircraft, conventional hydraulic actuators will be replaced by electromechanical actuators and electrohydrostatic actuators in primary flight controls, as well as electrically powered control units in high-lift systems—all with adjacent power and control electronics. “There is also demand for very compact units with higher power density for thin wing applications in modern wing geometries,” he explains.

Buecheler points out that higher power density requirements, such as those for trailing-edge actuators or leading-edge geared rotary actuators, require new development methods. They include optimized gear geometries, increased requirements with tolerance and enhanced production methods such as additive laser melting to achieve improved hydraulic valve blocks on actuators. “These methods are being continuously improved, and related prototypes are being tested permanently in our facilities,” he adds.

Architecture, Additive

Glynn Thomas, chief engineer for commercial actuation at the Moog Aircraft Group, cites recent trends indicating a movement away from conventional hydraulic servo actuation to more electrically driven actuators—based on two system architectures.

“These can be in the form of direct electromechanical, in which the power source and prime mover are electrical and electrohydrostatic, where the power source is electric but driving a pump and hydraulic prime mover, with an electric backup hydraulic actuator,” he explains. “The main incentive is to reduce or eliminate hydraulic systems weight and maintenance requirements. With the increased local complexity of the actuators, the control has also moved to be more localized with onboard electronics and digital communications reducing aircraft wire weight.”

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But at least for now, that presents an interesting situation, says Thomas, who points out that as actuators have moved toward more electric or electrohydrostatic systems, their weight and complexity “have typically increased,” with a consequential reduction in reliability traded against improved attributes at the aircraft-system level. “Power density of electrical motors and drives remains less than hydraulic in the current state, and it will be improvements in this field that will pave the way for further increased use of electro-mechanical actuators,” he notes.

Improvements, he predicts, will also be made on the manufacturing side, as future use of additive manufacturing will also reduce part count and consequently reduce assembly and maintenance times. “This technology will also permit faster manufacturing of spare parts in potentially lower batch quantities to support aftermarket turnaround,” he explains. “Also, one main driver of additive manufacturing is that now many parts can be combined into one more complex item, which would not be physically or economically possible using conventional subtractive techniques.” Doing so has the potential to save weight, cost and improve reliability.

Any actuator system weight reductions will likely involve the increased use of composite materials and lighter-weight electric drives, as well as additive manufacturing techniques, says Thomas.

The evolution from hydraulically powered to electrically powered actuators is “at the heart of Curtiss-Wright’s strategy as an electromechanical actuator specialist,” says Oliver Green, director of business development at Curtiss-Wright Actuation Group. He says there is increasing use of smart actuators featuring digital control—either located adjacent to, or as part of, the actuator itself.

“This trend is an enabler for building prognostics and health monitoring (PHM) functionality into the actuator, which results in improved direct operating costs and helps to make electromechanical actuators even more attractive for aircraft manufacturers,” he says. “We are also seeing continued and ongoing improvements in motor and power electronics technology that helps to significantly reduce actuator weight.” These trends have helped improve actuator reliability, “particularly with the control electronics, which are often located on the wing or in the landing gear bays and operate in very harsh environments,” he adds.

Engineering Challenges

While actuator technology advances, engineering challenges remain. Among them, Green cites the use of electromechanical actuators in primary flight controls, due to stringent safety requirements for airliners.

“Today, there is a lot of ongoing work between actuator manufacturers and aircraft manufacturers to improve electromechanical actuator reliability, to build in redundancy where required—without adding a significant cost/weight penalty—and to use PHM technology to provide advance warning that an actuator is degrading and could eventually fail,” he explains.

Green also notes that the use of direct drive architectures, which eliminate multiple stages of gearing between the motor and output, are helping to reduce the number of moving parts in electromechanical actuators. “This approach improves reliability by significantly reducing part count,” he says.

He adds that major improvements have been made in recent years to extend actuator life cycles. “One example of that is our most recent spoiler and flap electromechanical actuators, which are used on the Pilatus PC-24 business jet,” he says. “They are qualified for the life of the aircraft with no maintenance. We are actively exploring further actuator life-cycle improvements that will enable us to provide the same qualification on aircraft more heavily utilized than business jets.”

As promising as electromechanical actuation is, there are still some technology challenges on the motor side of the equation.

“Di-electric robustness is a concern,” says Mike Beasley, business development engineer for U.S. aerospace and defense, at Maxon Precision Motors. The Switzerland-based company is a major supplier of motors for commercial airliner interior actuation systems and fire-suppression actuators. “Di-electric breakdown happens when current flows between parts in the actuator that should have a conductive path, such as motor windings to motor housing. This can happen if there is a large voltage potential difference between these two parts,” he says. This is not a concern with hydraulic systems.

Beasley reports that shock, vibration and wide temperature swings also present issues for electromechanical systems. He notes that engineers are trying to develop electric actuators for application to extremely high-temperature environments, such as jet engines. “This is still in the research and development phase, since one of the main failure modes of an electric motor is thermal overload,” he says. “When ambient temperatures are extreme and without external cooling, the motor will reach its thermal limits.”

However, he stresses that optimizing the motor for the specific actuator design can also present challenges.

“This has to be balanced with what is available,” says -Beasley. “Building a brand-new motor design from the ground up is very expensive, thus the use of modified commercial off-the-shelf products is important in terms of cost. Fortunately, a wide range of options is available to help customers optimize the motor for their design without having to do a fully custom motor design.”

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