Articles
Radu-Stefan Dragan, industrial fellow at the Royal Commission for the Exhibition of 1851 and electrical design engineer at Magnomatics
Hydraulic actuators are used on all types of aircraft control surfaces – from primary surfaces such as ailerons, elevators and rudder, which provide roll, pitch and yaw, to secondary surfaces such as flaps, slats and spoilers, which increase or reduce lift and speed on take-off and landing.
Their main drawbacks are linked to the power delivery system, which is composed of hydraulic lines and fluid spanning across the aircraft from the hydraulic reservoir to the hydraulic piston. This requires the aircraft to have a secondary hydraulic power source and distribution systems, increasing weight while reducing efficiency.
The fault-tolerant Pseudo Direct Drive (PDD) we have developed at Magnomatics consists of a magnetically and mechanically integrated brushless permanent magnet motor and a magnetic gear. The inner high-speed rotor of the PDD is synchronously coupled to the fundamental component of the stator field to produce electromagnetic torque. This torque is transmitted to the output pole piece rotor through the interaction between the stationary permanent magnets on the stator and the asynchronous space harmonic field. This results from the modulation of the fields of the permanent magnets on the inner high-
speed rotor by the pole pieces on the output rotor. By employing a PDD, the mechanical gear stages of the electrical actuator drivetrain can be removed, and the inner high-speed rotor of the PDD motor connected to the mechanical drivetrain through the magnetic gear element.
Owing to its compliance and torque-limiting characteristics, the magnetic gear element isolates the mechanical drivetrain from the effects of the kinetic energy stored in the high-speed rotor in the event of mechanical overload. The reduced number of mechanical contacts across the proposed actuator drivetrain improves reliability, while reducing weight and size.
It is also compatible with the trend to pursue a “more electric aircraft” platform where some hydraulic and pneumatic power systems are converted to electric. By employing electrical systems for power generation, the aircraft weight can be reduced which can mean fuel savings. Electrical systems also require less maintenance.
But the aerospace sector has yet to take up this technology, preferring to stick with its tried-and-tested electro-hydrostatic actuators. In contrast the PDD topology is new to aerospace and needs to be developed, tested and certified for flight under civil aviation safety standards.
The magnetically geared fault-tolerant actuator is being designed to supply a certain balance between reliability, fault tolerance and weight savings. The main challenges to overcome include proving that the reliability of the new actuator topology is the same or better than an existing electro-hydrostatic system. Consideration also needs to be given to the manufacturing of the electrical machine to identify suitable processes and optimise the design for mass production.