More-electric aircraft are not without challenges

  • 08-Oct-2012 03:03 EDT
fig 1 2011-01-2701 health monitoring EMA.jpg

Currently two main types of EMAs with linear and rotary output are in the focus of research and development activities.

Flight-control systems of civil aircraft have undergone huge developments in the last decades. The increasing size as well as speed of civil aircraft led to higher control loads. This evolution forced the introduction of hydraulically powered flight-control systems.

However, recent advantages in high-performance magnetic materials, miniaturized and highly efficient power electronics, and gear technology make electromechanical actuators (EMAs) increasingly competitive compared to hydraulic actuators regarding power-to-weight ratio, considering the whole flight-control system.

EMAs are typically composed of a permanent magnet electric motor mounted on a reduction gear and/or a linear screw and a power electronics box. Currently two main types of EMAs with linear and rotary output are in the focus of research and development activities.

An obvious approach is the replacement of linear hydraulic actuators by EMAs using ball or roller screws.

Most ball-screw-based concepts require additional reduction gears due to the low gear ratio of the screws. Roller-screw-driven EMAs offer the advantage of a high gear ratio, usually making reduction gears unnecessary.

The rotary EMA approach corresponds to conventional servo drives widely used in robotics and factory automation. However, supplying the required output torque, the gearbox diameters usually exceed the space available in the wing to incorporate an actuator at the hinge point of a control surface. Thus, EMA installation in a thicker part of the wing is necessary and linkage mechanisms are required to interface with the control surface.

The use of EMAs in primary flight-control systems has been investigated for several years, but some central issues have still not been solved satisfactorily. For example, EMAs do not currently reach the necessary lifetime required for primary flight control actuators. Wear of the incorporated gears may further result in an unacceptable amount of backlash, which affects flight control quality and could lead to flutter.

A major concern regarding the flight-control architecture is the probability of EMAs jamming resulting from numerous mechanical contacts of the incorporated gear teeth and ball bearings. Simply introducing redundancy by actuator duplication even increases the probability of control surface jamming, because each actuator has the ability to jam the entire control surface.

Conservative solutions might be shear pins or couplers, but they are either irreversible and reduce the availability of the aircraft in combination with high maintenance costs, or increase system weight, making them an undesired solution for jamming concerns.

Keeping in mind that every component of an actuator can fail and that size and weight are critical parameters for an actuator, the direct drive concept seems to be suitable for aerospace applications. Additionally, it offers the advantage of a comparably small parts count. The components of a direct-drive-based actuation system can be grouped in three categories: electrics, structure, and gearing.

The electrics group is composed of motor control electronics, a permanent magnet electric motor with a resolver for measuring degrees of rotation, and a position measurement device such as a LVDT (linear variable differential transformer). An EMA housing and rod ends build the structure group, while the gearing group is composed of bearings as well as a screw drive.

Analyzing the effect of a malfunction in one of these groups in relation to the possible main failure cases of an actuator—which are actuator jam, actuator runaway, actuator disconnect, and the loss of control surface efficacy—shows that the gearing as well as the electrics part contributes to three of the mentioned failure cases, while the structure just contributes to one case.

Knowing these failure cases raises the question for appropriate countermeasures. These measures could either be active (e.g., monitoring systems) or passive (e.g., redundant or fail-safe design).

Structural faults could lead to disconnection of the actuator from the control surface or airfoil. The faults could be due to material fatigue, corrosion, or manufacturing defects in the rod ends or the housing. Fail-safe design of these components is an alternative to reduce the likeliness of this actuator failure case to occur.

Actuator jam caused by electrical malfunctions can have several reasons. The most obvious is the loss of power supply in combination with a self-inhibiting screw drive.

Another failure cause is a short circuit in the motor windings or a fault in the motor control electronics. Redundant design of the electronics with a voter or other fault-detection mechanism would inhibit safety-critical single-point failures and offer the ability to realize an operational system even in the case of a present fault.

An actuator runaway can arise as a result of a defect in the motor control or by a defective position sensor of the EMA. Redundant design of the electronics as well as plausibility checks of the measured position by the LVDT against the position calculated from the motor resolver signal can minimize the risk of this actuator failure.

Efficacy loss, similar to the performance loss of a hydraulic actuator with defective piston sealing, can arise in an EMA when demagnetization occurs caused, for example, by overheating of the permanent magnets. This leads to a reduction of the nominal output motor torque considering the same current consumption. Temperature monitoring of the actuator provides a measure to prevent this failure from arising.

Also corrosion of aged connectors could decrease efficacy, making periodic checks of the EMA mandatory.

The ability to use redundancy clearly differentiates the electric related failure cases from the ones originating from the gearing because faults in the gearing components can jam the load path. To bypass the jammed components, couplings would be required that are too large to be incorporated in an EMA with its limited installation space.

This article is based on SAE technical paper 2011-01-2701 by Hannes Wagner, Galin Nikolov, Andreas Bierig, and Holger Spangenberg, German Aerospace Center (DLR).

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