Hybrid actuation in primary flight control systems : a force-fight inhibiting system architecture

• Hybride Stellantriebe in Primären Flugsteuerungssystemen - Eine Kraftkonflikt vermeidende Systemarchitektur

Röben, Tobias; Stumpf, Eike (Thesis advisor); Moormann, Dieter (Thesis advisor)

Aachen (2018, 2019)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2018

Abstract

In civil aviation industry effort for optimization and increase of efficiency has been concentrated on single subsystems for decades. Nowadays the whole system architecture is getting into focus. A common approach to reduce system integration effort and increase efficiency is to substitute hydraulic and bleed-air powered systems by electrically powered ones. This applies especially for large aircraft, where the distribution of centralized generated hydraulic power is costly. Meanwhile alterations towards More Electric Aircraft (MEA) have been implemented for non-critical respectively backup systems. The consequent next step is to apply those concepts to critical systems, like Primary Flight Control (PFC), which so far rely on a triplex redundant hydraulic power distribution system. Herein pipes are connecting the central hydraulic power source with the actuation sinks distributed all over the aircraft dimensions. The integration of this rigid piping is elaborate and implies transfer losses as well as leakage of toxic fluid. Power-by-wire actuators in comparison facilitate the complete omission of hydraulics. Nevertheless, since there are safety and reliability challenges and less experience with electrically driven actuators, hybrid configurations arise in the meantime. Within those configurations hydraulic actuators are working adjacent to electric ones on the same control surface. Power distribution is of both electric and hydraulic type and therefore dissimilar. Considering such a hybrid actuator configuration to operate an aircraft control surface, an active/active mode is intended. This is most efficient and leads to less system size and weight, because the actuators are sharing their workload equally and can thus be dimensioned accordingly. Besides an active/passive mode is hardly applicable within a hybrid configuration featuring an electromechanic drive. Due to its transmission it bears high inertia on output level, when driven generatively. Furthermore dissipation of this electric energy becomes problematic. Ultimately electric drives have to be operated actively in future, in order to prove technology readiness, which is necessary to replace the conventional hydraulic flight controls in civil aviation. Consequently this publication addresses the feasibility and capability of an active/active hybrid actuator configuration. The presented system comprises three different principles of actuation, which are conventional servo-hydraulic, electrohydrostatic and electro-mechanic, within an academic fly-by-wire flight control system. It represents all physical and electric components, such as cockpit control interfaces, direct mode flight control computers and decentral actuator positioning electronics. Requirements and impacts of such a configuration are pointed out. The main challenge that comes with hybrid active/active surface actuation is the occurrence of opposing structural loads during operation, called Force Fight: Every single dissimilarity regarding the hybrid assembly and the actuator’s functional principle leads to imbalanced command response and interference behavior. These positioning mismatches result in stress within the hyperstatic system structure. In contrast the positioning behavior of homogeneous configurations is rather uniform. While unpredictable structural loads might reduce the fatigue limit drastically, hybrid active/active configurations are out of the question in today’s flight control systems. In the face of this current show stopper the presented hybrid actuation system has been set up physically. It demonstrates that Force Fight can be reduced to a minimum level even in such an exceptional configuration. For this purpose a model following feed-forward control concept is applied, in order to equalize the actuator’s specific response behavior and imprint uniform motion. In parallel to this feedforward position control a PID type force feedback and the position controller reduce remaining static deviations. Thereby both origins of force fight, static and dynamic, are addressed by dedicated means of control. The specific concept is embedded within a state of the art system architecture. It does not demand for additional electronics and therefore does not affect complexity. In general it is shown that the proposed architecture is competitive with conventional flight control systems not only in terms of performance, but also in terms of safety and reliability. It constitutes an intermediate solution and is thereby an enabler for further transition towards all electric flight controls.