Development of a Health-and-Usage-Monitoring-System for Ball Screws in Electro-Mechanical Actuators

The More Electric Aircraft is one of the main drivers in current aircraft systems' research and development. Indeed, higher electrification of aircrafts can reduce complexity as well as the weight of power supply systems. In this respect, one of the principal aims of MEA is to replace the hydraulic supply with electric technologies in the primary flight control surfaces. In order to do that, it is possible to employ an Electro-Mechanical Actuator (EMA) which mainly consists of a Brushless DC motor, a reducer and a ball screw. However, although this replacing seems feasible in theory, there are several issues that have to be overcome. One of these is represented by the wear mechanisms that affect the ball screw due to disadvantageous motion profiles of primary flight controls. In the light of this assessment, the principal goal of this thesis is the development of a suitable Health-and-Usage-Monitoring-System for ball screws embedded in EMAs. In attempt to do that, firstly an extensive literature research was carried out, in order to understand the state-of-the-art of primary and secondary flight controls, ball screw's structure, wear mechanisms as well as principal HUMS's concepts. After that, a high fidelity ball screw model was developed in Matlab/Simulink environement, in order to simulate the degradation in ball screws. In this respect several simulations was carried out to highlight the model's behavior when subjected to different parameters such as external force, preload and backlash. In the last part of the thesis a second model capable to simulate the EMA's nominal condition was derived from the high fidelity model. By comparing the parameters of the monitoring model with the high fidelity model, a health assessment of the actuator can be carried out. In particular, in this thesis two methodologies are proposed: one uses the frequency spectrum of the motor current and another utilizes the variations of several indexes capable to detect the deviations of e.g. motor current and nut position due to degradation. Finally, by using the motor current index, a methodology to identify the actuator's Remaining Useful Life (RUL) is proposed. On the basis of the obtained results, it is possible to conclude that the proposed methodologies should be capable to detect degradation in ball screws. Indeed, the deviations caused by degradation are high enough to detect decreasing in EMA's performances. For this reason, the results obtained in this thesis represent promising concepts for further analyses and experimental validation.