*3.2. Case Study and FMECA Analysis*

The flight-control system considered for analysis is based on a set of new direct-drive Electromechanical Actuators (EMA) responsible for the movement of a few secondary flight-control surfaces (active winglet and wingtip). As depicted in Figure 3, each actuator was driven by a Brushless-AC (BLAC) electric motor and controlled through 3 nested control loops, each monitoring the motor currents, its angular frequency, and the actuator linear position. A ball screw integral with the motor shaft was used to transform the rotary motion imposed by the BLAC into the sliding motion of the end user. Spherical joints connected the actuator to the aerodynamic surface on 1 end and to the airframe on the other. Each actuator came with a number of sensors used for control purposes; phase currents were measured, while a resolver was mounted on the motor shaft to monitor its position, dictate the phases commutation, and infer the angular speed. An LVDT was instead integral with the ball screw and used to precisely measure the position of the end-user, providing this information to the associated control loop. Phase voltages were also acquired. Such signals were then considered available for the development of the PHM system and the basis on which to evaluate the possible features and health indexes.

**Figure 3.** Architecture of the case study.

Once the available sensors were identified, guidelines provided in [1,15] suggested that an FMECA analysis tailored for PHM be performed, with the objective of assessing which failure modes needed to be prioritized when designing the PHM system. The most significant failure modes for each component were studied and their causes, symptoms, and failure effects were detailed, while a composite score was computed based on the fault's frequency of occurrence, the severity of effects, expected observability, and replaceability of the component. The results were hence ranked, and a priority list was derived, stating which failure modes were more interesting or more probably observable by a Health Monitoring framework. Such operations are described in more detail in [16] and allowed to select the following failure modes. For the electrical motor, 2 failure modes were selected: the occurrence of turn-to-turn shorts in the motor windings and the degradation of the motor's permanent magnets [17,18]. The reasoning behind this choice is that the first was the most probable failure mode involving short circuits [16], while the second was critical from a severity perspective since it involved the loss of the actuator damping capability, which prevented flutter in the case of a sudden loss of power. Two additional failure modes pertaining to the motor were addressed for fault detection only, due to their causes and dynamics: the occurrence of static eccentricity in the Brushless-AC motor and the occurrence of MOSFET Base-Drive Open circuit conditions within 1 leg of the inverter supplying the actuator. The first is related to mistakes during the assembly and is tracked to avoid the triggering of a false alarm on the "real" half-wing of the iron bird, while the second is often indicated as one of the most probable failure modes involving PWMdriven inverters [19,20]. Switching the attention to the mechanical drive, 3 failure modes were assessed as preferential. The first was the effect of lubricant degradation, and the consequent efficiency losses within the ball screw [21]. The other 2 were the occurrence of backlash due to wear within the mechanical transmission and within the spherical rod end connecting the actuator to the aerodynamic surface. The lubricant degradation was chosen since it was expected to be a frequent and inevitable occurrence over prolonged usage, while the progressive increase of backlash was selected due to its expected frequency of occurrence and severity of the possible effects, especially when considering the rod end [22]. All of the selected failure modes were expected to be observable or detectable.

The turn-to-turn short failure mode is covered in the literature [3,16], and several studies are available on the topic of magnet degradation and its detection [18,23]. Similar considerations can be taken for the selected failure mode for the MOSFET [20], the occurrence of static eccentricity [24], and the issues associated with lubricant degradation and backlash opening [25,26].
