Evaluation of the Correlation Coefficient As a Prognostic Indicator for Electromechanical Servomechanism Failures
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Evaluation of the Correlation Coefficient as a Prognostic Indicator for Electromechanical Servomechanism Failures Matteo D. L. Dalla Vedova 1, Paolo Maggiore 2 Lorenzo Pace 3 and Alessio Desando 4 1,2,3,4 Politecnico di Torino, Torino, 10129, Italy [email protected] [email protected] [email protected] [email protected] ABSTRACT 1. INTRODUCTION In order to identify incipient failures due to a progressive As defined by Vachtsevanos, Lewis, Roemer, Hess, & Wu wear of a primary flight command electromechanical (2006), the purpose of prognostics is to predict accurately actuator, several approaches could be employed; the choice the Remaining Useful Life (RUL) of a failing component or of the best ones is driven by the efficacy shown in fault subsystem. In fact, especially in aeronautics but also in detection/identification, since not all the algorithms might many other technological fields, the development of a be useful for the proposed purpose. In other words, some of prognostics health management (PHM) based fault-tolerant them could be suitable only for certain applications while control architecture can increase safety and reliability by they could not give useful results for others. Developing a detecting and accommodating impending failures thereby fault detection algorithm able to identify the precursors of minimizing the occurrence of unexpected, costly and the abovementioned electromechanical actuator (EMA) possibly life-threatening mission failures; reduce failure and its degradation pattern is thus beneficial for unnecessary maintenance actions; and extend system anticipating the incoming malfunction and alerting the availability / reliability. The advantages gained by means of maintenance crew such to properly schedule the PHM strategies are evident comparing the features of a servomechanism replacement. The research presented in the system developed according to this discipline with the ones paper was focused to develop a fault detection/identification of a classical design. The primary flight controls are a technique, able to identify symptoms alerting that an EMA critical feature of the aircraft system and are therefore component is degrading and will eventually exhibit an designed with a conservative safe-life approach, which anomalous behavior, and to evaluate its potential use as imposes to replace the related components subsequently to a prognostic indicator for the considered progressive faults certain number of flight hours (or operating cycles). This (i.e. frictions and mechanical backlash acting on approach fails in the detection of possible initial flaws, due transmission, stator coil short circuit, rotor static to the manufacturing process, that could generate a sudden eccentricity). To this purpose, an innovative model based failure, which could compromise the safety of the aircraft. fault detection technique has been developed merging Moreover, this design criterion merely requires the several information achieved by means of Fast Fourier component replacement, regardless of its effective status or Transform (FFT) analysis and proper "failure precursors" capability to operate still correctly, with the related (calculated by comparing the actual EMA responses with inefficiencies and additional costs. the expected ones). To assess the performance of the Instead, in a system suitably designed by taking into account proposed technique, an appropriate simulation test the PHM strategies, the failures could be managed in a more environment was developed: the results showed an adequate proper way, obtaining the following advantages: robustness and confidence was gained in the ability to early identify an eventual EMA malfunctioning with low risk of 1. lower operating costs; false alarms or missed failures. 2. less maintenance interventions are required; _____________________ 3. lower number of redundancies installed on board Matteo D. L. Dalla Vedova et al. This is an open-access article distributed aircraft; under the terms of the Creative Commons Attribution 3.0 United States 4. aircraft safety and reliability are improved; License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. International Journal of Prognostics and Health Management, ISSN 2153-2648, 2012 006 INTERNATIONAL JOURNAL OF PROGNOSTICS AND HEALTH MANAGEMENT 5. any maintenance work can be planned appropriately Finally, Section 6 was focused on generating several feature optimizing the necessary actions (limiting downtime maps for each fault modality with the previously found and related costs and allowing a more effective parameters. Then, the obtained maps were utilized for the organization of the maintenance and management of successful evaluation of the damage level affecting the spare parts warehouses) and limiting the logistical EMA. difficulties resulting from the manifestation of the fault. The research presented in this paper was focused on two 2. BACKGROUND main objectives. The first is the proposal of a numerical (a) EMA Faults and Degradations algorithm capable to simulate the dynamic behavior of a typical electromechanical servomechanism affected by one EMAs have only recently been applied in aeronautics. or multiple progressive faults. The developed numerical Therefore, the cumulated flight hours or on-board model takes into account dry friction, backlash, coil short installations of such units are not so much to achieve a circuit and rotor static eccentricity. The second consists in reliable statistics about the most recurring failures. Gökdere, the development of an innovative fault detection/evaluation Chiu, Keller & Vian (2005) show that it is possible to technique able to identify failure precursors (alerting that discern between four main categories of failures: the system is degrading) and to evaluate the corresponding 1. mechanical or structural failures; damage entity. The progressive degradation of a system component, indeed, does not initially create an unacceptable 2. BLDC motor failures; behavior, but only eventually leads to a condition in which 3. electronics failures; the whole actuation system operation could be 4. sensor failures. compromised. In order to fulfill these two objectives, an The present work has been mainly focused on the effects of innovative model-based fault detection and identification mechanical failures due to progressive wear, which causes technique has been developed merging several information an increase of backlash and friction, and on two typical achieved by means of FFT analysis and proper failure BLDC motor faults: the coil short-circuits ad the bearing precursors, i.e. system parameters whose variations could be wear generating rotor static eccentricity. associated with specific impending failure (Vichare & Pecht 2006). This technique was tested by means of numerical Sensor and electrical components failures are not less simulations on a typical aircraft primary command EMA, important than the other ones and their faults are often modeled in the MATLAB Simulink® simulation characterized by rather fast temporal evolutions. However, environment. The proposed method showed an adequate considering suitable time scales, it is possible to evaluate robustness, and more confidence was gained in the ability to precursors that can be used to take action (Ginart, Brown, early identify the malfunctioning, with low risk of false Kalgren & Roemer, 2010). Nevertheless, they are not alarms or missed failures. considered in this work because their time scales result often much shorter than the ones related to above mentioned The present work started with an extensive literature review, progressive faults. It is the intention of the authors to study focused both on the most common EMA fault modalities these types of failure in a next work. and propagation models and on the techniques and algorithms allowing their detection and evaluation. Then, Dry friction phenomena occur when two surfaces are in the numerical models, implemented in MATLAB relative motion: as the friction coefficient increases due to Simulink® and utilized to analyze the fault modalities, were wear, reaction torque becomes higher and the motor must described. The first model, described in Section 3, provide higher torques to actuate the control surface. As represents the typical electromechanical servomechanism, shown by Borello, Maggiore, Villero and Dalla Vedova and allows simulating the effects due to the four different (2010), increased dry friction, while still not causing the types of progressive faults previously mentioned. This seizure of the entire system, reduces the servomechanism model was coupled to the second one (see Section 4), which accuracy and, sometimes, influences the system dynamic represents the same EMA working in ideal conditions, i.e. response generating unexpected behaviors, such as stick-slip unaffected by any failure modality. The idea was to detect or limit cycles. the effects due to progressive faults by comparing the The mechanical wear could also generate backlash in EMA performance of the former with the response of the latter, moving parts such as gears, hinges, bearings and especially utilized as a monitoring model. By means of proper screw actuators. These backlashes, acting on the elements of algorithms, the obtained results could be later used to timely the mechanical transmission, reduce the EMA accuracy and identify the faults and evaluate their magnitudes. In Section can lead