State of Development of Advanced Sensor

The advent of advanced development in sensors, microelectronics, adaptive signal processing and predictive technologies have significantly shaped the fundamental approach to dealing with traditional maintenance and repair problems within the aerospace industry. The concept of aircraft Diagnostics, Prognostics and Health Management (DPHM) is increasingly becoming a main stream approach to dealing with aircraft maintenance within an advanced operational autonomic logistics structure. Driven by the requirement for increased safety, reliability, enhanced performance and platform availability at reduced cost, key sensor technologies such as shape memory alloy, piezoelectric materials, magnetostrictive and electrostrictive materials, triboluminescent materials, optical fibres, carbon nanotubes, comparative vacuum monitoring, micro and nano electromechanical systems (MEMS/NEMS) are expected to play a significant role in the development of such DPHM systems. These advanced sensors, often referred to as smart sensors, are further expected to provide functionality that is not matched by current technologies and nondestructive evaluation techniques within in an on-line in-situ environment. Regardless of the extensive number of sensors and sensor systems that could potentially be employed or integrated within a DPHM system, only selected few illustrating near commercial exploitation are introduced in this document. 1.0 INTRODUCTION The increasing mission complexity, operational constraints and stringent performance requirements of legacy and emerging air platforms have made on-line and in-situ health monitoring and management a critical component of aircraft operational logistics and maintenance programs. From an operator’s perspective, the goal of such health management system is to improve operational efficiency and safety, lower life cycle costs, increase maintenance cycle, increase platform operational availability and reliability, and improve asset management. It is estimated [1] that about 60% of the overall ownership cost of an aircraft is attributed to Operation and Support (O&S). Most of this cost is linked to personnel and materials used to support scheduled and unscheduled maintenance actions. Moreover, because it is difficult to diagnose faults in some components, such as engine compressors, more than 40% of compressors replaced on aircraft are not defective resulting in unnecessary false repairs at additional cost. Realizing the significant savings and increased operational availability that could result from the development of on-line in-situ integrated health management systems, the Joint Strike Fighter (JSF) Program Management Office (PMO) has deployed an advanced capability into its new fighter jet program to reduce O&S costs, enhance performance and increase the efficiency the management of the fleet. Such advanced capability, known as Diagnostics, Prognostics and Health Management (DPHM), was initially conceptualized during the introduction of the JSF program for engine diagnostics and prognostics. State of Development of Advanced Sensor Systems for Structural Health Monitoring Applications 29 2 RTO-MP-AVT-144 Traditionally, real-time operating parameters and damage information are captured and compared against both test and field data to assess the potential damage severity and required maintenance actions. Such global health monitoring approach is well known as Condition-Based Maintenance (CBM). This approach is intended to help manage the frequency and extent of maintenance procedures for various components within a complex integrated system. The incorporation of CBM with predictive tools is expected to increase the level of effectiveness of on-line in-situ health monitoring and management systems’ capability and provide advanced life cycle assessment tools. Such capability is known as the DPHM capability and is expected to provide the ability to predict and pinpoint potential damage and problem areas; thus, reducing or perhaps even eliminating the need for periodic inspections. This capability will not only reduce platform’s ownership cost, that is considered to be a critical concern for civilian aircraft operators, but also increase air safety, that is mandated by the Federal Aviation Administration (FAA) (e.g. 80% accident reduction by year-end of 2007). Faas et al. [2] reported that the ultimate results of a DPHM capability are to be able to reliably predict failure with high probability and high confidence. However, they claim that realistically one hopes that such systems provide tools to integrate with existing systems to break the ambiguity groups and then provide some insight into the predictive side. Furthermore, by deploying such capability, they identified that the specific war fighter benefits include reduced maintenance man-hours, restored aircraft to operational status sooner, reduced consumption of spare parts, reduced mission aborts due to system failures, reduced deployment footprint and increased warning time to order spares and invoke proactive repairs. Results reflecting the return on investment (ROI) for the deployment of an on-board monitoring system are illustrated in Table 1 [2]. Table 1: ROI for the deployment of on-board monitoring system. 100% On-Board 75% On-Board 50% On-Board 25% On-Board Return On Investment (ROI) 9.2 11.5 16.5 24.5 Time to Break Even (Years) 10 10 9 7 2.0 DIAGNOSTICS, PROGNOSTICS AND HEALTH MANAGEMENT (DPHM) 2.1 A Definition of DPHM A DPHM system is considered to be the central information system for on-line collaborative environment in aircraft autonomic logistics. DPHM has the capability to determine the state of health of a component to perform its function(s) (Diagnostics). It is also a predictive diagnostics tool that includes determining the remaining life or time span of proper operation of a component (Prognostics). It has the capability to report appropriate decisions making information about component maintenance based on diagnostics/prognostics information, available resources, and operational demand (Health Management) [3]. DPHM evolved from a vision of advanced diagnostics that uses advanced sensors to monitor and manage aircraft health and quite often is referred to as on-line structural health monitoring or advanced on-line nondestructive evaluation capability. 2.2 DPHM Architecture and Structure Depending on the application and desired outcome, a DPHM system varies in complexity from simple (e.g. detect and alert) to more complex (e.g. detect and advise). Figure 1 illustrates a conceptual implementation State of Development of Advanced Sensor Systems for Structural Health Monitoring Applications RTO-MP-AVT-144 29 3 approach of a DPHM system having moderate complexity; whereas, Figure 2 [4] illustrates the expected complexity for integrating an aircraft DPHM system within a military operational autonomic logistics structure. Figure 1: A moderate complexity conceptual structure of a DPHM system. Integrated Supply/ Engineering Info systems Lessons Learned/ Failure Rates Govt/Industry/Depot Activities Wholesale Stock point Supply of AOG item/ Replenishment of Ship Stock Return of Defective Item Automatic/JIT Re-Supply Automated Logistics Update PHM Download Operational Requirement Repair Required Replenishable Requirements Onboard Stock Repair Recommendation Aircraft Turn-round & Repair Just-in-Time Training Simulated World Haptic Maintenance Rehearsal Graphical Audio Reduced Log Footprint Aircraft Available