NAUČNOSTRUČNI I

 INFORMATIVNI ČASOPIS

 ZA OBLAST TEHNIČKE

 DIJAGNOSTIKE U TEHNIČKIM

 I SRODNIM DISCIPLINAMA

APPLICATION OF HEALTH MONITORING AND PROGNOSTIC SENSORS IN MECHANICAL

AND ELECRONIC SYSTEMS

PRIMENA SENZORA ZA MONITORING I PROGNOZU STANJA KOD MEHANIČKIH

I ELEKTRONSKIH SISTEMA 

Prof. dr Dragoljub Vujić, dipl. inž, Vojnotehnički institut, 11132 Beograd, Ratka Resanovića 1 

ABSTRACT 

This paper deals with application of health monitoring and prognostics sensors in mechanical and electronic systems. It is shown that health monitoring and prognostics of mechanical systems and structure has a much longer history than electronic systems. Some sensors for electronic systems are still in a development stage. 

Key words: sensors, health monitoring and prognostics, maintenance, reliability 

REZIME 

Rad prikazuje primenu senzora za monitoring i prognozu stanja kod mehaničkih i elekronskih sistema. Pokazano je da monitoring stanja i prognostika mehaničkih sistema imaju mnogo dužu istoriju nego elektronski sistemi. Neki senzori za elektronske sisteme su još uvek u fazi razvoja.  

Ključne reči: senzori, monitoring i prognoza stanja, održavanje, pouzdanost 

1.  INTRODUCTION 

Many systems and structures, such as avionic systems, civil infrastructures, nuclear facilities, and mining machinery, are designed for long-term operations and these operations need periodic maintenance or replacement for safety and reliability. Reliability is essential in these applications because failure of these systems and structures can be lead to serious loss of lives or properties.

Health monitoring and prognostics is a proactive system to identify fault and determine health conditions or likely failure occurrence. It provides information for maintenance and replacement so that a failure can be prevented in advance. In addition, prognostics in health monitoring and prognostics systems also provides prediction of life consumption and remaining life, which combined with other information on health conditions offers the possibility of maintenance and replacement on a complete per-need basis, and hence, the possibility of significant cost savings.

The health monitoring and maintenance of many systems and structures has so far been depending upon visual inspection and testing that are time consuming, labor intensive, and costly. Using sensors in health monitoring and prognostics enables continuous monitoring of a systems or structures with minimal human involvement and with no interruption of operation. It eliminates the need of disassembly or prior knowledge of likely damage locations in the health monitoring and maintenance operation.

Utilizing smart sensors developed with the latest technologies in microelectronics, micro-electro-mechanical systems, and nano-technology has become a clear trend in health monitoring and prognostics. These sensors provide extended sensing capability and performance, improved design flexibility for monitoring applications, and reduced cost. Combined with communication and networking technologies, particular the wireless networking technologies, the latest health monitoring and prognostics sensors offer possibility for developing a fully autonomous, condition-based maintenance management system.

At the beginning of the 80 s of the last century significant changes occurred in maintenance of turbojet engines, as in fighter aircraft so in engines for civil aircraft at commercial flights. Instead of maintenance to precisely defined number of flight hours appeared a new on-condition maintenance concept.  

Owing to sensors integreated into aircraft structure, in recently time in aviation construction, it follows a propagation of the crack. The constructions build from so-called smart materials, especially composite materials, find in the last time a more and more usage. In this way it is expanded a design life of the construction and significantly reducing the costs. It is very important at military fighters exposed more loads than airplanes during commercial flights. One of the main problem in this case, it is a processing of the signal from sensors. The sensors need a central device or distributed networks for processing. Based to incomimg results, this device desides about the preventive actions necessary to attempt. Many researchs are performed in domain of the artifical inteligence, expert systems, fuzzy logic and pattern recognition. The artificial neural networks promise the best results in the domain of effective paterrn recognition for damage detection. The significant progress is achieved in the past years in domain of determing of the optimal number and sensor position for damage location. The different optimization technique are developted. A new development in this area includes an application of genetic algorithm. 

2. HEALTH MONITORING AND PROGNOSTICS SENSORS IN TRADITIONAL APPLICATIONS 

Health monitoring and prognostics has traditionally been employed on safety-critical mechanical systems and structures, such as propulsion engines, aircraft structures, and railroad suspension systems. Even in today, the applications of health monitoring and prognostics systems on mechanical machineries and structures remain dominant compared with other applications, such as electronic systems. The reasons for this can be summarized from the following perspectives: 

First, if operations are carried out under designated conditions, majority of failures now seen on properly tested and qualified mechanical systems are of wear-out failure mechanisms of fatigue, creep and corrosion. People have gained sufficient knowledge and developed technologies to avoid unexpected sudden failures. Unlike over-stress failures, wear-out failures are a result of damages accumulated in one or multiple processes. These failures can be prevented using periodic maintenance and replacement provided that the damage can be detected in advance. Fortunately, people have obtained enough knowledge and have mature technologies of damage detection and fault diagnostics on mechanical systems. These knowledge and technologies provide a fundamental basis for developing a feasible and an effective health monitoring and diagnose system for mechanical machineries and structures. As a result, people have been able to ensure a reliable operation of a mechanical system in each mission and use maintenance to achieve the reliable operation of the system for its entire cycle of life.

It is well known in the reliability engineering community that redundancy is an effective approach to improve the overall operational reliability of the system without improving technology itself. Although redundancy can be integrated into mechanical systems in some cases, such as the use of multiple instead of one engine on an aircraft, designing redundant mechanical systems is feasible in engineering only for very limited cases. Therefore, the health monitoring and prognostic approach, combined with periodic maintenance, needs to play a major role in high-reliability operation of mechanical systems and structures. 

In addition, health monitoring and prognostics of mechanical systems and structure has a much longer history than electronic systems. An enhanced health monitoring and prognostics system utilizing the latest sensing technologies sees a broader need and electronic benefit on the mechanical systems and structures than for electronic systems. People have had a much more ready and mature sensing, fault diagnostics, and reliability prognostic technologies for mechanical systems than for electronic systems. Table 1 provides examples of health monitoring and prognostics applications on mechanical systems and structures.

Table 1. Examples of health monitoring and prognostics sensor applications on mechanical systems and structures 

3.   HEALTH MONITORING AND PROGNOSTICS APPLICATION IN MICRO-ELECTRONIC DEVICES AND SYSTEMS 

Compared to mechanical systems, using sensors in electronic devices for health monitoring and prognostics purpose is an emerging application. Some IC sensors and cells that are designed for the reliable operation of safety-critical ICs have been developed and introduced. However, in contrast to a long application history of health monitoring and prognostics sensing for mechanical systems, these sensors and cells for electronic system remain in a development stage. Many issues need to be addressed before any actual applications become possible. So far, no reports have yet shown services of these health monitoring and prognostics sensors and in any actual applications.

Unlike mechanical systems and structures, electronic devices lack technologies in general for fault detection and diagnose. Fault of electronic devices are not limited to physical damages or defects. They also vary significantly in nature. Even for the physical damages, many of them are in a micron, submicron or even nano-scale and do not necessarily link to failures or a loss of designated electrical performance or function. Therefore, health monitoring and prognostics of electronic systems lacks straightforward approaches to proceed, compared to that of mechanical systems and structures. It can help to improve the mission reliability of an electronic system by integrated health monitoring and prognostics system. However, considering the complexity of an electronic system, it can also be difficult to quantify the improvement on a solid theoretical or experimental basis.

The proper functioning of most electronic systems depends on both the hardware and the software reliability of the system. Hence, an electronic system failure can be a result of hardware problems, software problems, or both problems. However, sensors and health monitoring and prognostics systems are effective primarily on hardware failures. Therefore, using the health monitoring and prognostics systems alone is not able to ensure highly reliable operation of complex electronic systems.

In contrast to mechanical systems an structures, which encounter inherent difficulty in implementing redundancies, design of redundancy is almost always feasible to an electronic system from the engineering standpoint. A simple theory on how to qualify the reliability improvement with redundancy is also available. To be able to evaluate reliability in a qualitative manner is essential to many safety-critical applications, such as avionics and nuclear operations, due to specified limits in reliability that the federal government requires on the operation of certain electronic devices in these applications.

Because of these issues, health monitoring and prognostics is unlikely to replace the role of redundancy in the high-reliability operations of electronic systems. The potential application of health monitoring and prognostics sensors in electronic devices will be likely used more for the purpose of cost reduction in maintenance and replacement than for the purpose of reliability improvement.

For health monitoring and prognostics system to be implemented in actual electronic system, other issues that need to be addressed include:

·   Dependability of data interpretation and end of life prediction

·   Additional design, manufacturing and operating cost for integrating an health monitoring and prognostics system vs. the cost saving for using the system in maintenance and replacement

·   Operating reliability of the sensing and data processing system itself.

4.   PHYSICS OF FAILURE BASED LIFE CONSUMPTION MONITORING 

The parameters that are monitored in health monitoring and prognostics system can generally categorized as those that can be used to define the failures of the systems to be monitored and that cannot. The length of crack in aircraft structures in the health monitoring of ICs and oil consumption are examples of the first category of parameters. The measurement results of the parameters of this category in monitoring environment provide a direct indicator of health condition and reliability. The parameters of the second category are primarily those parameters that are measures of environmental and stress conditions, such as temperature, humidity, vibration and radiation. In general, the technologies that are required to monitor the parameters of the first category are application-specific, while those technologies required for monitoring the second categories are usually standardized and may have multiple options in selection of monitoring devices.

Based on parameters that need to be monitored, health monitoring and prognostics regime can also be in two categories: a test-based system and a physics of failure based system. A test-based system health monitoring and prognostics system requires direct measurement or monitoring of the parameters that can be used to define failures. With this approach, the health condition of an electronic system to be monitored can be determined by benchmarking the test results against the failure criteria defined in the remaining life prediction can be achieved by extrapolating the test data to the defined failure point with or without a degradation model available on the parameters.

However, in actual application of complex electronic systems, defining and monitoring failure including parameters can be difficult and sometimes it may not be feasible from the perspective of engineering and business. Another danger with this health monitoring and prognostics system is that it does not require a thorough understanding of failure mechanisms that govern the failure process. As a result, the parameters that are monitored in the process may not be sensitive enough for effective monitoring in the process may not be sensitive enough for effective monitoring until the ultimate failure occurs. The alternative approach is to adopt a physics of failure-based health monitoring and prognostics system. This system considers actual environmental and loading conditions, and requires the understanding of the dependence of failure on those external environmental and loading conditions. In return, the health monitoring and prognostics of an electronic system can be completed basically by monitoring the system’s life cycle environment.

SUMMARY 

With potential application in the health monitoring and prognostics of electronic systems, two types of cells and sensors have been developed. However, for these emerging technologies to be used in actual applications of electronic systems, many issues need to be addressed. Considering the engineering feasibility in sensor implementation health monitoring and prognostics sensor are unlikely to play a major role in applications, such as avionics, that require stringent reliability operation with specified quantitative limits imposed by the federal government. They will be likely used more for the purpose of cost reduction in condition based maintenance and replacement than for the purpose of reliability and safety.

REFERENCES  

[1]  VUJIĆ, D. Turbojet engine maintenance systems, Scientific Technical Review, Vol.LIII, No.2, 2003.

[2]  VUJIĆ, D. Sophisticated diagnostic systems in aircraft engines, Scientific Technical Review, Vol.LIII, No.4, 2003.

[3]  Bergaglio L., Tortarolo F., RB199 Maintenance Recorder: an application of „on condition maintenance methods” to jet engines, Condition-based maintenance for highly engineered systems, Universita’ degli Studi di Pisa, Pisa, Italy, September 25-27, 2000.

[4]  Boller C., Staszewski W., Worden K., Manson G., Tomlinson G., Structure Integrated Sensing and Related Signal Processing For Condition-Based Maintenance, Condition-based maintenance for highly engineered systems, Universita’ degli Studi di Pisa, Pisa, Italy, September 25-27, 2000.

[5]  JINGSONG X., MICHAEL P., Applications of in-situ health-monitoring and prognostic sensors, University of Maryland, College Park, MD 20742, USA.