Combining Health Monitoring and Control

Introduction

The safe and reliable operation of technological systems (cars, planes, trains, ...) and processes (energy, gas or water networks, chemical factories, ...) is of great significance for the protection of human life and health, the environment, and the invested economic value. The correct operation of those systems has an important impact also on production cost and product quality in manufacturing.

The emergence of new sensors/actuators in the complex technological systems and processes made possible the development of several sophisticated monitoring and control applications where a large amount of real-time data about the monitored environment is collected and processed to activate the appropriate actuators and to achieve the desired control objectives. However, the probability of failure of some of these components will increase exponentially with their number. Thus, the safe and reliable operation must take into account mechanisms for early detection of faults to avoid performance degradation and damage to the machinery or human life.

Maintaining the health of a complex system is a difficult task that requires the in-depth analysis of the target system, principles involved and their applicability and implementation strategies. According to Ofsthun (2002), a System Health Monitoring (SHM) module implemented in the target system will be able to:

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  diagnose the root cause of a system failure,

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  furnish data and recommend solutions in real time,

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  provide prognostic capability to identify potential issues before they become critical, and

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  capture and retain knowledge for predictive maintenance and new designs.

To this aim, a SHM system consists of instrumentation components, a fault detection, isolation and response module, diagnostic and prognostic software, as well as processes and procedures responsible for information gathering about system’s health and corresponding decision-making.

The scope of this chapter tries to combine two concepts. On one hand, the new concept of System Health Management (SHM) that integrates the tasks of diagnostic and prognostic modules, as well as processes and procedures responsible for information gathering about the system health enabling to make the right decisions on emergency actions and repairs (Jennions, 2010), is presented. On the other hand, the modern concept of Reliable Control (RC) that tries to design control strategies to allow a safe and a reliable operation in spite of faulty situations (Tang et al, 2008), is introduced. As a result of this combination a new paradigm appears: the Health-Aware Control (HAC) of the systems. In this paradigm, the information provided by the prognosis module about the component system health should allow the modification of the controller such that the system health is considered in the control objectives. In this way, the control actions will be generated to fulfill these objectives and, at the same time, to extend the life of the system components. HAC control contrarily to FTC adjusts the controller even when the system is still in non-faulty situation. The prognosis module will estimate on-line the component aging for the specific operating conditions. In the non-faulty situation, the control efforts are distributed to the system based on the proposed health indicator. An example will be used throughout the chapter to illustrate the ideas and concepts introduced.

The structure of the chapter is as following: first a background reviews the state of the art of the main concepts introduced in the chapter. Then, in the next section the integration of prognosis and control is discussed and the new concept of health aware control is introduced. The chapter concludes by presenting the conclusions and the main lines of future research.

Background

Currently in the field of security, availability and reliability of automated systems and processes we can find two different working strategies. On the one hand, there is the so-called Prognosis & Health Management (PHM) which originated in system engineering and considers aspects such as quality, reliability and maintenance. On the other hand, we have the Fault Tolerant Control (FTC) related to control engineering and that concerns issues such as safety and performance specifications. These strategies work with the fact that system components can fail either by an early damage or by accidental/fortuitous causes, respectively.