Condition assessment (CA) of bearings can be performed by using left-right hidden Markov models, because of the monotonically increasing pattern of the bearings degradation. Classical CA approaches assume that all possible system states are fixed and known a priori. The training of the system is performed offline at once with data from all of the system states. These assumptions significantly impede condition assessment applications in case that all the possible states of the system are not known in advance, or changes in environmental or operative conditions occur during the tool's usage. To overcome these limitations, we propose combining left-right continuous HMMs (CHMM) with a change point detection algorithm for (i) estimating, from historical observations, the initial number of the CHMM states and the initial guess of its model parameters and (ii) updating the state space as well as the model parameters during monitoring. Moreover, to deal with multidimensional sensor measurements, we propose using kernel principal component analysis for dimensionality reduction. Qualitative and quantitative evaluations of the proposed methodology have been performed using both simulated and real data from the NASA benchmark repository. Compared to state of the art techniques, the proposed methodology results in (i) an improvement of the HMM training phase in terms of iterations number; (ii) the detection of unknown states at an early stage; and (iii) an effective change of the CHMM's structure to represent the degradation processes more accurately in presence of unknown conditions.
IntroductionMachines and equipments breakdown in industrial environments can have significant impact in the profitability of a business. Maintaining the engineering assets in operating conditions is an industrial and economical requirement. Nowadays optimization strategies for maintenance are an important part of research for companies [1,2], since, next to energy costs, maintenance spending due to equipment or machine failure can be the largest part of the operational budget [3].Corrective maintenance [1,4], which is the system maintenance strategy subject of this work, is based on condition monitoring and is performed after a degradation modality emerges in a system, with the goal of preventing the system from failing. A fundamental requirement of this strategy is the detection of the abnormal behavior, that will lead to the system failure, at an early stage in order to give enough time to perform the repair operations.Industrial equipments and their components are often characterized by a very complex structure. For this reason the usage of mathematical models for condition monitoring can represent an arduous task. On the other hand, industrial machines are usually equipped with monitoring devices (sensors placed in critical parts of the system under study) which are able to record and collect large amount of measurements. Given the availability of these huge amounts of data, the methodology proposed in this paper will focus on datadriven mo...