Pundarikaksha Baruah

HMMs for diagnostics and prognostics in machining processes

Despite considerable advances over the last two decades in sensing instrumentation and information technology infrastructure, monitoring and diagnostics technology has not yet found its place in health management of mainstream machinery and equipment. This is in spite of numerous studies reporting that the expected savings from widespread deployment of condition-based maintenance (CBM) technology would be in the tens of billions of dollars in many industrial sectors as well as in governmental agencies. It turns out that a prerequisite to widespread deployment of CBM technology and practice in industry is cost efficient and effective diagnostics and prognostics. This paper presents a novel method for employing hidden Markov models (HMMs) for carrying out both diagnostic as well as prognostic activities for metal cutting tools. The methods employ HMMs for modelling sensor signals emanating from the machine (or features thereof), and in turn, identify the health state of the cutting tool as well as facilitate estimation of remaining useful life. This paper also investigates some of the underlying issues of proper HMM design and training for the express purpose of effective diagnostics and prognostics. The proposed methods were validated on a physical test-bed, a vertical drilling machine. Experimental results are very promising.

A neuro-fuzzy approach for estimating mean residual life in condition-based maintenance systems

This paper presents a framework for online reliability estimation of physical systems utilising degradation signals. Most prognostics methods promoted in the literature for estimation of mean-residual-life of individual components utilise trending or forecasting models in combination with mechanistic or empirical failure definition models. In the absence of sound knowledge for the mechanics of degradation and/or adequate failure data, it is not possible to establish practical failure definition models. However, if there exist domain experts with strong experiential knowledge, one can establish fuzzy inference models for failure definition. This paper presents a neuro-fuzzy approach for performing prognostics under such circumstances. The proposed approach is evaluated on a cutting tool monitoring problem. In particular, the method is used to monitor high-speed-steel drill-bits used for drilling holes in stainless steel metal plates.

An Industrial Strength Novelty Detection Framework for Autonomous Equipment Monitoring and Diagnostics

This paper presents a practical framework for autonomous monitoring of industrial equipment based on novelty detection. It overcomes limitations of current equipment monitoring technology by developing a “generic” structure that is relatively independent of the type of physical equipment under consideration. The kernel of the proposed approach is an “evolving” model based on unsupervised learning methods (reducing the need for human intervention). The framework employs procedures designed to temporally evolve the critical model parameters with experience for enhanced monitoring accuracy (a critical ability for mass deployment of the technology on a variety of equipment/hardware without needing extensive initial tune-up). Proposed approach makes explicit provision to characterize the distinct operating modes of the equipment, when necessary, and provides the ability to predict both abrupt as well as gradually developing (incipient) changes. The framework is realized as an autonomous software agent that continuously updates its decision model implementing an unsupervised recursive learning algorithm. Results of validation of the proposed methodology by accelerated testing experiments are also discussed.

Autonomous diagnostics and prognostics through competitive learning driven HMM-based clustering

A prerequisite to effective wide-spread deployment of condition-based maintenance (CBM) practices is effective diagnostics and prognostics. This paper presents a novel method for employing HMMs for autonomous diagnostics as well as prognostics. The diagnostics module exploits competitive learning to achieve HMM-based clustering. The prognostics module builds upon the diagnostics module to compute joint distributions for health-state transition times. The proposed methods were validated on a physical test bed; a drilling machine.

Autonomous diagnostics and prognostics in machining processes through competitive learning-driven HMM-based clustering

A prerequisite to widespread deployment of condition-based maintenance (CBM) systems in industry is autonomous yet effective diagnostics and prognostics algorithms. The concept of ‘autonomy’ in the context of diagnostics and prognostics is usually based on unsupervised clustering techniques. This paper employs an unsupervised competitive learning algorithm to perform hidden Markov model (HMM) based clustering of multivariate temporal observation sequences derived from sensor signal(s). This method improves autonomy of the diagnostic engine over the more traditional classifier based diagnostics models. Classifier models, such as the model presented by Baruah and Chinnam [Baruah, P. and Chinnam, R.B., 2005. HMM for diagnostics and prognostics in machining processes. International Journal of Production Research, 43 (6), 1275–1293] employ ‘labelled’ feature vectors for supervised model building and subsequent health-state classification during condition monitoring. Improving the autonomy of the diagnostics model also improves the autonomy of the prognostics module that often builds upon information extracted through the diagnostics module. We have validated the proposed methods on a physical test-bed that involved monitoring drill-bits on a CNC machine outfitted with thrust-force and torque sensors. Experimental results demonstrate the ability of this method to estimate on-line the remaining-useful-life of a drill-bit with significant accuracy.

Empirical prediction limit estimation methods for feed-forward neural networks

Feed-forward neural networks (FFNs) have gained a lot of interest in the last decade as empirical models for their powerful representational capacity, non-parametric nature and multivariate characteristics. While these neural network models focus primarily on accurate prediction of output values, often outperforming their statistical counterparts in dealing with sparse date sets, they usually do not provide any information regarding the confidence with which they make these predictions. Since prediction limits (PLs) indicate the extent to which one can rely on predictions for making future decisions, it is of paramount importance to estimate these limits. Two empirical PL estimation methods for FFNs are presented. The two methods differ in one fundamental aspect: the method employed for modeling the properties of the neural network model residuals. While one method uses a local approximation scheme, the other utilizes a global approximation scheme. Simulation results reveal that both methods have their relative strengths and weaknesses.