George Nenes

• Mechanical Engineer, MSc, PhD
• Professor (Full) at University of Western Macedonia

In some industrial environments flexibility is a key for the manufacturer, that is, the ability to switch frequently from the production of one product code to another and produce batches of different items during consecutive short production time intervals, called finite production horizons; such a setting is called a finite horizon process (FHP)....

This paper presents a fully adaptive multivariate statistical process control scheme for processes where multiple assignable causes may occur. The assignable causes affect both the mean vector and the covariance matrix, monitored by a T² chart and a control chart based on entropy, respectively. Markov chain theory is employed to model the stochasti...

Classic approaches of Statistical Process Control (SPC) include the collection of samples from a production process and the computation of some output concerning a specific quality characteristic. This output, which is called chart statistic, is then compared against appropriately designed limits to decide whether an investigation for an assignable...

Traditional measures of statistical performance of SPC control charts are widely used to evaluate the performance of a control chart. Pretty often, even if the control chart is designed from an economic point of view, statistical properties are set as necessary conditions (constraints), to control performance. The usual statistical measure used to...

This paper presents a new model for the economic-statistical optimization of a Variable-Parameter Shewhart control scheme. The proposed model can be utilized to monitor processes where apart from multiple independent assignable causes, affecting both the mean and variance, failures can also occur. Each time an alarm is issued by the control scheme,...

This paper presents a new statistical process control model for the economic–statistical optimization of a Variable-Parameter (Vp) Shewhart control scheme for monitoring processes where multiple assignable causes may occur. The assignable causes affect not only the mean but also the variance of a specific quality characteristic by leading to progre...

The increased quality volatility that characterizes returned used products in reverse supply chains, has led both academic research and industrial practice to support the establishment of, preferably fast and inexpensive, procedures for the quality assessment of returns. To this end, an initial classification of returns is typically performed based...

In manufacturing environments where the production horizon for a specific product can be limited to a few hours or shifts, statistical process monitoring based on control charts is strategic to cut scrap, rework costs and to meet due dates. In this paper, a Markov chain model is proposed to design a fully adaptive Shewhart control chart in a proces...

Today, many companies use job production to produce small batches of work output according to customised specifications required by customers. With job production, the quantity of parts to be produced is finite and can be limited to a few thousands or hundreds (low-volume production). Usually, job production requires high-quality standards. When 10...

In many manufacturing environments, the production horizon of the same part code between two consecutive set-ups should be limited to a few hours or shifts. When 100 % sampling is not possible, on-line quality control on a quality characteristic should be immediately started by means of a control chart. In this paper, we investigate the statistical...

Fully adaptive control charts are efficient statistical process control means to monitor a quality characteristic affecting the outcome of a manufacturing process. Usually, the performance of these adaptive charts is investigated in processes characterised by the possibility of the occurrence of a single assignable cause. However, this assumption i...

Gasification is generally considered as the most effective for low rank coal exploitation and CO2 gasification offers the advantage of upgrading an environmentally detrimental gas. Isothermal CO2 gasification tests of Greek low rank coal were performed in a tubular fixed bed reactor, to study the effect of the various reaction conditions. Greek lig...

In order to carry out acceptance sampling, companies often use sampling schemes recommended by easy-to-use quality standards. These standards, however, do not take quality costs directly into account. This research proposes a new model for the representation of the use of ISO 2859–1 (1999) standard. Using this new model, it is feasible to evaluate...

The requirement to be globally competitive requires companies to have a high level of flexibility to allow for the production of a large variety of products. To limit work-in-process, decision makers periodically schedule according to a make-to-order management strategy i.e. the production of finite batches of the same product code. Scheduling call...

This paper presents a new model for the economic optimization of a process operation where two assignable causes may occur, one affecting the mean and the other the variance. The process may thus operate in statistical control, under the effect of either one of the assignable causes or under the effect of both assignable causes. The model employed...

In order to obtain timely quality information of returns, remanufacturers have to rely on inexpensive, fast, yet inaccurate classification procedures. Quality assessment of returned products is usually based on the comparison of certain product characteristics or usage conditions to predetermined thresholds. Classification inaccuracies typically oc...

Recently, control charts plotting a statistic having a Student’s t distribution have been proposed as an efficient solution to perform Statistical Process Control (SPC) in short production runs where the shift size of the in-control process mean from μ0 to μ1 is known a priori. The shift size is usually measured as a multiple δ of the in-control pr...

We study the problem of optimizing the sampling and procurement decisions in a remanufacturing system under stochastic yield of returns in a single-period setting. Returned products are characterized by uncertainty regarding their ability to be successfully remanufactured. This uncertainty is formulated as a variable probability of each returned un...

This paper presents a new Statistical Process Control model for the economic optimization of a variable-parameter control chart monitoring a process operation where two assignable causes may occur, one affecting the mean and the other the variance of the process. Therefore, it is possible for the process to operate in statistical control, when none...

Starting the online monitoring of a quality characteristic by means of a control chart at the beginning of a short production run is often a challenging issue for quality practitioners: in fact, the frequent absence of preliminary information prevents from getting a precise estimate of the characteristic mean and standard deviation. Furthermore, fo...

During the last decades there has been a consistent need for companies to manufacture ‘green’ products in order to contribute to environmental protection. The utilisation of used products (literally, the extension of their useful life cycle) is an excellent, indirect way for companies to conform to this requirement and, at the same time, increase t...

This paper proposes a unified approach for the development of economically designed Variable-parameter (Vp) -Shewhart, -CUSUM and -EWMA control chart models, for monitoring the process mean in infinite-horizon production runs. The use of the models allows at each sampling epoch the determination of the scheme parameters that minimize the quality-re...

We develop a model for the economic design of a Bayesian control chart for monitoring a process mean. The process may randomly suffer failures that result in a non-operating state, and thus call for an immediate corrective maintenance action, as well as assignable causes that shift the process mean to an undesirable level. Quality shifts, apart fro...

This article presents a model for the economic design and optimisation of an adaptive -chart used to monitor the process mean in infinite runs. Assignable causes may randomly affect the mean of the process by shifting it from its target value to an undesirable level. The proposed model allows the determination of the scheme parameters that minimise...

We present the case of a Greek commercial enterprise facing the problem of managing the inventories of thousands of different items, supplied by more than 20 European and Asian manufacturers and sold to a large number of different-type customers. A key feature of the problem is that the demand for the vast majority of items is intermittent and lump...

When both purchase of new products and remanufacturing of returned products constitute options for a company, it faces a trade-off between the long purchase lead times and the high purchase costs versus the uncertainties generated by the unknown a priori quantity and quality of returns. In other words, although the remanufacturing of returns is gen...

This article analyses and evaluates the properties of a CUSUM chart designed for monitoring the process mean in short production runs. Several statistical measures of performance that are appropriate when the process operates for a finite-time horizon are proposed. The methodology developed in this article can be used to evaluate the performance of...

This paper proposes a model for the economic design of a variable-parameter (Vp) Shewhart control chart used to monitor the mean in a process, where, apart from quality shifts, failures may also occur. Quality shifts result in poorer quality outcome, higher operational cost and higher failure rate. Thus, removal of such quality shifts, besides impr...

To conduct Acceptance Sampling, companies often use plans that are determined by easy-to-use standards. However, these standards do not take quality costs directly into account. Motivated by the case of a Greek company, which uses the Greek equivalent to the ISO 2859 (1974) for the quality control of its incoming raw materials, this paper aims at e...

This paper compares the economic performance of CUSUM and Shewhart schemes for monitoring the process mean. We develop new simple models for the economic design of Shewhart schemes and more accurate ways to evaluate the economic performance of CUSUM schemes. The results of the comparative analysis show that the economic advantage of using a CUSUM s...

This paper studies a model for the economic design of an adaptive chart for short production runs that are subject to the occurrence of assignable causes, which may either increase or decrease the mean of the quality characteristic. At each sampling instance, the probabilities that the process operates under the effect of an assignable cause are up...

This paper proposes a model for the design of a CUSUM chart for monitoring the process mean in short production runs. The model allows the determination of the scheme parameters that minimize the relevant expected cost of the procedure as well as the calculation of several measures of statistical performance. To evaluate the economic effectiveness...

This paper develops a model for the analysis and design of a CUSUM chart for monitoring the process mean in short production runs and proposes statistical measures of performance that are appropriate when the process is not operating indefinitely. The model can be used to evaluate the performance of the CUSUM scheme for any given set of chart param...

We examine a reverse supply chain consisting of a collection site, where consumers return used products, and a remanufacturing facility. Some of the returned products are transported to the remanufacturing facility in order to be remanufactured and used to satisfy the stochastic demand for remanufactured products. The quality of returns is characte...