Robust CUSUM Control Charting for Process Dispersion

Process monitoring through control charts is a quite popular practice in statistical process control. From a statistical point of view, a superior control chart is one that has an efficient design structure, but having resistance against unusual situations is of more practical importance. To have a compromise between the statistical and practical purposes, a natural desire is to have a control chart that can serve both purposes simultaneously in a good capacity. This study is planned for the same objective focusing on monitoring the dispersion parameter by using a Cumulative Sum (CUSUM) control chart scheme. We investigate the properties of the design structure of different control charts based on some already existing estimators as well as some new robust dispersion estimators. By evaluating the performance of these estimators‐based CUSUM control charts in terms of average run length, we identify those charts that are more capable to make a good compromise between the aforementioned purposes in terms of statistical and practical needs. Copyright © 2013 John Wiley & Sons, Ltd.     

A New Distribution‐free Control Chart for Joint Monitoring of Unknown Location and Scale Parameters of Continuous Distributions

While the assumption of normality is required for the validity of most of the available control charts for joint monitoring of unknown location and scale parameters, we propose and study a distribution‐free Shewhart‐type chart based on the Cucconi 1 statistic, called the Shewhart‐Cucconi (SC) chart. We also propose a follow‐up diagnostic procedure useful to determine the type of shift the process may have undergone when the chart signals an out‐of‐control process. Control limits for the SC chart are tabulated for some typical nominal in‐control (IC) average run length (ARL) values; a large sample approximation to the control limit is provided which can be useful in practice. Performance of the SC chart is examined in a simulation study on the basis of the ARL, the standard deviation, the median and some percentiles of the run length distribution. Detailed comparisons with a competing distribution‐free chart, known as the Shewhart‐Lepage chart (see Mukherjee and Chakraborti 2 ) show that the SC chart performs just as well or better. The effect of estimation of parameters on the IC performance of the SC chart is studied by examining the influence of the size of the reference (Phase‐I) sample. A numerical example is given for illustration. Summary and conclusions are offered. Copyright © 2013 John Wiley & Sons, Ltd.     

Improving the Performance of Exponentially Weighted Moving Average Control Charts

A control chart is a graphical tool used for monitoring a production process and quality improvement. One such charting procedure is the Shewhart‐type control chart, which is sensitive mainly to the large shifts. For small shifts, the cumulative sum (CUSUM) control charts and exponentially weighted moving average (EWMA) control charts were proposed. To further enhance the ability of the EWMA control chart to quickly detect wide range process changes, we have developed an EWMA control chart using the median ranked set sampling (RSS), median double RSS and the double median RSS. The findings show that the proposed median‐ranked sampling procedures substantially increase the sensitivities of EWMA control charts. The newly developed control charts dominate most of their existing counterparts, in terms of the run‐length properties, the Average Extra Quadratic Loss and the Performance Comparison Index. These include the classical EWMA, fast initial response EWMA, double and triple EWMA, runs‐rules EWMA, the max EWMA with mean‐squared deviation, the mixed EWMA‐CUSUM, the hybrid EWMA and the combined Shewhart–EWMA based on ranks. An application of the proposed schemes on real data sets is also given to illustrate the implementation and procedural details of the proposed methodology. Copyright © 2013 John Wiley & Sons, Ltd.     

Improved Fast Initial Response Features for Exponentially Weighted Moving Average and Cumulative Sum Control Charts

Exponentially weighted moving average (EWMA) and cumulative sum (CUSUM) control charts have found extensive applications in industry. The sensitivity of these quality control schemes can be increased by using fast initial response (FIR) features. In this paper, we introduce some improved FIR features for EWMA and CUSUM control charts and evaluate their performance in terms of average run length. We compare the proposed FIR‐based EWMA and CUSUM control schemes with some existing control schemes, that is, EWMA, FIR‐EWMA, CUSUM, and FIR‐CUSUM. It is noteworthy that the proposed control schemes are uniformly better than the other schemes considered here. An illustrative example is also given to demonstrate the implementation of the proposed control schemes. Copyright © 2013 John Wiley & Sons, Ltd.     

Adaptation of the Natural Resources Conservation Service (NRCS) curve number (CN) model in estimating direct run‐off from humid tropical forest catchments

Suitability of the Natural Resources Conservation Service (NRCS) curve number (CN) model of run‐off prediction was evaluated on three humid tropical forested catchments in Kimakia, Kenya. The catchments were dominated by Pinus patula (catchment A), Arundinaria alpina (catchment C) and Pennisetum clandestinum (catchment M). The study used discharge and rainfall data collected between 1958 and 1986. Seventy‐three (73) isolated storms were graphically separated into baseflow, interflow and surface run‐off. Forest cover types significantly improved catchments characteristics that influence baseflow and interflow generation in catchment C but not those that influence surface run‐off production. In its original form, the NRCS CN model resulted in direct run‐off estimates that deviated from observed ones by between 43.8% and 55.3%. These discrepancies were minimized through modification of the β and CN parameters. CN generated empirically using storm rainfall predicted the direct run‐off satisfactorily. Therefore, the modified NRCS CN model adequately estimates direct run‐off from humid tropical forested catchments.     

DCS在工业级磷酸一铵装置中的应用

介绍传统法生产工业级磷酸一铵装置引入DCS控制的生产系统投运情况。试运行情况表明,传统工艺应用DCS控制,原料消耗和能耗降低,产品产量稳定,质量达标(w(P2O5有效)达61.7%,w(N总)达12%)。     

保持陶瓷企业质量管理体系有效运行的几个关键因素

管理体系运行有效性是每个建立和实施质量管理体系的组织所共同关心和需要面对的问题,也是社会有关各方普遍关注的问题,同时,它也是认证机构决定一个组织能否通过或保持质量管理体系认证的重要依据。笔者从如何保持和提高有效性方面,对管理体系的有效性问题进行了深入的剖析。     

Finding a trade-off between observability and economics in the fault detection of chemical processes

This paper presents a methodology to quantitatively gauge the potential economical loss due to unobserved faults when standard statistical monitoring charts are used. It is shown that in closed loop operation, a shorter time for detection may result from retuning the controller at the expense of higher product variability. Accordingly, an optimization approach is proposed for finding a trade-off between the economic losses resulting from lack of detection and losses resulting from higher product variability. In order to account for faults with different frequency contents, the method is applied in the frequency domain. The proposed optimization based methodology is later validated in the time domain.     

神华包头煤制烯烃项目GE水煤浆气化装置试车总结

介绍了神华包头煤制烯烃项目GE水煤浆气化装置试车、生产情况,重点阐述了试车过程中暴露的一些问题。     

Nonparametric procedures for monitoring a location parameter based on linear placement statistics

Nonparametric procedures are developed for the problem of monitoring the location parameter of a continuous process wnen the control value for the parameter is not specified. These procedures are based on linear placement statistics for comparing current samples with a standard sample taken wnen the process was operating properly. The linear placement statistics are used in versions of the Shewhart and cusum charts. Asymptotic approximations to the run length distribution are obtained and a new result for the Brownian motion process is derived for the case where there is an upper bound on the time that the process is observed.