Presentations - Wednesday (27/05/15)

Modified Control Charts for Processes with Complex Autocorrelation Structures

Authors: Renato Tigre Martins da Costa (UFRN), André Luís Santos de Pinho (UFRN), Carla Almeida Vivacqua (UFRN), Linda Lee Ho (USP)

Speaker: André Pinho

Abstract: This work proposes a modified control chart incorporating concepts of time series analysis. Specifically, we considerer Gaussian mixed transition distribution (GMTD) models. The GMTD models are a more general class than the autorregressive (AR) family, in the sense that the autocorrelated processes may present flat stretches, bursts or outliers. In this scenario traditional Shewhart charts are no longer appropriate tools to monitoring such processes. Therefore, Vasilopoulos and Stamboulis (1978) proposed a modified version of those charts, considering    proper control limits based on autocorrelated processes. In order to evaluate the efficiency of the proposed technique a comparison with a traditional Shewhart chart (which ignores the    autocorrelation structure of the process), a AR(1) Shewhart control chart and a GMTD Shewhart control chart was made. The criteria used to measure the efficiency were the ARL0 and ARL1. The comparison was made based on a series generated according to a GMTD model. The preliminary results point to the direction that the modified Shewhart GMTD charts have a better performance than the AR(1) Shewhart.  

Optimization of pultrusion process parameters via design of experiments and response surface

Authors: Antonio Faria Neto (UNESP); Antonio Fernando Branco Costa (UNESP)

Speaker: Antonio Faria Neto

Abstract: Experimental designs were implemented to investigate the effects of mold temperature and pulling speed on the tensile strength of cylindrical cables produced by pultrusion. A two-level factorial design with a central point revealed that the linear model is not applicable for describing the pultrusion process parameters’ influence on the tensile strength. Additional experiments were performed to investigate the contribution of quadratic terms. A three-level factorial design revealed that the simplest model with the highest performance has three terms, the linear components of the main effects and the quadratic component of the temperature. In the investigated range of the parameters, the tensile strength always increases with the mold temperature and decreases with the pulling speed. Thus, cables produced at the highest temperature and the lowest speed are the most resistant. Compared with the mold temperature, the pulling speed has a minor influence on the tensile strength. Based on this finding, during periods of higher demand, the pulling speed can be increased without significant loss in tensile strength. The gain in quality translated into cables with better resistance, which highly compensates for the additional energy consumption.