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Applications of the EWMA

capability sigma of 7j. The linę control operators say that the EWMA charts take morę time than the modified Shewhart charts, and early in the test program there were errors in the EWMA calculations. In the instance shown in Figurę 9, such an error resulted in two unnecessary adjustments. Morę recent experience shows that the operators have been able to avoid these problems. After the first year’s trial, this Algorithmic SPC approach was considered a success and it continues in use on all machines.

Case #2: Computerized EWMA/Shewhart Control In February, 1990, an automatic control system was implemented at Monsanto’s Greenwood, S.C. plant for improving the uniformity of a ąuality measurement on textured continuous filament carpet yam, which we shall cali Y₂ following the compact notation defined in Case #1. Variability in this property also has a direct relationship to finished carpet ąuality, and it is measured once every 12 hours for each machinę. Prior to implementation of the automatic system, operators were manually plotting Shewhart charts using the Ford rules (one point beyond three sigma limits and 7 of 7 on one side of target). There was a process troubleshooting guide for use in case of rule violations, but with few exceptions the corrective action was a pressure adjustment. This adjustment was madę with a hard-to-reach manuał valve located remotely from the pressure gauge. A sample of Y₂ data resulting from this Modified Shewhart approach is shown in

Figurę 10 to exhibit drifting behavior which fits the IMA model with X = .5. It was concluded at that time (June, 1988) that an automatic system would be capable of reducing variability to ct_k=.42, the same as a_a in Figurę 10,

because of morę timely and accurate adjustments.

For the automatic system, electronic pressure controllers were installed for each machinę with remote set-point capability, meaning that the pressure set-point could be changed by instructions from a host Computer. Yj data are obtained directly from the Iaboratory instrument by the host Computer, which runs the EWMA algorithm as described in the control algorithm section of this paper with the Shewhart limits applied to the forecast errors rather than the control errors. If a violation of the Shewhart 3-sigma rule occurs, the Iaboratory operator is notified before the new data point is passed to the EWMA adjustment algorithm. This operator then has the choice of releasing the result or performing a re-check on the same sample. If he releases the result, which occurs about 50% of the time, he also notifies the manufacturing operator that a Shewhart violation has occurred so that process troubleshooting can begin. Control samples are run in the Iaboratory every four hours to insure that instrument calibration problems don’t result in pressure adjustments.