Process Monitors and Controls (Page 2 of 2)

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Monitor results may just be jotted down on record books, but the simplicity of this method has a big drawback.  Numbers written on paper are difficult to analyze visually and will not catch the attention of the process owner when an anomaly arises.  This is why many companies employ control charts instead to record measurements from their monitors.

A control chart plots the measurement data for the parameter being monitored, usually with time on the x-axis and the measurements on the y-axis. These charts are special in the sense that they show the historical behavior of the parameter in terms of the mean and variance of past data. The variance is expressed in terms of upper and lower control limits, which are three (3) standard deviations away from the mean of the data distribution. 

During process monitoring, the process owners looks out for anomalous trends in the control charts.  For instance, any measurement outside the control limits is an automatic cause for alarm, because this is an outlier.  Four (4) or more consecutively increasing or decreasing points form a trend that is not normal, and therefore deserves attention.  Six (6) consecutive points on one side of the mean also deserve investigation. When such abnormalities are observed, the process owner must take an action to bring the process back to its normal behavior.

Process Control

Process Control, which is the means by which a process is kept stable within its normal behavior, comes in many forms.  It can be as simple as assigning a person to monitor the progress of the process (e.g., watching the cookies as they bake inside the oven) and responding in accordance with the state of the process observed (turning off the oven when the cookies are brown enough).  Or it can be as complicated as computer-based real-time monitoring of many parameters at the same time and automated control of equipment based on what the parameter readings are.

Again, cost and reliability play an important part in determining what process controls to implement on your manufacturing line. Remember, sophisticated computer systems need expensive maintenance, and a single downtime can result in total breakdown of your process control system. All things considered, many companies today (including the ones from the semiconductor industry) prefer to use Statistical Process Control (SPC) to keep their processes in check.

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See Also:  SPC;  Quality Systems;  Document Control;  The ISO9000 Standard;

Metrology and Calibration