SPC tags specify data that is to be collected for use in SPC operations. Once data is defined it can be dynamically analyzed (as SPC graphs and alarms) at run-time. SPC tags are similar to trend tags.
Like trends, CitectSCADA can collect and store any amount of SPC data. The only restriction on the amount of data that you can store is the size of the hard disk on your computer. (CitectSCADA uses an efficient data storage method to optimize the use of storage space on your computer's hard disk.) For long-term storage, you can archive the data to disk or tape (without disrupting your runtime system). You can also log data at regular intervals (periodic trendA trend that is sampled continuously at a specified period. You can also define a trigger (an event) to stop and start the trend (when a specified condition occurs in the plant).), or only when an event occurs (event trend), in the same manner as Trend Tags.
Note: SPC does not support Periodic-Event trends, which is a combination of the properties of Periodic and Event trends. In addition, if you use event-based SPC tags, your display screen refresh rate may be slower than desired or practical for your application.
When you define an SPC tag you will want to be sure to fill in the upper specification limit (USL) and lower specification limit (LSL) if you intend to perform a capability analysis. These values have to accurately represent the users requirements, and the target value lie midway between the two. If these fields are left blank the capability analysis will be meaningless.
To configure an SPC tag: