In manufacturing, it is important to know if the actual production values match the recipe values when the recipe is downloaded and throughout the run in the event they are changed from an outside source. Recipe values are written once when the recipe is first selected and it is important to confirm that the values were successfully set. In the case of the Settings and Changeover Module, when a recipe is selected, the values are written to the Ignition tags. This should happen successfully, but there can be expressions, scripts, etc. that prevent the value from being written correctly. This is more of an issue when the Ignition tag is configured as an OPC item connecting it to the PLC or other device. If a communication error occurs when the new recipe value was being written to the PLC or device, then it is very useful to know this before machinery is started.
It is very common to have operator interface terminals (OIT) or a standalone human machine interface (HMI) local to a machine that settings can be changed locally. Settings can also be changed from other sources besides the local OIT, and it is important to detect and log when any setting varies from the recipe value.
There are cases where it is normal for a live production value to vary after the initial recipe value has been written to the Ignition tag. In other cases, it might be okay for the live production value to change within a range. Recipe values can be configured to not monitor variances or to have a variance window that the live production value must fall outside of before the variance is logged. By default, the variance monitoring is enabled for each recipe value but it can be disabled by recipe value in the Designer. This allows for a mix of recipe values that variances will be monitored and other that will not to prevent irrelevant variances from being logged.
Refer to Recipe Values Settings - Enable Variance Logging for more information on setting up variance tracking.