The Calculation tool in version 19.1.1 and later version can be a powerful tool. It allows users to way more then they could in the past. In this instance we will walk through setting up a calculation tag to give the rate of change over a specific time period. This can then be viewed as a trend, used in applications, or even set up as an alarm event. In this example we will use the delta aggregate. The delta aggregate is the difference between the start and end bound values of the time interval. Here is in example of how to set this up.
- First name the calculation in this example we will call it rate of change.
- Next select the frequency. This can very based on the production process. The time interval of 10 minutes has been selected.
- Now the delta aggregate can be selected. Between the first set of quotes is where the tag name can be placed that we will be monitoring the rate of change for. The other set of quotes will be where the time period can be specified again. This equation will monitor the rate of change with an interval of 10 min and a time period of 10 min. It is also possible to choose to use a smaller interval then the evaluation period so that the calculation runs every min for a ten min period. This can even be set to run on a value change but you may see some performance issues is the change rate is to fast.
- The next step is to specify an output tag. The dataset should be specifed first followed by a period and the tag output name. In the below example the tag is being written to the calc tags.(dataset) RoC(tag name).
As a best practice we recommend creating a calculation dataset for all calculated tags.
- The last step is to evaluate the calculation to make sure it is returning values and apply the calculation. When closing the calculation out, the user will be presented with the main calculation page and the ability to start or stop evaluating the calculation. This tag can then be used as a trigger in an event if the values are climbing to rapidly.