Developing a control logic specification: Alarms and reports
By Michael Whitt
The primary purpose of a Supervisory Control and Data Acquisition (SCADA) system is to provide useful information to an operator in a timely, pertinent, fashion.
There are three primary ways to be sure this takes place: 1) Color animation and value displays at the HMI (human-machine interface); 2) Alarm management; and 3) Report generation.
Color animation and value displays: HMI graphic screens can provide a wealth of information at a glance by proper use of color animation. A well-written control specification will provide color animation and value display guidelines. These guidelines can take the form of a chart or list and need to cover all conceivable eventualities.
Each end device should be in the chart, with every animation possibility described.
Alarm management: HMI software packages all have some capacity for managing alarms. The control specification needs to define how to display and acknowledge the alarms. It should define their behavior under different conditions. Like the Animation Plan, the Alarm Management Plan can manifest in a chart.
The chart should include alarm priorities. Three levels of alarms are in this chart, but more or less can also work. Care should be taken to minimize confusion by the operators, so the simpler, the better.
Report generation: The single most under-specified item is generally the reports that will need to result upon completion of the project. Reports can serve to validate or refute assumptions, provide the basis of a cost/benefit analysis, or merely provide a list of alarms that came out over a 24-hour period. Report formats and content, perhaps even samples should be included in a well-written control specification. There are several fundamental reports:
Daily/weekly production reports
Daily/weekly raw materials usage reports
Daily alarm summary report
In addition, one must decide who and how to generate the report. Will the operator initiate the report from the HMI, or at a stand-alone PC? Which operator will generate the report, and at what time during the day? Will it come as a hardcopy, or as a file, or both?
A properly specified set of reports and alarms is critical to the eventual turnover and takeover of the system by Operations. The project people should vet this portion of the control specification very carefully and, if possible, give it to Operations for review and approval. This will greatly reduce misunderstandings and related rework, and will go far towards guaranteeing that the systems integrator delivers a system that will behave as desired.
ABOUT THE AUTHOR
Michael Whitt (firstname.lastname@example.org) is an ISA Senior Member and the Manager of Integrated Systems at Mesa Associates, Inc. His book is Successful Instrumentation and Control Systems Design, ISA Press, 2004.
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