Alarm Systems – Some key design considerations
By Vivek Dev
Alarm systems have come a long way since the days when these mainly comprised panel indicating lights, to today’s fully-featured human-machine interface (HMI)/PLC systems. Alarm systems used to consist of light boxes, audible alarms, and push buttons which presented problems to even the most experienced operators to trace the causes of equipment failure. HMI/PLC systems offer virtually unlimited alarm capabilities, software configured for maximum flexibility. One of the main features HMI alarm packages provide is the ability to custom-configure alarm notification.
Human intervention
But whatever the sophistication of an alarm system what needs to be understood is that for any automated equipment, the presence of an alarm is the control system’s way to get someone’s attention, mostly due to the system’s inability or failure to accomplish what it wanted to do. The alarm is at times also a notification that there was something the system needed and was unable to provide.
The function of an alarm is basically to alert someone that the control system is seeking human intervention or is trying to make someone aware of something. At a very basic level, alarm systems provide information through a human-machine interface (HMI) console. The systems often coupled with a PLC, work to provide a clear look at the details about historical and current alarms. The occurrence of an event triggers the alarm on the console. After operators acknowledge and clear the alarm, the information might then be stored to create charts and graphs that help in the identification of trends or events that led to the causative events and subsequent action.
Types of systems
Basic alarm systems differ from more advanced systems mainly in the manner of notification of an alarm. Basic systems require an operator to be located directly in front of the display to see and respond to the alarm while intermediate systems integrate the functionality to communicate with portable devices such as PDAs or other types of personal digital assistants. This ability comes in handy when companies share or divide resources, as it allows individual operators to attend to more than one machine. An alarm can be sent to the operator responsible for correcting the situation. Further a notification could be e-mailed to the plant manager or quality department to allow the occurrence to be logged and tracked, or other corrective action to be taken.
Alarms that provide information to keep processes running are no doubt essential; however what is of greater importance is the ability to identify trends that improve machine productivity. More advanced alarm management systems provide maintenance personnel the ability to analyse messages in the event history, which in turn allows users to view operational changes and identify root causes.
The current development trend is toward historization of alarms, to enable users to construct an historical database. With the available technologies users can view this database via a web interface. The technology receives notification of an alarm and allows different levels of personnel to access and look into the context of specific events.
User groups can be defined to suit the requirements of management that can receive specific inputs and different levels of notification can also be established. An alarm could be sent to an operator tasked with the responsibility for correcting the situation. Simultaneously a notification could be e-mailed to the plant manager or quality department to allow for the logging and tracking of the event, or for other remedial actions.
Less is more
Given the increasing complexity of systems, control system designers need to, at times, resist the temptation to take the easy way out and keep alarms at the bare minimum. Alarms often provide an easy way out for system designers for once the alarm is annunciated, the control system does not need to do anything else, since human intervention is required, which eases the designer’s job. However, at times there are simply no alternatives and providing an alarm is the only reasonable approach.
As a general rule more is definitely not good when it comes to alarms. The problem, however, is not with alarms it is with human psychology. It has been noted that the more often alarms occur, the more likely they are to be ignored or even disabled. Logic diagrams and flowcharts come handy while eliminating alarms that are redundant. System designers can often improve the performance of the control system by taking the minimalist approach. The system needs to be designed around ensuring that alarms are rare events as those events have consequences.
Alarms generally require operator intervention and if the designer were to work on the basis of the assumption that the operator was always busy adding value somewhere else, designers might design their controls better, especially regarding alarm events. The assumption would result in minimizing all events requiring operator interaction or attention that contribute to reduction of overall effectiveness.
Presentation of the data is another design consideration, and only necessary data should be presented. If an operator sees a large number alarms on one screen showing simultaneously with a lot of text associated with them, he can get overwhelmed quickly. The designer would, therefore need to identify which data would be needed to be presented and keep it succinct.
The control system designer should understand how the acknowledgement of the alarms is handled. Some alarm systems, for instance, allow other computers or a PLC to acknowledge an alarm. A good alarm system should display an alarm when it is active and has not been cleared and acknowledged.
Keeping track of who should receive what type of alarm can at times be a complex task. System designers can make use of software to help users send alarm information to the right person. Pre-scheduling alarm notification destinations allows users to accommodate holidays, weekends, and business trips.
