By Phil Burgert
Setting up alarms via HMI software provides many alternatives for alarm density, what to alarm, logging, ergonomics, color choice and other elements.
Andy Balderson, product manager for Parker CTC says there usually are two major strategies employed by different levels of HMI products. The first is to have all of the alarm decisions actually made in the controls, whether it be a PLC or motion controller. The controls then provide the HMI with an integer value that indicates which alarm is currently active and the HMI side displays a message related to that alarm.
Thats kind of the rudimentary or basic alarming capability, he says. You really still make all your alarm decisions in the controller, and the HMI just provides enunciation or display of those alarms as a more meaningful message and sometimes with an image to depict which alarm is active.
The second strategy is more sophisticated alarm management of the HMI, says Balderson. The HMI can look into the controller or controllers and do an expression-based analysis on the data or a combined value, he says. This analysis actually makes an intelligent decision on whether to create an alarm in the HMI, based on multiple inputs. If yes, it provides a message to the operator indicating the status, tools for acknowledging and clearing those alarm conditions, an option to log the alarm locally to keep an historical record and a way of grouping and prioritizing different types of alarms, notes Balderson.
Im not arguing for one or the other, but some engineers believe all of those decisions should be at the control level and that you should not build a machine whose operation or alarm information is dependent on the HMI, says Balderson. There are others who look at the HMI as a much more sophisticated data management tool than the controls in general.
Keeping alarms from overwhelming the operator depends in part on the properties of the alarm system, says Harold Muma, product marketing manager for HMI hardware for Siemens Energy & Automation (www2.sea.siemens.com). If you have a pretty flexible alarm system, this obviously can very quickly become overwhelming to the operator, he says. What we recommend is always to err on the side of caution.
An alarm system should be relatively easy to configure or manipulate, says Muma. Alarms should be capable of being reprioritized once an operator interacts with the system and indicates whether or not he is being overwhelmed, he says. The highest priorities obviously are those that present a safety risk to the personnel, says Muma.
The second priority should be alarms that indicate potential catastrophe to the manufacturing line or to the process, involving any potential physical or structural damage. All of those obviously should be top priority, he says.
The machine builder should understand how the acknowledgement of the alarms is handled, adds Muma. 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, he says.
Presentation of the data is another consideration, and only necessary data should be shown, says Muma. Obviously if an operator has 50 different alarms on one screen showing at one time with a lot of text associated with them, he can be overwhelmed very quickly, he says. You have to identify it and keep it succinct.
The most common complaint Mum says he runs into is alarm windows that pop up to such an extent that nothing else can be seen on a display. Operators dont want to be obstructed from seeing additional data when the alarm occurs, he says.
Color schemes can help to reduce alarm overload along with fonts, columns, rows, filtering and link configuration, says Tim Donaldson, director of marketing at Iconics. He noted, though, you might not be able to count on standardization of alarm color meanings from one industry to another.
Just as the operator is responsible for setting alarm conditions when configuring the alarming component, the same attention should be paid to overriding or other actions following alerts, says Donaldson. The alarm component should be robust enough to have settings in place in case of such an override with immediate, clear explanation to the operator as to the result of the alarm override on the entire system, he says.