Operator span of control is a much debated issue in the process control industry. The issue is unsettled and will remain so because it is clouded by many interacting factors including process dynamics, level of control system automation, operator experience and training, crew interaction, display and alarm system design, etc. One study, Moray, N. and Rotenberg, I. (1989) Fault management in process control: Eye movement and action. Ergonomics, 32 (11),1319-1342, helps shed some light on some of the factors that limit operator span of control.
The key limitation to operator span of control is the ability to respond to process upsets and emergencies. Operator response to a process upset requires that the operator 1) detect the cause of the upset (referred to as a fault), 2) correctly diagnose the fault's cause, and 3) effect the proper control action(s). In a severe process upset, many different faults, which may or may not be related, can occur.
In the research article, operators controlled a simulated process plant while the researchers introduced process faults into the system. System faults included plugged lines, faulty valves, etc. Researchers tracked several parameters of operator performance, including eye movement and fixation, number of control moves, and time between fault introduction and control action.
The study had several important findings. The first finding was that people tend to process faults in series; they only respond to one fault at a time and will not go on to another fault until the first fault is corrected.
The researchers also found a phenomenon which they refer to as “cognitive lockup”. When cognitive lock-up occurs, the operator focuses on the fault at hand and ignores the rest of the system, including new faults occurring in the system. The researchers further described cognitive lock-up as, when faced with an emergency or other high workload situations, operators' attention narrows to only a subset of the total instrumentation available to them. The result is that the operators fail to collect the information they need to diagnose the failures. The authors argue that to successfully control the process during emergency and high workload situations, the phenomenon of cognitive lock-up needs to be avoided.
What can be done to overcome the effects of cognitive lockup? One of the solutions is through better interface design. The critical process relationships must be defined and mapped onto the interface. If not defined and mapped onto the display and alarm system, the ability of the operators to detect critical information and diagnose fault is greatly diminished.
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