TOWARD AN UNDERSTANDING OF DCS CONTROL OPERATOR WORKLOAD

The relatively new technology of distributed control (DCS) has created a long list of new problems that must be solved and questions that mu st be answered. One perplexing question encountered by control system engineers is how to gauge DCS control operator workload and, more impo rtantly, how to discern if the operator's workload is excessive. The i ntroductions of local control networks and advanced control require mo re sophisticated metrics of workload than simply counting control loop s and using the 200 control loops per operator rule of thumb. Studies have also shown that there is little correlation between the number of control loops and many parameters used to gauge operator workload (su ch as control moves per hour). This paper will investigate current iss ues of DCS control operator workload in three main topic areas. First, the paper will briefly define factors that contribute to control oper ator workload and introduce a model of control operator workload. Cont rol operator workload is a multifaceted construct affected by a host o f interacting variables including the operator/control system interfac e (display and alarm system design), process dynamics, operator traini ng/experience, and crew interaction. Next, the paper will briefly desc ribe strategies that can be used to assess operator workload. Techniqu es used to assess workload are traditionally divided into three catego ries: (1) Subjective measures (NASA-TLX, developed by NASA, and SWAT, developed by the U.S. Air Force); (2) Physiological measures (heart ra te and eye blink rate); and (3) Performance based measures (such as th e number of control moves or alarms per hour, or where performance is measured on a secondary task that the operator completes). Also, the p aper will discuss which are the easiest and most appropriate technique s to use with control operators. Finally, the paper will present a wor kload study of a petrochemical plant DCS control operator job. The stu dy represents a typical control operator loading and how the workload ratings change in response to increasing job demands. The paper will d efine performance based parameters which are indicative of excessive w orkload. Also, the paper will touch on the effects of advanced control on operator workload and will discuss strategies that the control sys tem designer can use to reduce workload.