UNSCHEDULED MANUAL INTERVENTIONS IN AUTOMATED PROCESS-CONTROL

Preliminary observations during a human factors analysis of a recently automated process plant revealed that operators sometimes assumed con trol over the production schedule, overriding the process computer. In terviews with senior managers suggested that such behaviour was rare, and only occurred under close supervision. This paper presents an anal ysis of these actions, which are referred to as 'unscheduled manual in terventions' (UMIs), through a series of three studies carried out in the plant. A questionnaire study revealed that UMIs occurred much more frequently than were claimed, and that they also imposed a heavy work load on operators. These observations were confirmed by a formal analy sis of the process records for a full week's production, which allowed the authors to develop a taxonomy of UMIs according to their role in the control of either plant operations of the automatic schedule. It w as hypothesized that such actions were motivated by a need for persona l control in the increasingly automated workplace. This explanation wa s, however, rejected by a factor analysis of motivational items associ ated with a set of UMI activities. Comparison of factor scores across the different activities identified the desire to enhance production s peed and quality as the strongest motivational determinants, although the balance of these differed for plant and sequence interventions. A second analysis showed that operators considered such actions to be ma inly discretionary, rather than prohibited (unlike the official manage rial perspective found in interviews). The findings are discussed in t erms of their implications for system design in process plants, partic ularly plant reliability and accidents, and the function of tacit shop -floor knowledge and operator involvement in automated manufacturing.