In this paper, the effect of scan time (S/T) of Programmable Logic Controllers (PLCs) on machine cycle time is examined. The S/T is considered to be a an internal feature of the PLC, and it becomes economically not feasible to examine different PLCs in order to vary scan time and monitor the effect on cycle time. Although the relationship between the two parameters is expected to vary in inversely linear proportion, there exists an optimum region of scan time where beyond this the rate of change of cycle time is decreased. This region of scan time is of particular importance to practitioners as more investment in the processor's speed might not achieve a convenient reduction in cycle times.
The effect of scan time variation on discrete event processes has been investigated during this research and factors influencing machine cycle time, machine variability, and machine reliability have been identified. Also, the use of intelligent decision analysis tools in engineering has been considered through associated work in which the analytic hierarchy process (AHP) and fuzzy logic have been employed in an intelligent system. This paper attempts to combine both efforts in a model that aims at simulating and ultimately minimising scan time by considering different factors using the concept of multiple criteria decision making. In this paper, different criteria that affect the scanning time of a PLC are examined using a multiple criteria decision making tool called AHP (Analytic Hierarchy Process) giving a prioritised rank of alternatives contributing to those criteria.