Estimation of time delay of relay ladder in sequential control systems

The purpose of this paper is to establish the estimation methodology of the time delay of relay ladder in sequential control systems mainly experimentally. A structure of a programmable controller (PLC) is modeled as the process instruction described by relay ladder. First, the time delay in program execution of a ladder diagram (LD), i.e. the time delay in software, is examined to clarify the effect of increasing rungs and contacts of the LD on its execution time of the process program. Second, the time delay in process execution by the switching of an electromagnetic relay in the PLC, i.e. the time delay in hardware, is also examined. Experiments are conducted to clarify chatter vibration of an electromagnetic relay, which is the main reason for occurrence of the time delay in hardware. The time delay in hardware is discussed theoretically to explain the experimental results. Moreover, these experimental results are analyzed statistically to clarify the characteristics of time delay of the relay ladder in the sequential control systems.