Monitoring systems in sequential program control arrangements

Claim 1

A monitoring system comprising: operation detecting means for detecting operating states of a plurality of components in a sequential program control arrangement for controlling machinery, said components comprising an outlet component which causes a change in an effective action of the machinery and input components, wherein said output component causes the change when the input components are in respective predetermined operating states, memory means for storing reference operating states of said components, which are predetermined as normal operating states obtained sequentially under a normal operation of the machinery at every operation step in which a change in the operating states arises, therein in the form of reference operating states at sequential check steps, difference detecting means connected to both the operation detecting means and the memory means for detecting a difference between an actual operating state detected by said operation detecting means with the reference operating state stored in said memory means, difference detection control means for causing said difference detecting means to compare the actual operating states detected successively by said operation detecting means with the reference operating states at corresponding check steps respectively, then after a first check step at which the difference between the actual operating state and the reference operating state compared with each other is detected by the difference detecting means, to compare the actual operating state corresponding to the first check step with the reference operating state at each of check steps between the first check step and a second check step ahead of the first check step by a predetermined step number, wherein said second check step is selected such that the reference operating state at the second check step includes the change in the effective action of the machinery caused by the output element, and further after a third check step between the first and second check steps at which the reference operating state is coincident with the actual operating state corresponding to the first check step, to compare the actual operating states detected successively by said operation detecting means after the actual operating state corresponding to the first check step with the reference operating states at check steps ahead of the third check step respectively; and checking means for checking the operation of the machinery based on the difference detected by said difference detecting means.