Generation of restart states for manufacturing systems with discarded workpieces

In earlier work we presented a method for restart of manufacturing systems. After correction of an error, the system resumes normal production by returning to a certain restart state in the control function, and from there reexecutes parts of the work schedule. In the present paper the method is extended to handle also systems where some, but not necessarily all, workpieces are discarded during the error correction preceding the restart. The restart process must then replace the discarded workpieces with new ones. The workpieces that remain in the cell may limit the possibilities of executing certain operations needed to process the new workpieces correctly, thus complicating the restart. Properties of the restart states that guarantee a correct restart are defined, typical situations that make the system impossible to restart are presented, and some suggestions are given for modifications that make the system restartable.