Why is my supervisor empty? Finding causes for the unreachability of states in synthesized supervisors

Although supervisory control synthesis has been around for many years, adoption is still low. A weak point of synthesis is the absence of a reporting mechanism. When an empty or unexpected supervisor is returned, it is very difficult to explain why this is the case. It is desired to return an explanation for a question, like, “Why is my supervisor empty?”. In general, the information needed to provide such an explanation is not present in the synthesized result. In this paper, causes (explanations) are generated for questions regarding the absence of behavior in the synthesized system. To this end, it is first investigated what information is needed and how it should be stored. Based on these findings, information of the influence of each requirement is encoded in the supervisor. This is done by annotating colors. The resulting so-called colored predicates can be used after synthesis to derive a cause for a given question.