Hierarchical Interface-Based Supervisory Control with Data Events

Hierarchical Interface-based Supervisory Control (HISC) decomposes a discrete-event system (DES) into a high-level subsystem which communicates with $n\geq 1$ low-level subsystems, through separate interfaces which restrict the interaction of the subsystems. It provides a set of local conditions that can be used to verify global conditions such as nonblocking and controllability. As each clause of the definition can be verified using a single subsystem, the complete system model never needs to be stored in memory, offering potentially significant savings in computational resources. In this paper, we extend the range of the behavior of low-levels that interfaces can model by adding a new type of event, low data events, and by relaxing some restrictions in the HISC definitions. This allows us to have (i) request events that don't need to be followed by an answer event, (ii) to start a low-level on a task and then poll it for completion, (iii) to be able to send additional commands while a low-level is already processing a command (iv) to model low-levels that behave as buffers, and (v) to allow unsolicited information (status etc.) to be sent up from a low-level. Besides greatly enriching the behavior that can be modelled as interfaces and thus expanding the systems that HISC can effectively be applied to, the changes can enable behavior to be moved from the high-level to the low-levels. We demonstrate this when we discuss the application of our method to a large manufacturing system example based upon the AIP example, where we saw a 4.8 times reduction in computation time and a 6.5 times reduction in memory use. This helps prevent the high-level from growing too large, allowing the HISC method to apply to larger systems.