Exception Handling in Recipe-Based Batch Control

Sammanfattning: The focus of this thesis is exception handling in recipe-based batch control. Exception handling is a critical element for achieving long-term success in batch production. It is reported to constitute 40-60 percent of the batch control design and implementation effort. Correct handling of exceptions is a key element in process safety, consistent product quality, and production cost minimization. The previous work on Grafchart for sequential programming, batch process recipe handling and resource allocation is extended to also include exception handling. The work is based on two new language features: MIMO macro steps and step fusion sets. A MIMO macro step is a macro step with multiple input ports and multiple output ports. It can be conveniently used to represent hierarchical states in state-machine based control systems. The functionality is similar to the super-states in Statecharts. Step fusion sets provide a way to have multiple graphical representations, or views, of the same step. Using step fusion sets it is possible to separate the exception-handling logic from the normal operation sequences in a way that improves usability. An internal model approach is proposed where each equipment object in the control system is extended with a state-machine based model that is used on-line to structure and implement the safety interlock logic. The thesis treats exception handling both at the unit supervision level and at the recipe level. The goal is to provide a structure, which makes the implementation of exception handling in batch processes easier. The proposed approach uses the MIMO macro step functionality to implement the state machines and step fusion sets to provide separation between the exception handling logic and the logic for normal, fault-free operation. The approach has been implemented and tested on a batch pilot plant at UPC in Barcelona, Spain. The work follows the batch control standard S88. An inexpensive, portable, and flexible laboratory batch process has been developed. The process is used within the chemical engineering education at Lund Institute of Technology. The process can be used for teaching sequential, PID, multi-variable, and recipe-based control.