Patrik Bergagård

Sequence Planning Using Multiple and Coordinated Sequences of Operations

The sequential behavior of a manufacturing system results from several constraints introduced during the product, manufacturing, and control logic development. This paper proposes methods and algorithms for automatically representing and visualizing this behavior from various perspectives throughout the development process. A new sequence planning approach is introduced that uses self-contained operations to model the activities and execution constraints. These operations can be represented and visualized from multiple perspectives using a graphical and formal language called Sequences of Operations (SOPs). The operations in a manufacturing system are related to each other in various ways, due to execution constraints expressed by operation pre- and post-conditions. These operation relations include parallel, sequence, arbitrary order, alternative, and hierarchy relations. Based on the SOP language, these relations are identified and visualized in various SOPs and sequences. A software tool, Sequence Planner, has been developed, for organizing the operations into SOPs that visualize only relevant operations and relations.

Calculating restart states using reset transitions

This paper presents a supervisory control theory based offline approach for calculating restart states in a manufacturing control system. Given these precalculated restart states, an operator can be given instructions for how to correctly resynchronize the control system and the manufacturing resources during the online restart phase, as part of the error recovery process. Restarting from a restart state guarantees that all requirements on the nominal and the restarted productions are fulfilled. The paper includes an empirical comparison showing that the proposed approach enables restart states calculation for systems of sizes that could not be handled using an earlier presented approach.

Derivation of placement transitions for offline calculation of restart states

This paper presents a preprocess to an existing method for offline calculation of restart states for manufacturing systems modeled by operations. In the existing method, placement transitions are used to model restart in restart states from potential error states, and supervisory control theory is used to calculate which of these transitions are valid. With the proposed preprocess, the precedence and the alternative dependencies between the operations are exploited in order to reduce the number of such placement transitions which are required in the model used by the existing method. With such a reduced model, the valid restart states for larger and more complex systems can be calculated.

Deadlock avoidance for multi product manufacturing systems modeled as sequences of operations

This paper demonstrates how industrial models of the sequential order between manufacturing operations may be connected with academic resource deadlock avoidance policies. Given a set of product types each modeled as a sequence of operations, the operations can be adapted to control concurrent manufacturing of multiple product instances, without resource deadlocks and with maximal flexibility. The given sequences of operations may contain routing flexibility, conjunctive and/or disjunctive resource requirements, and product assembly. Using a known deadlock avoidance approach, this paper presents how to transform the given operations into models required for the deadlock avoidance approach and how the calculated policy may be retransformed back into the operations.

Error handling within highly automated automotive industry: Current practice and research needs

Fault tolerant systems, commonly found in literature, are implemented in various computer applications. Some of these methods have been studied and developed to aid manufacturing systems; however, they have rarely been integrated into the manufacturing process. Broadly, the problem seems to be integration of error handling procedures towards the end of physically building the manufacturing line, lack of a defined workflow, untested program logic and inadequately equipped personnel to name a few. To this end, a survey was conducted within the Swedish automotive industry to get an understanding of current error handling procedures and its shortcomings, and are presented here. Based on this data, and looking at the trends within the manufacturing industry, this paper also identifies research topics aimed towards defining methods to create next generation fault tolerant manufacturing systems.

On restart of automated manufacturing systems using restart states

Highly automated manufacturing systems have gained industrial popularity for their ability to combine high product volumes with high product quality. The high cost of investment in combination with many linked manufacturing systems in a factory, requires that the production runs smoothly with high utilization of the resources and that stoppages are avoided. One major reason for stoppages is the occurrence of errors. A wide variety of possible faults, such as badly fixated parts, broken actuators, and teething problems in the system, may cause errors that lead to an unsynchronization between the control system and the physical system that consequently lead to production stoppages [1,2]. The succeeding error recovery is often a complex and thereby time consuming process that typically requires operator involvement [3]. To plan for restart after errors already during the development of the system would therefore greatly support the online restart process and reduce the time the production is undesirably stopped. The common industrial practice to deal with such non-intended progress is to extend the control system with tailor-made solutions to account for foreseen errors [4,5]. This extension is both time consuming and there is no guarantee that all relevant errors are handled.