## 2 Methodology--Related Works
### Interactions and Dependencies between Factory Planning and Operational Control  
Based on the elaborations in the previous two chapters, both areas, the planning and design of the production system and the IT systems for operational control, play important roles concerning the vision of the future's self-controlled, flexible, and agile factory. Of course, the technological advancements in the fourth industrial revolution and the technologies encapsulated within it must be and are intended to be capitalized on. Therefore, it is necessary to rethink a core element of the factory planning process and the operational production control of the production system. This core element is the work or process plan. The work plan serves different purposes within the organization and management of the production system. Primarily, it is the core element connecting products and machines. In the work plan, the precise order of necessary processes for manufacturing any given product is specified, together with the respective processing times, changeover times, and other relevant information. The logic of the work plan is decades old. It is used in the general planning phase of the factory design process as a means to structure the manufacturing process and connect it with the favored technologies. Once the factory has been built, the same work plans are then used to steer and control the production system on a day-to-day basis (see Figure 4 as an example).  
The main problem with the concept of the work plan is its lack of flexibility. Alternative means to perform certain tasks are cumbersome to integrate in a way that allows for flexible and dynamic management of the production system. Furthermore, once a production order has been started, it is substantial and often manual additional effort to alternate the path of a product through the production system (e.g., because of a machine breakdown). Workers have to manually adjust the work plan and override specific parts to assign a specific order to a different machine than previously planned. This, of course, disturbs the resource utilization within the production system. It also creates potential new bottlenecks. Even though modern MESs can work with some challenges, they are still designed to work with different work plans and, thus, different predetermined sequences of machines for manufacturing a certain product. This problem is further enhanced by the fact that often, not all potential options for the transformation of a product at a certain stage in the process are known to the MES or ERP system. Often, this kind of knowledge exists in the minds of the workers. They will then reroute the production order to a different machine. In doing so, it is taken off the radar of the centralized planning system. Using the concept of the work plan to utilize this knowledge would require the creation of various work plans, each containing a specific route through the production system. If implemented consistently, the number of available work plans for most productions in a conventional SME would increase dramatically. It would make actual planning nearly impossible due to the number of available options, and IT system algorithms would continuously have to calculate and evaluate.  
In order to solve this problem, many authors have presented ideas for more decentralized and agile planning approaches [46; 47; 48]. Until recently, these approaches seemed impossible. The reason was the lack of operational real-time data and computing power. However, with the emergence of the IIoT, together with cloud computing, artificial intelligence, and the improvements in the computing power of PLCs, more and more authors  
Figure 4: Exemplary work plan.  
deem this concept of decentralized autonomous production planning and control feasible, even for SMEs.