# Increasing Resilience of Production Systems by Integrated Design
## 2 Materials and Methods
### Rapid Response to Changing Requirements Using Skill-Based Control Systems and Virtual Commissioning
This ensures that modules can be easily exchanged and arranged according to the automation task as well as to the hardware representation. Figure 7: The original workflow has been rescheduled to account for a defect of the milling machine. The task “Mill engraving” has been relocated to the end of the workflow and has been padded with “Repair milling machine”, “Insert work piece”, and “Take work piece from skid”, so the constraints imposed by the required and returned context data are still valid. This also triggers the main disruption in the programming of control devices: the developed code is no longer task-driven, but skill-driven and modular. The most important features of this controller programming approach are uniform interfaces and the provision of parameterizable module functions in specific skills. Afterwards, the functions can be orchestrated freely for the automation task by a higher-level system, whereby each module provides the necessary function descriptions, requirements, and boundary conditions [13]. The strengths compared to the established architectures lie in the free reconfigurability, interchangeability, and reusability of functional components at the control level. The concept of skill-based control can be derived from a product-process-resource model (see Figure 8). The benefit to the user is that the PLC programming is transformed from a programming effort to a combination of predefined skills and their parametrization. Hence, the controller is not only equipped with a specific program to solve one specific complex automation task (Figure 9, left), but bases it on a large set of modular basic skills, covering all of the possible or senseful abilities of the manufacturing unit. Here, skills can be parametrized and combined to jobs, leading to the complete automation task (Figure 9, right). To give an example, handling, measuring, orienting, and loading/unloading are regarded as jobs. All of the jobs consist of a sequence of skills, such as movement, opening/closing a gripper, call to a camera. The skill \"movement\" is furthermore a combination of basic skills such a move linear or move circular (Figure 9, right). The manufacturing cell is equipped with all hardware modules and a complete skillset. This skillset is taught and programmed by the automation technician but remains open for a variety of applications. The solution is extremely flexible in further applications and in individual utilizations. Further steps such as combining skills to jobs and programs can be conducted by skilled workers and do not require automation technicians. Figure 8: Capability concept based on product, process and resource inspired by [29]. Figure 9: Transformation of problem-based to skill-based automation. After a basic commissioning and software test, the machine operator can combine the skills and jobs to an automated process sequence or adapt the given sequence to a new setting with support of a graphical user interface (GUI). The fast commissioning as well as the software tests for the skill-based functions can be realized by virtual commissioning using a digital twin of the production system. Digital twins are already widely used by machine and plant manufacturers to virtually plan and optimize the most complex production machines and plants during their development [30]. Along the development process, the digital twin can be used for the design of the different domain-specific subsystems and to evaluate the dependencies between them. As such, at the end of the construction phase, there is a nearly exact digital twin of the machine that can be used for the development of the control functions while the hardware is in realization [31]. The virtual commissioning of control systems allows for a reduction of the commissioning time of the whole machine and results in a faster production start as well as reduced costs for the engineering process [32].