## 2 Method
### Assembly Sequence Model  
Based on these thoughts, this work proposes an assembly sequence model consisting of interconnected assembly steps. Each step must have at least one input component and one output component. The connection between the steps can then be realized over the components. An output component could be the input component of another step and vice versa. The assembly sequence must result in one coherent network of steps without circular connections. Therefore, among other things, each step must be directly connected to at least one other step. Input components that can or should not be connected to an output component are generated out of sources. In a real process, they must be supplied. Output components that can or should not be connected to an input component are products of the assembly and must be discharged. Each step can have a procedure type that characterized which kind of assembly must be done. Procedure types could be clustered into groups. Such groups could help designers describe a task that was not completely decided yet. The VDI 2860 norm defines assembly as the task of mating parts and all processes that are needed to achieve this, including handling, verifying, and adjusting [24]. As a basis for mating procedure types and their classification, DIN 8580 (manufacturing techniques) can be used [25]. It consists of six main groups of manufacturing, whereby mating is one of them. Each main group is separated into subgroups. The actual procedures are assigned to these subgroups. The classification as done by DIN 8580 may not be necessarily perfect for any use case. Nevertheless, it is a good starting point to gain an overview of a possible grouping and, more importantly, the entire range of procedures. For the field of assembly, more than 100 possible procedure types arise from this structure. This method proposes that all possible procedure types should be checked based on the chosen RMS. If a procedure type can be executed with a module or with a combination of modules, the procedure type should be selectable in the modeling of the assembly sequence. For this, a new grouping of the found types can be introduced. Picking a specific procedure type in the conceptual design phase may not be easy or sometimes even impossible. Furthermore, a designer voluntarily might not want to set it, because he/she simply does not care as long as an RMS can do the assembly afterwards. Therefore, the procedure type or its group need not be set. Still, more information will improve the quality of the resulting machine configuration by describing the task more precisely. Hence, users of the method can be supported by adding two types of properties: process properties and component properties. Process properties are tied to a specific assembly step, while component properties are applied to all steps that use the corresponding component. Both will influence the assembly and thereby the possible procedure types. Even if a designer is not able or willing to define a specific procedure type, he/she can set properties based on already chosen parts or the functional description of his/her product. For example, he/she can define if a joint should be reversible or if a component can endure high temperatures. On the other hand, when directly selecting a procedure or group, he/she should be able to see which properties his/her process and the involved components have to fulfill. Similar to the procedure type, component properties and process properties are optional parameters, so they do not need to be given for a result. Figure 3 shows a scheme of an assembly sequence model for a product design. Reasonable properties may differ strongly based on the field of application, for example if the method is applied to a micro-assembly or the manufacturing of a car part. Therefore, component and process properties must be found taking the actual use case into account. When deciding which properties should be used, the number of properties should be as small as possible.