they are used as a guideline to evaluate available manufacturing functionalities utilisable within the manufacturing process. In the second step the opposite direction of evaluation is used. In this case the most fitting manufacturing resources are selected based on the description of manufacturing functions provided by MUs. Within the third step basic engineering information are (re-)used like initial mechanical drawings or wiring planes of the MUs covering the unit internal wiring and its interfaces, or the control code fragments which are integrated in the entire control applications of the production system. An overview about the different engineering steps and their relation to each other is depicted in the lower part of Figure 6.
Figure 6: Global structure of mechatronical engineering process [17]
The project dependent MEP requires a library of useable MUs. These re-usable MUs have to be developed and tested prior to its application.The development of these re-usable objects starts with a requirement analysis, an investigation of available technologies at the market and further market conditions, and an analysis and modularization of best practice solutions. Based on these evaluations devices as well as components are planned, engineered, and realized in the same way as in the project oriented engineering process. Within a final step the engineered device or component is tested. Thereby, re-useable engineering artifacts are established which can become part of a library.
6. Application of mechatronical units within the mechatronical engineering process As mentioned above, MUs are applied within the MEP several times. Application
cases most relevant for project dependent MEP shortening are in the planning and the engineering phase of the MEP. They will be described in more detail in this section with a strong focus on control application design. The mechanical, electrical, etc. constructions are executed in a similar way.
Figure 7: Planning phase activities of MEP 6.1. Planning phase
Within the planning phase of the MEP appropriate MUs are selected usable for the execution of relevant Selection of manufacturing function to be realized
Are there mechatronical engineering units within the library fulfilling the required function?
Is it possible to divide the manufacturing ？ function in sub-functions? Apply mechatronical unit! Yes, no.
Divide function and proceed with sub-functions
Design a new mechatronical engineering unit for the manufacturing function
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Which devices or lower level mechatronical units are required?
Which execution interfaces are required to control the manufacturing function ? Which device interfaces are required to control the underlying devices and mechatronical units?
Which primary functions have to be fulfilled? Which secondary functions have to be fulfilled? How will the function behavior look like?
Which parameters are required for the execution of the required manufacturing function?
How the control application looks like? How the control interface looks like?
Design the mechatronical units based on the above questions yes no
Is this unit optimal for the intended function? yes ，no.
Integrate the mechatronical unit within the library manufacturing steps. This
selection process can directly exploit the information structure given in section 3. The process starts with the selection of a manufacturing function to be realized. To this manufacturing function an appropriate MU will be selected following the technological description given in the primary functions of the MUs.Two cases may happen, an MU can be found which is optimal for the execution of the required manufacturing function or no such MU exists. In the first case the next function can be considered.In the second case the function of interest should be divided into subfunctions if possible. If this is not possible a new MU has to be specified in the project independent MEP.
For this specification process a set of main questions has to be answered. The process starts with the questions about primary and secondary functions which have to be fulfilled.Based on the definition of these functions the overall behavior of the MU has to be specified. Therefore, the primary and secondary functions with its programmable organization units will be given. As next step, the parameters for the proper behavior of the functions, the required devices and lower level MUs, and the interfaces to them have to be named. Thus, the device interface+ parameter object and the devices object are detailed. Afterwards, the control application is characterized. Therefore, the programmable organization units and the sequences objects are applied. Finally, the interface to control the MU from a higher level is defined by completing the execution interface + parameter object.This process is given in Figure 7. 6.2. Engineering phase
The engineering phase of the MEP is dedicated to the detailed definition of the overall construction of the manufacturing system out of the selected. Therefore, it has to detail the behavior of the different MUs, mutually connect them, and generate the control code. For this process at first it has to be ensured that all necessary MUs
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are available within the library. If there are MUs missed they have to be developed as given above. If all necessary MUs are available within the library, the relevant amount of MUs has to be instantiated by defining MU occurrences within the planned system. Afterwards, all of them are detailed by the definition and instantiation of the necessary sub-units and devices within the device objects and the specification of the overall behavior of the MUs by defining new and connecting existing behavior descriptions within the controlled behavior part of the sequences object. If all relevant MUs and devices are given, they are connected by interlinking the signals given in the device interface + parameter objects of the different functions with the signals defined in the execution interface + parameters of the underlying MUs and devices. The same holds for the different parameters within these objects. After the complete interlinking of the MUs and devices the control code is generated Theoretically, all different available model based control code generation methodologies can be applied [19]. Generally, at first the control hardware (i.e. PLC structure) is defined, then the necessary code objects like function blocks are named,the mapping of the code object variables to signals is made, and the necessary variables are specified.Finally, the real code lines are generated. This structure is depicted in Figure 8.
Figure 8: Planning phase activities of MEP 6.3. Application of MU structure within MEP
One essential point connecting this process with the described information structure of the MUs are capabilities of the MU structure to support the process by the MU
structure.Two examples of these capabilities are the automatic generation of MU structures depending on different function structures of the MU and the automatic integration and connection of interfaces within the hierarchy of MUs.
As seen in Figure 5, the information structure of MUs is constant except the number of primary and secondary functions as well as the amount of MUs and devices within the MU hierarchy and, thereby, the amount of interfaces among them. Thus, the integration of MUs within an engineering project can be automated by application of appropriate templates for MUs, functions, and interfaces. These templates can be part of a library and automatically exploited. Within AD this can be implemented by using “e-blocks”.
The automatic integration and connection of interfaces within the hierarchy of MUs exploits the interface templates and creates the necessary interfaces and connections between a MU and its subunits or underlying devices. Therefore, for each execution interface of the underlying MU an appropriate interface within the device interfaces structure is created and automatically connected by “e-blocks” as depicted in Figure 9.
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Figure 9: Automatic integration and connectionof interfaces within MU hierarchy
7.Specification of re-usable MUs
Generally, the specification of re-usable MUs is a trade-off between detailed modeling and engineering of MUs and its applicability in special application cases.As more detailed a MU is given as small is the set of application cases it can be applied within. Hence, a good balance between both intentions (huge application range and large amount of re-use) has to be found.
Following [9] and [20], manufacturing systems can be structured hierarchically in 6 layers. The highest layer is the plant itself. It consists of manufacturing cells and manufacturing lines which all can be split to main groups of functional units like robots, milling machines or conveyer systems. These main groups consist of function groups like clamp sets, robot effectors, single conveyers, and so on.Each function group itself consists of subassemblies like motion groups within conveyers or axis of robots. Finally, each subassembly consists of single parts like drives and mechanical components. This structure is given in Figure 10.
From the authors point of view all layers between subassembly and manufacturing cell can be considered as MUs within the MEP. Single parts are considered as devices while the complete plant is out of scope.
Nevertheless, the definition of library elements is most efficient for MUs representing subassemblies or function groups.In special cases also the main groups are relevant for library element definition.
Figure 10: Hierarchical structure of MUs
within a manufacturing system (enriched from [9]) .The reasons for this view are the following.On the subassemblies and function groups layers sensor and actuator devices are directly coupled with information processing units like PLCs. Here usually a clear and strongly limited range of primary functions, the MU provides, is given. Hence, the versions variety of a MU is limited at this layer while its application range is large.Thus, maximal re-usable MUs can be specified.
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For the specification of MUs at this layer the sequence of the following questions should be answered.
? Which primary functions have to be fulfilled? ? How will the function behavior look like?
? Which parameters are required for the execution of the required manufacturing function?
? Which devices or lower level MUs are required?
? Which device interfaces are required to control the underlying devices and MUs? ? Which execution interfaces are required to control the manufacturing function? ? How should the control application look like? Based on these questions MUs can be specified.
On the main group layer several primary functions of the function group are combined. Here, in general the problem of complexity explosion is given.In parallel,the application range of the main function reduces in comparison to the function groups.Thus, theoretically, each possible combination of function groups has to be integrated within the library where the probability of reuse of each MU is much smaller.
Practically, in each industry usually only a limited amount of combinations is useful. For example, in the cases of soldering machines, welding robots, or chipboard press plants only a limited ranges of systems are useful. In such cases also main groups can be part o Practical considerations have shown that nearly each manufacturing cell is a one-time system. Hence, libraries of such system will not make sense.
Thus, the definition of library elements representing MUs based on the layers subassemblies or function groups seems to be most promising.The definition, engineering implementation, and test of these MUs requires a small amount of resources compared to the capability of re-use. Without re-use the necessary efforts for multiple engineering of the same information is much higher. But with increasing MU complexity and decreasing numbers of reuse this benefit is lost on the higher levels of the plant structure.
8 、Conclusion
Within a common research activity of Siemens AG and Otto-von-Guericke University Magdeburg the represented mechatronic engineering process has been executed for a laboratory plant using SIMATIC Automation Designer. The focus of this activity has been on the development of a library of re-usable mechatronical units, the development of automated support functions for engineering activities, and the evaluation of the MEP up to final generation of control code and control hardware configurations. The evaluation showed, that our modeling approach was practically feasible and consistent. The resulting project dependent engineering effort was strongly reduced
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