202215044 Auslandsfassung 1 Description Method for generating a model, computer product, system FIELD OF THE INVENTION The invention relates to a method for generating a model of a system, with the steps: (a) providing an engineering system with library elements, where the library elements are defined by interaction and a function model, wherein said interaction is defined by ports, incoming port entity types, outgoing port entity types, wherein said function model generates outgoing port enti- ties from incoming port entities, (b) providing a design of the system comprising defined sys- tem components of a specified plant area; (c) selecting library elements for the defined system compo- nents; (d) linking interactions of different selected library ele- ments according to the design. Further the invention relates to a system. BACKGROUND OF THE INVENTION In the field of plant engineering and mechanical engineering systems for engineering, i.e. planning and realization of au- tomation, of an industrial plant are commonly used. Automated processes in industrial plants like plants in general or ma- chines, production lines, chemical plants or power plants are complex. Often the system to be controlled is complex and the engineering process is multidisciplinary. The plant may com- prise a plurality of modules comprising several assemblies and subassemblies. Several of these assemblies and modules are multidisciplinary and interdependent. In an unfavorable case automation and control engineers may start working on the project not before providing a first complete plant draft. Such interdependency delay increases planning time and costs. In today’s agile industrial environment such delay can hardly be accepted. 202215044 Auslandsfassung 2 From WO 2014146716 A1 a method for engineering an industrial plant is known including weakly defined basic engineering ob- jects that can be detailed at a later project stage. Similarly, from WO 2015172814 A1 a method for generating li- brary elements for an engineering system is known comprising an automatic generation of library elements from e.g., pro- ject history. US 2015/106068 A1 relates to the accuracy and performance of simulations. This patent does not disclose providing unde- fined plant areas by providing non-specific library elements in a modelling environment and does not disclose generating a function model for such non-specific library element on the basis of the Buckingham Pi theorem. EP 3798 775 A1 discloses an industrial integrated develop- ment environment supporting a virtual design environment. US 2018/225409 A1 describes a procedure for creating a tur- bine Buckingham π theorem-model, which is subsequently devel- oped in a real-time application and used instead of an ana- lytical model. This is done to obtain a more stable behavior in the runtime application. Here a structure and essential parameters are already known and given in advance of creating the model. Engineering systems often provide a library with library ele- ments. Such libraries may contain standard assemblies and subassemblies as library elements, e.g., oil system compris- ing an oil tank, an oil pump, oil, piping and valves, a cooler, sensors for pressure and temperature and maybe fur- ther elements. These library elements may be specified and parametrized during the planning of the plant. Function blocks are commonly used library elements in graph- ical engineering tools. They provide an easy means to de- scribe a function required by a process and its connections to other functions. The functions can also be equivalent to 202215044 Auslandsfassung 3 specific pieces of equipment like pumps, valves, transducers, PID-controllers, or the like. Some products utilizing func- tion blocks are, e.g., Siemens’ Simcenter Amesim (simula- tion), Siemens’ Comos (process plant engineering) and Sie- mens’ CFC (automation engineering within PCS7 and TIA Portal, see figures below as an example). These tools require an engineer with considerable oversight of the engineering system´s library content to enable effi- ciency. Beginners or occasional users need a significant in- duction time and a deep existing knowledge of the function to implement these correctly. A development engineer working re- petitively in an established technical field may be able to efficiently apply parts of such libraries. Engineering systems such as Simcenter Amesim (simulation) in- vite to a more explorative working style. The simulation of an imperfect draft engineering design may lead to iteratively improved increasingly optimal solutions. The user in this case could be rather a research engineer, addressing a new solution approach in a rapidly developing field, e.g., the application of a memristor, a metamaterial with a negative Poisson ratio or a hydraulic device employing a ferrofluid. In such innovative application of the system, it may not be evident, which function block may be the best or even an ap- propriate choice. Further parametrizing a selected library element, e.g., a function block becomes difficult, as the function may be not fully known. SUMMARY OF THE INVENTION Based on the shortcomings of the state of the art it is one object of the invention to enable drafting of a plant or a system with approximate data, educated guesses and undetailed areas of only approximate function. In accordance with the invention there is provided a solution for the above-described problems by the incipiently defined method wherein in addition to defined system components, the 202215044 Auslandsfassung 4 system comprises at least one undefined plant area whose in- teraction with the specified plant area is specified, the method further comprising: (e) linking the interaction of the selected library ele- ments to the interaction of the undefined plant area by providing a non-specific library element, (f) generating a function model for the non-specific li- brary element for determining outgoing port entities from incoming port entities using the Buckingham Pi theorem. The step (f) is preferably a computer-implemented step. This enables a fully automized generation of a function model for the non-specific library element. According to a preferred embodiment the step (e) may preferably be a computer-imple- mented step. Most preferably steps (e), (f) are both com- puter-implemented. According to a preferred embodiment also steps (c) and/or (d) are computer-implemented steps. Most preferably the design is provided in a standardized format to enable a fully automatic processing. The object of the invention is achieved by the independent claims. The dependent claims describe advantageous develop- ments and modifications of the invention. The invention is led by the understanding that often interac- tions of an undefined system are known, which are – in the terminology of the invention the ports, incoming port entity types, outgoing port entity types – which basically reflect the natural-scientific function of the system and the number of the parameters or ports of a library element or a function block. By understanding the interaction but not knowing the exact function in the unspecified area of a system the user basically formulates a requirement specification without knowing the detailed specification. This situation reflects the reality of ordering components or services of engineering from a third party. The invention embodies and reflects this situation by providing the non-specific library element of 202215044 Auslandsfassung 5 the engineering system. Moreover, the invention provides a function model, which may be understood as a provisional function model of the non-specific library element embodying the undefined plant area on the basis of the interaction with the specified plant area. In an agile development environment of several teams provid- ing components of a system or plant such undefined plant ar- eas may be specified stepwise and replaced step by step with more detailed library elements comprising interactions and function models. Such the undefined area may be stepwise re- duced. In the meantime, all teams working on other components of the system are enabled to continue since the interaction with unspecified areas remains available all the time. Consequently, the invention enables to design a system with a flexible or agile approach such that the system design pro- cess may start with a group of at least one or several unde- fined plant areas. The project coordinator may split the sys- tem into assemblies and subassemblies and specify the inter- actions in-between. Every team working on one of these assem- blies or subassemblies may work nearly independent of the other teams since provisionally the interactions are availa- ble as if the components would be specified already. During the projects progress the interactions will change due to replacement of more and more non-specific library elements comprising function model on the basis of the Buckingham Pi theorem by library elements with function models reflecting design details and with higher accuracy. According to a preferred embodiment the system comprises a system control for controlling the system. This system con- trol may be tested by simulation using the model as the sys- tem to be controlled, the model being generated via the engi- neering system applying the method according to the inven- tion. 202215044 Auslandsfassung 6 Further, according to a preferred embodiment of the invention the system control may be a model-based control applying the model being generated according to the invention. The model of the model-based control may comprise at least one non-spe- cific library element, with function models for the non-spe- cific library element for determining out-going port entities from incoming port entities using the Buckingham Pi theorem. Applying the non-specific library element in the control al- gorithm of the model-based control may enable faster control reactions due to the nature of the Buckingham Pi theorem- based function model which often is a simplified version com- pared to a more detailed model. Further the method according to the invention enables to gen- erate reduced order models for control simulation or for model-based controlling with less effort. Since the scheme according to the invention enables a computer implemented au- tomized generation of function models for undefined plant ar- eas with known interactions the control method and a control module may be provided as a self-adaptive control module with a model-based control algorithm. The invention therefore further relates to a model-based con- trol method and a control module arranged and configured to execute said model-based control method for respectively con- trolling a system, wherein the model of the model-based con- trol is generated by a method according to the invention re- spectively according to claim 1. The problem underlying the invention is solved by a function block that generates an approximation of an analytical solu- tion to an engineering function based on a method from the field of dimensional analysis, known as “Buckingham π theo- rem”. (Dimensional Analysis is, e.g., used in the field of fluid dynamics (Reynolds number, Froude number, Mach number, etc.) and materials science, where complex relations that are hard to quantify and measure nevertheless need to be made 202215044 Auslandsfassung 7 available to engineering. The Buckingham π theorem is also used to validate hypotheses that have been developed by other means. The Buckingham π theorem is well established in engineering and science and the default way of application is well known to a person with ordinary skill in the art. Therefore only a few aspects are considered herein. The Buckingham π theorem states that just by knowing the parameters or variables with independent physical units involved in a problem a descrip- tive equation can be defined as an approximation that relies on few dimensionless parameters only, rather than a fully an- alytical physical description. The benefit is that few experimental data or an educated guess for these parameters can suffice to come to a numerical description of a function that is not yet fully understood in all physical details but generates solutions which are at least conclusive, and which often correctly reflect the phys- ical behavior with regard to proportionalities, trends, and tendencies, and which even may sufficiently accurate. The Buckingham π theorem basically neither qualifies fully as a numerical nor as a physical model. Instead, the Buckingham π theorem, when applied as herein proposed becomes synergisti- cally suitable to produce to generate a function model or a transfer function of a system, assembly, or subassembly due to its capability to automize and due to its characteristic simplification of the function. In contrast to an analytical solution for the function block engineering, where the function or the type of function block including their parameters and boundary conditions need to be known in detail in advance, the non-specific library element or such a Buckingham π theorem-function block requires only the interaction, namely a set of ports assigned with inde- pendent measurement units. For example, the user or engineer needs to know the type and respective number of independent measurement units used at the function block´s ports and as- signs them to a non-specific library element. The method 202215044 Auslandsfassung 8 according to the invention – in other words - provides for the non-specific library element evaluating the number and types of the ports and their respective physical units, checking for consistency, and setting up a set of equations and matrices and solving these to determine the function model. In general words, according to the invention, the structure of a Buckingham π theorem-model is built up according to the Buckingham π theorem-method known from the literature, which allows to make an analytically not yet fully clarified rela- tionship available for technically sufficient use by means of an approximation. A preferred embodiment provides that additional information e.g., some data from experience or preliminary experiments, they also are assigned to the Buckingham π theorem-function block (practically, this information would be submitted after defining the ports or posed as a question to the engineer, once the determination of the equation has been performed). The determination of the dimensionless factors can be per- formed for the function modules of the non-specific library element in different additional ways, which may be applied alternatively or in combination. These options may be used to be compared or compete to each other regarding accuracy and/or latency or other performance indicators of importance for the respective specific application. The options may com- prise one or several of the items listed here: a) solving the equation using experimental data b) searching for similar equations, c) presenting analogies and library elements, e.g. function blocks with similar behavior from library, this step may employ AI d) exploring and optimizing the solution by varying the constants which may be termed “constant explora- tion”: 202215044 Auslandsfassung 9 e) interactive involvement of the user. This may in- volve an Educated guess: the equation is presented to the engineer who can fill in the factors accord- ing to his judgement, available data, etc.. The invention makes it possible to work with objects, whose physical behaviors are at least partly known or postulated, but not yet fully understood. The invention further relates to a computer product arranged and configured to execute the steps of the method as ex- plained herein and as defined by the claims. The invention further relates to a computer-readable medium encoded with executable instructions, that when executed, cause the computer product according to carry out a method according to the invention. The invention further relates to a system for generating a model of a system, the system comprising at least one proces- sor being prepared by upload of computer-executable code to perform a method according to the invention. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention are now described, by way of ex- ample only, with reference to the accompanying drawings, of which: Figure 1 shows a simplified flow diagram illustrating the method according to the invention as well as a sys- tem for performing the method; Figure 2 shows schematically the stepwise application of the Buckingham Pi theorem; Figure 3 shows possibilities to determine dimensionless pa- rameters while applying the Buckingham Pi theorem; 202215044 Auslandsfassung 10 Figure 4 shows a stepwise reduction of an undefined plant area. It is noted that in different figures, similar or identical elements may be provided with the same reference signs. DESCRIPTION OF THE DRAWINGS Figure 1 shows a simplified flow diagram illustrating a method of Figure 1 shows a simplified flow diagram illustrating a method for generating a model of a system TFC according to the invention. Further, figure 1 illustrates a system SYS for generating a model of a system TFC, the system SYS comprising at least one processor CPU being prepared by upload of computer-executable code to perform the method according to the invention. The method may be handled as a computer product CMP arranged and configured to execute the steps of the method according to the invention. Such computer product CMP may be a data stor- ing media, like an optical or magnetic data storage or a download enablement e.g., a reading access of an URL where the respective data is provided to a user USR. Such data storage may be a computer-readable medium CRM encoded with executable instructions, that when executed, cause the com- puter product to carry out a method according to the inven- tion. As illustrated in Figure 1, during a first step an engineer- ing system EGT with library elements LET is provided, wherein the library elements LET are defined by interaction IAC and a function model FCM. Said interaction IAC is defined by ports PRT, incoming port entity PET types, outgoing port en- tity PET types of the library elements LET. Said function model FCM generates outgoing port entities PET from incoming port entities PET. 202215044 Auslandsfassung 11 During step (b) a design DSG of the system TFC is provided. This can be a design of any type of said technical sys- tem TFC. The design comprises defined system components DSC of a specified plant area SPA. Herein, plant area SPA means a part of the system TFC. In addition to defined system compo- nents DSC, the system TFC comprises an undefined plant area UDA. In step (c) library elements LET are selected for the defined system components DSC. This can be done automatically respec- tively fully computer-implemented or manually by a user. Dur- ing step (d) interactions IAC of different selected library elements LET are connected or linked to each other according to the design DSG. Most preferably the design is provided in a standard format enabling fully automized assignment of library elements LET. The automized assignment and linking of library elements LET may be done via an AI-module linked to a design-library data- base recognizing design patterns and retrieving corresponding library elements LET from an engineering system EGT library respectively database. An interaction IAC of said undefined plant area UDA with com- ponents of the specified plant area SPA is defined in a spec- ification of the interaction IAC. Based on this specification of the interaction IAC the method comprises step (e), wherein linking the interaction IAC of the selected library ele- ments LET to the interaction IAC of the undefined plant area UDA by providing a non-specific library element ULE is done. For the non-specific library element ULE in step (f) a func- tion model FCM for determining outgoing port entities PET from incoming port entities PET using the Buckingham Pi theo- rem is generated. For the purpose of better imagination in the figures the library elements LET may be interpreted as having incoming port entities PET on the left-hand side and 202215044 Auslandsfassung 12 outgoing port entities PET on the right-hand side. But this is not necessarily the case. A user USR can view the resulting function model FCM and the arrangement of respective function blocks corresponding to the library elements LET of the complete system TFC on a dis- play DSP. Figure 2 schematically shows the stepwise application of the Buckingham Pi theorem BPT. As a first step I) a non-specific library element ULE for an undefined plant area UDA of the system TFC is provided. Step II) illustrates that the interaction IAC with components of the specified plant area SPA is specified. During step III) the Buckingham Pi theorem BPT is applied to generate the function model FCM of the non-specific library element ULE for an undefined plant area UDA. Figure 3 shows possibilities to determine dimensionless pa- rameters while applying the Buckingham Pi theorem BPT. i) determining the parameters of the equation generated via the Buckingham Pi theorem BPT using experimental data EPD ii) searching for similar equations, present analogies and library elements with similar behavior from a database DBA or library, may employ AI iii) “Constant exploration” CXP: explore and optimize the so- lution by varying the constants or guessing preferably making an “Educated guess”: the equation is presented to the engineer who can fill in the factors according to his judgement, available data, etc.. Figure 4 illustrates that an undefined plant area UDE may be specified stepwise and replaced step by step with more de- tailed library elements LET comprising interactions IAC and function models FCM. Such the undefined area UDE may be step- wise reduced. The undefined plant area UDE as shown in the top of figure 3 may be split into at least two undefined plant subareas SUA corresponding to possible assemblies and subassemblies and specify the interactions IAC in-between as 202215044 Auslandsfassung 13 shown in the middle system TFC. During the project’s progress one or several of the undefined plant areas UDE become speci- fied plant area SPA and the non-specific library element ULE become library element LET – as shown in the bottom sys- tem TFC. The corresponding interactions IAC change due to re- placement of more and more non-specific library elements ULE comprising function model FCM on the basis of the Buckingham Pi theorem BPT by library elements LET with function mod- els FCM reflecting design details and with higher accuracy.

202215044 Auslandsfassung 14 Reference list DSC defined system components PET port entities LET library element EGT engineering system TFC system UDA undefined plant area DSG design PRT ports ULE non-specific library element FCM function model IAC interaction SPA specified plant area ULE non-specific library element SCT system control BPT Buckingham Pi theorem SUA undefined plant subareas