Integrating Rule Based Expert Systems into a Simulation framework for digital twins
Authors: Dominik Hüsener, Jürgen Roßmann
Abstract: The loss of expert knowledge due to experienced workers retiring in the coming years poses a significant challenge. To address this, we proposed to digitize and preserve this knowledge using the digital twin concept. Traditionally, expert systems have been used to emulate human experts. Rule-based expert systems, such as Prolog and CLIPS (C Language Integrated Production Systems), are one class of expert systems. Fuzzy CLIPS is an extension to CLIPS that can handle Fuzzy logic. Compared to machine learning techniques such as neural networks, rule-based expert systems are more reliable and can provide explanations for their reasoning.VEROSIM is a simulation framework for Digital Twins, into which Fuzzy CLIPS was integrated. Rules are executed depending on the status of the Digital Twin or user input and manipulate the state of the digital twin or generate an output. The Network Modeler is a graphical user interface built with QML that was developed to enhance the process of connecting simulation components, similar to OpenModelica, and visualize or manipulate the data flow between ports. The network modeler was extended to support the formulation of CLIPS rules. The existing Rule Visual Modeling Language was adapted for this purpose. The user can use the GUI to create thing templates, add properties called slots whose state can be fuzzy, such as cost (from low to high) or surface quality (low to high), and based on these templates, rules can be formulated. Rules have zero to several conditions and at least one action that is executed when the conditions are met. VEROSIM objects were created and mapped to a QML file for all required components, which defines the appearance inside the Network Modeler. A parser either parses FuzzyCLIPS code and produces a VEROSIM Model of the rules that can be visualized in the Network modeler or the user can graphically build the rules and the parser produces executable code. This approach allows non-experts to easily build a rule-based expert system, which can be extended or changed even during execution. Additionally, it can visualize which conditions lead to which actions in a more intuitive way than looking at the code. Finally, the integration into VEROSIM allows for direct interaction with the digital twin.This process is used to recommend the best tool to use based on optimization parameters given as user input and the geometry of a part to produce in the context of the production of individualized parts made out of fiber-reinforced plastics. The result will be stored in a production plan which is part of the digital twin and information about the produced part is fed back to the digital twin and used to update the ruleset of the expert system.
Keywords: expert system, digital twin, UML, CLIPS
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