# A Predictive Multibody Model of Paper Applied to Cash Recycling
###### Abstract  
Despite the spread of digital payment systems, banknotes still play a vital role in the lives of people, financial institutions, and businesses. Automation is crucial in all cases where a large number of deposits/withdrawals need to be handled. In spite of the large number of cash recyclers worldwide and their continuous evolution, one of the most significant parts of their design, i.e., the interaction with banknotes, is still predominantly based on a lengthy iterative process that includes testing. This has detrimental effects not only on time-to-market but also on the costs and on the willingness to explore multiple design solutions, thus potentially reducing the quality of the final product. The testing phase is made necessary by the lack of an effective and predictive model for the banknote and of its contact with the components of the cash recycler. The purpose of this work is to bridge this gap in the literature by introducing and validating an original multibody model of the banknote-cash recycler system. The proposed approach includes the possibility of including the nonlinear orthotropic paper material curves and of customizing quantities such as paper thickness and friction coefficients at specific locations, with potential applications in optimizing banknote validation, reducing wear in cash handling systems, and improving the design of next-generation cash recyclers. m +
Footnote †: journal: applied sciences  
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to increasing demands, e.g., seven banknotes handled per second, one at a time, without tearing. In this context, a computer-aided methodology to simulate the process of transporting banknotes through the cash recycler would be particularly beneficial. The presence of such a design simulation tool can be a great competitive advantage for companies. With a working methodology available, it would be possible to simulate banknote transport in different construction solutions without the need to physically build prototypes, thus reducing costs and product development time. Another advantage of simulation is the ability to investigate the behavior of the system under operating conditions that are not easily replicated experimentally. This includes analyzing the behavior of the banknote with a microsecond resolution, which is not feasible even with the use of powerful high-speed cameras. The numerical approach then leads to a better and deeper understanding of the physical phenomena underlying paper transport, thus paving the way to an effective process optimization. The tool can thus serve as a guide to the entire development process of machines that handle cash. In the literature, studies and computational models of the material behavior of paper and paperboard at different scales have been published [4; 5], as well as models considering the mechanical behavior of paper-based components, such as paperboard boxes [6; 7; 8]. To the authors' knowledge, however, there are no models available that consider paper sheets as components within more complex mechanical systems, e.g., a cash recycler, and therefore allow the interaction of paper with other mechanical components to be simulated. The objective of the present paper is to propose a methodology to simulate banknote transport in a Multibody (MB) framework. This task requires an effective model for the banknote. In the literature, models of paper sheets in a MB context are often limited to 1D simplified models [9], and while options to introduce flexible bodies in a MB simulation are available [10; 11; 12], they do not meet all the necessary requirements, as detailed in the following section. This paper proposes an original modeling strategy to correctly replicate the flexible orthotropic behavior of paper [13], while ensuring a robust description of the contact [14] between the banknote and the various components of the cash recycler.