## 5 Future Research  
Future research will be directed towards the open issues mentioned in the Conclusions section.  
In the modelling domain, nonlinear order-reduction techniques may be explored further as well as the ways of incorporating the experiments to fine-tune geometric and material properties of the developed models. The difficulty with non-linear models is that superposition does not work anymore. Therefore, nonlinear model reduction is still an active field of research. A sub-structuring approach that isolates the non-linear effects and keeps the rest of the model linear seems to be a viable approach to be pursued further.  
The system identification field offers new perspectives in control-relevant modelling, predictive maintenance and Digital Twinning. We plan to investigate possible ways of merging real-time plant observations with existing models derived in the initial modelling and commissioning phases. This will allow iteratively updating the models aiming to keep their fidelity under changing conditions during the whole lifetime of the machine. This may be a crucial step for development of performance assesment methods capable of recognizing gradual performance degradation of motion systems due to mechanical wear of the equipment.  
As for the low-order fixed-structure controllers, development of assisting software tools to aid with the complex calculations may be a promising direction, together with development of methodology of their employment in specific mechatronic applications. This is a necessary step to fill the gap between the theoretical research at the academic level dealing with complex model-based design methods and applications-driven employment of control systems performed by practising engineers demanding simple and reliable tools deployable to existing hardware. Promising new results were achieved in this field recently and will be published soon in a separate study.  
Conceptualization, M.G. and J.N.; Formal analysis, J.K.; Funding acquisition, J.N.; Investigation, M.G., J.K., R.K. and S.P.; Methodology, M.G., J.K., R.K. and S.P.; Project administration, M.G. and J.N.; Resources, R.K.; Software, R.K. and S.P.; Validation, M.G., J.K. and S.P.; Visualization, J.K. and S.P.; Writing--original draft, M.G., J.K., R.K., J.N. and S.P.; Writing--review and editing, M.G. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the H2020 ECSEL JU project I-MECH [37] under grant agreement number 737453. The first and the second authors acknowledge the support from the ERDF under project \"Research and Development of Intelligent Components of Advanced Technologies for the Pilsen Metropolitan Area (InteCom)\" No. CZ.02.1.01/0.0/0.0/17_048/0007267.  
Not applicable.  
The data that support the findings of this study are available on request from the corresponding author, M.G.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.  
The following abbreviations are used in this manuscript:  
\begin{tabular}{l l} FE & Finite Element \\ FEA & Finite Element Analysis \\ FEM & Finite Element Method \\ FMU & Functional Mock-up Unit \\ FRF & Frequency response function \\ LPV & Linear parameter-varying \\ LTI & Linear time-invariant \\ MDPI & Multidisciplinary Digital Publishing Institute \\ MEMS & Micro Electro-Mechanical Systems \\ PI(D) & Proportional-Integral-(Derivative) controller \\ NL & nonlinear \\ \(H_{\infty}\) & H-infinity norm of a linear time-invariant system \\ ROM & Reduced Order Model \\ \end{tabular}