# Realisation of a Novel Functionally Redundant Actuation System for a Railway Track-Switch
## 1 Introduction
### Contributions  
The first contribution of this paper lies in the demonstration and validation of the redundant actuation using one out of three fault-tolerance through redundancy of actuator bearers. This is important because it allows a significant improvement of the system-level reliability of the switch. Unlike traditional switches, this allows for continued use of the switch (and passing of trains) even after up to two actuator failures (see e.g., [14]). Three different operation scenarios are proposed; initially, operating the track switch using three actuators, which will be the normally expected case of the actuation; then, running the switch with two active bearers, simulating the failure of the third bearer; lastly, switching using only one of the three actuators (for the case where two actuators are failed).  
The second contribution of the work is the development and validation of a simulation model using multi-body simulation software. A modelling approach, similar to that presented in [22] for a traditional switch, is used. The models are useful during the design phase to calculate the actuation forces required and to specify the different electro-mechanical elements in sub-systems, they are also essential for design and validation of the feedback control loops (allowing safe testing before implementation). The modelling process starts with developing a flexible body model through finite element analysis in Abaqus, which is then imported to Simpack to construct the multibody simulation model of the system. The controller, which consists of three cascaded loops [13; 23], is developed and the actuation elements of the model are generated in Simulink. Then a co-simulation is carried-out using the SIMAT environment where the models are combined. The modelling results were validated with the experimental data after the assembly of the system [14].  
The third contribution of this research is to introduce closed-loop feedback control of three actuators in a track switch for the first time; to check synchronisation and to ensure proper movement of the switch rails. The control challenge was to ensure a proper movement of three redundant actuators, which means synchronisation of the movement of six motors driving to three electro-mechanical actuators. The synchronisation of the movement of two electric motors of one mechatronic actuator is tested first, in the laboratory to validate the modelling work before the assembly of the complete system.  
One of the major challenges that came to light while testing the demonstrator switch was the difficulty in getting proper load sharing across the three actuator bearers. The present controllers are developed with identical performance/profile such that each actuator might share the load equally. In the operation environment, it will be seen that one of the actuator bearers is subjected to significant load, while the other two actuator bearers bear much less of the load. This finding, which was not possible to solve within the time-frame of the experimental campaign, can be considered the fourth contribution of this paper.  
In the present paper, the experimental setup and mechanism design of the new switch is described in Section 2. In Section 3, the developed model of the track switch is presented. Section 4 describes the controller design, its implementation and model validation for the novel track switch. The experimental results, in a railway environment, of tests of the full-scale track switch, are presented in Section 5. Finally, conclusions are drawn and some ideas for future work are given in Section 6.