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Mr. John Smith

Job title



In the European railways market, the Wheel Slide Protection (WSP) function is regulated by the EN 15595 standard. This standard defines the basic features of the Wheel Slide Protection function and introduces the requirements for a watchdog (safety timer) functionality that prevents unwanted and extended reduction of brake effort, especially (but not exclusively), during an emergency braking activation. The typical WSP architectures developed according to this standard perform this safety timer function by means of an electronic hardware timer that counts the time during which the WSP electro-pneumatic valves are energized by the WSP control software, and forces the de-energization of these valves, thus disabling the WSP control function, when this time exceeds a given threshold. Once the timer has expired and has triggered the de-energization of the WSP valves, this state can be changed, and WSP control can be re-enabled, only if specific conditions are fulfilled, such as that the train has come to a full stop. These architectures, although widely deployed by all brake system suppliers for all the types of rolling stock, have in the recent years generated concerns about their safety integrity, because of the potential for the WSP control software to spuriously interrupt the hardware timer counting or to unduly reset the hardware timer after its triggering and before the system reaches a true “safe state”. This paper describes a solution to enhance the safety, yet improving the availability, of the WSP function by means of an electronic system developed with adequate Safety Integrity Level (SIL) that can be introduced in the traditional WSP architecture, offering an additional and independent safety barrier against a potentially unsafe behaviour of the WSP control software. The present paper describes the use of a supervising device, named WSP Supervisor, arranged to monitor the behaviour of the associated WSP system and, through direct or indirect actions on the timer devices of said WSP system, to increase its overall safety level, so that it can reach the expected safety integrity for a brake system during the emergency braking phase. The WSP Supervisor is arranged to: acquire the instantaneous linear speeds of the associated train axles and compare such instantaneous speeds with the linear reference speed of the railway vehicle monitor the state of the braking pressure available to the brake actuators Based on the above-mentioned inputs, the WSP Supervisor determines, for each axle in sliding phase, whether the WSP control software is working in a safe manner or not, depending on pre-determined operating conditions, and to cut-off the WSP control software when it is deemed to be working in an unsafe manner. Moreover, the WSP Supervisor can maintain or increase the preloaded time value for the safety timer associated with each axle when the WSP control software is deemed to be working properly, with the aim of fully “using” the available adhesion between wheel and rail to improve the braking performance. To guarantee an adequate safety integrity of the brake system especially during the emergency braking phase, the WSP Supervisor shall be developed with a Safety Integrity Level (SIL) 4 according to the standards EN50128/EN50657 and EN50129 respectively for its software and hardware. This invention introduces a high safety integrity calculator in traditional WSP architectures, that are typically designed to reach a maximum SIL2 integrity level, and revolutionize the way in which the WSP safety timer is performed, moving from a “non-intelligent”, purely hardware-based, safety timer to an intelligent safety timer function that has the technologies and the means to maximize the performance adapting the safety timer to the real conditions of the train (axle inertia) and the rail (available adhesion). This invention will bring the following benefits for future beneficiaries: Increased safety of the braking system, with reduced efforts in the Train Acceptance and Homologation phases Ensure an optimized availability of the WSP function, thus reducing the occurrence of wheel flats, with consequent wheel re-profiling Possibility to integrate the WSP Supervisor into existing WSP systems, with minimal integration and installation constraints

Wabtec: Mr. Pierre Debernardi, Mr. Marco Fasolini, Mr. Fabio Ferrara

Enhancing the safety and the availability of wheel slide protection function for railways applications

EB2022-IBC-009 • Full • EuroBrake 2022 • Rail brake systems and testing