Introduction - Major improvement to the wheel slide protection system
Wheel slide protection systems in rail vehicles work in a similar way to the ABS anti-lock braking system in motor vehicles by preventing the axle from locking if the adhesion conditions at the wheel/rail contact suddenly deteriorate while braking. This can happen, for instance, when the track is wet, if there are leaves on the line or in high humidity. Wheel damage, such as flat spots, can be a direct result of axles locking in this way. Rather than simply protecting the wheels, however, wheel slide protection systems are designed to make optimum use of the adhesion currently available at the wheel/rail contact in order to minimize braking distance.
Longer braking distances have a significant impact on the overall rail system, as they are factored in when calculating the minimum headway between two trains, along with the necessary safety margins. Conversely, if rail vehicles are to brake more reliably under all ambient conditions, this helps to shorten headways and thus utilize the existing rail infrastructure more effectively.
In view of this, Knorr-Bremse is aiming to significantly reduce braking distance variation and spread with its Reproducible Braking Distance (RBD) project. This involves an innovative deceleration control system, a train-wide adhesion management system and adaptive wheel slide protection control with optimized adhesion utilization working together as an integrated whole for the first time.
Conventional wheel slide protection systems are basically optimized for low adhesion conditions with maximum values in the UIC control range above ten percent slippage (macro-slip range), i.e. the range where vehicle speed exceeds wheel speed by ten percent. Targeted adjustment of the wheel slide protection control to adhesion conditions with a distinct peak in the micro-slip range (below five percent) has not been an explicit requirement of the applicable standards (UIC 541-05, EN 15595, NR - GM/GN2695) in the past.
This is where the new WSPA-3 wheel slide protection algorithm comes into play. In situations where the micro-slip range offers significant benefits in terms of braking force, the controller switches from macro-slip mode to the newly implemented Low Adhesion Mode (LADM) for micro-slip (Figure 1). If the conditions then change back again, the controller will revert to the mode for the standard control range with over ten percent wheel slip. In this way, the WSPA-3 algorithm enables greater braking force to be transmitted on extremely slippery tracks with adhesion values peaking in the micro-slip range, thereby exceeding the requirements stipulated in the standards. In order to determine the level of adhesion at the wheel/rail contact and thereby change between the operating modes at the right point, WSPA-3 comes complete with an adhesion monitor that analyzes the total braking force at each wheelset. (Implementation of the WSPA-3 algorithm requires it to govern all of the braking systems acting at a given wheelset.)