Theoretical background: Slip and slide protection
In order to utilize tractive effort to full effect, it is necessary to know all the relevant physical values and understand how they interact. This is the only way to achieve improvements in performance. For continuous improvements in wheel-slip prevention, Siemens employs systems and methods that provide insight into these interacting factors as well as the means to control them. This theoretical background offers a view behind the scenes.
Utilizing maximum physical values
Dynamic continuous traction control and a control system for limiting wheel acceleration are utilized in order to extract the physical maximum from the prevailing traction conditions. A wide range of different systems are available for utilizing the tractive effort to maximum effect:
Tractive resistance monitor
The tractive resistance monitor is an independent system for determining the present-time operating conditions of a vehicle from available parameters like the traction motor speed. It delivers a map of the train weight and line gradient, and its calculated values form the basis for further processing.
Determining the actual speed
The ground speed of the vehicle is an important parameter, and is calculated with the aid of the traction resistance monitor. This efficient system is independent of the radar systems that usually react with high sensitivity to different line conditions (for example, bridges) and weather conditions (like snow).
Wheel slip controller
The wheel slip controller controls the slip speed of the axles with respect to the actual ground speed according to the prevailing adhesion conditions. Dynamic controlling of the wheel peripheral speed maximizes the tractive effort and ensures that it develops uniformly depending on the quality of the wheel/rail interface.
Wheelset conditioning is a term used to signify the increase in the dissipated power input into the wheel-rail contact pair for influencing the boundary layer. This takes the form of a temporary increase in the wheel slip target values at low adhesion coefficients. This results in a roughening of the running surfaces, which promotes higher utilizable tractive effort and an accompanying improvement in the tractive effort/braking force level. The system is designed so that there is no appreciable effect on the life of the wheels or the quality of the running surfaces.
Tractive effort assistant
The tractive effort assistant transfers the tractive effort reference value of individual drives or drive groups that cannot be transferred to the rail to other drives or drive groups with comparatively low dynamic performance that are being less heavily utilized at a given moment. It thus serves to compensate for transfer of axle loads due to coupling moments in bogies and locomotive bodies, and also for the coefficients of friction of the drives or drive groups that on average can be utilized differently.
Protection of mechanical components against overstressing
Protecting mechanical components against overstressing means avoiding wheel flats and preventing wheel spin on stationary vehicles, as well as counteracting vibration in the drive train.
Wheel flat protection
Protection against wheel flats effectively prevents flats on electrically braked axles by reducing the maximum torques to zero at very low wheel speed. This rules out the possibility of the axles coming to a standstill as a result of the electric brake being applied while the locomotive is still in motion.
Chatter protection is a system for detecting and counteracting vibrations in the drive system. The level of vibration is heavily dependent on the rail conditions prevailing at that moment. If vibrations occur, they are detected immediately and effectively counteracted by the control system without losing the benefits of high tractive effort utilization.
Maximum operating reliability
Maximum operating reliability includes avoiding operating restrictions resulting from malfunction from the failure of sensors. A particularly important requirement is that the train protection systems must not be affected by these malfunctions.
Synchronous slip prevention
Synchronous slip prevention is a process for detecting and suppressing the gradual drifting of all axle speeds away from the locomotive’s ground speed, and is based on actual speed measurement and the expected speed change according to the tractive resistance monitor.
The function monitoring system monitors reductions in tractive effort and activates the fall-back levels to protect against malfunction in the event of implausible behavior or a sensor failure.
In the event of the pneumatic and electrodynamic brakes acting simultaneously on the same axle, the blending module controls the required reduction and the building up of the electric braking torques again, if possible. Otherwise, overbraking of the axle can occur or, in the least favorable case, both brakes can be reduced due to the action of two controllers on the same actual speed value. A standardized interface to the commonly used pneumatic slide protection devices (Knorr, Faiveley) allows different vehicle equipment to be used.