Slip-slide control system for railway vehicles

In railway transportation, braking and traction forces mainly depend on normal force and adhesion coefficient between wheel and rail. Regarding the restrictions on controlling normal force, maximization of adhesion coefficient seems to be the only way of increasing braking and tractive efforts. Moreover, efficient utilization of adhesion can also reduce operating costs with avoiding early wheel and rail damages and minimizing trip time. On the other hand, adhesion between the rail and the wheel is a highly dynamic function of many parameters such as environmental conditions, speed and slip ratio. The aim of this thesis is construction of a slip - slide control scheme which maximizes adhesion utilization in addition to avoid excessive situations resulting in component failures. Two different control approaches are proposed in the scope of this thesis. The first method relies on an Event Based Slip - Slide Control Scheme. In this approach, the excessive slip - slide situations are detected with wheel acceleration information which is derived from the measured wheel speed. If the wheel acceleration is higher than the predefined threshold that is chosen considering the maximum possible/permissible acceleration/deceleration of the vehicle, the controller takes action and decreases the magnitude of the reference motor torque in order to return the adhesion status back to the micro - slip area. During such a recovery mode, the phase shift between the input and the output of the traction system is observed using the orthogonal correlation method to accelerate the proposed control strategy by determining the adhesion status in advance. Then, the controller tries to maximize adhesion utilization by holding the phase shift value between the predefined upper and lower limits. Finally, the control action is terminated as soon as the braking/tractive effort fulfills the driver request due to recuperation on road conditions. The developed Event Based Slip - Slide Control Method forms the desired phase shift area with constant borders (upper and lower limits) for all possible vehicle velocities. Although the optimal lower limit of the desired phase shift area does not show any noticeable change with the varying vehicle velocity, using a constant value for the upper limit restricts the performance of the slip - slide control system since the adhesion formation process is largely affected by the vehicle velocity. The second slip - slide control system proposed in the scope of this thesis is an alternative form of the Event Based Slip - Slide Control System which uses adaptive upper limit for the desired phase shift area with respect to the vehicle velocity in order to improve adhesion utilization. Two different benchmark approaches which are Disturbance Observer Based Control and Direct PD Control of Phase Shift are tested with the proposed methods under different driving scenarios and adhesion conditions. The adhesion utilization of an Optimal Controller which has an access to both accurate vehicle velocity and friction coefficient is used as a reference performance criterion. The test results show that both the proposed Adaptive and Event Based Slip - Slide Control Systems provide rapid response to quickly changing adhesion conditions and driver requests while maximizing the braking/tractive effort even in poor adhesion conditions. In addition to their better performances in adhesion utilization, the proposed control schemes do not need the calculated wheel acceleration during the control phase and the possible stability problem of the Disturbance Observer Based Controller is prevented. What is more, the proposed controllers react extremely fast to the recuperated road conditions unlike the Disturbance Observer Based Controller, since the adhesion status is continuously estimated during the control phase using the observed phase shift information. Last but not least, the proposed approaches do not necessitate the persistent stimulation of the traction system as it is needed in the Direct Phase Shift Control method which might cause early damages in the traction system components. However, it is also shown that the adaptive form of the proposed method increases the adhesion utilization compared to the conventional one by using the phase shift information more efficiently.