RTR 4/2010 31 friction, which it is very difficult to predict. With the self-energising electro-hydraulic brake, on the other hand, it is possible to adjust the actual deceleration torque without needing any additional components. The advantages are obvious. Firstly, the braking distance can be shorted by reacting sensitively to the adhesion actually available at the wheel/rail contact. Secondly, the feedback of the deceleration force permits the use of a model-based estimation of velocity. The principle of the self-energising electrohydraulic brake is outlined in Fig. 1. One immediately striking feature is that the braking torque is taken up by the bogie through a support cylinder. The normal force generated on the brake actuator is converted into a tangential frictional force through friction contact. The brake calliper’s moveable suspension directs the friction force onto the support cylinder. The oil inside this is compressed, and a pressure is built up. The support cylinder is connected to the brake actuator by means of a valve control, making it possible for the self-energising loop to be closed. Closing this valve has the effect of interrupting self-energisation. 2 Review of the brake concept One consequence of self-energisation is that the build-up of the braking force is unstable. The pressure differential across the open valve increases with increasing braking force. If a hydraulic resistance is encountered, such as a regulating valve, the increasing pressure differential will result in an increased volume flow. This accelerates the build-up of the braking force, which is why it is essential to develop new types of valves for the self-energising electrohydraulic brake in order to compensate for this effect. A test rig was set up at the IFAS for the purpose of validating prototypes of regulating valves. Its concept includes inputs resulting from the experience with preceding projects. Attention is drawn to the following findings and aims: Self-energising electro-hydraulic brake (SEHB) The Institute for Fluid Power Drives and Controls (IFAS) forms par t of Aachen University of Technology (RWTH Aachen). This repor t tells about the development work it has been doing on a self-energising electro-hydraulic brake (SEHB) in the context of a project sponsored by the German National Science Foundation (DFG, Deutsche Forschungsgemeinschaft). The literature contains numerous reports on attempts to implement concepts for electro-mechanical or electro-hydraulic addons to conventional air brakes. A brake whose only mode of operation is electrohydraulic can do entirely without the production of compressed air and can thereby save on the energy needed for it. Most of the railway brakes in use today do not incorporate self-energisation. Their behaviour is stable and predictable. The use of the self-energising effect very considerably reduces the power needed by friction brakes. If a self-energising effect is applied to unadjusted brakes, however, fluctuations in friction may have drastic effects on the deceleration torque, which, in extreme cases, may even be the cause of instability. The self-energising electro-hydraulic brake (SEHB) developed at the IFAS minimises the risk of instability by making a continuous fluid-mechatronic adjustment to the deceleration torque in combination with simple hydraulic safety mechanisms. 1 How the self-energising electro-hydraulic brake functions The SEHB can be implemented in the form of a disc brake. It is fastened to the running gear in the same way as conventional brakes. Since it draws its energy from the braking process, it is possible to do without power connections along the length of the train. Conventional friction brakes have the disadvantage that the contact force of the brake pads pressing against the brake disc is predetermined. There can thus be very strong variations in the braking torque as a consequence of fluctuations in the coefficient of Member of the scientific staff Institute for Fluid Power Drives and Controls (IFAS), Aachen University (RWTH Aachen) michael.kuehnlein@ifas.rwth-aachen.de Dipl.-Ing. Michael Kühnlein Professor American University of Beirut, Hamra, Beirut matthias.liermann@aub.edu.lb Dr.-Ing. Matthias Liermann Head of the Institute for Fluid Power Drives and Controls (IFAS), Aachen University (RWTH Aachen) post@ifas.rwth-aachen.de Univ.-Prof. Dr.-Ing. Hubertus Murrenhoff Member of the scientific staff Institute for Fluid Power Drives and Controls (IFAS), Aachen University (RWTH Aachen) julian.ewald@ifas.rwth-aachen.de Dipl.-Ing. Julian Ewald
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