Analysis of Electric Shoegear Dynamics 1P. F. Weston, E. Stewart
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Analysis of Electric Shoegear Dynamics 1P. F. Weston, E. Stewart, C. Roberts, and S. Hillmansen The Birmingham Centre for Rail Research and Education, Department of Electronic, Electrical and Computer Engineering, The University of Birmingham, Birmingham, UK1 Abstract The mechanical interface between vehicle-mounted shoegear and the third rail is critically important to the smooth running of electrically powered railway vehicles. Research is being carried out into the static and dynamic interactions between shoegear and the third rail. This paper describes the instrumentation being developed for the shoegear of a class 375 railway vehicle in the UK. Introduction Many electrically powered railway vehicles operating in the UK pick up traction current from a third rail in parallel to the running rails. The interface between the vehicle-mounted shoegear and this third rail is critically important. Network Rail provides and maintains the conductor rail and associated lineside equipment, while train operating companies are responsible for the shoegear. The geometry and range of contact forces allowed between the shoegear and third rail are defined by standards that have evolved over many years of experience. In recent years however, a review of the shoegear and third rail standards in the UK has been initiated. This has been prompted by a) reported increase in the incidents of lost or damaged shoes, and b) anecdotal evidence that different shoegear designs perform differently in the presence of ice on the third rail. The contribution of the current research to this review is to design instrumentation to measure geometry, forces, and the dynamic response of the shoegear on the third rail. These measurements are being used to inform and validate a dynamic model of the system (being developed by Manchester Metropolitan University) intended to be used to test potential modifications to the system components, and also to inform the revised standards. The increase in the incidence of lost or damaged shoes might be attributable to degradation in the static lateral positioning of the third rail, or because of dynamic lateral movement of the bogie relative to the third rail. Some vehicles are reported to be more susceptible to a loss of traction than others when ice forms on the conductor rail (for example, in the early morning before the first train of the day). The detailed reasons for this reported difference are not known. Research has been carried out into various methods of clearing ice such as radio frequency induction and heating of the third rail [1, 4]. Some vehicle operators in the UK have proposed increasing the down force from the shoe during winter running in an effort to clear ice more effectively. The shoe manufacturers, meanwhile, have been experimenting with various profiles on the underside of shoes, essentially increasing the contact pressure by decreasing the contact area. Laboratory tests have, however, so far been unable to demonstrate significant ice clearing by either increasing the down force or by changing the pattern on the base of the shoe. In addition, the re-profiled shoes have a dramatically increased wear rate. In an attempt to better understand the interaction between the shoegear and the third rail, and what needs to be changed to decrease the incidence of shoe loss and improve winter running reliability, a number of related research projects are currently being conducted in the UK. The initial methodology is to develop a dynamic model of the shoegear and to instrument shoegear on various railway vehicles. The field measurements will be used to inform a model of the combined shoegear and third rail system. There is also a laboratory test rig in which a 3.9 m diameter third rail can be rotated while a stationary shoegear rides on top. Ice can be made to form on this rail by enclosing the whole rig in a cold chamber. This allows ice removal experiments to be carried out in a controlled environment. The various aspects of the work are being undertaken in collaboration with train operating companies and also with Network Rail, the infrastructure owner. All industrial partners have a joint interest in improving the system performance. This paper describes the instrumentation designed and built to obtain data from a class 375 railway vehicle, including strain gauges mounted on the arm of the shoegear. The instrumentation has been used in initial tests conducted on a spinning rail apparatus to establish forces present within the arm section. Shoegear and Conductor Rail Shoegear is the term used to describe the piece of apparatus that transfers electric current from a static lineside conductor rail to a moving railway vehicle. It positions the conductor shoe relative to the conductor rail; it provides electrical insulation between the vehicle and the high voltage supply; and it provides a nominal contact force of 250 N. Despite the relatively low vertical forces, the shoegear must be able to withstand far higher impact forces when hitting ramps or discontinuities in the vertical geometry of the third rail. In the event of impact forces in excess of 20 kN [2], the shoe is designed to detach. Further information on shoegear systems can be found in [2, 3]. The conductor rail runs parallel to the running rails and can be on either side. Vehicles therefore have shoegear on both sides. In certain regions of track, particularly around switches and crossings there are gaps in the conductor rail. Vehicles have multiple shoegear to span these gaps and also for redundancy. As a shoe loses contact with the conductor rail an arc may form between the shoe and the trailing ramp. Arc shielding on the vehicle prevents an arc from forming between the shoe and exposed metalwork. An example of the shoegear of a railway vehicle powered via a third rail conductor is shown in fig. 1. Current from the third rail is collected by a shoe mounted on a bracket attached at the end of an electrically isolating fibreglass arm. Substantial cables potentially carry the thousands of Amperes from the shoes to the vehicle bus bar. The shoe is held in contact with the rail by its own weight and the total down force is increased by means of a torsion spring. A downstop on the shoebeam (a bar mounted between the two axleboxes) stops the shoe from dragging along the ground at gaps in the conductor rail. The legs of the shoe bracket are thinned to form a frangible joint, designed to fail under excessive loading. Figure 1: Shoegear. Instrumentation has been designed to measure the movement of the bogie on which the shoegear is mounted, the distance between the bogie and the shoe bracket, and the bending of the fibreglass arm. Instrumentation This section describes the instrumentation being designed to mount on a class 375 railway vehicle. The instrumentation consists of an inertial measuring unit (3 accelerometers and 3 rate gyros), a non- contact displacement sensor securely attached to the bogie, and strain gauges to measure the bending of the fibreglass arm (from which vertical force on the end of the arm can be derived). Fig. 2 shows the general arrangements of a box attached to the bogie and the strain gauges on the fibreglass arm. Also visible is the displacement sensor that measures the distance from the bogie to the top of the shoe end of the fibreglass arm. The bogie box is mounted in place of the arc shielding. Removal of the arc shielding does not pose a risk because the conducting cables are not present during the trials. The eight strain gauges are arranged in two independent bridges, each measuring the bend of the fibreglass arm. In principle, collecting information from two bridges enables the position of the force on the shoe to be estimated. B ogie beam Displacement sensor Bogie IMU, strain gauge amplifiers, A/D conversion, fibre-optic link driver Strain gauges x 8 Figure 2: Bogie box, displacement sensor and strain gauges. The accelerometers and rate gyros are high quality units that provide excellent positional information with suitable processing [5]. The displacement sensor is also a high accuracy device, designed to withstand the high accelerations experienced on a bogie. In addition, particularly low noise strain gauge amplifiers were chosen. A block diagram of the electronics is shown in fig. 3. The sensor outputs are conditioned and then converted into digital information within the bogie box and relayed to a PC within the vehicle body via a fibre-optic cable. This removes the need for a multi-core signal cable and improves immunity to electrical interference. A microcontroller coordinates A/D conversions (carried out on separate 16 bit A/D devices) and serialisation of the converted data for transmission down a fibre-optic link. &x& y Fibre && Analogue Micro optic to &z& Signal controller link digital θ& conditioning driver φ& ψ& h F1 Strain gauge Power amplifiers supply F2 Figure 3: Electronics block diagram. Within the body of the vehicle is the PC, a custom-built USB to fibre-optic cable interface, and a 24 V power supply. The data stream from the instrumentation is collated and logged to disk for later processing and analysis. Train speed and GPS provided by the on board train monitoring systems will also be recorded. The instrumentation was tested on a spinning rail rig, as described in the next section. Laboratory and Spinning Rail Tests Some tests have been carried out in the laboratory and on a spinning rail test rig. For example, the eight individual strain gauges can be wired as two full bridges in various different ways. In one configuration it is possible to estimate the lateral position of the contact force between the conductor rail and the shoe (across the width of the rail).