Railway Track & Structures - November 2008 - (Page 22)

TTCI R&D collisions of the arms • Seal the gap between knuckles and protect the internal mirrors from dust and water, for example • Easily clean or replace the mirrors without affecting laser beam alignment. Each laser head is installed horizontally on an aluminum mount that is rigidly connected to the optical arm. The laser heads and stand are mechanically isolated through a double set of shock absorbers in order to prevent possible misalignments of the optical components (laser and optical arms). The entire laser optical path, from the laser head to the measurement unit, is totally shielded in order to guarantee full protection from direct and diffused laser exposure. The position of the ultrasonic laser sources and sensors is adjusted in realtime, in order to compensate geometrical changes in the rail section, by using the rail profile information provided by the profile measuring system. This last consists of two sub-units and allows evaluation of the entire profile of the rail, plus the relative position of the inspection unit in respect to the rail. The profile system is based on the triangulation between a laser beam and a digital camera. When there is a rail profile change, the system calculates the new angular positions of the collimators and the transversal displacements of the sensors and then drives the motors to the new positions to keep laser sources and sensors at the right locations on the rail surface. Figure 4 shows a close up photo of the bottom view of the inspection box, showing the profile system, sensor assemblies and laser collimators for railhead, web and base. Figure 5, top, shows waveforms acquired in the head without (left) and with a defect (right). Figure 6, middle, shows waveforms acquired in the web without (left) and with a defect (right). Figure 7 shows waveforms acquired in the base without (left) and with a defect (right). trolling the lasers. Position of the hi-rail vehicle is determined by encoder and GPS monitoring to provide a correlation between signal acquisition and progressive inspection distance for accuracy in identifying the location of defects. Architecture and interface PCs are used to accomplish all the required processing functions. There is a PC dedicated to the acquisition and storage of data, as well as the pre-processing of ultrasonic waveforms. Data are then transferred to the supervisor PC, where the user interface allows the operator to control all the functions and to visualize results of the real-time processing. Another PC acquires and processes the rail profile data, manages the movements of the laser collimators and sensor assemblies as well as con22 Railway Track & Structures Preliminary test results The U-Rail system was tested at the Rail Defect Test Facility at TTCI for a complete characterization of the entire cross section of the rail. The evaluation tests demonstrated that the laser-based technology is able to detect defects in the head, web and base at speeds up to 20 mph. In the current configuration, rail defects are expected to have the following effects on the ultrasonic waveform, as Figures 5 through 7 show: • Head defects are signified by attenuation of the longitudinal wave. • Web defects are signified by attenuation of the longitudinal wave. • Defects in the base are signified by a reflected wave received by the transducer when laser generation is between the flaw and the transducer. The system was able to detect TDs hidden by shells in the railhead, vertical split heads and base defects. The noncontact approach provides improved inspection efficiency by enabling inspection of rail locations in addition to the railhead. Expected signal response at defect and nondefect locations has been verified. There are some limitations with the current design, one of which is the effects of rail wear on the stability of the receiving transducers. These limitations have been identified and are being addressed to improve the system’s capability as the laser-based technology reaches maturity for use in revenue service. www.rtands.com November 2008 http://www.aar.com http://www.rtands.com

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Railway Track & Structures - November 2008

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