SENSING The Colour of Creep


Railway research project makes use of non-contact colour sensors to count wheel pulses to determine creepage between the wheel and the track.


T


he Institute of Railway Research (IRR) at the University of Huddersfield works in partnership with the University of Sheffield and other research organisations with funding from the Rail Safety and Standards Board (RSSB). The IRR specialises


in rolling stock and has significant expertise in the wheel to rail interface. This interface is highly significant in rolling stock


performance in terms of both traction efficiency and braking performance. A small contact area, low friction and high pressure combine to make this interface a primary area of interest for improving performance. A current research project at the IRR involves measuring the creepage or micro-slip. This is the difference between the rail speed and the wheel speed which arises from the transmission of force between wheel and the rail. This is being measured during brake applications at different coefficients of friction and at speeds of 18km/h up to 200km/h. Understanding the relationship between creepage is critical to optimal and reliable braking performance, particularly for low friction coefficients such as those that occur due to leaves on the line. If excessive creepage occurs, traction and braking force reduce and trains can fail to meet timetables. In extreme circumstances, wheel flats can form during brake application leading to the train being taken out of service for the wheelsets to be re-profiled in a maintenance depot, which is disruptive, time consuming and costly. Having a better understanding of creepage and the creep force relationship helps to ensure more reliable network operation and a higher operating life for the bogie wheelsets. This work complements other activities at the IRR, which already has a strong heritage in railway engineering and has helped to improve the knowledge of the way in which railway vehicles interact with the track, including key performance aspects such as suspension performance, wheel-rail contact, traction and braking. So far, this has led to a number of tools and techniques being


developed which are now used to predict deterioration of railway wheels and rails, to optimise the vehicle track interface, to increase safety and reliability levels, reduce cost and improve performance of the railway system.


TESTING FOR CREEPAGE According to Daniel Woodhead of the IRR, researchers have been able to see the effects on creepage and braking force


2 /// Climatic & Vibration Testing Vol 2 No. 2


when applying different coefficients of friction to the wheel-rail interface. “The test rig enables us to test under dry conditions, wet conditions and using leaves to simulate real life conditions on the tracks,” he says. The test rig is set up on the university’s “HAROLD” facility,


the Huddersfield Adhesion and Rolling contact Laboratory Dynamics rig. To measure creep, HAROLD has now been set up with a CFO200 non-contact colour recognition sensor from Micro-Epsilon. In conjunction with an encoder, the sensor is being used to monitor the rotational speed of a bogie wheelset in train wheel simulation tests. On the test rig, creepage is measured using the CFO200 sensor as an optical encoder to help calculate the rotational speed and position of the wheel. The colour sensor is used in conjunction with a Zebra encoder disc to calculate the rotational position of the wheel by counting pulses (using black and white coloured marks on the disc) as the wheel rotates. Due to the speeds that the wheels are run at and the amount of pulses being counted, the colour sensor needs to operate at a very high measurement rate of up to 30kHz. As Daniel Woodhead states: “We purchased the CFO200 colour sensor earlier this year and it has performed very reliably on the HAROLD test rig. Finding an ideal sensor was difficult due to the high vibration environment and the degree of measurement resolution and accuracy required to measure


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