Handbook of Railway Vehicle Dynamics

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442 Therailway represents an aggressive environment in which to perform test measurements,and careful attention to detail is required if reliable test set-ups on vehicles or track are to be implemented. Transducers and cabling may be subject to high levels of transient vibrations,particularly if mounted on unsprung components(e.g., axle boxes or rails). Acceleration levels of the order of 30–50gare not uncommon on axle boxes, and peaks of up to 100 ghave been recorded. Extremes of temperature and weathermust be cateredfor and all externally mounted componentsshould resist the ingress of moisture and dirt. Thetest data must not be affected by electrical noise from awide variety of source including high voltage AC or DC traction systems. Conversely, the test set-up must preclude the possibility of generating electro-magnetic interference that could adversely affect train control or signalling systems. Fortunately, most commonly used transducers are low voltage, low power devices and hence the problem does not arise. The following list highlights typical requirements for areliable installation: † All brackets should be rigid and robust and should prevent unwanted backlash between the bracket and transducer. † Cabling shouldbesecured to prevent (insofaraspossible) movement. This is particularly important in the case of connections to transducers or joints between cables. † Cables should be routed to avoid sources of electrical noise (traction motors, pantographs/collector shoes, wiring looms, generators etc.). † Where possible, signal conditioning should be carried out within the transducers. Where this is not possible (for example, with some types of accelerometer requiring aseparate charge amplifier), the distancebetweenthe device and the conditioning unit should be as short as possible as very low power signals may be rendered useless by even modest amounts of electrical interference. † Cables and connectors should be shielded tothe highest available standards. † All transducers, cables, and enclosures shouldbesealed to arecognised standard such as IP66/IP67. † Transducers should be selected to prevent the likely peak vibration transients exceeding the “shock loading” specification for the device. † Cabling lengths should be minimised and be routed inside vehicles at the earliest opportunity. Cables should be arranged tidily and long coils of spare cable should be avoided at the end of cable runs. † Care should be taken toensure that expensive transducers and data loggers cannot be exposed to damaging voltage “spikes.” Sources of power are an important consideration when conducting field testing. It is generally recommended that when using power other than from the mains (for example, from generators or vehicle sources),suitable voltage stabilisation and surge protection devices are used. In manycases, field power supplies cannot be guaranteed and an alternative battery backup should be arranged to guard against the loss of important test data. A. T RANSDUCER P OSITIONS ON V EHICLES Handbook of Railway Vehicle Dynamics One of the most common instrumentation applications that the vehicle dynamics engineer will encounter is the fitting of accelerometers and displacement transducers to the body, bogies and suspension, for ride test, passenger comfort, or dynamic response track tests. When choosing the locations for instrumentation, it is important to clearly understand the vibration modes that each transducer will “see”toensure that the correct data is gathered and that the desired information can be derived from it. It is evident from Figure 13.20 that avertical accelerometermounted on the vehicle floor in line with the vehicle centre of gravity(A 1 v )will largely sense to the bounce mode responses to the track © 2006 by Taylor & Francis Group, LLC
Field Testing and Instrumentation of Railway Vehicles 443 A 4 l input. As it is not practical to actually mount the accelerometer at the centre of gravity, some accelerations due to the change in floor height as the vehicle rolls will also be detected. Vertical accelerometers mounted on the vehicle floor above the bogie centre pivots ( A 2 v and A 3 v )will sense both the bounce and pitch mode responses. However, whilstaccelerations due to body bounce will be in phase at both transducers, the accelerations due to pitching will be 1808 out of phase. Assuming that the carbody is perfectly stiff, i.e., that no flexible modesoccur,the pitch and bounce components of the signal at accelerometer A 2 v can therefore be separated thus: A 2 bounce ¼ðA 2 v þ A 3 v Þ = 2 A 2 pitch ¼ðA 2 v 2 A 3 v Þ = 2 Similarly, accelerometers A 5 l and A 6 l will sense body lateral and yaw responses in and out of phase, respectively. A 4 l will likewise sense acombination of body yaw, lateral, and roll modes. Providing that sufficient transducers have been provided, and their locations chosen carefully, it should be possible to reliably establish the natural frequencies of the various modes of vibration of the vehicle body.The sensed accelerations on the body will alsoinclude the effects of flexible body modes. These may also be of interest to the dynamicsengineer (for example, whenconsidering the effect of the first body bending mode on passenger comfort). Accelerometers will also detect inputs from body mounted mechanical equipment such as internal combustion engines and compressors. However, these will often occur at constant frequencies, somewhat higher than the frequencies of interest to the dynamics engineer and may, if desired, beeasily removed by filtering. Similar considerations to those described above will also apply toaccelerometers mounted on the bogie frame or displacement transducers fitted across the primary or secondary suspension. Accurate records should be made ofthe mounting positions ofall transducers on the vehicle to allow for later correction of geometric effects. IV. DATA ACQUISITION A 2 v A 5 l A 1 v A 3 v A 6 l S1 P1 P2 P3 P4 FIGURE 13.20 Simple transducer layout. Key: A x v –vertical accelerometers, A x l –lateral accelerometers, Px–primary suspension LVDTs, Sx–secondary suspension LVDTs. Test data must be collected and store in asuitable form for later analysis. In the past this was often carried out using magnetic media such as tape recorderstoallow large volumes of data to be stored. However, modern computer systems are able to directly store such large volumes of data, making tape storage largely redundant. Modern data loggers are usually either in the form of aPCwith suitable additional hardware cards and software, or astandalone device with aPCcompatible up-link. Ineither case the analogue signals from the test devices must beconverted to digital form to allow the data logger tostore them. In addition to logging varying voltage or current signals from transducers, loggers may also have additional hardware inputs for digital signals and serial data (for example anRS232 connection to aGPS). As computers/data loggers store information in digital format, and most transducers provide an analogue signal, an analogue-to-digital converter (ADC) is required toconvert the signal. © 2006 by Taylor & Francis Group, LLC S2
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Handbook of Railway Vehicle Dynamics

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