Handbook of Railway Vehicle Dynamics

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428 Relative Sensitivity 30 dB 20 10 0 − 10 − 20 − 30 Useful Frequency Range 10% limit 0.3 f m 3dBlimit 0.5 f m Main Axis Sensitivity Transverse Sensitivity − 40 0.0001 0.001 0.01 Handbook of Railway Vehicle Dynamics Transverse Resonance Frequency Proportion of Mounted Resonance Frequency Mounted Resonance Frequency f m 0.1 1 10 FIGURE 13.5 Frequency response of an accelerometer (courtesy of Bruel and Kjaer). Source: From Serridge, M. and Licht, T. R., Piezoelectric Accelerometers and Vibration Preamplifier Handbook Bruel and Kjaer, Revised November 1987. With permission. will be dictated by the resonant frequencyofthe accelerometer. Acommonly used ruleofthumb is that the upper frequency limit shouldbenomore than one third of the resonant frequency. Forgeneral purposepiezoelectric accelerometers, the resonant frequencymay be of the order of 20 kHz, putting their upper useable limit way above anything likely to be requiredfor avehicle dynamics application. Piezoelectric accelerometers have one very important limitation with regard to vehicle dynamics test applications. Thelower frequency limit is determined not by the accelerometer itself but by the RC time constant of the charge amplifier used to condition the signal from the transducer. Whilst it is possible tosense very low frequencies using preamplifiers with very high impedance, general purpose equipment may limit the lower useable frequency limit to 1–3Hz. Clearly, this has important implications for the vehicle dynamics engineer. A general purpose piezoelectric accelerometer may be quite acceptable for mounting on unsprung masses such as the axle box, but may be operating near its lower limit when mounted on abogie frame. Vehicle body modes may occur at 0.5 Hz or less and therefore would be below the lower limit of ageneral purposepiezoelectric accelerometer. Piezoelectric accelerometers are generally not capable of measuring the quasi-static accelerations due to curving. In the case of bogie and body measurements, the capacitive accelerometer, described in Section II.B.2 below, will provide asolution to this problem. An ideal accelerometer design would only respond to vibrations applied to the main sensing axis. However, in practice, mostaccelerometers will exhibit somesensitivity to excitations at 908 to the main axis, known asthe transverse sensitivity. These are caused bysmall irregularities in the piezoelectric material causing the axis of maximum sensitivity to be slightly misaligned with the operating axis of the accelerometer. It can be imagined, therefore, that the transverse sensitivity will not be constant and subsequently there will be directions of maximum and minimum sensitivity at 908 to each other. Some accelerometer designs will indicate the direction of minimum transverse sensitivity on the accelerometer body to aid correct mounting of the device. It is generally found that the transverse resonant frequencyislower than the main resonant frequencyand therefore falls within the useable bandwidth of the accelerometer. However, at the relatively low frequencies of interest to the vehicle dynamics engineer, the maximum transverse sensitivity is usually less than 4% of the main axis sensitivity. 4 © 2006 by Taylor & Francis Group, LLC
Field Testing and Instrumentation of Railway Vehicles 429 FIGURE 13.6 Accelerometer mountings and cabling (courtesy of Bruel and Kjaer). Source: From Serridge, M. and Licht, T. R., Piezoelectric Accelerometers and Vibration Preamplifier Handbook Bruel and Kjaer, Revised November 1987. With permission. To achieve reliable results, care should be taken when mounting and cabling piezoelectric accelerometers.The device may be mounted usingeither astud screwing into ahole tapped directly into the testcomponent or by gluing it in place onto aclean flat surface. However, in manyrailway vehicle applications the accelerometer will be mounted on its stud to abracket which will in turn be bolted or clamped tothe vehicle. In this case, the mounting bracket and clamping arrangement should be as rigid as possible to ensure that the measured data is not degraded by vibrations or deflection of the mounting itself. Athin mica washer is often placed between the base of the accelerometer and the mounting which, whenused in conjunction with an insulated stud, increases the electrical isolation of the accelerometer from the test piece. It shouldbenoted that dropping an accelerometer onto ahard surface, such as aworkshop floor, can cause ashock load that exceeds the maximum design limit and damages the device permanently. Adescribed above, the piezoelectric sensor generates asmall charge when subjected to mechanical stress. Any noise generated between the accelerometer and the signal conditioning/ charge amplifier module can therefore adversely affect the accuracy of the results obtained. Flexing of the accelerometer cables can induceacharge as aresult of the separation of the layers within the co-axial cable, known asthe triboelectric effect. These charges can be sufficiently large to induce significant “noise” when measuring low levels of vibration. It therefore follows that accelerometer cables should besecurely clamped ortaped in position to prevent flexing of the cables that induce such charges (see Figure 13.6). In addition, cable runs between accelerometers and charge amplifiers should be as short as possible as the signal conditioning unit will generally outputastrong DC voltage which will be less sensitive to noise than the incoming signal from the transducer. An alternative (which may be preferable in many vehicle test applications) is to use an accelerometer with an in-built preamplifier that performs some orall of the required signal conditioning. For piezo-electric accelerometers, this is usually in the form of aminiature “charge amplifier” which produces an output voltage proportional to the charge generated by the accelerometer. As with all test equipment, cabling runs should avoid sharp bends and be routed away from sources of electrical and magnetic interference, such as traction equipment and current collectors (third rail shoes/pantographs). 2. Capacitive Accelerometers Although piezoelectric accelerometers have very high frequency upper useable limits, they can be limited to around1Hz at the lower end of their frequencyrange (dependent on the charge amplifier employed), as described in Section II.B.1. In contrast, capacitive accelerometers have no lower limit on their useable frequency range as they are capable of giving aDCorstatic response. They also have a number of other attributes that make them attractive for railway vehicle test © 2006 by Taylor & Francis Group, LLC
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Handbook of Railway Vehicle Dynamics

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