Handbook of Railway Vehicle Dynamics

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
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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
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