Acceleration Measurements Inside Vehicles: Passengers’ Comfort Mapping on Railways. Pablo Zoccali1, Giuseppe Loprencipe 1* and Robert Cristian Lupascu1

Figure 12. a_wx values on 10 m length segments along section 04 direction EW calculated for 4 different runs.
Analyzing then the vibration total value on the same section (Figure 13), a_v values greater than 0.8 m/² (uncomfortable level according to ISO 2631) were found just in correspondence of two zones located close to the end of the section.

In the same figure, it is also possible to note that the longitudinal RMS acceleration (a_wx) presents highest magnitude values at the beginning and at the end of the section due to, respectively, acceleration and braking actions in proximity of the stations, while lateral RMS acceleration (a_wy) was particularly affected by transiting on curve arcs. Looking at the vertical RMS accelerations (a_wz), it is possible to see that for most of the section length, about 92% (Table 11), they were lower than 0.315 m/s² corresponding to Not uncomfortable ISO 2631 level. The only exception was the area affected by the presence of the switch, where the peak values induced on passengers a fairly uncomfortable perception when passing on it.

To determine the percentage of the section length characterized by the ISO 2631 comfort levels, it was needed to define limit threshold values replacing the overlapping zones between some adjacent levels. For this purpose, the same limits used in [20] were adopted and reported in Table 11.


Figure 13. RMS acceleration values calculated on 10 m length segments along section 04 direction EW.

Table 11. Percentage of section 04 direction EW length belonging to the different ISO 2631 comfort levels.

	ISO 2631 comfort level
	Not uncomfortable (<0.315 m/s^2)	A little uncomfortable (0.315-0.565 m/s^2)	Fairly uncomfortable (0.565-0.9 m/s^2)	Uncomfortable (>0.9 m/s^2)
awx	72.92%	25.19%	1.89%	-
awy	70.56%	26.13%	3.31%	-
awz	91.98%	5.19%	2.83%	-
av	44.15%	38.11%	15.85%	1.89%

5. Conclusions

In the present work, the ISO 2631 approach to evaluate ride comfort on road and railway infrastructures was tested by means of experimental measurements inside trains traveling on a railway line characterized by Automatic Train Operation. In particular, repeatability analyses of the measured accelerations were performed.

In order to exclude an excessive variability of train acceleration and braking actions during the different measurements that may affect the results, correlation coefficients were calculated between the speed rates recorded during the different runs. The results showed that to obtained values greater than 0.82 both HDOP and VDOP have to be lower than 3 during all measurements time, ensuring in this way a high quality of the GPS signal.

The ride quality assessment was meant to provide a global characterization of the infrastructure in the real condition of vehicle ride. In this sense, two different analyses were carried out. The first one, aimed to assess long term ride quality, concerns the calculation of the RMS accelerations inside the vehicle along the three orthogonal directions (longitudinal, lateral and vertical) on the whole section between two consecutive stops.

The obtained values, also in terms of vibration total value, fell all within the Not uncomfortable level, with the highest value of 0.274 m/s² for a_v calculated along section 6 direction EW. Such results highlight that, in terms of global exposure of passengers to railway vibrations, the examined infrastructure together with train performances and traveling speed does not show any particular problem. The standard deviation calculated for all sections also highlighted the good repeatability of the performed acceleration measurements.

To properly assess the condition of an infrastructure, it is fundamental to locate the critical areas affected by excessive wear, irregularities or any other kind of distresses and problems. For this reason, the chance of using the approach based on the ride quality estimation according ISO 2631 approach on sub-sections of limited extension was tested.

Although the sections examined in this paper presented very low RMS acceleration values along the entire path, it was found that considering shorter segments of 10 m specific areas characterized by more uncomfortable level (up until Uncomfortable level in very rare cases) were localized. These areas, in particular, may be affected by significant acceleration or braking actions (high longitudinal RMS accelerations), particularly critical curve trajectories (high lateral RMS accelerations) or irregularities on the track (high vertical RMS accelerations).

Therefore, the chance of using such lengths for the boundary condition encounter on the examined railway (e.g. track geometry and type, trains and traveling speed) was verified and confirmed by the obtained results. The use of segments of 10 m was chosen starting from road recommendation but, in the future, it would be desirable to investigate the chance of dividing railway lines based on homogeneous sections with reference on specific parameters (e.g. speed rate, track geometry, track type). In fact, it was found that for greater speed the aforementioned length does not allow to capture and analyze the most critical frequency for the vertical accelerations.

In conclusion, the good results obtained in the present work confirmed the chance of using the ISO 2631 approach to assess long-term ride quality but also shorter segments of the traveled distance; more in general, this approach permits to evaluate the real condition of an infrastructure as a result of the trajectory and of the characteristics of vehicle operation. Such method would allow to continuously monitoring its status, ensuring a preventive maintenance intervention. The chance of analyzing and evaluating short sections, in particular, ensures the localization of the critical areas along the path in terms of vertical, transversal and longitudinal RMS accelerations. However, more investigation on the applicability and efficacy limits of this approach shall be conduct considering different type of infrastructures with different conditions (e.g. presence of irregularities or an increased wear on track and wheels) also to evaluate and determine whether and how they may affect the comfort measures.
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