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

Once the speed rate was verified, the repeatability of the acceleration measurements was investigated, in terms of frequency weighted RMS accelerations along the three directions (a_wx, a_wy and a_wz) and vibration total value (a_v). The values obtained for all passages performed along section 3 direction WE are shown in Table 8 and also depicted in Figure 6. As can be seen, acceleration values are pretty low and meaningfully lower than the maximum limit of the Not Uncomfortable level provided by ISO 2631 (i.e. 0.315 m/s²). In any case, it can be say that a good repeatability was found for this specific section and, more in general, for all examined sections as confirmed by the values reported in Table 9 and Table 10. The pretty low values found depends on the high maintenance requirements adopted for the line. In fact, vibration total values greater than 0.2 m/s² were found just for two section 04 and section 06 both direction EW (Table 10). In particular, section 06 direction EW was characterized by the highest vertical RMS acceleration value (i.e. 0.250 m/s²).

Performing the repeatability analysis on such low acceleration values does not allow to evaluate how irregularities and, more in general, worst ride comfort levels may affect measurements repeatability. In fact, in these cases, the final acceleration values are expected to be more sensitive to speed variability. However, although the examined sections do not present very high acceleration values, some acceleration peaks were located along each path.

For this reason, as already stated in the previous sections, in addition to the RMS acceleration values calculated on whole sections, they were also determined for segments of 10 m. In this way, the capability of the proposed approach to assess localized irregularities and discomfort causes along the path was also investigated.

Table 8. RMS accelerations for section 03 direction WE.

Measure N.	awx (m/s^2)	awy (m/s^2)	awz (m/s^2)	av (m/s^2)
1	0.07	0.1	0.11	0.16
2	0.06	0.1	0.11	0.16
3	0.06	0.09	0.1	0.15
4	0.09	0.08	0.1	0.16
5	0.1	0.08	0.11	0.17
6	0.08	0.09	0.11	0.16
7	0.07	0.09	0.11	0.16
8	0.1	0.09	0.11	0.17
9	0.07	0.09	0.11	0.16
10	0.08	0.07	0.12	0.16
11	0.07	0.08	0.12	0.16
12	0.09	0.09	0.12	0.17
13	0.09	0.09	0.11	0.17
mean	0.079	0.088	0.111	0.163
st. dev.	0.013	0.008	0.006	0.007

Figure 6. Frequency weighted accelerations for all measurements along section 3 direction WE.
Table 9. Statistical parameters of RMS acceleration values obtained for all sections direction WE.

Section ID.	Direction WE
	awx (m/s^2)		awy (m/s^2)		awz (m/s^2)		av (m/s^2)
	Mean	St. Dev.	Mean	St. Dev.	Mean	St. Dev.	Mean	St. Dev.
1	0.036	0.004	0.055	0.005	0.061	0.004	0.090	0.006
2	0.049	0.006	0.072	0.005	0.095	0.010	0.129	0.009
3	0.079	0.013	0.088	0.008	0.111	0.006	0.163	0.007
4	0.048	0.005	0.096	0.014	0.142	0.014	0.178	0.016
5	0.037	0.006	0.065	0.011	0.098	0.016	0.124	0.019
6	0.039	0.003	0.103	0.012	0.149	0.019	0.186	0.022
7	0.045	0.005	0.062	0.008	0.077	0.006	0.109	0.008

Table 10. Statistical parameters of RMS acceleration values obtained for all sections direction EW.

Section ID.	Direction EW
	awx (m/s^2)		awy (m/s^2)		awz (m/s^2)		av (m/s^2)
	Mean	St. Dev.	Mean	St. Dev.	Mean	St. Dev.	Mean	St. Dev.
1	0.039	0.005	0.043	0.007	0.058	0.007	0.082	0.009
2	0.044	0.013	0.064	0.011	0.081	0.009	0.112	0.016
3	0.039	0.004	0.050	0.006	0.076	0.009	0.099	0.009
4	0.039	0.003	0.132	0.013	0.167	0.006	0.217	0.010
5	0.055	0.012	0.064	0.007	0.098	0.008	0.129	0.013
6	0.042	0.004	0.102	0.012	0.250	0.007	0.274	0.010
7	0.052	0.010	0.077	0.005	0.101	0.003	0.137	0.006

Since the instrumentation used for the in-situ measurements does not directly provide distances, it is important to verify the accuracy and repeatability at the various passages of the distances calculated starting from speed and time measures.

For this reason, the whole lengths of each examined section measured during the various passages were compared. The obtained results are depicted in Figure 7 for direction WE and Figure 8 for direction EW where the error bars represent the standard deviation calculated for each section. In particular, in these figures, the variability of the calculated length compared to the effective ones is reported, together with the standard deviation expressed in percentage unit and referred to the effective length of each section. With regard to the latter aspect, it can be noted that, except for sections 2 and 7 in both directions characterized by the reduction of GPS signal quality along the paths, the percentage values found were always lower than 10%.

The accuracy found was evaluated to be acceptable for driving the acceleration signals measured at the various passages into 10 m length segments.

Figure 7. Calculated length analysis for all sections of direction WE.


Figure 8. Calculated length analysis for all sections of direction EW.
In order to better visualize the RMS acceleration values obtained for each segment, all results were inserted into a geographic information system (GIS), the Q-GIS platform, assigning each value to the middle point of the corresponding segment.

For brevity, the following figures are referred just to section 04 direction EW, but same results and considerations can be made for the other ones. The choice of this section depends on the fact that analyzing the variability of the a_wz along it, it was found a localized area characterized by values significantly greater than the ones on the remaining part of the section. What just said can be clearly seen in Figure 9, where the results obtained for 4 different measurements are depicted.

Figure 9. a_wz values on 10 m length segments along section 04 direction EW calculated for 4 different runs.
Looking at this area more in detail, the probable reason for these high a_wz values was found. As can be seen in Figure 10, in fact, in correspondence of the peak value there is a railway switch, which represents a localized macro-irregularity along the track. The proposed approach was capable of locating it at the various performed runs, although with an accuracy of about 10 m.

Similar results in terms of repeatability were also found for transversal accelerations (Figure 11), whose maximum values were found on the curve arc as expected, and for longitudinal accelerations in correspondence of acceleration and braking actions (Figure 12).

Figure 10. Magnification of the uncomfortable area.


Figure 11. a_wy values on 10 m length segments along section 04 direction EW calculated for 4 different runs.