Abstract
This paper shows the results of a study conducted on five different categories of vehicles in a specific test site. The aim was to investigate how the effect of the test site discontinuity determines variations of comfort related to the increase in speed and to the five selected road vehicles of different classes. Measurements were obtained by combining data relating to vibrations in the three reference axes, detected through a vibration dosimeter (VIB-008), and geolocation data (latitude, longitude, and speed) identified by the GPS inside a smartphone. This procedure, through the synchronization between dosimeter and GPS location, has been helpful in postprocessing to eliminate any measurement anomalies generated by the operator. After the survey campaign it was determined that a formulation allows defining a Comfort Index (CI) depending on velocity and five vehicles of different classes. This study showed that the presence of speed bumps, in the test site investigated, appears to be uncomfortable even at speeds well below those required by the Highway Code.
1. State of Art
Vibrations provoke different responses and sensations as a function of environmental conditions and user’s physical state. Human responses to vibrations can be foreseen if they are measured and estimated in relation to the frequencies that have most influence on body reactions to stress; this requires knowledge of different parameters (e.g., frequency, orthogonal axes direction, and duration) as to define values representative of the true level of exposure [1].
Several methodologies have been introduced using differences of acceleration, acoustic pressure, and combination of these [2–8] through the use of smartphones or precision instruments, to detect abnormalities of road infrastructures. These methodologies, although using the same variables, do not identify values of comfort on board the vehicle but allow identifying discontinuities of the road paving. As described [9] a body is considered “in vibration” if it describes an oscillating movement with respect to a specific reference system.
The level of vibration is determined by the square root of the instantaneous vibration corresponding to the time history measured on the whole body. The amount determined, represented in logarithmic scale, is defined as equivalent acceleration (), whose value can be made dimensionless [9] by means of the ratio between equivalent acceleration () and a reference value of 10−6 m/s2. Other studies [10] propose a methodology to measure the comfort on public transport vehicles using an index called Comfort Measuring System (CMS). The CMS (Figure 1) is comprised of three parts:(i)measurements obtained through the detection of smartphones’ sensors;(ii)database provided by operators of the transport system;(iii)algorithm for the determination of results, using measurements from smartphones and from the database.
According to ISO [11] acceptable values of vibration for the evaluation of comfort depend on many factors and specific analysis to which they are addressed. The same ISO [11] defines values to determine levels of comfort on public transport vehicles.
Values, however, depend on different variables, including the type of activity that passenger plays during the trip (reading, eating, writing, etc.) and other factors related to the environmental conditions inside the vehicles. In particular, the ISO [11] defines specific classes of comfort/discomfort:(i)less than 0.315 m/s2: not uncomfortable;(ii)0.315 m/s2 to 0.63 m/s2: a little uncomfortable;(iii)0.5 m/s2 to 1 m/s2: fairly uncomfortable;(iv)0.8 m/s2 to 1.6 m/s2: uncomfortable;(v)1.25 m/s2 to 2.5 m/s2: very uncomfortable;(vi)greater than 2 m/s2: extremely uncomfortable.
2. Instrumentation
The instrumentation used in this work, to identify the comfort on board, is as follows:(i)smartphone Samsung Galaxy S4 I9505 to geolocalize vehicles;(ii)vibration dosimeter VIB 008 (whole body mode) to detect, according to the European Directive [12, 13], the band limit frequency-weighted RMS acceleration in m/s2 (a) and the frequency-weighted RMS acceleration, Wd and Wk, filter, in m/s2 (), over the x-, y-, z-axis, and so forth (the main features are shown in Table 1. The analysis of data recorded by the VIB 008 has been realized with the software dB Maestro Ver.5.5, supplied by the manufacturer. This software allows analyzing and exporting data collected and stored by the instrument).
3. Methodology
The vehicles used to determine comfort on board are as follows:(i)Alfa Romeo Giulietta 2.0 140 CV (sport sedan);(ii)Land Rover Freelander 2.2 160 CV (SUV);(iii)Opel Zafira Tourer 2.0 160 CV (minivan);(iv)Ford Fiesta 1.4 96 CV (compact car);(v)Opel Insignia 2.0 160 CV (sedan).The vibration dosimeter was positioned, with appropriate orientation (Figure 2), on the driver’s seat (Figure 3), in order to determine vibrations affecting the spine while driving and caused by the presence of potholes, speed bumps, or other road abnormalities.
Parameters used to determine comfort on board are the (frequency-weighted RMS acceleration (Wd and Wk filters) in m/s2, along the x-, y-, and z-axis), determined by the mathematical formulation (1), and , overall whole body vibration acceleration, determined by the mathematical formulation (2).where we have the following:(i) is the weighted acceleration (translational or rotational) as a function of time (time history), in metres per second squared (m/s2). This value is automatically measured by the instrument and it can be approximated to a continuous function (>30 Hz);(ii)T is the duration of measurement (s).where we have the following:(i), , and are the weighted r.m.s. acceleration with respect to the orthogonal x-, y-, and z-axis, respectively;(ii), , and are multiplying factors with respect to the orthogonal x-, y-, and z-axis, respectively. The multiplying factors values are = 1.4, = 1.4, and = 1.0 (ISO [11]).
The use of these parameters has allowed creating a link between the traveling speed and the same parameter , resulting in a Comfort Index (CI). This index is a function, as shown by (3), of type of vehicle and speed:where we have the following:(i) are overall whole body vibration acceleration;(ii)Ty0 is overall whole body vibration acceleration for , measured and averaged in a temporal range () and for a type of vehicles and is overall whole body vibration acceleration for and for a type of vehicles;(iii) is the average speed of the vehicle.Once the value of CI is identified, some reference thresholds were chosen to determine the vibrational comfort degree inside the vehicle:(i)if CI is less than 0.315 m/s2, it is “not uncomfortable”;(ii)if CI = 0.315 m/s2 – 0.565 m/s2, it is a “little uncomfortable”;(iii)if CI = 0.565 m/s2 – 0.9 m/s2, it is “fairly uncomfortable”;(iv)if CI = 0.565 m/s2 – 0.9 m/s2, it is fairly “uncomfortable”;(v)if CI = 1.425 m/s2 – 2.25 m/s2, it is “very uncomfortable”;(vi)if CI is greater than 2.25 m/s2, it is “extremely uncomfortable.”
4. Experiments and Results
Instrumental analysis was carried out along a stretch of road, with excellent road surface condition and with a length of about 5.0 Km, with nonhomogeneous road paving because of the presence of stone artificial bumps.
The test site is located in the City of Rende (Cosenza) along Viale Principe (Figure 4).
Five vehicles of different categories were used for the instrumental analysis, for each of which 15 measurements were made, in order to have significant statistical relevance.
Data collected by the instrument, in any survey, were exported through the software dB Maestro Ver5.5 (Figure 5).
For each tested vehicle and for each measurement, the value of was determined. Then the model was calibrated by means of an exponential regression, described by (3), determining coefficients of dependent variables for each vehicle class useful to determine CI (Table 2).
For each of the five vehicles, the graph linking the traveling speed and the Comfort Index (CI) is determined.
Figures 6, 7, 8, 9, and 10 show the trend of Comfort Index varying the traveling speed and the type of car. As can be seen from the value of , for each type of car, the reliability of the exponential curve, which approximates the value of data obtained in the 15 measurements, is extremely high.
In order to have a more complete and clear view of differences between the various vehicles, we have shown a graph summarizing the CI trend for each type.
Figure 11 shows the CI trend at different speed for all types of vehicle, highlighting some differences between the same types of vehicle. In fact, at low speeds the most comfortable vehicle is the Opel Insignia (sedan); however at high speeds the most comfortable vehicle is the Land Rover Freelander 2 (SUV).
5. Conclusions
The user interprets the quality of road track through perceptions, often subjective, resulting in one of the possible variables of route selection.
The definition of Comfort Index (CI) allows identifying a measure of quality of the road track and can be a parameter for choosing the path. This study has identified an evaluation criterion of Comfort Index (CI) specific for five vehicles of different categories and travel speed. Some reference thresholds and six classes of vibrational comfort were also identified.
Starting from the analysis carried out on the test site, it was estimated that for speeds over 37 km/h, for each vehicle class, the road track results in being “fairly uncomfortable”; also for speeds over 45 km/h the road track results in being “uncomfortable.”
Therefore, while driving at speeds far below the maximum limit allowed by the Highway Code, in reference to all the five vehicles the road track results in being “uncomfortable.”
Competing Interests
The authors declare that they have no competing interests.
Acknowledgments
Thanks go to AESSE Environment s.r.l. and 01 dB Acoem for providing instruments used in this research and engineer Piertoni Cambiaggio for the fruitful cooperation offered in training authors to use instrumentation. Thanks go, also, to the dealership Tema Motori in Rende (CS) for the grant of some vehicles used in the surveys.