Vibration analysis of the vehicle seat with a finite element model

One of the significant factors affecting comfort in vehicles is vibration in seats. Passengers exposed to vibration face problems such as fatigue, lack of concentration, and low back pain. Therefore, eliminating or limiting the vibrations transmitted from the seat to the passenger is critical for comfortable travel. At this point, it is necessary to determine the dynamic properties such as natural frequencies, damping ratios, and mode shapes during the design phase, and accordingly to examine the transmissibility of the seat. In this study, modal analysis was carried out when the metal frame of a double passenger seat in a vehicle used in intercity transportation and wooden backrests are added to it. It was observed how the wooden backrests added to the backrest frame change the natural frequencies and mode shapes in the finite element software. It was noticed that wooden backrests slightly increased the natural frequencies of the seat in the 2nd, 4th and 5th modes, but decreased in the others, and it was seen some mode shapes changed compared to the beginning after the backrests were added. Additionally, the natural frequencies, damping ratios, and mode shapes of the frame structure were measured by the hammer test. It was seen from the experimental study that some natural frequencies of the seat were measured close to those belonging to the finite element model, and the similarities between them were drawn graph by comparing the relevant mode shapes and their Modal Assurance Criterion (MAC) indices. In the second stage, the back-and-forth vibration transmissibility of a double passenger seat against the vibration applied from the floor was investigated numerically using the ANSYS Random Vibration module. The Power Spectral Density (PSD) of the output acceleration from certain points on the upper seat frames versus the PSD input applied to the seat legs were calculated, and the transmissibility was obtained from the ratio of these two PSDs. It has been observed that some natural frequencies are slightly increased, and some are decreased if a wooden backrest is added. This shows that both the mass and stiffness properties of the added part affect the dynamic properties of the seat. In the studies, it was observed that the maximum transmissibility values decreased when the wooden layer was added. However, the transmissibility slightly increased when acceleration was applied especially, from the left foot at high frequencies. It is considered that these changes occur because the wooden part affects the damping and mode shapes of the structure. In the third stage of the study, the changes in profile, wood backrest thickness, and connection distances were compared according to the current situation, and the effect of transmissibility was examined. For the first situation, the profile thickness variation was examined. Natural frequencies increased as the thicknesses increased or decreased compared to the current situation. However, the transmissibility of the left seat at the first natural frequency increased when the thickness decreased and it decreased when the thickness increased. But in the right seat, transmissibility increased for both cases. In the second case, the change in connection distances was examined. It was observed that the natural frequencies increased compared to the current situation in both mid-connection and symmetrical-connection cases. In addition, in the left seat, the transmissibility values in the first mode increased compared to the current situation in both the middle connection and the symmetrical connection. In the right seat, the conductivity decreased in both connection conditions compared to the existing one. In the last case, the effect of the change in the thickness of the backrest on the frequencies and transmissibility was examined. As the thickness of the backrest wood decreases, the natural frequencies increase, and as the thickness increases, the frequencies decrease (except for the 6th mode). Again, considering the first mode of the structure, transmissibility decreased noticeably as the thickness increased in the left seat and it increased as the thickness decreased. A similar situation was observed in the right seat. The rates of change in transmissibility are similar both as thickness increases and decreases.