# A Study of a Ride Comfort Control System for Ultra-Compact Vehicles Using Biometric Information
## 1 Introduction  
In recent years, ultra-compact mobility has been used in car-sharing services at tourist destinations and for short-distance home delivery services, and the demand has increased as a new means of transportation [1]. Ultra-compact mobility has a remarkably simple miniaturized structure, and even at low speeds, unevenness and steps can deteriorate riding comfort. Therefore, our research group proposed a system in which the actuator is equipped with an active seat suspension that uses a voice coil motor which exhibits high responsiveness, and the input vertical vibration is reduced by active control [2]. To date, the improvement of ride quality by vibration-damping control has been evaluated based on the myoelectric potential of the neck. As a result, it has been confirmed through biological information that the physical burden is reduced by damping control. However, the ride quality tended to differ depending on the occupant's heart rate variability, which indicates the psychological state [3]. From this result, it was confirmed that damping does not always produce the best ride quality. It is essential to construction a new ride controlsystem. Additionally, the occupant's sense of ride comfort depends on an ever-changing physical condition and mood on the day. Furthermore, it is constantly changing due to the ever-changing driving environment. Therefore, the occupant's comfort level must be taken into account based on the ever-changing driving environment. Until now, it has been common to evaluate the level of ride comfort quantitatively by subjective evaluation. However, road surface conditions are constantly changing, it is not possible to evaluate ride comfort in real time through subjective evaluation. Therefore, the occupant calculates the comfort level (while driving) from the biometric information of the occupant in real time, and applies it to optimize vibration control. Although there are technologies such as vibration control [4, 5] that are actively performed based on the road surface condition and suspension adjustment according to the driver's preference, the comfort level is calculated from the occupant's biological information [6, 7, 8, 9, 10, 11, 12, 13, 14, 15]. Although many studies have been conducted to evaluate ride comfort using biometric information [16, 17, 18, 19, 20], the current state of the art remains limited to evaluation. There are no studies that predict occupant psychological state from occupant biometric information. Therefore, the authors aim to clarify the relationship between vibration and biological information and the relationship between biological information and psychological state and to construct a method for estimating the psychological state from biological information as a preliminary step to system construction. In addition, the authors have clarified that the vibration frequency affects the human vibration sensation using heart rate variability, biological information that reflects the activity of the autonomic nervous system; this finding is consistent with the results of previous studies [21]. However, as the autonomic nervous system responds to factors other than the psychological state, such as respiration and movement, it cannot be generalized that it is affected by vibration. Therefore, it is difficult to accurately estimate the psychological state of an occupant using only heart rate variability. Therefore, we evaluated the mental stress of the occupants using cerebral blood flow, an index that reflects the activity of the human central nervous system [22]. As a result, we have confirmed that the activities of the occupant's autonomic nerves and central nervous system become active when vibrations are applied. Furthermore, focusing on the difference in ride quality reported by [PERSON] et al. [23], we conducted a vibration experiment assuming actual driving conditions. The occupant is vibrated at multiple frequency components to clarify the relationship between the biological information and the psychological state by subjective evaluation.