The Effect of Vehicle Seat Vibration and its Control

The transmission of seat vibration to the human body contributes to vibration discomfort, particularly near the resonant frequencies of less than 10 Hz. ISO 2631-1 (1997) specifies measurement and assessing seated human body discomfort due to vibration. This thesis first describes an investigation of the effects of vibration on driver drowsiness in a laboratory-controlled study. This study found characteristic changes in the heart rate variability (HRV) domain that indicated progressively increasing neurological effort in maintaining alertness in response to low-frequency vibration, which became significant within 30 minutes. This thesis then describes an investigation of vibration's influence on mental workload and performance during a simulated driving task. Significant changes were detected in both steering entropy and heart rate variability (HRV), indicative of an increased mental workload after 30 minutes of exposure to vibration, whereas driving performance, as assessed by the Standard Deviation of Lateral Position (SDLP), did not degrade until 45 minutes. This finding has implications for the prediction of the risk of accidents. Even though vibration can cause riding discomfort and vibration-induced drowsiness, little consideration has been given to efficiently (with minimum increase in mass) controlling seat vibration to address these issues. Finally, this thesis focuses on the development of new devices for controlling vehicle seat structural vibration. The main structural vibration control methods are reviewed, and their distinctive characteristics are listed. A comparison among these methods is conducted, and consequently, piezoelectric-based devices being selected as the most suitable method for seat vibration control. Two types of piezoelectric active control systems are developed: One is based on a PID control, another uses Direct Velocity Feedback Control (DVFB). The effectiveness of the vibration control performance of both systems has been experimentally verified on cantilever beams. The piezoelectric multi-mode shunt damping (passive) method is developed and all the key factors which maximise the energy conversion via the piezoelectric effect are theoretically investigated. To control the lateral structural vibration of the seat, a piezoelectric stacker transducer is used with a shunt damping system, and the results indicate that it is effective for controlling the lateral structural vibration at the resonant frequencies of the seat. A piezoelectric patch with a shunt damping system is implemented on a vehicle seat to suppress the transmission of seat vibration to the occupant's body by controlling the seat foam vibration. The results indicate that the proposed method effectively controls seat vibration at the resonant frequencies of the seat and consequently improves the subjects' subjective comfort feeling. The application of a piezoelectric system holds some promise for the future development of lightweight vehicle seats with enhanced ride comfort and drowsiness reduction.