Human Fatigue Due to Automobile Steering Wheel Vibration

This study investigated the human fatigue caused by automobile steering wheel vibration. Discomfort data was collected from 15 subjects using a steering wheel vibration bench. A rigid wheel was used to apply sinusoidal rotational excitation of 0.08 m/s and 0.1 m/s amplitude at test frequencies of 4, 8, 16 and 32 Hz. Human discomfort was quantified after periods of 1, 5 and 10 minutes by means of a body part discomfort form. Significance testing showed that the discomfort ratings were dependent on vibration frequency, but not on vibration amplitude at a 5 % confidence level. Perceived discomfort was found to increase approximately linearly with increasing length of exposure, but the rate of increase was found to vary depending on the body region. Female subjects were found to perceive greater discomfort in the arm regions from 4 to 8 Hz than males, but no significant differences were found for the other body regions at p<0.05. On average, lighter subjects experienced greater discomfort than heavier subjects. Elbow angle was found to have no significant effect on perceived discomfort at p<0.05. The results suggest the importance of identifying the location of the discomfort when evaluating steering wheel vibration. 1 The Perception of Vibration in Automobiles The sensations produced by the vibrational stimuli which reach the vehicle driver can provide important information regarding the dynamic state of the vehicle, but can also provide annoyance and discomfort. Figure 1 illustrates the three main interfaces across which a vehicle transmits tactile information to the driver, namely the foot interface (floor and pedals), the body interface (seat cushion and backrest) and the hand-arm interface (steering wheel and gearshift). Of these, the hand contact at the steering wheel plays an important role in transmitting both information and discomfort. The area of research which investigates the vibrational and torque stimuli which inform the driver about the dynamic state of the vehicle can be termed that of Perception Enhancement Systems. The term Perception Enhancement System can be used to describe a device which optimises the feedback of vehicle movements and tyre-road dynamics to the driver. Such systems treat the vibrational stimuli from an information theoretic point of view, and thus are concerned with the optimisation of the man-machine interface so as to make the vehicle feel more like an extension of the driver’s body. Current research is attempting to define the vibrational frequency bands which contain the information regarding the vehicle dynamic state, so that steering components can be designed to better transmit the information to the driver. Hum. Fatigue due to Auto. Steering Wheel Vibr. J. Giacomin and O. Abrahams The University of Sheffield 27/10/2000 2/10 http://www.shef.ac.uk/uni/academic/I-M/mpe/dynam/ Figure 1) Vibration stimuli acting in the vehicle environment. A second and more traditional area of research is that of vibrational discomfort. Numerous studies have been performed to investigate the interaction between vibrating surfaces and the hand-arm system [3,7]. Reynolds et al. [16-19] investigated hand-arm response to vibrating handles when held with either the palm or the fingers. Miwa [14] measured threshold and tolerance limits for subjects holding their palm flat against a vibrating surface. Reynolds and Soedel [18] investigated the dynamic response of the hand-arm system to translational sinusoidal vibrations in the frequency range from 20 to 500 Hz. They concluded that arm position had little effect on the impedance of the hand while grip tightness and hand pressure were found to influence vibration response at frequencies above 60 Hz. They also suggested that once a method of grip had been established, the hand-arm system could be treated as a linear system. Burstrom and Lundstrom [1] assessed the influence of vibration direction, grip force, vibration level, and hand-arm posture on the absorption of energy by the hand-arm system. They concluded that energy absorption was dependent mainly on the frequency and direction of vibration. Absorption increased with both higher energy levels and firmer handgrips. Burstrom and Lundstrom also stated that varying handarm postures produced only small changes in the absorption of the translational energy, while the size and mass of the subject’s hand and arm greatly affected energy absorption. More recently, Giacomin and Onesti [6] investigated the effect of frequency and grip force on the perception of steering wheel rotational vibration using 10 second exposures to sinusoidal test signals in the range from 4 to 125 Hz. Discomfort was highly dependent on frequency. Large amplitude movements of the hand-arm system were found at low frequencies, while higher frequencies tended to localise the vibration response to only those parts of the hand which were in the immediate vicinity of the steering wheel. 30 Hz was found to be roughly the frequency above which the response movements localised to the hand. Giacomin and Onesti produced iso-comfort curves for the frequency range from 8 to 125 Hz for the reference amplitudes of 1.86 and 5.58 m/s. They concluded that a linear iso-comfort weighting might be acceptable at 5-10 % accuracy for evaluating typical steering wheel vibration signals, and that grip tightness would not greatly effect the evaluation. Mechan and Versmold [13] performed an investigation using 30 subjects which produced iso-comfort curves for the frequency range from 4 to 32 Hz. Merchan and Versmold also analysed the effect of the length of the vibration exposure on the perceived Steering Wheel Vibration Seat Vibration Floor & Pedal Vibration Subjective Perception of Comfort Hum. Fatigue due to Auto. Steering Wheel Vibr. J. Giacomin and O. Abrahams The University of Sheffield 27/10/2000 3/10 http://www.shef.ac.uk/uni/academic/I-M/mpe/dynam/ discomfort. A summary table was presented which indicated the frequencies at which the individual body parts were most prone to suffer discomfort The study described in this paper represents an extension of the previous research into the perception of steering wheel rotational vibration performed at Sheffield University. The objectives of the study were to investigate fully the effect of frequency in the range up to 32 Hz, and to analyse the importance of exposure duration. 2 Experimental Method 2.1 Test Equipment The tests were performed using the bench shown in Figure 1, which consisted of a rigid steering wheel connected to a shaft supported by five precision bearings. The shaft incorporates a lever arm which is connected to an electrodynamic shaker unit by means of a stinger rod. All mechanical components were rigid to frequencies in excess of 80 Hz. The seat and guide-rail were taken from a Fiat Punto, and the bench geometric dimensions (see Table 1) were chosen based on data from European B-segment automobiles. Seat travel and seat height were adjustable as in the vehicle, and the bench incorporated a scale and pointer system to measure the distance of the H point from the steering wheel hub centre. Figure 2) Steering wheel rotational vibration test bench Geometric Parameter Value steering column angle with respect to floor 23° steering wheel hub centre height above floor 700 mm steering wheel diameter 325 mm horizontal distance from h point to steering wheel hub centre 390–450 mm Seat h point height from floor 275 mm Table 1) Bench geometric dimensions which effect sitting posture Hum. Fatigue due to Auto. Steering Wheel Vibr. J. Giacomin and O. Abrahams The University of Sheffield 27/10/2000 4/10 http://www.shef.ac.uk/uni/academic/I-M/mpe/dynam/ The bench incorporated a G&W V20 electrodynamc shaker driven by a PA 100 amplifier [5], with internal sine wave generator. The acceleration obtained at the steering wheel was measured using an Entran EGAS-FS-25 accelerometer located on the top left side of the steering wheel. The accelerometer signal was amplified by means of an Entran MSC6 signal-conditioning unit [4] and monitored by means of a Tektronix TDS210 digital oscilloscope [21]. The experimental layout is illustrated in Figure 3. Figure 3) Experimental layout 2.2 Test Frequencies and Amplitudes Previous research [6] showed that at frequencies below about 30Hz subjects perceived vibrational discomfort over wide areas, whereas at frequencies above that value the discomfort was localised to the vicinity of the hand. In order to investigate this further, four sinusoidal test signals were chosen with frequencies of 4, 8, 16 and 32 Hz. Two test amplitudes were chosen for the current study based on the analysis of steering wheel vibration signals from European automobiles. The lower amplitude was chosen to be similar in root-mean-square terms to vibration which would be considered just noticeably uncomfortable in road vehicles. The higher amplitude was chosen to produce significant discomfort after about 10 to 15 minutes of continuous exposure. The constant peak velocity chosen for the low amplitude signals was 0.08 m/s, while that chosen for the high amplitude signals was 0.1 m/s. Table 2 presents the test frequencies, peak velocity amplitudes and the resulting peak acceleration amplitudes. For comparison purposes, a 0 Hz and 0 m/s signal (a static test) was included in the test programme so as to evaluate the level of background discomfort due to maintaining the static posture over the length of time of a test. Constant Velocity (0.08 m/s) Constant Velocity (0.10 m/s) Frequency (Hz) 4 8 16 32 4 8 16 32 Acceleration (m/s) 2.0 4.0 8.0 16.0 2.5 5.0 10.1 20.1 Amplitude (mV) 37.5 75.0 149.9 299.9 46.9 93.7 187.4 374.9 Table 2) Frequencies and amplitude of the test signals PA signal generator and power amplifier MSC6 signal conditioning unit TDS210 digital oscilloscope V20 shaker Steering wheel GGAS-FS-25 accelerometer