Steering System Vibration : Information and Perception Enhancement

Abstract

Automobile drivers are regularly exposed to vibrational and acoustic stimuli. These stimuli cause discomfort, and methods for analysing the noise, vibration and harshness (NVH) properties of automobiles are in regular use. Most manufacturers currently dedicate significant attention to the NVH characteristics of their products. NVH criteria are regularly applied to the design of the steering system, whose vibration spectra can reach frequencies as high as 300 Hz. The design of steering components has been the subject of several studies (Pak et al., 1991) and the human subjective response to steering vibration has also been investigated, in terms of perceived intensity (Giacomin et. al., 2004) and induced fatigue (Giacomin and Abrahams, 2000). While further research is required, much is known about the discomfort produced by steering vibration. A less well understood topic is the information transmitted to the driver. With electronically assisted and by-wire steering technologies (Jurgen, 1999) the question of what stimuli should reach the driver has become important. All current methods for estimating vibrational discomfort, whether hand-arm or whole-body, and whether based on frequency weightings (ISO 5349-1, 2001) or customer correlations (Schoeggl, 2001), are defined in such a way as to suggest that a uniform reduction in vibration level brings a uniform reduction in discomfort. Less vibration is considered better. This may not be appropriate in the case of information, however, since situations can be imagined in which an increase in vibration might prove useful towards understanding the nature of the road surface or of the vehicle dynamic state. The question of what information the steering should transmit to the driver is not a simple one. Vibrational stimuli help the driver to interpret many things including the type of road surface, the presence of water or snow, tyre slip (both longitudinal and lateral) and the dynamic state of subsystems such as the engine, the steering and the brakes. The stimuli are perceived, compared to models from long term memory and interpreted. A possible approach is the measurement of statistical information. Since the work of Shannon (1949), numerous researchers have applied the concept of information entropy to problems in human behaviour and control (Corning, 2001; Bea and Marijuan, 2003). In Shannon’s terminology information refers to the capacity to reduce statistical uncertainty, while entropy is the degree of uncertainty. The basic premise is that a communication channel can be analysed in terms of the symbols used, and that the probability of occurrence of the symbols can be used as a metric of information flow. In recent years information measures have occasionally been applied to automotive problems, one example being the measure of steering wheel entropy defined by Nakayama et. al. (1999). The availability of a metric for quantifying the information transmitted to the driver provides an important new type of evaluation. As shown in figure 1, stimuli can then be judged in terms of the two, often opposing, criteria of discomfort and information. Further, optimisation of the information transmitting elements of the steering system by the automobile designer leads to a perception enhancing interface, or, more specifically, a Perception Enhancement System (PES). A possible PES for a by-wire steering system is shown in figure 2, where movements at the tyre or wheel hub are returned to the driver through a perception enhancing electronic controller unit, which identifies significant features which are then amplified and transmitted. Research to define information metrics and perception enhancement systems for automobile steering systems is being performed as part of a collaboration between the Perception Enhancement Systems research group of Sheffield University and the Hyundai Motor Company central research laboratory. This article describes the results of two experiments which have measured the effect of the amplitude and the frequency bandwidth of steering vibration on the human ability to identify road surface type.