The dynamic steering system which debuts in the latest Audi A4 is a new departure for the Audi brand, and is available for all versions of the new A4 from 190PS upwards. A superimposed zero-play transmission, of a type already used successfully on astronauts’ vehicles, it varies the effective steering ratio according to road speed. Together with the ESP electronic stabilisation program, the dynamic steering system keeps the new Audi A4 stable by means of slight, extremely rapid steering movements.
The superimposed transmission is located in the steering column and combined with an electric motor. It is of the ‘Harmonic Drive’ type that has already proved highly successful in robotics and space travel. In 1971, a transmission of this type voyaged into space for the first time on board Apollo 15: it was part of the individual wheel drive for the ‘Lunar Rover’ car used on the surface of the moon. In 1997, Mars was its next destination, on the ‘Sojourner’ expedition vehicle used during the spectacular ‘Pathfinder’ mission. This type of transmission is also operating on the Hubble space telescope launched in 1990.
The harmonic drive transmission has only three main operating components. An electric motor turns an elliptical inner rotor which, by way of a ball bearing, alters the shape of a thin sunwheel connected to the steering input shaft. At the vertical axes of the ellipse, the sunwheel meshes with the teeth on the inside of an annulus or ring gear, which acts on the steering output shaft. When the inner rotor is turned, the main axis of the ellipse changes, and with it the extent of gear tooth meshing. Since the sunwheel has fewer teeth than the annulus, there is relative movement between them – the superimposed movement used to alter the effective steering ratio.
The dynamic steering ratio can vary by almost 100 per cent, depending on the car’s road speed and the chosen Audi drive select mode. Changes in the ratio take place continuously and imperceptibly.
When parking, the dynamic steering system is extremely direct, with only two turns of the steering wheel from lock to lock, and very little effort needed thanks to a high degree of power assistance. At typical country-road speeds, the direct response and level of power assistance are reduced slightly, but even then the driver will not have to move his or her hands to another point on the steering wheel when negotiating a sharp corner. At high speeds on motorways or similar roads, a more indirect steering ratio and less power assistance ensure that the car has a high degree of straight-line stability; in theory, four turns of the wheel from one steering lock to the other would be necessary.
The dynamic steering system cooperates closely with ESP in the vehicle dynamics and safety areas. It relieves the burden on the electronic stabilisation program because a steering correction can be performed about three times faster than the time needed to build up pressure for a brake application. This ability to intervene so rapidly reduces the number of brake applications required, making overall progress smoother. Although the steering corrections are highly effective, the driver remains unaware of them in most cases, especially since no noise is generated.
In the event of oversteer following a sudden load reversal, the dynamic steering system can rectify the situation alone by turning the steering in the opposite direction. A brake application will not be made unless the angle of float becomes too great, and in most cases is only needed to suppress residual movement.
If understeer begins to set in, the steering ratio is made more indirect for a short time, so that the driver is unlikely to turn the wheel beyond the limit of good tyre grip. The steering angle remains small and easily controlled, and understeer is almost completely suppressed. This function is exclusive to the new Audi A4.
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