Audi A4: The adap­tive steering

The magic word is dynamic steering. This new steering uses the vehicle speed to change the steering ratio and simul­ta­ne­ously adapt the steering torque. At high speed an indi­rect ratio and higher steering torque improves the direc­tional stability. The dynamic steering is extremely accu­rate with its smooth-running espe­cially when parking — only two turns of the steering wheel are needed from stop to stop.

Stability and dynamics

The system utilises an inter­ac­tive gearbox to supple­ment the power steering and the ESP. The gearbox absorbs manual impulses gener­ated by the steering wheel as well as those from a high-dynamic EC motor. Drivers have a direct ratio at their disposal, they can control their vehi­cles with preci­sion and react quickly: for example when parking, in city traffic or on winding roads in the low and medium speed ranges. The speed of the EC motor for moving the steering wheel is added in these situ­a­tions, which means that drivers have to turn the steering wheel much less — a consid­er­able increase in comfort and gain in agility.

In crit­ical situ­a­tions at high speed, such as when over­steering after sharp evasive action, the dynamic steering can prevent the rear from swerving. When under­steering — pushing the car to the outer edge of the curve — the dynamic steering also inter­venes help­fully: it becomes more indi­rect for a short time and assists drivers with using the area in which the tyres still have good adhe­sion to the road surface. The steering also becomes more indi­rect at high speeds in which the steering angle is reduced by the angle of rota­tion of the EC motor. The system checks several vehicle para­me­ters so that the motor is oper­ated in the correct direc­tion — direct or indi­rect. The main para­me­ters are the speed in addi­tion to the steering angle. The dynamic steering works hand in hand with the ESP to provide stabil­i­sa­tion. The major advan­tage of this inter­play lies in the fact that the ESP now acts on the dynamic steering rather than on the brakes according to situ­a­tion. In an emer­gency the adap­tive steering there­fore quickly helps the vehicle to achieve a stable line — at the same speed. Drivers expe­ri­ence the stabil­i­sa­tion more harmo­niously and more comfort­ably than without dynamic steering.

Quieter, smaller, more effi­cient

In the differ­en­tial steering systems currently known on the market, which were also devel­oped by ebm-papst together with ZF Lenksys­teme GmbH, the elec­tric motor had to be designed to cope with high envi­ron­mental influ­ences. Its unpro­tected instal­la­tion loca­tion in the engine compart­ment, in close prox­imity to the front axle, subjected it to dusty heat, perishing cold or damp­ness as well as aggres­sive fluids: from screen­wash through brake fluid right up to battery acid. On the A4, though, the drive sits in the column directly behind the steering wheel as an inte­gral compo­nent of the steering column. In this design, the steering shaft is routed right through the elec­tric motor and there­fore provides several advan­tages at the same time: it is protected from climatic influ­ences and can be installed univer­sally as opposed to previous solu­tions depen­dant upon the space avail­able in the engine compart­ment.

The inno­v­a­tive trick opened up a whole series of new hurdles that the devel­opers in St.?Georgen, at ZF Lenksys­teme and Audi had to tackle. The require­ments changed due to the fact that the motor was moved into the passenger compart­ment. Of course it no longer had to be sealed, but it now had to be quieter, smaller and more effi­cient. The basis for devel­op­ment was there­fore offered by a new EC motor design with a high power density opti­mised for the instal­la­tion space. The brush­less elec­tric motor stood out from the outset through its low noise level at top working speed. Further­more, it lasts throughout an entire vehicle life — and its energy effi­ciency is unri­valled. This inno­v­a­tive tech­nology allowed drive inte­gra­tion in the steering column. It would not have been possible to have realised this system and the enor­mous power density with conven­tional elec­tric motor systems.

The require­ments constantly increased during the devel­op­ment phase. According to Thomas Schrag, sales support manager at ebm-papst St. Georgen, the initial motor was completely different than the one finally used. “We had very little room avail­able anyway, and because of the crash spec­i­fi­ca­tions the EC drive for the dynamic steering has now become very short — one millimetre less space and it would have had to be built very differ­ently.” The solu­tion worked out by the engi­neers can not only no longer be heard, it is also unno­tice­able that a drive is working in the steering column. “The motor now has an absolute minimised detent torque, no torque ripple can be felt on the steering wheel, (torque fluc­tu­a­tions whilst the steering wheel is being turned and the elec­tric motor is working). Our devel­opers sat for months working on a solu­tion to this problem”, recalls Schrag.

Despite the secret helper, the driver always has direct contact to the front wheels via the steering wheel, which is also a legal require­ment. The driver always has full control, even in the unlikely event that the motor for the differ­en­tial steering system should fail. The intel­li­gent tech­nology now puts every manoeuvre to the test wher­ever driver error was frequently the cause of acci­dents to date. This is how the system helps in achieving extra driving stability and comfort – and there­fore greater joy at driving.

The heart of the power steering

The mech­a­nism of the dynamic steering system, the actu­ator, is inte­grated directly into the steering column. It consists of an EC motor with posi­tion sensors, an inter­ac­tive gearbox and a lock to secure the motor into a motion­less posi­tion. The Harmonic Drive gearbox ensures that both the manual pulses from the steering wheel and those from the EC motor are trans­mitted to the steering gear of the front axle. The inter­ac­tive gearbox works almost clear­ance-free and with great effi­ciency. The motor itself, as a load-bearing element, is a compo­nent of the steering column, the steering axle passes axially through the motor.