Bosch-ZF Active Steering

On the road to the totally-active bywire automobile, the deepest pothole engineers face is steering systems.

Safety regulators, customers, and liability attorneys have no problem with technology aimed at reduced parking drudgery and there is sincere appreciation for the great strides achieved with handling. But accepting wires and motors as the logical replacement for mechanical steering linkage demands a leap of faith lying somewhere between fuel cells and the tooth fairy.

The actuator mounts between a conventional ZF Servotronic-2 torquesensing power-assist control valve and the rack housing. An electric motor and electromagnetic locking device are externally attached.

One of the benefits of Active Steering is the ability to adjust the steering ratio — faster at low to medium speeds — slower at highway cruising speeds.

This is not to suggest that the quest for smarter chassis systems is hopeless. To the contrary, Bosch and ZF contend that by-wire steering is a viable near-term goal. Towards that end, they formed a 50:50 joint venture four years ago to pursue electronic steering systems. Notes Colin Carter, executive vice president responsible for sales at ZF Lenksysteme GmbH (the Bosch-ZF consortium), “The auto industry is interested in more intelligent systems capable of interacting with other systems. But standard hydraulically assisted power steering lacks that capability. The solution is to find some way to electrify these systems.”

Bosch began investigating electronic steering more than a decade ago. ZF Friedrichshafen AG’s gear cutting and mechanical-systems expertise made it the ideal partner. The first product of the 1999 Bosch-ZF union is Servolectric steering systems (with power assist supplied electrically instead of hydraulically) already available on European VW Golfs. Servolectric steering is also installed on BMW Z4, Audi A3 and VW Touran. The second step forward is an interesting blend of current and advanced technology called Active Steering. Without disconnecting the conventional mechanical linkage to the front wheels, equipment is added that facilitates a degree of computer control over the car’s direction of travel. This hybrid approach narrows the chasm between today’s largely passive steering systems and the fully active equipment that will inevitably guide cars of the future. (General Motors recently acknowledged it too has a motorized hybrid steering system under development.)

In the form adopted by BMW for optional use on its 2004 5-series sedans, Active Steering is an add-on module mounted to the pinion shaft between a conventional ZF Servotronic 2 torque-sensing power-assist control valve and the rack housing. Housed inside the module’s compact aluminum casting, there’s a compound planetary gear set.

An electric motor that drives the planetary gear set’s carrier on demand and an electromagnetic locking device that disables the active side of the steering system are externally attached.

A separate electronic control unit (ECU) gathers operating information from various car systems (steering wheel angle, pinion angle, road speed and individual wheel speeds) before dispatching electrical commands to both the hydraulic-assist control valve and to the Active Steering system’s electric motor. A conventional engine-driven pump supplies hydraulic pressure to the power-assist cylinder contained within the rack housing.

The aforementioned planetary gear train seamlessly blends mechanical and electrical steering commands. At its upper end, a sun gear is rotated by steering wheel motion. That gear meshes with three planet gears which in turn drive a second sun gear attached to the pinion shaft. As long as the planetary set’s carrier remains fixed, steering shaft input rotation is faithfully conveyed to the pinion shaft without alteration. According to Bosch-ZF engineers, the added friction from these eight meshed gears is inconsequential to the driver because it occurs downstream of the steering system’s hydraulic control valve.

The fun begins when the ECU disengages the electromagnetic locking unit (which assures normal mechanical steering in the event of any electronic systems failure) and tells the electric motor to spin the planetary gear set’s carrier. Active Steering is able to speed up or slow down the steering ratio independent of steering effort (which is coordinated and controlled by the same ECU through the Servotronic power assist control valve).

Ultimately the mechanical side of today’s Active System will wither and vanish like frog gills and tails on the evolutionary path to full steer-by-wire directional control. But today there are three key advantages available to enhance a BMW 5-series owner’s safety and driving pleasure. The first is a steering ratio approximately 30-percent quicker than normal at low to medium road speeds. Suddenly a large, long-wheelbase sedan is able to mimic a subcompact’s agile moves. The second benefit is that at highway cruising speeds the steering ratio is 25-30-percent slower than normal in the interests of placid straightline cruising. The third pay-off is much quicker reactions when corrections are necessary to stabilize a car on the verge of oversteer rear tires sliding wide). According to ZF Lenksysteme Engineer Peter Brenner, Active Steering corrections aimed at thwarting a skid are enabled approximately four times faster than the corrections possible with brake-based stability systems such as ESP.

Recent demonstration runs at Bosch’s Boxberg proving grounds in Germany revealed that spinning out a BMW 5-series with Active Steering is virtually impossible. In response to wildly aggressive steering inputs during abrupt lane-change maneuvers, the car reacts calmly and predictably with no tendency to wriggle its tail. Thanks to the quicker steering ratio available at moderate speeds, the blurred elbows and frantic wrist action usually necessary to negotiate tightly spaced slalom cones are unnecessary. An Active Steering BMW slinks through a serpentine path as if it’s hinged in the middle.

According to Carter, ZF Lenksysteme has the capacity to manufacture 100,000 Active Steering systems per annum. In addition to better coordination of steering ratio to driving circumstances and the ability to pre-empt ESP braking intervention with automatic steering corrections, the potential exists to incorporate new functions in the future. Interaction with powertrain and suspension systems is next on the list. ZF Lenksysteme’s plan is two-pronged: shrewd blend of hydraulic and electronic actuation for upper range models and simpler, less expensive computer-controlled pure-electric systems for lighter, smaller applications.

Full active steering is the Holy Grail for bywire protagonists. It has the potential to simplify the entire front-of-vehicle architecture and could deliver major cost, weight, NVH, safety, and occupant-comfort benefits. The one remaining engineering hang-up is the redundancy that’s necessary: two independent wiring harnesses, dual control units, a back-up power source, and some sort of mechanical linkage to guide the car in the event of a total electrical systems failure.

The most significant roles the Bosch-ZF Active Steering system will play is to demonstrate worthwhile progress toward full by-wire control and calming the fears of those reluctant to relinquish a mechanical link between their hands and the road.