Hanging by a wire - Engineer - BMW Technik researches drive-by-wire

Automotive Design & Production,  Dec, 2001  by Christopher A. Sawyer

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Then again, you might want to modify the pedal box in order to reduce foot and leg injuries. "If we don't need the vacuum booster right behind the pedal, and the pedal doesn't need to be where it is today," Guldner points out, "the pedal's length of travel can be lessened significantly. It becomes a variable-output switch." In addition, he suggests it would be possible to retract the steering wheel (remember, no column) in an accident, while adjusting the force of the airbag. "The opportunities are there," he says.

BUILDING ON THE ADVANTAGE Though Z22 was designed to show how a full drive-by-wire system would work, this three-dimensional concept also has altered the way BMW engineers look at the production process. "Because steer-by-wire systems will not have a steering column, this will not be the first item to go in once the body and chassis go together," Guldner explains. "Up until now it has been one of the determining factors in determining when this merger happens. Now we may be able to keep these sections apart longer and bring them together when they are nearly complete."

The steering wheel and column will become part of the instrument panel, needing no more than a quick mating of connectors to make the union complete. The steering actuators, on the other hand, will be added to the body after the engine is installed, or be included in a rolling chassis that includes the power-train, wheel assemblies (brakes, axles, dampers, etc.), and other underbody components. Either way, a quick electrical connection would be all that is necessary to enable a fully tested system.

"We haven't finalized how much this production method might save us," states Guldner, "or exactly what we might do differently in production, but we see a big chance to simplify production and greatly extend the phase where major systems and assemblies run down the line in parallel." BMW feels this will increase the value-added work, lower total production time, and reduce production costs when viewed on a systems basis.

IT TAKES TIME

Despite all of the apparent advantages, Guldner says BMW will not move from conventional to fully electronic systems overnight. "For safety reasons," he explains, "we have decided to follow a step-by-step procedure when it comes to drive-by-wire systems." Though BMW has a fully electronic throttle-by-wire system in production, the next generation of the company's vehicles will test the waters with hybrid electro-mechanical systems.

The first is a conventional steering system enhanced through the addition of a planetary gearbox between the steering wheel and road wheels. A small electric motor adjusts steering angle on demand, or in response to commands from the vehicle stability system. If the system fails, the conventional steering connection remains.

The second is an electro-hydraulic braking system. "We will be controlling the hydraulic actuator through electronics because we believe that there are advantages in the upper ranges to this type of system." says Guldner. He also states that in smaller vehicles like the 1 Series and 3 Series, direct electronic brake actuation might be the preferred technology, but with a big caveat: "We will need at least one level of redundancy for sensors and computation with a fully electronic system. It's similar to the aerospace model," he deadpans, "but without the seven layers necessary to get a stricken plane across the ocean." In limp-home mode the trip may be slower, but "it should be no less safe."