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Automotive Industry

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

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

Not satisfied with Theoretical Exercises, BMW Technik set out to see Whether or not Drive-by-Wire Systems Lived up to their Promise. The Best way to determine this, it Reasoned, was to Build a Drivable Technology Demonstrator. Six Years Later, the Z22 is still Teaching the Company About the Promise Inherent in Electronic Vehicle Control.

In 1995, BMW Technik, the automaker's in-house technology division, began work on project Z22. The idea behind it was to determine which technologies-from carbon fiber structures to drive-by-wire systems-were feasible, the approximate time it would take to bring them to production-ready status, and to determine the steps necessary to ensure the technologies were ready for prime time. Z22 didn't become visible to the outside world until 2000.

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Speaking about his area of interest, Dr. Jurgen Guldner, project manager, Drive-By-Wire Systems, says, "Initially, it was thought that we should see if it was feasible to electronically control a vehicle. To do that, we needed to see what had to be changed in the vehicle infrastructure, especially in terms of power generation, storage, and supply data communications; sensors and actuators."

PROTOCOL SEARCH

Drive-by-wire systems must interact seamlessly if they are to perform as expected, so Guldner says a common data communications protocol is of paramount importance. Researchers at the Vienna University of Technology have created a Time Triggered Protocol (TTP), which follows activities based on time: each of the by-wire nodes on the in-vehicle network share a common time reference and is given a slice of time (divided into milliseconds) so that there is no overlap.

However, a group consisting of BMW, DaimlerChrysler, Phillips, Bosch, and Motorola are, Guldner says, becoming increasingly interested in Flexray, an event-triggered protocol. He observes, "It looks like the industry is rapidly converging on the Flexray protocol," both in Europe and North America.

WORKING FOR REUSE

It's also convinced some components, and the basic software, will have multiple uses. "The actuator will need to be available in different sizes," says Guldner, "but it's not a big deal to put a bigger motor in place to make it vehicle-specific, though the motor itself will probably be off-the-shelf." So while Guldner doesn't see common actuators stretching from BMW's yet-to-be-built 1 Series small car to the 7 Series luxury sedan, he expects a reasonable overlap for the rest of the system. "I can see using common pieces within the 1 Series and 3 Series, the off-road vehicles, and the 5 Series and 7 Series," he remarks. "At least, that's what we re shooting for."

In addition, automakers are working to standardize the interface between sensors and the vehicle. "In cars right now," says Guldner, "we have three yaw rate sensors that measure practically the same thing. One is used for navigation, another feeds the stability system, and the third has various uses." Instead of using sensors that are system-specific, Guldner says the plan is to create sensors that, while a little more expensive, cover the complete range of measurement. "We might still want two sensors for redundancy," he says, "but definitely not three."

IN ACTION

"The steer-by-wire system on the Z22 was my area of responsibility," notes Guldner, "and we measured the forces through the tie rods, which gave us a pretty accurate picture of what was happening at the contact patch." This information was used to control the torque from the electric motor, and replicate the feel of a purely mechanical steering system. Work is continuing on this system to filter out the extraneous tactile information.

"In a mechanical system," says Guldner, "you either have good feedback-you feel everything-or you have poor feedback. The truth, however, lies somewhere in the middle." For example, when a tire comes close to its handling limit, the torque felt through the steering wheel decreases by an amount too small for most drivers to detect. Only when the tires begin to squeal and the amount of steering lock increases, do most drivers begin to take notice.

"This physical phenomena is suppressed by the hydraulic assist in a conventional system," states Guldner, "but we can enhance it electronically and amplify the effect at the steering wheel." Should the driver still add, or subtract, too much steering lock for the situation, the steer-by-wire system would change the angle of the front wheels based on information from the vehicle stability system, and independent of the driver's input.

"By tying into the other systems, you can cause the vehicle to respond much more quickly, and draw on a combination of systems to react to the situation," he says. "It's much like the way stability systems work today, except that you can draw on all of the vehicle's dynamic control systems to provide the safest, most efficient course of action."

In addition, Guldner expects drive-by-wire to provide features and functions not possible with conventional systems. One example he gives is a speed-dependent variable ratio steering ratio. At low speeds the steering wheel would be easy to turn, and small inputs would create large changes in the angles of the front wheels. Parallel parking could, for example, take as little as 20[degrees] of wheel movement in either direction. As road speed increases, the relationship between input and response would trend toward conventional values. "With an integrated system," says Guldner, "you can create a very agile, very responsive vehicle--even if the vehicle is a large one. So your limousine can feel like a sports car."

Automotive Industry

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

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

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