Steer-By-FlexRay

By-wire steering, braking and propulsion in vehicles may be years away from mainstream applications, but the strategy inches closer to fruition with General Motors Corp.'s Chevrolet Sequel hydrogen fuel-cell vehicle, which was made available for recent test drives.

The Sequel employs the advanced new FlexRay communications protocol, which allows mission-critical electronic devices within the vehicle to speak with each other at high speed, in real time and without interfering with each other.

The need for a more robust vehicle communications “bus” to accommodate a growing number of electronic devices with more processing power — particularly by-wire systems — led BMW AG and DaimlerChrysler AG to form the FlexRay Consortium with semiconductor suppliers Philips and Motorola in September 2000.

Additional core partners in the consortium are GM, Robert Bosch GmbH, Freescale Semiconductor and Volkswagen AG. The group includes 17 premium associate members and 57 associate members.

As soon as next year, production vehicles will employ the FlexRay communications protocol, sources say. Extensive discussions about FlexRay are expected at the Convergence auto electronics conference at Detroit's Cobo Center Oct. 16-18.

FlexRay enables electronic controllers in a vehicle “to talk to each other in ways that are not possible today,” says Brian Daugherty, director-global advanced development at Visteon Corp., which supplied the steer-by-wire system for the Sequel.

The traditional power steering components, such as the hydraulic pump and fluid supply lines, have been replaced with two electric motors to steer the front wheels and two additional motors in the rear — one for each wheel. The Sequel has 4-wheel steering to improve its turning radius.

The motor-driven system is similar to electric power steering, which Visteon manufactured until that product line returned to Ford Motor Co. last year as part of Automotive Components Holdings LLC.

But the Sequel's steering column represents a significant advancement beyond systems in production today. As the driver turns the wheel, three sensors that are part of the Driver Interface System collect the directional data and convey it almost instantaneously to four electronic controllers, Daugherty says.

Those four controllers are connected by a FlexRay network, which consists of a redundant, failsafe, wired link to allow rapid communication with each other and with other by-wire systems — namely, braking and propulsion.

Each of the four controllers on the steering bus has unique chipsets that speak FlexRay's language. The three steering sensors are designed to check each other's work, Daugherty says. If one sensor sends a faulty message that conflicts with the other two, it is overruled and ignored. The sensor data is constantly updated.

Once the four electronic controllers calculate the steering action based on the driver's command, they send signals to the steering motors to turn the wheels of the Sequel, while FlexRay shares the information with the rest of the vehicle.

It all sounds laborious, but in reality the information is conveyed and acted upon in 1/1,000th of a second, Daugherty says. FlexRay communicates at speeds 10 times faster than conventional high-speed controller area network (CAN) buses common on vehicles today.

The Sequel's steering system is constantly processing enormous amounts of data with regard to wheel position, wheel speed and steering wheel angle.

CAN buses may be ideal for certain applications, but they are not robust enough to enable such high-volume, high-speed communication, Daugherty says.

He compares a CAN bus to an old-fashioned telephone party line, over which several voices can be speaking at the same time, preventing effective communication. A malfunctioning device can become a “babbling idiot” on a CAN bus, hogging the communication line for too long.

FlexRay, on the other hand, uses a time-triggered protocol, which assigns each electronic controller a set time to speak on the bus, after which another device speaks, then another.

Redundancy is built in to FlexRay for this purpose, to allow a faulty message to be detected and ignored, without interfering with other vehicle operations.

Daugherty describes this “real-time, deterministic” philosophy with “built-in fault tolerance” as critical to “preventing collisions” of data on the FlexRay network. “With a CAN bus, you have collisions of data all the time,” Daugherty says. “With FlexRay, you can't put too many talkers on the same bus.”

He says Visteon could not have done the Sequel steer-by-wire system without FlexRay's ability to connect steering information with the rest of the vehicle. Copper wire is used for the FlexRay network on the Sequel, but other media, such as fiber optic cable, can be used as well.

The system sounds expensive, but Daugherty says FlexRay can be applied in ways competitive with today's technology. The Sequel's steer-by-wire system has a mechanical backup that allows for manual steering, with little power assist.

Such advanced technology could contribute to a steering system that provides inadequate feedback to the driver. But the Sequel's steering has drawn positive reviews from journalists who have driven the vehicle; several have noted the steering doesn't feel unusual or rubbery at all.

“It has good highway feel, and it's good for parking maneuvers,” Daugherty says.

A key advantage of steer-by-wire is the unlimited potential to tune the system precisely to match the desired handling characteristics as set out by the OEM customer. For the Sequel, GM told Visteon to tune the steering system for a sporty feel.

GM isn't saying when it will use FlexRay in a production vehicle program.

But GM R&D staff researcher Sanjeev Naik says there is considerable potential for production applications, particularly for achieving effective, high-speed communication among electronic components.

Like with any new technology, Naik says FlexRay has a few nagging issues to work out, particularly with regard to fault tolerance. “The consortium is working on this issue,” Naik says.

He declines to discuss the cost of deploying the FlexRay architecture in a production vehicle, but Naik agrees with Daugherty's assessment that the technology need not be cost prohibitive.

“One thing is for sure,” Naik says. “The trends are favorable.”