Detroit-Concerned about the looming prospect of drive-by-wire technology, automotive suppliers are pushing for the adoption of a time-triggered bus protocol for safety-critical systems, such as brakes and steering.
Steer-by-wire, brake-by-wire, and throttle-by-wire systems may need time-triggered electrical bus architectures.
Gathering at the Fourth Annual TTP (Time-Triggered Protocol) Forum, associated with the SAE 2000 World Congress, suppliers called on automakers to unify behind a standard protocol for safety-critical systems. The need to do so is critical, they say, because many automakers are planning to introduce drive-by-wire systems by the 2005 model year. "We need an OEM to step up and drive this technology into an industry standard, because 2005 isn't very far away," notes Jim Trent, operations manager for Motorola's Advanced Vehicle Systems Group.
Up to now, suppliers and automakers alike have expressed concerns that CAN buses may not be sufficient for future safety-critical applications. Still, no standard architecture exists as yet.
CAN buses are fading as a potential candidate because they are event-based, and therefore subject to the so-called "babbling idiot mode," in which a faulty sensor transmits a stream of meaningless data and locks up the controller. That, say engineers, is unacceptable for safety-critical systems. "If you lose your CD player, who cares?" says Mark DePoyster, chief engineer for chassis systems at Delphi Automotive Systems Customer Solutions Center (Dayton, OH). "But the chassis bus can't afford to go down."
To prevent that problem, engineers say they need a bus that offers fault tolerance and determinism. Fault tolerance enables the electrical bus and controllers to continue operating in the presence of a failed sensor, short circuit, transient software glitch, or other problem. CAN buses can often overcome such problems now, engineers say, but as electronic nodes proliferate in future vehicles, most believe that conventional CAN architecture will lack the determinism need for safety-critical systems.
Time-triggered buses solve that problem, suppliers say, because they are fault tolerant and not event-based. "If the controller is supposed to get a steering wheel input and it's not there, you need a system that will react and put the vehicle into a safe mode," notes Lawrence W. Tomczak, director of engineering for Delphi Automotive Systems (Saginaw, MI). "Time-triggered buses are always looking' for a signal, so they know if they've missed it."
At SAE, Motorola demonstrated a brake-by-wire system that uses Time-Triggered Protocol, a system developed by TTTech Computertechnik GmbH (Austria). Such systems could be in vehicles by 2005, engineers say. "People want to go to drive-by-wire because it allows enhancements that just aren't available any other way," explains Brian Murray of Delphi Automotive. "But you can't get there if you don't have the right electrical architecture."