Auto Service World
Feature   February 1, 2002   by Auto Service World

Knowledge Building: X-By-Wire

Automotive types have come to think of a car and its parts as a collection of interconnected systems, an anatomy of sorts: The brake pedal’s connected to the master cylinder, the master cylinder’s connected to the brake line, the brake line’s connected to the caliper, etc.

Sometime soon, this type of arrangement will be a thing of the past, displaced by drive-by-wire systems that connect the input device–gas pedal, brake pedal, steering wheel, gear shift–to the “business end” of the system by wires that carry electrical inputs; hydraulic lines, cables, linkages and rods would be a thing of the past. The benefits in automotive packaging are obvious: reduced weight and increased control.

We’re probably closer to having an influx of these X-by-wire technologies (so-termed to include an unknown multitude of systems) than you might think. Aircraft have been using similar systems for years, and several General Motors vehicles, including the C5 Corvette and the new Cadillac CTS, employ a throttle-by-wire system. In fact, virtually every major automotive manufacturer has something in the works for X-by-wire. How far from steer-by-wire are we when some cars already have a steering angle sensor to provide input for stability control? Add a good strong actuator to drive the steering rack and you could do away with any direct connection between steering wheel and the wheels that steer; goodbye steering column. That’s the theory anyway.

With this in mind, bearing maker SKF, in conjunction with auto stylist Bertone, put together the Filo concept car that includes a variety of X-by-wire systems. Steering, accelerator, brakes, gear change and clutch are all controlled by wire, the same technology used in the controls of a modern aircraft. Let’s look solely at the brake-by-wire system for brevity’s sake.

The Filo car’s braking system was developed jointly with Brembo, SKF’s partner in brake-by-wire.

At the heart of each system there is an SKF smart electro-mechanical actuating unit under intelligent control. Signals from the driver’s control unit are interpreted by logic systems developed by SKF that ensure appropriate behavior from the individual by-wire systems. All systems have built-in redundancy and a back-up power supply similar to those used in fly-by-wire systems. SKF’s drive-by-wire technology has also been chosen for use in General Motors’ skateboard-like AUTOnomy concept vehicle, that combines fuel cells with drive-by-wire systems.

The designers of the Filo made a decision to eliminate all pedals and incorporate all vehicle functions into what might be called the steering wheel, though that term seems inadequate when you consider that every function–braking, acceleration, wipers, etc.–is handled by the component. (That part strikes me as a bit far out for real-world application, but it does demonstrate the point that drive-by-wire allows for tremendous design flexibility.)

Control of the braking system is duplicated on both the left and right yokes of this “steering wheel,” and is activated by squeezing the handgrips. The mechanical design incorporates a progressive resistance and a small, but clearly discernible, free-play at the beginning of the movement, providing the driver with a tactile indication of when the brakes begin to operate. Squeeze harder to brake harder. The system controls each brake as an individual subsystem, under an umbrella control arrangement for the complete vehicle braking system.

These systems are obviously going to take a while to get to the aftermarket and quite a bit of technical training to understand, but any fears of the future of repair should be subdued to some degree.

Take a look at such a system. At each corner, there is an actuator integrated into the back of the caliper. This provides the driving force in place of the hydraulic pressure. What looks like brake line is actually heavily shielded data cable. Look closer still and you’ll see that within the caliper is a familiar piece of hardware: a set of brake pads. And, whenever these systems hit the market, there’s still going to be a need for wear parts, regardless of the technology behind them.

— Andrew Ross

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