X-by-wire systems promise to revolutionize the way cars steer and brake by eliminating physical connections and relying solely on electric signals. X-by-wire developers such as Delphi, whose steer-by-wire system is shown here, say that by going electric handling performance will be enhanced and safety increased.
The automotive industry is getting more and more disconnected–and that’s a good thing. Steering and braking systems that have always had physical mechanical connections will in coming years be replaced by those in which an electronic signal communicates the driver’s intention to turn or stop. Converting to these so-called “X-by-wire” systems will offer a panoply of benefits for both automotive design and production, but pose unique challenges as well.
Eliminating Hydraulics. Perhaps the most apparent benefit of switching to steer- and brake-by-wire systems is the reduction in parts and the elimination of hydraulics. Electric actuators placed at each wheel do the work of current hydraulically assisted steering and braking setups, eliminating the vacuum booster, master power, hoses, clamps and hydraulic fluid. So, the labor that is associated with the entire “evac and fill” process on the assembly line can be re-allocated to other value-added tasks. Suppliers envision corner modules that include all necessary electric components and can be quickly bolted on and plugged in. This arrangement would give designers much more packaging freedom and improve crashworthiness by removing large centrally located components that intrude into the cabin in a collision.
Development time for new vehicle subsystems will be greatly reduced, since most of the tweaking will be of software, not of cast or machined parts. Rex Struble, product business director, Steering, TRW Automotive, says of steer-by-wire development, “We can do in a week what would take a hydraulic system months in terms of taking a hydraulic valve and going through an iterative development process. Performance parameters can be calibrated with a laptop.” It is a whole lot quicker to write new algorithms than to produce and test prototype parts.
In production, parts can be commonized more easily both within one maker’s line up and across OEMs for similarly sized vehicles. Sensors for features like anti-lock braking and vehicle stability control can be shared, bringing costs down and potentially making it more feasible to equip even lower-priced vehicles with more safety features.
Fuel economy will be improved by removing the hydraulically assisted systems that constantly put a load on the engine via the accessory drive. Electric arrangements draw power only when it is needed. Jim Petrowski, manager, Customer Solutions, Delphi Automotive, says that switching from hydraulic to electric steering, “Is one of the single biggest things you can do for a car to increase energy efficiency.” He points to the electrically assisted steering unit supplied by Delphi for the current Fiat Punto as a real-world example of energy savings: he claims it only uses about 5% of the energy of a hydraulic system, largely because it is off whenever the car is going straight. Further fuel savings would come from electrically actuated brakes that produce zero drag. In conventional systems, the brake pads are always dragging slightly on the rotors because there is no mechanism to retract the pad, other than the mechanical force of a spring. But with brake-by-wire, the electric motor can be programmed to reverse the precise amount necessary to keep the pad out of contact with the rotor, but close enough to ensure quick braking performance.
On the environmental side, not only will better fuel economy reduce overall auto emissions, but the elimination of hydraulic fluid means one less noxious substance that has to be transported, stored, and cleaned up.
By-wire systems will also allow engineers much greater freedom to quickly dial in desired road manners. Common hardware can be programmed for different brand identity performance parameters, so automakers can get the cost and development benefits of commonized parts and still give each brand its own unique driving feel.
The Architectural Limit. The key drawback to implementing X-by-wire systems is the current 12-volt electrical architecture of vehicles. Electrically actuating brakes to stop a large car or an SUV requires more power than a 12-V system can reliably handle. Steer-by-wire can get by with the current power supply if it is applied to smaller vehicles, but it too needs more power to turn larger vehicles. That means widespread application of X-by-wire is dependent on the introduction of a 42-V architecture. “We are waiting until the 42-V system becomes a reality before we think we will find serious customers for brake-by-wire,” says Phillip Headley, chief engineer, Advanced Technologies, Continental Teves. “We have decided that we will do our designs with 42v because that’s what is coming in the future.”
In the meantime, X-by-wire developers are concentrating on surmounting another major obstacle: building in enough redundancy to ensure that the driver can steer and stop a vehicle if the primary power source fails. Since unlike in current systems there is no physical connection between the steering wheel or the brake pedals and the wheels they control, a complete electrical failure means both steering and braking are out. To avoid this, engineers are busy designing fault-tolerant systems in which a back-up component takes over immediately when a primary system fails. This means added complexity for the overall system, with the addition of a second battery (or an ultracapacitor), and redundant wiring and data busses. It also means a lot of up-front development effort. Phil Cunningham, product director, Chassis Control, TRW Automotive, observes, “The amount of work we put into ensuring that signals from one system do not generate an adverse reaction from another is already significant. When you take that to systems that have no hard connection, you have to make sure that those systems are even more robust than they are today.” This is time-consuming. And inasmuch as time is money, system costs will be higher initially than conventional setups based both on development complexity and the high level of redundancy. But given the much greater ease of assembly that can be achieved, experts think that the overall costs will be comparable once mass production-level volumes are reached. And since electronics have historically gotten cheaper as time goes on, in the long run X-by-wire should be clearly more cost-effective.
Early X. Though true X-by-wire systems are a few years away from mass production, the interregnum between the reigns of hydraulic and electric systems will see technology marketed that mixes aspects of both. Electrically assisted steering already exists on millions of cars in Japan and Europe and has just been introduced in North America on the Saturn VUE. Next year Delphi will introduce Quadrasteer which utilizes hydraulics to steer the front wheels and electric for the rear. Mark DePoyster, chief engineer, Advanced Chassis Systems, Delphi Automotive, envisions a similar braking system: hydraulically assisted front brakes and electric calipers in the rear. He says, “This allows the OEMs to get experience using electromechanical brake actuators without having to go to the full safety-critical, fault-tolerant architecture required by four corner brake-by-wire.”
Another variation on the theme is electrohydraulic technology that keeps some components of conventional systems while replacing others with more efficient electric parts. Electrically
Powered Hydraulic Steering (EPHS) essentially takes the steering pump off of the engine, eliminating belts and pulleys and simplifying hoses, and replaces it with an electric motor and a controller. Electrohydraulic Braking (EHB) gets rid of the vacuum booster and replaces the current modulator with one that includes a high pressure accumulator. Both systems offer greater ability to adjust performance characteristics, but they do not eliminate the use of hydraulic fluid which is one of the key benefits of an all-electric system.
Continental’s Phillip Headley thinks that the use of electrohydraulic systems is going to expand very rapidly. (His company will supply EHB systems for “more than one maker, and more than one platform within a maker” for the 2003 model year.) But instead of being replaced when true X-by-wire arrives, he sees the two systems co-existing for quite some time.
TRW’s Phil Cunningham sees the implementation of electrohydraulic systems as the way to get the right technologies in place to cost-effectively replace conventional setups. Once OEMs have sampled the environmental, weight-saving, and fuel economy benefits of a system like EHB, the ground will be prepared for true by-wire systems that will offer even better functionality.
Just Like the Game. Beyond its prosaic advantages, X-by-wire will lead to an era of unprecedented electronic control of vehicles’ basic functions–with or without driver input.
Collision avoidance systems will be able to take over control of a car’s lateral movement without yanking the steering wheel out of the driver’s hands. And intelligent highway systems will interact directly with cars to enable such practices as platooning, where vehicles travel at high speed in closer proximity than all but the most egregious tailgaters would be comfortable with. But perhaps the most disconcerting eventuality that X-by-wire will enable is the complete elimination of the steering wheel and brake and accelerator pedals. The safety and packaging advantages are undeniable, but customer acceptance would be a tough sell. Then again, by the time X-by-wire hits the market, a large part of the buying public will have spent countless hours maneuvering video game characters with joysticks, so maybe they won’t mind driving their cars with them, too.