By-Wire Cars Turn the Corner

Replacing a car’s hydraulic systems with wires, microcontrollers, and computers promises better safety and handling—but will drivers buy it?

14 min read
Illustration of a by-wire system.
Illustration: Motorola

Picture this: you’re driving down the highway on a gray and rainy day when you hit a puddle and start to skid, veering into the adjacent lane. Automatically, the brakes on each wheel adjust to stabilize the car as you and the car steer in the direction of the skid. Dampers and roll sensors feed information back to a computer that returns optimum damping levels for the car’s suspension, helping to stabilize the car and keep the wheels firmly on the pavement.

Better fuel economy, too, can be attributed in part to the car’s by-wire electronics, specifically throttle-by-wire. An electronically controlled throttle can maintain optimal airflows for all driving conditions. One controlled by your foot on the accelerator, however, doesn’t deliver the same performance.

Carmakers, according to Joe Ziomek of JFZ & Associates, an automotive consulting firm in Islamorada, Fla., see reductions in the number of traffic fatalities and injuries with the use of by-wire systems. Industry experts believe such systems may prevent up to 30 percent of traffic fatalities, the same percentage as airbags.

Someday, by-wire systems will automatically steer the car and reduce its speed from 80 km/h to 40 km/h to 0 km/h without specific action on the driver’s part. Instead, input will come from a built-in positioning system, road-condition sensors, and nearby cars to determine the safest path for vehicles likely to collide. The necessary calculations will be made by powerful on-board computers. But that is a decade or more away and requires more computing and control than by-wire systems are initially expected to have.

“Fundamentally, though,” said Brian Murray, manager of safety systems engineering for Delphi Automotive Systems Corp., Saginaw, Mich., “this is what [near-term] by-wire systems are all about—putting a computer between the driver and whatever is being controlled with the intention of making the driver or the car’s performance better, not [taking complete control of the car from] the driver.”

The basic concept of by-wire sounds simple enough: replace the car’s mechanically linked hydraulic systems—steering and braking, for example—with electronic ones. By-wire systems began to be installed well over a decade ago, first in military and then in commercial aircraft.

In a “true” by-wire system, there would be no hydraulic backup to the electronic system; therein lies a cause for carmakers’ concern. Drivers count on the fact that the brakes and steering work when and how they are supposed to, thanks to hydraulic systems. Carmakers just don’t know how drivers will react to the wires, computers, and microcontrollers. They see drivers asking “Will I need to reboot my brakes instead of adding brake fluid?” and walking away from by-wire cars.

Another basic hurdle automakers face is that no industrywide standard exists for a by-wire system. There is no set specification for the electronic control of a safety-critical system like braking or steering. While automakers agree that having such a standard will help both in winning public confidence in by-wire systems and in designing and implementing such systems, they have yet to agree on one. What’s more, this standard needs to work for all safety-critical functions under the by-wire umbrella [see illustration] like steer-by-wire (for front and rear steer), throttle-by-wire, and brake-by-wire.

By-Wire System illustration

By-Wire Systems Replace Mechanical Systems: As the mechanical systems in cars give way to wires and microcontrollers, performance and safety will improve. Electric throttle valve control and direct fuel injection have served some car models for at least five years. Active suspensions will work with brake-by-wire systems to enhance handling and stability when the latter start appearing on 2004 model year cars; steer-by-wire may come as early as 2002. Electrically assisted power steering is on some luxury cars today. Additional power will be provided by 42-V technology. Click on the image to enlarge.

Making the move

The transition from mechanical systems to the electronic systems of by-wire is being done piecemeal, for now at least. Take, for example, throttle-by-wire, also called drive-by-wire. It has been used in diesel-fueled cars for about five years now. For at least three years, it has been on some high-performance gasoline-fueled cars, like the Corvette. In conventional cars, the throttle plate regulates the amount of air used in the combustion process. Pressing the accelerator determines the position of the throttle plate within the air intake system for the engine cylinders. A mechanical linkage transmits the accelerator pedal position to the throttle plate.

In throttle-by-wire, “the throttle [plate] is now controlled by a motor controlled by computer or microcontroller that is independent of the accelerator,” explained Murray. In a throttle-by-wire car, a microcontroller determines the correct throttle plate position. If the driver needs sudden acceleration and stomps on the accelerator pedal, a sensor on the pedal transmits the driver’s pressure on the pedal to a microcontroller, which calculates and relays the correct throttle position to the motor actually moving the throttle plate.

Electronic control of this plate offers improved fuel economy and emissions by maintaining optimal throttle conditions at all times, something human drivers cannot do. Because it’s under the hood, hidden from consumers, many drivers may not even know that their foot on the accelerator no longer controls the throttle plate. Carmakers introduced this by-wire system first, because the throttle’s primary impact is car performance, and not safety. The idea is for drivers to gain confidence in by-wire systems in general, thus paving the way for more of them.

As with throttle-by-wire, the steer-by-wire and brake-by-wire systems replace mechanical with electronic links. Or at any rate, they will do so in model years 2002 and 2004, respectively, when they begin to appear on cars and even in pickup trucks.

Steer-by-wire comes in two flavors, one for front steer and one for the rear wheels. Front steer-by-wire eliminates the mechanical connection between a vehicle’s front wheels and its driver, so turning the steering wheel does not directly turn them, as it would in a car today. Rather, it sends a signal to a motor at each of the front wheels. The signal instructs the motors to turn the wheels.

Rear steer-by-wire tightens the turning radius and increases vehicle stability, so the rear wheels don’t just follow the lead of the front wheels. For example, when parking or driving at lower speeds, the wheels in Delphi’s rear-wheel steer-by-wire system turn in the opposite direction to the front wheels during tight turns, providing any size car or pickup truck with the agility of a small car. Delphi reports that with this system, called Quadrasteer, it has reduced a full-sized sport-utility vehicle’s turning circle radius from 13.7 meters to 10.3 meters. A Honda Civic turning radius is 10 meters.

Brake-by-wire does everything: the antilock and traction-control functions of today’s antilock braking systems plus brake power assist, vehicle stability enhancement control, parking brake control, and tunable pedal feel, all in a single, modular system. With today’s antilock brakes, the electronics do the brake-pedal pumping instead of the driver, but otherwise they are still mechanical systems. Pressing the brake pedal produces hydraulic pressure on the brake pads, which squeeze the brake disk to produce friction and stop the car.

Helping the driver

Safety and stability are at the heart of the push to develop automotive by-wire technology, especially for a braking-and-steering combination. “The goal is to make the average driver as skilled as a professional test course driver in bringing the vehicle back to a safe and stable condition from an unsafe one,” explained Delphi’s Murray. Controlling the brakes at each wheel individually, as is done in brake-by-wire, and helping the driver steer through a skid may prevent an accident.

There are vehicle stability systems, also called yaw control packages by automotive suppliers, in cars today that do something similar, noted Jim Petrowski, development program manager at Delphi Automotive. They, too, are primarily focused on braking. These systems electronically control the brake pressure applied separately to each wheel, which therefore gets the amount of braking it needs to help guide the car out of a skid, for instance. Models with brake-by-wire in 2006 or 2007 could be using computer control to adjust the steering by braking each wheel individually on the basis of input from collision avoidance radar, yaw rate sensors at each wheel, and lateral acceleration sensors.

But even when brake-by-wire and steer-by-wire systems start appearing on production vehicles, “some in the industry may not consider it ‘true’ by-wire technology since hydraulic backup systems will be in place, just in case,” cautioned Delphi’s Murray.

By-wire systems also have manufacturing and design advantages. Brake-by-wire modules would plug into the corners of a car, near each wheel, connected by wires to a microcontroller connected by wires to the brake pedal, making car assembly easier and faster than “putting a plumbing system in a car,” noted Petrowski [see figure]. “Plus, things need not be quite so rigidly aligned with by-wire.” Also, no hydraulic lines or fluid means that there is no fluid to renew or spills that can ruin paint.

On a steer-by-wire system the actuators can be localized at the corners of a car and at the steering wheel. Without a mechanical steering shaft, designers can rethink the crumple zone to increase safety. Crumple zone is the name given to the sheet metal surrounding the engine, so called because it is designed to crumple up like an accordion in the event of a head-on collision. This design absorbs the force of a collision so the steering column remains in the engine area instead of being pushed against the driver’s chest. Without a steering column, structural support placement and materials can be reexamined.

Cars with steer-by-wire may not even have a driver’s wheel. “Concept cars have put a joystick in place of the steering wheel. Without the need to put the mechanical linkage in an exact spot for steering—the wires can run to anywhere in the cockpit of the car—joysticks make sense,” noted Pat Torossian, manager of advanced driveline and chassis at Visteon Corp., Dearborn, Mich. “Carmakers would no longer need to adjust their designs to accommodate left- and right-hand drive configurations in the same vehicle model,” he concluded.

Plus, electronic systems—wires and microcontrollers—weigh less than hydraulic pumps and mechanical actuators. That translates into improved fuel economy, though just how much is uncertain at this point.

Gently apply the brakes...

Luxury carmakers like BMW, Mercedes, and Audi are targeting the 2004 model year for the introduction of brake-by-wire, according to The Hansen Report on Automotive Electronics, an industry newsletter. Whether or not these will be fully electronic brake-by-wire systems without hydraulic backup is still being debated. Ziomek expects that European carmakers will implement brake-by-wire without the backup sometime within model years 2005­07, U.S. carmakers a year or two after that.

There are “lots of forms of brake-by-wire,” explained Jim Trent, chassis systems operations manager with Motorola Inc., Austin, Texas. Electromechanical braking relies on electronic control of an electric motor to provide braking without a hydraulic backup. It’s likely to enter vehicles within the 2008­10 model years, and it will need 42-V technology, according to Trent. He expects to see electromechanical braking employed in some compact cars by 2004­05, so that reliability data can be collected and analyzed.

The 42-V technology Trent refers to is the system voltage being embraced by carmakers and suppliers who see the standard 12-V battery as insufficient to supply the power demands of cars expected to roll off assembly lines by 2005. Whether the 42-V system voltage will be supplied by a second lead-acid battery and the existing 12-V battery remains uncertain. Carmakers will probably decide the fate of the 12-V battery individually, just as they would decide whether a car has two doors or four.

Considered “true” brake-by-wire because it does away with the hydraulic link between the brake pedal and brake pads, electromechanical braking puts a motor-driven caliper supplied by 42-V power at each wheel. Electronics control and operate the caliper through a multiplexed signal sent by a sensor in the brake pedal. Microcontrollers on each wheel are connected to a master controller with the ability to interrupt the signal between the pedal and the caliper. (The caliper is basically the mechanism that squeezes the brake pads against the brake disk to produce friction that slows the car.)

4.Drivebywire.f4b Compare that to the conventional disk brakes on a car today. Pressing the brake pedal applies hydraulic pressure (via the hydraulic fluid) to force the pads in the caliper assembly against the spinning brake disk on each wheel. The friction between disk and pads slows the car.

Electrohydraulic braking falls somewhere between conventional hydraulic systems and electromechanical systems. It will show up on some luxury cars in the 2002 model year, and could also be considered brake-by-wire, according to Trent. Though each wheel is assigned a conventional hydraulic caliper, an electronically controlled valve controls the actual braking force (the pressure of the fluid it releases). Instead of the pedal directly pushing the hydraulic servo, brake fluid pressure is governed by an electric pump connected to the brake pedal by wires. Press the pedal, activate the pump, brake the car.

Beyond just working out the technical kinks, winning over the driving public to brake-by-wire will be crucial. When antilock brake systems (ABS) first came out, for example, high accident rates for cars equipped with ABS made consumers wary. In addition, ABS cost more and required a different approach. Once drivers learned to apply steady pressure and not pump the brake pedal themselves, wariness gave way to acceptance. In fact, nearly a decade after their introduction, antilock brakes are considered standard equipment by most car owners, or at least as an option worth the purchase price.

Carmakers hope drivers will come to find brake-by-wire equally desirable. They are introducing by-wire systems slowly, initially “making brake-by-wire transparent to the customer,” explained Visteon’s Mike Bullion, manager of advanced technology for chassis. “Then, as incremental improvements are made, and hydraulics eliminated from even backup roles, public confidence in the technology will build.”

...then steer the course

With the 2002 model year, one Detroit carmaker plans to put a steer-by-wire system on the rear axle of a pickup truck. (Concerns about customer acceptance of the by-wire system prompted the carmaker to request anonymity.) The Delphi-built system, known as Quadrasteer, integrates an electromechanical actuator into the rear axle, while the front-wheel system is a conventional hydraulic system. The combination gives the truck four-wheel steering, but the rear-wheel steering can be turned off, leaving the truck with two-wheel steering on the front wheels. In effect, the front-wheel steering on the truck will work like the power-steering system on cars today: turning the steering wheel opens the control valve proportionately to the turn, releasing hydraulic fluid that (through flow maintained by an engine-driven pump) activates the actuator. In turn, the actuator moves a piston connected to the steering linkage providing extra torque to assist the driver in turning. The steering wheel is mechanically linked by the steering column to the steering linkage (the rack-and-pinion in the conventional rack-and-pinion steering), which then turns the wheels of the car.

The goal [of by-wire] is to make the average driver as skilled as a professional driver in stabilizing the car

Though rear wheels usually have no steering components, the steer-by-wire rear wheels on the pickup will have them. In general, steer-by-wire omits the control valve, the hydraulic pump and fluid, and the mechanical link between the steering wheel and steering linkage. Steer-by-wire has its own set of parts: an actuator to manage the steering forces under the hood of the car, typically an electromechanical device with a motor to steer the wheels; a distributed control system made up of a network of controllers; and a mechanism to electrically transmit the road feel back to the steering wheel and the amount of steering wheel turn to the control system [see figure]. “The idea is to re-create the driving feel by providing some feedback,” explained Petrowski.

Drivers “feel” the road today through the mechanical link to the wheels: grooved pavement, for example, makes the wheels vibrate. The mechanical links between the wheels and steering wheel vibrate in turn. Similarly, the mechanical links provide a certain "feel" to the steering wheel when the driver turns it that must be recreated in a steer-by-wire system using sensors on the steering wheel, the suspension, and the wheels.

That’s a huge challenge, Petrowski added. “The idea is to emulate the steering feel from all of the inputs of the car. That requires an active system constantly changing in response to the conditions of the vehicle.” Designers have looked to the videogame world for pointers on force-feedback technology. But real cars require a more rugged technology than their on-screen counterparts. “After all,” Petrowski said, gaming “joysticks went from no feedback to some. With steering, we have to start by delivering what people get now—and that’s a lot.” Petrowski’s group is working on the technology, but a delivery time on a working product is not yet set.

Like brake-by-wire, steer-by-wire also offers greater configurability. “Carmakers could put a luxury feel into an economy car, or adjust the steering during cruise control operation to be a touch less responsive, so that a sneeze doesn’t prompt a lane change,” noted Murray.

“From a technology perspective, it was electrically assisted power steering that started us on the steer-by-wire path,” said Petrowski. Instead of an engine-driven hydraulic pump, the system uses an electrically controlled motor activated when the driver turns the steering wheel. Electrically assisted power steering, he explained, “is an on-demand system instead of one that’s always on. It uses 5 percent of the energy that the hydraulic system uses. That is a powerful motivator to develop the technology further.” Electrically assisted power steering is currently in use on several smaller car models, including the Fiat Punto in Europe and Japan’s Nissan NSX.

Besides winning public confidence, carmakers and by-wire system suppliers hope for a clarification of legal issues. Who is to blame in a lawsuit if the by-wire system is correcting an oversteer on the driver’s part, say, and an accident still occurs? Is the driver 50 percent responsible, the carmaker 25 percent, and the steer-by-wire supplier the rest? These issues may begin to be resolved only after the first lawsuits are settled.

Clearing the speed bump

If by-wire is to deliver on the safety promise of reducing deaths and injuries in accidents, the by-wire systems in a car must be able to communicate with one another. That means there must be a communication network that will enable by-wire systems to work both individually and together, smoothly, safely, and efficiently, emphasized Kurt Sievers of Philips Semiconductors, in Stuttgart, Germany, where he is system marketing manager in the global automotive sector.

By-wire systems integrated in one network offer a functional advantage as well, noted Delphi’s Murray: “To control the car requires a combination of steering and braking, so coordination is the key,” he observed. Having one network of controllers for both the brake- and steer-by-wire systems is clearly more efficient than having a separate network for each, because signal delay is reduced and system resources are shared.

To handle the communication needs of the by-wire network, the automotive industry seems to be converging on a time-triggered architecture, said Murray. Time-triggered architecture simply means that actions are carried out at well-defined points in time. So all the nodes in such a network have a common time reference based on their synchronized clocks. Messages are prioritized so that time is made available to higher-priority actions on the network, even if another message is being sent about another action or event.

The front-runners

The two leading time-triggered architecture candidates are time-triggered protocol (TTP) and FlexRay. TTP has been around for some 15 years and is backed by Audi, Volkswagen, and TTTech Computertechnik AG, among others. Late last year a consortium formed by BMW, DaimlerChrysler, Motorola, and Philips Semiconductors announced its own time-triggered architecture, dubbed FlexRay. A third protocol candidate—Time-Triggered Controller Area Network (TT-CAN)—is considered a remote possibility because its development is just beginning. Still, it cannot be ruled out because it builds off the protocol governing the networks that control such systems as power windows and door locks in cars today.

At first glance, FlexRay and TTP seem almost identical. Both are time-triggered architectures but have different data rates and transmission media. FlexRay is designed for optical fiber for a 10-Mb/s data transmission rate; but it can also run on copper. TTP uses copper for only 2 Mb/s, though developers at the University of Vienna are exploring fiber.

Sievers feels that FlexRay will first appear on copper, which is today’s state of the art in cars. But fiber is likely to be the future state of the art. It offers speedy data transmission plus weight and electromagnetic compatibility advantages over copper; but bending fiber to run throughout a car is not yet practical. Interestingly enough, FlexRay was designed for both copper and fiber because BMW and DaimlerChrysler, the two automakers in the consortium, are backing different options.

According to Sievers, the thinking behind FlexRay, which is based on a BMW protocol called ByteFlight, goes something like this: “The wires must provide the same safety as a mechanical system. So what is safeguarded in a mechanical system? The hydraulic fluid that provides the working force. The information the wires carry is the equivalent of the hydraulic fluid, so it must be protected. The protocol controls and organizes the information. The information must arrive at a clearly deterministic point of time—that is to say, at the correct time. To avoid an indeterministic point in time requires predictability.”

“Deterministic” simply means that data is sent at a predetermined time (time-triggered), in contrast to any time (event-triggered). In terms of FlexRay and TTP, the data messages to and from each node on the network are scheduled against the network’s global clock. Thus clock synchronization and periodic resynchronization are required to keep all network nodes on the same schedule and in the same time frame. TTP and FlexRay have built-in mechanisms for clock synchronization.

The by-wire protocol must also be fault tolerant. A car operates in a rugged environment. Wires run from a very hot engine compartment to an icy-cold trunk. And as the car moves, things shift, including wires and connectors. So both the bus system (that is, the wires) and the protocol must safeguard against wire breaks, corrosion at connectors, and shorts. It does so with redundant systems.

The dark horse protocol candidate is Time-Triggered Controller Area Network, or TT-CAN. Basic CAN (without the time trigger) is used in cars now for control and communication at up to 1 Mb/s by way of serial data transmission. But it is an event-triggered protocol, meaning that it processes commands as they occur, not by priority. Because of its slow speed and inability to prioritize, automakers think CAN is not suited for safety-critical applications like electronically controlled braking or steering. Whether TT-CAN developers are able to overcome these problems is unknown. “It’s always difficult to expand a protocol to something it wasn’t originally designed for,” said Sievers.

Avoid the pothole and drive on

One last pothole on the by-wire road is power. The 12-V system standard in cars today already falls short of anticipated needs for future models [see IEEE Spectrum, May 2000, “Automotive electronics power up,” pp. 34­39]. But despite the addition of power-hungry by-wire systems, the amount of hydraulic pump power saved is expected to produce a net savings in energy consumption—once the hydraulic backup systems are gone. Petrowski’s steering group is recommending the 42-V system as the most appropriate to handle steer-by-wire power needs for most cars on the roads.

Robert LeFort, automotive group vice president at Infineon Technologies Corp., Northville, Mich., agrees. LeFort’s company makes power drivers for the electrically actuated motors that are replacing the hydraulic systems. “Depending on the application, current requirements can range from 20 amps to over 100 amps,” he explained.

Beyond fixing the power shortage and settling on a single communication protocol, what’s next on the by-wire horizon? Intelligent highway systems, responded Visteon’s Bullion and Torossian. They envision a day when a car will drive itself to a destination set by the driver by using its by-wire systems in conjunction with intelligent highway systems. But given the lack of infrastructure and the prohibitively high costs, they warn, don’t expect to see such a scenario for at least another decade or two.

To Probe Further

In the fifth edition of Understanding Automotive Electronics, published by Newnes in Woburn, Mass., author William B. Ribbens covers the basics ofsuch systems with clear explanations and diagrams.

The basics of 42-V technology and why it is needed in future automobiles are outlined in “Automotive electronics power up,” IEEE Spectrum, May 2000, pp. 34­39.

Both of the primary time-triggered architecture candidates—FlexRay and Time-Triggered Protocol—maintain comprehensive Web sites with information on the protocol specifications, partners, and development timelines. For FlexRay, see, and for TTP, see

This article is for IEEE members only. Join IEEE to access our full archive.

Join the world’s largest professional organization devoted to engineering and applied sciences and get access to all of Spectrum’s articles, podcasts, and special reports. Learn more →

If you're already an IEEE member, please sign in to continue reading.

Membership includes:

  • Get unlimited access to IEEE Spectrum content
  • Follow your favorite topics to create a personalized feed of IEEE Spectrum content
  • Save Spectrum articles to read later
  • Network with other technology professionals
  • Establish a professional profile
  • Create a group to share and collaborate on projects
  • Discover IEEE events and activities
  • Join and participate in discussions