X-By-Wire: For Power, X Marks the Spot

Oct. 1, 2004
Drive-by-wire, x-by-wire (XBW) or simply by-wire technology is fly-by-wire technology applied to vehicles. This is just another example of sophisticated

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Drive-by-wire, x-by-wire (XBW) or simply by-wire technology is fly-by-wire technology applied to vehicles. This is just another example of sophisticated aerospace technology being used to solve automotive problems. In this case, the problems range from the need for more precision in electronic engine controls to eliminating continuously driven loads for improved fuel economy. XBW is required for advanced collision control avoidance systems that override the driver's input for brakes, throttle, etc. As a result, a transition from mechanical or hydraulic systems to electronically controlled systems has been under way for almost a decade. One of the main reasons that automakers can investigate and implement XBW approaches today is the availability of a variety of semiconductor ICs that meet the cost targets to provide the control, power and communications required for these systems.


X-by-wire is usually discussed in the context of replacing vehicle controls such as throttle, steering, braking, shifting and clutch. In these systems, the actual control of the vehicle is initially supported by electronically assisted control prior to the elimination of the mechanical linkage. As shown in Figure 1, the only system to go directly to a by-wire technology is throttle-by-wire — a system where the power consumption is low. However, some industry experts have applied the term to a broad range of mechanical system replacements including HVAC-by-wire, wiper-by-wire, lighting-by-wire and door-by-wire [1]. However, the most frequently XBW systems are those shown in Figure 1.

XBW systems eliminate the continuous parasitic losses by providing on-demand use. For example, electric power steering (EPS) only operates when requested by the driver rather than continuously driving a traditional power steering pump. As a result, a fuel economy savings of about 4% can be realized, making the system comparable to manual steering. EPS has an added benefit of providing torque for control when the engine inadvertently loses power making the vehicle safer. EPS is required when power is intentionally removed in hybrid vehicles. In this case, the engine is turned off to conserve fuel, so systems such as power steering must be electrically powered.


Electronic throttle control (ETC) is one of the first major successes for an XBW system with projection of more than 20 million units being sold in 2004 according to Strategy Analytics, a U.K.-based market research firm. Some systems are more electrical control where the computation is performed at the engine control unit (ECU) and the motor on the throttle body simply responds to the ECU's commands according to Ike Ogbuik, applications engineer for ETC at Kolbenschmidt Pierburg. When the throttle body controls the interaction and communicates with the ECU, it truly is an electronic throttle control. European diesel engines usually have an electronic throttle control.

Sensors are required for the accelerator pedal to provide the driver's intent and at the throttle to indicate the position of the throttle blade. In the case of the ETC system, both of these sensors can be resistive or non-contact sensors, such as Hall effect sensors. “The market in the future is going more non-contact,” said Peter Hradowy, vice president of sales and engineering. One of the leading suppliers of ETC in Europe, Kolbenschmidt Pierburg AG, recently introduced its next-generation ETC unit shown in Figure 2a that is priced as much as 20% less than its current models [2]. The cost reduction came from modular design, a redesigned housing and common components.

An example of enabling technology that has made ETC a high penetration rate system and allowed the cost to be reduced is the availability of application-specific standard products (ASSPs). The semiconductor industry developed these ICs specifically for ETC and other automotive motor control applications. An example is the TLE6209 power IC from Infineon, shown in Figure 2 (b) [3]. The circuitry is optimized for controlling dc motors and delivers up to 6 A continuous and 7 A peak current. Operating at supply voltages up to 40 V with low RDS(on) of 150 milliOhms (typical) per switch, minimum heatsinking is required to keep the circuit within its safe operating temperature even in underhood temperatures of 125°C. The TLE6209 can be used for pulse width modulated (PWM) applications up to 25 kHz, minimizing both audible noise and electromagnetic interference.


A number of suppliers manufacture electrohydraulic power steering systems where an electric motor drives the hydraulic pump rather than the engine driving the pump continuously. Delphi provides an EPS system called E-Steer that incorporates a steering gear, assist mechanism and electronic controller to reduce the steering force and eliminates the need for a power steering pump, hoses, hydraulic fluids, and a drive belt and pulley on the engine. The components that are in the E-Steer system are shown in Figure 3. The column-mounted motor drives an intermediate shaft attached to a Rack and Pinion steering gear mounted on the axle. Sensors for this system measure the steering shaft torque and hand-wheel position. These inputs are used with the vehicle speed signal and other system variables by the E-Steer's electronic control module that verifies their integrity and then determines the direction and amount of steering assist.

Delphi also has a rear steering system, called Quadrasteer, installed on some General Motors' vehicles. The rear steering is a steer-by-wire system with no mechanical linkage between the driver and the rear subsystem. Delphi views the Quadrasteer system as an intermediate step between E-Steer and full steer-by-wire.

With the power requirements for EPS being in the 500 to 1,000 range, highly efficient power MOSFETs are required, especially in today's 14 V systems to minimize the power dissipation. Fortunately, the new higher power automotive electronics systems being developed will directly benefit from improved efficiency power semiconductors. This results when trench MOSFET technology, which has been widely used in many portable consumer products, is used instead of the planar MOSFETs that were previously designed into automotive applications. For example, a frequently used figure of merit for the technology capability of power MOSFETs is the lowest on-resistance that can be obtained in a TO-220-type package. Until recently, this has been 4 milliOhms for 40 V rated planar MOSFETs used in 14 V systems. With trench technology, 2 milliOhms has been achieved in products such as International Rectifier's IRF2804S, reducing the power lost in the package to 50% of the previous level.


Similar to electrohydraulic power steering, electrohydraulic brakes (EHB) are the first step toward full by-wire technology for braking. In these systems, traditional hydraulic brakes apply the braking force but a sensor on the brake pedal provides the driver's input to an electronic control unit instead of the brake pedal linkage. An example of EHB is Robert Bosch Corp's Sensotronic Brake Control (SBC) system that was first used on Mercedes-Benz SL sports car in the 2003 model. In the system, sensors gauge the pressure inside the master brake cylinder, as well as the speed with which the brake pedal is operated, and passes this data to the SBC computer [5].

Continental provides an EHB system for the Ford Escape Hybrid [6]. To improve the vehicle's efficiency and increase miles per gallon, the series regenerative braking system provides electrical power to the battery. The system controls the distribution between hydraulic brake force and electrical regenerative braking. The braking capability of the high-voltage generator in the powertrain is maximized to recharge the batteries. Traditional friction brakes provide additional stopping torque as required by the system.


Volvo developed a suspension system called Four-C (Continuously Controlled Chassis Concept), which adjusts damping for each shock absorber at a speed that sets a new standard for production cars. A Freescale Semiconductor 40 MHz microcontroller processes signals from five accelerometers and two height sensors, as well as signals from dynamic stability, traction control/ABS and engine control systems. Steering wheel sensors provide both speed and position inputs. The Four-C system detects wheel hop, dive and lift, oversteer, understeer and roll, and adjusts the dampers at each wheel to compensate. Response is extremely rapid — around 5 ms — achieved by electromagnetic control valves used in the shock absorbers [7].

The ultimate suspension-by-wire, a fully active suspension, will require as much as 2,000 W to activate motors that would react prior to encountering significant road surface variations. This is one of the loads frequently cited as requiring the 42 V architecture.


Automatic transmissions routinely use high-speed solenoid valves for controlling the hydraulic fluid to shift gears. In transmissions such as Daimler Chrysler's 7G-tronic, a 7-speed automatic, the electronic control is used to achieve 0.1 s faster response time, 0.1s to 0.2s faster downshifts and 7% faster kickdown acceleration form 60 kph to 120 kph (37 mph to 75 mph).

Magna Steyr Powertrain supplies an electronically controlled multiplate clutch for BMW's xDrive, which is standard on the X5 and X3 series. The electric motor provides a reaction time of 100 ms. A Texas Instruments TMS470 MCU with an ARM core monitors the torque to the front wheels every 20 ms.


Today, x-by-wire technologies are implemented separately in the engine control, braking or steering system. In concept, vehicles such as General Motors' Hywire and Paradigm, by-wire control is an integral part of the entire vehicle design. To address the higher-powered loads and provide for regenerative braking, 42 V and even higher bus voltages will be used.

With the ultimate XBW technologies, redundant mechanical linkages are no longer used. This puts the entire system redundancy into the electronics. Figure 6 shows the various aspects of power and communication that could be present on future vehicles [8]. The integral starter alternator (ISA) and the battery are 42 V units. A dc-dc converter provides power for traditional 14 V loads. Newer x-by-wire systems are powered by 42 V, except for lower- power loads such as ETC. Another major change is the use of a time-triggered architecture (TTA) to communicate information.

Today's vehicles frequently use CAN and LIN for communicating instructions and receiving diagnostic information. True drive-by-wire architectures will require a safety-critical data bus with inherent fault tolerance and higher bandwidth. Currently, two architectures, Time Triggered Protocol (TTP) and FlexRay, are being considered by automakers. The time-triggered technology in both of these protocols is viewed as essential to ensure that important messages always get through on the data bus. The combination of intelligence communicated on this bus and next-generation semiconductors will be required to take x-by-wire systems to the next level.


  1. Nico A. Kelling and Patrick Leteinturier, “X-by-Wire: Opportunities, Challenges and Trends, 2003-01-0113, SAE World Congress, Detroit, MI, March 3-6, 2003.
  2. Kolbenschmidt Pierburg Introduces Next-Generation Electronic Throttle Control, press release, Detroit, MI, March 29, 2004.
  3. Andreas Pechlaner, Hermann Kern and Frank Auer, “Throttle Control with Smart Power Bridges and Microcontrollers of the C500 and the C16x-Families,” Infineon SE0899.
  4. Sanket Amberkar, Barbara J. Czerny, Joseph G. D'Ambrosio, Jon D. Demerly and Brian T. Murray, “A Comprehensive Hazard Analysis Technique for Safety-Critical Automotive Systems,” 2001-01-0674 SAE 2001 World Congress Detroit, MI, March 5-8, 2001.
  5. “Electrohydraulic brake SBC by Bosch — Now also in the Mercedes-Benz E-Class,” www.Bosch.com.
  6. Continental Supplies Advanced Braking System for Ford Escape Hybrid SUV, Detroit, MI, July 27, 2004.
  7. Intertech's “Power Management in Today's and Future Automotive Systems” including the 2004 update http://www.intertechusa.com/studies/PowerManagement/PM_Study.htm.
  8. Randy Frank, “Toward the Intelligent Power Network,” 2002-21-0060, Convergence 2002 Proceedings, Detroit, MI, Oct. 21-23, 2002.


Randy Frank is a freelance writer and president of Randy Frank & Associates Ltd., a technical marketing consulting firm based in Scottsdale, Ariz. He can be reached at [email protected].


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