Driving by wire

DN Staff

October 7, 2002

8 Min Read
Driving by wire

Three laps around Stile Bertone's Caprie facility in the Susa Valley are enough to learn how to drive the car of the future. It takes one lap to get used to the brand new sensation of driving without steering column and wheel, clutch, brake, or acceleration pedals. A second lap is needed to settle down. After the third, it becomes difficult to imagine that all cars aren't built like this.

Left/right steering control yokes are linked mechanically and offer a full travel of 20 degrees.

The test drive was organized by Bertone and SKF, supported by other project contributors Brembo, Nokia, and Bose. The drive's mission: Introduce a totally new concept car to the public. Dubbed "Filo" (Italian for "wire"), the car not only demonstrates the possibility of drive-by-wire technology for automotive vehicles, but its reality. After all, SKF's avionics and aerospace arm, known as SARMA, has been providing by-wire technology to the civil aviation industry for 15 years. SKF, along with its partner companies, are betting that the mechatronic, actuation, and control loop systems now common to aircraft will work their way into automobiles as well.

Technical considerations. How does driving the Filo differ from conventional vehicles? By-wire technology controls steering, gear change, acceleration, and braking. Instead of using traditional mechanical and hydraulic interfaces, the driver communicates his or her intentions to the vehicle's major operating systems electronically. Smart electro-mechanical actuating units (SEMAUs) with integrated sensors and measurement systems do the rest.

Each SEMAU, supplied by SKF, translates rotational motion into linear motion via a brushless motor/ball screw assembly. The motor features an external stator and internal rotor mechanically connected to the ball screw. Motor rotor and ball screw run in bearings at either end. "One of those bearings," explains Richard Hannis, responsible for SKF Drive-by-Wire product development, "is an SKF Sensorized Bearing with built-in Hall sensor. That sensor gives the commutation signal for the brushless motor to fire transistors in the electronics at specific angular events."

A microprocessor on the SEMAU allows digital closed-loop control and supports application-specific logic. All SEMAUs connect as separate nodes to Filo's high-speed (2 Mbaud capacity) data backbone. The backbone architecture runs a time-triggered protocol (TTP).

"Because each smart electro-mechanical actuating unit communicates with the vehicle control system," says Hannis, "the drive-by-wire system is very flexible. Tradeoffs can be made between force, response time, and/or actuating travel to tailor system operation." Hannis adds that the fully programmable SEMAUs provide diagnostic information and can be configured fail safe or fault tolerant.

Filo's interior -- without pedals, gear shifter, and steering column -- offers plenty of space. Rear seats position higher than those in front for better forward visibility.

Shift-by-wire. When entering the Filo for a test drive, the first thing one notices is the vehicle's spacious interior. The familiar elements of driving are strangely missing-steering wheel and steering column, pedals, as well as the gear selector. In their place is the Guida-Filo ("Drive Filo") for driver control of the by-wire systems. Even the configuration of this physical interface is not necessary, but Bertone designers wanted to retain some semblance of a conventional steering wheel for easy acceptance.

Plus and minus buttons on the Guida-Filo correspond to the up and down shift. The driver shifts up through the gears by simply pressing the plus button. A complex series of messages based on engine speed, torque, and other sensed data ensures the following simplified sequence:

The initial request goes to the clutch actuator control unit, which carries out instructions to disengage the clutch. With this operation complete, it sends another signal to the gearshift ACU, giving it permission to shift up. Once this operation is successfully executed, the gear-shift ACU notifies the clutch ACU that the clutch may be re-engaged. Reverse gear is selected via a dedicated button. Logic in the actuator control unit prevents inappropriate selection.

Power is supplied to the actuator's electric motor. A ball screw ocnverts rotary motion in to linear motion. Integrated electronics and sensors control position and/or force applied to a mechanical load, which turns the front wheels, engages the brak calipers or shifts the gears.

To demonstrate that the by-wire systems fit within the confined spaces of a modern car platform, notes SKF's Hannis, Filo employs a conventional H-pattern manual gearbox. This, he says, requires an actuator with both linear and rotational motion to accomplish the second-to-third and fourth-to-fifth movements of the original selector mechanism inside the gearbox.

Steer-by-wire. Once Filo is in gear, the driver opens the throttle and releases the brakes by easing his or her grip on the left/right Guida-Filo steering control yokes. Offering a 20-degree travel swing, the steering control yokes interface with Filo's steering control system. The steering control system maps movement of the front wheels with vehicle speed to set the appropriate level of steering sensitivity. Existing SKF knowledge regarding aircraft side stick feedback systems forms the basis for the driver feedback incorporated into Filo's steering control.

Two SEMAUs drive the steering rack. One as the master; one as a standby. The master actuator communicates with a steering feedback actuator. Two control loops determine rack position dependent on 1) driver demand and 2) force feedback from actual drive conditions. A motor generates resistance torque in the steering Guida.

Just as the relationship between movement of the yokes and movement of the front wheels is fully programmable, so is the level of "feel" experienced by the driver. "It is possible," Hannis points out, "to program a non-linear transfer function between steering device and rack position. One can therefore make it easy to negotiate tight maneuvers such as parking."

Filo's data bus integrates all of the by-wire subsystems (4 brake, 3 steer, 3 transmission, and 2 for vehicle control) into a single vehicle system.

Brake-by-wire. Unlike conventional braking systems, every wheel in Filo's braking system works autonomously and features an independent actuating control unit. A joint development between Brembo S.p.A. and SKF, the Filo brake-by-wire system employs a SEMAU integrated into the brake caliper of each wheel. Squeezing the left/right handgrips on the steering control yoke signals the brake system's four actuating control units. These, in turn, calculate the braking forces applied by the individual calipers according to Filo's control logic.

The by-wire caliper design is common to front and rear axles. Likewise, the Filo brake system employs the same size discs front and rear. The discs are made of a composite ceramic-fiber material for low weight, thermal stability, and long life. In addition, closed lubrication ensures that the brakes are environmentally friendly.

"Because the brakes are controlled both as a single vehicle system and individually as four separate subsystems," Hannis claims, "Filo effectively increases the level of protection over conventional split-circuit hydraulic systems."

Future prospects. Based on a cursory walk-around, one might be tempted to dismiss Filo as another attention-grabbing concept car intended for the international auto show circuit. "Not so," counters Roberto Piatti, managing director of Stile Bertone. "Working with the Drive-by-Wire Business Unit of SKF, we are utilizing innovative technology as an opportunity for design." By demonstrating the enormous potential of drive-by-wire, he states, the Filo directly challenges traditional automobile design.

When will the driving public actually experience by-wire technology? Filippo Zingariello, head of the SKF Drive-by-Wire Business Unit, envisions a progressive introduction, beginning with electric command of the gearbox, perhaps in 2003 or 2004. Brake-by-wire, he predicts, will follow with the first production systems entering the market in 2005. "First we need 42V electric systems," he cautions. Steering-by-wire will be the last implementation because of current legislation requiring a mechanical link between the driver and wheels.

To date, SKF has filed 35 patents for automotive by-wire functions, covering brake-by-wire, steer-by-wire, clutch-by-wire, gear shift-by-wire, and CVT-by-wire. Other possibilities, such as turbo-by-wire and suspension-by-wire, are under consideration. Evidence enough that the company views the Filo not as a concept car, but a prototype "car of the future."

Sign up for the Design News Daily newsletter.

You May Also Like