A new advance could revive brake-by-wire technology, which has been stalled since global automakers decided not to adopt 42-volt electrical systems.
Siemens VDO Automotive is developing a system that addresses a technical obstacle of by-wire brakes -- providing sufficient power to make the caliper clamp hard enough on the rotor to stop the car.
The developers of early by-wire brakes anticipated carmakers moving to 42-volt in-car electrical systems from 12-volt to provide enough power for effective clamping forces. But automakers decided to avoid the cost and complexity of 42 volts.
The Siemens VDO 12-volt prototype employs metal wedges in the brake system that use the car's momentum to mechanically amplify clamping forces.
"We are confident we will launch a complete brake-by-wire system in a premium car before the end of the decade," said Bernd Gombert, Siemens VDO's chief technology officer of body and chassis electronics.
He said Siemens VDO's system will cost no more than a hydraulic system, perform better and save fuel.
Industry still wary
It is hard for new braking solutions to compete against effective, well-priced hydraulic brakes. Delphi just found its first customer for a hydraulic system that doubles braking forces in a small package.
Delphi didn't name the customer.
By-wire brakes had a shaky start. Pioneered by Mercedes-Benz and Robert Bosch on the E class in 2002, the complex system was recalled twice within a year. In addition, the by-wire brakes provided few noticeable customer benefits.
"It had to have a hydraulic back-up, which unfortunately came into play quite often," said a senior industry executive.
The experience created mistrust and suspicion toward innovative braking solutions. As a result, 98 percent of European new cars have conventional hydraulic disc brakes.
Automakers remain cautious. A BMW spokesman declined comment on future brake technology. But last year, BMW board member Burkhard Göschel said that brake-by-wire would come to BMW and would be used first on the rear axle.
Bosch, Continental and TRW Automotive lead European brake sales. A Frost and Sullivan study estimates 2005 European advanced brakes sales at E5 billion. Globally, braking is a E17 billion segment.
Joseph Pickenhahn, TRW vice pre-sident for brake engineering, said future brake growth will be in combined braking and steering systems.
"Collision mitigation systems will dictate new requirements," he said. "We want to apply the brakes as fast as possible. It's 600 milliseconds before we get 1g braking. We want to get the 1g within 300 milliseconds."
Siemens VDO's Gombert says by-wire's all-electrical signals can link to a car's existing chassis computer.
"You can really save money by combining electronic units," he said. "The ABS [antilock brakes] and ESC [electronic stability control] are not hydraulic valve blocks, but just pieces of software."
Some have doubts about by-wire.
Krishnasami Rajagopalan, author of the Frost and Sullivan study, believes the Siemens VDO system will be "quite well-priced" but its need for a backup battery will limit use.
TRW is only monitoring developments of by-wire systems.
Bosch will focus on hydraulic brakes for the next 10 to 15 years. "Bosch is still researching electro-mechanical braking systems, but sees technical hurdles without clear solutions to overcome them," spokes-man Stephan Kraus said.
Continental said it is researching and developing by-wire braking systems, but declined further comment.
Italian brake supplier Brembo has two by-wire projects with carmakers. "Brake-by-wire will come on one part of the vehicle first," said Roberto Vavassori, business development director. "Manufacturers want to be very cautious [after] the E class."
But by-wire brakes will come, he said. "We favor small fully electric or hybrid vehicles to exploit this technology first."
Advantages
- Quicker ABS cycle time shortens stopping distance
- Reduces car weight, fuel consumption
- Uses conventional disc, pads and 12-volt system
How it works
The Siemens VDO electronic wedge brake is a brake-by-wire unit that does not need a costly 42-volt car electrical system.
Conventional electromechanical brake calipers need to develop a clamping force of six to eight tons on the disc to slow a car. But the wedge brake uses the vehicle's kinetic energy to mechanically amplify braking pressure, eliminating the need for huge mechanical effort or strong electric motors.
The system also retracts the friction pad when the brake is off, reducing drag, increasing pad life and allowing six-times faster ABS cycle times. Compared with ordinary brakes, the braking distance at 100kph is 2m shorter.
If it goes wrong
A conventional dual-circuit hydraulic braking system has two levels of redundancy. If one hydraulic line fails, the back-up circuit still brakes two or three wheels. If boost assistance is lost, the backup works, but the driver must push the brake pedal harder.
Brake-by-wire systems must protect against a total loss of electricity. The Mercedes E class used a "wet" backup system of a conventional hydraulic circuit.
Siemens VDO chose a "dry" method: a backup battery to power the brakes if the main system fails. Each wheel has a separate circuit, so three remain if one fails.
