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Auto Electronics – Braking Systems
A revolution in braking technology is in the making. A new electronic wedge brake currently being developed for market launch by Siemens can be used for any type of vehicle. But that’s not all. The new brake is also faster, less expensive and more efficient than conventional hydraulic systems.
Bernd Gombert cranks up the metal disk until it rotates at speed. "Now, press here and make it stop," he says. It requires strong pressure applied with the palm of the hand before the plastic brake pad brings the disk gradually to a halt. "Friction," says Gombert in his dry engineer’s manner. "That’s how today’s cars brake. Now try the wedge." Once again, he cranks the disk up, only this time harder. The small wedge is mounted at the side, between a metal pin, which sits parallel to the disk, and a guide mechanism. Although the metal disk is rotating very quickly, a prod of the index finger is all it takes for the wedge to be literally dragged onto the disk, which snaps to a halt immediately. "That’s how they used to brake horse-drawn vehicles," says Gombert with a grin. "And, in principle, that’s how brakes will function in the future. A lowenergy, low-cost system with a simple mechanical design!" Not that Gombert is advocating a return to the horse-drawn era, when wedge brakes were used to provide a highly effective but very abrupt means of bringing the wheels of a cart or carriage to a halt. Instead, he’s referring to the Electronic Wedge Brake (EWB), a development project that Gombert heads at Siemens VDO Automotive (SV). "The difference is that we stop the wedge from completely locking the wheel by preventing it from being fully dragged in between the mounting and the brake disk," he explains. The trick here is to allow the wedge to be pulled in until the desired braking moment has been achieved, but no further. This in turn requires sophisticated sensors and a precisely controllable electric motor (see info box).
For each wheel, the electronic wedge brake has a control unit (see diagram above) consisting of a brake pad, a mechanical transfer system, two electric motors for precision control, and sensors to measure movements and forces. Around 100 times a second, a total of four sensors measure wheel rotation and therefore the speed of the vehicle, the forces on the brake and the position of the wedge. Whenever the driver presses the brake pedal, the system transmits the force electromechanically to the wheels, which are electronically networked with one another. Depending on the sensor readings and the braking signal coming from the driver, the two electric motors move the brake pad over a series of rollers along a slanted surface—the actual wedge. The position of the rollers on the inclined surface determines the pressure point of the brake pad. When the pad presses against the disk, the latter is immediately braked. As soon as a high braking moment is generated by increasingly powerful frictional forces, the electric motors either hold the brake pad in position or move it back over the roller bearing and into an optimum position. The distances involved are a matter of micrometers, and the response times are measured in milliseconds. The vehicle’s onboard 12-V network is perfectly suited to driving the electric motors. In fact, a flashlight battery would also be powerful enough.
"The intelligently controlled wedge converts the vehicle’s kinetic energy directly into braking energy," explains Gombert. As a result of this self-reinforcement, the EWB only requires one tenth of the actuating energy required by today’s hydraulic braking systems. What’s more, it also responds substantially quicker. Given this significantly enhanced efficiency, the EWB will also have smaller dimensions and therefore reduce total vehicle weight. At the same time, there will no longer be any need for brake lines, a power brake unit or a brake fluid reservoir, which will free up a volume of around 22 l in the engine compartment and thereby give vehicle designers added scope. "And you won’t need a hand brake any more," adds Karsten Hofmann, head of Chassis Product Marketing at SV.
Likewise, the well-nigh ubiquitous ABS anti-lock braking system and the less prevalent electronic stability program (ESP) will be replaced by the integrated software in the EWB system. "We’ve developed our own algorithm that reproduces these functions," explains Hofmann. "But our system is much quicker." All in all, it takes 140 to 170 ms for a conventional ABS to generate full braking power. The EWB only needs around 100nbsp;ms, an advantage which, according to Siemens engineers, should make a difference on uneven and icy roads. "Our system can control each individual wheel faster than any hydraulic system and thus keep them on course better," says Hofmann.
With the EWB project, Siemens VDO is also preparing for the time when electric vehicles will claim a much greater market share. In fact, successful hybrid cars can already be seen on our roads today, including the Toyota Prius, which enjoys cult status in California and has been voted Car of the Year 2005 by European auto magazines. Future electric cars may well be equipped with an individual motor for each wheel. This would generate substantially higher torque directly at the wheel and therefore liberate more power to accelerate the vehicle. Another possibility would be to recover kinetic energy during braking. Here, the electric motor fitted to the wheel would also function as a generator and produce electricity to recharge the battery.
Brake by wire. The EWB would be a perfect complement here, as it would be able to supply whatever additional and emergency braking capacity is required. It could also be used in conventionally powered vehicles to provide a range of electronic "brake-by-wire" features. These might include a "soft-stop" function, whereby the system brings the vehicle to a smooth halt by automatically reducing the braking force just before it stops, or an antistall assistant controlled by software, which would prevent the vehicle from rolling back when pulling away on a gradient. Such a feature would certainly be welcomed by student drivers. Although the EWB requires a constant electrical current, it is immune to problems associated with the onboard power supply. "If the current to one of the wheels were disrupted, the system would automatically compensate by distributing the braking power to the others," says Hofmann. And if the vehicle’s entire power supply system failed, an emergency battery would ensure that all vital functions were maintained until the fault was remedied.
However, Gombert isn’t too worried about problems related to the new system. He’s much more interested in its potential. Gombert was a top researcher at the German Aerospace Center (DLR) before leaving to devote more time to the EWB project. In 2000 he set up his own company, eStop, which was taken over by Siemens VDO in 2005. Gombert holds almost 120 patents, 40 of them with eStop, for various aspects of the EWB system. In September of this year, his team demonstrated the principle at the International Motor Show in Frankfurt, Germany, and are currently testing the system in a vehicle that will be made available to automakers at the end of the year for further tests. The pilot customer is a major European automobile manufacturer. If everything goes according to plan, the first vehicles fitted with the EWB as standard should hit the roads in 2009. Before then, however, this completely new technology will have to prove that it can compete both technically and economically with a braking system that has been successfully employed for more than 70 years. Aside from passenger cars, Gombert also sees big potential for the new system in heavy-duty vehicles. Today’s trucks are fitted with air brakes, and it can take up to a second for the brake signal to reach a rear trailer. Using the EWB system, it would be possible to brake a trailer more quickly and in a more controlled way. In principle, any wheeled vehicle can be braked using this new system, including high-speed trains, which are currently equipped with maintenance-intensive and therefore expensive brakes. Gombert also has an eye on automation technology, which employs all sorts of motors. His vision is that his technology might one day be used to brake and regulate anything driven by a motor.