Brake by Wire

By Tom Ripley

You might soon be saying goodbye to brake cylinders, hydraulic lines and brake fluid. Siemens VDO, a global supplier of electronics to the auto industry, has developed a vehicle braking concept that turns the conventional braking system of your car on its ear. Called the electronic wedge brake (EWB), the system is a low-power, low-energy 12-volt by-wire braking technology that uses a car's kinetic energy to stop itself. The system not only rids cars of potentially troublesome master cylinders and leaky hydraulic lines, it also eliminates the need for brake boosters and traditional antilock braking control.

Just as hydraulic-activated brakes replaced mechanical brakes in the 1920s, Siemens VDO expects its system to prompt a switchover from many manufacturers because of its advantages over conventional hydraulic systems. It lowers overall weight and makes for greater reliability and improved safety with reduced servicing requirements. By doing away with the hydraulic braking system, it also helps to make the vehicle more environmentally friendly. In addition, the EWB works much faster than conventional hydraulic brake systems, enabling shorter stopping distances on challenging driving surfaces, such as ice and snow.

Because it uses the standard 12-volt vehicle electrical system, the system can be readily installed in a wide variety of vehicles, opening up new design potential because the hydraulic-free wedge brake takes up less space both in the engine compartment and in the chassis. EWB is based on technology developed by eStop, a firm which was acquired by Siemens VDO in early 2005, and its control-related foundations originate from German Aviation and Aerospace Center applications. During braking, a brake pad attached to a wedge is pressed between the brake caliper and the brake disk. As the wheel turns, the wedge effect is automatically intensified. This allows any level of braking power with a minimum of intricacy.

Vehicles using the electronic wedge brake will have an intelligent wheel-braking module fitted to each wheel. The module consists of brake pads, a wedge attached to the wedge-bearing mechanism, a mechanical power transmission between two electric motors, and a sensor system for monitoring movement and force. When the driver engages the brake pedal, the EWB system electronically transmits the activation signal to the interconnected brake modules. When the brake activation signal is received, electric motors actuate a wedge-bearing mechanism to move the wedge into the required position, according to the sensor feedback values. This causes the brake pad to press against the brake disk. Based on the principle of self-energization, the braking effect builds up very rapidly, while the intelligent control prevents any danger of the wedge locking up. The mechanical decoupling of the brake pedal and the brake also will prevent the often-misunderstood pulsing of the brake pedal when antilock brakes near maximum braking effort.

While it might seem this system would be vulnerable to failure, the principle of "unstable" control structures was taken from high safety-critical systems for aviation and aerospace applications and adapted for automotive purposes. Similar to airplanes, the EWB has a sophisticated redundant architecture, making the system extremely safe and reliable.

A final bonus of the electronic wedge brake is that it also can be used as an automatic parking brake. This allows vehicle designers to do away with the traditional manual parking brake, while preventing the vehicle of the future against rolling away when parked.

Driving Today Contributing Editor Tom Ripley writes about automobiles and the human condition. He lives in Villeperce, France.

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