- The Washington Times - Friday, March 29, 2002

It was back in the 1930s when automotive brakes were first revolutionized that's when troublesome mechanical brakes were replaced by a more reliable hydraulic system. Hydraulic brakes have been improved over the years with the addition of the dual master cylinder, proportioning valves, power boosters, and recently antilock technology.

Even so, the basic operation of the hydraulic system hasn't changed much. When the driver steps on the brake pedal, a rod pushes on a piston in the master cylinder that forces brake fluid to each wheel. This pressure buildup slows the vehicle by squeezing friction pads against brake rotors inside each wheel. This time-proven hydraulic brake system is used in nearly all cars currently on the road. That's all changing now.

The new 2003 Mercedes-Benz SL500 is the showcase for an electronic braking system that vastly expands a driver's capability to control the car. Now, lightning-fast electronics replace many of the mechanical and some of the hydraulic parts. Integrated with the data network of the car, a powerful microcomputer oversees the system's operation by processing information from a series of sensors and control units.

With split-second accuracy, the system can change brake pressure on each wheel over uneven surfaces and can even increase brake pressure on just the outside wheels when braking in turns, taking advantage of the higher loading during cornering. Even the anti-lock and stability control systems work more efficiently since they are more deeply integrated with the brakes, instead of functioning as parallel systems.

Although the whole system makes extensive use of electronics, computers, and software, several hardware changes are necessary to make it all work. An electrically-driven hydraulic pump, supplying a high-pressure reservoir/accumulator, provides an always-on source of brake pressure.

Computer-operated modulator valves control how much pressure actually operates the brakes at each wheel. In the new system, the bulky vacuum brake booster is obsolete.

Several more-subtle features improve braking performance. If the driver abruptly moves his foot from accelerator to brake pedal, pedal sensors recognize the early signs of an emergency situation and prepare the brake system for quick action. Before the brake pedal is touched, the caliper pistons are moved hydraulically from their relaxed position, where they create no drag, to a position just touching the rotor poised to spring into action with full force as soon as the brakes are applied. Engineers at Mercedes-Benz claim this pre-loading of the braking system reduces stopping distance at highway speeds by about 3 percent.

Since all SL500 electrical components can "talk" to each other over a common computer network, the electronic brake system knows when the windshield wipers are operating and imperceptibly applies the brakes just enough to keep the rotors dry, so that brake operation remains fast and consistent in the rain.

So how does it all work? When the driver steps on the brake pedal, hydraulic fluid is forced into what the Mercedes engineers call a pedal travel simulator. It's a spring-loaded hydraulic accumulator that accepts fluid when the pedal is depressed and sends it back to the master cylinder when the pedal is released. Other than that, the fluid never goes anywhere else, except during a complete electrical failure when pedal pressure is diverted directly to the front wheels as a backup system.

The brake pedal is instrumented to measure travel, and the simulator has a sensor to measure pressure. The electronic braking microcomputer calculates exactly the right brake force for each wheel by processing signals from the pedal unit and from sensors all over the car. For example, it considers data on wheel speed from the ABS system, steering angle and cornering g-forces from the stability control system, and signals about the engine braking effect and which gear is engaged from the engine-transmission microcomputers. The result of these highly complex calculations is instantaneous brake commands, which means maximum braking and driving stability.

Impressive as electronic brakes are, the SL500 features even more high-tech developments such as active suspension, high/low beam Xenon headlights, adaptive cruise control, keyless ignition, LED brake lights, a pop-up roll bar and of course, voice control of the radio and phone.


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